Research On Off-shore Net Cage For Flatfish And Land-sea Relay Mariculture Technology

Flatfish which has been extensively cultured under intensive systems in the north coastal regions is a very important mariculture species in China. Flatfish farming has become an important industry of mariculture. However, the high energy sources consuming and cost have blocked its production in recent years. Therefore, more attentions should be paid for the mariculture mode of flatfish culture in order to maintain its stable, healthy and sustainable development.A square steel off-shore net cage for flatfish culture was designed and used to culture Brown flounder (Paralichthys olivaceus) in the semi-opening sea area of Ailian bay, Rongcheng, Shandong province, in the year 2008 and 2009. Meanwhile, the land-sea relay mariculture technology was discussed.1 Design of off-shore net cage for flatfish cultureA safe, practical and economical square steel off-shore net cage for flatfish culture is designed to meet the benthic habit of flatfish and the working condition of low velocity flow, strong waves in the semi-opening sea area of Ailian bay, Rongcheng, Shandong province.The principal part of the cage which is square frame configuration is trigonal, jointed by three 060mm galvanized seamless steel tubes. The flat-bottomed frame is jointed by six 038mm galvanized seamless steel tubes, with the font style"田".Four 200L controllable plastic float buoys in the four angles of the cage and one bottomed controllable float buoy are utilized to accomplish the raise and submergence, the cage could float up and down like a submarine. A normal gridding mooring is adopted in the mooring system of the cage, and the depth of the underwater rope grid is 2m. The cage links to the rope grid first and then connects with rope stays via the rope grid.The square steel off-shore net cage is designed to resist the wind force of 10 and maximum current velocity ups to 0.82m/s with the dimension of 5m×5m×4m, acreage of 25m2, and the life-span of the cage system is 3 years.2 Experiment on off-shore flatfish net cage mariculture of brown flounderThe experiment on off-shore flatfish net cage mariculture of brown flounder was set out in the semi-opening sea area of Ailian bay, Rongcheng, Shandong province, in the May to November of the year 2008 and 2009.In 2008, the average body weight of brown flounder reached to 656.0g,515.0g,761.1g after 34,78,169 days from brown flounder seeds with the average body weight of 376.0g,221.2g,144.6g, the survival rates of net cage mariculture were 91.2%,98% and 97.3% respectively.In 2009, the average body weight of brown flounder reached to 614.0g,885.3g,1030.0g after 174 days from brown flounder seeds with the average body weight of 59.3g,136.0g,202.Og, the survival rates of net cage mariculture were 93.3%,87.5% and 93.3% respectively.The experimental results indicated that the condign size brown flounder seeds which were hatched and domesticated in closed recirculating fish culture systems on land could be transported to the net cage to cultivate when the surface temperature of the sea area climb up to 12.0℃above in Spring, the growth speed of brown flounder is 1.4-3.6 times more than which were cultured in the closed recirculating fish culture systems, and the experimental results showed that larger size, faster growth speed will be get.And the experimental results also showed that the quality of brown flounder cultured in the cage is closer to the wild brown flounder, the content of fat and cadmium of brown flounder cultured in the cage is 41% and 50% less than which were cultured in the closed recirculating fish culture systems respectively.3 Experiment on land-sea relay mariculture of brown flounderThe experiment on off-shore flatfish net cage relay mariculture of the condign size brown flounder which were hatched and domesticated in closed recirculating fish culture systems on land was set out in the semi-opening sea area of Ailian bay, Rongcheng, Shandong province, in the year 2008 and 2009.The operation flow of brown flounder land-sea relay mariculture are as follows: Choosing the condign size brown flounder→Catching the brown flounder from land-based pool→Enclosing the brown flounder with oxygen bag→Transporting the oxygen bags to ship→Putting the condign size brown flounder seeds to the net cages to culture.The best time of transporting the condign size brown flounder seeds to the net cages is the last ten-day of May when the surface temperature of the sea area climb up to 12.0℃above, and the best time for net cage mariculture is June to November and the mariculture days of brown flounder could get to 180d or more. Therefore, the hatching and domesticating time of brown flounder seeds in closed recirculating fish culture systems on land could be shorten, energy consumption could be saved, the mariculture cost could be reduced and large size merchandise brown flounder could be cultured within 18 months.It is the most economical and costless way to culture the brown flounder in the net cage when the seeds which were hatched and domesticated in closed recirculati-ng fish culture systems on land with the average body weight of 60g. In additional, the yield brown flounder of the cage could get to 800g above when the seeds which were hatched and domesticated in closed recirculating fish culture systems on land with the average body weight about 140g, therefore, the merchandise brown flounder could get more price and market superiority.The test farming results of brown flounder in the year 2008 and 2009 showed that the density of yield could get to 18.6-23.1 kg·m-2 when the density of fish released is 3.3-4.8 kg·m-2, therefore, the density of fish released could be considered as the best.