Movement, migration, and smolting of Atlantic salmon (Salmo salar)

A variety of movements characterize the behavioral plasticity of Atlantic salmon (Salmo salar) in fresh water, including movements of fry from redds, establishment of feeding territories, spawning movements of sexually mature male parr, movement to and from winter habitat, and smelt migration in spring. Smelting is an adaptive specialization for downstream migration, seawater entry, and marine residence. While still in fresh water, smelts become silvery and streamlined, lose their positive rheotaxis and territoriality, and begin schooling. Physiological changes include increased salinity tolerance, olfactory sensitivity, metabolic rate, scope for growth, and altered hemoglobin and visual pigments. Through their impact on the neuroendocrine system, photoperiod and temperature regulate physiological changes, whereas temperature and water flow may initiate migration. Smelt survival is affected by a limited period of readiness (a physiological "smolt window") and the timing of seawater entry with environmental conditions such as temperature, food, and predators (an ecological "smolt window"). Smelt development is adversely affected by acidity, pollutants, and improper rearing conditions, and is often more sensitive than other life stages. Unfortunately, the migration corridor of smelts (mainstems of rivers and estuaries) are the most heavily impacted by pollution, darns, and other anthropogenic activities that may be directly lethal or increase mortality by delaying or inhibiting smelt migration.