Replicated whole-lake experiment reveals the ineffectiveness of stocking five example fish species in small lakes

Abstract

Fish stocking is common practice in fisheries management, but its effectiveness to support or increase stock size or catches is debated. In Lower Saxony, Germany, a controlled, replicated stocking experiment of five native fish species in eight gravel pit lakes <20 ha was conducted to examine the potential additive effects of fish releases on abundance, indexed by catch per unit effort (CPUE) in electrofishing and gillnet samples. Fish were stocked twice in four study lakes, while the remaining four control lakes were not subjected to any stocking over a 6-year study period. Recapture rates of individually marked fishes were extremely low in all five species. Consequently, stocking did not increase species-specific and total fish community abundance. In additional control experiments in separate ponds all stocked fish species, except bream (Abramis brama), showed high survival in at least one study year, indicating general survival of the stocking material. Fish marking with passive integrated transponders did not affect fish survival, except for roach (Rutilus rutilus), and can thus be ruled out as an explanation for the high post-stocking mortality. We conclude that stocking failed to enhance stocks of five native fish species in small gravel pit lakes.