Responses of crucian carp populations to differential predation pressure in a manipulated pond

Theoretical analyses have suggested that the magnitude of antipredator responses of prey should be related to the intensity of predation. To examine this proposal at the population level, we removed all fishes from a natural pond in Finland, divided the pond into sections with plastic curtains, and stocked each section with equal populations of crucian carp (Carassius carassius), a common lentic cyprinid. Stocking of perch (Perca fluviatilis) and recolonization by pike (Exos lucius) created a gradient of predation pressure across three sections (I > II > III). Within 1 month, fewer crucian carp, particularly the more vulnerable small fish, remained in section I than in II and III. Crucian carp remaining in section I had higher growth rates and condition factors than fish in sections II and III. Greater proportions of carp were active offshore and during the day in section III, compared with greater inshore and nocturnal activity in sections of higher predator density (I and II). Directly and indirectly, crucian carp did respond differentially to varying intensities of predation under field conditions, although responses were not always strictly proportional to measured levels of predation pressure.