Sea otter (Enhydra lutris) COSEWIC assessment and status report 2007L chapter 9

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Population Sizes and Trends

• Search Effort
• Abundance
• Trends
• Rescue Effect

Search Effort

The first census of sea otters in British Columbia was conducted in 1977. Between 1977 and 1987, surveys were made mainly by fixed-wing aircraft with some counts conducted from boats (Bigg and MacAskie 1978; Morris et al. 1981; MacAskie 1987; Watson et al. 1997). Most of the sea otter range has been surveyed annually since 1988 by boat or helicopter, although there were gaps in survey coverage from 1996 to 2000 (Watson 1993; Watson et al. 1997). Most population surveys from 2001 to 2004 were led by Fisheries and Oceans Canada, although portions of the west coast of Vancouver Island were also surveyed in 2002 and 2004 by biologists with the Nuu-chah-nulth Fisheries (Nichol et al. 2005; Dunlop et al. 2003; Dunlop pers. comm. 2006).

These surveys are direct counts that provide measures of relative abundance for trends in population growth.

Abundance

Global

Sea otters were hunted by indigenous peoples of the North Pacific prior to European contact, but it was the maritime fur trade commencing after 1741 that drove the species to the brink of extinction. Prior to the fur trade the total range-wide population of sea otters is estimated to have been 150 000 to 300 000 otters, although some authors suggest the number may have been even larger (Kenyon 1969; Johnson 1982). By 1911, the world population numbered fewer than 2000 animals scattered amongst 13 remnant colonies (Kenyon 1969). Since protection in 1911, sea otter populations have been recovering. Some from remnant colonies, others as a result of reintroductions made in the late 1960s and early 1970s Until the early 1980s the bulk of the global population (~ 165 000 animals) occurred in the Aleutian Islands (55 100 to 73 700 individuals) (Calkins and Schneider 1985). However, dramatic declines in the Aleutian Islands started in the mid-late 1980s (Estes et al. 1998; Doroff et al. 2003). The most recent total population estimate (North America and Russia) of about 126 000 otters dates from the late 1990s (Gorbics et al. 2000), However, the global population is now likely lower than this estimate because this estimate was made while the decline in Western Alaska was underway. The continued decline in the Aleutian Islands to 8742 individuals (CV = 0.215) in 2000 led to sea otters in Western Alaska being listed as Threatened in 2005 under the US Endangered Species Act (USFW 2006).

Canada

The size of the population of sea otters in British Columbia prior to commercial exploitation is unknown, but there is evidence that they were abundant. As many as 55 000 sea otter pelts were landed in British Columbia between 1785 and 1809. It is difficult to determine the geographic source of these pelts but at least 6000 came from the west coast of Vancouver Island (Fisher 1940; Rickard 1947; Mackie 1997). Some pelts landed in British Columbia may have been from Washington, Oregon and Southeast Alaska By 1850, sea otters in Canada were considered commercially extinct, and they may have been ecologically extinct (and ceased to function as a keystone species) earlier than this (Watson 1993).

Although 89 sea otters were reintroduced to British Columbia in 3 translocation efforts (1969 to 1972), many did not survive and the initial population may have declined to as few as 28 animals (Estes 1990). An aerial survey in 1977 resulted in a count of 70 otters in 2 locations on the west coast of Vancouver Island. In 1995, boat surveys resulted in a count of 1522 sea otters, of which 1423 occurred along the west coast of Vancouver Island and 99 occurred along the central mainland coast in the Goose Islands (Bigg and MacAskie 1978; Watson et al. 1997). Surveys in 2001 resulted in a count of 2673 otters along the Vancouver Island coast and 507 on the central British Columbia coast for a total of 3180 otters (Nichol et al. 2005). Surveys were also made in 2002, 2003 and 2004, but some segments of the range were missed in each year. Using interpolation to estimate numbers of otters in the missed segments (which represented less than 10% of each annual count) resulted in population estimates of 2369 in 2002 (a decline from 2001), 2809 in 2003 and 3185 in 2004 (Nichol et al. 2005).

Trends

North America

Following protection in 1911, sea otter populations began recovering. Remnant colonies in western and central Alaska and in California began recovering without intervention. Populations in southeast Alaska, British Columbia, and Washington were established by translocating sea otters from the remnant populations in Alaska.

Population growth is highly variable among sea otter populations. Reintroduced populations in Washington, British Columbia and Southeast Alaska were successful, whereas attempts to reintroduce sea otters to Oregon in 1970 and 1971 failed (Jameson et al. 1982). Once established, all of the translocated populations (except Oregon) increased initially at 17-20% per year, likely as a result of abundant invertebrate prey, which increased when sea otters were extirpated. Population growth seems to be more variable in remnant populations (Table 1).

The California population has undergone periods of growth (5-7% per year) and declines (-5% per year). Declines in the 1970s were partially attributed to entanglement in sunken gill nets (Estes 1990; Estes et al. 2003b; USFW 2003). Mortality is high in this population, with disease and anthropogenic factors considered to be contributing causes (Estes et al. 2003b). In Alaska, the Prince William Sound sea otter population declined following the Exxon Valdes oil spill of 1989. It has not increased appreciably since 1994 (13 234 CV = 0.198; 1999 population estimate) and is not thought to be at its pre-spill level (USFW 2002a). Remnant populations in the Aleutian Islands increased following protection in 1911 and represented about 80% of the world population by the 1960s (Kenyon 1969; USFW 2002b). The population increased up until the early 1980s with estimates of 55 100 to 73 700 sea otters. However, a steep decline of 17.5% per year starting in the mid- to late 1980s reduced the population in the Aleutian Islands to 8742 (CV = 0.215) by 2000 (Calkins and Schneider 1985; Estes et al. 1998; USFW 2002b; Doroff et al. 2003). A decline has also been detected along the Alaska Peninsula and Kodiak Archipelago (USFW 2002b). Predation by mammal-eating killer whales has been suggested as the most likely cause of the sea otter decline in the Aleutian Islands. Seals and sea lions, likely the preferred prey of killer whales in the Aleutian Islands have drastically declined in abundance in Western Alaska and Estes et al. (1998) hypothesize that with the decline of pinnipeds, killer whales have switched to preying on sea otters.

Canada

Watson et al. (1997) estimated population growth to be 18.6% per year from 1977 to 1995 on Vancouver Island. Since 1995, the growth rate on Vancouver Island appears to have slowed. A simple log-linear regression applied to counts from 1977 to 2004 resulted in a rate of 15.6% per year along Vancouver Island. A piece-wise regression which allowed for an inflection in the log-linear regression provided a slightly better fit and estimated the growth from 1977 to 1995 to 19.1% per year, and then 8.0% per year from 1995 to 2004 (Nichol et al. 2005) (Figure 4).

Figure 4: Trend in sea otter population on Vancouver Island

A) Thin line – simple log-linear regression (growth 15.6% per year, r2 = 0.950, n =18). Thick line – piece-wise regression (growth 19.1% per year until 1995 and 8.0% per year from 1995 to 2004, r2 = 0.975, n = 18). B) The same trends presented on an ordinal scale. From Nichol et al. (2005).

In the absence of density-independent factors such as predation (e.g. in Western Alaska), sea otter populations are thought to be regulated by food through density- dependent processes that result in elevated juvenile mortality. As the number of sea otters in an area increases and food becomes limiting, otter density in the area is maintained at equilibrium (K) through mortality and emigration (Estes 1990). Rapid initial growth rates of 17-20% (~ r_max for the species) and a subsequent slowing of growth as parts of the population reached equilibrium are typical of reintroduced sea otter populations (Estes 1990). Some parts of the population near the centre of the range on Vancouver Island have been at equilibrium since the mid-1990s and additional areas are now at or nearing equilibrium, suggesting density-dependence may at least in part explain the reduced population growth rate on Vancouver Island (Watson et al. 1997; Nichol et al. 2005). However, counts from 2001-2004 may have been affected by survey conditions which were generally poorer than from 1988-1995, and other factors, such as a mortality event or illegal shooting, may also have contributed to the decline in growth rate.

Surveys on the central British Columbia coast started in 1990 following a sighting in 1989 of females with pups in the Goose Islands (British Columbia Parks 1995; Watson et al. 1997). A simple log-linear regression indicates the population on the central coast grew at 12.4% per year between 1990 and 2004 (Nichol et al. 2005) (Figure 5). This estimated rate of growth seems low given the amount of unoccupied habitat available. Nichol et al. (2005) were unable to explain this low growth rate but noted that the fit of the regression was poor compared to the Vancouver Island regression.

Rescue Effect

Sea otter populations adjacent to British Columbia exist in both Washington State and southeast Alaska. However, in the event of a catastrophic incident such as an oil spill, with widespread effects on the Canadian sea otter population, movement and colonization by sea otters from either Washington State or Southeast Alaska (whose populations are presently below K) would be unlikely. Dispersal of adults from adjacent populations, below equilibrium density, is unlikely because of the limited movement, high site fidelity and small home ranges that sea otters have (Jameson 1989, Bodkin et al. 2002). Colonization of adjacent unoccupied area generally occurs when sea otters in an occupied area approach equilibrium density and male sea otters move en masse into unoccupied habitat. The females follow once the males have moved on (Loughlin 1980; Garshelis et al. 1984; Jameson 1989).

The pattern of population recovery following the Exxon Valdez Oil Spill was studied at two island sites in Prince William Sound. Population growth at oiled and unoiled sites resulted from internal reproduction and immigration of juveniles and not from the widespread redistribution of adults from other parts of Prince William Sound (Bodkin et al. 2002).

Figure 5: Trend in Sea Otter Population Growth on the Central Mainland Coast

A) The line represents simple log-linear regression (growth 12.4% per year from 1990 to 2004, r2 = 0.737, n = 10). B). The same trend presented on an ordinal scale. From Nichol et al. (2005).