Study of Acoustic Backscatter Properties of Walleye Pollock (Gadus chalcogrammus, Pallas, 1814) in Deep-Water Areas of the Sea of Okhotsk in Spring

Abstract— According to spring trawl-acoustic surveys, estimates of the nautical area scattering coefficient (population density) for walleye pollock measured in situ in deep-water areas of the Sea of Okhotsk are significantly lower than those calculated based on trawl catches. The discrepancies are attributed to the physiological state of the recorded aggregations. After spawning, walleye pollock individuals have minimal indicators of gonad development and swim bladder filling, which contributes to a weakening of their backscatter properties. The relationships between the target strength of postspawning walleye pollock individuals and their zoological length have been studied in natural environmental conditions. The regression equation for target strength (TS) of postspawning walleye pollock in the Sea of Okhotsk, normalized to 20 log(L) with an average length (L), takes the form: \(TS = 20\log \left( L \right) - 71.5~\) (at 38 kHz). The derived target strength values for postspawning walleye pollock are 5.5 dB lower than those for feeding walleye pollock, and 2.5 dB lower than those for prespawning and spawning specimens. The application of the refined target strength will enhance the accuracy and reliability of acoustic assessments of the walleye pollock abundance in the Sea of Okhotsk during the spring season.