Grey whale (Eschrichtius robustus) COSEWIC assessment and status report: chapter 9

Population sizes and trends

Models of historical catches and available habitat suggest that the size of the eastern North Pacific population of grey whales was probably between 23 000 and 35 000 individuals before the onset of commercial whaling in 1846 (Reilly 1992; Punt and Butterworth 2002; Wade 2002). Grey whales were hunted during the migration from shore-based stations between Baja California and British Columbia from 1854 to 1901 (Sayers 1984). In addition, grey whales were also hunted from ships in the breeding lagoons between 1846 and 1874 at which time the whalers considered the population economically extinct (Henderson 1984). Between 1914 and 1946, pelagic whaling ships killed a reported 940 grey whales (Reeves 1984). There is evidence that Japanese and Soviet catches were underreported and continued past 1937 when grey whales were internationally protected (Baker et al. 2002). Rice et al. (1984) suggest that the eastern North Pacific population of grey whales reached its low point in the late 19th century at approximately 4000 individuals.

The western North Pacific population of grey whales has yet to recover from commercial exploitation. This population is estimated to number approximately one hundred individuals (Weller et al. 1999; Weller et al. 2002a) and is considered to be one of the most endangered populations of baleen whale (Clapham et al. 1999).

Modern abundance estimates of the eastern North Pacific population are precise by standards of cetacean population data. Since migrating grey whales travel close to shore, the entire population can be counted at strategic points along the migration corridor (e.g., Reilly et al. 1983; Rugh 1984; Buckland and Breiwick 2002) and standard errors for estimates of population size are generally small (4-8%; Buckland et al. 1993). Shore-based counts along the California coast during the southbound migration have been conducted approximately every other year since 1967 and have detected an increase from 12 921 individuals in 1967 to 20 869 individuals in 1988 and 26 635 in 1998 (Buckland et al. 1993; Gerber et al. 1999; Buckland and Breiwick 2002). This indicates an average rate of population growth of approximately 2.5% (Buckland and Breiwick 2002).

In the winter of 1998-1999, a drastic change of mortality and recruitment patterns occurred in the eastern North Pacific population -- mortality on the breeding grounds and during the 1999 northbound migration increased three- to fourfold (Le Boeuf et al. 2000). At the same time, calf production (usually between 4.6 and 5.2%; see section ‘Reproduction’) dropped to 1.7% (Perryman et al. 2002). Mortality was also high in 2000, but returned to normal in 2001 (Le Boeuf et al. 2000; Moore et al. 2001; International Whaling Commission 2003b). Calf production remained low in 2000 and 2001 (1.1% and 1.4% respectively; Perryman et al. 2002; International Whaling Commission 2003b), but increased to approximately 4.8% in 2002 and approximately 4.4% in 2003 (Perryman, pers. comm.). Based on ice distribution in the Arctic in summer 2003, Perryman (pers. comm.) predicts 2004 calf production to fall within the normal, pre-1999 range.

The estimated size of the eastern North Pacific population was 18 761 in 2001 and 17 414 in 2002 (International Whaling Commission 2003b) indicating an annual decrease of approximately 10% since 1998. It is unclear whether these lower population estimates reflect a true decline or are the result of many animals not migrating all the way south to the breeding lagoons in those years. In either case, close continued monitoring of the population is warranted to reassess population trends. However, no population census was conducted during the 2002/2003 southbound migration and none is scheduled for the 2003/2004 migration (Rugh, pers. comm.)

A preliminary analysis suggests that summer-resident grey whales are not genetically distinct from the eastern Pacific grey whales summering on arctic feeding grounds (Steeves et al. 2001). They probably migrate to the same lagoons to mate (Deecke 2003). Summer-residents therefore do not form a distinct population. However, factors that adversely affect summer residents or their habitat could cause local extinction due the high degree of site fidelity of summer-resident individuals and the possibility of site-specific recruitment (Calambokidis et al. 2002). For this reason and because of the cultural and economic importance of summer-resident grey whales in British Columbia (see section ’Special Significance of the Species’), this feeding aggregation may need to be managed as a separate unit. Darling (1984) suggests that 35-50 individuals feed along the west coast of Vancouver Island in the summer. Deecke (1996) found that between 12 and 20 individuals feed on a 57 km stretch of coastline near Cape Caution at any time during the summer. In the years 1996 to 1998, Calambokidis et al. (2002) identified a total of 155 summer-resident grey whales along the west coast of North America of which at least 80 individuals have been seen in British Columbia waters (Deecke 2003). Mark-recapture estimates using the 1997 and 1998 sightings suggest a size of 179 (95% C.I.: 171-187) individuals for the summer-resident community from northern California to Southeast Alaska (Calambokidis et al. 2002).

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