Summary
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 7,1.5, 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.
Methodology
Tests were conducted in the laboratory using bluntnose minnows, Pimephales notatus, as the shoaling species. Largemouth bass, Micropterus salmoides, a natural predator of bluntnose minnows in lakes (Scott & Crossman 1973), were used as predators. All fish were captured using a beach seine from Lake Opinicon at the Queen's University Biological Station, Chaffey's Lock, Ontario (44° 30' N, 76° 30' W). Fish were transported to the main campus laboratory of Queen's University.