Ayu (Plecoglossus altivelis altivelis), has one of important freshwater fisheries targets. Landlocked ayu in Lake Biwa have two groups with different migratory patterns, large fluvial-type (“o-ayu”) and small lake-type one (“ko-ayu”). The former migrates from the lake into inlet rivers from early spring until summer, while the latter remains the lake until autumn and migrates into river only for spawning.

Shiga Fisheries Experiment Station conducts investigation spawning of ko-ayu in tributaries of L.Biwa but only for 11 on main rivers every year.

This study was performed at 26 rivers around L.Biwa including 10 main spawning rivers, from September to November in 2011.The density of egg and its survival were recorded We compared them with environmental factors(current velocity, size distribution, etc.).

As a result, there were many rivers where Ayu intensively spawned than the main rivers. In addition, the number of spawned eggs was significantly related with the distance from the river mouth to the first riffle(spawning site).