Abstract
A method to assess size-selectivity of gillnets on the basis of variation of fish body forms was considered. Size-selectivity of a gillnet was defined as the probability both if a girth at anterior (around operculum) section of fish does not exceed a mesh perimetre and if the girth at the maximum body section exceeds it. In the present method, the probability distribution at each section against fish lengths were calculated according to normal distributions of the girth approximated with its means and standard deviations linearly correlated with lengths. Size-selectivity was derived from multiplication of the respective probability distributions. A series of tank experiments were conducted to obtain cartch ratios by tested gillnets for various sizes of tilapia, carp and rainbow trout. Selectivity curves of the tested gillnets for the species were calculated on measurements of body forms of the tested fish individuals, which were compared to the catch ratios. Regarding the wedged capture, the calculated selectivity curves were well supported by the catch ratios. It was concluded that the present method assesses e%-selectivity lengths of gillnets with errors less than 4% for species and size ranges caught by mainly the wedging function of gillnets.
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