Target strength of an oily deep-water fish, orange roughy (Hoplostethus atlanticus) I. Experiments

Target strengths (TS) of orange roughy (Hoplostethus atlanticus) are diffic ult to measure in situ because this species lives at 800-1200 m, has low TS at 38 kHz, shows strong avoidance behavior, and sometimes co-occurs with s pecies having much higher TS. Orange roughy have wax-invested swimbladders and fish are similar to 20% wax esters by weight. Estimates of orange rough y near-dorsal aspect tilt-averaged target strength ([TS]) are controversial , varying from [-39, -35] dB to [-52, 48] dB for a 35-cm fish in different studies. [TS] is estimated using experiments on both live and dead fish, an d new swimming orientation data. Target strength of previously frozen and t hawed fish decline rapidly, stabilizing after similar to 12 h as air and oi l diffuse out of tissues during rehydration. A [TS] of -46.3 dB is predicte d for a 35-cm live orange roughy (bootstrapped 95% confidence intervals = - 48.0 to -45.5 dB). This value is 3.7 dB higher than Australian estimates of approximate to-50 dB for [TS] of a 35-cm live orange roughy. It is shown t hat the magnitude of the difference between in situ estimates of [TS] and e xperimental measurements of [TS] on live fish can be explained by the effec t of strong avoidance reactions to a towed transducer on the tilt angle of the fish. Previously much higher estimates of dead orange roughy [TS] were an artifact of air pockets caused by freezing. Comparison of orange roughy TSmax measured in our experiments with a large dataset for nonswimbladder a nd swimbladder fish shows that orange roughy have an unusually low target s trength at 38 kHz. (C) 1999 Acoustical Society of America. [S0001-4966(99)0 5007-9].