Sea otter (Enhydra lutris) herd size, distribution and foraging behavior have been monitored on a weekly basis within the vicinity of the Diablo Canyon Nuclear Power Plant since 1973. The 15-km study area includes the recognized southern front of the sea otter's range. The sex composition of the population at the front is predominantly male. Periodically moving southward, the herd has made 18 major rafting-site relocations over six years within the study area. The southern front herd exhibits a seasonal fluctuation in size, with the largest numbers of animals present in the winter and the lowest in the fall. This phenomenon may be related to breeding behavior. Abalones, sea urchins and crabs are the predominant food items consumed by otters within the study area. Feeding observations and supportive subtidal foraging evidence suggest sea otters have significantly contributed to the decline of large herbivorous invertebrates within the vicinity of the Diablo Canyon Nuclear Power Plant. Sea urchin densities have dropped to less than 1% of their original levels at subtidal study sites within the range of sea otter foraging activity.
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