Ontogeny of swimming performance of hatchery‐reared post‐larvae and juvenile fish: a case of two threatened Mediterranean species

Swimming performance is a well‐established key physiological parameter of fish that is highly linked to their fitness in the wild. In the context of fish restocking purposes, it therefore appears crucial to study this specific behaviour. Here, the authors investigated intra and interspecies differences in the swimming performance of hatchery‐reared post‐larvae and juveniles belonging to two Mediterranean candidate threatened species, the common dentex, Dentex dentex (Sparidae), and the brown meagre, Sciaena umbra (Sciaenidae), with body sizes ranging from 8 to 37 mm total length (TL, from 24 to 58 days post‐hatch). The swimming abilities were estimated through the calculation of their critical swimming speed (U (crit)), their relative U (crit) and their Reynolds number (R (e)). Two different patterns were observed between D. dentex and S. umbra, showing a different effect of ontogeny on the performance of both species. The relative U (crit) of S. umbra decreased linearly through ontogeny, whereas the relative U (crit) and U (crit) of D. dentex increased linearly through the range of sizes tested. The ontogenetic change in U (crit) of S. umbra occurred in two stages: a first stage of sharp improvement and a second stage of a slow decrease in performance. Both stages were separated by a breakpoint that coincided with the appearance of a refusal to swim behaviour that occurred shortly after the end of metamorphosis and can potentially be associated with the establishment of this species sedentary behaviour. The swimming performance of both species showed ontogenetic differences. Sciaena umbra had the highest relative performance when its body sizes were the smallest, whereas D. dentex showed the highest relative performance when its body sizes were the largest. These results will be linked to future research on both of these species concerning their escape, exploratory and predatory behaviours, and for restocking purposes to draw a more realistic overview of hatchery‐reared juvenile performance. Knowledge of both species’ behavioural and swimming performance through ontogeny is important to consider when using hatchery‐reared fish juveniles for restocking, as size‐at‐release can have a large impact on fish survival and thus on restocking success.