A method of determining selectivity curve of separator grid

Abstract

This paper describes a theory and method for determining a grid selectivity curve from the data of a fishing experiment using a grid separator with several bar spacings. Grid selectivity is defined as the probability that a fish does not pass through a grid given that it has encountered the grid. Assuming that grid selectivity is regarded as a sieve process, grid selectivity, s g, can be expressed as a function of the 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 . Application of the model is demonstrated by a fishing experiment using three grids 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 Trachypenaeus curvirostris and mantis shrimp Oratosquilla oratoria), two 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 revealed that, for each species, a single selectivity curve describes accurately data of each of the grids as a master curve of grid selectivity. The master curve of grid selectivity allows estimation of the selectivity curve of grids other than those tested in this experiment.