Estimation of Desirable Gillnet Mesh Size for an Exploited Population of a Pangasiid Pangasius bocourti in Thailand’s Fishing Ground of the Mekong Mainstem

The mesh selectivity of gillnets for pangasiid Pangasius bocourti was investigated using four different stretched mesh sizes (4, 7, 9 and 14 cm). Fishing experiments were conducted in Nongkhai Province, Thailand, between September 2007 and July 2008. A total of 2,673 P. bocourti was caught with the size range (total length) of 9.3-55.5 cm. Length at 50% maturity estimated as 23.3 cm, was used as the criterion for selecting desirable mesh size. The optimal retention length was estimated for each mesh size employing Baranov-Holt method using the model for various mesh sizes. The optimal retention lengths of P. bocourti were estimated as 11.6, 20.3, 26.1 and 40.6 cm for the stretched mesh sizes of 4, 7, 9 and 14 cm respectively. Accordingly, the desirable gillnet mesh size for P. bocourti corresponding to length at 50% maturity was determined to be 9.0 cm. The asymptotic total length and the curvature parameter estimated by length-based stock assessment methods were 57.6 cm, and 0.56 yr^-1, respectively. Meanwhile, the natural mortality rate was 1.04 yr^-1. The relative yield per recruit was used to analyze long-term biological effect, which was based on different length at first capture of each studied mesh size.