Foraging Rules for Group Feeders: Area Copying Depends upon Food Density in Shoaling Goldfish

The decision rules governing forage area copying behaviour were investigated in shoaling fish. Shoaling goldfish were offered two equal food patches, one of which was adjacent to an equal‐sized shoal feeding behind a transparent barrier. When food was low, goldfish foraged according to an area copying rule, but under high and zero food area copying disappeared. Only under high food density did equal numbers of fish feed at both sites as predicted by foraging theory. Under zero food the fish were less certain about where to forage. Precise visual cues from feeding fish were required: non‐feeders did not attract area copiers. Furthermore, area copying was task‐dependent since it reappeared strongly if fish were not able to forage on patches like their fellows. Control experiments eliminated an increase in group size for anti‐predator advantage as an explanation. Two sequential decisions: to stay or move, and to join or leave may explain the results, which are not accommodated by simple optimality models. These decisions may be based on a comparison of current food intake with the anticipation of a higher reward by foraging socially.