Growth, survival and interspecific social learning in the first hatchery generation of Eurasian perch (Perca fluviatilis)

Härkönen, L., Hyvärinen, P., Mehtätalo, L. and Vainikka, A.

New insights into present-day cultivation practices are needed when new species are taken into aquaculture. Understanding different features of survival selection operating on wild fish in artificial hatchery environment is topical also in the contexts of fish conservation and reintroduction programs. We used wild Eurasian perch (Perca fluviatilis) to produce the first generation hatchery stock, and studied survival and growth of the offspring at the age of two years through the transition from a seminatural pond to the hatchery, and the first growth season in the tank environment. First, we tested whether hatchery-raised brown trout (Salmo trutta) could be used as tutors for tank-naïve perch in training to accept commercial dry feeds, and thereby increase survival and growth of the perch in tank culture. Second, we tested whether supplementing the dry feed with minced prey fish would increase perch survival and enable faster growth compared to dry feed diet only. The brown trout tutors did not increase survival or growth of tank-naïve perch, but both improved markedly if the perch were habituated to tank culture conditions already during cold-water seasons. Survival and consequent biomass gain were the highest when the pre-habituated perch were on the fish-supplemented dry feed diet. Survival on the fish-supplemented diet was significantly higher (73.1%) compared with the survival on the dry feed diet (44.4%). The perch exhibited bimodality in final size distribution in both experiments. Although the mean body size of the fast-growing perch did not depend on the diet, the total biomass gain during the first growth season was substantially higher on the fish-supplemented diet (+ 104%) than on the dry feed diet (+ 41%). The difference was mainly due to higher survival of the large perch on the fish-supplemented diet but also because the fish supplement supported significant growth among the small fish. To conclude, commercial feeds may exert a strong survival selection on wild fish in hatcheries, which can be mitigated by adding natural elements to the diet. Although perch ability to copy the feeding behaviour from interspecific tutors was negligible for large-scale aquaculture purposes, our experiment encourages further studies on the potential of inter- and intraspecific social learning in the context of feed training in aquaculture.