Effects of captive-breeding conditions on metabolic and performance traits in an endangered, endemic cyprinidontiform fish

Captive breeding and stocking are commonly employed strategies for enhancing fisheries and conserving endangered fish species. However, hatchery-raised fish often exhibit reduced performance in the wild, displaying alterations in physiological, morphological, and behavioral traits. We tested for differences in swimming capacity and metabolic traits between wild and hatchery-reared individuals of the Spanish toothcarp (Aphanius iberus) from two different populations. Furthermore, we experimentally tested if these changes translated into fitness differences after their stocking into the wild. There were significant differences in swimming capacity and metabolic traits between wild and hatchery-reared individuals and also between the two populations. Captive-bred individuals displayed consistently lower metabolic rates than wild individuals from the same population (30-76% lower). Critical swimming speed rather differed between the two populations. Sex-specific differences were observed in maximum and standard metabolic rates, with wild individuals and females generally exhibiting higher values but with some exceptions. During a three-month experiment, survival rates did not significantly differ between wild and captive-bred fish. Captive-bred individuals started smaller but exhibited rapid growth during the experiment. Initially, larger captive-bred fish had lower body condition than their wild counterparts, but these differences progressively diminished. In summary, captive-bred individuals of this fish species showed lower metabolic rates, although the differences with wild individuals slightly depended on sex and size.