Classification and behavior of free-ranging Weddell seal dives based on three-dimensional movements and video-recorded observations

Abstract

The goal of this study was to classify free-ranging (FR) dives of Weddell seals (Leptonychotes weddellii) and to compare them to isolated hole (IH) dives. Classification and comparisons were based on 58 descriptors for three-dimensional dive paths computed from data obtained by attaching video and data recorders to the backs of 12 adult Weddell seals that were free-ranging in eastern McMurdo Sound, Antarctica. We then inferred behavioral functions for the dive classes based on video-recorded observations. Three of the four dive types previously identified from seals diving at an IH occurred in FR seals. Although there were differences associated with location, Types 1, 2 and 3 dives clustered in a similar pattern in the discriminant analysis for FR and IH dives. Most prey (79%) captures occurred during Type 1 dives, and the primary (99%) prey was Antarctic silverfish (Pleuragramma antarcticum). Type 1 dives were the deepest (mean maximum depth 324–378m), longest in duration (15.0–27.0min), covered the greatest total distance (1470–2197m), and had the steepest dive angles (descents: −30°; ascents: +27°). Types 2 and 3 dives formed a continuum from short duration (3.6–7.5min), shallow (mean maximum depth 30–66m) dives that were close to the ice hole (farpoint distance 75–130m) and often involved aggressive interactions with other seals for breathing opportunities (Type 2) to progressively longer (7.9–17.2min), deeper (mean maximum depth 81–143m) dives that covered greater total distances (878–1194m) and were associated with transiting between holes, exploring and occasionally foraging (Type 3). Very long distance Type 4 exploratory dives that were identified in the IH study were completely absent in FR seals.