Steller sea lion (Eumetopias jubatus) COSEWIC assessment and status report: chapter 9

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Population Sizes and Trends

Between the late 1950s and 1970s, overall abundance of Steller sea lions in the North Pacific (range-wide: California to Japan) was believed to have been stable at about 250 000-300 000 individuals (Kenyon and Rice 1961; Loughlin et al. 1984). Abundance subsequently declined to about 116 000 by 1989, 97 500 by 1994-95, and 95 000 by 1999-2002 (Braham et al. 1980; Merrick et al. 1987; Loughlin et al. 1992; Trites and Larkin 1996; Sease et al. 1999; Burkanov 2000; Olesiuk 2003; Pitcher et al. 2003; Sease and Stinchcomb 2003).

The drop in overall abundance of Steller sea lions was attributable to declines in the western part of their range. Historically, the western population (Gulf of Alaska to Russia) was much larger than the eastern population (California to southeast Alaska), and accounted for roughly 90% of total abundance between the 1950s and 1970s (Kenyon and Rice 1961; Loughlin et al. 1984; Trites and Larkin 1996). The decline appears to have begun in the eastern Aleutian Islands in the mid-1960s, and spread to the western Aleutian Islands and Gulf of Alaska in the late 1970s. Numbers dropped precipitously during the 1980s, and continued at a much slower rate through the 1990s (York et al. 1996). By 1999-2002, the western population was estimated to have numbered about 50 000 individuals (Burkanov 2000; Sease and Stinchcomb 2003), a decline of about 80% from levels present during the 1950s to 1970s.

The eastern population of Steller sea lions (California to southeast Alaska) was historically much smaller than the western population, accounting for roughly 10% of total abundance between the 1950s and 1970s (Kenyon and Rice 1961; Loughlin et al. 1984; Trites and Larkin 1996). However, in contrast to the western population, the eastern population has been growing in recent years (Calkins et al. 1999; Olesiuk 2003; Pitcher et al. 2003). In 2002, the eastern population was estimated to have numbered about 45 000 individuals (Pitcher et al. 2003), almost the same size as the once more abundant western population.

To understand the present status of Steller sea lions in BC, it is important to put it in context of the historical kills and management actions that significantly affected abundance and distribution (Bigg 1985). Between 1913 and 1969, an estimated 49 100 sea lions were destroyed in predator-control programs, and another 5 700 animals were taken in commercial harvests (Fig. 3). The most intensive culling occurred at Virgin and Pearl Rocks in the Sea Otter Group. In an attempt to protect the Rivers Inlet salmon fishery, federal fishery officers visited these two rookeries annually during mid-June (1923-1939) and shot as many breeding animals as possible before landing and clubbing pups, most of which were too young to escape into the water. A total of about 20 000 animals (including 7 000 pups) were killed. These kills essentially eliminated new recruitment to the rookery, and pup production declined from about 1200 pups when the control program was initiated to fewer than 10 by the time it had ended. Virgin and Pearl Rocks have not been used as a rookery since the culls. Pupping has occurred there only sporadically, although the sites are used as haulout sites.

Once the Sea Otter Group rookeries were extirpated, control programs were directed toward the Scott Islands, where about 7500 animals (including 2800 pups) were killed during 1936-39. There was some attempt to commercially harvest animals for hides, but this proved to be uneconomical. Large-scale culls were suspended during World War II, although the Canadian air force and navy may have killed significant numbers of animals during bombing practices. However, no records exist of the magnitude of these kills (Bigg 1985). Several individuals are known to have hunted pups on the Scott Islands during the early 1950s, in a scheme in which they removed and altered their snouts and fraudulently claimed harbour seal bounties (Olesiuk, unpub. data). Predator control kills resumed during 1956-66 and included not only the Scott Islands, but for the first time the rookeries at Cape St. James and North Danger Rocks. During this period, about 11 600 animals were killed, including approximately 5 000 animals that were commercially harvested for mink food, but again this venture proved to be economically unfeasible. In addition to predator control and commercial harvests, 764 animals were reportedly killed from 1913-69 for research.

Figure 3: Total Numbers of Steller Sea Lions Reported to have been Killed in BC as Part of Control Programs and Commercial Harvests During 1913-70

Data have been grouped and totaled into 5-year periods, and are colour-coded by major breeding area. Comparison with Figure 4 shows the impact these kills had on populations. (Data from Bigg 1984.)

Bigg (1984; 1985) compiled historical sightings and counts of Steller sea lions during the period 1892-1984, which he used to reconstruct historic trends in numbers of breeding animals on rookeries. His analysis meticulously considered the likely reliability of observations, the timing of counts in the context of the life history of Steller sea lions, and the potential effect of disturbance. Bigg (1985) concluded that control programs and commercial harvests had depleted breeding populations, and estimated that the BC rookeries were inhabited by about 14 000 animals (all ages, including pups) when the first counts were made in 1913-16 (Figure 4). The extirpation of the Sea Otter Group rookeries, concurrent with a slight increase in numbers breeding on the Scott Islands, resulted in an overall reduced population of about 12 000 by 1938. By 1956, numbers on rookeries had been further reduced to about 8900-9400 sea lions (including 2850 pups) (Pike and Maxwell 1958; Bigg 1985). The population declined sharply with the resumption of predator control and commercial harvesting during 1956-66. Numbers on rookeries in BC had been reduced to only about 4550 by 1961, and 3390 animals (including 940 pups) by the time the first aerial survey was conducted in 1971. Thus Steller sea lions in BC were depleted to about one-quarter of their historic size by predator control and commercial harvesting (Bigg 1985; Olesiuk 2003). It should be noted that the subjectivity involved in interpreting the earlier counts precludes any formal statistical analysis of historic population trends.

Aerial survey procedures for counting Steller sea lions were developed in the mid-1960s (Mathisen and Lopp 1963) and have been standardized since the early 1970s (Mate 1977; Withrow 1982; Bigg 1985; Olesiuk et al. 1993). Typically, oblique 35 mm photographs are taken from a small fixed-wing aircraft as it circles a haulout or rookery during the breeding season. The censuses provide an index of relative abundance because some animals are always foraging at sea and are missed. Surveys in BC are conducted during the last week of June and first week of July, which represents the time by which most pups have been born, but most are still too young to have begun to disperse from rookeries (Olesiuk 2003). In recent years, high-resolution vertical medium-format photography has been used, particularly to census pups (Snyder et al. 2001; Olesiuk 2003). Life tables provide an estimate of the ratio of pups to older age-classes, and thereby a means to estimate the total population size (Calkins and Pitcher 1982; Loughlin et al. 1992; Trites and Larkin 1996; Sease et al. 1999; Olesiuk 2003). It should be noted, however, that the only life tables available were derived from specimens collected in the Gulf of Alaska just prior to major population declines, and the multipliers to estimate total population size might vary with status of populations.

Aerial surveys have been conducted in BC at 4-5 year intervals since the species was pindicate that numbers of pups and non-pups on rookeries both increased at a mean rate of 3.2% annually, which has resulted in a doubling in the size of the breeding population since the early 1970s (Figure 5, Appendix Appendix1 and Appendix2) (Olesiuk 2003). However, most of the increase appears to have occurred since the 1982 survey for non-pups, and the 1987 survey for pups. Overall, the number of animals on rookeries in BC has increased to about 8900, which represents about 65% of the animals present prior to the large scale kills (Figure 4). Total pup production in BC in 2002 was estimated at about 3300 - 3600 (Olesiuk 2003; Olesiuk et al. 2003). Applying corrections for animals at sea and missed during surveys, the total number of Steller sea lions inhabiting coastal waters of BC during the breeding season was estimated at about 18 400 - 19 700 individuals (all ages, and including non-breeding animals associated with rookeries in southeast Alaska) (Olesiuk 2003).

As noted previously, population trends in BC are difficult to separate from those in SE Alaska due to the large rookery that has become established on Forrester Island just north of the BC-Alaska border. Steller sea lions were not known to breed in SE Alaska during the early 1900s, so the 14 000 animals on BC rookeries represented the total breeding population for BC and SE Alaska. However, while control programs were underway in BC, animals began to breed at Forrester Island. The numbers of breeding animals increased dramatically from fewer than 100 in the early 1900s, to approximately 2400 animals (including 1100 pups) by 1961, and 6160 animals (including 2370 pups) by 1973 (Bigg 1985). This represented a mean annual growth rate of 6% in pup production, but the rate of increase fell during the 1980s and pup production on Forrester Island was stable during the 1990s (Calkins et al. 1999). However, at about the same time Forrester Island stabilized, animals began to establish new rookeries further north in SE Alaska (Calkins et al. 1999), and the growth rate on BC rookeries accelerated (Olesiuk 2003). The combined breeding population in BC and SE Alaska has thus sustained a fairly steady growth rate of about 2.4% since the 1960s, resulting in almost a 3-fold increase in abundance (Olesiuk 2003), and is now about 50% above the abundance level of the early 1900s prior to any large scale kills (Figure 4).

Figure 4: Historic Trends in Total Numbers of Steller Sea Lions (pups and non-pups) on Breeding Rookeries in BC (●), Forrester Island, Alaska (▲), and Other New Rookeries in SE Alaska (■)

The thin lower lines show distribution among main breeding areas in BC. Pup counts made from 35 mm slides were adjusted by a factor of 1.05 for BC rookeries and 1.25 for Forrester Island, to account for pups that were obscured in the oblique photographs (see Olesiuk et al. 2003) (modified from Bigg 1985).

Figure 5: Recent Trends in the Numbers of A) Non-pups (rookeries and haulout sites) and B) Pups (rookeries) Counted During Aerial Surveys of Sites in British Columbia Between 1971-2002

Dashed lines show overall average trends, whereas solid lines allow for a change in the population growth rate. Pups counts have been inflated by a factor of 1.05 to account for those obscured in oblique 35 mm photographs (Olesiuk et al. 2003).

Steller sea lions do not breed in Washington, although the occasional pup has been born at haulout sites (S. Jeffries, Washington Department Fish and Wildlife, Tacoma, WA, pers. comm.). The relatively small rookeries in Oregon also appear to be increasing slowly, and have doubled in size since the 1970s (Brown and Reimer 1992; Angliss et al. 2001). In contrast, numbers of Steller sea lions have been stable in northern California, and declined in southern California, since the 1960s.