Reexamination of migration patterns of masu salmon Oncorhynchus masou inferredfrom new otolith techniques

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The habitat shifts of the juvenile of masu salmon, Oncorhynchus masou, were studied at four streams in Hokkaido. Otolith Sr:Ca ratios of smolts in the Betsubetsu, Ninaibetsu and Moshosanbetsu rivers, showed low values, suggesting that these fishes remained in freshwater habitat until the smolt stage, while the ratios of the smolts in the Tokotan River increased after the fry stage suggesting that they might have experienced the brackish water habitat. A similar pattern was observed in all of the three years studied. The Tokotan River is the steepest and the stream width is narrow causing rapid flow, although the annual water temperature of the Tokotan River is intermediate among streams studied. These results suggest that the topography and resultant flow conditions may influence the habitat shifts of masu salmon, and the juveniles in the Tokotan River may be flushed out of the river with the strong flows from melted snow in early spring.