Diet of southern Buller's albatross (Diomedea bulleri bulleri) and the importance of fishery discards during chick rearing

Abstract

Southern Buller's albatrosses (Diomedea bulleri bulleri Rothschild) are frequently associated with fishing vessels off southern New Zealand, and may depend substantially on discards from fishing vessels, particularly trawlers, during chick rearing. To test this hypothesis, the diet was determined from 188 chick regurgitations collected at The Snares (May 1996, July 1996 and 1997) and Solander Island (May and July 1997). Fish (mostly discards but also some small species obtained naturally) was the dominant prey category, occurring in 92% of samples, and forming 65% by weight of solid food consumed. Salps (Pyrosoma sp. and lasis zonaria) were the most abundant prey items (44% of all items), and second most important by frequency of occurrence (78%) and weight (24%). Cephalopod remains (mostly arrow squid (Nototodarus spp.) and Histioteuthis atlantica) occurred in 53% of samples, but contributed only 17% by number and 7% by weight. Other dietary items included crustaceans (chiefly Munida gregaria, Nectocarcinus antarctica, and Lepas anatifera), bird feathers, and flotsam. Fishery discards were present in 70% of samples, constituted 60% of the diet by weight, and comprised 91% of the weight of all fish consumed. Discard composition was dominated by Gadiformes (mostly hoki (Macruronus novaezelandiae) and macrourids) in 1996, and by jack mackerel (Trachurus sp.) in 1997. Breeders at The Snares took more discards in July 1997 (68 g per sample) than May or July 1996 (31 and 28 g respectively), those at Solander Island took more in May (56 g) than July 1997 (29 g). In May 1996, the diet of birds at The Snares tended to be biased towards fish (both discards and natural prey) after long foraging trips, but towards cephalopods after short trips. Sexual differences in diet and discard consumption tended to be more pronounced amongst samples from The Snares. The high, and apparently increased proportion of fish in the diet, strongly suggest that discards from fishing operations have a beneficial effect on the population of southern Buller's albatrosses breeding on The Snares.