Detecting mechanisms that structure ecological communities often requires investigations at appropriate spatial and temporal scales. A 15-mo, whole-lake experiment revealed direct and indirect mechanisms by which a fish predator structured its prey population, primarily by affecting recruitment of young-of-the-year (YOY) fish. Piscivorous Eurasian perch (Perca fluviatilis) were added to two of four sections of a divided lake in eastern Finland previously stocked with crucian carp (Carassius carassius). Although the stocked crucians quickly grew beyond vulnerable sizes, recruitment of their offspring through their 1st yr of life was reduced by 90% in sections with perch. Surviving YOY in predator sections were completely confined to vegetated inshore areas. This restriction to refuges lowered their growth rates in comparison with YOY in predator-free sections, which can further reduce recruitment by prolonging susceptibility to size-limited predation and decreasing energy reserves needed for anaerobic overwintering. In the presence of piscivores, surviving prey subsequently benefit from reduced intraspecific competition, increasing growth rates to achieve large, invulnerable body sizes rapidly. Resulting populations of crucian carp, consisting of relatively few but relatively large individuals, contrast with populations living in the absence of piscivores, which are characterized by high densities of small, stunted individuals. Such divergent population patterns are displayed by a number of prey species in Scandinavia and North America, suggesting that a combination of direct and indirect effects of predation on younger age classes, such as documented for crucian carp, may be common in fish populations of small northern lakes. Detecting these patterns and identifying their underlying mechanisms may often require a combination of extensive and intensive studies.
