COS 3-3 - Cooperation between apex predators drives foraging success despite unpredictability in prey abundance

Abstract: Foraging theory proposes that individuals adapt their foraging behavior relative to environmental conditions and prey availability. One key adaptation to overcome the challenge of finding and capturing unpredictable prey is social foraging. However, the role of environmental conditions and prey availability on the expression of social foraging is rarely tested empirically. Here, we test these links empirically in an interspecific case of social foraging, the traditional cooperative fishing between wild dolphins (Tursiops truncatus gephyreus) and artisanal human fishers to catch migratory mullet (Mugil liza)—a patchy and ephemeral prey—in southern Brazil. First, we used a multiplatform tracking system to quantify prey and predator abundance and foraging behavior moment-by-moment, at the fine spatiotemporal scale that matters for this social foraging tactic. We recorded in situ the local environmental conditions that influence mullet migration; used underwater sonar imaging to record fast-moving mullet schools in the murky estuarine waters and developed a novel deep learning model to quantify their abundance; and combined behavioral observations with drone imagery to quantify the number of dolphins and fishers, the synchrony of their foraging behavior, and resultant number of mullet caught. We then use confirmatory path analysis to test the multifaceted relationships among environmental conditions, prey and predator abundances, and predators’ behavior and foraging success. We first show that local environmental conditions at the interaction site do not predict mullet abundance, and that mullet abundance alone does not predict dolphin or fisher foraging behavior. Instead, significantly more dolphins are present during flood tides, and their presence results in significantly more human fishers in the interaction site. A greater abundance of both dolphins and fishers increases the rates of their cooperative foraging interactions and, together with higher mullet abundance, increases the number of mullet caught. Taken together, our moment-by-moment analyses demonstrates that the success of this cooperative foraging tactic is more strongly dependent on the co-occurrence and willingness of predators than it is on the fine-scale prey availability, and both of these are largely independent of local environmental conditions. Our results reveal novel aspects of a century-old traditional fishery that involves a rare type of cooperation between humans and wildlife. More broadly, these results suggest that by foraging cooperatively, animals that forage socially can disconnect their foraging success from the underlying environmental conditions, which might be a benefit to overcoming prey unpredictability and ephemerality.