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

Abstract

Target strengths (TS) of orange roughy (Hoplostethus atlanticus) are difficult 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 species having much higher TS. Orange roughy have wax-invested swimbladders and fish are ∼20% wax esters by weight. Estimates of orange roughy 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, and new swimming orientation data. Target strength of previously frozen and thawed fish decline rapidly, stabilizing after ∼12 h as air and oil diffuse out of tissues during rehydration. A 〈TS〉 of −46.3 dB is predicted 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 ≈−50 dB for 〈TS〉 of a 35-cm live orange roughy. It is shown that the magnitude of the difference between in situ estimates of 〈TS〉 and experimental measurements of 〈TS〉 on live fish can be explained by the effect 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 and swimbladder fish shows that orange roughy have an unusually low target strength at 38 kHz.