Design And Selectivity Of Escape Vent In Cage Net For Charybdis Japonica In The Artificial Reef Area

Cage net, as a common fishing gear in the artificial reef area, is widely used infishing reef benthos, and Charybdis japonica is one of the object captures. Dueing tothe small mesh and poor selectivity, cage net badly destruct the crab resource in theartificial reef area. In order to improve the gear selectivivty for C. japonica,reasonable and orderly use the crab resource, laboratory and field experiments areconducted, respectively. Laboratory experiment is to observe ingress behavior of C.japonica and design the most appropriate size of escape vent, field experiment is toproduce the vented cage nets and operate test fishing experiment in the artificial reefarea. This study aims to provide reference and technical support for the improvementof cage net and accessory fishing in the artificial reef area. The main results are shownas followed:1.In order to lay the foundation for protocolling the most appropriate size of escapevent, ingress behavior of C. japonica is observed under different light and baitconditions, covered net method is used to obtain the selectivity of different ventlengths for C. Japonica, and Logisitic model is used as the selective model to estimatethe selective parameters and simulate the selectivity curve. Experimental results showthat: C. japonica are photophobic, under bright conditions they prefer to be in the fourcorners of the water tank. Under dark conditions, they are randomly distributed. Bothbait and the colour of black entice C. japonica into the cage. The rectangular escapevents at the bottom of the cage net were4cm×3cm,4.5cm×3cm,5cm×3cm and5.5cm×3cm length by height, which corresponded to L50(the correspondingcarapace length with50%retention) of C. japonica being3.75cm,4.24cm,4.54cmand5.08cm, respectively, and SR (the selection range) being0.65cm,0.45cm,0.62cm and0.82cm, respectively. L50has a linear relationship with the length of theescape vent, being about92.3%of its length. Based on the minimummarket-acceptable carapcae length standard of C. Japonica4.0cm, the most appropriate size (length×height) of the escape vent is protocolled at4.3cm×3cm.2.Comparative fishing method is used to conduct comparative fishing experimentsbetween conventional and vented(4.3cm×3cm, length×height) cage nets in theartificial reef area of Zhuwang, Laizhou. The experimental results show that: Duringthe30fishing trials, conventional and vented nets get174and311valid hauls,respectively, the conventional types catch136undersized crabs(carapacelength<4.0cm) and265marketable crabs(carapace length≥4.0cm) with a meancarapace length of4.47cm and CPUE of2.30(0.86and1.44for the undersized crabsand marketable crabs, respectively), ratio of the undersized is33.9%; while the ventedtypes catch112undersized crabs and759marketable crabs with a mean carapacelength of4.89cm and CPUE of2.80(0.45and2.34for the undersized crabs andmarketable crabs, respectively), ratio of the undersized is12.9%. The SELECTestimated-spilt model is used as the selectivity model for vented cage net,corresponding to L50of C. japonica being4.1cm; through simulating the catchincomes, conventional cage nets get1841.1Yuan with10.6Yuan/haul, and the ventedcage nets get4624.6Yuan with14.9Yuan/haul. The installation of escape vent canrelease some undersized crabs, improve the catch efficiency for marketable C.japonica, and increase the average yield per haul.