Factors affecting adult survival and inter-colony movement at the three South African colonies of Cape gannet

摘要

Marine systems are under pressure from both climate change and exploitation. While many of these ecosystems are inherently variable and hard to monitor, seabirds can be used as ecological indicators that provide early warning signals of deeper environmental change. The Agulhas-Benguela marine ecosystem around southern Africa has exhibited long-term changes in sea surface temperature, and the distribution of pelagic fish in this system has shifted. The Cape gannet Morus capensis is a seabird endemic as a breeding species to the Agulhas-Benguela ecosystem. Cape gannets breed at just 6 locations and are listed by the IUCN as Vulnerable. Knowledge of the survival and movements of a species is important for understanding of factors influencing its conservation. A random effects multistate capture-recapture model was used to estimate the annual survival probabilities and movement between colonies for adult birds at the 3 South African colonies of the species. The effects on survival of environmental and fisheries-related covariates were explored. Survival over the 20 yr period did not exhibit any long-term trend at the 2 southern colonies (Malgas and Bird Islands) but decreased at Lambert's Bay between 1996 and 2007. At all 3 colonies, adult birds showed a high degree of site fidelity. It may be that for Cape gannets, the primary effects of climate and fishing are on recruitment rather than on survival. The continued use of sub-optimal conditions by the west coast colonies has been referred to as an 'ecological trap' and necessitates the introduction of spatial considerations into fisheries management.