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A re-evaluation of the role of killer whale (Orcinus orca) predation in the decline of sea otters (Enhydra lutris) in the Aleutian Islands

University of British Columbia

Sea otters in the Western Aleutian Islands have experienced a drastic population decline since the 1990s and Estes et al. (1998) hypothesized that killer whale predation is responsible. This hypothesis has not been challenged nor tested empirically. The aim of this study was twofold: 1) to conduct a literature review of the evidence that killer whales caused the sea otter populations to decline, re-examining alternative explanations, and 2) to empirically test the "killer whale prédation hypothesis" by comparing sea otter behavioural responses to artificial killer whale cues (playbacks of killer whale vocalizations and blows) in the Aleutian Islands to the response of sea otters to the same cues in British Columbia, where they do not suffer intensive killer whale prédation. The literature review revealed that the existing data are inconclusive and further research into other possible causes is needed. For example, high contaminant levels observed in sea otters coupled with intensive military occupation in the Aleutian Islands warrants further investigation into the role that toxins have played in the health of otters. Increases in shark populations in the Aleutian Islands concomitant with the sea otter population declines also calls for further research into alternative marine predators. The empirical test revealed that sea otters in the Aleutian Islands responded to both treatment and control playbacks of killer whale vocalizations while the sea otters in British Columbia did not. This suggests that there is some form of environmental difference between the two sites, but, because sea otters did not respond more strongly to the sounds of killer whales than to controls, it does not support the notion that killer whale prédation caused the discrepancy. This is the first experimental evidence that killer whales may not have preyed on sea otters to the extent suggested by Estes and colleagues, if at all. Together, the results from the review and the playback experiments indicate that the situation is more complex than previously acknowledged and fiirther research into alternative mechanisms driving the sea otter population down is required to ensure proper conservation and management of the species.

Thesis/Dissertation

application/pdf

2009-03-06

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/5637

Zoology

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/