Many fishes in Lake Biwa migrate between the lake and its tributaries during their life cycles. Japanese dace, Tribolodon hakonensis , runs to spawning into tributary from L. Biwa from spring to early summer. Though there are not enough information on its migration because of the methodological difficulty to observe the migrating fishes. Fortunately a good amount of information can be provided by the survey at the migrating fishes as a commercial fishing weir, “Yana”, prevents the migration of fishes and traps most of them.

Field surveys were carried out at a fish weir, “Yana”, 1 km apart from the river mouth of River Ado, a tributary located in the northwest of L. Biwa. Fishes trapped into “Yana” were counted every 2 hours over 24 hours. The survey has been from late March to early June in 2006 - 2011.

In this presentation I will report diel activity and yearly fluctuation, and estimate environmental trigger to stimulate spawning run of Japanese dace.