Size Selectivity of Square-Mesh Codends in Stownet for Coilia mystus in the South China Sea

Stownet is one of the most important items of fishing gear for catching <i>Coilia mystus</i> in China's coastal waters. To improve the selectivity of stownet for <i>C. mystus</i>, a series of selectivity experiments were conducted using the covered net method with 4 square-mesh codends of different mesh sizes of 25 mm, 30 mm, 35 mm, and 40 mm. The selectivity curves of <i>C. mystus</i> under different mesh sizes were fitted using the Logistic and Richards models, and the selectivity parameters were compared among codends. The results showed that the 50% selection length (L50) increased with mesh size, reaching 105.83 mm, 109.02 mm, 114.36 mm, and 116.66 mm, respectively. The proportion of juvenile <i>C. mystus</i> in the codends decreased correspondingly, indicating that larger mesh sizes improved the selectivity of the square-mesh codend. Based on the linear relationship between L50 and mesh size, the minimum mesh size for <i>C. mystus</i> was estimated to be 43.81 mm. Considering the management regulation of the minimum landing size (120 mm) for <i>C. mystus</i>, the L50 obtained for the 40 mm square-mesh was still below this standard, suggesting that the selectivity of the current codend requires further optimization to promote the sustainable utilization of <i>C. mystus</i> resources.