Maternal swimming exercise training improves survival and the heritability of thermal tolerance and length in brown trout offspring

Hatchery-reared fish released to sustain wild populations exhibit low survival in the wild. Possible reasons are rearing conditions that preclude fish from displaying their natural swimming behaviour, which negatively affect their cardiorespiratory physiology. This might also make them vulnerable to heatwaves, reducing their survival. Here, we investigate whether the negative effects of captive rearing on cardiac performance and fitness-related traits could be mitigated by maternal exercise in brown trout (Salmo trutta). We reared the mothers with two different exercise training programmes: control (0.12 m*s-1) and exercise (0.41 m*s-1). After 5 weeks of training, exercise-trained mothers showed significantly higher cardiac thermal tolerance, measured by the thermal performance curve for maximal heart rate (fHMAX) and the temperature of onset of cardiac arrhythmia (TARR). Exercise training not only improved cardiac thermal performance and reproductive success in mothers but also enhanced the survival of embryos via transgenerational plasticity. However, maternal exercise did not affect the thermal tolerance in the offspring, as measured by critical thermal maximum (CTMAX), fHMAX and TARR. Interestingly, the heritability and, therefore, the evolutionary potential of body length and thermal tolerance increased in the offspring from trained mothers. Our results suggest that exercise training represents a valuable tool for improving the physiological performance of hatchery-reared fish, having cascading beneficial effects on the next generation. This study strengthens the idea that transgenerational plasticity and Genotype-by-Parental-Environment interactions can be harnessed to improve conservation and restoration programs by producing more resilient offspring.