Interpopulation variation in reproductive traits of female masu salmon, Oncorhynchus masou

Several life history models predict that larger eggs and lower fecundity should be favored in a low-growth environment. We applied the model of Sibly et al. to seven Japanese populations of masu salmon ( Oncorhynchus masou ) in order to test the predictions that populations in which individual growth rate is low are characterized by larger eggs and lower fecundity. Two populations, Shumarinai and Shikaribetsu, had the lowest growth rates of individuals, largest egg sizes and lowest fecundities of all populations examined. In contrast, the Shiribetsu, Chitose and Uono populations had highest growth rates of individuals, smallest egg sizes and highest fecundities. The Shibetsu and Toya populations were intermediate between these two groups. The gonadsomatic index (GSI) of the Shumarinai and Shikaribetsu populations was smaller than that of all other populations. Correlation analysis indicated that populations with lower individual growth rates had larger eggs and lower fecundities. The results were consistent with the predictions of the Sibly et al. model: increased egg size which results in decreased fecundity is probably an adaptation to low-growth environments. Therefore, in masu salmon, growth differences among populations may explain interpopulation variation in egg size and fecundity.