SPAWNING BEHAVIOR AND SIZE-ASSORTATIVE MATING OF JAPANESE CHARR IN AN ARTIFICIAL LAKE-INLET STREAM SYSTEM

We describe the spawning behaviour of Japanese charr, Salvelinus leucomaenis, which have begun to change their life history style from fluvial to lacustrine-fluvial after the construction of an artificial lake by damming. There was a discrete size distribution between mature lake run individuals (larger body size) and mature stream resident fish (smaller body size). Lake run males use pairing tactics, while stream resident males use satellite or pairing tactics depending on the presence of lake run males. Sneaking success of stream resident males is low relative to pairing success of lake run males. Pair formation is strongly assortative by body size and consequently by type. We suggest that both male-male competition and female choice of similar sized males, play an important role in pair formation. We hypothesize that the number of stream resident type relative to lake run type, especially stream resident females, will decrease in future years.