The presence of mahi-mahi at multi-predator feeding aggregations modifies striped marlin attack behaviour.

Terrestrial multi-species predator feeding aggregations (MPFAs) are characterized by contest competition, but scramble competition is likely more common at MPFAs in pelagic waters. However, empirical investigations are rare. Aerial video was used to estimate abundances of striped marlin and mahi-mahi alongside their attack and capture rates and spatio-temporal positioning as they competed at prey schools. Mahi-mahi had higher capture efficiencies than marlin, and the presence of marlin did not affect mahi-mahi predatory behaviour. Conversely, the presence of mahi-mahi in smaller abundances may benefit marlin. Marlin attack rates on the school were not affected by mahi-mahi presence, but net capture rates increased, likely explained by a shift in marlin hunting dynamics where marlin increased the proportion of their total captures that were individual fish isolated from the prey school. Estimated abundances of mahi-mahi, however, showed a negative relationship with marlin abundance. As mahi-mahi monopolized the space closest to the prey school, where they made rapid and efficient attacks on the prey school, in large abundances they may interfere with marlin hunting behaviour, causing them to abandon the hunt. The persistence of scramble or contest competition at MPFAs in pelagic waters may therefore be partially modulated by group size. This article is part of the theme issue 'Multi-predator feeding aggregations in pelagic waters'.