Abstract
Although oceanic tropical waters occupy almost 50% of the total area of pelagic oceans, knowledge of the foraging ecology of top predators in these low productivity waters is limitied. This is particularly the case for tropical seabirds that are believed to rely on scarce and unpredictable resources and have developed specific foraging strategies to exploit these resources. Frigatebirds are tropical seabirds that rely on subsurface predators such as tuna or cetaceans to feed. We studied the foraging strategy at sea of great frigatebirds breeding on Europa Island in the Mozambique Channel using satellite transmitters and altimeters. When foraging, birds moved at slow speeds (average 16.4 km h–1) and stayed at an average altitude of 180 m, continuously climbing and descending. During climbs, they reached high altitudes (maximum 2867 m) and during descent rarely came close to the surface to feed. Birds came to the surface on average 6.2 times d–1 . Feeding opportunities occurred only during the day, peaking early in the morning and late in the afternoon. Frigatebirds foraged over extensive distances, up to 612 km from the island, usually during the incubation or post-breeding periods, concentrating their effort in the western oceanic waters of the channel where overall productivity, although low, was still higher than in the eastern part of the channel. The higher productivity in the western waters is due to the presence of a persistent field of mesoscale anticyclonic gyres. Birds tended to avoid the centre of cold eddies and remained at the edge of eddies. When brooding chicks, birds foraged closer to the island, at an average distance of 94 km, mainly in the northwest of the island, in the vicinity of shallow waters of the Bassas da India Bank. During both long and short trips, birds did not return to the same area. Diet was composed essentially of flying-fish and Ommastrephid squids. The results of the study indicate that the strategy of frigatebirds is based on wide ranging foraging directed toward scarce prey patches that are encountered irregularly, and are not predictable in exact location at a coarse scale. At a mesoscale level, birds appear to favour large areas with slightly enhanced productivity such as a zone of strong eddies. Since they feed in close association with subsurface predators, mainly tuna in our area, it can be predicted that large predators such as seabirds or tuna are spread over extensive areas and have an unpredictable localisation at a coarse scale, but have some specific preferences on a regional scale.