Grey whale (Eschrichtius robustus) COSEWIC assessment and status report: chapter 10
Limiting factors and threats
The principal threat to the eastern North Pacific population of grey whales probably lies in increased human activity in the breeding lagoons (Clapham et al. 1999). Certain lagoons or parts of lagoons have already become unsuitable because of boat traffic and salt extraction (Rice et al. 1981) and any continuation of this trend could eventually put the entire population at risk.
No coordinated program to determine the cause of mortality of stranded whales is currently in place in western Canada, and information on causes for the unusually high mortality grey whale strandings in British Columbia during the northbound migrations of 1999 and 2000 is currently limited. Studies in Mexico and the US (Le Boeuf et al. 2000; Moore et al. 2001) noted that many of the animals stranded there were extremely emaciated implicating nutritional stress as the cause for mortality and low recruitment. Perryman et al. (2002) documented that the Bering Sea remained ice free for an unusually short time in the summers of 1998 and 1999, thus substantially shortening the feeding season in these years. The sudden increase in mortality and decrease in calf production were therefore probably caused by the combined effect of the population approaching the carrying capacity of its feeding ground and two subsequent suboptimal feeding seasons (Moore et al. 2001; Perryman et al. 2002; Wade 2002). Chronic or acute nutritional stress is also likely to increase the animals’ susceptibility to disease (Moore et al. 2001).
As the eastern North Pacific population returns to its pre-whaling size, the population is increasingly limited by the amount of available habitat on the feeding grounds. The high mortality and low recruitment from 1998 to 2000 suggest that any changes in the availability of feeding habitat, through climate change or through anthropogenic factors will affect grey whales in the North Pacific.
Oil and gas exploration and the associated anthropogenic noise and potential of spills can cause loss of habitat on arctic and temperate feeding grounds (Jayko et al. 1990; Moore and Clarke 2002). Airgun pulses used in the seismic exploration for oil and gas reserves have been shown to elicit strong behavioural avoidance in migrating grey whales at distances of up to 5 km (studies reviewed in Richardson et al. 1995; Moore and Clarke 2002), and seismic exploration appears to strongly displace feeding grey whales as well (Weller et al. 2002b). Clapham et al. (1999) consider oil and gas exploration as the greatest threat to the western North Pacific population of grey whales. Similarly, offshore mining and dredging, through noise pollution and by removing and covering feeding substrate, can lead to loss of feeding habitat (Jewett et al. 1999). The opening of the Alaskan North Slope to oil and gas extraction could mean a loss of grey whale feeding habitat. Lifting the moratorium on oil and gas exploration in British Columbia waters will lead to increased noise pollution along the migratory pathway and on the feeding ground of summer-resident grey whale.
In addition to noise pollution, increases in vessel traffic in the breeding lagoons, along the migratory corridor and on the feeding grounds can lead to increased mortality of grey whales from boat strikes. Laist et al. (2001) mention that grey whales are commonly struck by boats off the coast of California, and some individuals identified off British Columbia bear prominent propeller scars (Deecke 2003).
Entanglement in fishing gear and other marine debris is another source of mortality. Heyning and Lewis (1990) report that grey whales are the most common species of baleen whale involved in entanglement off southern California. Both Calambokidis and Baird (1994) and Ford et al. (1994) suggest that entanglement in fishing gear represents an important anthropogenic threat to grey whales off British Columbia. Fishing gear found on entangled whales includes offshore drift nets used for swordfish, inshore gill nets used for seabass, halibut, salmon, and shark nets, as well as longlines, and crab and lobster pots (Sumich and Harvey 1986; Heyning and Lewis 1990; Baird et al. 2002). Off British Columbia, seine and gillnet fisheries for salmon, as well as longline fisheries for bottom fish, are probably the greatest source of mortality (Baird et al. 2002).
Their near-shore distribution and their benthic or epi-benthic feeding mode make grey whales potentially susceptible to environmental toxins. Biotoxins include paralytic shellfish poisoning and domoic acid, although confirmed cases of poisoning from these sources are rare (e.g., Moore et al. 2001). Since grey whales regularly ingest bottom sediments when feeding, it has been proposed that they may be severely affected by human pollution. However, aside from somewhat elevated levels of copper in one stranded individual (Méndez et al. 2002), no alarming levels of organochlorines or heavy metals have been detected (Varanasi et al. 1994; Jarman et al. 1996; Krahn et al. 2001; Tilbury et al. 2002). The fact that grey whales feed at a relatively low trophic level probably prevents them from accumulating the high concentrations of toxins found in some odontocetes (O'Shea and Brownell 1994).
Since the end of commercial whaling in 1937, grey whales are hunted off Chukotka under subsistence quotas issued by the International Whaling Commission, and the Makah tribe in Washington State resumed hunting grey whales in 1999. Annual subsistence catches have ranged between zero (1944, 1992, 1993) and 374 (1967) individuals (International Whaling Commission 2003a). Population models suggest a harvest of up to 463 animals to be sustainable for the eastern North Pacific population (International Whaling Commission 2003b). While the current take levels are low enough to be of little significance to the population as a whole, they could still cause local extinctions due to the high site fidelity of grey whales. For example, the take of up to five grey whales annually allotted to the Makah can be sustained by the migratory population, but could lead to local extinction if the hunt was to target summer-resident grey whales. The fact that Atlantic grey whales were extirpated long before the onset of large-scale industrial whaling suggests that grey whales as a species are susceptible to coastal community-based whaling.
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