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.

Responsible fishing operations for clean oceans,healthy ecosystems and sustainable fisheries

This special issue contains selected papers presented at the ICES-FAO Working Group on Fishing Technology and Fish Behavior mini-symposium on“Responsible Fishing Technology for Healthy Ecosystems and Clean Environment”held in Shanghai,China between 8 and 12 April 2019.The symposium was co-sponsored by Food and Agriculture Organization of the United Nations(FAO),International Council for the Exploration of the Seas(ICES)and Shanghai Ocean University,China.The meeting was chaired by Pingguo He(USA),Haraldur Einarsson(Iceland)and Liming Song(China).The special issue was edited by the following guest editors:Pingguo He(managing guest editor),Haraldur Einarsson,Liming Song,and Michael Pol,with the assistance of a number of anonymous reviewers.

Release of unwanted flat-bodied fish from a horizontal-bar grid system as revealed through comparative fishing trials and underwater video observations

Grid systems separate fish species primarily through physical means:fish size and body shape.On Georges Bank off the northeast USA,many species of flounders are overfished,and their catch needs to be reduced.Flat-bodied skates are also often discarded.We tested a European style horizontal-bar grid system to reduce these flat-bodied low quota species in a trawl targeting the haddock(Melanogrammus aeglefinus),whose population is robust.The grid system consisted of 4 grid sections,two on each side,with horizontal bars 70 mm apart.The alternating tow method was used to compare the catch characteristics between a trawl with a grid section and the same trawl without a grid section.A video camera was used to observe fish escape in the grid section.The results indicate that the grid system reduced the flounder catch rate(mainly winter flounder,Pseudopleuronectes americanus)by 51.3%,and skates(mainly winter skate,Leucoraja ocellata and little skate,Leucoraja erinacea)by 29.4%,while there were no differences in the catch of Atlantic cod(Gadus morhua).While haddock was reduced by 37%by weight,the reduction was primarily small undersized individuals.There was no reduction in large haddock greater than 50 cm.Video observations indicates that flounders and cod mainly escaped from the bottom half of the grid while haddock were from the top half.These finding suggest that this system reduced low quota flounders,as well as discarded skates and small haddock,while retaining Atlantic cod and large size haddock.Underwater observations indicate that differential spacing(narrower on the top and wider on the bottom)may improve the system performance by releasing more flat-bodied fish,that were observed to escape from the bottom part of the grid,while retaining more haddock,which typically escape from the top part of the grid.