On the depth dependence of fish target strength

Abstract

Data on the depth dependence of the target strength of live kokanee (Oncorhynchus nerka), a physostome, at 50 kHz (Mukai, T., and Iida, K., ICES J. mar. Sci., 53: 245-248 (1996)) have been reexamined. In the cited study, data collected over a range of depths on a total of 50 tethered, anesthetized specimens were averaged at each depth, mainly at 5, 10, 20, and 40 m, then fit with a model consistent with Boyle's law. Here the data are analyzed for each individual fish, and a variety of models are used to fit the respective data set. The goodness of fit is gauged by the standard error of the regression. The applicability of Boyle's law, or that giving the inverse pressure-volume relationship of a constant mass of gas under isothermal conditions, is examined in the light of the several fits and is seen to be confirmed. In a speculative application, the depth dependence for kokanee is assumed also to describe that for herring. Incorporation of this into the standard target strength equation for Norwegian spring-spawning herring yields the following expression: TS=20 log l - 67.3 -6.7 log (1+z/10), where TS is the target strength in decibels, 2 is the mean fish length in centimeters, and z is the depth in meters.