Foraging gannets on the front line
Date: September 21, 2016
Source: University of Exeter
A team of scientists led by Plymouth University and the University of Exeter examined the feeding behaviours of breeding northern gannets (Morus bassanus) in the Celtic Sea.
They showed that in the majority of cases, the birds performed shallow and short dives with less swimming when at ocean fronts, signifying the possible presence of abundant food closer to the water's surface.
Writing in Royal Society Open Science, researchers say the findings have important implications for conservation because these habitats could be earmarked for designation as marine conservation zones.
Ocean fronts occur where bodies of water meet and are typically marked by strong gradients in temperature and salinity, alongside converging surface current flows.
Previous studies have shown they are important for marine ecosystem functioning, with accelerated photosynthesis creating more plankton and thus attracting more fish, which may be concentrated at the water's surface.
Gannets feed by flying high and then circling before plunging into the sea to catch fish. The RSPB estimates there are 220,000 gannet nests in the UK, and because they only breed at a few selected sites, they are on the organisation's Amber List for conservation.
For this study, which ran over two years, scientists tracked 53 breeding birds using bird-borne GPS devices and time depth recorders, examining the relationship between 1,901 foraging dives and satellite-detected ocean fronts in the Celtic Sea.
The results showed that in the majority of cases around fronts (~94% of dives), birds deployed a short V-shaped dive, indicating food was easier to catch close to the surface. In some instances, where food is less easily available, gannets may use a U-shaped dive, spending more time underwater so as to reach food at deeper depths, but this rarely occurred in frontal dives (~6% of the time).