Piscivory in larval perch (Perca fluviatilis):: mechanisms structuring larval roach (Rutilus rutilus) cohorts

Perch (Perca fluviatilis) can act as a piscivore from larval stage VI (body size 10.3 mm) on newly hatched larval roach (Rutilus rutilus), bream (Abramis brama) and smaller siblings of its own cohort. Consumption rates at this stage were approx. 0.5 prey/perch*h at 21 degreesC. Larval perch predation was strongly gap-limited, and the maximum size of roach consumed by perch (perch length interval 10.3-62.0 mm) under experimental conditions followed the linear regression, Pprey-max. = 0.478 * L-Pred. + 1. 829 (r(2)=0.99, P <0.001, n= 12). Under experimental conditions, predatory 0(+) perch substantially affected the size distributions of 0(+) roach prey cohorts, since smaller prey individuals were predated more frequently than larger ones. In both unimodal and bimodal size distributions of prey roach, the distributions changed according to the maximum prey size consumed by the added predatory perch. Unimodal prey distributions were positively skewed when piscivorous perch were added compared with controls without predators. According to the size distributions of lake-living 0(+) roach and 0(+) perch and the relative size difference between prey and predator, the vulnerability of 0(+) roach cohort to 0(+) perch predation changed from June to September. Prey vulnerability was extremely sensitive to the relative size difference between predator and prey. Therefore differences in hatching time and growth rates between the two species will strongly influence the potential for predator-prey interactions.