Problems with acoustic target strength measurements of a deepwater fish, orange roughy (Hoplostethus atlanticus, Collett)

Abstract

In situ target strength measurement of fish at 600 to 1200 m depth were made around a spawning aggregation of orange roughy off the east coast of Tasmania in 1992. The target strength data showed many modes, none of which could be definitely and uniquely attributed to orange roughy, partly because the orange roughy avoided the towed body housing the acoustic. Dominant modes at -50 and - 55 dB were attributed to myctophid fishes with standard length modes at 8.2 and 5.3 cm; these fish have gas-filled swimbladders and were undisturbed by the towed body. Small modes at - 44 dB and 31 dB were attributed to the macrourid Coryphaenoides subserrulatus and the morid Halagyreus johnsonii, respectively. The swimbladder of H. johnsonii is gas-filled, while that of C. subserrulatus contains a gas-filled spongy tissue matrix. No evidence was found of a separate mode at - 36 or - 41.3 dB, the previously reported target strength values of orange roughy. Modelling and tethered experiment on orange roughy suggested the target strength range for a 35 cm standard length fish was - 47.2 to - 53 dB. The modelling indicated values at the higher end of the range; measurements taken at depth of a tethered fish indicated the lower end. The dominant mode in the in situ data at - 50 dB (which ranges from about - 48 to - 52 dB) was probably associated with orange roughy as well as myctophids. We concluded that the in situ target strength for a 35 cm standard length orange roughy is between - 48 and - 52 dB. Such a low target strength (compared to other species from the same depth that have gas-filled swimbladders) makes acoustic assessment techniques using echo integration very sensitive to the number of fish with gas-filled swimbladders.