A variety of movements characterize the behavioral plasticity of Atlantic s
almon (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 spr
ing. Smelting is an adaptive specialization for downstream migration, seawa
ter 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 toler
ance, 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 affec
ted by a limited period of readiness (a physiological "smolt window") and t
he timing of seawater entry with environmental conditions such as temperatu
re, food, and predators (an ecological "smolt window"). Smelt development i
s adversely affected by acidity, pollutants, and improper rearing condition
s, and is often more sensitive than other life stages. Unfortunately, the m
igration corridor of smelts (mainstems of rivers and estuaries) are the mos
t heavily impacted by pollution, darns, and other anthropogenic activities
that may be directly lethal or increase mortality by delaying or inhibiting
smelt migration.