Identification of foraging dives in free-ranging Weddell seals Leptonychotes weddellii: confirmation using video records
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Understanding foraging behavior is important for addressing ecological questions about air-breathing marine vertebrates. However, the inability to directly observe them underwater has made it difficult to classify and understand their foraging behaviors. We equipped 8 free-ranging, adult Weddell seals with animal-borne video and data recorders (VDRs) to monitor their underwater behavior. Eighteen dive descriptors summarizing the duration, depth, speed, stroking frequency, gliding, and energetic cost of 234 dives were calculated. Dive descriptors were included in non-hierarchical cluster analyses that identified 5 groups of dives. Eight of the 18 dive descriptors contributed strongly to the discrimination between dive groups. Presence of prey on the video record confirmed Groups 1, 4, and 5 as foraging dives. Group 1 dives were deep and exceeded the estimated aerobic dive limit, while Group 4 dives were also deep but probably remained aerobic. Group 5 dives were shallow and aerobic. Comparisons with prior classifications showed that deep aerobic dives were similar in depth and duration to foraging dives identified in previous studies, but shallow aerobic dives represented a previously undescribed foraging category. Deep anaerobic dives also differed from previous classifications and were not indicative of benthic foraging, as was previously thought. The proportion of successful foraging dives (at least on prey encounter observed on video) varied among the groups and among locations with different bathymetry. The distribution of foraging dive types also varied among these locations. These results will help characterize foraging activity of Weddell seals and provide a better understanding of the energetic costs and foraging strategies of diving marine predators. Inter-Research 2008.
