Effect of oceanographic features on fine-scale foraging movements of bottlenose dolphins

Physical processes play an important role in structuring predator–prey interactions by influencing the abundance, distribution and behaviour of prey. Many biological hotspots are associated with ocean currents, fronts and eddies, as a result of their predictability as a prey source. In this study, we determined the effect of topography, tidal currents and fronts on the movements and behaviour of a marine top predator, the bottlenose dolphin Tursiops truncatus, within a known foraging area in the Moray Firth, Scotland. Three main fronts were identified and characterised. Dolphins concentrated their search effort in close proximity to these small-scale fronts. Areas with steep seabed gradients were also preferred, but different sites were used depending on the direction of the current, indicating an effect of topography on hydrodynamics and predator–prey interactions. The mean swimming speed over ground for all dolphin tracks (n = 26) was 1.15 m s–1, but their mean speed through the water was higher (2.18 m s–1). This is close to the most energetically efficient swimming speed for bottlenose dolphins. On average, the dolphins therefore appeared to be expending minimal energy during movement. These results suggest that the interaction between topography and hydrography provided a mechanism for improving foraging success and/or efficiency for the bottlenose dolphin. The narrowness of the channel may concentrate prey and the presence of topographically controlled fronts may further increase the availability of prey, as well as its predictability. Marine predators can focus their foraging efforts on such locations to improve efficiency and reduce energetic costs.