Sea Otters Current Threats and Conservation
Introduction
Sea otters, Enhydra lutris, are rather large members of the Mustelidae family (Morrison et al. 218). They range in size from 48 to 58 inches in length and from 44 to 100 pounds in weight (Sea World). These diurnal animals are noted for their dense, water-resistant fur (Morrison et al. 219), their use of tools (218), and their strong swimming abilities (Lubina 539).
Historically, the habitat range of the sea otter extended from Japan to the Pacific coast of Mexico (Doroff et al. 55). They typically prefer isolated, rocky subtidal kelp-forest habitats (Lubina 534). Though sea otters generally live in cold water regions, their range extends as far south as Baja California (Morrison et al. 225, 228). There are actually three subspecies of sea otters which are separated geographically Enhydra lutris lutris (Russian or Asian sea otter), Enhydra lutris kenyoni (Alaskan sea otter), and Enhydra lutris nereis (southern or California sea otter) (Sea World).
Sea otters are carnivorous animals known for their voracious appetite and substantial maintenance requirement (Morrison et al. 225). In fact, sea otters eat approximately 25 of their body weight each day (Miller et al. 1005). The primary food source for sea otters consists of benthic invertebrates such as crabs, sea urchins, abalones, clams, mussels, and snails found in subtidal zones (Lubina 532). Sea otters forage for their food in water that is no more than 30m deep. As such, much of their population is concentrated near coastlines (532).
Under the Endangered Species Act, the California sea otter is listed as threatened, meaning that there is a high risk of extinction (Sea World). The animals precarious existence largely arose because of commercial hunting for the extremely valuable furs (Morrison et al. 218). In 1911, the year that commercial hunting was outlawed, the California sea otter was thought to be extinct (Lubina 529), and the once abundant populations in the westernmost Aleutian Islands had been exterminated (Estes and Duggins 78). Fortunately, smaller colonies still existed, and populations gradually began to rebuild through a combination of reproduction and migration. In 2000, the minimum population estimate for the Aleutian archipelago was 8,742 (Doroff et al, 55). Estimates of the California sea otter population are around 2,300 (Miller et al, 1003). It should be noted that it is extremely difficult to estimate populations especially in northern sea otters. For these populations in vast and remote regions, aerial surveys, though problematic, provide the only practical means of counting the animals (Doroff et al. 61). California sea otters, on the other hand, are relatively easy to count from the shore (Rall et al. 1529).
Despite recovery efforts including the establishment of new colonies, population growth, has not been steady throughout the decades. Since the end of commercial hunting, otters have faced, and will continue to face, a large number of threats. Consideration of each of these threats fishing, surf conditions, predation, infection, and oil pollution illustrates the vulnerability of this species. As this species is considered a keystone predator, its success is vital to the stability of its ecosystem.
Discussion
Fishing Practices
In the 1970s, the recovery of the California sea otter population suffered a setback due to the incidental take of sea otters in gill and trammel nets (Rall et al. 1530). Regulations on net fishing helped restrict the number of sea otter deaths (1531). Though sea otters do not currently face huge problems from commercial fishing practices, that these practices occurred historically should always be kept in mind when regulatory practices are updated.
Surf Conditions
Alaskan sea otters face problems due to heavy surf conditions, a significant source of mortality. Females with pups will generally travel to sheltered inlets to protect themselves and their offspring. These conditions may also be affecting California otters, with juveniles having a more difficult time foraging for food (Lubina 54).
Predation
Sea otter populations are currently declining due to the activities of two predators, killer whales and sharks, with the former affecting Alaskan populations and the latter affecting California populations. In a study on the sea otter population in the Aleutian archipelago, Doroff et al. found that the population increased in the 1980s only to abruptly decline in the 1990s. Because of the uniformly low population density, Doroff et al. hypothesized that the decrease is due to increased predation (55). Killer whales appeared to be the likely suspect as sea otter numbers in Clam Lagoon, an area inaccessible to killer whales, remained stable while numbers in Kuluk Bay, an area inhabited by killer whales, declined by 76 (Estes et al. 1998). This was a rather surprising finding as sea otters are not killer whales typical food however, the whales might have had to change their diets after numbers of their preferred prey, harbor seals and Steller sea lions, declined (Doroff et al. 63).
Predation by killer whales has been visually established, with the first attack being witnessed in 1991. Since this time, nine more attacks have been seen. It is estimated that approximately 6,788 sea otters die from killer whale predation in the Aleutian Islands each year (Estes et al. 1998).
Southern sea otters are susceptible to attack by sharks. Kreuder et al. found that 13.3 of southern sea otter deaths were caused by shark attacks (500). For sharks, however, sea otters are not a food source. The wounds found on otters were typically large gashes similar to those seen when sharks attack humans and not consistent with bite patterns reserved for intended prey (504).
Infection
Infectious diseases pose an enormous threat to sea otter populations. Kreuder et al. found that a high percentage of mortality in southern sea otters from 1992-1995 was due to infectious disease (495-496). Southern sea otters appear to be particularly affected by Toxoplasma gondii. Miller et al. found that 42 of live otters and 62 of dead otters had T. gondii in their blood (1000). In this study, frequency of infection was found to be positively associated with male gender, increasing age, dead versus live status, and proximity to freshwater flow at the time of sampling (1001). It was also found that rates of infection were particularly high near the towns of Morrow Bay and Cayucas, California (1001). The researchers believe that the sea otters primary food source, benthic invertebrates, contribute to the high-rate of infection as these invertebrates are filter-feeding (1005).
While Miller et al did not examine mortality rates from T. gondii, this aspect was examined by Kreuder et al. The researchers found that T. gondii infections can lead to death from encephalitis in sea otters and that this is one of the leading causes of death in the southern sea otter (Kreuder et al. 499). This infection also makes sea otters more susceptible to death from cardiac disease and shark attack (502, 504).
Oil Pollution
According to Rall et al., the most serious threat the southern sea otter population faces is a major oil spill (1528). Oil pollution, such as that arising from the Exxon Valdez Oil Spill, poses both immediate and long-term consequences to all subspecies of sea otters. Short term consequences include harm to the otters fur. This fur is essential to the maintenance of body temperature (1530), and otters spend a great deal of time grooming their fur.
With regards to the effect of oil spills on sea otters, the Exxon Valdez case is instructive. Immediately following the Exxon Valdez Oil Spill in 1989, between 1000 and 2800 sea otters died in the Prince William Sound, Alaska area (Peterson et al. 2082). The otters primary food source, benthic invertebrates, also suffered mass mortality from toxicity, smothering, and physical displacement from the habitat due to pressurized wash-water (2082).
Despite intensive clean-up efforts, much oil remained in the ecosystem, and this oil affected primarily those areas populated by sea otters. Oil settled in the intertidal zone with gravelly shores as well as under mussel beds (Peterson et al. 2082). From these locations, oil has had a long-lasting entry point into many vital food chains (2083). Indeed, sea otters have been found to suffer from chronic exposure to oil both from sedimentary concentration as well as from food sources, and this chronic exposure has increased mortality rates for a number of years (2083). This increased mortality rate has significantly affected population growth rates in areas strongly affected by the oil spill. For example, at Knight Island, which was heavily oiled during the spill, the sea otter population had not begun to increase as late as 2000 while at Montague Island, which was not subjected to any of the spilled oil, the population doubled in the period from 1995 to 1998 (2083).
One explanation for this lack of increase in population is the otters food source. The clams and mussels they eat have chronically elevated tissue contamination because they very slowly metabolize hydrocarbons (Peterson et al, 2083). When the otters eat these animals, they are ingesting concentrated amounts of hydrocarbons. This hypothesis seems to be supported by the finding of significantly higher levels of cytochrome P4501A, a biomarker of hydrocarbons, in Knight Island sea otters (Bodkin et al. 237).
Ecological Implications of the Presence Absence of Sea Otters
Sea otters are essential to the Gulf of Alaska kelp ecosystem because they help control sea urchin populations. This, in turn, helps prevent sea urchins from overgrazing on the kelp and macroalgae that provides structural habitats for fish and invertebrates (Peterson et al. 2085). The sea otters role in this ecosystem has been studied by Estes et al (1998). In this study, the researchers found that, as a result of sea otter population decline, sea urchin size and density increased to produce an eight-fold increase in biomass, while kelp density declined by more than a factor of 12 (Estes et al, 1998). This constitutes an enormous change in the ecosystem.
Because sea otters predation on sea urchins has such a large effect on the ecosystem, these animals have been labeled keystone predators (Mills et al. 220). Otters, in fact, control the density of numerous other species through their eating habits. This ecosystem is, therefore, controlled from top-down forces, with carnivores limiting herbivores, and herbivores limiting plants (Estes and Duggin 98).
As killer whales are now feeding on sea otters, their role in this food web must be considered. Post World War II industrial whaling had greatly reduced the number of whales. In 1976, commercial whaling was outlawed, and numbers of whales began to increase again. However, harbor seal and Steller sea lion populations the primary food choices for killer whales began to decline in the late 1960s and 1970s (Estes et al. 2009, 1649). As whales turn more and more to sea otters for nourishment, the ecosystem will likely become more and more misbalanced.
SummaryConclusion
Sea otters have historically faced high threat levels from commercial hunting and fishing practices. Placement on the endangered species has drawn attention to the plight of the sea otter, and considerable progress has been made in allowing the populations to recovers. However, numerous threats still exist. While some, such as surf conditions, are not able to be controlled, others result from human activities and can be regulated. Infection from T. gondii comes from freshwater run-off, often from inadequately treated wastewater containing domestic cat feces (Miller et al. 998). As oil spills pose the most serious threat to sea otter populations, everything possible must be done to ensure that oil does not leak into the ecosystem. With regards to predation by killer whales and sharks, much more research must be done to determine the causes of these events. What has caused the decline in the population of killer whales food Why are sharks attacking sea otters and not eating them
More work also needs to be done to establish populations. As part of the initial recovery efforts, a new colony of sea otters was established by scientists to minimize the risk that the entire population would be wiped out (Rall et al. 1530). This colony at San Nicolas Island proved to be largely unsuccessful as the majority of the 139 transplanted sea otters returned to their original location, with only seventeen remaining in the new colony (1531). Research into the reasons behind this migration might allow the establishment of other colonies in more suitable environments.
Because the success of the entire kelp ecosystem is dependent upon the success of sea otter populations, much more research and policy initiatives need to be undertaken in this area.
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