HASSRICK, J/L; CROCKER, D/E; ZENO, R/L; BLACKWELL, S/B; COSTA, D/P; LE BOEUF, B/J; Univ. of California, Santa Cruz; Sonoma State Univ.; Sonoma State Univ.; Greeneridge Sciences, Inc.; Univ. of California, Santa Cruz; Univ. of California, Santa Cruz; ; : Swimming speed and foraging strategies of northern elephant seals Mirounga angustirostris
The northern elephant seal, Mirounga angustirostris, is known as the deepest continually diving phocid seal. Workers have puzzled over how this species consistently exceeds calculated aerobic dive limits. In this study we examined swimming speed as a key variable to successful foraging strategies between adult males and females and adult and juvenile females. Foraging strategies were determined using a combination of speed, time, and depth data from datalogging equipment carried by the seals. Adults were also equipped with satellite transmitters for accurate location data. In addition, ultrasonic and morphological measurements were taken with mass to determine body composition upon departure and return from foraging migrations. Contrary to model predictions from previous investigations, seals did not reduce swimming speed with increased dive depth and retained bottom durations of dives irrespective of depth. The two � dimensional shapes of dives varied between sexes and age classes, but all individuals exhibited characteristic dives and their putative functions. Cycles of acceleration and deceleration were present in all study groups. Individual variation within classes and across dive types was significant. Sex differences in diving behavior and between young and adult females were significant. Mean swim speeds were similar across all classes despite significantly different predicted MCT calculations. Buoyancy appeared to affect two-dimensional dive shapes and seals� ability to utilize extended gliding behaviors between the surface and deep water foraging zones. Unknown behavioral and physiological adjustments appeared to allow juveniles and adults to defend time in the foraging zone irrespective of swimming speed on descent.
