Geographic variations in life-history characteristics of white-spotted charr (Salvelinus leucomaenis)

Abstract

Breeding-population structure, age and size at maturity, age and size at smolting, and riverine growth rate were investigated for three populations of white-spotted charr (Salvelinus leucomaenis) in the Nairo, Haraki, and Takahara rivers in Japan. The breeding populations in the Nairo and Haraki rivers on Hokkaido Island exhibited size polymorphism, comprising smaller mature parr and larger migrants. On the other hand, the population in the Takahara River on central Honshu Island showed a monomorphic structure, being composed of mature parr only. Maturation at the parr stage was positively associated with body size. The patterns of length-specific maturation ratios in the three rivers were similar, although females matured at a larger size than males. Smolt sizes differed between the polymorphic populations in the Nairo and Haraki rivers, but size relationships between the sexes were similar in the two populations. The incidence of smolts of a given length within populations was much higher in the northern Nairo River population than in the more southerly Haraki River population. These observations indicate that among-population variation in the structure of white-spotted charr populations might be related to differences in the relative proportions of mature parr and smolts in the population. We suggest that the relative difference in size thresholds between smolting and maturation determines the relative incidence of mature parr and smolts, both within and among populations and between the sexes, which in turn regulates the breeding-population structure.