Foraging distribution of Cape gannets in relation to oceanographic features, prey availability and marine protected areas

摘要

Seabirds forage in a dynamic environment of heterogeneously distributed prey resources. Many seabird species use oceanographic features that promote prey aggregation, as a means of locating prey patches. The combination of tracking data, remote-sensing data and estimates of prey availability is useful in determining how seabirds locate prey. GPS tracks of foraging Cape gannets Morus capensis were collected across 3 breeding seasons and tested for inter-annual changes in home range size and foraging effort, and compared against the availability of their predominant prey (sardine Sardinops sagax and anchovy Engraulis encrasicolus) as determined from acoustic surveys. Biophysical features associated with foraging were compared to a random point dataset using a binomial general linear mixed model, while foraging distributions of Cape gannets were compared against the placement of marine protected areas (MPAs). The total home range of foraging Cape gannets, breeding at Bird Island, South Africa, propagated westwards over the 3 breeding seasons, which coincided with a concurrent westward contraction in the distribution of sardine and anchovy. Foraging effort showed an apparent increase in response to low prey densities and occurred mostly outside MPAs. Although gannets seemed to forage in relatively cold waters, biophysical features were generally unreliable predictors of gannet foraging distribution. The relationship between home range and prey distribution, coupled with recent declines in local prey availability, demonstrates the ability of gannets to track the distribution of their prey resources. However, the ephemeral and dynamic nature of these prey resources places a major constraint on conservation-based spatial planning involving Cape gannets.