Abstract
The diurnal variation of the dorsal aspect target strength of encaged fish was investigated, and an attempt was made to estimate the fish tilt angle distributions. The experiment was conducted at Lake Kuttara in Hokkaido. In this lake, kokanee Oncorhynchus nerka are the dominant species. The net cage was made of nylon monofilament, had a volume of approximately one cubic meter, and was nearly acoustically transparent. Five kokanees, caught by gill net, were released into the net cage. During the experiment periods, the net cage was kept at a depth of 10m under the transducer which was driven by a 50 kHz transmitter. About 600 echoes from encaged fish occurring over a 24 hour period were measured and analyzed. The results obtained were as follows:
1) The shape of the dorsal aspect target strength probability density functions (PDFs) of encaged fish showed a large variation before and after sunrise and sunset.
2) Diurnal variation of the averaged dorsal aspect target strength of encaged fish was about 18 dB.
3) Some of the observed target strength PDFs agree approximately with estimated target strength PDFs based on fish tilt angle distribution which was assumed to truncated normaldistribution.
