ABUNDANCE OF AUSTRALIAN FUR-SEAL PUPS, ARCTOCEPHALUS-PUSILLUS-DORIFERUS, AT SEAL-ROCKS, VICTORIA, IN 1991-92 FROM PETERSEN AND BAYESIAN-ESTIMATORS

Seal Rocks is the largest colony of the Australian fur seal, Arctoceph alus pusillus doriferus; annual pup production in the late 1960s and e arly 1970s was estimated to be 2100-2200 by counting and tagging. In t he 1991-92 pupping season, the abundance of pups at Seal Rocks was est imated by counting and by mark-recapture with repeated recapture sessi ons. Counts of pups from elevated positions on the island in early Dec ember resulted in an estimate of 2440. In all, 816 pups were marked by shearing guard hairs from the head in late December; pups were resigh ted in six recapture sessions. Mark-recapture estimates of the number of pups alive in late December were calculated with a modified Peterse n formula and with Bayesian statistics (2817 and 2819 pups, respective ly). These approaches require the same basic assumption to be satisfie d: that marked and unmarked pups have an equal probability of being re corded in recapture sessions. About 29% of pups sighted in the recaptu re samples were marked. More-conservative 95% confidence intervals res ulted 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 estim ate and that based on direct counting for the 1991-92 breeding season indicates that pup production at Seal Rocks has probably been higher t han reported previously, by a factor of 1.15.