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

Limiting Factors and Threats

The severe depletion of the bowhead population by commercial whalers is the main reason that the species is listed as endangered in several parts of its range. A recent report suggests that killer whales may be the number one threat to the bowheads in the eastern Canadian Arctic (Moshenko et al. 2003). An additional concern comes with increased interest in offshore developments in the Beaufort Sea and the eastern Canadian Arctic, which could possibly affect whale populations with the traffic, noise, and possible oil spills associated with such developments. Illegal hunting could directly affect population numbers. It has been suggested that climatic factors, which are influencing ice conditions, may also affect both the survival and the distribution of bowhead whales in the future (Tynan and DeMaster 1997). None of the other threats discussed below is expected to pose a serious threat to the population.


Killer Whales

The distributions of killer whales and bowhead whales in the Bering Sea overlap, at least during fall (George et al. 1994). Killer whales have never been seen during the 15 years of ice-based censusing in spring at Barrow (George et al., 1994), and only two sightings have been reported from the Beaufort Sea (Lowry et al., 1987) where bowheads spend the summer months. George et al. (1994) examined bowhead whales of the Bering-Chukchi-Beaufort population harvested by Alaskan Inuit for scars from killer whale injuries. They estimated that the frequency of scars from killer whale attacks ranged from 4.1% to 7.9%. The relatively low frequency of bite marks on the Bering-Chukchi-Beaufort population likely reflects a relatively low frequency of killer whale attacks and predation pressure, although most bowheads that are successfully killed probably go unrecorded (George et al. 1994).

Predation by killer whales has been suggested to be a higher source of mortality for the Baffin Bay-Davis Strait population (than the Bering-Chukchi-Beaufort population), where about one-third of the bowheads have killer whale scars (Finley 1990). Killer whales regularly appear in Cumberland Sound and Pond Inlet (eastern Baffin Island) where they would have frequent access to bowhead whales (Mitchell and Reeves 1982). Inuit report the presence of killer whales in all regions of Nunavut (NWMB 2000). Many stranded bowheads have been found with evidence suggesting that killer whales were responsible for the deaths (NWMB 2000). Some Inuit have observed interactions between bowheads and killer whales with the end result of the killer whale killing the bowhead (NWMB 2000). According to elder Inuit, young bowheads are especially susceptible to predation by killer whales (Finley 1990). Moshenko et al. (2003) rated the killer whale as possibly the greatest threat to the recovery of eastern arctic bowhead whales.

Predation by killer whales on bowhead whales may increase in the future if the refuge provided by ice coverage is reduced by global warming.


Illegal Hunting

Due to the relatively old age of first birth (age 25 yr) and low birth rate (1 calf every 3-4 yr), the sustainable removal rates from bowhead whale populations are extremely low. For example, the sustainable removal rate from the Hudson Bay-Foxe Basin population has been estimated as one whale per two or three years based on historical hunting and the associated assumption that the population has been increasing or stable despite the removals. For the Davis Strait-Baffin Bay population, the sustainable removal rate has been estimated as one whale every thirteen years.

From 1993-2004, one bowhead whale was reported landed from the Davis Strait Baffin Bay population, and five were reported landed from the Hudson Bay-Foxe Basin population. Regulated hunting does not appear to be a threat under current management. However, there have been unsubstantiated reports from the eastern Canadian Arctic of bowhead whales being shot at illegally and, on at least a few occasions, killed over the past few decades (Reeves pers. comm. 2005). Illegal hunting could pose a threat to bowheads given their low abundance.


Strandings

Reports of stranded bowhead whales are exceedingly rare due perhaps to the remoteness of such events from human habitation. Nonetheless, nine stranded bowheads were found in the western Canadian Arctic from 2000-2004 (Carpenter pers. comm. 2005). The reason(s) for these strandings are uncertain but may range from natural mortality to illegal shootings. No similar strandings have been reported in Alaska (Nelson pers. comm. 2005).


Toxins (pollution contaminants)

Jayko et al. (1990) developed a model to quantify the probability of bowhead whales encountering spilled oil in Alaskan waters. Their spill scenarios resulted in the oiling of an average of 0.1-2.0% of the Bering-Chukchi-Beaufort bowhead population. The model simulations discussed by Jayko et al. (1990) indicate that oil spills in the Beaufort Sea are unlikely to affect a significant portion of the bowhead whale population. There is no evidence that any of the large oil spills to date have had a significant impact on a baleen whale population (Geraci and St. Aubin 1990). St. Aubin et al. (1984) showed that oil fouling of the fringes of the baleen reduces feeding efficiency but that the impact would be short-term if the animal does not remain in the affected area. Offshore oil/gas exploration is rapidly increasing in Davis Strait (off West Greenland), and Lancaster Sound is known to have some significant deposits of oil and natural gas (though not yet commercially viable to extract) (Moshenko et al. 2003). Many Inuit fear that oil spills may pose a hazard for bowheads and other sea mammals in Nunavut (NWMB 2000). Moshenko et al. (2003) ranked pollution and contaminants as a high threat to bowheads in the eastern arctic populations.

Baleen whales generally have lower tissue contaminant levels than the toothed whales (O’Shea and Brownell 1994). Chemical pollutants are believed to accumulate at low concentration levels due to the low trophic level of bowhead (O’Hara et al. 1998). Bratton et al. (1993) describe various aspects of contaminants but data and understanding of physiological mechanisms are limited. The limited available information suggests that contaminant exposure poses no present threat to bowheads; however, productivity damage by chemical contaminants on planktonic food resources could potentially affect them.


Man-made Noise

The main sources of man-made noise pollution are ships, aircraft, seismic exploration, marine construction, drilling, and motor boats (Richardson and Malme 1993).Most of the research on bowhead reactions to industrial activities has been done on the Bering-Chukchi-Beaufort population that summers in the Beaufort Sea (Richardson et al. 1985; Richardson and Malme 1993). Bowhead whales are known to react to man-made sources of underwater noise by avoiding the area where the noise comes from. Reactions appear to vary by season, habitat and behavioural state (Richardson et al. 1985; Richardson and Malme 1993). Inuit hunters from the eastern Alaskan Beaufort Sea reported that noise affected whale behaviour when there was seismic activity in the area (Galginaitis and Koski 2001). They observed that the whales moved further offshore, and that they were easily spooked when they did see them. Some Inuit report that bowheads react negatively to noise from snowmobiles and small motorized boats, although many Inuit have also reported that bowheads do not seem to be adversely affected by the same noises (NWMB 2000). Inuit in Clyde River are concerned with the increasing numbers of large tour ships, and their unregulated operations (Moshenko et al. 2003). With an increasing number of motorized boats, both from tourism and local recreational activities, the potential for interference with bowhead activities increases (Moshenko et al. 2003). Moshenko et al. (2003) ranked man-made noise and tourism and recreation as a high threat to bowheads in the eastern arctic.


Climate Change

Direct effects of climate change on arctic marine mammals include the loss of ice-associated habitat (Tynan and DeMaster 1997). Indirect effects include regional or seasonal shifts in prey availability, which can affect nutritional status, reproductive success, and geographic range. It may also alter the timing or patterns of migrations, which may produce changes in species distributions and population structure of bowheads (Tynan and DeMaster 1997). Schell (2000) found isotopic evidence that the Bering Sea ecosystem underwent a reduction of between 30-40% in average seasonal primary productivity from 1966 to 1997. Gough and Wolfe (2001) compared two generations of a climate model using the impact of a CO2 doubling on the Hudson Bay region. One model suggests that sea ice might virtually disappear in Hudson Bay, leading to a substantially higher regional temperature. This degree of climate change would likely affect the bowhead population by altering the food web, although it is not known whether this would be a positive or negative effect.

In the high Arctic, the base of the food chain consists of ice algae (Alexander 1995) formed on the underside of the ice at the ice-seawater interface. With spring warming and ice melt, algal cells are sloughed off into the surrounding water column, where a seasonal bloom of phytoplankton is initiated. Ice edge habitat generates a restricted zone of high productivity (Sakshaug et al., 1994). Many species of copepods (the primary prey of bowheads) reproduce under the ice before the phytoplankton bloom and feed on sedimenting ice algae (Drolet et al., 1991). With a loss in ice habitat, there would be less ice algae produced and possibly less food for copepods (bowhead prey). Given the coupling between the ice-edge habitat and the prey of many species of arctic marine mammals, Tynan and DeMaster (1997) speculated that a sufficient reduction in the extent of the ice edge, and its associated community, may have deleterious consequences for marine mammals that have evolved with these unique systems. There is the additional concern that species which rely on the ice-edge community for foraging could be adversely affected by a reduction in the areal extent and latitudinal shift of ice-edge habitat (Tynan and DeMaster 1997).

In the Bering-Chukchi-Beaufort population, bowhead whales stay with the ice edge as it advances and retreats each year (Goering and McRoy 1974). Reductions in the southern extent of seasonal sea ice could displace southern ranges of bowheads northward. Interannual changes in the onset and severity of seasonal sea ice may also affect the length of feeding seasons, timing of migration, fecundity, and survivorship of marine mammal species (Tynan and DeMaster 1997). Inuit hunters from the eastern Alaskan Beaufort Sea report more whales migrate later in the season than they used to (Galginaitis and Koski 2001). It is not possible at this time to determine the impact (whether positive or negative) of climate change on the bowhead population. Moshenko et al. (2003) ranked climate change as a high threat to bowhead whales.


Ship Collisions

George et al. (1994) examined bowhead whales of the Bering-Chukchi-Beaufort Seas population harvested by Alaskan Eskimos for scars from ship-collision injuries. They estimated that the frequency of scars from ship-collisions was about 1%. These low numbers suggests that the incidence of ship collisions with bowheads is quite low, because few vessels pass through most of the bowhead’s range, but it may also be that bowheads do not survive the collision (Kraus 1990). It is unlikely that many bowheads come in contact with ships in any of the bowhead ranges, due to the low number of ships that enter the arctic waters, and bowhead are generally associated with ice, where ships would generally travel in the ice-free areas. Moshenko et al. (2003) ranked ship collisions as a low threat to bowheads in the eastern arctic.

Richardson et al. (1987a) reported that most bowheads show avoidance reactions to approaching ships more than 1 km distant in the Eastern Canadian Beaufort, which would reduce the likelihood of a ship collision. However, they found these reactions were short-term and suggest that summering bowheads could habituate to an ongoing stimulus from offshore drilling or vessel operations, especially when they are feeding.


Ice-entrapment

The close association of bowheads with ice places them at risk. Mitchell and Reeves (1982) describe accounts of fatal ice entrapment, but whether the whale was frozen in the ice after it died could not be determined. Inuit have observed ice-entrapped bowheads on a few occasions (NWMB 2000). One bowhead was discovered entrapped on March 20, 1997 in a polyna in Admiralty Inlet (NWMB 2000). The whale was still alive when last seen just before ice breakup, and was believed to have survived since no carcass was ever found (NWMB 2000). Short of direct mortality due to ice entrapment, there is little doubt that bowheads can be prevented from reaching preferred feeding grounds during heavy ice years (Mitchell and Reeves 1982), which may affect their survival. Inuit report bowheads avoid areas where the ice cover is very extensive or apparently continuous (NWMB 2000). Ice-entrapment is likely the lowest threat to bowheads due to their ability to navigate through extensive ice fields and punch holes up through the ice in order to breathe.


Diseases and Parasites

There is limited information on causes of natural mortality in bowhead whales. What is known has come from the study of what were otherwise apparently healthy whales taken in the subsistence hunt (Philo et al. 1993). Bowheads carry parasites, such as whale lice, internal roundworms, tapeworms, and potentially pathogenic microbes (yeasts, bacteria and viruses), but none of these are thought to contribute to bowhead mortality (Philo et al. 1993). They also are known to occasionally suffer from developmental and degenerative conditions. Smith et al. (1986, 1987) suggests there is evidence that bowheads may be hosts for viruses such as caliciviruses, of which symptoms may include formation of cysts, open sores, inflammation of the lungs, brain, heart, stomach and intestine lining, and abortion. How these viruses affect mortality and reproductive rates is unknown (Philo et al. 1993). Moshenko et al. (2003) ranked disease and parasites as a low threat to bowhead whales.


Entanglement

There are no data to estimate the number of entanglement fatalities that occur in bowhead whales, although smaller bowheads are more likely to die from entanglement than larger whales because they may not be powerful enough to break the ropes or have the stamina to drag the gear (Philo et al. 1992). Reports of bowhead whales entangled in harpoon lines or ropes from fishing gear have occurred (summarized in Philo et al. 1992), but are rare. Inuit have reported bowheads swimming into nets set for belugas, narwhals and fish in Cumberland Sound and near Pangnirtung, resulting in destroyed nets and entanglement (NWMB 2000). The effect of entanglement mortality on the bowhead whale population is unknown, but is likely small at present. Moshenko et al. (2003) ranked entanglement as a low threat to bowhead whales at this time.