The food and feeding ecology of the white-chinned petrel Procellaria aequinoctialis at South Georgia

The diet of the white-chinned petrel at Bird Island, South Georgia was studied during the chick-rearing period in 1986 by quantitative analysis (by weight, frequency of occurrence and number of individuals) of regurgitated or lavaged adult stomach contents. Antarctic krill was the most important constituent of the diet, comprising over 90% of prey items and forming 47% of the diet by weight; fish and squid occurred In 67% and 35% of samples and formed 33% and 19% of the diet by weight, respectively. Decapods, amphipods and salps occurred in a few samples. The fish were mainly lanternfish (Myctophidae) of 8 species (chiefly in the genera Electrona and Gymnoscopelus) , forming 80% by numbers and 52% by mass of fish prey and the notothenioid Patagonotothen guntheri (14% by number and 35% by mass). Of the squid species taken, the ommastrephid Martialia hyadesi comprised 57% by number and 52% by mass and the gonatid Gonatus antarcticus 14% by number and 42% by mass. These dietary data confirm white chinned petrel as the most important avian consumer of fish and squid at South Georgia (and the third most important consumer of krill). In 1986 the notothenioid fish were probably obtained via commercial fishing operations but the myctophids and squid were probably live-caught, most likely at night. Meal size increased rapidly until chicks were three weeks old and then remained constant until the chicks were within 10 days of fledging, when it decreased. Meal delivery rate was high (one per day) for young chicks (1-10 days old) and thereafter fluctuated between 0.56 and 0.88 meals per day until close to fledging, when it was halved. These provisioning rates, and the proportion of krill in the diet, are higher than those recorded previously at South Georgia and Indian Ocean sites probably reflecting high local availability of krill at South Georgia in 1986. In many respects white-chinned petrels at South Georgia are intermediate ecologically between prions and albatrosses, although specialised in their extensive consumption of myctophids. Because krill and all their main fish prey are currently the targets of substantial commercial fishing and the main squid prey (Martialia) is a potential target, the role and status of white-chinned petrels is of additional importance.