Acoustic Counting Method of Upstream Juvenile Ayu Plecoglossus altivelis by Using DIDSON

Plecoglossus altivelis, often called ayu in Japan, is one of the important inland aquatic resources. Ayu is a typical amphidromous fish migrating between coastal waters and rivers. In order to assess the abundance of ayu, the direct visual count has mainly been taken so far. Unfortunately, this kind of counting method is not only labor-intensive, but also is affected by weather conditions and turbidity. Recently, an ultrasonic instrument called DIDSON (Dual-frequency IDcntification SONar), which can provide almost-video-quality images to identify objects even in turbid water, has been found applicable for fish counting. This instrument is useful for counting big fish which is several dozen centimeters large. However, as for the small fish whose length is only 8 centimeters or less, the counting becomes technically difficult because its echo image is too weak and too unstable to identify fish from images and to track it. In this paper we present a method for counting juvenile small size ayu that migrates upriver using a DIDSON. We first subtract the stationary background from the acoustic image and remove noises. Then we track the remaining targets. After the removal of the garbage going downstream, we reached the correct counting of the upstream fishes. The above acoustic method for counting young small size ayu has been proven to be useful and effective by the experiment conducted on May 31st, 2006.