Estimation of dorsal aspect target strength of deep-water fish using a simple model of swimbladder backscattering

A practical model for estimating target strength of gas-filled swimbladdered fish is described using the scattering function of a finite cylinder. Two equivalent cylinders are derived from measurements of swimbladder length, surface area, and volume. The scattering functions of both equivalent cylinders were tested using data available for gadoids, then applied in the estimation of target strength of deep-water hoki (Macruronus novaezelandie). Molds of 23 hoki swimbladders were constructed for measurements of their internal dimensions. Estimates of target strength using the scattering function of the equivalent surface area cylinder agree closely with estimates by other methods for gadoids. Estimates using the scattering function of the equivalent volume cylinder are lower, but approach the former estimates when the swimbladders are fully inflated. The advantage of using the latter method is the simplicity in measuring the swimbladder volume. The dorsal aspect target strength of hoki were estimated at four frequencies, 38, 49, 70, and 120 kHz for several distribution functions of fish tilt angle.