The effect of temperature on growth and early maturation in a wild population of Atlantic salmon parr

Atlantic salmon Salmo salar parr exposed to heated water effluent were larger than parr living upstream of the discharge. There was no effect, however, of temperature on the incidence of male parr maturation, despite the link between growth and maturation found in other studies. Mature parr upstream of the effluent were smaller than mature parr downstream of it, suggesting a higher size threshold for maturation in those parr exposed to the heated water. The small distance between upstream and downstream sampling sites probably precluded genetic differences between the parr, thus implying an environmental influence on the threshold size above which male parr matured.