ABSTRACT: Abundance of pilot whales at Santa Catalina Island, California, USA, declined from several hundred to between 0 and 33 per winter following the 1982-83 El Niño-Southern Oscillation. Starting in 1987, abundance of Risso's dolphins, rarely seen before that time, increased. During a 9-winter-long study at Santa Catalina Island, parallels between the behavior and distribution of these 2 medium-sized, squid-eating delphinids were documented. One possibly aggressive encounter between pilot whales and Risso's dolphins was observed and others were reported. These factors led to the hypothesis that, while the ranges of these 2 species overlap, competitive displacement will generally prevent co-occurrence of these species in restricted geographic areas with limited food resources such as seen at Santa Catalina Island.

KEY WORDS: Pilot whale . Risso's dolphin . Competitive displacement . El Niño

Published in MEPS Vol. 123 (1995) on July 20
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1995