Use of a growth model to assess the suboptimal growth of Atlantic salmon parr in a subarctic river

This study tested the hypothesis that Atlantic salmon parr grew suboptimally according to the water temperature regime during the ice‐free season (from May to September) in the subarctic River Reisa (northern Norway). A laboratory‐based growth model was used to estimate the optimal growth which was then compared with the observed growth of the parr in the river. Water temperatures were recorded every second hour with a submersible data logger, and observed and predicted growth of several age classes were compared at two nearby nursery habitats. Except for parr aged 2, most age classes showed no significant change, or even a weight loss, from May to July, whereas from July to September, positive growth was found for all age classes coinciding with favourable temperatures above a critical temperature for growth. The parr growth rates were suboptimal, being lower than the predicted values, but values for observed and predicted body mass were not significantly different in five of 12 comparisons. Hence, the fish were sometimes unable to grow according to the optimal baseline set by the temperature regime. Therefore, factors other than temperature, such as limited food resources and resource competition, may also be a major determinant of Atlantic salmon parr growth. Our study demonstrates how growth predictions based on mathematical models may be employed by fish ecologists to identify growth deficiencies in wild fish populations.