A METHOD OF DETERMINING SELECTIVITY CURVE OF SEPARATOR GRID

This paper describes a theory and method for determining a grid select ivity curve from the data of a fishing experiment using a grid separat or with several bar spacings. Grid selectivity is defined as the proba bility that a fish does not pass through a grid given that it has enco untered the grid. Assuming that grid selectivity is regarded as a siev e process, grid selectivity, s(g), can be expressed as a function of t he ratio of cross-sectional diameter to bar spacing as follows: s(g)(d ,L)=S-g(R), where L and d are the cross-sectional diameter of the fish body and bar spacing of the grid, respectively, and R=L/d. Applicatio n of the model is demonstrated by a fishing experiment using three gri ds with bar spacings of 8, 10 and 15 mm, carried out in a shrimp beam trawl in the Inland Sea of Japan, Grid selectivity was plotted against length by bar spacing, of two shrimp species (southern rough shrimp T rachypenaeus curvirostris and mantis shrimp Oratosquilla oratoria), tw o crab species (charybdid crab Charybdis bimaculata and portunid crab Portunus hastatoides), and frog flounder Pleuronichthys cornutus. The selectivity analysis in terms of the non-dimensional parameter R revea led that, for each species, a single selectivity curve describes accur ately data of each of the grids as a master curve of grid selectivity, The master curve of grid selectivity allows estimation of the selecti vity curve of grids other than those tested in this experiment.