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

Abstract

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 6 0.4 vs. 2.7 6 0.3 m) and longer (10.1 6 1.0 vs. 5.0 6 0.2 s), and more U-shaped dives (dives where birds stayed at more or less one depth) were performed (52% 6 7% vs. 7% 6 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 5 122 6 81 km vs. 62 6 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.

East or west? Migration routes and wintering sites of Northern Gannets Morus bassanus from south-eastern Iceland

“…Flexibility in their choice of size and species of fish is well known in Northern Gannets and enables them to respond to such changes in prey fields (e.g. Hamer et al 2007;Montevecchi et al 2009;Garthe et al 2011). Breeding gannets at Skrúður depend nonetheless on herring and mackerel (Scomber scombrus) for chick rearing (Vigfusdottir et al 2009).…”

Mesoscale fronts as foraging habitats: composite front mapping reveals oceanographic drivers of habitat use for a pelagic seabird

“…Gannets are large, mediumranging marine predators, which feed on a wide variety of piscivorous prey [7,[39][40][41]. Foraging plasticity in gannets has been linked to oceanographic variability over a range of scales [40,[42][43][44]. We here assess the influence of mesoscale frontal activity on gannet foraging behaviour and evaluate the utility of composite front mapping for elucidating oceanographic controls of habitat selection.…”

Relative importance of local enhancement as a search strategy for breeding seabirds: an experimental approach

“…In contrast, spawning capelin aggregations can be predictably located near a cluster of annually persistent deep-water (15e40 m) spawning sites (Gull Island, Fig. 1b;Davoren, 2013a; located within foraging ranges of seabirds breeding on Funk Island (60e80 km and 60e120 km for murres and gannets, respectively; Garthe et al, 2011). Murres and gannets appear to use both memory-based foraging and local enhancement to locate prey during foraging trips from the colony under these changing prey conditions (Davoren, Montevecchi, & Anderson, 2003a, 2003bGarthe et al, 2007Garthe et al, , 2011Montevecchi et al, 2009).…”

“…Numerous studies have documented marine predators making foraging trips to a few distinct locations over the short term during breeding seasons (e.g. Garthe, Montevecchi, & Davoren, 2007;Garthe, Montevecchi, & Davoren, 2011;Hamer, Phillips, Hill, Wanless, & Wood, 2001;Irons, 1998;Montevecchi, Benvenuti, Garthe, Davoren, & Fifield, 2009), nonbreeding seasons (e.g. Gende & Sigler, 2006;Sigler et al, 2012), as well as on an annual scale (e.g.…”