The results of the spring trawl-acoustic surveys indicated that the estimates of the backscatter coefficient (density) for walleye pollack measured in the deep-water areas of the Okhotsk Sea 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. Relationships between the target strength of post-spawning walleye pollock individuals and their zoological length are investigated in natural environmental conditions (in situ). The regression equation for target strength (TS) of post-spawning walleye pollock in the Okhotsk Sea, normalized to 20log(L) with an average length (L) takes the form: TS = 20log(L) – 71.5 (at 38 kHz). The derived target strength values for post-spawning walleye pollock are 5.5 dB lower than those for feeding walleye pollock, and 2.5 dB lower than those for pre-spawning and spawning walleye pollock. The application of refined target strength will enhance the accuracy and reliability of acoustic assessments of the walleye pollock abundance in the Okhotsk Sea during the spring season.
Kuznetsov et al. (Wed,) studied this question.