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COSEWIC Assessment and Update Status Report on the Bowhead Whale in Canada

Biology

General

Bowheads are slow swimmers, averaging speeds of 4.50 ±1.22 km/h during their fall migration (Koski et al. 2001) and 4.0 km/h (Rugh 1990) to 3.89 ±1.48 km/h (Richardson et al. 1995b) during their spring migration. Bowheads are among the more vocal of baleen whales (Clark and Johnson 1984). Würsig and Clark (1993) suggested that calls may function to maintain social cohesion of groups and monitor changes in ice conditions. Bowheads may use the reverberations of their calls off the undersides of ice floes to help them orient and navigate (Ellison et al. 1987; George et al. 1989).


Reproduction

Sexual activity occurs during much of the year, although studies of fetuses indicate that most conceptions occur in late winter or early spring (Koski et al. 1993). Gestation lasts 13-14 months (Nerini et al. 1984) or 12-16 months (Tarpley et al. 1988), with one offspring per pregnancy. Calves are usually born during the spring migration between April and early June (Koski et al. 1993), with a peak in May (Nerini et al. 1984). Calves are 4.0-4.5 m long when born and grow at a rate of 1.5 cm/day for the first year (Koski et al. 1993). Weaning occurs between the ages of 9-15 months (Nerini et al. 1984) with about 95% of the yearlings weaned by the next spring migration (Rugh et al. 1992). Yearlings are 6.6-9.4 m long in spring (Nerini et al. 1984) and 7.0-8.7 m in summer (Koski et al. 1993). The growth rate appears to slow after weaning, with rates of less than 1 m/yr estimated for small bowheads re-identified in successive years (Davis et al. 1986; Schell et al. 1989; Koski et al. 1992).

Carbon isotope analysis suggests bowhead whales grow and develop slowly, taking about two decades (Schell et al. 1989; Schell and Saupe 1993) or longer (Zeh et al. 1993; George et al. 1999) to reach sexual maturity. Ovarian evidence suggests that females attain sexual maturity at lengths of 14.2 m or more, and perhaps at least occasionally when they are as small as 12.3 m (Nerini et al. 1984; Tarpley et al. 1988). Aerial photogrammetry indicates that whales as small as 12.2 m can be accompanied by calves (Davis et al. 1983; Cosens and Blouw 2003). George et al. (1999) estimated sexual maturity to occur at around 25 years of age (age at length 12-13 m for males and 13-13.5 m for females) using an aspartic acid racemization technique to age individuals. They also found that females grow faster than males. Smaller females tend to calve later in the spring migration than larger females: 1.5% (1/68) of the adults with calves photographed in the spring were <13.5 m long compared to 12% (7/59) in the summer (Koski et al. 1993). Evidence such as ivory or stone harpoon heads found in killed whales suggests that bowhead whales can live >50 years (Philo et al. 1993) or >75 years (George et al. 1995). Using an aspartic acid racemization technique, George et al. (1999) estimated four individuals (all males) to be more than 100 years of age.

Aerial photogrammetric surveys from 1985-92 found that 5.2% of the sampled population in the Barrow area (Bering-Chukchi-Beaufort population) in spring were calves (<6 m), 53.7% were juveniles (6-13 m) and 41.1% were adults (>13 m) (Angliss et al. 1995). The sex ratio of bowheads landed by Alaskan Eskimo hunters from 1973 to 1992 was equal (Braham 1995). Cosens and Blouw (2003) estimated the ratio of bowheads by size classes in northern Foxe Basin from surveys in 1996, 1997 and 1998. Combining their data over the three years showed that 17% of the photographed population were calves, 71% were juveniles, and 12% were adults. Cosens and Blouw (2003) thus considered northern Foxe Basin to be a “nursery” area.

Annual pregnancy rates have been estimated as 0.21 (Tarpley et al. 1988) or 0.20-0.35 (George et al. 1995) calves/mature female, with a possible increase since 1985 (George et al. 1992). These pregnancy rates imply calving intervals of three to five years (Miller et al. 1992; Rugh et al. 1992). The mean calving interval appears to be 3-4 yr based on aerial photogrammetric data along with rates of sightings of calves by ice-based observers (Rugh et al. 1992; Withrow and Angliss, 1992; Koski et al. 1993). The gross annual reproduction rate (calves/non-calf/year) from all available information from 1982-89 (summarized in Koski et al. 1993) was 0.052, but year-to-year variation was substantial. Even with this low rate, the Bering-Chukchi-Beaufort population is increasing, suggesting a very low rate of natural mortality (Koski et al. 1993).


Survival

Zeh et al. (2001) used a Bayesian statistical approach to calculate an annual survival rate of 0.980, with a lower confidence limit of 0.898. This estimate is for adults since whales were identified using patterns of scars on their rostrums, backs or flukes, which are more abundant on older whales.

Growth of calves during the first three to four years after weaning is thought to be limited because baleen plates are short and feeding efficiency is poor (Schell and Saupe 1993). Calves are the age class that likely suffers the highest mortality (Moshenko et al. 2003). Most beached carcasses found in the eastern Arctic are of young bowheads and young bowheads are considered more vulnerable to lethal attacks by killer whales (Finley 1990; Moshenko et al. 2003).

The managed subsistence hunt of bowheads by Alaskan Inuit is the only measurable bowhead mortality. A quota of 280 bowhead whales was set for 1999-2002, of which a total of 67 (plus up to 15 unharvested in the previous year) could be taken each year (NMFS 2000). Although bowhead whale mortality caused by the subsistence hunt is fairly well documented (summarized in Stoker and Krupnik 1993), little is known about natural mortality of bowhead whales (Philo et al., 1993). Since 1964 (through the early 1990s), at least 36 cases exist in which the cause of death could not be established (Philo et al. 1993). Nine stranded bowheads were found in the western arctic from 2000-2004 (Carpenter pers. comm. 2005). Some of these deaths may have been the result of hunter-inflicted wounds. Types of human-induced injuries include embedded shrapnel and harpoon heads from hunting attempts, rope and net entanglements from harpoon lines and crab pots, and collisions with vessels (summarized in Philo et al. 1993). Exposure to man-made noise and contaminants may produce short- and long-term effects (Richardson and Malme 1993; Bratton et al. 1993) that compromise the health and reproductive performance of some whales.

The killer whale (Orcinus orca) is the only known predator of bowheads, besides man. Of 195 bowheads examined during the Alaskan subsistence harvest (1976-92), 8 bore evidence of encounters with killer whales (George et al. 1994). The frequency of attacks on bowhead whales in the Bering-Chukchi-Beaufort population is thought to be low (George et al. 1994), although it is not possible from the available data to assess this in a quantitative way (Shelden and Rugh 1995). Predation by killer whales may be a greater source of mortality for the Baffin Bay-Davis Strait population, where about one-third of the bowheads observed in a study of living animals bore killer whale scars (Finley 1990). Killer whales regularly appear in Cumberland Sound and Pond Inlet (eastern Baffin Island) where they have at least occasional access to bowhead whales (Mitchell and Reeves 1982). A high proportion of stranded bowheads have exhibited evidence suggesting that killer whales were responsible for their deaths (NWMB 2000). Some Inuit have observed killer whales killing bowheads (NWMB 2000).


Physiology

Enlargement of the head region to accommodate baleen is carried to an extreme in the bowhead whale (Lowry 1993). In adults, the head may be as much as two-fifths of the total body length. The head profile is distinctive, with the blowholes elevated by a kind of crown that allows the whale to push up through ice to breathe (Finley 2001).

The blubber layer is particularly thick -- from 5.5 cm on the chin to about 28 cm thick over the majority of the trunk (Fetter and Everitt 1981 in Haldiman and Tarpley 1993; Haldiman et al. 1981 in Haldiman and Tarpley 1993). Maximal thickness is about 50 cm (Tomilin 1957 in Lowry 1993). The thick blubber layer is probably, at least in part, an adaptation for maintaining normal body temperature in the Arctic and sub-Arctic (Lowry 1993).


Movements/Dispersal

Bering-Chukchi-Beaufort Population

The Bering-Chukchi-Beaufort population is widely distributed in the central and western Bering Sea in winter (November to April), generally associated with the marginal ice front (Bogoslovskaya et al. 1982; Brueggeman 1982; Braham et al. 1984). From April through June, these whales migrate north and east into the eastern Chukchi Sea until they pass Point Barrow where they travel east toward the southeastern Beaufort Sea (Braham et al. 1980; Braham et al. 1984). Most of the summer (June through September), bowhead whales range through the Beaufort Sea (Hazard and Cubbage 1982; Richardson et al. 1987a; Richardson et al. 1987b), spending a large portion of time feeding in Canadian waters. Eskimo hunters from the eastern Alaskan Beaufort Sea have observed large concentrations of bowheads east of Hershel Island in July (Galginaitis and Koski 2001). In September and October, bowheads migrate west from the Canadian Beaufort Sea into the Alaskan Beaufort, and then into the Chukchi Sea. Eskimo hunters from the eastern Alaskan Beaufort Sea, describe the migration as noticeably starting in August and lasts through October (Galginaitis and Koski 2001). During fall, unlike spring, there is open water along the north coast of Alaska, where many bowheads are seen within 25 km of the shore (Richardson et al. 1987b; Moore and Reeves 1993; Moore 2000; Moore et al. 2000; Griffiths et al. 2001; Koski and Miller 2001).

Age class segregation has been documented during the spring migration past Barrow, Alaska (Zeh et al. 1993; Angliss et al. 1995; Richardson et al. 1995b; Koski et al. 2004); on the summering areas in the Canadian Beaufort Sea and Amundsen Gulf (Cubbage and Calambokidis 1987; Koski et al. 1988); and in the eastern Alaskan Beaufort Sea during late summer and autumn (Koski and Miller 2001). During spring at Barrow, subadults tend to occur in deeper water with more ice. During summer in the Canadian Beaufort Sea, Cubbage and Calambokidis (1987) found a significant inverse correlation between longitude and size class, with larger animals occurring further east. During fall migration in the eastern Alaskan Beaufort Sea, subadult whales tend to occupy shallow near­shore areas and adults tend to be found further offshore (Koski and Miller 2001). Eskimo hunters from the eastern Alaskan Beaufort Sea described the whales in the fall as segregated temporally by size, with larger whales seen later in the season, and they also described smaller whales tending to be closer to shore (Galginaitis and Koski 2001). During both spring migration past Barrow (Rugh 1990; Withrow and Angliss 1992, 1994; George et al. 1995; Koski et al. 2004) and fall migration past Kaktovik (Galginaitis and Koski 2001; Koski and Miller 2001), the initial segment of the bowhead whale migration is made up primarily of subadults, followed by adult whales with much overlap in dates. The mothers and calves tend to migrate late in the season, especially in spring.

Hudson Bay-Foxe Basin Population

The Hudson Bay-Foxe Basin population is believed to winter in Hudson Strait. Some of the bowheads winter in the waters off Mansel Island and some of the more southern islands of the east side of Hudson Bay (Low 1906). Inuit from Igloolik, Repulse Bay, and southwestern Southampton Island have reported a few bowheads at the floe edges in some winters, although they are a rare occurrence (NWMB 2000). McLaren and Davis (1981, 1983) observed bowheads wintering in western Hudson Strait. From April through May, these whales migrate west into Hudson Bay until they reach the northwest region of the bay (Reeves and Mitchell 1990), while others may move directly from Hudson Strait north into Foxe Basin appearing in northern Foxe Basin (Igloolik floe edge) by late June (Cosens et al. 1997b; NWMB 2000). Historically, Hudson Bay whales arrived at the floe edge off southwestern Southampton Island in May and June, then moved north, following ice breakup, through Roes Welcome Sound, where they remained abundant until September, but eventually moved into Foxe Basin (Reeves et al. 1983; Reeves and Mitchell 1990; Ross 1993). Recent tagging data show that some of the Foxe Basin bowheads move west through Fury and Hecla Strait (Dueck pers. comm. 2002). In September and October, bowheads apparently migrate east from Roes Welcome Sound and south from Foxe Basin into Hudson Strait.

Ross (1974) found little evidence of bowheads in Hudson Strait during the commercial whaling era. The few sightings in Hudson Strait occurred as early as May and as late as early August (Reeves et al. 1983). Most ship navigation occurred between mid-July and mid-September. It is possible that the whales usually cleared the strait in advance of the whalers in summer and after them in fall (Reeves et al. 1983). There is some evidence that whalers left Hudson Strait before the bowheads did. From October to November, Finley et al. (1982) observed bowheads swimming east in Hudson Strait. Low’s (1906:257) assessment of the migration of bowheads into and out of Hudson Bay was based on the information received from whalers: “The whales are known to enter Hudson Strait early in the spring; they have been captured around Big Island in April and May, and at the western end of the Strait in the latter part of May. They then cross to the west side of the bay along the edge of the open water, being found in June and early July along the land-floe on both sides of the southern part of Roes Welcome. As the Welcome clears of ice they proceed north to Repulse Bay, and, still later, pass through Frozen Strait into Fox Channel. Late in the autumn they again pass through Hudson Strait going eastward.”

Age class segregation has been documented in the Hudson Bay-Foxe Basin population (Cosens and Blouw 2003). Whales summering in northern Foxe Basin (aerially photographed in August of 1996, 1997 and 1998) were largely juveniles ranging up to 13.5 metres in size (Cosens and Blouw 2003). Calves and juveniles made up 89%, 97% and 79% of photographed bowheads in each of the three years. The numbers of adults and calves photographed suggest that adults summering in northern Foxe Basin are nursing females. Cosens and Blouw (2003) suggested that adult males and non-calving females aggregate in northwestern Hudson Bay. Similar data are not available for northwestern Hudson Bay.

Davis Strait-Baffin Bay Population

From their wintering grounds in central Davis Strait, southern Baffin Island, and west Greenland, bowheads migrate north into northwestern Baffin Bay in spring. Heide-Jørgensen et al. (2003) tracked two bowheads from Disko Bay (West Greenland) to North Water-Southeast Bylot Island (Canada) in May-June 2001. A single whale photographed in Canada (NE Baffin Island) in late September (1986) was re-identified in Greenland in early April (1990) (Heide-Jørgensen and Finley 1991). Bowheads that appear at the Cumberland Sound floe edge in April and May are believed to migrate north along the Greenland coast to northern Baffin Bay and westward in May and June (Davis and Koski 1980; Holst and Stirling 1999). Between Cumberland Sound and Pond Inlet, Inuit have observed bowheads migrating northward along the Baffin coast from May to July (NWMB 2000). Bowheads summer in the inshore waters and fiords of northern Baffin Island and along the northeast coast of Baffin Island from May through August (NWMB 2000). Fall migration begins in late August and September and occurs casually over the next two to three months, with periods of directed swimming, resting, or feeding. Migration past Cape Adair (71°30’ N), northeast Baffin Island, peaks in late September to early October (Davis and Koski, 1980). Movements slow as the whales feed on the autumn grounds (western Baffin Bay) (Finley 1990). Bowheads eventually reach Cumberland Sound, southeast Baffin Island, in late October and November. The peak movement of bowheads past Cape Hopes Advance (Hudson Strait) occurs in late November (Finley et al. 1982).

Age class segregation has been observed in the Baffin Bay-Davis Strait population (Finley 1990). Aerial photogrammetry on 28-29 September 1986 in Isabella Bay (east coast of Baffin Island) showed that adult bowheads primarily used the area at that time (89% were >13 m long) (Finley 1990). Results from the aerial photogrammetry support the impression that most whales observed from shore or from kayaks were adults or large subadults (Finley 1990). Most of the whales taken during the years of commercial whaling on the autumn grounds (east coast of Baffin Island) were large males (Reeves et al. 1983). During commercial whaling, the predominance of young whales and females with calves in early season catches at the Pond Inlet floe edge and in the summer catches well inside Lancaster Sound and Prince Regent Inlet suggests that this may be the nursery grounds for the Davis Strait-Baffin Bay population (Reeves et al. 1983).


Nutrition and Interspecific Interactions

Most of the prey species identified in the stomachs of 35 bowhead whales (21 males and 14 females) taken by Alaskan Inuit between 1975 and 1989 were crustacean zooplankton, particularly euphausiids and copepods ranging in length from 3 to 30 mm (Lowry 1993). Epibenthic organisms, mostly mysids and gammariid amphipods, were also common in the stomachs, with only a small representation of benthic species (Lowry 1993). Slightly higher proportions of epibenthic organisms were found in the stomachs of small whales (<10.5 m in length) (Lowry 1993). It appears that copepods are more important in the diet of larger whales (Schell et al. 1987). Some may feed on benthic invertebrates as they migrate east along the Yukon North Slope based on observations of bowhead whales coming to the surface in summer with mud streaming out of their mouths (Carpenter pers. comm. 2005).

Based on patterns of isotope variation in the visceral fat and muscle sampled from three adults and six subadults, Schell and Saupe (1993) and Schell et al. (1987) proposed that significant feeding occurs outside the eastern Beaufort Sea, particularly by older whales in the fall and early winter when they are in the Chukchi and Bering Seas. Older animals may feed in different areas or on different prey types than younger animals (Schell and Saupe 1993). In contrast to the Schell and Saupe (1993) data, Hoekstra et al. (2002) found that seasonal fluctuations in stable carbon isotope values were consistent for all age classes, suggesting that the Bering and Beaufort seas are both important feeding grounds.

Inuit believe that bowheads and other marine mammals are strongly influenced by the tidal cycle and tide-induced sea currents (NWMB 2000). Bowheads are said to be very active and to feed heavily in areas where the currents are strongest around the full moon when tidal variation is greatest. Feeding behavior has been observed during spring and summer in the Beaufort Sea and fall in the Chukchi Sea, but not in winter in the Bering Sea (Lowry 1993). Lowry (1993) suggested that bowheads rely on the abundant food supply in late summer and fall to acquire lipid reserves to sustain them during the winter.


Behaviour/Adaptability

Bowheads are exquisitely adapted to living in an arctic environment (McLeod et al. 1993). Among their obviously adaptive features are great longevity, massive energy storage capability, a sophisticated acoustic sense for ice navigation and long-range communication, and a crown on the head for pushing up through ice to breathe (Finley 2001).

Bowheads often exhibit a fright reaction to the presence of killer whales. They move into areas of broken sea ice or shallow near-shore waters, apparently seeking protection from the killer whales (NWMB 2000). Inuit believe that killer whales have influenced bowhead distributions and migrations (NWMB 2000). For example, bowheads may avoid or abandon the Isabella Bay area in summer/early fall if killer whales are present (NWMB 2000).

Bowhead whales are known to react to man-made sources of underwater noise, with reactions varying by season, habitat and behavioural state (Richardson et al. 1985; Richardson and Malme 1993). Fall migrants appear to react more strongly than summering whales. Startle reactions such as hasty dives and avoidance behaviour may occur in response to aircraft flying at altitudes below 460 m (Richardson et al. 1985; Richardson and Malme 1993). Summering bowheads react to some vessel traffic at distances of 1 to 4 km by moving away from the ship track, whereas drillships appear to induce avoidance reactions at distances of up to 10 km. Bowheads avoid seismic vessels at distances of 6-8 km (Richardson and Malme 1993), or even at distances of 20 km (Richardson et al. 1995).