Effect of an escape vent in accordion-shaped traps on the catch and size of Asian paddle crabs Charybdis japonica in an artificial reef area

Accordion-shaped traps are widely used in China to catch the Asian paddle crab C harybdis japonica but traps of conventional design often catch juvenile crabs. A new type of accordion-shaped trap with an escape vent(L×W=4.3 cm×3.0 cm) was designed and a comparative study between the newly designed and conventional traps was performed in the artifi cial reef area of Zhuwang, Laizhou Bay, China from June to August 2012. The mean catch per unit effort(CPUE) of undersized crabs was signifi cantly lower in the vented traps than in the conventional traps(paired t-test, n =30, P <0.001), while the CPUE of marketable crabs was signifi cantly higher in the vented traps(paired t-test, n =30, P <0.001). The mean size of crabs(carapace length) caught in the vented traps was signifi cantly larger than in conventional traps(paired t-test, n =29, P <0.001). The ratio of undersized crabs was 35.05%±2.57% in conventional traps and 12.53%±0.69% in vented traps(signifi cantly lower, paired t-test, n =29, P <0.001). Therefore, a 4.3 cm×3.0 cm escape vent was considered appropriate for C. japonica fi shing in the artifi cial reef area. This fi nding will assist the development of more sustainable and effi cient crab fi shing methods using accordion-shaped traps.

Effectiveness of escape vent shape in crab pots for releasing swimming crab Portunus trituberculatus in the East China sea

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.

Effect of escape vents on retention and size selectivity of crab pots for swimming crab Portunus trituberculatus in the East China Sea

An increase in the crab pot fishery in the East China Sea has caused great pressure on swimming crab Portunus trituberculatus resources.Thus,it is essential to implement suitable measures to release sublegal-sized crabs to increase the number of recruits for legal-sized crabs.One of the measures considered is the installation of escape vents on crab pots.We tested crab pots with one and two escape vents and compared the catchability and size selectivity of these pots with control pots without an escape vent.Pots with one escape vent located on single side of the pot,top or bottom(SS),and another on both sides at top and bottom edges(BS)were tested in sea trial experiments following typical commercial fishing practice.The results show that both SS pots and BS pots caught significantly smaller number of sublegal-sized crabs(<116 mm carapace width,CW),but the difference in catch between the two types of crab pots is not significant.The 50%selective CWs(CW_(50))and the selective ranges(SRs)of both BS pots are slightly larger than those of SS pots.However,95%confidence intervals of CW_(50)s and SRs are overlapped,indicating that the discrepancy between them is small.Then,in a mixed effect model,the effect of locations of escape vents,taken as the fixed effect,on selectivity parameters and indexes was analyzed by hypothesis testing.The results show that null hypothesis of no effect of location of vents on size selectivity cannot be rejected,indicating that there is no significant difference in size selectivity between the two types of pots for P.trituberculatus.