Movement of the Juvenile Shirauo (Salangichthys microdon) in a Canal between the Main Stream of the Ishikari River and an Oxbow Lake

Abstract

Shirauo Salangichthys microdon inhabiting the Ishikari River hatched in the estuary during May and June, and migrated upstream on the salt wedge. It seemed that juvenile Shirauo moved to the oxbow lake through the Shibi Canal after reaching the mouth of the canal located 5.5 km from the river mouth in July. To reveal the immigration processes of juvenile Shirauo to the oxbow lake through the canal, water current and daily change in fish distribution int he canal were observed in July, 1995-1997. Vertical and horizontal distributions of juvenile Shirauo changed mainly with the flow directions and daily change, and two immigration processes were suggested from the changes of juvenile distribution. One process was that juveniles stayed around the canal mouth in the main stream during ebb tide, and they were carried to the oxbow lake by the fast flood tide in the deeper layer at center of the canal. The other process was that juveniles distributed near the bank int he canal to avoid the fast flow during the ebb tide and concentrated int he center with fast flow during the flood tide. These processes would be reasonable for immigration in to the oxbow lake.