Abstract: The typical approach to the problem of indirect fish target strength estimation from data collected using single-beam system is based on inversion of probability density function (PDF) of measured fish echo level. Due to ill-conditioning of most inversion algorithms, small errors in input data, and inaccuracies in the assumed form of the kernel, may result in large errors at the output. Therefore, al! modern inversion algorithms use sophisticated techniques to achieve satisfying results, assuming fixed kernel in the inversion procedure. The paper presents different approach, which uses adaptive construction of the kernel, under assumption of non-negative constraints in the obtained estimate of fish target strength PDF. As a result, the more reliable beam pattern PDF is obtained which leads to unbiased and more accurate estimate offish target strength PDF, than in the conventional (fixed kernel) methods.