Changes in the abundance of New Zealand fur seals, Arctocephalus forsteri, on Kangaroo Island, South Australia

Abstract

Kangaroo Island was an important seal-harvesting site during the early years of European colonisation of Australia. A recent survey of the New Zealand fur seal, Arctocephalus forsteri, in South and Western Australia indicates that Kangaroo I. is still an important centre for the species. In order to determine changes in the abundance of the population, numbers of pups were determined at four colonies on Kangaroo Island by mark-recapture in up to five breeding seasons from 1988-89 to 1992-93. Clipping was the preferred technique for mark-recapture estimation of pups because it was quick, easy and effective. Recaptures were conducted visually; they were repeated several times in each season to improve precision of the estimates. No pups were marked between recaptures in order to minimise disturbance. Assumptions made in estimating population size by the mark-recapture technique pertinent to this study are reviewed. Pup numbers increased at three colonies: at Cape Gantheaume, from 458 to 867 over five years (with exponential rate of increase r = 0.16, n = 5); at Nautilus North, from 182 to 376 over five years (at r = 0.19, n = 4); and at North Casuarina Islet, from 442 to 503 over four years (at r = 0.043, n = 2). Rates of increase in the first two colonies are similar to those at the most rapidly increasing fur seal populations in the Southern Hemisphere. The Kangaroo I. population is estimated to be 10000 animals in 1992-93. It is likely to be at the recolonisation phase of growth, with high rates of increase at individual colonies (or parts of colonies) resulting from local immigration. As space does not appear to be limiting expansion in these colonies, fur seal numbers may continue to increase there.