Comparing acoustic model estimations and in situ target strength measurements of juvenile walleye pollock (Theragra chalcogramma)

Abstract

Recently, many acoustic surveys for the juvenile walleye pollock have been done around the Hokkaido, Japan. Dependence of target strength on fish length is a most important factor in the conversion of acoustic data to fish biomass. However, the relation between target strength and length of juvenile walleye pollock is not well investigated. This study examined the target strength length relationship (3–10 cm) for the echo integration measurements of the juvenile walleye pollock at 38 kHz from acoustic theoretical model predictions and in situ measurements. A prolate spheroid model was used to compute the target strength of the fish relative to its orientation. Swimbladder morphologies of live juvenile walleye pollock caught by set net were obtained from transmission of light or soft x‐ray, and converted their morphological parameters. Biological samplings using framed mid‐water trawl were carried out near the spawning area of walleye pollock. During fish samplings, in situ target strength measurements wer...