Optimising the timing of visual surveys of crabeater seal abundance: haulout behaviour as a consideration

Abstract

The most practical means of estimating pack-ice seal abundance is by conducting visual surveys from ships and aircraft. However, only those seals hauled out on the ice are ‘available’ to such surveys, and additional information on haulout behaviour is required to adjust counts of seals on the ice to estimate the total population size. Consideration of the optimal time to undertake visual surveys with respect to availability is important to ensure that bias and uncertainty in the abundance estimate are minimised for a fixed survey effort. In order to assess the optimal time for conducting visual surveys of crabeater seals (Lobodon carcinophaga), satellite-linked dive recorders were attached to 24 adult seals in the pack-ice off east Antarctica to record haulout behaviour over a 4-month period from mid-September to mid-January. The optimal time for visual surveys within these four months was December to mid-January (after the pupping season) when a high, relatively constant proportion of seals were hauled out over a period of 6–7 h during daylight, and when variation in haulout behaviour between seals was low. Despite the necessity for breeding seals to haul out continuously for extended periods during the pupping season, this was not a preferred time for visual surveys because variability in haulout behaviour between breeding and non-breeding seals was high. The efficiency of surveys before pupping was limited by the relatively short time during daylight when both the proportion of seals hauled out was high and variability in haulout behaviour among seals was small.