Space partitioning within groups of social coral reef fish

Site-attached fish that form social groups may face a trade-off between the advantages of group living and the disadvantages related to intra-group competition for food. A possible solution for the latter is space partitioning among group members. Technological limitations related to individual tagging and underwater tracking hindered such spatial studies in grouping fishes. Here, using underwater video cameras and recent developments in deep learning tools, we successfully tracked the 3D movements of individually tagged fish in 4 groups of the damselfish Dascyllus marginatus in the coral reef of Eilat, Red Sea. Our findings, based on tracking sessions lasting 3–11 min that were recorded during a period of > 1 month, show that the individual fish kept separate foraging spaces with minimal overlap and that this separation was stable in time. When the tidally driven current reversed, the separation was kept, and a corresponding reversal was found in the positions of each fish relative to the coral and its neighbors. We propose that the stable spatial partitioning observed in our study is a primary mechanism through which site-attached species can organize themselves in order to reduce intra-group competition.