Effects of environmental factors on jumping behaviour of the juvenile ayu Plecoglossus altivelis with special reference to their upstream migration.

The jumping behaviour against a waterfall, of ayu is known to be closely related with the tendency to migrate upstream in a river.In order to determine the triggering mechanism for upstream migration in the ayu, the effects of environmental factors on jumping activity were observed.Jumping behaviour was active in the time corresponding to daytime even under constant light conditions, LL and DD photoperiods, as well as 16L8D.The jumping activity was facilitated by changes in light intensity, a raise in the water temperature, shallow water depth, and deprivation of food.Accordingly, these changes in environmental factors appeared to have the potential to act as a trigger of the migration.