Australian fur seals at Seal Rocks, Victoria: pup abundance by mark recapture estimation shows continued increase

The abundance of Australian fur seal pups was determined at Seal Rocks, Wes ternport, Victoria in late December 1997 using a mark recapture procedure w ith repeated recapture sessions. Pups (n = 1291) were marked by clipping th e black guard hair on the head to reveal lighter underfur. Recaptures from the whole colony were made on eight occasions 1 3 days later. In the recapt ure sessions, a mean of 32% of sighted pups had been marked. Estimates of p up numbers over the eight recapture sessions were calculated using the Pete rsen estimate and then combined by taking their arithmetic mean. The combin ed estimate was 4024 (95% confidence range 3908 4141). In 1991 92, a simila r procedure led to an estimate of pup numbers of 2817 (95% confidence range 2703 2930). For both breeding seasons, estimates of pup numbers from each recapture session were also combined assuming a joint hypergeometric distri bution; there was little difference in the results from the two procedures, although the confidence intervals for the hypergeometric mean were smaller than those for the arithmetic mean. From 1991 92 to 1997 98, pup numbers increased by 43%, at an exponential ra te of 0.059 (95% confidence range 0.0526 0.0664), equivalent to 6.1% per an num (5.4 6.9%). This is greater than the rate of increase of pups at the co lony between 1968 69 and 1991 92, which was 0.023 (95% confidence range 0.0 198 0.0268), equivalent to 2.4% per annum (2.0 2.7%). Because of the rapid rate of increase of the Australian fur seal colony at Seal Rocks and the importance of the locality for tourism, we recommend tha t the abundance of pups there be determined every 3 5 years using a mark re capture technique.