Sea otter (Enhydra lutris) COSEWIC assessment and status report 2007L chapter 10
Limiting Factors and Threats
Sea otters were hunted for subsistence by indigenous people throughout the North Pacific, but the maritime fur trade of the 18th and 19th centuries resulted in extirpation of the sea otter from much of its range. Sea otter populations have been recovering since then, from surviving remnant populations, and from reintroduced animals from surviving remnant stocks. In general the availability of habitat and food is thought to be the primary factor limiting growth in sea otter populations, although predation is presently the principal factor in Western Alaska (Riedman and Estes 1990; Estes 1990; Estes et al. 1998; Doroff et al. 2003).
Aside from humans, predators include bald eagles, Haliaeetus leucocephalus, (Sherrod et al. 1975), killer whales Orcinus orca (Riedman and Estes 1990; Estes et al. 1998), and, sharks (at least in California); Ames and Morejohn 1980). Eagles scavenge adult carcasses and prey on live sea otter pups. In the Aleutian Islands, sea otter pups comprise 5 to 20% (by frequency) of the eagle diet during the pupping season (Anthony et al. 1999). Sources of mortality have not been studied in British Columbia, but pup carcasses found at eagle nests suggest that eagle predation may be a significant source of pup mortality (Watson et al. 1997).
Although there are some anecdotal accounts of killer whales pursuing sea otters in British Columbia, there is no evidence that such predation is a significant source of mortality (Watson et al. 1997). Unlike in Western Alaska, populations of pinnipeds are considered abundant in British Columbia and southeast Alaska (Trites et al. In Press), thus a sea otter population decline as a result of killer whale predation seems unlikely. However, it is important to note that the sea otter decline observed in the Aleutian Islands, proposed to be from killer whale predation, is unprecedented in the current knowledge of sea otter populations, and occurred in a very short period of time (<15yrs). One of the challenges to sea otter conservation is accurately estimating population size and thus detecting population trends (Bodkin 2003). It is because of the high variance typical of sea otter counts that the decline in the Aleutian Islands was not detected for almost 10 years and even then hotly debated. Bodkin (2003) suggests that sea otters may be regulated by new and different factors (which are poorly understood) as they attain equilibrium densities.
Exposure to a variety of diseases has been documented in sea otters in Alaska, Washington, California and British Columbia (Thomas and Cole 1996; Reeves 2002; Lance et al. 2004; Gill et al. 2005; Shrubsole et al. 2005). Thus far, disease-caused mortality does not appear to be a threat in most otter populations with the exception of California. In California, 40% of the beach-cast carcasses were animals that died from disease and diseases appear to be affecting high numbers of prime-age animals, which may be a major factor explaining the low observed rate of population growth (Thomas and Cole 1996; Estes et al. 2003b). The emergence of infections from Toxoplasma gondii and Sarcocystis neurona (two pathogens found in humans and terrestrial mammals), for which sea otters are not considered a normal host, is of particular concern in California (Thomas and Cole 1996; Estes et al. 2003b). In California the presence of T. gondii and S. neurona in the marine environment may be linked to domestic sewage and urban and agricultural runoff that transports these pathogens into coastal waters where they infect prey species consumed by sea otters (Lafferty and Gerber 2002; Miller et al. 2002; Kreuder et al. 2003).
Since 2000, sea otter beach-cast carcasses have been examined to determine cause of death in Washington State (Lance et al. 2004). In 2000, one of 6 animals died from dual infection with T. gondii and S. neurona. In 2002, 1 of 8 animals examined died from infection with S. neurona and 6 died from infection with Leptospirosis. In 2004, 2 of 3 animals examined had died of infection from S. neurona and 1 from Canine Distemper (CDV) (the first reported case of CDV in sea otters), although 81% of 32 live-captured sea otters in 2000 and 2001 tested seropositive for exposure to morbilliviruses such as CDV (Lance et al. 2004).
In British Columbia, beach-cast carcasses are rarely retrieved because of scavenging by eagles, bears and wolves and the remoteness of the sea otter range. However, in 2006 one animal from the west coast of Vancouver Island died from infection with S. neurona (Raverty pers. comm. 2006). Among 42 animals live-captured on the British Columbia coast in 2003 and 2004, 8 were seropositive for morbilliviruses and 2 tested positive for T. gondii (Shrubsole et al. 2005).
Butter clams (Saxidomus spp.) and other bivalve species form an important component of sea otter diet, and can accumulate the biotoxin responsible for Paralytic Shellfish Poisoning (PSP) (Anderson 1994). A die-off of sea otters at Kodiak Island in 1987 was partly attributed to PSP poisoning (DeGange and Vacca 1989), suggesting that PSP may represent a source of mortality in sea otter populations. Research suggests that sea otters can detect toxic levels of PSP and may avoid feeding on prey items with toxic levels, unless other prey are not available (Kvitek and Bretz 2004). Domoic acid, a biotoxin produced by some species of diatoms and marine algae, can accumulate in filter-feeding invertebrates and fish. Domoic acid has been identified as the cause of several large die-offs of sea birds and sea lions in California as well as mortality in southern sea otters (Kreuder et al. 2003). The frequency of PSP and domoic acid events in British Columbia is monitored at least to the extent that it supports the commercial bivalve fisheries and shellfish aquaculture, but the effect of PSP or domoic acid on the British Columbia sea otter population is unknown.
Threats to sea otters include environmental contamination, entanglement in fishing gear and collisions with vessels, illegal killing, disease and possibly human disturbance.
Environmental Contamination - Oil Spills
Oil is a significant threat to sea otters. It destroys the water-repellent nature of the pelage which eliminates the air layer, and reduces insulation by 70%. The result is hypothermia and death (Costa and Kooyman 1982; Williams et al. 1988). Once fouled, a sea otter grooms itself obsessively and stops feeding, resting and caring for young (Ralls and Siniff 1990). Furthermore as it grooms, the otter ingests oil and inhales toxic fumes which damages internal organs. Methods for cleaning and rehabilitating sea otters exist, but they are costly and the benefits at a population level are questionable (Estes 1991; Williams and Davis 1995).
Several behavioural characteristics predispose sea otters to oil exposure. Sea otters typically rest in sexually-segregated aggregations (rafts) of up to 200 animals, meaning that large numbers of otters can be oiled simultaneously. In addition, rafts of otters often form in or near kelp beds, which accumulate and retain oil (Ralls and Siniff 1990). Finally, otters may be chronically exposed to oil through ingestion of contaminated prey (e.g. mussels) long after the spill has occurred (Bodkin et al. 2002).
On December 23, 1988, the oil barge Nestucca was rammed by its tug and spilled 875 000 l of Bunker C oil into the water off Grays Harbor, Washington (Waldichuk 1989). Within 7 days, oil had spread northward to Cape St. James, Queen Charlotte Islands, and was observed throughout the entire British Columbia sea otter range. The spread of oil from this spill, which killed at least one sea otter in British Columbia, demonstrated the vulnerability of the British Columbia otter population to oil spills (Watson 1990). The Nestucca spill, which affected both the Washington State and British Columbia sea otter populations, suggests that in the event of a catastrophic oil spill, it is likely that adjacent otter populations will also be affected.
Tankers, barges, fuel tanks and bilges of marine vessels, shore-based fuelling stations and shore-based industries are the main sources of water-borne oil in British Columbia (Shaffer et al.1990). In the early 1990s, there were 7000 transits per year of freighters and tankers along the British Columbia coast. Of these, at least 1500 were tanker trips to or from Alaska; each year more than 350 loaded tankers entered the Strait of Juan de Fuca (Burger 1992).
Risk models for southern British Columbia and Washington State, developed in the 1980s, predicted the following oil spill frequencies: spills of crude oil or bunker fuel exceeding 159 000 litres (1 000 barrels) could be expected every 2.5 years, and spills of any type of petroleum product exceeding 159 000 litres (1 000 barrels) could be expected every 1.3 years (Cohen and Aylesworth 1990). The actual frequency of large spills affecting British Columbia between 1974 and 1991 was fairly close to the predicted frequency (Burger 1992). In addition to large spills, small chronic spills are also of concern. Environment Canada tracks all spills of more than 1113 litres (7 barrels). There are at least 15 such reportable spills annually along the west coast of Vancouver Island (Burger 1992). The effect of contamination from such small chronic spills on sea otter populations is not known.
The existing transport of oil along British Columbia’s coast poses a significant threat to the British Columbia sea otter population because of its small size and limited distribution. A recent development proposal to deliver crude oil by tanker from Kitimat, British Columbia to Asia Pacific and California markets (Enbridge Inc. 2005) increases the probability of a significant oil spill occurring in British Columbia. There are also proposals to allow drilling for oil and gas in Hecate Strait and Queen Charlotte Basin, which could also increase the threat of oil spills (British Columbia Ministry of Energy, Mines and Petroleum Resources).
In the spring of 1989, the oil tanker Exxon Valdez ran aground in Prince William Sound, Alaska, spilling 42 million litres of crude oil. Nearly 1000 sea otter carcasses were recovered, but estimates of total mortality ranged from 2650 (Garrott et al. 1993) to 3905 animals (DeGange et al. 1994). Subsequent studies of this spill illustrate the long-term impact of the oil. Population modelling showed decreased survival rates in all age-classes in the 9 years following the spill and indicated that the Prince William Sound sea otter population has not yet completely recovered (Monson et al. 2000b). As well, elevated levels of cytochrome P4501A, a biomarker for exposure to hydrocarbons, still occur in blood samples from otters in areas that were heavily oiled, suggesting continued exposure (Bodkin et al. 2002).
Controlled experiments on mink (Mustela vison) showed the effects of oil on reproduction in this mustelid. Female mink were fed low doses of crude and bunker C oil to simulate residue levels measured in invertebrates in Prince William Sound 4 years after the Exxon Valdez spill. These mink had significantly fewer kits/birth than controls. In addition, female kits born to exposed mothers had poorer survival to weaning, and those that survived had lower reproductive success than controls (Mazet et al. 2001).
Environmental Contamination - Other Contaminants
Organochlorine contaminant levels have not been measured in British Columbia sea otters. However, polychlorinated biphenyls (PCB), organochlorine pesticides including DDT, and butyltin have been measured in sea otters from California, Washington and Alaska (Bacon et al. 1999; Kannan et al. 2004; Lance et al. 2004). PCBs concentrations were higher in Alaskan otters from the Aleutian Islands (309μg/kg wet weight) compared to otters from California (185μg/kg wet weight) and southeast Alaska (8μg/kg wet weight) (Bacon et al. 1999). Total DDT concentrations were highest in California sea otters (850μg/kg wet weight), compared to the Aleutian Islands (40μg/kg wet weight) and southeast Alaska (1μg/kg wet weight). The levels of PCBs measured in California and Aleutian sea otters is considered to be of concern since similar levels caused reproductive failure in mink, a closely related species (Risebrough 1984 in Riedman and Estes 1990). Although the levels of DDT measured in California sea otters were not considered to be exceptionally high when compared to other marine mammals (Bacon et al. 1999), reduced immune competence is a well-documented side-effect of contaminants in marine mammals and is considered a possible factor in the high rate of disease-caused mortality in the southern sea otter population (Thomas and Cole 1996; Reeves 2002; Ross 2002). Among a small sample of beach-cast carcasses retrieved for contaminant analysis in California, those that died from infectious disease contained on average higher concentrations of butyltin compounds (components in antifouling paint), and DDTs than animals that had died from trauma and unknown causes (Kannan et al. 1998; Nakata et al. 1998).
Sea otters can limit abundance of their prey, and influence the distribution and size of their prey (Morris et al. 1979; 1981; Breen et al. 1982; Watson 1993; Watson and Smith 1996). In the presence of sea otters, invertebrates are unlikely to reach commercially harvestable densities or sizes. In fact, in British Columbia commercial fisheries for subtidal invertebrates such as geoducks (Panopea abrupta), sea urchins (Strongylocentrotus spp.), sea cucumbers (Parastichopus californianus) and possibly Dungeness crab (Cancer magister) were likely made possible by the extirpation of sea otters combined with new diving technology and growing international markets (Watson and Smith 1996). Sea otters also affect the abundance and size of intertidal clam species, and affect subsistence, commercial and recreational harvests. As the range of sea otters has expanded concerns about the sustainability of invertebrate resources on the part of commercial, First Nations, and recreational harvesters have intensified and led to requests to regulate or control otter population growth.
Entanglement in Fishing Gear and Collision with Vessels
Sea otters have become entangled and entrapped in fishing gear in Alaska, California, Washington and Japan (Rotterman and Simon-Jackson 1988; USFWS 2003; Lance et al. 2004; Hattori et al. 2005). Sea otters were reportedly entangled and killed in salmon fisheries particularly in central and western Alaska during the 1970s and 1980s and there was some concern in the late 1980s that the numbers might be significant and increasing (Rotterman and Simon-Jackson 1988). High levels of sea otter mortality from entanglement in sunken gill nets in <30 m depths was a serious problem in California during the late 1970s and early 1980s Restrictions on net fisheries within the sea otter range in California are now in place (USFWS 2003). In Washington State, the treaty gill net fishery is allowed to operate within the sea otter range. Entanglements and mortality are reported infrequently, but may increase as sea otters increase in abundance and range (Gerber and VanBlaricom 1998; Lance et al. 2004). Incidental mortality in trap fisheries for crab has been known to occur in California and Alaska, but no entrapments have thus far been reported in Washington State (Bodkin 2003; Lance et al. 2004).
The extent of accidental drowning of sea otters in fishing gear in British Columbia has not been investigated, although there appears to be limited geographic overlap between sea otters and net fisheries at this time except possibly in Queen Charlotte Strait. There is, however, considerable overlap between sea otters and the crab fishery and there are anecdotal reports of otters being drowned in commercial crab pots (J. Watson unpub.). As the sea otter range continues to expand, more overlap may be anticipated between sea otters and net and trap fisheries. The increase in shellfish aquaculture may result in some interactions (e.g. entanglement in gear), and this may be a future consideration.
Incidents of collisions with vessels have not been investigated in British Columbia, but are reported from other regions. Vessel strike was the primary cause of death of 5 of 105 beach-cast carcasses examined between 1998 and 2001 in California (Kreuder et al. 2003). Vessel strikes are also reported from Alaska (Rotterman and Simon-Jackson 1988). In British Columbia, the occurrence or frequency of vessel strikes has not been investigated, although one incident of probable vessel strike mortality is reported by Watson et al. (1997). Although the significance of vessel strikes as a source of mortality is unknown for the British Columbia sea otter population, such incidents may increase has sea otters expand into more areas that are near human habitation.
Illegal killing does occur in British Columbia and is reported to occur in other regions (Rotterman and Simon-Jackson 1988; Bodkin 2003). In other jurisdictions sea otters are shot both legally and illegally for their fur and in an effort reduce their effects on invertebrate stocks. There are no estimates of the magnitude of this source of mortality in British Columbia, but in 2005 and 2006 a total of at least 5 shot and skinned carcasses of sea otters were reported or recovered on Vancouver Island, which suggests that illegal killing may be an emerging threat (DFO unpubl.).
Sea otters in British Columbia are typically wary of humans, and rafts of sea otters are often difficult to approach and are easily disturbed by boat traffic. Females with pups are most sensitive to disturbance; however, rafts of males routinely exposed to boat traffic seem to be able to habituate and single males are most tolerant of the presence of boats. Overall the impact of humans inhabiting the shoreline (noise and presence of people) or from operating boats in the vicinity of sea otters does not appear to be a major concern at this time. Certainly, observation from California, e.g. in Monterey Bay, indicate sea otters appear capable of habituating to human activity (e.g. Woolfenden 1995).
Disease is a potential threat. In California, the high number of prime-age animal mortalities resulting from infection with Toxoplasma gondii and Sarcocystis neurona (pathogens thought to be terrestrial in origin) may be linked to sewage and agricultural runoff (Miller et al. 2003). These pathogens have been identified in the British Columbia sea otter population (Shrubsole et al. 2005; Raverty pers. comm. 2006).
Report a problem or mistake on this page
- Date modified: