Shoal size, patch profitability and information exchange in foraging goldfish

Goldfish in shoals of 2 spent less time foraging than fish in shoals of 5, probably because of increased individual vigilance. When feeding on three patches with different food densities, goldfish in shoals of 2 allocated more of their foraging time to the most profitable patch than individuals in a shoal of 5, who carried out twice as much sampling of the poorer patches. Sampling behaviour, which reduced individual food intake in a trial below the maximum possible, enabled fish in the larger shoal to recover feeding performance more rapidly after a switch in the profitability of the food patches. The experimental protocol ensured that one fish in each shoal was familiar with the new patch arrangement. Such ‘informed’ fish continued to forage more effectively than their ‘misinformed’ fellows for more trials after the switch when they were in shoals of 2 rather than 5. Analysis of visits to the patches immediately after the profitability switch revealed that sampling behaviour rapidly overrode the incorrect information of the misinformed majority in shoals of 5. The precise mechanism generating greater sampling behaviour in the larger shoal is not clear, but was probably neither ‘area copying’ nor direct observation of a successful food gatherer.