Size selectivity of combined square mesh and diamond mesh codends of shrimp beam trawl in South China Sea

In order to investigate the size selectivity of combined square mesh and diamond mesh codend of shrimp beam trawl in the South China Sea, we conducted fishing experiments using three combined square-mesh and diamond-mesh codends: codend with 25 mm square-mesh+25 mm diamond-mesh (H₁ codend), codend with 30 mm square-mesh+25 mm diamond-mesh (H₂ codend), codend with 35 mm square-mesh+25 mm diamond-mesh (H₃ codend). The logistic curve was used to analyze selective parameters of codends for main target species, and model deviance was compared with the degree of freedom to test the fit of models. The target species of combined mesh codends were Metapenaeus ensis, M.joyneri, Exopalaemon carinicauda and Caranx (Atule) kalla. The 50% retention lengths (L₅₀) of M.ensis were 63.93 mm and 67.96 mm for H₁ and H₃ codends, respectively. The L₅₀s of M.joyneri were 62.08 mm, 66.30 mm and 67.32 mm for H₁, H₂ and H₃ codends, respectively. The L₅₀s of E.carinicauda were 53.83 mm and 56.69 mm for H₁ and H₃ codends, respectively. The L₅₀ of C.(Atule) kalla were 59.24 mm, 61.63 mm and 64.64 mm for H₁, H₂ and H₃ codends, respectively. According to the L₅₀ values and minimum landing sizes of main target species, H₃ codend had the best efficiency both in maintaining fishing product and protecting fishery resources.