7 - Seawater to Freshwater Transitions in Diadromous Fishes

Abstract

Diadromous fishes have evolved mechanisms to transition between seawater and freshwater environments. For anadromous species, and probably also amphidromous species, movement into freshwater is obligate for successful spawning. Landlocked populations that do not migrate to seawater indicate that movement into marine habitats for feeding and rearing is facultative in anadromous species, but some amphidromous species may be obligate. For catadromous species, movement into freshwater is facultative as fish seek areas of greater productivity, but some individuals may complete their life cycle without entering freshwater. Conditions for spawning, therefore, appear to be more strongly dictated than habitats for feeding. This chapter describes the behavior and timing of movements from seawater to freshwater in diadromous fishes, the physiological and endocrine changes that enable successful transition between the two environments, mechanisms for selecting freshwater habitats, and the effect of movement between seawater and freshwater on genetic population structure of diadromous species.