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

Orange roughy consist of approximately 18% lipids by weight, mostly as wax esters, and the lipids must be taken into account when modeling target strength. A deformed cylinder model incorporating the effect of temperature and pressure on sound speed through wax ester was used to scale experimental measurements of target strength to the temperatures and pressures where orange roughy live (approximately 6 degrees C, depths approximately 800-1300 m). The effect of decreasing temperature and increasing pressure is to increase the sound speed in orange roughy lipids. Modeling shows that the net effect of this is to reduce tilt-averaged target strength, 'TS', by approximately 2 dB. Adjusting experimental results to compensate for temperature and pressure effects gives a predicted 'TS' for a 35-cm orange roughy of -48.3 dB. Adjusting in situ estimates of orange roughy 'TS' for avoidance behavior [McClatchie et al., J. Acoust. Soc. Am. 106, 131-142 (1999)] suggests the correct 'TS' is approximately -47.5 dB, rather than -50 dB as previously reported [Kloser et al., ICES J. Mar. Sci. 54, 60-71 (1997)]. We conclude that experimental and in situ estimates now converge at a 'TS' of approximately -48 dB for a 35-cm fish.