Fig. 4. Predicting migratory behaviour in wild brown trout in the mainstream (River Lille Aa). Numbers of predicted (broken lines) and actual (solid lines) future migrants and residents in the mainstream (River Lille Aa) as a function of threshold probability of migration (or threshold gill Na⁺/K⁺-ATPase activity), as predicted from the logistic regression equation relating premigratory gill Na⁺/K⁺-ATPase activity in mid-April and future migratory behaviour of wild brown trout from the two tributaries (Haldum Brook and River Granslev Aa).