EFFECTS OF TIME-DEPTH RECORDERS ON MATERNAL FORAGING AND ATTENDANCE BEHAVIOR OF ANTARCTIC FUR SEALS (ARCTOCEPHALUS-GAZELLA)

We evaluated the difference in average durations of foraging trips and nursing visits to shore between one group of female Antarctic fur seals (Arctocephalus gazella) carrying radio transmitters only and another group carrying time-depth recorders (TDRs) and radio transmitters during their first five postpartum foraging trip - nursing visit cycles. Data were collected from 105 different fur seals in five breeding seasons (1989-1990 to 1993-1994) on Seal Island, Antarctica. Average foraging-trip and nursing-visit durations were significantly greater for the seals carrying TDRs and radio transmitters than for seals carrying radio transmitters only (two-way ANOVA, year x instrument type; trip, P = 0.004; visit, P = 0.04). Historically, instrument-effect studies on marine animals have focused on smaller species and larger instrument to body size ratios. Because of small sample sizes (the number of instruments successfully deployed and retrieved) and the typically variable nature of data from TDRs, the statistical power to detect significant differences due to instrument effects has been low. The evidence of instrument effects on Antarctic fur seals has possible implications for studies utilizing devices attached to similar-sized animals: results may not be representative of the natural (non-instrumented) population.