VARIABILITY IN ANTARCTIC FUR SEAL DIVE DATA: IMPLICATIONS FOR TDR STUDIES

We analyzed 19 dive records obtained from female antarctic fur seals ( Arctocephalas gazella ) during three austral summer breeding seasons on Seal Island, Antarctica, to assess the extent of individual variation and the potential for using time-depth recorder (TDR) statistics to detect annual changes in six measures of foraging behavior. We report the mean values and typical variability among individuals for dive duration, dive depth, proportion of time submerged, transit time, vertical distance dived, and diving intensity. Dive duration was the least variable and vertical distance dived was the most variable among individual seals. The results were used to estimate the sample sizes required to detect—with acceptable precision and power—differences in the six measures between sites, years, or species. Statistics that vary most among individuals require the largest sample sizes to reliably detect a given percentage difference between annual means. Interestingly, we also observed the most significant interannual differences in those same statistics. These results emphasize that specifying the magnitude of the (interannual, intersite or interspecific) difference that is biologically significant to the study population is an important, though sometimes difficult, component of TDR survey design.