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 predation and reduced starvation risk, which reduces mortality risk.
In the early critical life stage, more active brown trout are better survivors. This finding, which contradicts general assumptions about the balance between predator exposure and food intake, could possibly be due to trout being highly territorial species in which active individuals can claim the best territories. We also find that young trout are likely limited in growth rate by environmental conditions in the wild, as growth compensation following food restriction is possible in the lab, but not realized in natural streams.
KeywordsCompensatory growth State-dependent behavior Salmonidae Behavioral repeatability Survival Activity-mortality trade-off
We thank Anders Hargeby and Robert L. McLaughlin for useful comments on a previous version of the manuscript. We also thank all reviewers for their insightful and constructive comments.
JN and JIJ conceived the experiments. JN, PSC, and JIJ designed the experiments. JN and PSC performed the experiments. JN and PSC analyzed the data. JN and PSC wrote the manuscript, with editorial advice from JIJ.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable international, national, and institutional guidelines for the care and use of animals were followed. The experimental procedures were approved by the Ethical Committee on Animal Experiments in Gothenburg, Sweden (ethical license number 8–2011).
The datasets generated during and analyzed during the current study are available in the figshare repository, DOI: https://doi.org/10.6084/m9.figshare.4789492.
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