Target strength measurements of fish

Abstract

During the summer of 1971, target strength measurements of fish were made at two frequencies, 38 kHz and 120 kHz. The relationships between dorsal aspect target strength and fish length were worked out for four species. The results for the gadoid fishes were in accordance with the results reported from previous studies and also in accordance with observations from field measurements. For fish of lengths 6-12 cm, the dorsal aspect target strengths of gadoids and clupeoids are approximately equal. For bigger fish the dorsal aspect target strength of clupeoids was found to be lower than of the gadoids. No significant differences in side aspect target strengths were found between the two groups. As the dorsal aspect target strength of fishes depends heavily on the inclination of the fish, more information on fish behaviour will improve both abundance estimation and length determination by acoustic equipment.