A re‐evaluation of the role of killer whales Orcinus orca in a population decline of sea otters Enhydra lutris in the Aleutian Islands and a review of alternative hypotheses

Abstract

ABSTRACT During the past 15–20 years, sea otters Enhydra lutris in the Aleutian Islands, Alaska, USA, experienced a drastic decrease in population size. It has been hypothesized that an increase in killer whale Orcinus orca predation was the primary cause of this decline. Causation of the decline by increased killer whale predation is now considered a textbook case of top‐down predator control. The purpose of this review is to re‐evaluate the evidence for killer whale predation and to review evidence for alternative causes. The killer whale predation hypothesis is based on three lines of evidence: (i) there was an increase in the number of observed killer whale attacks on sea otters during the 1990s, coincident with a decline in sea otters, (ii) sea otter populations did not decline in areas considered inaccessible to killer whales, while they declined in adjacent areas considered accessible to killer whales, and (iii) the estimated number of attacks necessary to account for the rate of decline is similar to the observed number of attacks. Our re‐evaluation indicates that although the killer whale hypothesis is by no means disproved, the supporting data are limited and inconclusive. Increases in shark populations in the Aleutian Islands concurrent with the sea otter population declines indicate the need for further research into the role of alternative marine predators in the population decline. High contaminant levels observed in sea otters in the Aleutian Islands warrant further investigation into the impact of these toxins on sea otter health and vital rates, and their possible role on the population decline. Disease has not been ruled out as a significant contributor to the population decline, particularly in the early stages of the decline.