Narwhal (Monodon monoceros) COSEWIC assessment and status report: chapter 6

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Biology

General

Age determination of narwhals continues to be a source of debate and a problem for population biologists. Layers of periosteal bone and tooth dentine and cementum are deposited throughout most of the animal’s life as it continues to grow. These growth layer groups (GLGs) provide a record of the animal’s growth and longevity. Unfortunately, mark-recapture experiments have not been conducted to confirm the rate of layer deposition, which may vary with age (Hay 1980). Young fast-growing animals may deposit three growth layers annually, while adult females may only deposit one in the tooth and mandible annually. As females approach physical maturity, they appear to resorb layers as fast as they deposit them. For population management purposes, narwhals in Canada are assumed to deposit one layer of periosteal bone on the mandible and one dentinal layer on the unerupted tooth annually (Hay 1984). The maximum number of mandibular periosteal growth layers that have been recorded is about 50 in males and 30 in females. Based on aspartic acid racemization in narwhal teeth, this may underestimate the actual age of narwhals (Bada et al. 1983). While the species’ longevity may be about 50 years, most animals probably do not reach the age of 30 (Bada et al. 1983; Hay 1984). Recent studies by Neve (1995b) suggest that two growth layer groups may be deposited annually on the mandible and tooth cementum of male narwhals. The function of the male narwhal’s spectacular ivory tusk, which has long been sought after by humans, continues to be a matter for speculation.

Reproduction

Narwhals are seasonal breeders (Best and Fisher 1974; Hay 1984; Hay and Mansfield 1989). The females are polyoestrous and experience up to four consecutive ovulations during the breeding season (Hay 1984).

Conception occurs between 20 March and 19 May, peaking in mid-April (Hay 1984). The gestation period has been estimated at between 14 (Best and Fisher 1974) and 15.3 months (Hay 1984). Two foetal length classes are found in narwhals that are hunted during the summer, one that is very small (10-40 cm) and another that is near term (140-170 cm) (Hay and Mansfield 1989; Stewart et al. 1995; Gonzalez 2001). Neonates are typically about 160 cm and 80 kg (Hay 1984; Neve 1995a). Most calves are born in July and August (Mansfield et al. 1975; Hay 1984; Hay and Mansfield 1989). However, the presence of newborn narwhals in Lancaster Sound on 27 May, and regularly thereafter during the spring of 1986 (Cosens and Dueck 1990), suggests that the breeding and calving periods are either broader than reported or vary significantly between years and/or locations. There are few newborn calves at the Repulse Bay ice edge in July (Gonzalez 2001).

Mature females produce a calf about once every three years on average (Hay and Mansfield 1989; Kingsley 1989). About 20% of mature females calve every 2 years (Hay 1984). The proportion of females becoming pregnant in any one year is between 0.30 and 0.38. The crude birth rate is about 0.07 for Lancaster Sound (Hay and Mansfield 1989). Calves are weaned at 1 to 2 years of age (Hay 1984).

Lacking accurate information on the true age of narwhals beyond the age of sexual maturity, population biologists must assume that they live, grow and reproduce much like belugas and have used data from the beluga as estimates for the biological traits of narwhals (JCNB/NAMMCO2001).

Vital rates for narwhals are uncertain due to the lack of a proven method to estimate age. The available data suggest that females mature at 5 to 8 years (Braham 1984; Kingsley 1989)--possibly 4 to 9 years (Neve 1995a), and produce their first young at 7 to 13 years (Kingsley 1989). Reproductive senescence in females may begin at about 23 years (Hay 1984). Male narwhals are believed to mature at 11 to 13 years (Hay 1984; Kingsley 1989)--possibly 12 to 16 years (Neve 1995a).

Lacking reliable age estimates, generation time cannot be calculated accurately using discrete age classes. Braham (1984) estimated the generation time of narwhals at about 10 years (y), which seems low. Assuming ages of maturity (α) of 7 y and reproductive senescence (ω) of 23 y, a simplistic calculation of generation time (T), where T =(α+ω)/2, yields an estimate of 15 years. This calculation ignores the fact that many females die before reaching reproductive senescence. However the effect of this omission may be more than offset by uncertainty in the estimate of reproductive longevity, which may be conservative. Instantaneous adult mortality and net recruitment rates for narwhals are unknown.

There is disagreement between biologists and Inuit as to the reproductive rate of narwhals (Remnant and Thomas 1992; Stewart et al. 1995; Gonzalez 2001). Scientific reproductive rates are based on examination of the ovaries and uterus. Because the gestation period of narwhals is greater than 12 months, as demonstrated by the presence of two foetal age classes in summer, narwhals cannot breed annually. Based on the proportion of the mature females in the population that are pregnant at any one time, narwhals appear to breed about every three years on average. Many Inuit believe that narwhals give birth more frequently because they have seen females accompanied by more than one calf, sometimes by a neonate, yearling, and two-year-old. Their assumption that the female gave birth to each of these calves has not been tested.

Survival

There are no direct estimates of survival rates for narwhals, only reasonable guesses based on analogies to other odontocetes. In modeling narwhal population dynamics, Kingsley (1989) found that uncertainty in the values of survival rates of adults and young contributes twice as much to the uncertainty in population growth rates as does uncertainty in reproductive rates. Removal by humans is perhaps the greatest and most consistent cause of mortality among narwhals. The rate of mortality from predation by killer whales and polar bear (Ursus maritimus) is unknown and may vary significantly depending on the annual presence of killer whales and ice. The potential for large-scale mortality due to entrapment by ice or disease is unpredictable.

Ice entrapment

Reports of narwhals being entrapped by ice come from a number of sites. Whales from the Baffin Bay population have been trapped by ice in Eclipse Sound (Munn 1932); at Moffet Inlet and near Pond Inlet (Degerbøl and Freuchen 1935); in Adams Sound (Remnant and Thomas 1992; Stewart et al. 1995); at Dundee Bight (76°04’N, 100°10’W) of May Inlet, Bathurst Island; in Agu Bay and near Pond Inlet (Mitchell and Reeves 1981); and in the vicinity of Fury and Hecla Strait (Stewart et al. 1995). Narwhals from the Hudson Bay population have been trapped by ice in Lyon Inlet (Degerbøl and Freuchen 1935), near White Island, and in Ross Bay (66°52’N, 85°00’W) (Gonzalez 2001). Few large entrapments have been reported from Canadian waters. The largest on record was at Moffet Inlet in March 1924, when at least 600 narwhals were entrapped (Degerbøl and Freuchen 1935; Mitchell and Reeves 1981). About 400 whales were killed, some drowned, and the remainder escaped when a lead formed in the ice.

When these entrapments occur early in the winter the mortality rate is likely high but the survival rate of animals trapped later in the season may be better, provided they are not hunted or found by bears. These catastrophic events are a significant and unpredictable cause of mortality for the species.

Predation

The rate of predation on narwhals by killer whales and polar bears is unknown but may be significant. When killer whales are present narwhals hide in broken pack ice or shallow nearshore waters (Freuchen and Salomonsen 1958; Steltner et al. 1984; Campbell et al. 1988; Reeves and Mitchell 1988; Gonzalez 2001). They breathe quietly to avoid detection and stop vocalizing instantly when killer whales approach (Ford 1987). Their fear is such that they will ignore humans. Killer whales appear to prefer non-tusked narwhals (Gonzalez 2001).

Hunters in the Repulse Bay area see killer whales more frequently now than in the past and have expressed concern about killer whale predation on narwhals (Gonzalez 2001). The frequency of seasonal visits by killer whales to Hudson Bay, and their effect on the narwhals is unknown (Reeves and Mitchell 1988; DFO1998a; Stewart et al. 1991). Killer whales may have driven narwhals close to Cape Dorset in the 1960s (Higgins 1968), south to Arviat in 1988 (W. Angalik, pers. comm. in Stewart et al. 1991), and into shallow water in the Repulse Bay area in 1999 (Gonzalez 2001). The latter resulted in an unusually large harvest of narwhals by Repulse Bay (Tables 1 and table2). Killer whales also made hunting narwhals easier in the Repulse Bay area in 1998, and in the Lyon Inlet area in 2000.

Polar bears will kill narwhal calves (Kingsley 1990) and prey upon stranded narwhals (Smith and Sjare 1990). Hunters have described parallel claw scars from unsuccessful bear attacks on the backs, sides, and tails of narwhals (Kingsley 1990; Stewart et al. 1995). Greenland shark (Somniosus microcephalus) scavenge dead narwhals, but it is not known whether they also prey upon narwhals (Beck and Mansfield 1969; Stewart et al.1995). Walrus have been found eating dead narwhals (Gray 1939 in Hay and Mansfield 1989).

Diseases and parasites

Little is known of the diseases of narwhals and their response to pathogens (Murray et al. 1995; Nielsen et al. 2000).

The skin fold at the base of the tusk and wounds on the body of narwhals are frequently infested by the amphipod whale lice Cyamus nodosus Lütken and C. monodontis Lütken (Porsild 1922; Hay and Mansfield 1989). Five species of endoparasites have been found in narwhals (Hay and Mansfield 1989).

Physiology

Little is known of the physiological requirements of narwhals or of the species’ ability to adapt to changes in its environment.

Movements/dispersal

Narwhals display a pronounced seasonal migratory cycle, the timing of which can vary by a month or more from year to year, depending upon ice conditions. They generally travel in groups (pods) of <10 animals that are dispersed during localized movements in summer, but gather into concentrations of many hundreds of animals during directed migrations in the spring and fall (Silverman 1979; Koski 1980a; Guinn and Stewart 1988; Cosens and Dueck 1991; Koski and Davis 1994; Richard et al.1994). Most travelling to new areas occurs at the surface and their swimming speed averages 5.0 km/h whether they are travelling horizontally or diving vertically (Heide-Jørgensen et al. 2001). Narwhals can reach peak speeds of 20 km/hr (Richard 2001).

There is good agreement between scientific (e.g. Finley 1976; Koski and Davis 1994; Richard et al. 1994; Dietz et al. 2001; Heide-Jørgensen et al. 2003) and traditional knowledge (e.g. Remnant and Thomas 1992; Stewart et al. 1995) of the general timing and progression of the Baffin Bay populations’ migrations. Narwhals from this population move northward along the ice edge offshore the east coast of Baffin Island in April and May (Remnant and Thomas 1992; Stewart et al. 1995; Stewart 2001; Heide-Jørgensen et al. 2003). They then move westward into the sounds of eastern Baffin Island and into Lancaster Sound and adjoining waters as the ice permits, following cracks and leads, typically during June and July, to reach their summering habitats in the Eclipse Sound area, Admiralty Inlet, Prince Regent Inlet, Barrow Strait and Peel Sound. Some animals may travel south out of Prince Regent Inlet through Fury and Hecla Strait and into Foxe Basin (Brody 1976; Stewart et al.1995). When landfast ice begins to form, typically in September or October, the whales begin to move out of the sounds and inlets adjoining Lancaster Sound and the sounds of eastern Baffin Island. Some of the animals from the Lancaster Sound region continue eastward, moving offshore from southeastern Devon Island towards Baffin Bay (Koski and Davis 1994). Most of them move eastward out of Lancaster Sound and then south along the east coast of Baffin Island, visiting many of the fiords on their way to wintering areas in southern Baffin Bay and northern Davis Strait. The narwhals that visit Fury and Hecla Strait may retrace their route to exit via Lancaster Sound, or they may continue southward through Foxe Basin and Hudson Strait to complete the circle.

Thirteen narwhals tagged in August at Tremblay Sound remained in Eclipse Sound and its tributaries until they moved south in late September (Dietz et al. 2001; Heide-Jørgensen et al.2002). However, three animals travelled west into southern Admiralty Inlet and to Creswell Bay area of Prince Regent Sound in late August and early September before moving east and south. They visited many fiords along the east coast of Baffin Island on their way, passing Cape Adair in late September or early October and reaching their wintering grounds in the pack ice of southern Baffin Bay and Davis Strait in late October or early November. The speed and range of their movements declined from 85 km/day enroute to the wintering grounds to 29 km/day once the wintering ground was reached. Their late summer and winter kernel home ranges were about 3,417 km² (SD 3,900, range 874-11,275 km²) and about 12,000 km², respectively (Heide-Jørgensen et al.2002). At least three of the animals tagged at Tremblay Sound wintered in the same general area of Davis Strait as narwhals tagged at Melville Bay, Greenland (Dietz and Heide-Jørgensen 1995).

Sixteen female narwhals tagged at Creswell Bay (72°45’N, 94°05’W) in August 2000 and 2001 were also tracked by satellite (Heide-Jørgensen et al. 2003). They remained within <200 km of the tagging site during August and followed specific migration corridors east and south in the fall to their wintering grounds in the consolidated pack ice of Baffin Bay. They remained there from November through April but in a different area than the animals from Tremblay Sound. In May through July they followed the receding ice edge back into Lancaster Sound. The areas of their summering and wintering grounds were 9,464 km² (7 whales, 95% common kernel range, SD = 4718) and about 25,846 km² (3 whales), respectively. Two of the animals were followed for a complete migratory cycle and returned to the summering grounds they occupied in Peel Sound the previous year.

The degree of site fidelity shown by narwhals, particularly those of the Baffin Bay population, is an important unanswered question. Only two narwhals have been followed for an entire migratory cycle and both returned to Peel Sound in two consecutive years (Heide-Jørgensen et al. 2003). Thisdemonstrates strong site fidelity on the part of at least some female narwhals. It does not confirm whether this behaviour is widespread among narwhals or consistent over the life of individual animals. Likewise, while none of the Tremblay Sound and Melville Bay narwhals traveled to another area of aggregation after they were tagged, it is not known whether there was exchange between the summering aggregations earlier in the season or between years (Dietz et al. 2001). Narwhals that summer in Eclipse Sound are vulnerable to being killed by hunters from Pond Inlet in the summer and by hunters from Clyde River in the fall. It is not known whether other animals that summer in Canadian waters are also vulnerable to hunting by hunters from several Canadian communities (JCNB/NAMMCO 2001).

The seasonal movement patterns of the Hudson Bay narwhals are not well known. In the spring, they likely migrate westward from putative wintering grounds in eastern Hudson Strait (Richard 1991; Koski and Davis 1994), traveling offshore through Hudson Strait and Foxe Channel until they reach the floe edge east of Repulse Bay in late June (Gonzalez 2001). They move into their summering grounds in western Foxe Channel, Frozen Strait, Lyon Inlet and Repulse Bay as ice conditions permit and typically remain until late August or early September, when they travel southeastward out of the area through Frozen Strait, following the east coast of Southampton Island. Narwhals are seldom seen west of Southampton Island or along the west coast of Hudson Bay unless killer whales are present, but they are seen on occasion at the floe edge near Coral Harbour in late June or early July, and in late August or early September.

Narwhals from the Hudson Bay population are not known to move north of Lyon Inlet (Richard 1991; Gonzalez 2001), but observations of whales passing east of Igloolik Island on their way north to Fury and Hecla Strait (Stewart et al. 1995) suggest that there may be some northward movement of narwhals into the region. It is also possible that these whales are resident in the region or are members of the Baffin Bay population returning northward. Tagging studies have not followed whales from the Hudson Bay population between their summer and winter habitats or whales from the Igloolik area to their wintering habitat.

Nutrition and interspecific interactions

Narwhals eat a small variety of fishes and invertebrates. The composition of their diet varies with season and location, likely in response to dietary preferences and the seasonal or geographical availability of prey species (Neve 1995a). They appear to feed year-round but may increase their food intake prior to migration (Remnant and Thomas 1992; Stewart et al. 1995). Tomilin (1957) described them as “teuthophages” or squid eaters, citing their reduction in teeth, wide rostrum, coarse palatal surfaces, and deep-diving ability as adaptations for feeding on squids.

The diet of narwhals hunted from mid-June through August in the Pond Inlet area consists mostly of fish and squid (Finley and Gibb 1982). Stomach samples (n=93) pooled over two years contained: Arctic cod B. saida (51% wet wt.), turbot Reinhardtius hippoglossoides(37%), squid Gonatus fabricii (7%), and polar cod Arctogadus glacialis (6%). Small amounts (<1% wet wt.) of other fishes (dark belly skate Raja hyperborea,snailfish Liparis spp., redfish Sebastes marinus, eelpouts Lycodes spp. and sculpin Cottidae) and invertebrates (octopus Bathypolypus arcticus, and crustaceans Boreomysis spp.) were also present. Most (61%) of these samples were taken from adult male narwhals. Narwhals had more food in their stomachs when they were taken at the ice edge or from ice cracks than from open water. The largest item found was a whole turbot weighing 2.4 kg. [Note: in Greenland B. saida is known as polar cod and A. glacialis as Arctic cod; turbot are also known as Greenland halibut].

Other scientific studies of the diet of narwhals in Canada and West Greenland have found the same prey species but in different proportions (e.g. Vibe 1950; Bruemmer 1971; Hay 1984; Hay and Mansfield 1989; Roberge and Dunn 1990; Heide-Jørgensen et al. 1994; Neve 1995a). Inuit hunters have made similar observations (Remnant and Thomas 1992; Thomsen 1993; Stewart et al. 1995). Arctic and polar cod, turbot and squid are all eaten in quantity during the open water season, and narwhals feed heavily on shrimps (Pasiphaea tarda) as they migrate through breaking fast ice of Pond Inlet, Eclipse Sound, and Admiralty Inlet in late June and July (Hay and Mansfield 1989). Inuit hunters have also found Greenland cod (Gadus ogac) in narwhal stomachs (Stewart et al. 1995) as well as Arctic charr (Salvelinus alpinus) (Remnant and Thomas 1992). Arctic cod are abundant under the ice edge of Admiralty Inlet in June, coincident with the presence of narwhals (Crawford and Jorgenson 1990).

Little is known of the narwhal diet from late autumn to spring. Neve (1995a) found mainly squid G. fabricii in the October-November diet of narwhals taken in West Greenland (Uummannaq) and shrimp Pandalus spp. in March-April samples from the Disko Bay area.

Little is known about the interactions between narwhals and other species for food and habitat. Their preference for deepwater habitat effectively separates them from belugas for much of the summer. They do participate with belugas, harp seals (Phoca groenlandica) and seabirds in mass feeding frenzies on Arctic cod, which they occasionally drive into shallow water in late summer (Finley and Gibb 1982; Welch et al. 1993).

Commercial fisheries in Baffin Bay, Davis Strait, and Cumberland Sound may increasingly compete with narwhals for turbot, redfish and other species (Topolniski 1993; Treble and Bowering 2002; M. Treble, pers. comm. 2002). These fisheries take place in open water or through landfast ice and are rarely, if ever, coincident with the presence of narwhals.

The question of whether food availability is a limiting factor for narwhal abundance has not been examined. While narwhals feed heavily on cod and turbot, the extent of their dependence on these species for food is unknown. A significant change in the abundance or distribution of these fishes might adversely affect narwhals.

Behaviour/adaptability

Key aspects of narwhal behaviour that may affect their population numbers and assessment include their sensitivity to noise, affinity for heavy ice cover, fear of the killer whale, and ability to dive deeply and remain under water for long periods.

Noise sensitivity

Narwhals make a variety of sounds and are sensitive to underwater noise. They appear to use click-sounds for orientation and echolocation of prey and squealing, growling, and whistling sounds for communication (Reeves 1977; Ford and Fisher 1978; Ford 1987; Miller et al. 1995). Narwhals can detect approaching ships at a distance of 80 km and show behavioural responses at distances of 55-40 km (Finley et al. 1984, 1990; Miller and Davis 1984; Cosens and Dueck 1988, 1993). Unlike beluga, which show a “flight” response by aggregating into large herds and moving rapidly away from approaching ships, narwhals show a “freeze” response similar to that seen when killer whales are present (Finley et al. 1983, 1984, 1990; Miller and Davis 1984). They return to disturbed areas and resume normal activities faster than beluga, when the noise level from ice-breaking operations is up to 120 dB. Cosens and Dueck (1988) did not observe this “freeze” reaction, perhaps due to the lack of an ice edge in the survey area or habituation to ship traffic. They did observe an increase in slow directed movements. Inuit hunters have observed that narwhals are sensitive to, and avoid, noise from machines and explosions (Remnant and Thomas 1992; Stewart et al. 1995; Gonzalez 2001).

Diving behaviour

The narwhal’s ability to dive deeply and hold their breaths for long periods enables them to move long distances under water to avoid hunters and to locate areas where they can surface to breathe. In the deep waters of Baffin Bay, narwhals dive to at least 1500 m and daily make dives to depths of over 500 m (Heide-Jørgensen and Dietz 1995; Heide-Jørgensen et al.2002). They can remain under water for at least 26.2 minutes when foraging (Laidre et al. 2002) and up to 30 minutes when pursued by Inuit (Gonzalez 2001). Their diving ability makes it difficult to obtain accurate population estimates. Variations in narwhal diving behaviour related to season, location, and sex of the animal complicate the correction of population survey data for animals that were submerged deeply enough to be invisible to the survey. Instead of using a single general correction factor, specific values may have to be determined for each locale and season (Heide-Jørgensen et al. 2001).

Ice

Narwhals frequent heavy pack ice for much of the year and follow leads in the ice to their summering grounds. They can be entraped in the ice, and their use of narrow leads also makes them vulnerable to hunting. Narwhals will explore tracks in the ice created by icebreakers (Finley et al. 1984; P. Richard, pers. comm. 2002). The effects on narwhals of shipping activities that alter ice habitats are unknown.

Avoidance of killer whale

Narwhals normally avoid people but will ignore them in the presence of killer whale (DFO 1998a; Gonzalez 2001). They seek shallow water and remain immobile, making them easy targets for Inuit who take advantage of this behaviour to hunt the frightened whales. This behaviour contributed to the unusually large number of narwhals killed by Repulse Bay hunters in 1999 (Table 2) and made hunting easier in 1998 and in the Lyon Inlet area in 2000.

Source material: Canadian harvests 1977-87 (Strong 1989), 1988 (DFO1991), 1989 (DFO 1992a), 1990 (DFO 1992b), 1991 (DFO 1993), 1992 (DFO 1994), 1993 (DFO 1995), 1994 DFO1996), 1995 (DFO 1997), 1996 (DFO 1999), 1997-2001 (DFO unpubl. data); Greenland harvests (JCNB/NAMMCO 2001).

Adaptation to captivity

Narwhals have not been held successfully in captivity. Six whales were captured in 1969-70 for exhibit in public aquaria and all died within four months, likely of pneumonia (Newman 1970; Reeves 1977). In 1987, an attempt to capture narwhals at Pond Inlet for on-site assessment in a large holding tank was unsuccessful (P. Hall, pers. comm. 2002). Animals that are captured for tagging are restrained in the water and released immediately after the tag has been attached (Dietz et al. 2001; Laidre et al.2002; Heide-Jørgensen et al. 2002).