The Estimation of Fish Length Distribution from its Acoustical Measures

The paper concerns the problem of estimating fish length PDF from its target strength PDF obtained from acoustic surveys. It has been shown that the target strength of a single fish can be treated in the first approximation as a function of two variables: one, which depends on fish size and the other, which depends on its angular orientation (aspect). Extending this simplified relationship to the case of fish populations allows to formulate relationship between PDFs in the form of convolution function. The study further suggests the possibility of reconstructing size distribution using inversion technique when directivity pattern of a single fish is known. This is because its angular statistics could be calculated if fish tracking procedure is applied for consecutive pings. However, the problem is generally more complex, as in reality directivity pattern depends also on fish size and requires introduction of conditional PDF for fish directivity pattern.