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

Abstract

The size of the breeding population of Brandt's Cormorants Phalacrocorax penicillatus on Southeast Farallon Island, off the coast of central California, USA, shows large annual fluctuations and has declined by 50% since the 1970s. We investigated patterns of variation in adult survival, breeding probability, resighting probability, juvenile survival and reproductive success, to determine the extent to which variation in demographic processes reflected variation in environmental conditions (e.g. food availability) versus densitydependence, using observations of breeders and non-breeders from 1976 to 1995. Resighting and breeding probabilities varied significantly among years, and both were positively correlated with an annual index of prey availability — the abundance of juvenile rockfish. Adult survival between years was significantly correlated with changes in juvenile rockfish abundance 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 Niño 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 population size. The return rate of juveniles, an index of survival during the first three years of life, was negatively related to sea surface temperature, both in the year of hatching and in the third year of life, when individuals first return to the colony to breed. We conclude that this population is strongly susceptible to environmental fluctuation. All parameters of survival and reproduction deteriorate under poor environmental conditions, with no evidence of buffering, and there was no evidence of density-dependence. Thus extrinsic, not intrinsic, forces appear most important in explaining recent population fluctuations.