White Shark (Carcharodon carcharias) Atlantic population: COSEWIC assessment and status report 2021

Official title: COSEWIC assessment and status report on the White Shark (carcharodon carcharias) Atlantic population in Canada

Endangered 2021

COSEWIC assessment summary

Assessment summary – April 2021

Common name: White Shark - Atlantic population

Scientific name: Carcharodon carcharias

Status: Endangered

Reason for designation: This highly mobile species is a seasonal migrant in Atlantic Canada and considered to be part of a widespread Northwest Atlantic population. The status of the Canadian population is considered to be the same as that of the broader Northwest Atlantic population. That broader population is estimated to have declined by >70% over the past 1.5 generations (since the 1960s) because of incidental mortality from fishing. However, the population appears to have remained stable since the 1990s and is projected to remain stable or increase slightly. Although measures to improve fishing practices have been introduced, the primary threat continues to be mortality from incidental capture in fisheries. The species is still vulnerable to this threat because of its long generation time (42 years) and low reproductive rate.

Occurrence: Atlantic Ocean, New Brunswick, Newfoundland and Labrador, Nova Scotia, Prince Edward Island, Québec

Status history: Designated Endangered in April 2006. Status re-examined and confirmed in May 2021.

COSEWIC executive summary

White Shark

Carcharodon carcharias

Wildlife species description

White Shark (Carcharodon carcharias (Linnaeus, 1758)) is the only species of this genus. In French it is called grand requin blanc. Genetic evidence combined with satellite tracking information clearly shows that this species is wide-ranging in the Pacific and Atlantic oceans. Gene flow among populations across ocean basins is restricted based on high site fidelity and reproductive philopatry of females (returning to same spawning site). There is no known genetic distinction across the Canadian and U.S. population in the Atlantic Ocean. As such, the entire Northwest Atlantic population should be considered a single designatable unit.

Cultural significance

All species are significant and are interconnected and interrelated. As part of COSEWIC status assessments, Aboriginal Traditional Knowledge (ATK) reports are prepared by the Aboriginal Traditional Knowledge subcommittee (ATK SC) based on publicly documented ATK compiling and summarizing the relevant ATK to the status assessment. There is no ATK report for White Shark.

Distribution

White Shark is widely distributed, from 60°N to 60°S, but is most frequently observed in temperate waters over the continental shelves of the western North Atlantic Ocean, Mediterranean Sea, the North Pacific Ocean, and off the coasts of southern Africa, southern Australia, and New Zealand. On the Atlantic coast of Canada, White Shark is known from fewer than 100 confirmed or probable records since 1874, in addition to tracking of White Shark tagged in the Atlantic waters of both the U.S. and Canada. White Shark occur throughout Atlantic Canada and are seasonal migrants in the late summer and early fall. Tracking shows that the White Shark uses most of Canada’s Exclusive Economic Zone (EEZ) south of 52°N but occasionally ranges off the continental shelf northward to waters near Greenland.

Habitat

White Shark range widely in coastal and oceanic waters. Juveniles are common in coastal habitat but move off the continental shelf seasonally as adults. Bathymetric range is from near the surface to just above the bottom, to a depth of at least 1,128 m but predominately in waters from <50 m to 500 m. White Shark is most common in water temperatures between 14-25°C but can be found in waters from 1.6 to 30.4°C.

Biology

White Shark are ovoviviparous with gestation period estimated from 10-20 months. Litter size averages seven pups. The Mid-Atlantic Bight is a likely pupping area, with the New York Bight being a nursery area. Length of the reproductive cycle is estimated at up to two years and may be more than three years. Maximum lifetime reproductive output has been estimated to be 45 pups. In the Northwest Atlantic, males are estimated to reach sexual maturity at 26 years and a length of ∼3.8 m while females likely reach maturity at an age of 33 years and a length of 4 to 5 m. Lifespan in this population is estimated to be 40-73 years based on limited sampling, particularly for females. Generation time has been estimated at about 42 years, with a low natural mortality of 0.063-0.125/year. Intrinsic rate of population increase is also low and estimated at 0.035-0.056/year.

Population sizes and trends

There are no estimates of population size in Canadian Atlantic waters. There are only around 100 records of White Shark off the Atlantic coast of Canada since 1874, although sightings are increasing, with more than 40 since 2009. Abundance in Canada has likely always been much lower than in adjacent U.S. waters, given the low encounter rate in commercial and recreational fisheries in Canada and apparent seasonal presence. The White Shark population trend in the Northwest Atlantic is uncertain but is likely stable or increasing. While all analyses agree on the major decline of the White Shark population from the 1960s to the 1990s, there is substantial uncertainty about any subsequent increase.

Threats and limiting factors

Human activity is the most significant threat to White Shark, which is taken as sport fish, commercial bycatch, and for international trade of their body parts. In the Northwest Atlantic White Shark occur as bycatch in commercial fisheries in USA waters. Recorded incidental capture in Canada is rare. Their late age at maturity, small litter sizes, and longevity make White Shark highly vulnerable to even low levels of mortality. Their position as top predators and ovoviviparous reproduction also make them vulnerable to pollution by environmental toxins, particularly organochlorines.

Protection, status, and recovery activities

At the start of 2005, the Convention on International Trade in Endangered Species of Flora and Fauna (CITES) listed White Shark in Appendix II. In 2009, the International Union for the Conservation of Nature listed White Shark globally as ‘Vulnerable’. The species was protected in 1997 in U.S. waters under the federal Fisheries Management Plan. In Canada, the species has been listed as Endangered under the SARA since 2011. Under the provisions of section 32 (1) of the Act it is illegal to kill, harm, harass, capture, or take White Shark, and under section 32(2) it is illegal to possess, collect, buy, sell, or trade White Shark. All sharks captured must be released with minimal harm in U.S. and Canadian fisheries but release mortality is unknown for most fisheries.

Technical summary

Carcharodon carcharias

White Shark, Atlantic population

Grand requin blanc, population de l’Atlantique

Range of occurrence in Canada (province/territory/ocean): Atlantic Ocean, New Brunswick, Newfoundland and Labrador, Nova Scotia, Prince Edward Island, Québec

Quantitative Analysis

Is the probability of extinction in the wild at least [20% within 20 years or 5 generations, or 10% within 100 years]? Not undertaken

Threats (direct, from highest impact to least, as per IUCN threats calculator)

Was a threats calculator completed for this species? Yes, July 9, 2020

5.4 Fishing and harvesting aquatic resources

1. Bycatch in fisheries – Low impact

What additional limiting factors are relevant? Late maturity, low fecundity.

⁺ See Table 3 (Guidelines for modifying status assessment based on rescue effect)

Data sensitive species

Is this a data sensitive species? No

Status history

COSEWIC Status History: Designated Endangered in April 2006. Status re-examined and confirmed in May 2021.

Status and reasons for designation:

Status: Endangered

Alpha-numeric codes: A2bd

Reasons for designation: This highly mobile species is a seasonal migrant in Atlantic Canada and considered to be part of a widespread Northwest Atlantic population. The status of the Canadian population is considered to be the same as that of the broader Northwest Atlantic population. That broader population is estimated to have declined by >70% over the past 1.5 generations (since the 1960s) because of incidental mortality from fishing. However, the population appears to have remained stable since the 1990s and is projected to remain stable or increase slightly. Although measures to improve fishing practices have been introduced, the primary threat continues to be mortality from incidental capture in fisheries. The species is still vulnerable to this threat because of its long generation time (42 years) and low reproductive rate.

Applicability of criteria

Criterion A (Decline in Total Number of Mature Individuals): Meets Endangered A2bd. The number of mature individuals is estimated to have declined by 73-79% over 1.5 generations based primarily on incidental mortality from past fishing practices.

Criterion B (Small Distribution Range and Decline or Fluctuation): Not applicable. EOO of >1,000,000 km² and IAO >2000 km² exceeds thresholds.

Criterion C (Small and Declining Number of Mature Individuals): Not applicable. Absolute number of mature individuals is unknown. Most recent analyses indicate some increase in relative population indices.

Criterion D (Very Small or Restricted Population): Not applicable. Absolute number of mature individuals is unknown; criteria not applied but effective population size analysis suggests population numbers may be <1000 though this analysis is highly uncertain.

Criterion E (Quantitative Analysis): Not applicable. Analyses not done.

Preface

Since the writing of the 2006 COSEWIC status report there have been new studies on the genetics, life history, movement, and abundance of White Shark in the Northwest Atlantic, although there is little indication of major changes to bycatch mortality, the main threat to the population. Recent global genetic analyses suggest the Northwest Atlantic population is demographically isolated from other populations, even in the Mediterranean, and may be inbreeding, having recently experienced a genetic bottleneck. New aging studies have updated the maximum ages, age at maturity, productivity, and susceptibility to bycatch mortality. Recent tagging and tracking work expanded understanding of movement between the U.S. and Canada.

New studies of this population’s relative abundance suggest a decline of 73-79% from the 1960s to the 1990s. One of these recent studies estimated trends up to 2010 using three standardized indices and suggests a gradual increase since the 1990s; however, there is substantial uncertainty about the magnitude of this increase. The absolute population size of White Shark in the Northwest Atlantic is not known.

COSEWIC history

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list. On June 5, 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.

COSEWIC mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.

COSEWIC membership

COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal entities (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biodiversity Information Partnership, chaired by the Canadian Museum of Nature), three non-government science members and the co-chairs of the species specialist subcommittees and the Aboriginal Traditional Knowledge subcommittee. The Committee meets to consider status reports on candidate species.

Definitions (2021)

Wildlife Species

A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for at least 50 years.

Extinct (X)

A wildlife species that no longer exists.

Extirpated (XT)

A wildlife species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E)

A wildlife species facing imminent extirpation or extinction.

Threatened (T)

A wildlife species likely to become endangered if limiting factors are not reversed.

Special Concern (SC)*

A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats.

Not at Risk (NAR)**

A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances.

Data Deficient (DD)***

A category that applies when the available information is insufficient (a) to resolve a species’ eligibility for assessment or (b) to permit an assessment of the species’ risk of extinction.

* Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.

** Formerly described as “Not In Any Category”, or “No Designation Required.”

*** Formerly described as “Indeterminate” from 1994 to 1999 or “ISIBD” (insufficient scientific information on which to base a designation) prior to 1994. Definition of the (DD) category revised in 2006.

The Canadian Wildlife Service, Environment and Climate Change Canada, provides full administrative and financial support to the COSEWIC Secretariat.

Wildlife species description and significance

Name and classification

White Shark (Carcharodon carcharias (Linnaeus, 1758)) is the only living species of this genus. Over the years, there have been proposals to name separate regional populations, but to date morphometry, meristics, colouration, and skeletal anatomy from different ‘centres of abundance’ are not recognizably separable. The accepted French name for the White Shark is grand requin blanc.

Morphological description

The following description is taken primarily from Compagno (2001).

The snout is bluntly conical (Figure 1a). The interior teeth are enlarged and the anterior, intermediate, and lateral teeth are compressed and form a continuous cutting edge. The intermediate teeth are enlarged and are over two-thirds the height of adjacent anteriors. The total tooth count is 44 to 52. The body is usually stout with the dorsal fin origin usually over the pectoral inner margins. The origin of the anal fin is under or slightly posterior to second dorsal fin insertion. The total vertebral count is between 170 and 187 with total length of adults between 3.8-6 m and possibly longer. Typically, there is a black axillary spot at the insertion point of the pectoral fin; and the pectoral fin tips are usually abruptly black on their ventral surfaces.

Figure 1. Field characters useful for identifying White Shark (Carcharodon carcharias), A. Lateral view, B. detail of upper and lower anterior teeth. Diagrams by R. Aidan Martin.

Field Marks: Heavy spindle-shaped body with a moderately long conical snout. The teeth are large, flat, and triangular with serrated edges. The gill slits are long. The first dorsal fin is large with a dark, free rear tip; the second dorsal and the anal fins are minute; the caudal peduncle has a strong horizontal keel; and the caudal fin is large and shaped like a crescent.

The dorsal surface is a grey or brownish-grey to blackish above and the ventral surface of body is white. The margin between the dark dorsal and white ventral surfaces is sharply delimited. The iris of the eye is conspicuously black.

Population structure and variability

No genetic work has been conducted on White Shark in Canadian waters but there has been one recent genetic analysis in U.S. waters. White Shark in the Northwest Atlantic are relatively genetically isolated from the South African population (O’Leary et al. 2015). Based on mitochondrial DNA (mtDNA), two distinct lineages and three distinct haplotypes have been identified in global White Shark populations: the Mediterranean/Indo-Pacific lineage and the Northwest Atlantic and Indian Ocean/South Africa lineage, with an additional unique haplotype found in South Africa (Pardini et al. 2001; Andreotti et al. 2016b). White Shark in the Northwest Atlantic are most closely related to White Shark in South Africa; however, the populations have likely been separated for at least a half million years and are genetically distinct (mtDNA ΦST = 0.10, p < 0.00001; microsatellite FST = 0.1057, p < 0.021) (O’Leary et al. 2015; Andreotti et al. 2016b). White Shark in the Mediterranean have separate haplotypes from those in the Northwest Atlantic but the sample sizes in the analysis are too small for definitive conclusions about gene flow between these populations (Gubili et al. 2011). Satellite tracking information from other jurisdictions suggests that although this species is highly migratory, it rarely disperses across ocean basins, having high philopatry to particular pelagic and coastal aggregation sites (Pardini et al. 2001; Boustany et al. 2002; Jorgensen et al. 2010 – see Dispersal and Migration). For instance, both mtDNA and biparentally inherited DNA suggest Australian White Shark display high reproductive philopatry to coastal aggregation sites and have limited gene flow across ocean basins, even to New Zealand (Blower et al. 2012). Reproductive philopatry was also found from mtDNA in White Shark at Guadalupe Island, confirming low dispersal (Diaz-Jaimes et al. 2016). While mtDNA evidence indicates there may be no gene flow between females from the Northwest Atlantic and those from other populations, the possibility of genetic dispersal by males cannot be excluded despite the apparent philopatry in all populations studied..

Genetic structuring within the Northwest Atlantic population has not been studied and is required. However, a low M-ratio (0.71, p < 0.004) and a high inbreeding coefficient (F_IS = 0.222, p=0.001) indicate low genetic diversity, a potential for inbreeding, and a recent genetic bottleneck in this population (O’Leary et al. 2015). Additionally, Northwest Atlantic White Shark likely have a low effective population size, at least smaller than that of the South African White Shark population based on a comparative genetic study (O’Leary et al. 2015; Andreotti et al. 2016a). Populations in each of the Northeast Pacific and South Africa form single populations, based on genetic studies, and since these have a similar geographic range size to the Northwest Atlantic population, it is unlikely that genetic structuring exists for it either (Jorgensen et al. 2010; Onate-Gonzalez et al. 2015; Andreotti et al. 2016b; Diaz-Jaimes et al. 2016). These genetic studies, along with active tracking of seasonal migration between the U.S. and Canada (Skomal et al. 2017 – see Dispersal and Migration), suggest that White Shark of the entire Northwest Atlantic are a single population. However, genetic structuring between eastern and southwestern coasts of Australia suggests that philopatry in White Shark can be strong enough to create genetic structure over small scales (Blower et al. 2012).

Thermal fronts and the reproductive philopatry and high site fidelity of both males and females form the largest barriers to dispersal (Jorgensen et al. 2010; Gubili et al. 2011; Blower et al. 2012). The 15°C isotherm may be the main northern limit to its distribution in the Northwest Atlantic (Casey and Pratt 1985). Although possibly rare, ocean-wide dispersals do occur, even across warm tropical waters usually avoided by White Shark (e.g., Bonfil et al. 2005). It is unknown how much gene flow these rare long dispersal events create.

Designatable units

The designatable unit (DU) identified in the 2006 COSEWIC report is “Canada’s Atlantic population”. However, under new COSEWIC guidance, the population is best described as the Northwest Atlantic DU, as subpopulation structure within the Northwest Atlantic is unlikely (Curtis et al. 2014).

In the Atlantic Ocean, the most closely related White Shark populations (Northwest Atlantic and South Africa) have FST values of ∼0.1 (microsatellite DNA), indicating some level of discreteness, although clustering analysis identified some individuals from the Northwest Atlantic having genotypes very consistent with those from S. Africa and vice versa (suggesting dispersal in both directions). Analyses with mtDNA are inconclusive in terms of significance because sharks are known to have high female philopatry, which may produce high levels of population differentiation for maternally inherited mtDNA markers. Such markers do not reflect population differentiation across the genome if males mediate dispersal. However, satellite tracking indicates White Shark rarely cross ocean basins and have high site philopatry in all populations studied. Therefore, until a broad examination of population structure using nuclear heritable markers is conducted, the Northwest Atlantic White Shark is considered as a separate DU. Although tracking and genetic information indicate the Atlantic population of White Shark in Canada is distinct from those in other ocean basins, it is not distinct from the population in U.S. waters.

Special significance

White Shark is the largest predatory fish and one of the few sharks that regularly preys upon marine mammals (Compagno 2001). The species has been known to the Mi’kmaq people of Atlantic Canada for thousands of years: a tooth has been found in an oyster midden dated 1,000 to 2,000 years B.P. at Pig Island, Northumberland Strait, Nova Scotia (Gilhen 1999). There is no species-specific ATK in the report. However, White Shark, like all species, is important to Indigenous peoples who recognize all interrelationships within an ecosystem.

White Shark is notorious for its attacks on humans and boats (Miller and Collier 1981; Burgess and Callahan 1996). Four attacks by White Shark on boats are known from Atlantic Canada (Table 1): 1) in 1873 or 1874 a 4 m White Shark attacked a dory off the St. Pierre Bank, Newfoundland; it was identified by tooth fragments embedded in the hull (Putnam 1874); 2) in June 1920, a 4.6 m White Shark attacked a boat off Hubbard Cove, St. Margaret’s Bay, Nova Scotia; it was identified from scars on the boat and description of a tooth embedded in it (Piers 1934); 3) in July 1932, a 4.6 m White Shark attacked a boat 16 km NW of Digby Gut, Nova Scotia; it was identified from a tooth embedded in the hull (Piers 1934); 4) on 9 July 1953, a 3.7 m White Shark attacked and sank a dory off Fourchu, Cape Breton Island, Nova Scotia; neither of the fishermen on board was attacked but one of them drowned. The shark was identified from teeth embedded in the hull (Day and Fisher 1954).

Due to its large size, striking appearance, predatory prowess, and potential danger, White Shark has been both revered and demonized in popular culture (Ellis 1994). The celebrated cultural status of White Shark makes its jaws and teeth particularly sought-after as curios; and its fins are used as a food additive for markets catering to Asian delicacies and traditional medicines. Even in the face of protective legislation, the high prices paid for White Shark parts remains a powerful incentive to stimulate and maintain black market trading in such goods, but the origin and species of shark parts on the black market are uncertain.

Distribution

Global range

White Shark is widely distributed in sub-polar to tropical seas in both hemispheres, from 60°N to 60°S, but aggregates in temperate continental shelf waters of the western North Atlantic, Mediterranean Sea, southern Africa, southern Australia, New Zealand, and the eastern and western North Pacific (Figure 2; Compagno 2001; Fergusson et al. 2009). In the western North Atlantic, the White Shark ranges from Hare Bay, Newfoundland, to northern Brazil (Templeman 1963; Gadig and Rosa 1996). In the eastern North Pacific, it ranges from the central Bering Sea to Mazatlan, Mexico (Kato 1965; Cook pers. comm. 1987).

Figure 2. Global distribution of White Shark. Source: IUCN 2019.

Canadian range in the Atlantic

This species occurs regularly and seasonally in Canadian Atlantic waters, known from 85 confirmed or probable records since 1874 in addition to tracking of tagged White Shark into Canada and 21 sharks observed (19 tagged) during tagging efforts in 2018 and 2019 off Nova Scotia (Tables 1–3, Figure 3). Observations throughout Atlantic Canada include, but are not limited to, the northeast Newfoundland Shelf, the northern coast of Newfoundland, the Grand Banks, the St. Pierre Bank, Sable Island Bank, St. Margaret’s Bay, Passamaquoddy Bay, the Bay of Fundy, off Grand Manan, the Northumberland Strait, Chaleur Bay, and in the Laurentian Channel as far inland as the Portneuf River Estuary (on the north shore of the St. Lawrence Estuary) (Putnam 1874; Piers 1934; Vladykov and McKenzie 1935; Day and Fisher 1954; Leim and Day 1959; Vladykov and McAllister 1961; Templeman 1963; Arnold 1972; Mollomo 1998, Skomal et al. 2017). White Shark records from Atlantic Canada (Figure 3, Table 1) consist primarily of incidental captures and observations plus four cases of attacks on boats (Templeman 1963; Mollomo 1998). Tracking shows White Shark using most of the shallow continental shelf off Nova Scotia and Newfoundland out to the limits of Canada’s Exclusive Economic Zone (EEZ), with few movements north of Newfoundland (Figure 3, Skomal et al. 2017). The occurrence of White Shark in Atlantic Canada is seasonal and inter-annual and is likely the result of sharks with distribution further south migrating north during the late summer and early fall months (Figure 4b, Curtis et al. 2014; Skomal et al. 2017); these movements are potentially correlated with the seasonal shift of the warm Gulf Stream toward the coast (Hogg 1992). Of the 76 Atlantic Canada sighting records for which the month is known, all occur from June to December; the most (33) occurred during August, while the remainder occurred mainly in June, July, or September (Figure 4a). Sharks tagged by the private organization Ocearch in 2020 migrated into Canadian waters mostly from June to November with only a few sharks tracked in Canada briefly during December, January, and February (Table 2,3, Ocearch 2020).

Collectively, the foregoing suggests that the Canadian component of this species’ distribution represents the northern edge of their distribution in the Northwest Atlantic.

Figure 3. Canadian range of White Shark in the Atlantic Ocean based on sightings and incidental capture of White Shark and tracking of White Shark in Atlantic Canada from PSAT (n=27) and SPOT (n=5) tags modified from Skomal et al. 2017. Source: DFO (2020) White Shark sightings and strandings database, and digitized position data from maps in Skomal et al. (2017).

Figure 4. (a) The frequency of sightings by month of White Shark in Atlantic Canada. Source: DFO 2020 White Shark sightings and strandings database. (b) The seasonal distribution of sightings and catches of the White Shark in the Northwest Atlantic, from Curtis et al. (2014). Dashed line is the minimum latitude of Canada’s EEZ in the Atlantic.

Extent of occurrence and area of occupancy

Augmenting the limited sightings database with tracking work by Skomal et al. (2017), the extent of occurrence (EOO) of the White Shark in Atlantic Canada is approximately 1,029,903 km² (1,346,138 km² including land area). However, recent tagging work by Ocearch in 2018 and 2019 that has tracked White Shark near the EEZ on the northeastern Grand Banks suggests the EOO could be at least 1,062,590 km² (1,380,744 km² including land area) (Ocearch pers. comm. 2020; 2020). There are insufficient data to calculate a reliable index of area of occupancy (IAO), but the same data used to calculate EOO suggest an IAO much higher than 2000 km², given White Shark’s wide-ranging movements throughout the most of Canada’s Atlantic EEZ south of Newfoundland. There were insufficient data in the previous report (COSEWIC 2006) to judge if the EOO has increased, but it is unlikely to have changed.

Search effort

The first recorded observation of a White Shark in Atlantic Canada was in 1874 from an attack on a dory on St. Pierre Bank, Newfoundland (Table 1). There have been active commercial fisheries in Atlantic Canada that could catch White Shark for over half a century, including groundfish and herring fisheries and pelagic longline fisheries. These are historically the primary sources of White Shark bycatch in the U.S. and Canada (Skomal et al. 2012, Table 1). There has been no survey effort in Atlantic Canada other than two tagging programs conducted over a few weeks in Fall 2018 and 2019 by the NGO Ocearch and DFO in Canada. Ocearch has spent 412.5 hours over 35 days total, with an average of 5-6 hooks per day deployed, across September to October 2018 and 2019 (Ocearch pers. comm. 2020).The reporting of opportunistic sightings and incidental catch has increased in recent years since the White Shark was listed as Endangered under the Species at Risk Act (SARA) in 2011. The DFO efforts to educate the public and harvesters on reporting and the implementation of mandatory reporting of interactions with White Shark in commercial (2018) and recreational (2019) fisheries have also increased the number of White Shark sighting records.

Habitat

Habitat requirements

White Shark occurs in coastal inshore waters, offshore on the continental shelf, and off the shelf into open ocean pelagic habitat, from the intertidal to the upper continental slope and mesopelagic zone. Known bathymetric range is from just below the surface to just above the bottom down to a depth of at least 1,128 m in the Northwest Atlantic (Skomal et al. 2017). It occurs in the breakers off sandy beaches, off rocky shores, and readily enters enclosed bays, lagoons, harbours, and estuaries, but does not penetrate brackish or fresh waters to any known extent (Compagno 2001). Sand habitat is particularly important for White Shark foraging in South Africa (Kock et al. 2018).

White Shark have a wide thermal tolerance, having been recorded in temperatures between 1.6 and 30.4°C (Figure 5d, Skomal et al. 2017). Despite this wide thermal range, there is evidence that temperature plays a role in habitat choice. In South Africa, White Shark abundance is higher during the winter when there is no upwelling, conditions are more stable, and water temperature is warmer (Towner et al. 2013). Catches of White Shark in eastern Australia’s shark control programs peak at temperatures 19°C or below (Payne et al. 2018). Tracking studies show White Shark spend most of their time in temperatures between 14 to 25°C with the highest occurrence between 18–20°C when not moving offshore (Weng et al. 2007b; Bruce and Bradford 2012, 2015; Lee et al. 2018; Wintner and Kerwath 2018), including in the Northwest Atlantic (Figure 5b,d, Curtis et al. 2014; Skomal et al. 2017). Here, median temperature of incidental captures of White Shark was 19.5°C for young-of-the-year sharks, but was 16°C for adult sharks, reflecting a possible ontogenetic change in thermal habitat use (Curtis et al. 2014).

Figure 5. (a) Distribution of bottom depths and (b) sea surface temperatures of White Shark sightings and captures in the Northwest Atlantic, reproduced from Curtis et al. 2014. (c) Percentage of time (mean ± SE) spent at depth and (d) sea surface temperature for 21 PSAT tagged sharks, reproduced from Skomal et al. (2017).

White Shark is a wide-ranging, migratory species capable of crossing ocean basins, including the Atlantic and Pacific (see Dispersal and Migration). However, White Shark primarily occupy coastal waters <100 m deep, seasonally migrating to deeper, oceanic habitat to mate or feed (Boustany et al. 2002; Carlisle et al. 2012; Curtis et al. 2014, 2018). Their rate of foraging is higher in coastal habitat (Carlisle et al. 2012), which is particularly important for the growth of young-of-the-year and juveniles that use it throughout the year in the Northwest Atlantic (Skomal et al. 2017; Curtis et al. 2018). Juveniles predominate in this habitat (Curtis et al. 2014). The high abundance of young-of-the-year White Shark in the New York Bight suggests it may serve as important nursery habitat (Casey and Pratt 1985, Curtis et al. 2014, 2018). Larger sharks use coastal habitat primarily in summer (Weng et al. 2007a,b; Jorgensen et al. 2010; Bonfil et al. 2010; Francis et al. 2012). In the Northwest Atlantic, adults move off the continental shelf to mesopelagic depths between fall and spring (Skomal et al. 2017, Figure 6, see Dispersal and Migration).

Figure 6. The positions (coastal=black, pelagic=white) of White Shark tagged with both SPOT (n=5) and PSAT (n=24) tags in the Northwest Atlantic by season. Reproduced from Skomal et al. (2017).

White Shark may occupy the entire water column but have higher occurrences at particular depths. In coastal habitats, they occupy shallow depths (Bruce et al. 2006; Weng et al. 2007b; Bonfil et al. 2010) and in the Northwest Atlantic are reported to spend over half their time at <20 m depth (Skomal et al. 2017, Figure 5a,c). Over 92% of the 564 opportunistic sightings and incidental catch records in this coastal habitat since 1800 occurred at depths <100 m, with a median of 30 m (Figure 5a, Curtis et al. 2014). Beyond the continental shelf in oceanic habitat, White Shark punctuate their time at shallow depths (<100 m) with deeper dives to the mesopelagic zone 200-1000 m deep, likely to forage (Figure 5c, Boustany et al. 2002; Bonfil et al. 2005; Weng et al. 2007a,b; Skomal et al. 2017; Gaube et al. 2018).

White Shark can show homing and philopatric behaviour to specific aggregation sites or “hotspots” often associated with foraging, such as sea lion colonies (Johnson et al. 2009; Jorgensen et al. 2010; Kock et al. 2013). In the Northwest Atlantic, White Shark presence around coastal Nova Scotia, the Gulf of St. Lawrence, and Sable Island may indicate foraging on Atlantic grey seal (Halichoerus grypus) (Lucas and Natanson 2010; Hammill et al. 2017). However, in the eastern Pacific some individuals may forage for dispersed prey over wider areas (Domeier and Nasby-Lucas 2013).

Habitat trends

It is unknown to what degree habitat deterioration has contributed to the apparent global decline of this species. In Atlantic Canada, there are no known activities altering the habitat and hence abundance or distribution of White Shark. Given its coastal association, effects on White Shark biology from U.S. coastal developments and offshore energy projects are possible (Curtis et al. 2018). Habitat in White Shark is determined partially by prey, and the increase in abundance of the Atlantic Grey Seal (Halichoerus grypus) in Canada and the U.S. has improved prey availability (NMFS 2009; Skomal et al. 2012; Hammill et al. 2015, 2017). The increasing frequency of warm anomalies in Atlantic Canada (Brickman et al. 2018) could expand White Shark thermal habitat northward while warmer waters could also retract distribution to the south (Payne et al. 2018).

Biology

The most recent and comprehensive biological information on White Shark comes from populations in California and Australia; there has been little research on White Shark in Canadian waters. However, recent tagging and genetics studies in the Northwest Atlantic demonstrate connectivity between sharks in the U.S. and Canada. Tagging and tracking work is ongoing in the North Atlantic by several organizations, including DFO, Ocearch, Massachusetts Division of Marine Fisheries, and the Atlantic White Shark Conservancy. There is also some updated life history information on White Shark in the Northwest Atlantic, mostly from incidental fisheries catch in the U.S. (Curtis et al. 2014).

Life cycle and reproduction

Lifespan in Northwest Atlantic White Shark was recently estimated to be 40 years for females and 73 for males, based on bomb radiocarbon dating (Hamady et al. 2014; Natanson and Skomal 2015). While the maximum age estimate for females is accurate for the samples examined, Natanson and Skomal (2015) noted that there were limited samples of large females. Studies on other pelagic sharks (Natanson et al. 2002) suggest similar lifespans for males and females and this is likely to be the case for this species. Additionally, Harry (2018) has suggested that maximum ages in many sharks, including White Shark, are underestimated even when using reliable methodologies such as bomb carbon validation. It is believed the much lower estimate of lifespan for female White Shark is an underestimate and the maximum age is more likely to be similar to that of males.

Intrinsic rate of population increase (r) is estimated at 0.035–0.056 depending on method and assumptions, an intermediate value among elasmobranchs (Smith et al. 1998; Dillingham et al. 2016; DFO 2017). Instantaneous natural mortality (M) is estimated to be between 0.063 and 0.125 (Smith et al. 1998; Mollet and Cailliet 2002; DFO 2017). Using the values for r and the number of female offspring produced over a lifetime (m) for this population estimated by DFO (2017) based on assumptions of a longer lifespan (70 years), the generation time (G) is:

G = ln(m)/r = ln(4.4)/0.035

This method gives a generation time estimate of 42 years. Assuming a shorter lifespan, m becomes 5.6 and r becomes 0.101 (DFO 2017) and gives a generation time of 17 years. However, the estimate of 42 years is assumed to be more accurate based on recent studies of Northwest Atlantic White Shark growth and lifespan. Estimated juvenile annual survival from mark-recapture research in the eastern Pacific is high (0.63, SE = 0.08), and increases steadily with age, reaching 0.95 (SE = 0.02) in adult sharks, likely due to greater experience in prey capture and reduced predation risk (Benson et al. 2018; Kanive et al. 2019).

The maximum size of White Shark is unknown, but growth studies estimate it at 5 to 6 m total length (TL) (Tanaka et al. 2011; Christiansen et al. 2016). Reports of White Shark reaching total lengths greater than 7 m have not been verified (Figure 7, Compagno 2001). The largest verified White Shark from Atlantic Canada was 5.6 m caught in a cod gillnet off Alberton PEI in 1983 (Scott and Scott 1988; Curtis et al. 2014).

Figure 7. The distribution of estimated White Shark (a) fork length from the DFO 2020 White Shark sightings and strandings database in Atlantic Canada and (b) total length from the sightings database of the whole Northwest Atlantic reproduced from Curtis et al. (2014).

Estimates of age at sexual maturity range from four to 33 years depending on geographic location (Compagno 2001; Dudley and Simpfendorfer 2006; Tanaka et al. 2011; Natanson and Skomal 2015; Christiansen et al. 2016). Similarly, size at maturity ranges from 3.1 to 4.1 m TL for males and 4 to 5 m TL for females (Francis 1996; Pratt 1996; Tanaka et al. 2011). Females can be a metre longer than males at the same age, and a thirty-three-year difference in age has been estimated to exist between males and females of the same size (Hamady et al. 2014). There is only one estimate of size of maturity for male White Shark (∼3.8 m TL) in the Northwest Atlantic, based on clasper morphology (Pratt 1996). Natanson and Skomal (2015) used bomb radiocarbon dating combined with the minimum estimates of size at maturity and estimated age at maturity at 33 and 26 years for females and males, respectively. However, these estimates are high given the minimum estimates of lifespan, indicating there is still uncertainty on White Shark growth in the Northwest Atlantic.

White Shark are ovoviviparous (aplacentally viviparous), with yolk sac reserves augmented by histotrophy of maternal lipids and late term oophagy (Gilmore 1993; Francis 1996; Uchida et al. 1996; Sato et al. 2016). Gestation period is still uncertain, potentially as high as 18-20 months (Francis 1996; Mollet et al. 2000; Bruce 2008) but is at least 10 months in the Northwest Pacific (Christiansen et al. 2014). Litter size varies from two to 10 and possibly to 17, with an average of seven, and fecundity increases with size of the female (Cliff et al. 2000; Compagno 2001; Christiansen et al. 2014). Litter size in the Northwest Atlantic is unknown. Maximum lifetime reproductive output of a female White Shark has been estimated to be 45 pups (Compagno 1991). Length at birth is assumed to be between 1.09–1.65 m (Compagno 2001). The length of the reproductive cycle is unknown but could be two years based on tracking and photo-identification (Dewar et al. 2013; Domeier and Nasby-Lucas 2013; Chapple et al. 2016). It may also be more than three years, or unpredictable, as post-partum females may require a year or more between pregnancies to rebuild energy stores (Compagno 1991).

Mating may occur anywhere throughout the northeastern coastal waters of the U.S., with Cape Cod being a possible aggregation spot based on the observation of possible mating scars (Pratt 1996; Skomal et al. 2017). Opportunistic data from strandings and bycatch have recorded reproductively mature White Shark of both sexes along the Atlantic Coast of Canada (Figure 7, Table 1). Parturition likely occurs in the spring and summer, but in an unknown area, from where young-of-the-year sharks then migrate to potential nursery habitat in the New York Bight and the Mid-Atlantic Bight (Casey and Pratt 1985; Curtis et al. 2014; Skomal et al. 2017).

Physiology and adaptability

White Shark can tolerate a wide range of temperatures and environmental conditions (see Habitat, Payne et al. 2018). They are observed from sub-polar to tropical waters, including in the Northwest Atlantic from the coast to the Gulf Stream and the Mid-Atlantic Ridge (Skomal et al. 2017).The ability of White Shark to remain agile predators in colder water is in part due to countercurrent vascular heat exchangers that allow them to maintain a body temperature higher than the ambient water temperature (Carey et al. 1982; Goldman 1997; Compagno 2001). Tracking studies demonstrate White Shark regularly enter water as cool as 4°C in the Northwest Atlantic (Skomal et al. 2017). Their large size, and thus large thermal inertia, and their thermoregulatory abilities likely modulate potential metabolic effects caused by changes in water temperature (Goldman 1997; Skubel et al. 2018).

White Shark is a highly visual predator with a duplex retina, featuring a low rod-to-cone ratio (about 4:1) for acute, and possibly full-colour vision (Gruber and Cohen 1985). It visually investigates surface objects (Strong 1996; Collier pers. comm. 1986; Fallows pers. comm. 2000) and that often brings it into contact with humans (Miller and Collier 1981; Collier pers. comm. 1986; Burgess and Callahan 1996; Fallows pers. comm. 2000).

The life history of White Shark, especially its late age at maturity, suggests vulnerability to high fishing mortality (Francis 1996; Wintner and Cliff 1999; Hamady et al. 2014, Dapp et al. 2015). Its widespread distribution combined with an opportunistic feeding strategy may allow the species to readily disperse from localized threats and move to better feeding areas. However, its life history and ovoviviparous reproduction also increases its vulnerability to organic pollutants and the likelihood that pollutants will transfer from mother to offspring during gestation (Lyons et al. 2013a; Mull et al. 2013).

Dispersal and migration

White Shark can swim long distances, e.g., from Hawaii to California (Compagno 2001; Boustany et al. 2002), from South Africa to Australia (Bonfil et al. 2005), and from Florida to an area 930 km south of Greenland (Skomal et al. 2017). This ability to cross ocean basins results in sporadic observations off oceanic islands such as Hawaii (Taylor 1985, Jorgensen et al. 2010; Block et al. 2011) and the Azores (Compagno et al. 1997; Skomal et al. 2017).

In the Northwest Atlantic, conventional and acoustic tagging information shows that White Shark range widely throughout U.S. and Canadian waters (Figure 6). Individuals showed movements all along the Atlantic coast of the U.S. and Canada, with regular seasonal migrations between Florida and as far north as Newfoundland (Figure 6, Skomal et al. 2017). Nine White Shark tagged by Ocearch in the U.S. (50% of all U.S. Ocearch-tagged White Shark) migrated into Canadian waters, as far north as the Magdalen Islands, Chaleur Bay, and the Grand Banks between summer and fall (Table 3, Figure 3b, Ocearch 2020; pers. comm. 2020). Conversely, 17 White Shark tagged by Ocearch in Canadian waters in fall moved south into US waters (Table 2, Ocearch 2020; pers. comm. 2020).

Telemetry studies focused on the Canadian population have only recently been started, with DFO implementing an acoustic tagging program and acoustic receivers along Nova Scotia’s eastern coast in 2018 (Bowlby pers. comm. 2019). Acoustic tagging of White Shark in the U.S. showed ∼60% of all sharks tagged acoustically, including immature and mature individuals of both sexes, have migrated into Canada from the U.S. since 2011 (Chisholm pers. comm. 2019). Overall, about 18% of all acoustically tagged sharks migrated into Canadian waters (Chisholm pers. comm. 2019). Satellite tagging of the Canadian population has also recently started off Nova Scotia by DFO (n = 2 shark) and Ocearch (n = 17 sharks, Table 2, Ocearch pers. comm. 2020). An additional six sharks were tagged off Cape Cod from a collaboration of DFO, MDMF, and the Atlantic White Shark Conservancy, and this collaboration will continue tagging sharks in Canada and the U.S. into the future. The sharks tagged so far by DFO have mostly either migrated to or remained in the U.S., with only two (including one U.S. tagged shark) spending significant time in Canadian waters (from June to October 2017 for the U.S. tagged shark) before migrating southward (Bowlby pers. comm. 2019). Preliminary evaluation of dispersal and movement in Canada is consistent with foraging behaviour. The sharks tagged by Ocearch in Fall 2018 spent limited time moving along the Nova Scotian coast or offshore in Canada’s EEZ and beyond before moving south to the Carolinas or Florida; by July 2019, all six of the sharks tagged in 2018 had returned to Nova Scotia by summer or fall 2019 (Table 2, Hueter pers. comm. 2018; Ocearch 2020).

The frequency of long-range movements along coastlines and across ocean basins changes ontogenetically and seasonally. Tagging data to date show this population moves widely throughout the Northwest Atlantic, spending significant time in both coastal and oceanic habitat, with a few seasonal patterns (Skomal et al. 2017; Curtis et al. 2018). Smaller sharks (<3 m) seasonally migrate from summer habitat in the northeast shelf of the northeastern U.S. and Atlantic Canada, to winter in the southeastern U.S. and the Gulf of Mexico, following prey and thermal regimes (Figure 6, Casey and Pratt 1985; Curtis et al. 2014; Skomal et al. 2017). As White Shark grow larger, their coastal migrations increase in distance and depth (Skomal et al. 2017; Curtis et al. 2018). Sub-adults and adults are also shelf-oriented during summer, but often move offshore for the rest of the year, lacking the clear seasonal migratory patterns displayed by juveniles (Figure 6). These offshore movements, likely related to foraging, extend over a 30° latitudinal range, with some White Shark moving near the Azores (Skomal et al. 2017). In fall and winter, larger sub-adult and adult sharks move as far south as the Bahamas and in summer as far north as Newfoundland and southeast of Greenland (Skomal et al. 2017). It is also mostly the larger juveniles and adults of this population that make the longer migration into Canadian waters each summer based on the size distribution of sharks here (Figure 7, Curtis et al. 2014). White Shark of all sizes have been recorded in sightings and fisheries databases on the continental shelf in each season, but sightings increase in New England and Canadian waters and reach their most northerly distribution in late summer and early fall (Figure 4, Curtis et al. 2014). Both the acoustically tagged sharks and the sharks tagged by Ocearch migrated into Atlantic Canada from June to November, although some acoustic detections have occurred in February and March (Table 2,3, Chisholm pers. comm. 2019; Ocearch 2020). In winter, White Shark observations were concentrated below Cape Hatteras (∼35°N), closer inshore, where they are rarely observed in summer (Curtis et al. 2014).

Similar patterns exist for other White Shark populations (Bruce et al. 2006; Bonfil et al. 2010; Domeier et al. 2012; Andreotti et al. 2016b) but fewer individuals may migrate over the open ocean (Bonfil et al. 2005; Bruce and Bradford 2012). Long-range, offshore, and seasonal movements of adult White Shark are also more common elsewhere (Domeier and Nasby-Lucas 2008; Jorgensen et al. 2010; Duffy et al. 2012). Dispersal in White Shark is limited by high philopatry and site-fidelity (Jorgensen et al. 2010; Blower et al. 2012; Bruce and Bradford 2012). However, it is still unclear whether such site-fidelity and philopatry for foraging and reproduction, exists for the Canadian Atlantic population. Increasing seal populations off Cape Cod in the Northeastern U.S. may be causing aggregation at these sites (Skomal et al. 2012).

Interspecific interactions

White Shark are born between 1.09 and 1.65 m TL, and such a large size at birth minimizes predation risk. Humans have been identified as the single largest cause of mortality to White Shark (Compagno 2001). However, there is one reported attack of an Orca (Orcinus orca) on a White Shark off California (Pyle et al. 1999). At least two Orcas killed White Shark at hotspots in South Africa, resulting in the disappearance of sharks from these areas for a time (Engelbrecht et al. 2019).

White Shark is an apex predator with high energetic requirements, metabolic rates, and a feeding periodicity less than two weeks (Semmens et al. 2013). As such, it exploits a broad prey spectrum covering multiple trophic levels. The broadening of its dentition with growth suggests that at a length of about 300-340 cm this species undergoes a dietary shift from bottom-dwelling fishes to marine mammals (Tricas and McCosker 1984; Estrada et al. 2006; Hussey et al. 2012).

Smaller White Shark (<2.5 m TL) tend to consume relatively small demersal prey, including teleosts, small elasmobranchs, and invertebrates, while larger individuals tend to take larger nektonic prey, including pinnipeds, odontocetes, and large elasmobranchs (Klimley 1985, 1994; Cliff et al. 1989; Bruce 1992; Fallows and le Sueur 2001; Amorim et al. 2018). Trophic level, ranging from 4.2 to 5.0 or higher (Hussey et al. 2015), generally increases with age (Estrada et al. 2006; Hussey et al. 2012). White Shark will also switch from hunting sea lions to fish when sea lion pupping season ends (Kock et al. 2013). They are generalists but there is intraspecific variation, sometimes spanning more than one trophic level, that suggests a degree of individual specialization on prey (Kim et al. 2012). However, at every growth stage White Shark is highly opportunistic and can kill a wide variety of prey (LeMier 1951).

Scavenging on floating cetacean carcasses represents a significant, and previously underestimated, component of White Shark ecology (Pratt et al. 1982; Dudley et al. 2000; Curtis et al. 2006), with a recent study showing up to 40 White Shark scavenging for up to 13 hours at a time on a single carcass in False Bay, South Africa (Fallows et al. 2013). White Shark also hunt dolphins (Wcisel et al. 2010, Sprogis et al. 2018), and have attacked and consumed Harbour Porpoises (Phocoena phocoena) in the Bay of Fundy (Day and Fisher 1954; Templeman 1963; Arnold 1972; Turnbull and Dion 2012). In Atlantic Canada, it is probable White Shark regularly scavenge on, and potentially hunt, cetaceans (Table 1, Pratt et al. 1982; Casey and Pratt 1985; Taylor et al. 2013); with the historical decline of pinnipeds in the Northwest Atlantic, cetaceans probably formed an important component of White Shark diets in the region (Carey et al. 1982). However, White Shark in the U.S. Atlantic are incorporating more seals into their diets as seal populations rebound (Skomal et al. 2012). White Shark have been implicated in attacks on Grey Seal off eastern Canada (Brodie and Beck 1983).

Population size and trends

For the Northwest Atlantic, in over 210 years (1800-2010) of sighting records and fishery observer reports, there were 649 confirmed records of White Shark (Curtis et al. 2014). Only a fraction (43) came from Canadian waters. There are no surveys for White Shark in Canadian waters other than the recent tagging programs (see Distribution and Dispersal and Migration sections). Until these tagging programs, most records came from opportunistic stranding and sightings, published historical observations, and occasional reported incidental catch. There is substantial fishing effort in the form of pelagic longline, small pelagic weir and purse seines, and groundfish trawls and gillnets that could potentially catch White Shark in Canada. Fishing effort is monitored in part by at-sea observers, dockside monitoring, and Fishery Officers, but observer coverage is incomplete and variable in many fisheries and thus inadequate to fully characterize White Shark occurrence. For instance, the majority of Atlantic groundfish fisheries have annual observer coverage varying from <1-5%, with Div. 3NO Yellowtail at 25% and Northern Shrimp at 100%, while swordfish and tuna longline fisheries have targeted observer coverage below 10%. International longline fishing effort for pelagic species occurring outside of Canada has increased exponentially in the Northwest Atlantic over much of the last fifty years (Figure 8). Reporting also increased in Canada after the species was listed as Endangered under the SARA in 2011.

Figure 8. Estimates of the total longline fishing effort, represented by total number of hooks, in the Northwest Atlantic. Source: ICCAT (2019).

Abundance and trends

Information on the global population size of White Shark is sparse, but most sources agree population numbers are low compared to other wide-ranging shark species (Fergusson et al. 2009; Chapple et al. 2011; Towner et al. 2013; Andreotti et al. 2016a; Hillary et al. 2018), with a low natural rate of increase (Smith et al. 1998; DFO 2017). Partially from this rarity, information on abundance of White Shark globally is data-limited, especially with respect to different life-history stages (Huveneers et al. 2018). As it is an apex predator, population size is understandably low (Cortés 1999; Estrada et al. 2006).

There are no estimates of White Shark abundance in the waters of Atlantic Canada; however, between 1874 and September 2018, 85 White Shark observations were recorded, with 45 of those records (33 authenticated) since 2009 (Table 1). In addition, 19 sharks were tagged in Canada in 2018 and 2019 with an additional three observed during these expeditions (Table 2). While less than half of the sightings records come from fisheries catch (see Threats – Biological Resource Use), the low level of observer coverage does not permit conclusions about the regularity of occurrence of White Shark in these fisheries. There are 6,087 records of White Shark, albeit unconfirmed, in the U.S. pelagic longline database from 1986-2000 extracted from over 200,000 sets but most of these (80%) were from areas south of Florida (Baum et al. 2003). Those are disputed as having been accurately identified as White Shark (Burgess et al. 2005). The International Commission for the Conservation of Atlantic Tunas (ICCAT) reports only six mt, representing 10–16 individual sharks, caught in the Northwest Atlantic between 1987 and 2018, primarily by U.S. fleets (ICCAT 2019).

The number of mature White Shark in the Northwest Atlantic is unknown. The South African population has relatively low abundance based on both photo mark-recapture (n=438 95% CI = 353−522) and genetic effective population size analysis (Ne=333; 95% CI = 247−487) (Andreotti et al. 2016a). A separate genetic study compared the effective population size of South African White Shark (Ne = 365, 95% CI=188–1998) to that of Northwest Atlantic White Shark (Ne=44, 95 CI=31–66), but only the relative difference in effective population size can be interpreted and the absolute numbers are unreliable. Nonetheless, analysis suggests the Northwest Atlantic population is, or recently was, less abundant than the South African population (O’Leary et al. 2015).

Only three published studies have examined trends in White Shark abundance in the Northwest Atlantic (Baum et al. 2003; McPherson and Myers 2009; Curtis et al. 2014). Additionally, one genetics study has suggested this population suffered a recent genetic bottleneck (O’Leary et al. 2015). Baum et al. (2003) calculated trend information based on catch per unit of effort (CPUE) logbook data from the U.S. pelagic longline swordfish and tuna fleets in the Northwest Atlantic from 1986 to 2000, estimating a decline of 79% in CPUE over this period (95% CI: 59 to 89%). This decline was based on 6,087 trip records primarily from the southeastern seaboard of the U.S. and Caribbean, although the species in many of these records are unverified. McPherson and Myers (2009) also estimated a decline in historical White Shark sightings in Atlantic Canada up to 2005, but due to the opportunistic nature of the data, the magnitude of this decrease was highly uncertain. Additionally, the decrease they inferred is now contradicted by the increase in sightings since 2005 (Table 1).

Curtis et al. (2014) estimated relative abundance trends for White Shark based on three data sources (Figure 9): the U.S. Northeast Fisheries Science Center’s (NEFSC) fishery-independent longline survey (1961-1989, 2009), catches in five recreational fishing tournaments (1965-1996), and U.S. observer data from commercial bottom longlining (1994-2010). These trends are based on a Bayesian hierarchical analysis combining the three indices, although one of the indices (observer data) is not temporally coincident with the other indices (excepting the single 2009 overlap of the NEFSC survey series with observer data). Their analysis showed approximately the same decline from the 1960s to the 1990s (-73%) as the logbook data (-79%) used by Baum et al. (2003). However, the analysis suggested that since the 1990s, and continuing into the 2000s, the White Shark population in the coastal Northwest Atlantic has been increasing (Figure 10). The estimated increase is exclusively dependent on the observer dataset from the bottom longline fishery, which started only in 1994, and showed a highly variable but increasing trend until its final year (2010) (Figure 9). Considering only the point estimates, the data suggest the population approximately doubled since the 1990s.

Figure 9. The separate trend in relative abundance of White Shark for each of the three indices in U.S. waters (Northeast Fisheries Science Center fishery-independent longline surveys, tournament database, and observer program of directed longline fishery) used in the analysis of Curtis et al. (2014). The dotted red line is when the Fisheries Management Plan for the White Shark was introduced and the solid red line is when the White Shark was prohibited from commercial and recreational capture. Reproduced from Curtis et al. (2014).

Figure 10. The trend (red line) in relative abundance of White Shark based on hierarchical Bayesian analysis of three indices from U.S. waters (Northeast Fisheries Science Center fishery-independent longline surveys, tournament database, and observer program of directed longline fishery). Black line is the posterior mean and the grey area represents the 95% credible interval around the trend. Reproduced from Curtis et al. (2014).

Curtis et al. (2014) acknowledged that the understanding of trends based on their analysis of this White Shark population is highly uncertain. Indeed, they state that their analysis had the largest amount of uncertainty associated with the trend in relative abundance, with confidence intervals being 4-5 times greater than the magnitude of the point estimates (Figure 10). They did attempt to validate the estimated increase through examination of sightings records (Figure 11) and found some corroboration of an increase. The increase in Grey Seal population off the northeast coast of North America (Wood et al. 2020), a major prey item for White Shark, would also lend support to some increase in the shark population.

Figure 11. Comparison of the trend in relative abundance of White Shark observations between (a) the hierarchical Bayesian analysis of three indices from U.S. waters (Northeast Fisheries Science Center fishery-independent longline surveys, tournament database, and observer program of directed longline fishery); and (b) the database of sightings and capture records subset from 1959-2009. Reproduced from Curtis et al. (2014).

However, there are several reasons to question the magnitude of the estimated increase derived from the Curtis et al. (2014) Bayesian analysis. First, the analysis suggests an increase from the late 1990s to the late 2000s of approximately 155%. This implies a population growth rate of approximately 11%/yr, an unrealistically high rate for this species. Second, the estimated increase associated with sightings is highly sensitive to the assumption of fixed observation effort. It is likely that both sighting effort and the verification of White Shark sightings has increased since management plans, observer programs, and tagging programs have been implemented in Canada and the U.S., leading to some overestimate of White Shark increase. Lastly, the sole data set spanning the entire time period analysed (NEFSC fishery-independent survey) shows very little change in White Shark relative abundance from the late 1990s to the late 2000s, although there is only a single data point for the survey after 1990.

Bowlby and Gibson (2020) conducted simulation analyses to evaluate how many animals must have been removed from the population to have caused the historical decline rate, and then accounted for these removals when projecting the population forward, to evaluate its capacity for subsequent increase. If White Shark in the Northwest Atlantic are as long-lived as research shows, then their analysis suggests that the population is unlikely to have doubled in abundance over the past 30 years. Mitigation measures implemented in fisheries would have had to be 100% effective at eliminating all sources of incidental mortality for the population to have doubled. Concerning whether the CPUE indices used in Curtis et al. (2014) to construct trends analyses are likely to index abundance, Bowlby and Gibson (2020, p. 4999) state:

“That the abundance index [referring to the Curtis et al. trends analysis] increased so rapidly from the 1990s suggests that climatic or environmental variation has affected the distribution of White Shark in the Northwest Atlantic and thus encounter probabilities in the fishery-independent data (e.g., Hobday and Evans 2013), or that there have been changes in fleet behavior that increase susceptibility to capture (e.g., Tidd, Brouwer, and Pilling 2017). The recent trend is very unlikely to be solely due to changes in abundance over time.”

Bowlby and Gibson (2020) further suggest that the life history of White Shark confers high vulnerability to extremely low levels of incidental mortality and that their widespread distribution and potential for interaction with multiple international fleets (Queiroz et al. 2019) is cause for concern.

While all analyses agree on the decline of the White Shark population from the 1960s to the 1990s, there is substantial uncertainty about the magnitude of any subsequent increase. Based on the only consistent data set throughout the entire time period (the NEFSC fishery-independent survey), the population has at the least been stable since the 1990s. Assuming that the population has remained stable since the estimated declines between the 1960s and 1990s, the population would currently be depleted by 73-79% compared to 1960 abundance level. This time period represents approximately 1.5 generations.

Some of the apparent recent increases in White Shark sightings in Atlantic Canada may also be associated with rebounding Atlantic Grey Seal population. It is also possible that the sightings increase could represent range shifts and extended migrations further into Canada from the U.S., caused by changes in thermal habitat. Changes in the interaction of the Labrador Current with a northerly shifting warm Gulf Stream could be causing increased frequency of warm anomalies off the Scotian Shelf (Peterson et al. 2017; Saba et al. 2017; Brickman et al. 2018), bringing Canadian Atlantic waters further within the thermal preferences of the White Shark. Finally, increased reporting of sightings to DFO is likely responsible, at least in part, for the rapid increase in records in Atlantic Canada since the 2006 report (Bowlby pers. comm. 2019). Anecdotally, this recent increase in sightings throughout the Northwest Atlantic is still ongoing, even excluding the increases observed at Cape Cod (Curtis pers. comm. 2019). There is then an indication of an increasing trend throughout the Northwest Atlantic, even if its magnitude is uncertain, but there is also strong indication of depletion relative to historical abundance.

White Shark populations outside of the Northwest Atlantic have shown evidence of declines. Targeted shark control programs can quickly reduce White Shark populations, with marked reductions (∼80–90%) from historical abundance in Australia (Reid and Krogh 1992; Roff et al. 2018) and irregular declines off Natal, South Africa (Cliff et al.1989). The decline of White Shark sightings and fisheries interactions in the Northwest Pacific (Christiansen et al. 2014), the decline of sightings at a hotspot in False Bay, South Africa (Hewitt et al. 2018), and their apparent extirpation in South America (Amorim et al. 2018) suggest that even the removal of only a few individuals can have a noticeable effect. However, increased protection in New South Wales has stabilized, and possibly increased, White Shark interactions with a shark meshing program (Reid et al. 2011; Lee et al. 2018). A gillnet ban and reduction in fishing effort may be responsible for increased incidental catch of White Shark in the Southern California Bight (Lowe et al. 2012). Ultimately, White Shark population growth rates are slow (Smith et al. 1998; Braccini et al. 2017); population modelling of the West Australia population suggests only a 2–6% increase can be expected per year when there is no fishing (Braccini et al. 2017).

Rescue effect

Tagging information and genetic studies indicate regular movement of White Shark between Canada and adjacent U.S. waters. However, rescue of the Canadian segment of the population could not be based on migration of fish from U.S. waters because those fish are from the same Endangered DU.

Based on analysis of both the mitochondrial control region and multiple nuclear-encoded microsatellite loci, and tagging data, the White Shark population in the Northwest Atlantic is considered demographically isolated, even from a geographically close population in the Mediterranean, and is therefore dependent primarily on intrinsic growth (Gubili et al. 2011; O’Leary et al. 2015). There is evidence of restricted gene flow among females with South Africa population, the closest population genetically to White Shark in the Northwest Atlantic (Gubili et al. 2011; O’Leary et al. 2015). This makes an extra-regional rescue effect unlikely, and this population may even be experiencing a genetic bottleneck and inbreeding (O’Leary et al. 2015).

Threats and limiting factors

Threats

The IUCN Threats Calculator was used to assess the scope and severity of risk to the population from current and imminent threats (Master et al. 2012). Scope of a threat is defined as the percentage of the population expected to be impacted by the threat within 10 years if current circumstances and trends continue. Severity is the level of damage (percent population loss) to the population within the scope identified for the threat that can reasonably be expected if current circumstances and trends continue over the next 10 years or three generations, whichever is longer. Timing is defined as whether the threat is occurring now or only expected in the future. An IUCN Threat Calculator is provided for the White Shark DU (Appendix 1).

IUCN 5. Biological resource use – low impact

Human activity is the most significant source of White Shark mortality worldwide, most often as bycatch in commercial longlines or as sport fish targeted for their lucrative jaws, teeth, and fins (Compagno et al. 1997; Fergusson et al. 2009). Generally, White Shark were not historically targeted by commercial fisheries, rather they are captured as bycatch in the eastern Pacific (Lowe et al. 2012; Santana-Morales et al. 2012; Lyons et al. 2013b), Australia (Bruce and Bradford 2012), and the Northwest Atlantic (Baum et al. 2003; Skomal et al. 2012; Curtis et al. 2014). Additionally, they are targeted by beach meshing programs meant to control their numbers or exclude their presence near beaches in South Africa and Australia, and these have indicated declining populations in those areas (Cliff et al. 1989; Reid et al. 2011; Roff et al. 2018). Because of their coastal habitat use, young-of-the-year and juvenile sharks are susceptible to capture in near-shore entangling nets, such as gillnets and seine nets, and to longlining and recreational angling (Figure 12, Lowe et al. 2012; Santana-Morales et al. 2012; Lyons et al. 2013b; Ramirez-Amaro et al. 2013; Curtis et al. 2014).

Figure 12. White Shark caught by fishing gear type and life history stage in the Northwest Atlantic sightings and incidental capture data base of Curtis et al. (2014). FD: fishery-dependent; FI: fishery-independent. Reproduced from Curtis et al. (2014).

The degree to which fishing mortality affects the Northwest Atlantic population is highly uncertain due to low levels of observer coverage in almost all fisheries. In the U.S., fishing effort in the fisheries (groundfish and bluefin tuna longline) historically responsible for the most bycatch has been decreasing over the last 20-30 years (Skomal et al. 2012; Curtis pers. comm. 2019) but the effect of this effort reduction on the White Shark population is unknown.

The late age of maturity of White Shark results in slower recovery from overexploitation, even with low levels of fishing mortality (Hamady et al. 2014; Braccini et al. 2017). From active tracking in California, natural mortality (0.08) was estimated to be six times less than the rate of interactions with gillnets (0.48), indicating incidental fishing interactions would impact population growth (Benson et al. 2018). In fact, incidental fishing mortality likely caused the extirpation of the South American White Shark population in the Atlantic (Cione and Barla 2008; Amorim et al. 2018).

Mortality from incidental capture is poorly estimated in Atlantic Canada, because interactions with fisheries are underestimated due to low observer coverage and species misidentification (DFO 2017). Based on a sparse data set of observations, 38 of 85 (45%) of White Shark records from Atlantic Canada since 1874 were incidental capture (including two records from the Scotia Fundy Observer Program, one from the Quebec Region Observer Database, and one from the Newfoundland Observer Database), with ten records of incidental capture since 2009 (Table 1). Thirty-two of these fisheries interactions resulted in mortality (DFO 2017, Table 1), and the last time two were killed in one year was 1977 (although potentially a double record of the same shark). There are undoubtedly more unreported mortalities and it is unknown how such mortality contributes to changes in population size. However, the White Shark population likely remains susceptible to very low levels of fishing mortality. Using a life history simulation model, theoretical removals of 20 animals per year caused 72% of population trajectories to decline when projected into the future (DFO 2017).

Management measures have recently been put in place to understand and reduce the chance of mortality from incidental capture. Under the SARA, reporting of interactions with White Shark in logbooks was made mandatory in all relevant fisheries by 2019. However, enforcement of recording in SARA Logbooks or other reporting tools is critical to the effectiveness of this measure; otherwise, shark bycatch continues to go unrecorded unless an observer is aboard. Additionally, in 2019, hook size and line strength were limited in the DFO Maritimes Region shark recreational and charter licences to reduce the risk of harm to White Shark. Hook size and line strength restrictions were also introduced for Newfoundland shark recreational licences in 2020 (Dunne pers. comm. 2020).

IUCN 9. Pollution – unknown impact

As long-lived, apex predators consuming fat-laden prey, White Shark are expected to bioaccumulate pollutants in their tissues, as has been found for other elasmobranchs (Cagnazzi et al. 2019). Zitko et al. (1972) found that muscle and liver tissue from White Shark taken in the Bay of Fundy-Gulf of Maine area had higher levels of PCBs and chlorinated hydrocarbon pesticides than other fishes. While the health impacts of these pollutants on White Shark have not been investigated, they may negatively impact reproductive fitness of males, possibly via compromised gametogenesis or impaired sperm motility (Cadbury 1997). Studies in carcharhinids suggest toxicity can result from exposure to organic pollutants and can affect stress responses in stingrays (Cullen et al. 2019, Lyons et al. 2019). Juvenile White Shark in the Southern California Bight had mean mercury levels six times higher than the established wildlife screening value of concern (0.5 μg/g) and had significant levels of organic contaminants in their livers, with DDT concentrations (72.37 μg/g) the highest reported for any elasmobranch (Mull et al. 2012). In South Africa, White Shark had plasma concentrations of mercury and arsenic so high they would be toxic in other animals, but these sharks showed no evidence of adverse health effects, suggesting protective mechanisms may exist (Merly et al. 2019). The potential for maternal offloading in White Shark is also likely increased due to their high trophic position (Lyons et al. 2013b). As such, organochlorines and heavy metals may be a concern, but the extent and biological effects of such toxicity are unknown.

Limiting factors

The degree to which the Northwest Atlantic White Shark population has recovered to baseline levels since its protection in the U.S. in the 1990s is uncertain, although abundance is thought to be increasing (Curtis et al. 2014). The status of White Shark in Canada is largely determined by events outside of Canadian waters. The main factor limiting the recovery of these White Shark is the species’ late age of maturity and low fecundity, which limit its population growth rate, as is typical of the majority of shark species (Smith et al. 1998; Hamady et al. 2014; Natanson and Skomal 2015). Furthermore, although adults can readily migrate to adapt to climate change, changes in prey type, increasing water temperatures and ocean acidification, the latter two factors may negatively affect White Shark pups and nursery grounds, because pups cannot move long distances to find waters conducive to their survival and growth.

Number of locations

White Shark is highly mobile and migratory, meaning an individual may span across its Northwest Atlantic range throughout its lifetime. All individuals within Canada likely form a single population and the primary threat is incidental bycatch, which applies throughout the range in Canadian waters. However, White Shark in the DU are wide-roaming and the threat of mortality is random and ephemeral, therefore the Locations concept does not apply.

Protection, status and ranks

Legal protection and status

Due to their high profile and low numbers, White Shark are one of the most highly protected shark species internationally. At the beginning of 2005, CITES listed White Shark in Appendix II. White Shark is also listed in Appendices I and II of the Convention on Migratory Species, a United Nations Treaty Organization. The species has been protected from both commercial and recreational fisheries along U.S. Atlantic and Gulf Coasts under the federal Fisheries Management Plan since 1997 (NMFS 1997). In Canada, finning (keeping only the fins) of all shark species was made illegal in 1993 (DFO 2006), and White Shark was officially listed under Schedule 1 (‘Endangered’) of the SARA in 2011. Under the provisions of section 32 (1) of the Act it is illegal to kill, harm, harass, capture, or take White Shark, and under section 32(2) it is illegal to possess, collect, buy, sell, or trade White Shark. As such, within U.S. and Canadian EEZ waters, White Shark is relatively well-protected from fisheries; however, White Shark does move frequently into unprotected, international waters (Skomal et al. 2017; Huveneers et al. 2018). Canadian and U.S. prohibitions do not reduce shark interactions with fishing gear, some of which result in 100% mortality in gillnets and deeper-water trawls.

Non-legal status and ranks

White Shark was designated as Vulnerable by the IUCN in 2019 based upon criteria A2bd due to past and ongoing global declines in mature individuals (Rigby et al. 2019). White Shark is not yet ranked on the national scale by NatureServe (2019), but has a rank ranging from not ranked to imperilled on the global scale. White Shark’s General Status rank is also unrankable at the national level, but the Northwest Atlantic population is imperilled, indicating risk of extirpation due to few occurrences for both the migratory (S2M) and non-breeding populations (S2N).

Habitat protection/ownership

Generally, there has been little effort to protect White Shark habitat. White Shark receive some protection from various time-area closures and closed areas along the U.S. east coast as well as a few marine protected areas (MPAs) in Canada, such as the Gully and St. Anns Bank MPA; however, these protected areas are small compared to the home range size of White Shark and the use of these MPAs by White Shark is unknown.

Acknowledgements and authorities contacted

The writers thank Dr. Heather Bowlby at the Bedford Institute of Oceanography for providing DFO sightings data and comments; Erin Dunne at the Northwest Atlantic Fisheries Centre for comments on shark recreational licence restrictions in Newfoundland; Dr. Tobey Curtis at the National Marine Fisheries Service for discussion; Dr. Bob Hueter at the Mote Marine Laboratory for discussion on Ocearch data; Dr. Nigel Hussey at Windsor University for discussion and communication about Ocearch tagged sharks; Dr. Gregory Skomal at the Massachusetts Department of Fish and Game for discussion on his tracking work; and Dr. John Chisholm for discussion on White Shark acoustically tagged by Massachusetts' Division of Marine Fisheries. Finally, the writers would like to thank the writers of the previous COSEWIC Status Report for the White Shark, Dr. R. Aidan. The writers would also like to thank Dr. Scott Wallace again for the groundwork he did in writing the Species Appraisal Summary for White Shark in 2017, which greatly helped in the writing of this report.

Authorities contacted

In addition to those acknowledged above, the following authorities were contacted:

Andrew Boyne at the Canadian Wildlife Service; Shelly Pruss at Parks Canada; Tom Dooley, Department of Fisheries, Forestry and Agriculture in Newfoundland; Dr. Sherman Boates, Department of Biology, Acadia University; Mary Sabine, Department of Natural Resources, New Brunswick; the Huntsman Marine Science Centre; Don McAlpine, New Brunswick Museum; Andrew Hebda, Nova Scotia Museum; Adam Durocher, Conservation Data Centre; Steven Campana, University of Iceland.

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Biographical summary of report writer(s)

Geoffrey Osgood, BSc, completed his Bachelor of Science, Honours in Ecology and Zoology at the University of Calgary in 2014 and since then has been working on his PhD in Biology at the University of Victoria studying the use of marine protected areas for shark conservation.

Julia K. Baum is Associate Professor of Biology at the University of Victoria in British Columbia, Canada, and a 2017 Pew Fellow in Marine Conservation. She earned her BSc from McGill University (1999; Montréal), and her MSc (2002) and PhD (2007) from Dalhousie University (Halifax), all in Biology. Julia subsequently held a David H. Smith Conservation Research Fellowship at Scripps Institution of Oceanography, UC San Diego, followed by a Schmidt Ocean Institute Postdoctoral Fellowship at the National Center for Ecological Analysis and Synthesis (NCEAS), UC Santa Barbara. Amongst other foci, Julia’s research has documented precipitous declines in shark populations and the cascading effects of the loss of apex predators.

Appendix 1. Threats calculator

Species or ecosystem scientific name: White Shark (Carcharodon carcharias) Atlantic population

Element ID: N/A

Elcode: 899

Date: July 9, 2020

Assessor(s): D. Lepitzki (facilitator), B. Leaman (Co-Chair), G. Osgood (report writer), J. Baum (report writer), R. Claytor (Co-Chair), B. McBride, D.Keith, J. Shaw, M. Trudel, M. Trebel, S. Lenhert, A. MacNeil, D. Sam

References: Post-provisional status report

Assigned overall threat impact: D = Low

Impact adjustment reasons: The overall calculated threat impact of Low was retained as the assigned overall threat impact; however, the threats assessment group concluded that the lower end of the range of the predicted population decline in the next 100 years caused by the threats acting in the next 10 years could be zero, instead of the 1-10% range projected due to the combined scope of pervasive and severity of slight for threat 5.4.

Overall threat comments: The species is globally distributed in sub-tropical and temperate waters, but absent from cold polar waters; hence Atlantic and Pacific populations in Canada are isolated from each other and are separate designatable units. This very large apex predator is rare in most parts of its range, but particularly so in Canadian waters, which represent the northern fringe of its distribution and occurrences are primarily seasonal. However, there are increasing records of its occurrence in Atlantic Canada associated with warming ocean waters, although still fewer than 100 since 1874. No abundance trend information is available for Atlantic Canada. Numbers have been estimated to have declined by about 73-79% over 1-2 generations in areas of the Northwest Atlantic Ocean outside of Canadian waters. The species is highly mobile, and individuals in Atlantic Canada are likely seasonal migrants belonging to a widespread Northwest Atlantic population; hence the status of the Atlantic Canadian population is considered to be the same as that of the broader population. Additional considerations include the long generation time (∼42 years) and low reproductive rates (estimated gestation is 10-20 months and average fecundity is 7 live-born young) of this species, which limit its ability to withstand losses from increase in mortality rates. Bycatch in commercial fisheries, particularly the pelagic long line fishery, is considered to be the primary cause of mortality, along with recreational fishing. Time frame for scoring severity and timing is 100 yrs (maximum allowable). Threats assessed to Canadian population, including threats outside Canadian waters. Population trend in NW Atlantic waters is highly uncertain but appears to be increasing and thought to be due to increased measures to mitigate mortality from incidental capture in commercial fisheries.