Target-strength measurements of walleye pollock (Theragra chalcogramma) and Pacific whiting (Merluccius productus)

The importance of knowledge about the target-strength (TS) characteristics of the surveyed fish population to the accuracy of acoustic assessments using echo integration is well known. The most commonly used TS to length (L) relationship for walleye pollock is based on swimbladder morphology measurements and in situ TS measurements TS (in dB)=20 log L−66.0 for fish length in centimeters. For Pacific whiting, TS measurements have not been made over an adequate range of fish lengths to determine the appropriate relationship for use in scaling echo-integration surveys. However, results presented in this paper and elsewhere suggest a smaller TS (about 2 dB for a given length) for 40–60 cm fish. Measurements of the TS of walleye pollock and Pacific whiting were made with a SIMRAD EK500 in the present study. The appropriateness of the currently used TS to length relationship for echo integration scaling is discussed for pollock. The application of a TS to length relationship for Pacific whiting is considered. An attempt to use a system with a transducer lowered through the water column to reduce noise-induced bias is described.