In situ target strength measurements of a deep‐water fish, orange roughy Free

In situ target strength methods have been reported to be the best way to convert echo integration acoustic surveys into biomass. Detailed in situ target strength measurements were obtained by both drifting over and towing through aggregations of orange roughy at 700–800‐m depths. The aggregations displayed a marked avoidance response at times to the deep‐towed body and it was difficult to ascertain when single orange roughy targets were being detected. Using three frequencies, it was possible to include/exclude various targets in the selection process and have more confidence that the observed targets were the species of interest. Calibration of the data was simplified by suspending a sphere under the transducer for all the drift experiments. The extracted target strength histograms for the drift experiments were very broad with many modes, and distribution mean target strengths were sensitive to the density of the schools and also not surprisingly the concentration of other species. Towing experiments were highlighted by a marked avoidance response as the towed body neared the orange roughy schools. The extracted in situ target strength histograms showed that the mean target strength for diving orange roughy can decrease by 3.1 dB (50%).