Aquaculture and Fisheries (May 2023)
Effectiveness of escape vent shape in crab pots for releasing swimming crab Portunus trituberculatus in the East China sea
Abstract
To stop the decline of swimming crab (Portunus trituberculatus) stocks in the East China Sea, escape vents on crab pots are urgently required to improve pot selectivity and enable release of sublegal-sized crabs. This study presents field comparative fishing experiments, which examined the effectiveness of different shapes and sizes of escape vents in crab pots for the release of swimming crabs. Two sizes each of rectangular, elliptic, and circular escape vents were tested. The results of comparative fishing experiments indicated that experimental pots with escape vents, regardless of their configuration, caught significantly fewer sublegal-sized crabs (<49 mm carapace length, or <116 mm carapace width) than unmodified control pots. Although legal-sized swimming crabs were also less caught, analysis of size selectivity showed that rectangular vents produced steeper selectivity curves than elliptic and circular escape vents. This indicates that rectangular vents may be more suitable and controllable when escape vents will become a requirement and their use will be enforced in fisheries. Underwater observations were conducted using GoPro cameras and LED lights during the first 2.5 h of the deployments (which usually lasted between 5 and 12 h). Video recordings of experimental pots equipped with rectangular or elliptic escape vents showed that swimming crabs trapped in the pots could detect the vents, and readily approached and passed through these. However, no significant difference was detected in the response behavior of swimming crabs between rectangular and elliptic vent pots when quantitative indicators, derived from video recordings, were compared. The effect of different vent shapes on the escape behavior of swimming crab gradually manifested as an increased soak time and by the onset of the escape of larger crabs. Future underwater observations should thus last the entire duration of pot soaking to study how crabs escape from the pots.
