Foraging ecology and ecophysiology of Cape gannets from colonies in contrasting feeding environments

Using a multi-disciplinary approach, we evaluated the potential consequences of long-term contrasting prey availability on the condition Cape gannets Morus capensis. We compared breeding adults from a decreasing colony on Malgas Island off the west coast of South Africa, where the abundance of small pelagic fish has decreased, with an increasing colony on Bird Island off the south coast, where pelagic fish are more abundant. We investigated (1) pelagic fish density using data from a hydro-acoustic survey, (2) gannet diet (stomach content analyses, fatty acid and stable isotope analyses), (3) gannet foraging effort and at-sea feeding areas (nest attendance patterns, GPS-tracking and blood haematocrit), (4) chick growth rates, and (5) adult body condition (morphometric measurements and breast muscle thickness). Our data confirmed contrasting prey availability between colonies, although pelagic fish were more abundant on the west coast than in previous years. Gannets exhibit dietary plasticity, feeding on pelagic fish and trawler discards, but favour natural prey when available. Stomach content samples showed that gannets from both islands mainly ate natural prey in 2009, but there were differences in their stable isotope and fatty acid signatures, supporting evidence of long-term diet differences. Nevertheless, chick growth rates and adult body condition were similar at both colonies, which was surprising for breeding adults from the west coast colony that had been feeding extensively on energy-poor fishery waste for several years. Breeding gannets' behavioural flexibility seems to have succeeded in maintaining body condition. However, this might have long-term costs, as adult survival at Malgas Island has decreased in recent years. Population decreases at this colony are exacerbated by low juvenile survival and perhaps differential recruitment to Bird Island. Higher foraging effort by adult breeding on the large colony of Bird Island compared to birds from the west coast, despite greater pelagic fish abundance on the south coast, probably suggests a greater intra-specific competition there. Monitoring of population trends, population health indices and foraging behaviour, as well as prey availability is necessary to better understand the mechanisms underlying the population trends.

► Gannets from a larger colony worked harder to feed their chicks.
► Long-term dependence on low quality diet did not affect breeding gannet body condition.
► Behavioural plasticity apparently offsets local prey shortages.
► Highlights importance of monitoring of population health indices and behaviour.