Modeling of available size selectivity of the SURF-BRD for shrimp beam trawl

The System of Unwants Ramp-way Filtered-Bycatch Reduction Device (SURF-BRD) was developed to reduce shrimp bycatch in a shrimp beam trawl. It consists of a pair of rectangular net panels, namely, the front panel (FP) and rear panel (RP), and two fish escape vents. In this study, numerical models for expressing available selection by the SURF-BRD were proposed and assessed in several fishing experiments conducted using two types of the BRD: a prototype and an improved type with the BRD attached at a higher point on the side-nets . Contact probability and selectivity parameters of the FP and the RP for four major species were estimated for each BRD type. The improved type with the higher BRD showed a larger contact probability for the FP. Size selectivity of the FP and RP for two species (cinnamon flounder and spotted swimming crab) was almost equivalent to the mesh selectivity of the net panel, but that for lizard fish seemed to depend not on mesh size of the FP but on the swimming behavior of the fish. These results suggest that the smaller mesh size of the RP would be helpful for releasing more fish of smaller size and to exclude more spotted swimming crab, which hinder on-deck sorting by fishermen.