Synopsis
Shoals of 3, 5, 7, 10, 15, and 20 bluntnose minnows,Pimephales notatus, were allowed to forage in the absence and presence of a fish predator, which was separated from the shoal by a clear plexiglass partition. A typical dilution effect was observed in that individual fish in larger shoals were approached less frequently by the predator. In the absence of a predator, foraging latency decreased significantly and the rate of foraging increased with increasing shoal size. Foraging latency for each shoal size tended to increase in the presence of a predator and foraging rate decreased, significantly for shoals of 7, 15, and 20 fish. Members of larger shoals were safer and enjoyed a greater level of food consumption, perhaps due to decreased individual vigilance for predators and social facilitation. However, foraging effort decreased when a predator was present, as more time was allocated to predator avoidance.
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Morgan, M.J., Colgan, P.W. The effects of predator presence and shoal size on foraging in bluntnose minnows,Pimephales notatus . Environ Biol Fish 20, 105–111 (1987). https://doi.org/10.1007/BF00005290
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DOI: https://doi.org/10.1007/BF00005290