Does Fish Conditioning in Aquaculture Increase Survival Success in the Wild? A Case Study on a Cyprinid Fish

Many endangered fish species in the wild are artificially bred, and their populations are strengthened by the stocking of aquaculture-raised juveniles. Because fish from aquaculture are generally not well prepared for the challenging life in the wild, we tested whether training for selected challenges could improve fish survival after stocking. We chose conditioning on predation pressure (by learning predator image of northern pike Esox lucius using predator chemical cues and visual stimuli), increased rearing water velocity 20 cm × s −1 , and direct exposure to predation. The juvenile cyprinid fish asp ( Leuciscus aspius ) was used as a model prey species. A total of 7949 asp were reared in four groups using a combination of high flow, predation, and control treatments (low flow, no predation; 2018, 2149, 1929, and 1856 individuals, respectively). Of these, 1800 individuals were released into three ponds with pike, and their mortality rates were monitored in relation to predation for two months after stocking using passive telemetry arrays. The remaining 6149 aquaculture-reared individuals were released directly into a large reservoir, while 1426 individuals that survived pike predation for two months were released after the pond experiment ended. Fish survival in a reservoir was monitored in 2020 and 2021 by boat electrofishing and passive telemetry. The effect of training in aquaculture was not detected in pond conditions, but the fish that survived direct predation from pike for two months in the semi-natural treatment were more likely to survive in the wild than their aquaculture-reared counterparts. In the laboratory environment, asp responded to predator chemical cues with an increase in shoal cohesion and swimming activity, which demonstrate their ability to detect chemical cues. However, exposure to more fluvial conditions did not result in increased critical swimming speed. The study suggests that conditioning tested in aquaculture may not be sufficient to prepare fish for the wild, while exposing fish to direct predation could increase fish survival.