Seabird assemblages, abundance, and distribution in the African sector of the southern Indian Ocean

Seabird distributions in the Southern Ocean are influenced by the location and accessibility of suitable breeding sites, but also by the environmental factors that influence the distribution and availability of their prey. For example, oceanic fronts, concentrate prey at their surface and therefore present important foraging areas for many seabirds. This study investigated the latitudinal distribution and abundance of seabirds in the African sector of the Southern Ocean. In particular, we investigated the relationship of seabird assemblages and densities to key biophysical environmental parameters (SST, sea surface height, bathymetry) and the main oceanic fronts. There was a high density of seabirds north of the Subtropical Convergence (STC), which is situated at approximately 39°S, with declining densities farther south. There was latitudinal segregation between several species, e.g. black-browed albatross (Thalassarche melanophris) occurred north of the STC, and grey-headed albatross (T. chrysostoma) occurred to south of it. The Subantarctic Front (SAF) and the Antarctic Polar Front (APF) had less influence on seabird populations than the STC. Latitude was the greatest predictor of seabird assemblages and densities, reflecting environmental gradients in physical and biological parameters and their influences on prey distributions. Of the environmental parameters, sea surface temperature and bathymetry were the most important physical features influencing seabird assemblages. In particular, the density of seabirds north of STC declined with increasing sea surface temperature and had a negative relationship with bathymetry, with most seabirds occurring in shallower waters. In contrast, seabird density had a positive linear relationship with sea surface height. Relationships with other environmental parameters, such as wind, salinity and chlorophyll concentration (as a proxy for productivity), were less well-defined.