Summary
Traditionally many marine mammal populations are estimated by visual surveys of the number of ‘available’ animals (eg pack-ice seals hauled out on ice floes) with correction for the number not available (those in the water). However, for those species or stages of a specie’ life cycle where the majority of animals are not available to visual surveys, this approach may not be effective. Here we investigate whether acoustic surveys offer promise for estimating the distribution and abundance of populations using the Antarctic pack-ice seals as a model. This study looks at the acoustic survey of pack ice seals over four surveys conducted in October 1996 and 1997, and December 1997 and 1999. No crabeater seals were heard, whereas leopard and Ross seals were highly vociferous. These seals use underwater acoustic displays as part of their breeding behaviour and December coincides with their breeding season. A preliminary estimate of 0.13 adult male leopard seals per km2 was made of the number of vocalising leopard seals by predicting the area acoustically surveyed, through modelling transmission loss and measurements of received background levels, and by modelling the seals calling behaviour. This is in the very high range according to the literature.