Lihua CHEN, Yanhui LIU, Tiegang LIU, Bingkun YANG, Xiuying LI, Zhanwei WANG, Xiujie ZU*

1.Baicheng Yueliangpao Reservoir Administration, Da’an 131307, China; 2.Jilin Academy of Fisheries Sciences, Changchun 130033, China

Abstract [Objectives]To explore the ecological aquaculture model in ponds in North China, several single techniques were assembled into the same culture system.[Methods]Three ponds were selected, the species and stocking rate were exactly the same, the water was not changed during the culture period, and the water loss due to evaporation and leakage was recovered.Since the middle of May, the hydrochemical indicators such as ammonia nitrogen, nitrite nitrogen, water temperature, dissolved oxygen and pH were monitored every 10 d.According to the monitoring results of ammonia nitrogen, carbon sources were added to the culture ponds to adjust the ratio of C to N, and carbon sources were added 9 times during the culture period.The stocking rate and yield per unit area were accurately measured at the beginning and end of the experiment, and no less than 30 fishes were randomly sampled to calculate the relevant growth indicators and feed coefficients.[Results]Except that the nonionic ammonia in pond 3# exceeded the standard by 10.3% on July 25, all other hydrochemical indicators met the Fisheries Water Quality Standard, and there was no significant difference in all hydrochemical indicators at the same time(P>0.05).The survival rate in 3 ponds was more than 95.0%, the average body weight of individuals out of the pond had no difference(P>0.05), and the feed coefficient was 1.41-1.43.There was no disease during the culture period, and the water was saved by 46.6% compared with the traditional culture model.[Conclusions]This study can provide a basis for the construction of a new model suitable for ecological aquaculture in ponds in North China.

Key words Ponds in North China, Water Saving, Ecology, Model, Construction

1 Introduction

Over the past 40 years of reform and opening up, aquaculture has developed rapidly, in which freshwater pond culture plays an important role in aquaculture[1].However, due to the excessive pursuit of profit maximization, the traditional aquaculture industry makes excessive use of land and water resources and increases the use of inputs while increasing the output.High density and high input lead to frequent diseases, deterioration of aquaculture water quality, and environmental pollution of water caused by frequent water exchange.Since the 18thCPC National Congress, the national fishery system has taken accelerating the green development of fishery as the fishery center, vigorously developing healthy aquaculture, and the mode of industrial development is undergoing revolutionary changes[2].Therefore, changing the traditional extensive production mode and optimizing and adjusting the pond green ecological aquaculture model is the fundamental way to solve the problem of pond culture[3].At present, all provinces and cities in the south are vigorously promoting new culture models such as container fish culture[4], pond circulating water fish culture[5], fish and vegetable symbiosis[6].The northern region is limited by climatic conditions and natural environmental conditions and other factors, so it is difficult to popularize and apply many new models.Therefore, it is necessary to explore an efficient, simple and easy-to-popularize pond green ecological aquaculture model.

Since the implementation of the national bulk freshwater fish industry technology system construction project in 2018, a number of new pond culture technologies have been studied and established to meet the needs of healthy culture[7].A number of technologies have been demonstrated and popularized, such as immune prevention and control of two-year-old grass carp, biological flocculation technology to regulate and control water quality, accurate feeding, comprehensive disease prevention and control, micropore oxygen enrichment in feeding areas.The purpose of this study is to integrate the above technologies into the same culture system to construct a new model suitable for ecological aquaculture in ponds in North China, which has the "ecological, clean and accurate" characteristics, so as to lay a foundation for the rapid development of freshwater aquaculture.

2 Materials and methods

2.1 Test materialsThe test time is from May 6 to October 14, 2021, and the test site is Jinyuan Aquatic Farm, Changyi District, Jilin City.There are three experimental ponds(pond 1#, pond 2#, pond 3#), the area is 10 667 m2, the depth of the pond is 3.2 m, the maximum depth of water is 2.8 m, and the source is groundwater.The fries of two-year-old grass carp and two-year-oldC.carpio2 were cultivated on this farm, and the average body weight was 164.747 2 and 147.336 6 g, respectively.The fries ofP.fulvidracowere purchased locally, with an average body weight of 19.782 g.Silver carp and bighead carp were introduced from South China, and the average body weight was 0.210 07 and 0.260 06 g, respectively.Sucrose molasses was selected as the carbon source, and the content of active constituent was 48%.Quadruple vaccine was selected as grass carp vaccine(enteritis, gill rot, red skin disease, hemorrhagic disease).The oxygen enrichment equipment was the combination of microporous aeration equipment(Roots blower + microporous aeration plate)and traditional aeration equipment(paddle wheel aerator).Positive pressure 360°air-assisted feeding machine was selected as feeding equipment.

2.2 Experimental designThe experiment was conducted with two-year-old grass carp as the main fish for culture, combined with the fries of carp,P.fulvidraco, silver carp and bighead carp for mixed culture.The stocking situation of fries in 3 experimental ponds was exactly the same, and the stocking quantity and specifications are shown in Table 1.

Table 1 Stocking of experimental fish

ΔCH=20×H×S×Cammonia nitrogen

(1)

The water level was about 1.5 m before the middle of June, and 2.5-2.8 m from the middle of June to September 20.After September 20, the water level gradually lowered with the drop of temperature, in preparation for coming out of the pond.There was no change of water during the whole test period, only the water lost due to evaporation and leakage was added.100 m2microporous aeration facility was installed under the feeding table, the microporous aeration equipment was turned on during feeding, to increase the dissolved oxygen in the feeding area, promote the feeding of fish and improve the feed utilization rate.Feed nutrition and feeding were mainly for grass carp.According to the principle of accurate feeding, the feeding characteristics, living habits and metabolic rules of grass carp were studied to determine the feed nutrition, grain size, feeding frequency, feeding amount and so on.When the average body weight was lower than 250 g, the feed protein content was 29%, it was fed 4 times a day, and the grain size of feed was 3.5 mm.When the average body weight was more than 250 g, the feed protein content was 27%, it was fed 4 times a day, and the grain size of feed was 4.0-6.0 mm.The feeding amount should be flexibly controlled according to the weather and the intake of fish, until most of the fishes were full.

2.3 Data monitoring

2.3.2Growth indicators monitoring.No less than 30 fishes were randomly selected during stocking and catching, the body weight of each fish was measured one by one, and the average body weight during stocking and catching was calculated.During the catching, all kinds of fish were weighed separately, the yield per unit area was calculated, and the catch was calculated according to the average body weight and yield per unit area.Each growth indicators was calculated according to formula(2)-(4):

Nt=W/Wt

(2)

SR(%)=100×Nt/N0

(3)

FCR=Wfeed/(Wyield+Wdeath-Wstocking)

(4)

whereSRis the survival rate, %;FCRis the feed conversion rate;NtandN0are the number of fishes during catching and stocking, respectively;WandWtare the yield per unit area and the average body weight at the end of the trial, respectively, g;Wfeed,Wyield,WstockingandWdeathare feed amount, total yield of carp and grass carp, total stocking amount of carp and grass carp, and total death amount during culture of carp and grass carp, respectively, g.

2.4 Data analysisThe data were analyzed by Excel, and the data were expressed as mean ±standard deviation(Mean±S.E.).The significance was tested by single factor analysis of variance(ANOVA).

3 Results and analysis

Table 2 Addition time and amount of carbon source(kg/667 m2)

Fig.1A reflects the changes of NH3-N in three experimental ponds, showing a trend of rising first and then descending.The content of NH3-N in all three ponds reached peak on July 25, and the ranges were 0.004 3-0.018 3, 0.003 7-0.020 3, 0.003 3-0.022 3 mg/L, respectively.Only the content of nonionic ammonia in pond 3 was 0.022 3 mg/L on July 25, exceeding the standard by 10.3%.After reaching the peak, nonionic ammonia decreased slowly.There was no significant difference in the change of NH3-N at the same time point(P>0.05).

Fig.1C reflects the change of pH in three ponds.The change rule of pH was completely the same, rising first and then decreasing, but the peak time was different.The peak value of pond 1# was 7.80 on July 15, and the peak value of pond 2#, 3# was 7.89 and 8.04 on July 25, respectively.After the peak value appeared in three ponds, the pH gradually decreased with the experiment.There was no significant difference in the change of pH in three ponds at the same time(P>0.05).

Fig.1D reflects the change of DO in three ponds.From the change curve, there was no obvious change rule, and the range of variation was 4.99-6.00, 5.38-6.11 and 5.27-5.99 mg/L, respectively, and the lowest value was 4.99 mg/L, which met the requirements of fishery water quality standards for dissolved oxygen in aquaculture water(close to or greater than 5.0 mg/L).

Note: A.nonionic ammonia; B.nitrite; C.pH; D.DO.

3.2 Growth indicators and feed utilizationThe growth indicators and feed utilization were shown in Table 2.There was no significant difference in individual average body weight of all kinds of fish caught in 3 ponds(P>0.05).Other growth indicators and feed coefficients were very similar.

Table 3 Growth indicators and feed utilization(667 m2)

3.3 Water saving and emission reductionDuring the experiment, there was no change of water, fresh water was added 8 times in 3 ponds, the average fresh water(16.0 cm)was added each time, the total depth of fresh water was 1.28 m, and the amount of water added per 667 m2was 853.8 m3.In the traditional culture model in the northern region, the water was added and changed 12-13 times in the production season, with water of about 20.0 cm each time.The average annual water replacement per 667 m2was about 1 600.0 m3[10].Compared with the traditional culture model, this culture model saved water 46.6% a year.

3.4 Occurrence of fish diseaseAfter the middle of July, due to the increase of water temperature, the large feeding amount and the deteriorating water quality of culture ponds, regular testing found that some carp and grass carp suffered from gill rot disease.Fresh water was added for 2 consecutive days and aeration equipment ran for a long time from July 26 to 27, and no disease was found in early August.During the whole culture period, except for regular inspection of fish disease and sampling, only a few sporadic dead individuals were found in each pond, and the fish was found to be rotten and the cause of death could not be examined.Only the survival rate ofP.fulvidracocultured in pond 1# was slightly lower than 95.0%(94.7%), and the survival rate of other fishes in pond 1# and all fishes in pond 2#, 3# was greater than 95.0%.

4 Discussion

Through 360°air-assisted feeding, the feed was distributed evenly, and the feeding area was large, which avoided the intense fish intake and physical energy consumption.The microporous aeration in the feeding area satisfied the feeding needs of fish under the condition of high oxygen environment, and the feed conversion rate was improved.According to the results of culture in Table 3, the average specification and total yield of all kinds of fish in three ponds were not lower than those under the traditional culture mode in North China.Through the precise feeding combination, the feeding amount, feeding frequency, feeding mode and nutritional requirements at each growth stage were effectively controlled, so that the feeding rate was close to the best intake rate under the condition of maximum benefit[18], and the feed utilization rate was improved.The FCR was low, only 1.41-1.43, which was significantly lower than that under the traditional culture model[19].