Technique for Ecological Pond Breeding of Charybdis japonica

Lijun SHI, Liwei SHI, Hua YANG, Wenping JIA, Zhiru FU

1.Tianjin Binhai New Area Huawei Marine Technology Co., Ltd., Tianjin 301800, China; 2.Tianjin Fisheries Research Institute, Tianjin 300221, China; 3.Tianjin Agricultural Development Service Center, Tianjin 300061, China

Abstract In 2016 and 2017, the ecological pond breeding experiment of Charybdis japonica was carried out, with a total of experimental area up to 4 ha.Through two years of experimental research, it obtained a total of 48 950 kg of C. japonica, with an average output of 12 237.5 kg/ha.The production output value was 2.367 5 million yuan, the total production cost was 831 700 yuan, the net profit was 1.535 8 million yuan, the average profit was 384 000 yuan/ha, and the input-output ratio was 1∶2.85.The average proportion of C. japonica population with a production weight of 150 g or more was 7.9%.The proportion of the group of 120-150 g was relatively large, with an average of 80% in two years, and the proportion of the group below 120 g was relatively small, with an average of 12% in two years.The cumulative harvest of shellfish in two years was 65 700 kg, with an average output of 16 425 kg/ha, the production output value of 738 100 yuan, the production cost of 295 500 yuan, the net profit of 442 600 yuan, and the average profit of 110 700 yuan/ha.

Key words Charybdis japonica, Shellfish, Ecological breeding

1 Introduction

Charybdisjaponicabelongs to genusCharybdis, subfamily Portuninae in family Crustacea of class Portunidae, also called Asian paddle crab and Japanese shore swimming crab.It is a medium-sized marine edible crab with high economic value.It is widely distributed in shallow waters such as the Bohai Sea, the Yellow Sea, and the East China Sea.It belongs to coastal settlement species and is one of the common crab species in China[1].C.japonicameat is fresh and tender, rich in nutrition, and has the functions of clearing away heat, nourishing and reducing swelling[2], and has gradually become one of the important marine species in China.C.japonicahas the characteristics of fast growth, strong adaptability, low temperature resistance, and is suitable for ecological breeding in ponds.

At present, domestic researchers have carried out research onC.japonicaoverwintering and fattening, temporary breeding in autumn and winter, growing up, artificial breeding, and broodstock indoor temperature-raising cultivation[3-8].Based on this, we made full use of the pond’s ecological material circulation system, and used bivalve molluscs(such asPotamocorbulalaevis,Mactraveneriformis,Ruditapesphilippinarum,NassariusvariciferusandC.japonica)that can filter a large amount of water, clean up humus, and provide food forC.japonicato conduct pond ecological breeding experiments, and used discarded ground cage nets, stacked sand dunes as reef shelters,etc.to increase the survival rate ofC.japonica.

2 Experimental conditions

2.1 Selection of pondsIn 2016 and 2017, we used a pond with an area of 1.33 ha and a pond with an area of 2.67 ha in Hangu, Binhai New Area, Tianjin.The two ponds were both north-south and rectangular.The bottom of the ponds was muddy and sandy.The highest water level of the ponds could reach 3.0 m, and it was convenient for seawater to enter and discharge.The water inlet and outlet were equipped with conical dense nets to prevent harmful organisms from entering the pond andC.japonicafrom escaping.

2.2 Pond dredgingIn 2016 and late August 2017, pond dredging was started, and the bottom of the pond was thoroughly disinfected with quicklime immediately after cleaning.We spread 2 250 kg/ha of quicklime evenly in the pond, and raked the bottom of the pond manually.Then, poured 25-35 cm water.After 4-5 d, drained the water.Then used water to drain while washing the pond.At the same time, conducted renovation work at the pond dam, inlet and outlet.After cleaning the bottom of the pond, drained water to dry the pond.

2.3 Water disinfectionPoured 30-40 cm of water 3-4 d before putting in the fry, so that the remaining substances at the bottom of the pond were fully infiltrated by seawater.Later, disinfected water with a chlorine-containing disinfectant.The dosage of bleaching powder, bleaching powder essence and sodium hypochlorite was 10-20 mg/L of available chlorine.After 2-3 d, drained the disinfectant water, and dried the pond for another 2-3 d.

2.4 Water fertilizationBefore putting the fry, conducted water fertilization to cultivate and reproduce some basic food organisms that are beneficial to the growth of shellfish, including benthic organisms and plankton.First fed water 40-50 cm, used a small amount of urea(15 kg/ha)and potassium dihydrogen phosphate(3 kg/ha), then fed water once a day, each time 10-15 cm, slowly fed water to the pond depth of 1.3 m.And according to the water color and transparency, added urea and potassium dihydrogen phosphate to keep the ratio of nitrogen and phosphorus in the water at 15∶1-20∶1.Alternatively, some bait organisms could be transplanted, such as the transplanted and culturedSkeletonema,Phaeodactylumtricornutum,Nitzschiaclosterium, andChlorella.Through water fertilization, it was suitable to make the pond water appear yellow-green, light brown, or tea-brown, with a transparency of about 40 cm.

2.5 Making and release of reefsC.japonicais fierce and aggressive, and often kills each other.In order to reduce the chance of killing each other as much as possible, discarded ground cages were used to make ecological reefs of 40 cm×40 cm×50 cm, with one reef body per 3 m2.Placed small sand piles around the pond, one sand pile every 8 m2, and each sand pile was 0.4-0.5 m2.

2.6 Construction of ecological breeding modelBivalve molluscs(such asP.laevis,R.philippinarum, andM.veneriformis)andN.variciferusthat feed on organic detritus, which can filter large amounts of water, clean up humus, and provide food forC.japonica, are used for ecological pond breeding.In 2016, the ecological breeding area ofC.japonicapond was 1.33 ha, which was a greenhouse-style pond.A total of 5 800 kgC.japonicawith an average individual of 39.1 g/pc were released with an average of 4 360.9 kg/ha.Its ecological farming model wasR.philippinarum,P.laevis,N.variciferus, andC.japonica.In 2017, the ecological breeding area ofC.japonicaponds was 2.67 ha.It was an open pond.A total of 11 840 kgC.japonicawith an average individual of 38.17 g/pc were released, with an average of 4 434.5 kg/ha.Its ecological breeding model wasR.philippinarum,P.laevis,M.veneriformis,N.variciferus, andC.japonica.The ecological pond breeding ofC.japonicawas shown in Table 1.

Table 1 Ecological pond breeding of Charybdis japonica in 2016 and 2017

3 Experimental methods

3.1 Source and selection of organismsAfter water fertilization and reef release in the pond, first sowed benthic shellfish, then added natural seawater, injected seawater every day as required, and put the purchasedC.japonicafries into the pond at any time.

Water source: natural seawater.Conditions of seawater: clear water quality, no pollution, stable physical and chemical factors all year round, pH of 8.2-8.4, salinity of 30‰-31.4‰, water temperature of 10-25 ℃, ammonia nitrogen less than 0.5 mg/L, and dissolved oxygen(DO)greater than 4 mg/L.

Source and selection of shellfish:M.veneriformis,P.laevis,R.philippinarum, andN.variciferuswere all purchased from the coastal areas of Tianjin and Hebei, and were acquired by special personnel after they were caught in Bohai Bay.

Source and selection ofC.japonica: Mid-September to October is the peak season forC.japonicacapture, and it is also the best season for the source ofC.japonicapond ecological breeding fries.We purchased Small size ofC.japonicafrom the fishermen in Tianjin and Hebei Province as fries, and selectedC.japonicawith bright body color, intact body and limbs, no damage, strong vitality, and short dehydration time.It is best to select those with short capture time and light limb integrity and damage, dry-pack them in plastic boxes or foam incubators, and transport them quickly to reduce the drying time and improve the survival rate of fries during transportation.

3.2 Stocking ofC.japonicaFrom September 16 to September 29, 2016, 5 800 kg ofC.japonicawith an average weight of 39.11 g/body were put into the pond(1.33 ha), and the average stocking rate was 4 360.9 kg/ha.From September 14 to September 28, 2017, 840 kg ofC.japonicawith an average weight of 38.17 g/pc was put into the pond(2.67 ha), and the average stocking rate was 4 434.5 kg/ha.When releasingC.japonica, the temperature difference between the acquisition site and the pond water should not exceed 3 ℃, and the salinity difference should not exceed 5‰.

3.3 Breeding management ofC.japonica

3.3.1C.japonicais carnivorous and it likes to eat small and low-value fresh shellfish such asSolidiscorbulatunicata.When the water temperature dropped below 5 ℃,C.japonicaate so little.No feeding is required for ecological breeding ofC.japonica.

3.3.2Water quality control.With the continuous growth ofC.japonicain mid-September, the water depth of the pond was gradually increased, so it was necessary to inject water 10-15 cm daily.C.japonicalikes a clean water environment.When the water temperature was above 15 ℃, the amount of water should be changed appropriately.The water should be changed by 20%-30% every day to keep the pool water fresh and have sufficient dissolved oxygen content, and the transparency should be controlled above 50 cm.Sprinkled the whole pond with 1-2 g/m3quicklime water every 10 d, which not only improved the water quality of the pond, but also facilitated shelling and disease prevention ofC.japonica.Kept the water level higher when the water temperature dropped below 10 ℃ until it reached the highest water level.The water should be changed once every 4-5 d and the water change volume was 10%-20%; when the water temperature dropped below 5 ℃, it was basically not necessary to change water.

3.3.3Patrol inspection of the pond.During the ecological breeding ofC.japonica, it was necessary to conduct patrolling the pond every morning and evening to observe the water quality, water level and activity ofC.japonicain the pond, and observe whether there is hypoxia inC.japonica, and check whether the escape prevention facilities are intact, pay close attention to weather changes, raise the water level of the pool to the highest water level of 2.8 m especially before strong winds and drastic changes in temperature.Besides, it was necessary to make a record of daily patrol and promptly find and deal with problems.

3.4 Harvest ofC.japonicaAfter about 5 months of breeding from autumn to winter, the vast majority ofC.japonicahave grown fat and can be caught and sold in February of the following year(around the Spring Festival).C.japonicawas caught by harvesting in drained dry ponds, using multi-tooth rakes, nets or manually.According to the demand of the consumer market, the harvest can be completed in about 10 d.The capturedC.japonicabody surface and the sludge in the gill cavity were washed clean with fresh sea water to prevent death from suffocation, and then they were classified and selected, weighed and sold.ForC.japonicaweighing more than 150 g/pc, the two claws should be tightly bound with rubber aprons immediately to prevent mutual pinching and injury.C.japonicais more resistant to low temperature.When the water temperature is low in winter, it is basically in a dormant state, which is conducive to temporary breeding and timely high price sale, so as to obtain high economic benefits.

4 Experimental results

4.1 Effects of ecological pond breeding ofC.japonicaIn 2016,C.japonicawent through 5 months of ecological breeding management, and the capture ofC.japonicabegan on January 26, 2017.After 6 d of harvesting, a total of 15 630 kg ofC.japonicawere captured from the pond, with an average of 11 751.9 kg/ha, an average individual weight of 124.54 g, and an average individual weight gain of 2.95 times.Among them, 1 297.29 kg of individuals above 150 g, accounting for 8.3%; 12 722.82 kg of individuals of 120-150 g, accounting for 81.4%; 1 609.89 kg of individuals below 120 g, accounting for 10.3%.According to statistics, the survival rate ofC.japonicaecological pond breeding in 2016 was 84.84%.In 2017,C.japonicawent through 5 months of ecological breeding management, and the capture ofC.japonicabegan on January 23, 2018.After 10 d of harvesting, a total of 33 320 kg ofC.japonicawere captured from the pond, with an average of 12 479.4 kg/ha, an average individual weight of 121.34 g, and an average individual weight gain of 2.89 times.Among them, 2 499 kg of individuals above 150 g, accounting for 7.5%; 26 222.84 kg of individuals of 120-150 g, accounting for 78.7%; 4 598.16 kg of individuals below 120 g, accounting for 13.8%.According to statistics, the survival rate ofC.japonicaecological pond breeding in 2017 was 92.76%.The ecological pond breeding ofC.japonicain 2016 and 2017 was shown in Table 2.

Table 2 Ecological pond breeding of Charybdis japonica in 2016 and 2017

4.2 Ecological breeding effect of shellfishThe bottom-seeded shellfish were harvested in the first 10 d of April every year, and 65 700 kg of shellfish were harvested in total, including 33 520 kg ofN.variciferus, 20 100 kg ofR.philippinarum, 840 kg ofP.laevis, and 11 240 kg ofM.veneriformis.The harvest of ecological breeding shellfish in 2016 and 2017 was indicated in Table 3.

Table 3 Harvest of ecological breeding shellfish in 2016 and 2017

4.3 Production costsC.japonicaproduction costs included acquisition fees(C.japonica, shellfish), utilities, transportation, labor, pond rental and other costs.

In 2016,C.japonicaproduction costs:C.japonicapurchase fee of 41 500 yuan, shellfish purchase fee of 28 100 yuan, water and electricity fee of 4 500 yuan, transportation fee of 12 600 yuan, labor cost of 34 500 yuan, pond rental fee of 30 000 yuan, and other expenses of 15 300 yuan, 166 500 yuan in total.In 2017,C.japonicaproduction costs:C.japonicapurchase fee of 86 500 yuan, shellfish purchase fee of 55 600 yuan, water and electricity fee of 7 500 yuan, transportation fee of 36 600 yuan, labor cost of 75 000 yuan, pond rental fee of 60 000 yuan, and other expenses of 48 500 yuan, 369 700 yuan in total.

Shellfish production costs include labor costs for harvesting shellfish, transportation and other costs.In 2016, we selectedN.variciferus,R.philippinarum, andP.laevis.Because the cost of purchasing shellfish fry has been amortized in the costs ofC.japonicaecological breeding, the production costs included the labor cost of harvesting shellfish 53 400 yuan, the transportation fee of 26 200 yuan, and other expenses 26 100 yuan, 105 700 yuan in total.In 2017, we selectedM.veneriformis,N.variciferus,R.philippinarum, andP.laevis.Because the cost of purchasing shellfish fry has been amortized in the costs ofC.japonicaecological breeding, the production costs included the labor cost of harvesting shellfish 95 600 yuan, the transportation fee of 48 500 yuan, and other expenses 45 700 yuan, 189 800 yuan in total.

4.4 Economic benefits of breeding

4.4.1Breeding benefits of 2016.In 2016, a total of 19 680 kg of shellfish ofN.variciferus,R.philippinarum, andP.laeviswere harvested, with an output value of 238 600 yuan, production costs of 105 00 yuan, and a net profit of 132 900 yuan.In 2016, the output value ofC.japonicaecological pond breeding was 496 700 yuan, the production cost was 166 500 yuan, the net profit was 330 200 yuan, the average profit was 247 500 yuan/ha, and the input-output ratio was 1∶2.98.In 2016, the experimental area ofC.japonicaecological pond breeding was 1.33 ha, the total output value was 735 300 yuan, the total cost was 272 200 yuan, the total profit was 463 100 yuan, the average profit was 348 200 yuan/ha, and the input-output ratio was 1∶2.70.

4.4.2Breeding benefits of 2017.In 2017, a total of 46 020 kg ofM.veneriformis,N.variciferus,R.philippinarum, andP.laevisshellfish were harvested, with an output value of 499 500 yuan, production costs of 189 800 yuan, and a net profit of 309 700 yuan.In 2017, the output value ofC.japonicaecological pond breeding was 1.132 7 million yuan, the production cost was 369 700 yuan, the net profit was 763 000 yuan, the average profit was 285 800 yuan/ha, and the input-output ratio was 1∶3.06.In 2017, the experimental area ofC.japonicaecological pond breeding was 2.67 ha, the total output value was 1.632 2 million yuan, the total cost was 559 500 yuan, the total profit was 1.072 7 million yuan, the average profit was 402 000 yuan/ha, and the input-output ratio was 1∶2.92.

4.4.3Total breeding benefits of 2016 and 2017.In 2016 and 2017, the total ecological pond breeding area ofC.japonicawas 4 ha, and theC.japonicaharvest of ecological breeding was 48 950 kg, the average output was of 12 237.5 kg/ha.The total output value was 2.367 5 million yuan, the total cost was 831 700 yuan, the total profit was 1.535 8 million yuan, the average profit was 384 000 yuan/ha, and the input-output ratio was 1∶2.85.In 2016 and 2017, a total of 65 700 kg ofN.variciferus,R.philippinarumandP.laevisshellfish were harvested.The output value was 738 100 yuan, the cost was 295 500 yuan, the net profit was 442 600 yuan, the average profit was 110 700 yuan/ha, and the input-output ratio was 1∶2.50.

5 Conclusions

Through the experimental studies in 2016 and 2017, we found that the proportion ofC.japonicaweighing more than 150 g/pc was low, with an average of 7.9% in two years; the proportion of the population of 120-150 g was relatively large, with an average of 80%, and the proportion of the population below 120 g was 12%.The average survival rate ofC.japonicaecological pond breeding was 88.8%, which was relatively high, due to the sand piles and ground cage nets used as reef shelters.

It is worth noting that when purchasingC.japonicafries, individuals with incomplete or injured limbs should not be purchased.During the production period ofC.japonicaecological pond breeding, it is the season of autumn and winter.Special attention should be paid to the sudden change of weather.Before the sudden change of weather, the water level of the pond must be increased to the highest level, at least 2.8 m.If the pond conditions permit, it is best to choose a greenhouse that can survive the winter for ecological breeding.In ecological pond breeding,C.japonicapreferred to eatP.laevisfirst, followed byR.philippinarum, which was consistent with the research results of Zhao Jingetal[9].The ecological pond breeding mode ofC.japonicacan continue to be explored and improved, and the ecological breeding technology and mode can be optimized, for example, reduce the amount ofM.veneriformis,N.variciferus,R.philippinarum, or reduce the density and quantity ofC.japonica,etc.The experimental research of the project has achieved remarkable results, with an average profit of 384 000 yuan/ha and an input-output ratio of 1∶2.85.The experimental research adopted off-season ecological breeding, which effectively improves the utilization rate of biological resources, ponds and seawater resources, enriches the market supply, increases the income of fishermen, and finds a new way for fishermen to increase their income.In summary,C.japonicaecological pond breeding technology is a green and healthy breeding technology and a model for the development of carbon sink fishery.