CONSEQUENCE OF SWIMBLADDER MODEL CHOICE AND FISH ORIENTATION TO TARGET STRENGTH OF 3 NEW-ZEALAND FISH SPECIES

Information on fish orientation has lagged behind the development of m odels to estimate target strength from fish swimbladders, despite fish tilt angle being an important variable influencing target strength, F ew studies compare models, because authors generally compare their own model results with experimental data. We contrast three models for es timating target strength from fish swimbladders, and compare the magni tude of the differences between models to the effects of fish tilt ang le on target strength. The swimbladder models compared were the mappin g method, a deformed cylinder model, and the equicylinder model. The d eformed cylinder model should give average target strengths with accur acy between the ''exact'' mapping method solution and the ''approximat e'' equicylinder solution, The effect on average target strength of ha ving a 5 degrees or a 15 degrees standard deviation of tilt angles is far more significant than the choice of model used to estimate target strength. We estimate the first averaged target strengths for three Ne w Zealand commercial fish species: southern blue whiting (Micromesisti us australis Norman, 1937), red cod (Pseudophycis bachus Bloch and Sch neider, 1801), and barracouta (Thyrsites atun Eupharasen, 1791). We su ggest that the greatest gains in target strength accuracy may be made from acquiring better information on fish orientation, rather than fro m the development of more elaborate modelling methods. (C) 1996 Intern ational Council for the Exploration of the Sea