Threatened fish populations worldwide are relying on stock supplementation by hatchery-reared fish. Although stocking with non-native fish may lead to hybridization, thereby disrupting local adaptations, it could also improve the adaptive potential of small populations by increasing variability in essential behavioral traits. In this study, we crossbred the critically endangered Finnish landlocked salmon with a geographically close Baltic anadromous salmon population to compare boldness-related behavioral traits among the crossing groups. We studied postrelease exploration in seminatural streams and tested stress tolerance of the fish, as reflected by their response during swimming trials and commencement of feeding after the trials. In the stress tolerance experiments, we compared fish with different rearing backgrounds to gain insight into environmentally induced variability in stress tolerance. When compared with the anadromous salmon, the landlocked salmon showed more active postrelease exploration and higher stress tolerance. The hybrids displayed intermediate behaviors compared with the purebred salmon, indicating heritability of these traits. The landlocked salmon showed less variable exploration than the hybrids with anadromous salmon as the maternal strain, but we did not find rearing-dependent responses in stress tolerance. In summary, our findings suggest that hybridization could help in reversing likely domestication-related negative behavioral traits in the landlocked salmon.
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