White shark (Carcharodon carcharias) COSEWIC assessment and status report: chapter 6

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Biology

Life cycle and reproduction

Due to the rarity of this species, knowledge of the reproduction and life history of the white shark is incomplete. In some cases inferences are drawn from other closely related species. Reproductive mode is ovoviviparous (aplacentally viviparous), with yolk sac reserves augmented at late term via oophagy (Gilmore 1983; Francis 1996; Uchida et al. 1996). Gestation period may be similar to that of its close relative, the shortfin mako (Isurus oxyrinchus), in which gestation has been estimated to last 14 months (Mollet et al. 2000). Litter size varies from 2 to 10 and possibly to 17 with an average of 7 (Compagno 2001; Cliff et al. 2000), with fecundity increasing with size of the female. Length at birth is assumed to be between 1.09-1.65 m (Compagno 2001). Possible white shark pupping areas on the west and east coasts of North America include off southern California (Klimley 1985) and the Mid-Atlantic Bight (Casey and Pratt 1985), respectively. The length of the reproductive cycle in the white shark may be more than three years as post-partum females may require a year or more between pregnancies to rebuild energy stores (Compagno 1991). Maximum lifetime reproductive output of a female white shark has been estimated to be 45 pups (Compagno 1991). Pup survival is thought to be low (Australia and Madagascar CITES Proposal 2004).

Males reach sexual maturity at an age of 8 to 10 years and a length of 3.5 to 4.1 m (Pratt 1996; Compagno 2001) while females reach maturity at an age of 12 to 18 years and a length of 4 to 5 m (Francis 1996; Compagno 2001; Australia and Madagascar CITES Proposal 2004).

Longevity in this species is estimated to be 23-60 years (Cailliet et al. 1985; Mollet and Cailliet 2002; Australia and Madagascar CITES Proposal 2004). Generation time has been estimated at 23 years and instantaneous natural mortality (M) between 0.077 and 0.125 (Smith et al. 1998; Mollet and Cailliet 2002). Intrinsic rate of population growth (i.e., annual population increase) is estimated at 4-5.6% (Smith et al. 1998).

The maximum size of the white shark is unknown, with length and especially mass of large individuals notoriously difficult to validate (see Mollet et al. 1996). The largest captured specimens measured between 5-5.8 m in total length. Reports of white sharks reaching total lengths greater than 7 m have been reliably reported but have not been verified (Compagno 2001).

Life history information from Canadian waters

There have been no scientific studies or surveys of white sharks in Canadian waters. Opportunistic data from strandings or bycatch have shown that reproductively mature white sharks of both sexes have been recorded from Pacific and Atlantic Coasts of Canada (Figure 3, Table 1).

The largest verified white sharks from Canadian waters are a 5.2 m individual found stranded at Long Inlet, Graham Island, Queen Charlotte Islands, British Columbia, on 16 December 1986 (Coad 1995), and another of the same length and 907 kg in mass captured between Bliss and Whitehorse Islands, southern New Brunswick, in August 1971 (Arnold 1972).

Behaviour

In cool temperate waters, this species is often observed scavenging on floating cetacean carcasses, either with aggregates of conspecifics (Pratt et al. 1982; Fallows pers. comm. 2000) or with other large sharks (Dudley et al. 2000). Natural predation by white sharks on pinnipeds is observed seasonally at the Farallon Islands, California (Klimley 1994; Klimley et al. 1992, 1996), and off the Western and Eastern Cape, South Africa (Stewardson and Brett 2000, Fallows and le Sueur 2001). A single case of possible white shark courtship and mating has been reported (Francis 1996). Recent studies reveal that the white shark exhibits a variety of social behaviours and may travel in small but stable groups (Compagno 2001; Collier pers. comm. 1986; Fallows pers. comm. 2000).

The white shark is a highly visual creature with a duplex retina featuring a low rod-to-cone ratio (about 4:1) that is well adapted to acute, and possibly full-colour vision (Gruber and Cohen 1995). This species will visually investigate virtually any object at the surface, from boats and surfboards to floating kelp and rubbish (Strong 1996; Collier pers. comm. 1986; Fallows pers. comm. 2000). The white shark’s curiosity about novel objects and activities at the surface often brings it into contact with humans (Miller and Collier 1981; Burgess and Callahan 1996; Collier pers. comm. 1986; Fallows pers. comm. 2000).

Herbivory/predation

White sharks are born at a total length of 1.09-1.65 m. Their large size at birth precludes predation from most marine animals.  Humans have been identified as the single largest cause of mortality to adult sharks (Compagno 2001). Newborns are likely taken by other shark species and possibly marine mammals but there is no evidence of this. There is one reported attack of an orca whale (Orcinus orca) on a white shark off California.

Physiology

White sharks can tolerate a wide range of temperatures and therefore have been observed from sub-polar to tropical waters (5-27°C). There is some evidence that white sharks have a temperature preference around 14-15°C, but they are regularly found in warmer temperatures.

The ability for white sharks to perform as agile predators in colder water is in part attributable to countercurrent vascular heat exchangers which allow them to maintain a body temperature higher than the ambient water (Compagno 2001).

Dispersal/migration

White sharks are able to swim long distances over extended periods with an average cruising speed of 3.2 kph (Compagno 2001). Seasonal and size-class distribution of 109 white sharks captured along the West Coast of North America suggests pregnant individuals may migrate south to southern California waters to give birth (Klimley 1985), but this remains to be demonstrated. Sonic telemetry studies conducted at the Farallon Islands, California, suggest that larger white sharks concentrate predatory effort in relatively small areas, while smaller individuals cruise larger areas (Goldman and Anderson 1999). Size-based differences in white shark activity space at the Farallon Islands may reflect learning in larger sharks to concentrate predatory effort in areas that have proved successful in previous years and/or displacement of smaller sharks by larger conspecifics. A satellite telemetry study revealed that four of six white sharks tagged off the Farallon Islands travelled at depths of 300 to 500 m far into the open Pacific – in one case, a 4.7-m male, travelled from the Farallon Islands to off Kahoolawe, Hawaii, a distance of 3,800 km; the two other sharks remained close to the Farallons (Boustany et al. 2002).

Of 36 white sharks tagged in the western North Atlantic through the National Marine Fisheries Service (NMFS) Cooperative Shark Tagging Program, only two were recaptured, both female (Kohler et al. 1998). Of these, one travelled north from off Assateague Island National Seashore, Virginia, to off Gloucester, Massachusetts, the other south from off Moriches, Long Island, New York, to Charleston, South Carolina.

A study of mitochondrial and nuclear DNA of 95 white sharks from South Africa, Australia and New Zealand suggests sex-biased dispersal of this species: males migrate long distances (up to and including crossing entire ocean basins) while females occupy relatively small ranges (Pardini et al. 2001). Based on the non-coding ‘control’ region of the maternally inherited mitochondrial genome, two divergent genetic lineages were revealed. Populations from Australia and New Zealand are not significantly different, but these individuals are significantly different from South African sharks (P<0.0001 for each). This result was further supported by an analysis of restriction-fragment-length polymorphisms in mitochondrial DNA that found only 1 of the 95 animals to be out of place (a South African haplotype male captured in Australia). In contrast, an analysis of five polymorphic nuclear encoded microsatellite loci revealed no significant difference. Pardini et al. (2001) concluded that there is sufficient male mediated gene flow to homogenize allele frequencies and that dispersal of individuals is more extensive than previously thought. This differential dispersal may reflect sex differences in parental investment between males and females, allowing the latter to conserve energy stores toward nourishment of pups.  In a recent oral presentation, unpublished data were presented that showed satellite tracking of a female white shark (3.8 m total length) that travelled from Gansbaai, South Africa, to Exmouth, Western Australia – a distance of approximately 10,000 km – in three months; possibly disproving Pardini et al.’s (2001) hypothesis of female philopatry in this species (Bonfil pers. comm. 2004). It is worth noting that the observed movement of this individual does not necessarily indicate gene flow.

No tagging or telemetry studies have been published on white sharks in Canadian waters and available data (Table 1) are inadequate to draw any meaningful conclusions about dispersal in Canadian waters.

Interspecific interactions

The white shark is an apex predator exploiting a very broad preyspectrum. Teleosts, elasmobranchs, and marine mammals are the main prey categories, but it also takes cephalopods, other molluscs, decapods, marine birds, and reptiles (Cortéz 1999). The white shark also scavenges marine mammal carcasses, fisheries offal, and fishes caught on lines (Compagno 2001). Its dentition broadens with growth and it has been suggested that, at a length of about 300 cm, this species undergoes an ontogenetic dietary shift from bottom-dwelling fishes to marine mammals (Tricas and McCosker 1984). Smaller white sharks (<2.5 m total length) 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; Cliff et al. 1989; Bruce 1992). However, at every growth stage, the white shark is highly opportunistic and apparently can capture and kill a wide variety of prey. For example, LeMier (1951) reported that the stomach of a 4.4 m white shark captured in Willapa Harbor, Washington, September 1950 contained four partly digested salmon (Oncorynchus sp.), vertebral columns of Pacific hake (Merlucius productus) and rockfish (Sebastes sp.), the hides of two harbour seals (Phoca vitulina), and 150 crabs, primarily Dungeness crabs (Cancer magister) with the remainder rock crabs (C. productus). Feeding periodicity in an adult white shark (~ 4.5 m total length), based on calculated metabolic rate and calorie content of blubber-rich food, has been estimated to be between 45 and 90 days (Carey et al. 1982; Klimley et al. 2001).

In Canada, diet and feeding behaviour of the white shark in Canadian waters have only been reported from the Atlantic coast. In August 1953, between Passamaquoddy Bay and Grand Manan Island, a pale grey shark more than 4.3 m long – believed to be a white shark – was observed to bite a full-grown harbour porpoise (Phocoena phocoena) in half and consume the posterior part while the anterior part was retrieved by a fisherman with a gaff (Day and Fisher 1954). In August 1953, an adult male harbour seal was shot near Docet Island, in the St. Croix River between Maine and New Brunswick; its tail was severed and bore fresh tooth slashes of a size that strongly suggested they were made by a white shark (Day and Fisher 1954). In July 1962, a 3 m white shark caught near Wallace, Nova Scotia, had a “small porpoise” (probably P. phocoena ) in its stomach. In September 1969, near the mouth of Passamaquoddy Bay, New Brunswick, two specimen collectors lost a harbour porpoise to a shark at least 4 m long (Arnold 1972). In August 1971, a 5.2 m white shark was caught between Bliss and Whitehorse islands, New Brunswick; its stomach contained three harbour porpoises, estimated to have been between 1.2 to 1.5 m long, in variable stages of digestion and with their tail stalks severed (Arnold 1972). The white shark has also been implicated in attacks on grey seals (Halichoerus grypus) off eastern Canada (Brodie and Beck 1983). In Atlantic Canada, the white shark probably also feeds on a wide variety of fish and invertebrate prey, but this remains to be documented.

Based on reports from Washington State (Bonham 1942; LeMier 1951) andsoutheastern Alaska (R.A.Martin-author, unpublished data), diet of white sharks in Canada’s Pacific waters probably includes green sturgeon ( Acipenser medirostris), sockeye salmon (Oncorhynchus nerka) and other Pacific salmonids, rockfishes, hakes, Pacific halibut (Hippoglossus stenolepis), harbour seal, and Steller sea lion (Eumetopias jubatus).

Adaptability

The life history of the white shark suggests that it is unable to withstand high anthropogenic mortality. The widespread distribution combined with an opportunistic feeding strategy may allow for the species to adapt by dispersing from localized catastrophic events and changing prey sources.