The Adaptive Significance of Variations in Life History among Local Populations of Atlantic Salmon in North America

Abstract

In this paper variations in life history data among local populations of Atlantic salmon have been examined. The following patterns emerge: (1) The mean age of first spawning increases with the difficulty of upstream migration as estimated by the distance the fish ascend into freshwater. Other indices of river harshness yield similar results. (2) The effect of commercial fishing has been to eliminate larger and older fish from the run. Available evidence suggests that this has selected for an early age of first return on the Miramichi River and that the frequencies of genes coding for different ages of first spawning have been altered. (3) The mean age of first spawning is positively correlated with marine growth rates after the grilse stage. Rapid growth at sea subsequent to the grilse stage is associated with delayed reproduction; slower growth with an earlier age of first breeding. This result suggests different paths of high—seas migration. (4) The variability about the mean age of first spawning first increases and then decreases as one moves north over the salmon's range from Maine to Ungava. In addition, we have observed that these results are in accord with predictions made by recent theoretical analyses of the optimal reproductive response to differing environmental conditions. We, therefore, conclude that the observed patterns of variation in life history are adaptive. We further take the agreement between theory and nature as a validation of the hypothesis that populations will in general differ from each other in the manner of their respective optima.