Aggregation of avian predators and zooplankton prey in Otago shelf waters, New Zealand

Abstract

In Otago shelf waters surface swarms of krill (Nyctiphanes australis), hyperiid amphipods (Parathemisto spp.) and galatheid crab larvae (Munida gregaria) provide an abundant summer food source for planktivores. We tested the hypothesis that aggregation of avian planktivores depends upon the spatial distribution of their prey. Gulls (Larus scopulinus, L.bulleri, L.dominicanus), sooty shearwaters (Puffinus griseus) and white-fronted terns (Sterna striaXa) showed significantly aggregated distributions. The distribution of birds sitting on the sea surface was correlated with the abundance pattern of krill but was not correlated with the distribution of smaller hyperiid amphipods. The distribution of flying red-billed gulls (L.scopulinus), black-billed gulls (L.bulleri) and sooty shearwaters was correlated with the krill distribution but black-backed gulls (L.domini- canus) were not. Stomach contents of black-billed gulls were dominated by krill, in contrast to sooty shearwaters, which ate a higher proportion of Minida, and black-backed gulls, which contained Munida and fish. No amphipods were found in bird stomachs. There was no significant correlation between bird distributions and the hydrographic regime, water depth or distance offshore. The distribution of prey rather than hydrographic regime was a more important determinant of bird distributions at this spatial scale (2.6-12 km) and location.