Energetic state and the continuum of migratory tactics in brown trout (Salmo trutta)

Alternative migratory tactics, like partial migration, are common in many taxa. The proximate and ultimate drivers underpinning these strategies are unclear, though factors like condition and energetic status have been posited as important predictors. We sampled and PIT tagged 1882 wild brown trout prior to the first so-called decision window, and explored the links between migratory tactics (residency, autumn or spring migration) and body metrics (length and condition), lipids (triglycerides and cholesterol), and sex, in 150 randomly selected individuals. We found that more females adopted the autumn and spring migration tactic than males, while more males adopted the residency tactic than females, likely reflecting sex-biased benefits in anadromy. We also found that autumn migrants were in poorer condition prior to the presumed first decision window than spring migrants and residents. Lastly, we found that both condition and cholesterol were positively correlated to the timing of migration, such that individuals in poorer condition and/or with lower cholesterol migrated earlier. Collectively, these results suggest that energy depletion is an important factor in determining migratory strategy, including timing.