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

In situ target strength measurements of fish at 600 to 1200 m depth were made around a spawning aggregation of orange roughy oV 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 acoustics. Dominant modes at "50 and "55 dB were attributed to myctophidfishes with standard length modes at 8.2 and 5.3 cm; thesefish 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 experiments 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. ? 1997 International Council for the Exploration of the Sea