Eels are marine teleosts that have ancestrally evolved a continental growth phase during which diadromy is facultative. This migratory plasticity reflects a fitness trade-off between search for the most productive habitats and competition avoidance favoring shift to less productive areas, while migration costs (increased mortality and reduced growth) hinder movements. To cope with these conflicting selective pressures acting in heterogeneous and unpredictable environments, eels have evolved a conditional evolutionarily stable strategy (ESS) with alternative tactics as a regulation mechanism for migratory plasticity. In this model, diadromy is considered as a threshold reaction norm to an individual's energetic status. In the population, this reaction norm evolves rapidly in response to changes in the cost/benefit fitness ratio of diadromy. This conditional ESS model may explain how the collapse of freshwater eel stocks results not only from decreased recruitment but also from an adaptive drop in the proportion of the diadromous phenotype in populations. Hence, I suggest that the decline in populations of eels has been less steep in salt than in freshwater habitats.