Future migratory behaviour predicted from premigratory levels of gill Na+/K+-ATPase activity in individual wild brown trout (Salmo trutta).

The relationship between premigratory gill Na+/K+-ATPase activity, determined at two dates during spring, and future migratory behaviour was investigated using non-lethal gill biopsies and PIT-tagging in wild brown trout (Salmo trutta) from two tributaries. No significant relationship between future migratory strategy (individuals eventually becoming migrants or residents) and gill Na+/K+-ATPase activity was found in late February-early March. By contrast, in mid-April, a highly significant logistic regression equation identifying the migratory strategy in 93% of the 75 individuals was obtained. The ability of this regression model from the tributaries to predict future migratory behaviour in an independent group of trout caught in early April in the mainstream was evaluated. A threshold probability of migration was used to predict the behaviour of the mainstream individuals as either future migrants or residents. The maximum percentage of correct predictions of future migratory behaviour in mainstream fish was observed at threshold probabilities between approximately 0.15 and 0.45 (corresponding to threshold gill Na+/K+-ATPase activities between 2.7 and 3.7 micromol ADP mg(-1) protein h(-1)), with an average of 91% of the predictions being correct. The present study shows that a non-lethal premigratory biochemical measurement can successfully select individual brown trout with high probability of migration.