Evidence for a by-product mutualism in a group hunter depends on prey movement state

1. Group-hunting animals have been shown to engage their prey in long pursuits (kms) to increase capture success, but the evidence for this is limited to a few terrestrial species. This predation strategy may be effective in the open-ocean, where group-hunts are characterised by large predator–prey size ratios and there are few places for prey to hide. 2. However, recording the attack rates, capture rates and capture success probabili- ties of grouping predators during long-distance pursuits through the open ocean, remains a significant challenge. 3. Here we use a combination of underwater and aerial video to assess the attack and capture behaviour of striped marlin, Kajikia audax, in relation to the move- ment of their schooling prey (‘stationary’ and ‘mobile’ movement states) during group hunts. 4. We found that the attack rate was higher when the prey school was ‘stationary’ and, during these times, individual marlin could make longer sequences of unin- terrupted attacks on the prey school, effectively monopolising the prey resource for short periods over conspecifics. 5. When the prey school was ‘mobile’, the attack pattern changed and individual marlin alternated their attacks on the prey school more frequently. Capture success probability (likelihood of capture per attack) was not affected by prey movement state. However, attacks on ‘mobile’ schools resulted in more prey fish becoming isolated from the school, and these isolated prey were easily captured— not by the attacker but by conspecifics swimming nearby. 6. The likelihood of a prey becoming isolated from a ‘mobile’ school was predicted by prey school speed, with a 0.1 m s−1 increase leading to an 18.01% increase in the chance of isolation. 7. Altogether this meant the capture rate at the group level was highest when the prey school was ‘mobile’ rather than ‘stationary’ because the predators that were pursuing the ‘mobile’ prey school had an increased availability of isolated and easy to capture prey. 8. This mechanism provides evidence for a by-product mutualism selecting for con- specific group formation and we discuss our findings in consideration of the social structure of the hunting group and the type of prey