Mesh Selectivity of the Shrimp Trawl Fishery in the Western Coastal Waters of Sri Lanka

The selection properties of the shrimp trawl gears in the west coast of Sri Lanka were investigated using the covered cod end method. Trawling was conducted using two craft types, non-mechanised and mechanised in the seas north and the south of the sea mouth of the Negombo lagoon, respectively. The cod end mesh sizes experimented with non-mechanised trawls were 16, 18, 20 and 26 mm while with the mechanised trawls along with the above, an additional mesh size of 23 mm was used. Both trawls exploited juveniles of valuable shrimps. The shrimp catch rates of mechanised trawls for depths 5 m were significantly higher than the same estimated for the depths 5 m (P 0.05). The estimated 50% retention lengths for shrimps and fish increased with the elevating cod end mesh size for both craft types. For Metapenaeus dobsoni minimum and maximum relative yield/recruits were estimated for fishing with 18 and 23mm cod ends, respectively. For Parapenaeopsis coromandelica maximum values for relative yield/recruit were estimated for 18 mm cod ends. The estimated optimum exploitation levels were highest for fishing with 23 mm cod ends for two major shrimp species exploited by the trawl fishery. The corresponding relative values/recruit for both species were also highest for the same.