The acoustic target strength of live fish

• Published in: ICES Journal of Marine Science
• Main Authors: Goddard, G. C., Welsby, V. G.
• Format: Text
• Language: English
• Published: Oxford University Press 1986
• Subjects: Articles; Canicula; Gadus morhua
• Online Access: http://icesjms.oxfordjournals.org/cgi/content/short/42/3/197; https://doi.org/10.1093/icesjms/42.3.197

Experiments to measure the sonar target strength of free-swimming live fish were made in a sea-flooded quarry, the target fish being constrained within a netting cage which did not affect the tests. The cage was kept at a mean depth of 10 m and the acoustic beam could be trained on it either vertically downwards or at an angle of 22½° to the horizontal. Measurements were made of pulse amplitude, calibrated to an accuracy of ±1 dB, irrespective of the position of the fish in the cage. One thousand successive pulses were recorded in each case and the mean value subsequently computed to give one data point. Species and numbers involved were: cod, Gadus morhua (102); haddock, Melanogrammus aeglefinus (41); saithe, Pollachius virens (8);and dogfish, Squalus acanthius and Scyllium canicula (23). Tests were made at dorsal aspect and at 22½° aspect, at 10, 30, and 100 kHz, respectively, making a total of 6000 individual recorded measurements on each of 174 fish. The results are presented in the form of scatter diagrams and also as empirical regression formulae. An additional experiment investigated the combined target strength of groups of fish containing 2, 4, 8, 16, and 32 individuals, respectively. If each echo were unaffected by the others there should have been a rise of 3 dB per doubling of number of fish. While “shadowing” effects would cause a rise less than 3 dB, the observed results showed an as yet unexplained rise slightly greater than 3 dB per doubling of numbers.