Position choice by red-spotted masu salmon: applicability of a bioenergetics model to habitat evaluation of headwater streams in southwestern Japan

Habitat loss and degradation due to human activities are major threats to stream fishes. To predict and/or monitor habitat changes, quantification of suitable habitats for target fish species is indispensable. For drift-feeding salmonids, bioenergetics models that estimate energetic benefit (net energy intake: NEI) can be used for their habitat evaluation. In this study, we examined applicability of a bioenergetics model, which had been developed for Arctic grayling (Thymallus arcticus) in Alaska, North America, to habitat evaluation for red-spotted masu salmon (Oncorhynchus masou ishikawae) in southwestern Japan, by assessing whether their position choice can be predicted by the model. The results showed that the model successfully predicted their position choice in 12 of the total 16 study sections. Further, in two sections inhabited by several individuals, strong positive correlations were found between NEI potential and fish body length, being consistent with a widely reported trend in drift-feeding salmonids that larger individuals occupy more profitable positions. We concluded that the bioenergetics model we tested is useful in evaluating habitat quality for red-spotted masu salmon. Our results suggested that NEI models can be regarded as a site-independent, widely applicable tool for habitat evaluation for drift-feeding salmonids.