Abstract
Acoustic surveys for studying the behavior of kokanee Oncorhynchus nerka in LakeKuttara, Hokkaido, have been conducted over the past ten years and several interesting results have been obtained. Kokanee were recorded on echograms as school echoes in the daytime and as numerous individual echoes in the nighttime. Their swimming layer changes seasonally in accordance with the depth change of thermocline in the lake. Kokanee also have a diurnal behavioral pattern. Namely, they form schools after sunrise and they disperse after sunset. The shifting process from individuals to schools or from schools to individuals is strongly related to illuminance, and it takes 50 minutes at sunrise and sunset for the change to occur. The swimming speed of kokanee in the daytime is faster than that in the nighttime, and the fastest speed of the day occurs at twilight. Average swimmingspeeds were estimated to be 57cm/s in the twilight, 48cm/s in the daytime and 20cm/sin the nighttime periods, respectively.
