Mitochondrial efficiency impacts nocturnal sheltering in juvenile salmon ( <i>Salmo salar</i> ), affecting the trade-off between foraging and predation risk

At cold winter temperatures, juvenile salmonids typically spend much of their time sheltering from predators, which negatively impacts foraging for food. Previous work shows that inter-individual variation in mitochondrial efficiency explains variation in food intake, growth and metabolic rate. Here, we examine whether inter-individual variation in mitochondrial efficiency predicts sheltering as a proxy of foraging patterns for overwintering juvenile Atlantic salmon (<i>Salmo salar</i>). PIT-tagged salmon were housed individually under winter conditions, and their use of a custom-built shelter was recorded automatically. In line with the previous research and estimates of relative predation risk, fish showed a broad preference for sheltering during the day and emerging to feed at night. However, they exhibited marked among-individual variation in their use of shelter, which was unrelated to body size but was predicted by mitochondrial function: there was a positive relationship between muscle mitochondrial phosphorylation efficiency and proportion of time spent in the shelter during the night but not during the day. Individuals with the most efficient mitochondria at producing ATP were thus able to spend more time sheltering from predators. This suggests that individual heterogeneity in cellular function may drive variation in the trade-off between foraging and sheltering, which has implications for selection pressures acting on wild populations.