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Sea otter foraging behavior and hydrocarbon levels in prey

January 1, 1994

Following the Exxon Valdez oil spill (EVOS), Prudhoe Bay crude oil from the vessel spread on the sea surface and covered coastal shores from western Prince William Sound (PWS) to the Alaska Peninsula. In PWS alone. acute mortality of sca otters at the time of the spill was estimated to be greater than 2000 (Doroff et al. 1993; Garrott et al. 1993).

Shoreline oiling was observed on approximately 24% of the 1891 km of coastline surveyed within PWS (Exxon Valdez Oil Spill Damage Assessment Geoprocessing Group 1991). The effect of oil on the abundance of nearshore marine invertebrate populations is unclear, and the concentration and persistence of hydrocarbons present in tissues of most of these invertebrate species still remains unknown. What is known is that marine bivalves can accumulate petroleum hydrocarbons from both chronic and acute sources (Blumer et al. 1970; Ehrhardt 1972; Boehun and Quinn 1977). Potential long-term chronic effects of oiled intertidal and subtidal prey on the sea otter population are of concern.

Sea otters prey on a wide variety of benthic marine invertebrates (Riedman and Estes 1990) and forage in shallow coastal waters (Wild and Arnes 1974), which vary widely in exposure to the open ocean, substrate type, and community composition. Sea otters have high metabolic demands relative to other marine mammals and can consume 20-25% of their body weight per day in invertebrate prey (Kenyon 1969: Costa and Kooyman 1984). Sca otters have occupied southwestern PWS since at least the early 1950s (Lensink 1962; Garshelis et al. 1986). The sea otter population in the PWS spill region was likely near equilibrium density and limited by prey availability before the oil spill (xcurrel (Estes et al. 1981; Garshelis et al. 1986; Johnson 1987). Sea otters in this region spent 59% of the daylight hours foraging, while otters in recently reoccupied habitats of eastern PWS spent only 27%. (Garshelis et al. 1986). Therefore, small differences in abundance of prey or net caloric availability due to heavy oiling in portions of southwestern PWS may have led to reduced carrying capacity and delayed recovery for the sea otter population in this region.

Recovery of the PWS sea otter population may be influenced by several factors. Decreased food availability caused by oil-related prey mortality or consumption of contaminated prey may be detrimental. Prey availability in western PWS may have declined due to increased mortality of invertebrates at the time of shoreline oiling. of by oil-removal activities. In addition. relative prey availability may have been decreased by sea otters avoiding invertebrate prey contaminated with petroleum hydrocarbons. However, we lack the baseline data on abundance and distribution of near shore invertebrates necessary to estimate a reduction in prey availability. In addition. the effects of ingesting prey contaminated with petroleum hydrocarbons on sea otters are unknown.

Our objectives were to determine if sea otter foraging success and prey composition differed between oiled and nonoiled areas and to assess hydrocarbon levels in sea otter prey between oiled and nonoiled areas.

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