Application Study Of Y-shaped Juveniles Exclusion Devise In The East China Sea Offshore Stow Net Fisheries

In recent years,the fishery resources in the East China Sea continued to decline,and the fishery pressure was increasing.Meanwhile,the codend mesh size was small in the used of filter fishing gear,such as trawl,stow net,and capture a large number of juvenile fish in the fishing process,especially in season of spring and summer,in part in juvenile fishing ratio even up to 99%,which makes all kinds of filter fishing gear in the East China Sea juvenile ratio is outstanding and bring a lot of negative influence to fisheries management and fishing resources.In order to effectively release bycatch,since 1992,foreign fishery researchers began to study the application of rigid exclusion devise,the test proved that the rigid exclusion devise in fish and shrimp separation,juvenile release and other aspects of the application effect is significant,some of the countries and regions fisheries regulatory organization is obliged to use rigid exclusion devises in filterable fishing gear.China's rigid exclusion devise for the study of less,no research report before 2000,even entering the 21 st century.In order to study the application value of rigid exclusion devise in fishery in our country,a new type of rigid exclusion devise was designed in this study.The exclusion devise was mainly used to release the economic fish juvenile species and other small bycatch with low value.The exclusion devise shown in this study consists of three parts,each part of the framework between with hinge connection,the overall exclusion devise similar Y-shaped.Use the upper two framework screening and release small individual juveniles,while use the lower frame to guide large individuals Catch into the net codend.Three sea trials were conducted in December 2014,December 2015 and June 2016 to test the performance of the exclusion devise and to study the relevant influencing factors.The main structure of the device in the three sea trials is similar,but the main dimensions of the Y-shaped juvenile exclusion devise used in the three tests are slightly different because of the differences in the specifications of the stow net used for the test.In addition,the Y-shaped juvenile exclusion devise used in the second and third sea trials stainless steel was replaced by a less dense epoxy resin.The application test of the Y-shaped juvenile exclusion devise was carried out in the winter of 2014 in the East China Sea,and the catch of the Y-shaped juvenile exclusion devise was compared with the catch of the control net.Analyze the release effect of this device on the size of different individuals.The results show that the Y-shaped juvenile exclusion devise with an angle of 60 ° between the two bar the interval is 2 cm and the upper two frames are installed in the head of the web.The juvenile fish exclusion devise can effectively release 90% of the young juveniles and keep the large Partial(94.6%)large individual catch,achieved good juvenile release effect.In 2015,three sets of barrier spacing were set at 10 mm,15 mm and 20 mm,respectively,and the separation frame was 60 ° in the direction of the flow,a total of 20 trials were conducted.Logistic model was used to analyze the selectivity curve.Choose body length of 50%(L50)increased with the increase of the bar interval,and the selection range(SR)decreased with the increase of the bar interval.This indicates that the different spacing is different for the release of the target juvenile fish but may be different for the loss of the target species.Therefore,the optimal spacing of the Y-shaped juvenile exclusion devise in the East China Sea should be to find a balance between the loss rate of the fish and the release rate of the fish.In the 2016 sea trials,a total of five groups were set up,with the barrier spacing set at 10 mm,15 mm,20 mm,of which 15 mm group set the fish frame height of 10 cm,15 cm,two installation angle of 45 °,60 °.The results show that with the increase of the barrier spacing,the catch release effect increases,the fish frame 10 cm high,the installation angle of 45 ° release effect is better.The results of the three trials in the study show that the Y-shaped juvenile fish exclusion devise has great potential for the release of juvenile fish in the target species of the mixed fish species in the East China Sea.The angle of the Y-shaped juvenile exclusion devise can be adjusted to suit the requirements of different sizes of net,different mounting positions and the degree of expansion of different netting,to improve the applicability of the device,and to extend the device by simple structural adjustment And can be used as reference for the future study on the release of juvenile fish.Finally,some technical problems encountered in the design,installation and use of the device were discussed.