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Recovery Strategy for the Sea Otter
The following are categories of human-caused mortality, or threats, to sea otters. Disease is included because of the apparent anthropogenic influences emerging in California. See Appendix II for definition of table headings and terms.
|1||Threat #1 (Environmental Contaminants - Oil Spill)||Threat Information|
|Threat Category||Accidental Mortality and Habitat Loss or Degradation||Extent||Localized|
|General Threat||Transport of oil and use of hydrocarbons to fuel vessel||Occurrence||Anticipated|
|Specific Threat||Oil spill||Causal Certainty||High|
|Stress||High mortality from hypothermia, inhalation of fumes or ingestion of oil from fur causing damage to internal organs. Reduced reproductive success; chronic contamination through exposure to contaminated sediment and prey||Level of Concern||High|
|2||Threat #2 (Environmental Contaminants - Persistent Bioaccumulating Toxins)||Threat Information|
|Threat Category||Pollution||Extent||Widespread or localized|
|General Threat||Deposition of industrial and agricultural chemicals in marine food webs||Occurrence||Current|
|Specific Threat||Bioaccumulating toxins||Causal Certainty||Low|
|Stress||Reduced reproductive success, reproductive impairment, reduced immune competence, mortality||Level of Concern||Low|
|3||Threat #3 (Disease and Parasites)||Threat Information|
|Threat Category||Accidental Mortality, Changes to Ecological Dynamics, Pollution||Extent||Localized and Widespread|
|General Threat||Introduction of diseases and parasites||Occurrence||Anticipated|
|Specific Threat||Exposure to novel disease||Causal Certainty||High|
|Stress||High mortality, loss of reproductive potential||Level of Concern||Low|
|4||Threat #4 Entanglement in fishing gear||Threat Information|
|Threat Category||Accidental Mortality||Extent||Localized|
|General Threat||Entanglement or entrapment||Occurrence||Anticipated|
|Specific Threat||Entanglement/entrapment in fishing or aquaculture gear||Causal Certainty||High|
|Stress||Increased mortality (drowning)||Level of Concern||Low|
|5||Threat #5 Collisions with vessels)||Threat Information|
|Threat Category||Accidental Mortality (or Injury)||Extent||Localized|
|General Threat||Vessel traffic||Occurrence||Anticipated|
|Specific Threat||Collisions with vessels||Causal Certainty||High|
|Stress||High mortality, loss of reproductive potential||Level of Concern||Low|
|6||Threat #6 Illegal kill||Threat Information|
|General Threat||Shooting, trapping||Occurrence||Current|
|Specific Threat||Illegal kill||Causal Certainty||Moderate|
|Stress||High mortality||Level of Concern||Low-Moderate|
|7||Threat #7 Human Disturbance||Threat Information|
|Threat Category||Disturbance and Persecution||Extent||Localized|
|General Threat||Human activities on the water, vessel traffic, sea otter viewing||Occurrence||Current|
|Specific Threat||Behavioural disruption||Causal Certainty||Low|
|Stress||Reduced reproductive success (possible displacement from preferred habitat)||Level of Concern||Low|
1.5.2 Description of threats
Oil contamination has both immediate and long-term effects on sea otters and the recovery of their populations. The following five points summarize sea otter vulnerability to oil contamination.
- Sea otters depend upon the integrity of their fur for insulation. Oil destroys the water-repellent nature of the fur. As it penetrates the pelage, it eliminates the air layer and reduces insulation by 70% (Williams et al. 1988). This usually results in hypothermia.
- Once the fur is fouled, sea otters ingest oil as they groom themselves. Ingested oil damages internal organs, which in turn has chronic and acute effects on sea otter health and survival.
- Sea otters are nearshore animals with strong site fidelity, and will remain in or return to oiled areas, additionally, they often rest in kelp beds, which collect and retain oil.
- Sea otters are found in single sex aggregations, which can include 100 or more animals. Thus, large numbers of sea otters, representing a substantial portion of the reproductive potential of a population, can become simultaneously fouled by oil. The loss of a raft of male otters may have less reproductive impact than the loss of a raft of female otters because of the species’ polygynous mating system.
- Sea otters feed on benthic invertebrates, which can accumulate and store toxic hydrocarbons during, and after, an oil spill.
The status of the sea otter population in Prince William Sound illustrates both short-term and long-term impacts of oil contamination. In the spring of 1989, the oil tanker Exxon Valdez ran aground in Prince William Sound, spilling 42 million litres of crude oil. Nearly 1000 sea otter carcasses were recovered within six months, but total mortality estimates ranged from 2,650 (Garrott et al. 1993) to 3,905 (DeGange et al. 1994). Presently, sea otters in parts of the Sound that were most heavily oiled continue to have significantly higher levels of cytochrome P4501A, a biomarker for hydrocarbons, than otters in less heavily oiled areas. This suggests continued exposure to residual oil in prey and habitat. Population growth is significantly lower in the heavily oiled area, as well, and it is thought that recovery is constrained by residual oil effects, despite an adequate food supply, and by emigration (Bodkin et al. 2002). Population modelling using data from 1976 to 1998 shows that sea otters in Prince William Sound had decreased survival rates in all age-classes in the nine years following the spill. The effects of the spill on survival appear to have dissipated mostly as those animals alive at the time of the spill have died (Monson et al. 2000b), but the Prince William Sound sea otter population has not yet fully recovered to pre-spill levels.
The risk of oil spills in BC has been of considerable concern for sometime, particularly since the Nestucca oil spill, December 22, 1988 (Waldichuk 1989), and the Exxon Valdez spill that occurred less than six months later (Loughlin 1994). The Nestucca spill released 875,000 litres of Bunker C oil off Grays Harbour, Washington. The current, combined with onshore winds, carried the oil slick northward fouling the shoreline of western Washington and the west coast of Vancouver Island. Weathered oil reached as far as the Goose Islands Group on the central coast of BC (Watson 1989). Sea otter surveys made soon after the spill found one oiled sea otter carcass on an offshore islet in Checleset Bay and wolf scats containing oiled sea otter fur on Vancouver Island in the affected area. While there is little doubt sea otters did die from oil contamination, the exact number could not be established because wolves and bears quickly scavenge beach-cast carcasses. Boat-based surveys made the following summer found no detectable effect on the population (Watson 1989), although variation among sea otter counts can be quite high, making trends often difficult to ascertain. Although the impact of the spill appears to have been minimal, the event, nonetheless, demonstrated the vulnerability of the sea otter population to oil contamination.
Sources of oil spill threats in the marine waters around BC include cargoes of tankers and barges, bilges, fuel tanks of marine vessels, shore-based fuelling stations and even shore-based industries such as pulp mills (Shaffer et al.1990). In the early 1990s, more than 7000 transits were made annually by freighters and tankers in Pacific Canadian waters, including at least 1500 tanker trips to or from Alaska, and more than 350 loaded tankers entered the Strait of Juan de Fuca (Burger 1992). The greatest volume of petroleum and risk comes from shipments of crude oil and refined petroleum products. Based on data from 1988 and 1989, over 26 million cubic metres of crude oil were transported annually into and out of the Strait of Juan de Fuca, mostly carried by tankers, and an additional 15 million cubic metres of refined petroleum products, carried mostly by barges (Shaffer et al.1990). About 15% of these loads were delivered to coastal depots along the west coast of Vancouver Island (Shaffer et al.1990).
It is unlikely that the volume of petroleum transported has declined since the late 1980s, in fact it is more likely to have increased with the growing human population (Schaffer et al. 1990). Risk models developed at that time predicted the following oil spill frequencies for the marine waters of southern BC and northern Washington:
- spills of crude oil or bunker fuel exceeding 159,000 litres (1000 barrels) could be expected every 2.5 years;
- spills of any type of petroleum product exceeding 159,000 litres (1000 barrels) could be expected every 1.3 years (Cohen and Aylesworth 1990).
The actual frequency of large spills affecting BC between 1974 and 1991 was fairly close to the predicted frequency (see table in Burger 1992). In addition to spills of at least 159,000 litres, there are numerous smaller spills. Spills over 1,113 litres (7 barrels) are considered significant by Environment Canada and are tracked. Along the west coast of Vancouver Island, there are at least 15 reportable spills of more than 1,113 litres (7 barrels) annually (Burger 1992). A recent development proposal to deliver crude oil by tanker from Kitimat, BC, to Asia Pacific and California markets (Enbridge Inc. 2005) and proposals to allow drilling for oil and gas in Hecate Strait and Queen Charlotte Basin (BC Ministry of Energy Mines and Petroleum Resources) pose additional risks and could alter the above predictions about the size and frequency of spill events.
Environmental Contaminants – Persistent Bioaccumulating Toxins
Organochlorine contaminant levels have not been measured in Canadian sea otters. 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), likely reflecting the greater degree of agricultural activity in California than in Alaska. The levels of PCBs measured in California and Aleutian sea otters is considered to be of concern, since similar levels cause 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).
Disease and Parasites
In general, disease is not thought to be a major cause of mortality among most sea otter populations (Riedman and Estes 1990). The southern sea otter population has a much lower rate of growth than other populations and a higher rate of mortality, of which 40% is disease-caused (Thomas and Cole 1996). This is true even during periods of population increase (Estes et al. 2003). Although high rates of disease-caused mortality have been noted in the southern sea otter population for several decades, of recent concern is the emergence of infections arising from parasites for which sea otters are thought not to be the normal host. In addition, diseases seem to be affecting high numbers of prime age animals (Thomas and Cole 1996; Estes et al. 2003). A large number of recent mortalities have been the result of protozoal encephalitis caused by Toxoplasma gondii. Cats and other felids are the terrestrial parasite’s definitive host. Runoff from urban and agricultural areas into streams and rivers may be linked to the transport of the parasite to coastal marine waters. (Miller et al. 2002;Lafferty and Gerber 2002). Sarcocystis neurona, a disease thought also to be terrestrial in origin and typically associated with opossums, is causing mortality among southern sea otters as well (Kreuder et al. 2003). Peritonitis induced by acanthocephalan parasites has increased in recent years (Thomas and Cole 1996). The observed prevalence of disease and variety of diseases are of concern, and it is speculated that decreased immune function may be a factor. Reduced immune competence could result from environmental toxins, genetic factors, or habitat degradation leading to nutritional stress (Thomas and Cole 1996; Reeves 2002).
Exposure to a variety of diseases has been documented in sea otters in Alaska, Washington, and BC (Thomas and Cole 1996; Reeves 2002; Lance et al. 2004; Gill et al. 2005; Shrubsole et al. 2005). 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 six animals examined died from dual infection with T. gondii and S. neurona. In 2002, one of eight animals examined died from infection with S. neurona and six died from infection with Leptospirosis. In 2004, two of three animals examined had died from infection with S. neurona. One animal died from Canine Distemper Virus (CDV), a member of the genus Morbillivirus. This was 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 BC, 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 was examined and found to have died from infection with S. neurona (Raverty pers. comm. 2006). Among 42 animals live-captured on the BC coast in 2003 and 2004, eight were seropositive for morbilliviruses and two tested positive for T. gondii (Shrubsole et al. 2005). CDV has recently been detected in river otters living in the marine environment in BC. Transmission is thought to occur via terrestrial hosts (Mos et al. 2002). The disease can cause mortality in populations that have not previously been exposed. Persistent organic pollutants that suppress immune function appear to exacerbate morbillivirus-related outbreaks in other marine mammals (Ross 2002).
Entanglement in fishing gear and collisions with vessels
Mortality from entanglement in fishing gear can have a substantial impact to a population, particularly where prime age animals are killed. Incidental drowning in sunken gill nets was a significant cause of mortality in California during the late 1970s and early 1980s and contributed to a population decline (UFWS 2003). As a result, restrictions in the use of gill and trammel nets in waters less than 65 metres were implemented (Riedman and Estes 1990) and the population decline reversed. Increased mortality in fishing gear is again under consideration, along with disease, as a cause of the current decline in southern sea otters (USFWS 2003).
Incidental entanglements in fishing gear have been reported in Alaska (USFWS 1994) and Washington. There have been accidental takes in the Makah tribal set-net fishery for salmon (Gearin et al. 1996; Gerber and VanBlaricom 1998). The extent of accidental drowning of sea otters in fishing gear in coastal BC is unknown, but not thought to be significant at this time. However, as the sea otter population expands into areas of gill-net fisheries, there may be local effects and entanglement may emerge as a threat of concern in the future (Watson et al. 1997). Sea otters die from drowning in various crab and fish trap fisheries in California and Alaska (reviewed in Lance et al. 2004). Crab traps may present a threat to sea otters, particularly since they are set in shallow waters within the species’ diving depth range.
Collisions with vessels are not well documented. In BC, one sea otter carcass recovered from Kyuquot Sound had injuries that could have been caused by a boat propeller, but the occurrence of collisions is probably minor and localized at this time (Watson et al. 1997).
Illegal killand Human Disturbance
There are few verified reports of illegal killing of sea otters in BC, although it has long been suspected based on unconfirmed reports. Four skinned carcasses were reported and verified in 2006 and one shot carcass in 2004 (DFO unpubl.). The extent of illegal killing and its impact is unknown.
The extent of disturbance of resting and foraging otters from boat traffic is largely unknown, but unlikely to be significant at this time. Disturbance may become a more significant local effect in the future as the sea otter population expands its range into more human-populated areas, and public awareness and interest in watching the BC sea otter population grows.
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