Allan D. Ligon and Robin W. Baird
In Abstracts of the 14th Biennial Conference on the Biology of Marine Mammals, Vancouver, Canada, December 2001.
Cetacean dive depths are sometimes inferred from knowledge of the habits of their prey. For false killer whales an early published report suggested they might dive to 500 m, based upon general feeding habits. However, dive depths have not previously been documented for this species. In Hawaiian waters, this species has been observed feeding on fish such as mahimahi and yellowfin tuna, which spend most of their time in surface waters. We hypothesize that such observations are biased towards surface-oriented prey, and false killer whales are also likely feeding at depth. During 1999 and 2001, we deployed three suction-cup attached time-depth recorder/VHF radio tags on this species around Maui and Lana'i, Hawaii, obtaining just over 16 hours of depth/velocity data. Dives were relatively shallow (maximum 22, 52 and 53 m, average from 8 - 12 m) and were not limited by bottom depth. Regressions of dive depth versus duration for all whales were positive, however the r-squared values were relatively low (0.07 - 0.55), suggesting that dive shape is highly variable (and dive duration cannot be used as a predictor of dive depth). A comparison of day and night dive parameters (depth, duration, velocity) for one whale showed some differences, however dive parameters suggested foraging both during the day and at night. In general, the dive depths of the three false killer whales were shallow relative to depths documented for several smaller species of odontocetes, suggesting that diving behaviour is likely driven more by ecology than body size. Given the relatively shallow depth profile of the whales our hypothesis was not supported. Although a larger sample size would be preferred, our results suggest that surface-based observations of predation may be relatively unbiased for this species in near-shore Hawaiian waters.
Link to:Stacey, P.J., and R.W. Baird. 1991. Status of false killer whales, Pseudorca crassidens, in Canada. Canadian Field-Naturalist 105:189-197.
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