Marine predators forage hotspots at oceanic fronts.
The gannet has been admired for its wonderful abilities in diving for centuries. The areas they travel in order to perform these feats of acrobatic feeding have been the subject of a study of the locations of food for many marine predators.
Shelf sea fronts form foraging habitats because of their influence on water temperature, salinity and density. Seasonal fronts are often persistent and are used by gannets frequently, while transient fronts were more difficult to assess.
Many large marine predators, whether mammals, birds, turtles or sharks find their prey by long distance travel. They have patterns of movement that can be targeting, very consistent and obviously have to be successful. They seek oceanic fronts, eddies or tidal flows where their prey aggregate and become more available. On ocean fronts, primary production and biomass accumulation are predictably enhanced although the study of the processes involved has been limited by the difficulties of measurement. Gannets in many of their habitats have been observed feeding at a site of increased frontal activity.
Investigating gannets also has an advantage in that they possess 2 different diving techniques. The short V-dive is aimed at surface fish such as herring or mackerel while the U-dive is shaped so as to enable the bird to swim underwater and venture 25m where necessary to catch prey like sand eels (Ammodytes spp.) or capelin (Mallotus villosus). Near Grassholm, a Welsh island, gannets nest during July and were be caught for a 12 minute period in this research to be sampled and fitted with data gatherers which do not impede their normal life. The Irish (Celtic) Sea stretches west from Grassholm, with gannets foraging far and wide. At times of high frontal activity the male birds were found diving more often when the front was close. The dives of female gannets were associated with seasonal fronts. Persistent front activity saw both sexes, but especially females, diving.
Subsurface movement depended obviously on the prey behaviour involved. V-dives were predominant around the fronts, indicating prey were easy to catch or were in high densities at these points. Fish can prefer the area above the near-surface thermocline here and mackerel are known to avoid cool water near the bottom. Such research is proving valuable in assessing the nature of these shelf fronts and how their productivity can influence the ecology of all the species noted here. The authors from Plymouth University,Plymouth Marine Lab.,UK, the Universities of California, US (Santa Cruz) and Exeter and the RSPB and NOAAs SW Fisheries Science Center were led by Sam L Cox, who is completing her Ph. D thesis at Plymouth. The title is
Seabird diving behaviour reveals the functional significance of shelf-sea fronts as foraging hotspots, published in the Royal Society Open Science journal today.