Preferable summering habitat of returning adult masu salmon in the natal stream

The preferable summering habitat of returning adult masu salmon Oncorhynchus masou Brevoort was examined in 31 study pools of a small stream in northern Hokkaido, Japan. A total of 23 salmon were captured and were present in only 10 of the study pools. Female salmon were found to be significantly larger than male salmon. Habitat characteristics were expressed by six environmental variables: mean water depth, maximal cover size, mean current velocity, pool volume, substratum coarseness and cover area. There were no significant differences between the pools with salmon and those without salmon in terms of substratum coarseness and cover area. However, significant differences were found in mean water depth, maximal cover size, mean current velocity and pool volume. The analysis found that the pools with salmon were discriminated from those without salmon at a level of 93.5% which was determined by the six environmental variables. Our results revealed that the returning adult salmon selectively used the pools with greater depth, large cover and slow current velocity as their preferable summering habitat. The results also found that the summering habitat of salmon can be estimated by measuring only six environmental variables. Therefore, these variables can be considered useful parameters in determining the summering habitat components of returning adult masu salmon and can contribute to the conservation and restoration strategy of this species by providing a better understanding of the relationship between fish and their habitat.