Depth dependence of target strength of live kokanee salmon in accordance with Boyle's law

The target strength (TS) offish varies with a number of factors, especially the presence or absence of a swimbladder. This study investigated the eVect of the swimbladder on the TS of live kokanee (Oncorhynchus nerka) in relation to depth. Experiments were conducted at Lake Kuttara, Hokkaido, Japan, which is abundant with kokanee. Fish samples were caught with a fishing net. The live fish were suspended on the sound beam axis, and tilted from "50) (head-down aspect) to +50) (head-up aspect). The dorsal aspect TS functions were measured at 50 kHz. Measurements were performed when the fish were forced to change depth rapidly. The TS values were measured as a function of tilt angle and depth, and the maximum dorsal aspect TS (TSmax) values and the averaged dorsal aspect TS (TSavg) values were calculated and compared. TSavg values were calculated with respect tofish tilt-angle distribution, which was assumed to have a mean value of "5) and a standard deviation of 15). Both TSmax and TSavg decreased with depth in accordance with Boyle’s law, i.e. a reduction rate of 6.7 dB per 10 atm was observed. ? 1996 International Council for the Exploration of the Sea