Structure of foraging dives by Weddell seals at an offshore isolated hole in the Antarctic fast-ice environment

Knowledge of the biology and ecology of diving animals has been constrained by the inability to make direct observations of their underwater behaviour. We deployed an animal-borne video and data recorder on 10 adult Weddell seals (Leptonychotes weddellii) near Ross Island, McMurdo Sound, Antarctica, during the austral spring, to examine the structure of foraging dives. Three-dimensional dive profiles for 109 foraging dives were reconstructed, and nine behavioural states were identified based on geometry of the travel path and changes in speed and stroking. Five kinds of prey-related events were identified using video imagery. Most foraging was in midwater; rarer benthic foraging dives were especially distinctive. Successful and unsuccessful midwater foraging dives did not differ in energetic cost, but benthic foraging dives required significantly more energy than midwater dives and approached the seal's aerobic dive limit. We described the dominant sequence of behaviours in successful midwater, unsuccessful midwater, and unsuccessful benthic foraging dives. We also examined the dominant sequence leading to the first prey encounter in a dive and found that it usually followed an ascent, which in turn followed descent and/or horizontal swimming. We confirmed that discontinuous trajectories in time–depth profiles occur during midwater prey encounter. There were two kinds of descent behaviour (descent and meandering descent), both with reduced stroking and a large amount of gliding, which appeared to be when searching occurred. There is also some indication that increased amounts of gliding during descent behaviours could increase the seal's ability to capture prey by reducing the amount of self-generated noise. These findings provide a baseline for comparison with entirely free-ranging seals and for more detailed study of foraging strategies of diving seals.