Sexual dimorphism, moult and body condition of seabirds killed by longlinevessels around the Prince Edward Islands, 1996-97

A total of 393 seabirds from nine species killed by longline fishing in Sou th African waters around the Prince Edward Islands during summer 1996-97 we re sexed, measured, and their body fat levels and primary moults scored. Ma les of Whitechinned Petrels Procellaria aequinoctialis and Greyheaded and ( Indian Ocean) Yellownosed Mollymawks Thalassarche chrysostoma and T. [chlor orhynchos] bassi averaged 1-9% larger than females. Measures of bill depth were the most dimorphic characters in all three species. I recommend that i f only a single measure is taken to estimate sex of adults, bill depth at t he base be the standard measure for petrels, and bill depth at the nail be the standard measure for mollymawks. Discriminant functions are described f or predicting the sex of unknown individuals; the proportion of misclassifi ed birds ranged from 2-10%. With the exception of giant petrels Macronectes spp., only two young birds were actively moulting their primary flight fea thers. Analysis of wear patterns among adult Greyheaded Mollymawk primaries conforms with studies of moult in this species at South Georgia. Fat score s did not differ between sexes. They tended to decrease during the breeding season, but trends were masked by great inter-individual variation. There was no significant relationship between fat scores and the presence of inge sted plastics. Plastic debris was found in the stomachs of 90 birds from fi ve species. Incidence in Whitechinned Petrel stomachs (37%) was less than t hat reported from birds collected off South Africa in the early 1980s (57%) , but this is probably due to the predominance of breeding adults. The freq uency and size of plastic loads decreased through the breeding season, whic h is consistent with the hypothesis that inter-generational transfer is imp ortant in the dynamics of ingested debris.