Hidden rivals: the negative impacts of dolphinfish on seabird foraging behaviour

Marine predators often aggregate at the air-sea boundary layer to pursue shared prey. In such scenarios, seabirds are likely to benefit from underwater predators herding fish schools into tight clusters thereby enhancing seabirds' prey detectability and capture potential. However, this coexistence can lead to competition, affecting not only immediate foraging strategies but also their distribution and interspecies dynamics. We investigated both the longitudinal relationships and instantaneous interactions between streaked shearwaters (<i>Calonectris leucomelas</i>) and common dolphinfish (<i>Coryphaena hippurus</i>), both preying on Japanese anchovy (<i>Engraulis japonicus</i>). Using GPS data from 2011 to 2021, we calculated behavioural parameters for streaked shearwaters as an index of time spent and distance travelled. Despite the abundance of Japanese anchovies, we found that streaked shearwaters might increase their foraging time in the presence of underwater predators. Moreover, video loggers provided direct evidence of streaked shearwaters and common dolphinfish attacking the same fish schools, potentially interfering with bird foraging by dolphinfish. Our results suggest that the presence of underwater predators in a given patch might increase the time spent by seabirds foraging without affecting the distance travelled. This highlights the need for future studies that consider the potential adverse effects of other top predators on seabird prey availability.