Performance of wild brown trout in relation to energetic state and lab-scored activity during the early-life survival bottleneck

The early life stage is typically a selective bottleneck during which individual performance is important for survival. We investigated size, energetic state, and activity, in relation to recapture probability in the youngest free-swimming stage of a territorial fish, the brown trout. In two experiments, we induced compensatory growth in wild-caught brown trout fry, using a restriction-refeeding protocol. Upon refeeding in the laboratory, the restricted trout showed compensatory growth in mass, but not in length. During this compensatory growth phase, we released the fish into their native stream habitat and then recaptured them after 1 month to assess survival and growth. Despite not having fully compensated body size at release, restricted fish did not show continued growth compensation in the stream, indicating that the natural environment limits growth capacity during early life. Individual baseline activity was scored in open-field tests before and after food restriction and was found repeatable but not significantly affected by growth manipulations. Under natural conditions, we found a positive association between open-field activity and survival (as indicated by recapture probability), but no significant differences between food-restricted and control fish. Initial body length positively influenced survival in the first experiment (early summer), but not in the second (late summer). These results contrast with the assumption that high baseline activity should be riskier in natural environments. For territorial animals, we hypothesize that activity is associated with high aggression and territoriality, which facilitates access to high-quality territories providing both shelter from prédation and reduced starvation risk, which reduces mortality risk.