In situ target strength measurement for acoustic fish stock assessment

Abstract

In an acoustic fisheries assessment survey, the target strength distribution of the fish being surveyed is an important parameter, primarily as a factor to scale the output of an echo integration system to fish density; however, good techniques to evaluate target strength of the surveyed fish are not readily available. Results of target strength measurements using tethered or caged fish are available but, because little is known about the orientation and aspect of the surveyed fish population, the scaling of echo integration to fish density using these results is questionable. In situ estimation techniques are the most promising alternative. Indirect in situ methods which statistically convert echo strength distributions to target strength distributions are available but often provide poor results. A dual beam method (J. E. Ehrenberg, Proc. 1974 IEEE Conf. Engr. Ocean Environ. 1, 152–155) utilizes a dual beam transducer system and corrects echo strength to target strength for each detected single fish target. Unlike the indirect techniques, the dual beam method allows the effect of an inherent bias caused by a system noise induced threshold to be reduced. Field results using a standard target (ping‐pong ball) and fish targets are reported and comparisons with theoretical predictions are made.