Consequence of swimbladder model choice and fish orientation to target strength of three New Zealand fish species

Information on fish orientation has lagged behind the development of models to estimate target strength from fish swimbladders, despite fish tilt angle being an important variable influencing target strength. Few studies compare models, because authors generally compare their own model results with experimental data. We contrast three models for estimating target strength from fish swimbladders, and compare the magnitude of the differences between models to the effects of fish tilt angle on target strength. The swimbladder models compared were the mapping method, a deformed cylinder model, and the equicylinder model. The deformed cylinder model should give average target strengths with accuracy between the “exact” mapping method solution and the “approximate” equicylinder solution. The effect on average target strength of having a 5° or a 15° 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 New Zealand commercial fish species: southern blue whiting (Micromesistius australis Norman, 1937), red cod (Pseudophycis bachus Bloch and Schneider, 1801), and barracouta (Thyrsites atun Eupharasen, 1791). We suggest that the greatest gains in target strength accuracy may be made from acquiring better information on fish orientation, rather than from the development of more elaborate modelling methods.