Skewed reproductive success among male white‐spotted charr land‐locked by an erosion control dam: Implications for effective population size

Abstract

In 1996 and 1997, the spawning behavior of fluvial white‐spotted charr Salvelinus leucomaenis, was observed in the upstream area of an erosion control dam. A small number of males with relatively large body size mated successfully with females as a pair, while almost all satellite males did not sneak successfully, resulting in a non‐random mating system. The low sneaking success of subordinate males, in addition to the monopolization of spawning opportunities by a few dominant males, is one of the most important causes of skewed reproductive success among males. The total number of adult fishes in the study area (N: approximately half of the whole tributary above a dam) was estimated as 148 and 102 in 1996 and 1997, respectively. Based on these findings and some further assumptions, the estimated effective population size (Ne) was low in both years. The Ne/N ratio ranged from 0.33 to 0.36 in both years. In addition to reduced population size by construction of an impassable dam, the above‐dam population suffered low Ne due to skewed reproductive success among males. The low Ne may be one cause of extinction in above‐dam populations of fluvial charr, especially just after the construction of impassable barriers.