We analyzed 19 dive records obtained from female antarctic fur seals (
Arctocephalus 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 d
etect annual changes in six measures of foraging behavior. We report t
he mean values and typical variability among individuals for dive dura
tion, dive depth, proportion of time submerged, transit time, vertical
distance dived, and diving intensity. Dive duration was the least var
iable and vertical distance dived was the most variable among individu
al seals. The results were used to estimate the sample sizes required
to detect-with acceptable precision and power-differences in the six m
easures between sites, years, or species. Statistics that vary most am
ong individuals require the largest sample sizes to reliably detect a
given percentage difference between annual means. Interestingly, we al
so 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 biolo
gically significant to the study population is an important, though so
metimes difficult, component of TDR survey design.