Inter-annual changes in prey fields trigger different foraging tactics in a large marine predator

We report on inter-annual comparisons of the foraging behavior of Global Positioning System-equipped chick-rearing northern gannets (Morus bassanus) in the western Atlantic during years with contrasting oceanographic and prey conditions. We hypothesized that the predators would modify their foraging tactics when small fishes (capelin [Mallotus villosus]) and large pelagic fishes (mackerel, saury) varied in inter-annual abundances. We predicted differences in (1) diving behavior, (2) spatial, and (3) temporal patterning of foraging behavior. Predictions 1 and 2 were supported, prediction 3 rejected. Dives were significantly deeper (4.3 +/- 0.4 vs. 2.7 +/- 0.3 m) and longer (10.1 +/- 1.0 vs. 5.0 +/- 0.2 s), and more U-shaped dives (dives where birds stayed at more or less one depth) were performed (52% +/- 7% vs. 7% +/- 2%) in the year with higher abundance of forage fishes. Flight patterns exhibited remarkable spatial and geographic differences: gannets flew significantly (17%) more and foraging ranges were about twice as long when they pursued large pelagic fishes (mean = 122 +/- 81 km vs. 62 +/- 12 km). The 95% kernel feeding range was 34 times larger when large pelagic fishes were available. Yet foraging trip durations were not different between years. Inter-annual variation in foraging tactics by the same species at the same colony in successive years was strongly related to prey availability, showing that spatial foraging parameters can be determined largely by ocean and prey conditions.