Changes in diets of river otters in Prince William Sound, Alaska: effects of the Exxon Valdez oil spill

We studied the effects of the Exxon Valdez oil spill on the diets of river otters (Lutra canadensis) from oiled and nonoiled areas of Prince William Sound, Alaska, U.S.A., in 1989 and 1990. On the basis of identification of prey remains in their feces, otters fed principally on marine, bottom-dwelling fishes. Marine gastropods, bivalves, and crustaceans composed most of the invertebrates in the diet of otters; freshwater and terrestrial food items seldom occurred in their feces. The diets of otters included 149 different taxa, most of which rarely occurred in their feces. Sixty-five taxa occurred greater than or equal to 5 times in our combined data set. Species richness and diversity of prey remains in otter feces were similar on oiled and nonoiled study areas in late winter(April) 1989 (before the oil spill) and during summer (June-October) 1989 following the spill. By summer (July-September) 1990, however, there were significant declines in the richness and diversity of species (mostly bony fish, molluscs, and bivalves) in otter diets on the oiled area. Likewise, the relative abundance of prey remains in otter feces showed strong differences between areas and years, and an area by year interaction. Members of the Perciformes and Archaeogastropoda declined from 1989 to 1990 on the oiled area while they increased on the nonoiled site; Malacostraca exhibited the opposite pattern. These outcomes, when considered with other data on body mass and blood chemistry, strongly suggest that some effects of the oil spill on otters were delayed.