Low target strength fish in mixed species assemblages: the case of orange roughy

Use of acoustics to estimate biomass remains problematic for mixed species assemblages, and the problem is particularly difficult where the main species of interest has low acoustic target strength, as is the case for orange roughy. Despite their size and relative abundance in mixtures, orange roughy have low acoustic backscatter due to their oil-filled swimbladder. Trawl catches and acoustic target strength-length relationships at 38 kHz were combined to estimate the species-specific area backscattering coefficients from a mixture of species on the flat areas around the North West Hills on the Chatham Rise to the east of New Zealand. Orange roughy constituted a large proportion (x¯ = 48%) by weight in trawl catches, but their contribution to the area backscattering coefficient was very low (<1%). Robust cardinalfish (30.1%), three species of rattails (White rattail = 17.4%, Notable rattail = 13.8% and Serrulate rattail = 9.7%) and Johnson's cod (10.8%) made much larger contributions to the area backscattering coefficient (values in brackets) despite their relatively low catch rates (totaling an average of 7.6% of the catch by weight). We calculated the effect of changing the mean density of orange roughy on the backscattering coefficient by simulating order of magnitude increases or decreases in orange roughy densities. Increasing the density of orange roughy by a factor of 10 times would only increase the area backscattering coefficient from the fish community by 3%. We conclude that changes in orange roughy density of this order are not likely to be detectable in acoustic backscatter using echo integration data. This does not mean that orange roughy cannot be separated from mixtures, but to detect them it will be necessary to use single-target discrimination techniques such as the phase difference-〈TS〉 methods [Barr, R., Coombs, R.F., Macaulay, G., 2000. Can we discriminate between different deepwater fishes using a standard acoustics target strength ping? In: Proceedings of the 2000 Conference on International Society for Acoustic Remote Sensing, Sydney, Australia, pp. 77-80].