The effect of escape vents in collapsible pots on catch and size of the mud crab Scylla olivacea

The effect of installing an escape vent on catch and size of the mud crab Scylla olivacea (Herbst) in collapsible pots was investigated. Initially, the escape behavior of crabs was observed in a pond in the laboratory when the rectangular-shaped escape vents were located at five different positions on the pot; the aim being to determine the most suitable position for escape vents. The study also included field trials to assess the effect of different sizes of escape vents (3 cm × 4 cm, 3 cm × 4.5 cm, 3 cm × 5 cm, 3 cm × 5.5 and 3 cm × 6 cm) and was carried out in a commercial fishing ground in Klong Ngao mangrove swamp, Ranong Province, Thailand (latitude 9°51′ to 9°56′N and longitude 98°31′ to 98°37′E). The escape vent located at the bottom on the side panel of the pot produced the highest number of escapees. The selectivity curve of the 3 cm × 6 cm escape vent allowed more small crabs to escape than other vent sizes. Further, the percentage of non-target species catches decreased as the size of the escape vent increased. The 3 cm × 6 cm escape vent was the most effective for releasing undersized S. olivacea. The vents did not affect the rate of mud crab entry into the pot.