The influence of energetic requirements on the preferred temperature of overwintering juvenile Atlantic salmon (Salmo salar)

We used a horizontal temperature gradient to investigate the effect of alte rnate life history strategies and nutritional state on the preferred temper ature of overwintering juvenile Atlantic salmon (Salmo salar L.). Contrary to our prediction, there was no significant difference in final preferred t emperature between juvenile Atlantic salmon that will migrate to sea the fo llowing spring (early migrants) and those that show reduced growth and dela y migration for at least another year (delayed migrants). Both migrant grou ps preferentially selected relatively low temperatures (< 10 degreesC), lik ely owing to their low appetite and growth rates. Food deprivation resulted in a significant increase, rather than our predicted decrease, in the fina l preferred temperature of the juvenile Atlantic salmon of approximately 2 degreesC. We suggest that this is due to the need for an increased foraging effort to offset the projected energy deficit later in the winter. The fin al preferred temperature of delayed migrants increased from winter to sprin g, as predicted, coincident with increases in natural food availability and endogenous seasonal increases in appetite and growth rates. We conclude th at the preferred temperature of overwintering juvenile Atlantic salmon may be influenced by future energetic requirements rather than the current leve l of energy reserves.