Late summer sampling of pelagic age-0 year fish communities in five Czech reservoirs and one Dutch reservoir revealed extremely small age-0 year pikeperch Sander lucioperca (mean 24 mm standard length, LS, minimum 13 mm LS) alongside more normal-sized S. lucioperca that are found at the end of the first growing season (mean 50 mm LS), resulting in two clearly size-separated cohorts. Reference to such small age-0 year S. lucioperca in lakes or reservoirs at this time of year and in such large numbers are almost absent the scientific literature, and the presence of these small S. lucioperca is contradictory to the common understanding of the reproductive biology of this species. This overlooked phenomenon may have a major effect on the population dynamics of this valuable species because of size-dependent winter mortality.
Keywords: extremely small pikeperch; fry trawling; population dynamics; Sander lucioperca