UC Irvine

We investigated the relationship between surface-foraging prions Pachyptila spp. and their zooplankton prey by comparing counts of foraging birds with near-surface concentrations of zooplankton from net samples. Zooplankton abundance was assessed by employing a new design of sampler that combined a frame net with a pump at the cod-end and which was deployed at the sea surface. Six transects using the sampler with concurrent bird observations were made over a short stretch of Stewart Strait near Bird Island, South Georgia. Concentrations of a variety of zooplankton species occurred patchily along each transect. Peak numbers of prions were recorded consistently towards the southern end of each transect. These peaks coincided with peaks in zooplankton numbers, but with different prey species predominant on different occasions. Comparison of the distribution of foraging and sitting birds with the presence or absence of zooplankton peaks, after pooling zooplankton species into 2 broad classes, showed that birds were consistently observed over copepod maxima, but were not found to be associated with aggregations of larger zooplankton more often than expected by chance.