Abundance of Australian fur seal pups, Arctocephalus pusillus doriferus, at Seal Rocks, Victoria, in 1991-92 from Petersen and Bayesian estimators

Abstract

Seal Rocks is the largest colony of the Australian fur seal, Arctocephalus pusillus doriferus; annual pup production in the late 1960s and early 1970s was estimated to be 210C-2200 by counting and tagging. In the 1991-92 pupping season, the abundance of pups at Seal Rocks was estimated by counting and by mark-recapture with repeated recapture sessions. Counts of pups from elevated positions on the island in early December resulted in an estimate of 2440. In all, 816 pups were marked by shearing guard hairs from the head in late December; pups were resighted in six recapture sessions. Mark-recapture estimates of the number of pups alive in late December were calculated with a modified Petersen formula and with Bayesian statistics (2817 and 2819 pups, respectively). These approaches require the same basic assumption to be satisfied: that marked and unmarked pups have an equal probability of being recorded in recapture sessions. About 29% of pups sighted in the recapture samples were marked. More-conservative 95% confidence intervals resulted from the Bayesian method (2709-2933) than from modified Petersen statistics (2725-2908), but the logic underlying confidence intervals is different in the two cases. Comparison of the mark-recapture estimate and that based on direct counting for the 1991-92 breeding season indicates that pup production at Seal Rocks has probably been higher than reported previously, by a factor of 1.15.