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Recovery Strategy for the Sea Otter
1.1 Species Assessment Information from COSEWIC
Date of Assessment: April 2007 Common Name: Sea Otter Scientific Name: Enhydra lutris Assessment Criteria: Not applicable Status: Special Concern Reason for Designation: The species had been extirpated in British Columbia by the fur trade by the early 1900s, and was re-introduced from 1969-72. It has since repopulated 25-33% of its historic range in British Columbia, but is not yet clearly secure. Numbers are small (<3,500) and require careful monitoring. Their susceptibility to oil and the proximity to major oil tanker routes make them particularly vulnerable to oil spills. Canadian Occurrence: BC Pacific Ocean Status History: Designated Endangered in April 1978. Status re-examined and confirmed Endangered in April 1986. Status re-examined and designated Threatened in April 1996 and in May 2000. Status re-examined and designated Special Concern in April 2007.
Sea otters are legally listed as Threatened under Schedule I of SARA (June 2003). The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) recently reassessed the sea otter population as Special Concern in April 2007. Consideration for a change to the legal listing of sea otters under SARA based on the reassessment will follow the regular SARA listing process.
Sea otters are the second smallest marine mammal, and the second largest member of the Mustelidae, or weasel, family. Worldwide there are 12 species of otters. All have streamlined bodies, thick fur and amphibious habits, but the sea otter is the only species that carries out all aspects of its life in the marine environment. Sea otter possess several important adaptations. These include development of hind flippers for aquatic locomotion, flattened premolars and molars for crushing the hard-shelled marine invertebrates and enlarged kidneys to process the large amounts of ingested sea salt (reviewed in Riedman and Estes 1990).
On average, sea otters weigh between 19.5 kg and 29.5 kg (reviewed in Riedman and Estes 1990). Adult male sea otters tend to weigh more than females, and can weigh up to 50 kg and reach lengths of 1.5 m (R. Jameson pers. comm. 2002). The presence of the penile and testicular bulge is the only reliable method for determining sex when observing free-ranging otters. Newborn pups are characterized by a light brown, or yellowish, woolly natal fur that is completely replaced by adult fur by 13 weeks (Payne and Jameson 1984).
Three subspecies of sea otter are recognized, based on detailed skull measurements. Enhydra lutris kenyoni, which is thought to have historically ranged from the coast of Oregon to the Aleutian Islands, Enhydra lutris nereis, which occurs along the California coast and Enhydra lutris lutris, which ranges from the Kuril Islands to the Kamchatka Peninsula and the Commander Islands (Wilson et al. 1991). Genetic analysis of mitochondrial DNA (mtDNA) variation supports this, although there are some similarities in the frequencies of mtDNA haplotypes between Enhydra l. lutris and Enhydra l. kenyoni (Cronin et al. 1996). Recent genetic analysis also indicates some gene flow occurred between California and Prince William Sound, Alaska prior to the Maritime fur trade (Larson et al. 2002a).
Sea otters have little or no body fat. To survive in an aquatic environment, they maintain an exceptionally high metabolic rate and rely on the integrity of their dense fur for insulation. The fur consists of an outer layer of protective guard hairs below which is an extremely fine dense under fur of approximately 100,000 hairs per cm2 (Kenyon 1969). Oil from glands in the skin helps to enhance the water repellency of the fur. Sea otters must groom their fur frequently to maintain its insulative quality and water repellency. During grooming, the fur is cleaned, hair shafts are straightened and aligned to maintain loft, oil is distributed and air is blown through the fur where it is trapped as tiny bubbles that enhance the insulative capacity of the fur (reviewed in Riedman and Estes 1990).
The metabolic rate of the sea otter is 2.4 to 3.2 times higher than that of terrestrial mammals of a similar size. To fuel this internal heat production, free-ranging sea otters consume the equivalent of 23 to 33% of their body weight per day (reviewed in Riedman and Estes 1990).
1.3 Populations and Distribution
Sea otters are found in coastal areas throughout the North Pacific (Figure 1). The species once ranged fairly continuously from Northern Japan to central Baja California (Kenyon 1969), but the Maritime fur trade caused near extinction of the species by the mid-1800s. Today, the sea otter occupies about half of its historical range. Small remnant populations in California, the Aleutian Islands and Russia survived and eventually became re-established. Yet large areas to the south of the Gulf of Alaska, with the exception of California, remain unoccupied except where sea otters were intentionally re-introduced (Southeast Alaska, BC, Washington). Sea otters are found in Washington State and Southeast Alaska, the US jurisdictions bordering BC. In Southeast Alaska, sea otters range into Dixon Entrance (USFWS 2002c). In Washington State, sea otters range along parts of the west coast, north to Cape Flattery and eastward into the Strait of Juan de Fuca to Pillar Point (Lance et al. 2004)
Figure 1 Historical and current global range of all three subspecies of sea otters.
In an effort to re-establish sea otters to BC, 89 sea otters were reintroduced to Checleset Bay, BC, from Alaska (Bigg and MacAskie 1978) (Table 1). Until 1987, sea otters occupied two locations along the west coast of Vancouver Island, Checleset Bay and Bajo Reef off Nootka Island which is 75 km southeast of Checelest Bay. By 1992, the range of the population extended continuously along Vancouver Island from Estevan Point northwest to Quatsino Sound (Watson et al. 1997). By 2004, sea otters along Vancouver Island ranged from Vargas Island, in Clayoquot Sound, northward to Cape Scott and eastward to Hope Island in Queen Charlotte Strait (Nichol et al. 2005) (Figure 2). In 1989, females with pups were reported near the Goose Islands on the central BC coast indicating establishment of sea otters in the area (BC Parks 1995). By 2004, sea otters on the central BC coast ranged continuously from the southern end of the Goose Group, northward through Queens Sound to Cape Mark at the edge of Milbanke Sound. Single sea otters are periodically reported outside the continuous range.
Table 1. Sex, maturity, and health of 89 sea otters released in Checleset Bay 1969 to 1972. From Bigg and MacAskie (1978.)
|July 31, 1969||Amchitka||29||9||19||1||Fair-good|
|July 27, 1970||Prince William Sound||14||6||8||Excellent|
|July 15, 1972||Prince William Sound||46||8||22||7*||9*||Excellent|
* includes 4 male and 2 female pups
Figure 2 Range of the sea otter in BC and place names mentioned in the text regarding range. Shaded areas on Vancouver Island represent range in 1977. Gray line represents the range by 1995, black line the range by 2001 and the dashed line, range expansion on Vancouver Island in 2004.
Approximately 5 to 10% of the global distribution of sea otters occurs in Canada in the coastal waters of BC. In terms of population size, sea otters represent 3 to 4 % of the global population, however should declines in the sea otter populations of Southwestern Alaska continue, this percentage could increase.
Population Size and Trends
Estimates of the historic number of sea otters that occurred throughout the North Pacific prior to the maritime fur trade are crude and uncertain, but range from 150,000 to 300,000 (Kenyon 1969; Johnson 1982). Kenyon (1969) reported a world population in the late 1960s of about 30,000 sea otters, occupying about one fifth their former range. From this, he surmised, conservatively, that the pre-fur trade population could have been 100,000 to 150,000 animals. Johnson (1982) followed Kenyon’s approach, but used 60,000 as an estimate of the population in the late 1960s.
Although the maritime fur trade was a period of intensive hunting of sea otters, native peoples hunted sea otters prior to the trade. Examination of midden data from some sites in the Aleutian Islands show alternating periods of abundant urchins and sea otter remains that have been interpreted as evidence that humans may have caused periods of local extirpation long before European contact (Simenstad et al. 1978). Yet it was over-exploitation by European and American trade with aboriginal peoples during the maritime fur trade that drove sea otters to the brink of extinction by the mid-1800s. The International Fur Seal Treaty of 1911, signed by Japan, Russia, the United States and the United Kingdom (for Canada), intended to protect the Northern fur seal, included an article that prohibited non-natives and anyone hunting for commercial purposes from hunting sea otters in international waters (three miles from shore). This would have afforded some protection. By 1911, however, less than 2000 otters remained scattered amongst 13 remnant populations (Kenyon 1969). Several of these remnant populations declined to extinction (Kenyon 1969).
Until the early 1980s, most of the world population of sea otters (~ 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). Gorbics et al. (2000) provides the most recent total population estimate for North America and Russia of about 126,000 otters based on data from the late 1990s. However, the global population is now likely lower because this estimate was made while the decline in Western Alaska was underway. Precipitous declines in the Aleutian Islands to 8,742 individuals (CV = 0.215) by 2000 has meant that sea otters in Western Alaska are now listed as Threatened (2005) under the US Endangered Species Act (USFW 2006). Table 3 provides a summary of recent counts and estimates from North America and Russia, using a variety of different survey methods and survey effort. Some are minimum counts, while others have been adjusted with correction factors to account for missed animals.
Table 2. Recent counts and estimates of sea otter populations in the North Pacific.
|Region||# of otters||Year of estimate||Source|
|USA- California||2,735a||2005||USGS 2005|
|USA- Washington||814a||2004||Jameson and Jeffries 2004|
|Canada- BC||3,185a||2001, 2004||Nichol et al. 2005|
|USA- Southeast Alaska||12,632b||1994, 1995, 1996||USFW 2002c|
|USA- Southcentral Alaska||16,552b||1996, 1999, 2002||USFW 2002b|
|USA- Southwestern Alaska||41,474b||2000, 2001, 2002||USFW 2002a; Doroff et al. 2003|
|Russia- Commander Islands||5,546a||2002||A. Burdin pers. comm. 2003|
|Russia- Kamchatka Peninsula and Kuril Islands||16,910a||1997||Gorbics et al. 2000|
|Japan – Cape Nossapu||1a||1997||Gorbics et al. 2000|
a = direct counts, b = estimates corrected for unseen animals
Canada(Pacific) - British Columbia
The size of the population of sea otters in coastal BC prior to commercial exploitation is unknown, but records from the maritime fur trade give an indication of the magnitude of the hunt and the supporting population of sea otters. Sea otter pelt landings in BC between 1785 and 1809 total 55,000. Without a complete record of ship logbooks from which it would possible to ascertain where each trading event occurred, it is difficult to determine the geographic source of these pelts. Some of them could have come from Washington, Oregon or Southeast Alaska, but at least 6,000 of these came from the west coast of Vancouver Island (Fisher 1940; Rickard 1947; Mackie 1997). From surviving 18th century logbooks and voyage accounts between 1787 and 1797, at least 11,000 pelts were obtained in trade in the Queen Charlotte Islands alone. The aggregate landings from the Queen Charlotte Islands of four ships in 1791 alone was at least 3,000 pelts (Dick 2006). 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, Estes et al. 1989) earlier than this (Watson 1993).
Although 89 sea otters were reintroduced to the coast of BC in three translocation efforts (1969 to 1972), many did not survive, and the initial population may have declined to as few as 28 animals (Estes 1990). Seventy sea otters were counted during an aerial survey in 1977 in two locations on the west coast of Vancouver Island. In 1995, 1,522 sea otters were counted, of which 1,423 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 2,673 otters along the Vancouver Island coast and 507 on the central BC coast for a total of 3,180 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 2,369 in 2002, 2,809 in 2003 and 3,185 in 2004 (Nichol et al. 2005).
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 and the average annual growth rate between 1977 and 2004 was 15.6% per year (Nichol et al. 2005).
Sea otter populations are density dependent. As the number of sea otters in an area increases and food becomes limiting, otter density is maintained at equilibrium through mortality and emigration (Estes 1990). Rapid initial growth rates of 17-20% per year (~ rmax for the species) and a subsequent slowing of growth as parts of the population reach equilibrium are typical of reintroduced sea otter populations (Estes 1990). Such high rates are likely a result of unlimited food and habitat resources following the long absence of sea otters. 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, in part, explain the reduced population growth rate on Vancouver Island (Watson et al. 1997; Nichol et al. 2005). Surveys on the central BC coast started in 1990 following a sighting in 1989 of females with pups in the Goose Islands (BC Parks 1995; Watson et al. 1997). Nichol et al. (2005) estimated population growth between 1990 and 2004 to have been 12.4% per year, however, they noted that this estimate seemed low given the amount of unoccupied habitat available. There may be greater inter-survey variability in this area obscuring the trend and/or unknown sources of mortality.
USA(California, Alaska and Washington)
Following protection from commercial hunting by 1911, sea otter populations began to recover from remnant populations (western and central Alaska and California). However, even by the 1960s, sea otters had not repopulated the area from southeast Alaska to northern California. Translocations to southeast Alaska, BC, Washington and Oregon were attempted in the 1960s and 1970s in an effort to re-establish the species in its historic range (Jameson et al. 1982). The reintroduction efforts were successful in all cases except in Oregon (Jameson et al. 1982). In general, translocated populations have exhibited maximum growth rates of 17- 20% per year, whereas remnant population growth has been lower and more variable and has often included periods of decline (Estes 1990; Bodkin et al. 1999; Doroff et al. 2003). The reasons for these apparent differences are not entirely clear. Whereas the high rates of growth in translocated populations have been attributed to unlimited food and habitat resources in the areas of reintroduction, the low and variable rates among remnant populations are at least in part due to continued illegal harvest following protection in 1911, as well as incidental mortality related to fisheries in the later part of the twentieth century (Bodkin et al. 1999; Bodkin 2003). Certainly, some of the remnant populations that existed after 1911, such as the remnant population in the Queen Charlotte Islands, declined to extinction after 1911 (Kenyon 1969).
Positive growth has ranged from 5 - 7% per year, although there have also been periods of decline (Estes et al. 2003; USFWS 2003).). A decline of about 5% per year was detected in the mid-1970s and was attributed to mortality from entanglement in submerged fish nets. The trend reversed following restrictions on net use, and by 1995 surveys indicated a minimum population of 2,377. The southern sea otter population continues to exhibit a high rate of mortality compared to other sea otter populations. Disease, in particular from parasites for which sea otters may not be the natural host, anthropogenic factors including sewage and runoff, as well as entanglement in coastal gill and trammel nets, are considered contributing factors (Estes et al. 2003; USFWS 2003). Recent population surveys indicate a minimum population size of 2,735 in 2005 (USGS 2005).
In Southcentral Alaska, sea otters have recolonized most of their former range. The population in Prince William Sound was, however, significantly affected by the Exxon Valdez oil spill in 1989. Since the spill, the sea otter population in Prince William Sound has recovered, but not to the level expected (USFWS 2002b). An estimated 16,552 sea otters occur in Southcentral Alaska (USFWS 2002b).
In Southwestern Alaska, sea otters re-established to a large population size as early as the late 1950s, by which time the Southwestern Alaska population accounted for about 80% of the world population which was estimated to be 30,000 animals in the late 1960s (Kenyon 1969). By the 1980s, the Aleutian Island sea otter population alone numbered between 55,100 and 73,700 (Calkins and Schneider 1985), but began to decline precipitously in the late 1980s. By 2000, the population had declined to 8,742 (CV = 0.215),at a rate of -17.5 %per year (Doroff et al. 2003). Surveys of other parts of Southwestern Alaska suggest the decline may extend eastward to include the Alaska Peninsula and the Kodiak archipelago (Doroff et al. 2003). The most recent total population estimate for all of southwestern Alaska is 41,474 animals (USFWS 2002a).
Between 1965 and 1969, 412 sea otters were re-introduced to Southeast Alaska from Southwestern Alaska (Jameson et al. 1982). Population growth averaged 18 % per year between 1969 and 1988, but has since slowed to 4.7% per year overall (1988 to 2003), despite ample amounts of unoccupied habitat still available for expansion (Esslinger and Bodkin 2006). The slow growth rate does not appear to be attributable to disease, predation or limiting resources, but sea otters are hunted in Southeast Alaska (Esslinger and Bodkin 2006). The population is estimated to include 12,632 animals (including Yakutat and the north Gulf of Alaska)(USFWS 2002c).
In 1969 and 1970, 59 sea otters were re-introduced to Washington State from Amchitka Alaska. The population grew rapidly in the early years (~20% per year) but since 1989 the rate has averaged 8.2% per year (Estes 1990; Jameson and Jeffries 2004). In 2004 the Washington sea otter population included 814 animals (Jameson and Jeffries 2004). It has been suggested that the population may be approaching equilibrium density in some rocky habitat along the outer coast (Gerber et al. 2004; Jameson and Jeffries 2002).
Russia(Kuril Islands, Kamchatka Peninsula, and Commander Islands)
Gorbics et al. (2000) compiled counts from Russia including counts from 1997 of 16,910 sea otters in the Kuril Islands and Kamchatka Peninsula. The results of surveys of the Commander Islands in 2002 indicate a total of 5,546 animals, and the population there is likely at carrying capacity (Bodkin et al. 2000; A. Burdin pers. comm. 2003). Sea otters are not considered endangered in Russia, but the population is still considered to be below previous (historic) levels. The population is considered to be threatened by poaching, habitat contamination and fisheries conflicts. Poaching is of particular concern because a black market is believed to exist in Russia to illegally export pelts to China, Korea and Japan (Burdin 2000).
Translocation of sea otters
Translocation as a means of re-establishing sea otter populations into parts of their former range was successfully used in the late 1960s and early 1970s in Southeast Alaska, BC, Washington and Oregon (see above). Although sea otters reproduced and remained in Oregon for several years, they eventually disappeared. The reason for the failure in Oregon is unclear (R. Jameson pers. comm. 2003). Early translocations in the 1950s to a variety of Aleutian Islands (Kenyon and Spencer 1960) and a translocation in 1966 of 55 sea otters to the Pribilof Islands were likewise considered unsuccessful (Jameson et al. 1982). At present there are less than 50 sea otters in the Pribilof Islands, and there is some question as to whether these are descendants of the re-introduced animals, or animals that have dispersed from the Alaska peninsula (R. Jameson pers. comm. 2003). Many of these early translocations were conducted to determine if sea otters could be successfully relocated, and to assess capture and transport techniques. A summary of all these early sea otter translocations can be found in Jameson et al. (1982).
More recently, translocation was used in California as a recovery strategy to increase the distribution of the southern sea otter population, thereby reducing the impact of an oil spill, and to establish another breeding population (Benz 1996). The following summarizes the results to date of this approach to achieving recovery of southern sea otter.
In 1982, the Southern Sea Otter Recovery Plan (1982) called for the United States Fish and Wildlife Service (USFWS) to establish a second breeding group of southern sea otters in California, which would expand the distribution, and increase the population size, thereby reducing the threat of a catastrophic oil spill (Riedman 1990). At that time, the southern sea otter population had not grown significantly since 1973, and oil spills were considered a major threat in California (VanBlaricom and Jameson 1982).
From 1987 to 1990, USFWS translocated 140 southern sea otters from central California to San Nicolas Island, located in the Channel Islands off Santa Barbara, more than 200 km southeast of the mainland population and about 100km west of the coast. In addition to reducing the effects of a catastrophic oil spill on the southern sea otter population, scientists further hoped to refine the techniques used to capture, hold and relocate sea otters, gather data on population dynamics and ecological relationships, and determine if removing sea otters affected the source population (Benz 1996).
The decision to translocate sea otters was extremely controversial. As part of the translocation the USFWS was legally obliged to restrict the “experimental population” of sea otters to the translocation site at San Nicolas Island, and to ensure that the existing sea otter population did not extend south of Point Conception. This “zonal management” strategy was instituted because shellfish fishers demanded that a no sea otter zone be created to ensure the continued availability of commercially valuable shellfish resources south of Point Conception. Sea otters moving into the no sea otter zone were captured and relocated back to the approved sea otter zone (Benz 1996).
By the end of the first year of translocation, more sea otters had dispersed from San Nicolas Island than was expected and the translocation strategy changed several times to try and address this problem. The last otters were released in 1990. Of the 140 sea otters moved to San Nicolas, 36 returned to their capture location on the mainland. Eleven were captured in the no sea otter zone and returned to the mainland. Seven were found dead in the no sea otter zone. Three were found dead at San Nicolas Island, and at least 13 are believed to have established at San Nicolas Island. The fate of the remaining 70 translocated animals is unknown, although they are suspected of having returned to the mainland or to the no sea otter zone and died (USFWS 2003). However, the results of the earlier translocations to Washington State suggested that high mortality and dispersal following translocation was normal, and even with a very small founder population, sea otters eventually became established in Washington State (Benz 1996). This was also true in BC and Southeast Alaska (see sections above).
In terms of establishing a breeding population, the translocation project has been less successful than hoped. The number of otters at San Nicolas Island has increased slowly since 1993 with 27 animals in the population as of 2002 and at least 73 pups known to have been born since re-introduction (USFWS 2003). In terms of containing the population, the project failed. Zonal management has proven ineffective, costly, and potentially detrimental to the parent population. In July 2000, the USFWS decided that the containment of sea otters by attempting to maintain the no sea otter zone was jeopardizing recovery of the southern sea otter population and stopped removing sea otters from the exclusion zone (Federal Register January 22, 2002, Volume 66:14:6649-6652). The decision to stop capturing sea otters was contested by commercial fishers who filed a lawsuit against the USFWS. The courts, however, found in favour of the USFWS and sea otters have been allowed to expand into the no sea otter zone. A final decision regarding options for the translocation program, whether to revise the program or whether to terminate the program, is pending (USFW 2005).
It is not clear why the translocation has had such limited success (Benz 1996). At least one third of the adult sea otters dispersed from San Nicolas, often returning to where they were captured and to other areas beyond San Nicolas Island. The requirement to capture and relocate otters dispersing from the translocation zone, and especially to limit the range of the existing population, was extremely expensive and difficult to monitor, and possibly detrimental to the original sea otter population. Had the no sea otter zone not been in effect and the relocated population been left alone, the effort to establish a new breeding population beyond the current range in California might have been more successful than currently thought (R. Jameson pers. comm. 2003). The Exxon Valdez oil spill in 1989 illustrated that a spill of a similar magnitude in California would have affected both the existing population and the experimental population at San Nicolas Island. As such the translocation could not reduce the threat from such a large spill, although the threat posed by smaller spills might be reduced.
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