Differences in the Impact of a Weir on the Reproductive Activities in White-spotted Charr and Dolly Varden in a Japanese Pond-associated Stream System.

Impacts of a weir on reproductive activities of introduced charrs, white-spotted charr Salvelinus leucomaenis and Dolly Varden S. malma, were investigated in a pond-associated stream system in south-western Hokkaido, Japan. Fish trapping during a breeding season revealed that only a few spawners of both species migrated upstream across the weir. White-spotted charr primarily spawned in two spawning grounds upstream and downstream of the weir. Length-distribution and scale annuli sug gested that all the upstream spawners of the species were stream-residents, whereas downstream spawners were considered to include impoundment migrants. The sex ratio of the downstream spawners was about even, whereas that of the upstream spawners was strongly biased towards males. In contrast, Dolly Varden which had an even sex ratio, were observed to spawn only in the upstream ground. The original strain of white-spotted charr was anadromous form, in which most females gener ally descended to the sea, but that of Dolly Varden is stream-resident. The difference in strains could lead to the male-biased spawning group of white-spotted charr in the upstream reaches of the weir.

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