Plasma prolactin and cortisol concentrations of stressed coho salmon, Oncorhynchus kisutch, in fresh water or salt water.

Juvenile coho salmon, Oncorhynchus kisutch, adapted to fresh water or seawater were either acutely handled or continuously stressed by severe confinement. Chronic stress, independent of external salinity, caused a gradual increase in the concentration of circulating prolactin that persisted for 1 to 5 days but lagged behind the cortisol response which peaked much more rapidly and remained elevated. Acutely stressed fish showed a rapid, more transient increase in plasma cortisol titer with no apparent effect on prolactin. Confinement appeared to be more stressful to fish in salt water than to those in fresh water, as judged by their sodium regulatory ability, hormone profiles, and mortality. Stress always elevated plasma prolactin concentrations, regardless of medium or developmental stage.