Tracking changes in relative body composition of southern elephant seals using swim speed data

Changes in buoyancy during an animal's time at sea are a powerful tool for inferring spatial and temporal foraging success. Buoyancy can be difficult to measure, but in some species of seal, drift components of dives can be used. We used swim speed data from adult female southern elephant seals Mirounga leonina using geo-locating velocity-time-depth recorders during 2004 post-lactation (PL; n = 7) and 2002, 2004 and 2005 post-moult (PM; n = 18) foraging trips to detect periods of passive drifting during diving. In addition to the characteristic drift dives of elephant seals, drifting also occurred during putative foraging dives. We used generalised linear models (GLMs) to examine the relationship between body lipid content measured on land and several diving variables collected within a week of these measurements being taken. The strongest support (deviance explained = 90%) was for the model including drift rate (77%), seal length (12%) and descent rate (2%). Estimates of body lipid, based on the GLM, were predicted for each day of the foraging trips. Areas where seals increased their relative lipid content from one day to the next corresponded well with areas in which the seals spent the greatest amount of time. Inferring foraging success from positive changes in drift rate has so far been limited to elephant seals which perform characteristic drift dives, but the addition of swim speed data to detect short periods of stationary behaviour allows for this method to be expanded to a greater range of ocean predators.