Mark-recapture estimates of pup production for the Australian sea lion (Neophoca cinerea) at Seal Bay Conservation PArk, South Australia

Australian sea lion (Neophoca cinerea) pup counts have been conducted at monthly intervals at Seal Bay Conservation Park, South Australia, since 1983. The protracted breeding season of the Australian sea lion causes difficulties for estimating pup production as simple counts of pups are likely to underestimate pup abundance. Here we describe the use of a mark-recapture technique, the modified Petersen estimate, to estimate the total number of sea lion pups produced at Seal Bay by the end of the breeding season in late June 2003. Our aims were to compare the estimate of pup numbers from direct counting with that from mark-recapture, and evaluate methods used to estimate pup abundance at the end of the pupping season. Pups (n = 74) were marked over a four month period by clipping the hair on the rump. Pups were also given individually identifying microchips (23 mm TIRIS (TM) RFID) that were inserted subcutaneously. These identifying microchips were used to test for unequal catchability and estimate the number of marked pups in the population at the time of the mark-recapture (n = 74). The mark-recapture was conducted after peak pup production in the seventh month of the breeding season. The best estimate of the number of sea lion pups in the Seal Bay colony was 230 (95% CI 203-257). These incorporate the mark-recapture estimates. In their absence, the estimate from direct counting of pups in the whole colony was 163 (95% CI 147-179). Thus the mark-recapture estimate was 187% of the direct count (95% CI 173-201), highlighting that pup counts underestimate pup production at Seal Bay Conservation Park.