Introduction

The anadromous life history entails early development in fresh water followed by movement to the ocean and subsequent return to fresh water for spawning. In contrast to other euryhaline fish, in which movement from fresh water to sea water is often frequent, anadromy usually results in seasonal movement of juveniles from fresh water to sea water. For many (but not all) anadromous species seaward migration occurs only once in an individual's life. Some anadromous salmonids have evolved a preparatory adaptation (known as the parr–smolt transformation or smolting) in which salinity tolerance and other adaptations for ocean life develop at the time of seaward migration (McCormick & Saunders, 1987; Hoar, 1988). The relative ‘strength’ and developmental stage of the parr–smolt transformation varies widely among salmonids. Although smolting is by definition strictly a salmonid phenomenon, similar preparatory adaptations may exist in other anadromous species (Youson, 1980) but to date their presence has not been widely examined.

For most anadromous species seaward migration is highly seasonal, often spring or fall, at a time of rapid temperature change. Under these conditions temperature may be an important factor for determining the timing of development and migration, and may impose further physiological challenge to animals whose osmotic tolerances are being altered. Here we will review the known effects of temperature on the osmoregulatory physiology of anadromous fish with emphasis and speculation on aspects that may affect the survival and distribution of these important fish.