Foraging dive patterns of humpback whales, Megaptera novaeangliae, in southeast Alaska: a cost–benefit analysis

Abstract

The foraging behavior of humpback whales (Megaptera novaeangliae) summering in southeast Alaska was studied from 1979 to 1984 using surface observations, a 120-kHz echo sounder, and remotely operated underwater cameras. Considerations of foraging costs as well as profits appear to play a dominant role in determining feeding dive patterns of the whales. The energy costs of diving, based on observed blow rates, estimated tidal volume, and O2 extraction efficiency, have previously been shown to increase with increasing depth of dive during foraging episodes. Time spent diving into and returning from a prey patch, and time spent at the surface recovering from a dive represent the time costs of foraging. The percentage of time available for actual capture of prey decreased with increasing depth of dive. Thus, time and energy costs were highly correlated with the depth of the target prey patch. In conjunction with acoustic scans, the underwater cameras made possible the accurate determination of potential prey patch depths, densities, and vertical extent. Prey patches were ranked using a derived weighted density value and associated time and energy expenditure required of the whales to feed upon the patch (viz., gain–cost representation). Foraging models incorporating prey patch density value and time and energy costs of diving resulted in the best fit when compared with observed foraging patterns of the humpback whales.