Environmental and individual effects on the foraging success of an apex predator, the northern elephant seal (Mirounga angustirostris)

The use of marine organisms as platforms for ocean sensing is beneficial to the biologists interested in their behavior as well as oceanographers. However, sensor calibration, performance over time and the effects of animal behavior and sampling frequency on data quality must be examined before the data can really be useful. I examined the performance of Wildlife Computers Mk9 time-depth recorders (TDRs), with a rapid response external thermistor, in controlled lab conditions and in the field. I found a small correction of 0.05°C brought 83% of TDRs calibrated within +/-0.1°C, the manufacturer specified accuracy. Performance of the temperature sensor was stable through time and I did not detect any effects of instrument performance as a result of being attached to a seal rather than a CTD cast.;I used validated temperature measurements from Mk9s to address questions about the sub-surface thermal structure or regions of the north Pacific important to adult female northern elephant seals (Mirounga angustirostris). Satellite telemeters and TDRs were deployed on 75 adult females between May 2004 and May 2006. The area restricted search zones (ARS) of the females, as derived by the fractal landscape method, were concentrated in the subarctic frontal zone on their post molt foraging trip and in the subarctic domain/California current on their post breeding trip. Logistic regression revealed that sub-surface thermal structure was significantly different within ARS versus other areas outside ARS. Further, mixed models revealed temperature change within the thermocline as the model with the most efficient fit (lowest AIC score) relative to foraging success.;Finally, I examined the satellite tracks of 15 females instrumented over multiple seasons or years. I predicted that females successful on their first trip should show higher at-sea fidelity on their second trip than unsuccessful females. Although there was a high degree of fidelity to at-sea locations, there was no relationship between fidelity and foraging success, and little spatial overlap in ARS. Examination of the sub-surface thermal structure of ARS may reveal that females are utilizing the same hydrographic features between seasons or years, but because of the dynamic nature of the marine environment, this does not necessarily correlate to using the same geospatial locations.