Fine-scale foraging cues for African penguins in a highly variable marine environment

Abstract

Breeding seabirds often need to locate prey in spatially confined search areas on short temporal scales. Ocean physical features such as thermoclines are used as foraging cues since they concentrate and thus increase the likelihood of locating prey. However, in highly variable environments, it is less well understood how these features act as foraging cues. African penguins Spheniscus demersus foraging in Algoa Bay, South Africa, were fitted with GPS-TD loggers to determine the cues they use to locate prey on fine temporal (< 24 h) and spatial (<100 km) scales in a bay with changing thermal properties. African penguins showed a preference for cooler surface waters associated with upwelling, avoiding warm surface waters associated with the Agulhas Current. Thermocline presence and characteristics were an important foraging cue; penguins consistently foraged at and below the thermocline even though its depth and gradient shifted over time. Dive ascent and descent rates were quicker in the presence of thermoclines with strong gradients, which form a distinct separation between the warm upper mixed layer and the cooler lower layer. Foraging dives occurred predominantly below the thermocline, while search dives occurred around the thermocline depth. Penguins dived deeper in search of prey when there was no thermo cline. Our results demonstrate that top predators can cope with highly variable environments by adjusting their search strategy to target environments suited to their main prey.