Statistical power and plausible alternate hypotheses of gametic recapture estimates from wintering grounds of humpback whales

In Garrigue et al. (2004) we present multiple lines of evidence to suggest that humpback whales Megaptera novaeangliae from the winter grounds around New Caledonia (NC) form a small autonomous population unit that is relatively closed to demographic and reproductive interchange with other populations in the South Pacific. Our evidence included a 7 yr database of sighting–resighting records for both individual identification photographs (photo-identification) and microsatellite genotypes from small skin samples collected with a biopsy dart. From these we derived estimates and 95% confidence intervals (CI) of abundance for the non-calf population and for the sex-specific components of the population using standard closedpopulation, capture-recapture models (Fig. 1). The low coefficient of variation (CV) of these estimates, reflecting a high proportion of recapture for individuals across the 7 yr study, and the limited interchange documented with other breeding grounds (Garrigue et al. 2002) supported our conclusion that the population was small and closed demographically. From the total sample of 133 non-calf males, we also inferred the paternity of 5 calves from a total sample of 16 calves (representing about 42% of all cow–calf pairs sighted in the small population over 7 yr). Following the logic of capture-recapture models, we used the proportion of inferred paternities as a male ‘gametic’ recapture estimate of abundance for comparison to the male ‘organismal’ recapture estimate. These 2 estimates were essentially identical (Fig. 1), suggesting that the population of males wintering near NC were likely to be the fathers of the calves. Formally stated, we were unable to reject the null hypothesis that the number of paternities inferred from the sample of calves was greater or less than expected given the estimated number of males in the local population. This finding was unusual, as the success rate of paternity assignments in populations of marine mammals is generally below expectations, even when sampling of males is considerably more complete than in the case of the NC humpback population (e.g. WorthingtonWilmer et al. 1999, Gemmell et al. 2001, Krutzen et al. 2004). We considered that the evidence of relative