Many artificial structures exist to control flood in rivers. And fish ways are to secure migration of fish across such weir. Now, we must consider artificial structures and fish ways from a viewpoint of ecosystem conservation in rivers because many fishes need to migrate (Nakamura 1995).

A Japanese sculpin, Cottus reinii is endemic to Lake Biwa and has been designated as an endangered species IB. The species is an amphidromous fish between Lake Biwa and its tributaries. But there is little information on its distribution in a river and migration pattern.

I investigated distribution of C.reinii in Ado, Kamo, Wani and Inukami rivers around Lake Biwa to analyze if artificial weirs prevent the migration of C.reinii .

From May 4th to November 26th 2010, I sampled fish by hand by 3 to 5 people during 20 minutes at lower and upper areas of 15 weirs. I measured standard length of fish and recorded height and slope of weirs. Measured fish were released in the originally collected area.

C.reinii was able to pass over weir with less than 45 cm height, but unable to go up 120 cm more height. C.reinii could pass go up at 90°of slope.

Thus, for this fish, the height of weir seems to be critical to secure its migration along river, and change below at least 120 cm height of artificial structures.