The relationship of target strength (TS), tilt angle and the effect of swimbladder resonance were examined using a theoretical scattering model for Maurolicus japonicus (30.4-46.8 mm standard length (SL)) at 38 kHz and 120 kHz, based on swimbladder measurements. The effect of tilt angle on TS was seen at 120 kHz, but was minimal at 38 kHz. The effect of resonance on TS was clear at 38 kHz, but was minimal at 120 kHz. This was due to the small size (average 11 % of SL) and oval shape with a relatively high aspect ratio (average 0.48) of the swimbladder. Considering the effect of resonance, the TS-SL relationships at 38 kHz were estimated: TS = 10.0 log₁₀SL - 61.3 at 200 m depth. Considering the effect of tilt angle, the average TS (TS_avg) -SL relationship at 120 kHz was estimated: TS_avg = 16.5 log₁₀SL - 75.1.

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Ayu is a typical amphidromous fish that migrates from coastal waters to rivers. In order to estimate the number of ascending ayu, direct visual counting has mainly been performed thus far. Unfortunately, this counting method is labor intensive, and its results are affected by weather conditions and water turbidity. Even the presence of persons acting as counters on river banks may affect the ayu's movements. Recently, a dual-frequency identification sonar (DIDSON), which can provide near-video-quality images for identifying objects even in turbid water, has been used for fish counting. The counting function is simple and only capable of counting big fish that are several dozen centimeters long and produce good quality echo images. However, counting small fish with lengths of less than 8 cm becomes technically difficult; their echo images are weak and unstable, and therefore, it is difficult to extract the fish from the images. Fallen leaves flowing downstream also cause miscounts. This paper presents a fully automated acoustic method to count juvenile small-sized ayu by using a DIDSON image, in which a Kalman filter algorithm was designed to track moving objects and distinguish ayu from fallen leaves. Experimental results have shown that the proposed counting method is useful and effective.

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