Industry-collected target strength of high seas orange roughy in the Indian Ocean

Visually verified in situ target strengths (TS) are the state of the science for determining the conversion from acoustic echo-integration surveys to biomass. Here, we show how these measurements can be made by high seas fisheries during normal operations using a net-attached acoustic optical system (AOS) without specialized personnel on board. In situ TS were collected from similar to 45cm standard length (SL) orange roughy (Hoplostethus atlanticus) in the southern Indian Ocean at 38 and 120kHz. We use a multiple lines of evidence approach to demonstrate that the previous TS-SL equation developed for similar to 10cm smaller fish in Australia and New Zealand is not suitable for the larger orange roughy and instead propose new TS-SL equations. Our findings show that biomass estimates at 38kHz will be reduced by similar to 58% when using this new TS-SL compared to the existing TS-SL for smaller fish. This highlights the error of extrapolating TS-SL equations outside the measurement range. For this high sea region, the net-attached AOS represented a practical cost-effective method to obtain measurements and provide a result that could be used to inform the management of the stocks. We suggest that this method would be useful in all deep-water fisheries to monitor the TS of the fish for a range of environmental and ontogenetic conditions.