Survival, breeding probability and reproductive success in relation to population dynamics of Brandt's cormorants Phalacrocorax penicillatus

The size of the breeding population of Brandt's Cormorants Phalacrocorax pe nicillatus on Southeast Farallon Island, off the coast of central Californi a, LISA, shows large annual fluctuations and has declined by 50% since the 1970s. We investigated patterns of variation in adult survival, breeding pr obability, resighting probability, juvenile survival and reproductive succe ss, to determine the extent to which variation in demographic processes ref lected variation in environmental conditions (e.g. food availability) versu s density-dependence, using observations of breeders and non-breeders from 1976 to 1995. Resighting and breeding probabilities varied significantly am ong years, and both were positively correlated with an annual index of prey availability the abundance of juvenile rockfish. Adult survival between ye ars was significantly correlated with changes in juvenile rockfish abundanc e and differed between sexes (0.77, males; 0.71, females). Female, but not male, survival appeared age-specific: older females showed reduced survival , especially during El Nino years. Intermittent breeding appeared common in this population: 54% of males and 49% of females estimated to be alive in a given year were not observed breeding in that year. Reproductive success varied in relation to the juvenile rockfish index but not in relation to po pulation size. The return rate of juveniles, an inner of survival during th e first three years of life, was negatively related to sea surface temperat ure, both in the year of hatching and in the third year of life, when indiv iduals first return to the colony to breed. We conclude that this populatio n is strongly susceptible to environmental fluctuation. All parameters of s urvival and reproduction deteriorate under poor environmental conditions, w ith no evidence of buffering, and there was no evidence of density-dependen ce. Thus extrinsic, not intrinsic, forces appear most important in explaini ng recent population fluctuations.