Effects of annual changes in primary productivity and ocean indices on the breeding performance of tropical roseate terns in the western Indian Ocean

We assessed the influence of inter-annual changes in primary productivity and local, regional and large scale ocean indices on the breeding parameters of roseate terns S. dougallii on Aride Island, Seychelles, western Indian Ocean. Productivity (chicks/breeding pair), timing of breeding, clutch and egg sizes were monitored annually for 8 years and correlated with local ocean productivity (denoted by SeaWiFS estimates of chlorophyll concentration, CC), sea surface temperature and indices recording the status of the Indian Ocean Dipole and of El Nino. The rate of increase in CC (between mean laying date and CC peak value) was positively related to roseate tern productivity and mean clutch size over the 1998-2005 study period. Colony productivity seemed also to be influenced by the Multivariate El Nino Index. In most years, the breeding phenology of roseate terns corresponded to the local increase in CC around Aride, and failure to adjust timing of reproduction to the timing of the phytopankton bloom decreased the probability of breeding success. This is the first study showing that a tropical seabird species is sensitive to inter-annual variations in the intensity and timing of the phytoplankton bloom, which should be connected to annual variations in the availability of its main fish prey (juvenile goatfish). Overall, these patterns indicate that the reproduction of this top marine predator is dictated by the temporal variability in oceanographic conditions. We suggest that CC data available over the world’s oceans may be a useful tool to develop models predicting the fate of colonies of inshore feeding seabirds when other, more conventional monitoring methods cannot be used.