Effects of Biological Flocs on Growth Performance of Cyprinus carpio var.Furui No.2 and Aquaculture Water Quality

Chunshan GAO, Bingkan YANG, Tiegang LIU, Xiuying LI, Yanhui LIU, Xiujie ZU

Jilin Fisheries Research Institute, Changchun 130033, China

Abstract [Objectives]To study the effects of biological flocs on growth performance of Cyprinus carpio var.Furui No.2 and aquaculture water quality.[Methods]The comparative experiment method was adopted.An experimental group and a control group were set up, with 3 replicates in each group.The experimental group used beet molasses as the carbon source which was added once a week.The breed, quantity, and specifications of stocked fish in each group were exactly the same.The experiment lasted for 60 d.Fish growth indicators were measured every 7 d.After one week of the experiment, ammonia nitrogen and nitrite were monitored once a week, and dissolved oxygen was monitored once a day.[Results]The survival rate of fish, the average weight of , the weight gain rate, and the specific growth rate of the experimental group were significantly higher than those of the control group(P<0.05), and the feed conversion rate was significantly lower than that of the control group(P<0.05).One week after adding the carbon source in the experimental group, the ammonia nitrogen was lower than the control group, and after three weeks it was significantly lower than the control group(P<0.05); after adding the carbon source, the nitrite in the experimental group was lower than the control group, there was no significant difference in the first three weeks(P>0.05), and there was a significant difference after three weeks(P<0.05).The feed conversion rate of the experimental group was significantly lower than that of the control group(P<0.05).[Conclusions]Adding a carbon source can significantly reduce ammonia nitrogen, nitrite and other toxic and harmful substances in the aquaculture water, promote the growth of aquaculture fish and increase the feed utilization rate.

Key words Carbon source, Cyprinus carpio var.Furui No.2, Larvae, Growth, Water quality

1 Introduction

At present, high-density intensive aquaculture in ponds mainly adopts artificial compound feed.Due to a large amount of feed put into the aquaculture water body, it leads to a large accumulation of residual bait manure, which causes aggravation of the pollution of the aquaculture water body and severely damages the aquaculture water environment.In order to make the cultured fish grow rapidly, a large amount of culture wastewater has to be discharged, and the culture tail water contains a lot of toxic and harmful substances, which causes environmental pollution and causes great harm to human life.In this situation, solving the problem of water quality in the process of intensive aquaculture has become a major problem for aquatic scientific research workers.In recent years, the biological flocs technology is an emerging technology that has been widely used for controlling the aquaculture water quality.Without water exchange during aquaculture, the C/N in the aquaculture water is adjusted by adding carbon sources, increasing the number of heterotrophic bacteria in the water, and transforming toxic and harmful substances such as ammonia nitrogen and nitrite in the aquaculture system into bacterial protein, which can be used as nutrients for fishes.This realizes the reuse of nutrients, improves the feed utilization rate, purifies the aquaculture water quality, reduces the amount of water exchange, saves feed, and increases the survival rate of cultured fishes.Biological flocs are mainly used in shrimp farming in aquaculture, and most of the research focuses on water quality control, biological disease prevention, growth promotion, and immunity enhancement.The application in fishes is mainly concentrated on the regulating aquaculture water quality of

Oreochroms

mossambcus

, improving feed utilization rate, reducing the feeding amount in

Ctenopharyngodon

idella

, and increasing the growth rate and reducing feed conversion rate of

Aristichthys

nobilis

.However, there is no research report on the application of biological flocs technology to

Cyprinus

carpio

larvae.Thus, during the culture stage of

Cyprinus

carpio

var.Furui No.2 larvae, we added carbon sources to explore the feasibility of biological flocs technology in the culture system of

C.

carpio

var.Furui No.2 larvae to adjust the culture water environment, improve growth performance, survival rate, and feed utilization rate, so as to provide basic data for the application of biological flocs technology in the culture stage of

C.

carpio

var.Furui No.2 larvae and apply it to the production practice.

2 Materials and methods

2.1 Experimental materials

The experimental

C.

carpio

var.Furui No.2 larvae were introduced from Freshwater Fisheries Research Center of Chinese Academy of Fishery Sciences, and cultured to body weight of(1.74±0.56)g for experiment.

Hypophthalmichthys

molitrix

and

A.

nobilis

summerlings were introduced from the south and cultured to the body weight of(0.51±0.04)g and(0.73±0.08)g, respectively.Six experimental ponds with area of 1.6 × 667 mwere arranged.The carbon source was 48% molasses produced by Jilin Sugar Co., Ltd.; the aeration equipment adopted YL-3.0 impeller aerator; the experimental water used the groundwater.

2.2 Experiment design

We set up an experimental group(adding carbon source)and a control group(not adding carbon source), each group had 3 replicates, the experimental group and the control group adopted the same stocking amount of larvae, with 3 000

C.

carpio

var.Furui No.2, 500 H.molitrix, and 300

A.

nobilis

for each 667 m.The experimental group was only replenished with evaporation and leakage water, while the control group water was replenished or replaced in time according to changes in water quality.Each pond was installed with a 3.0 kW impeller aerator and automatic feeder.The feeding amount of each group was the same.The feed used was commercial carp feed with a protein content of 34%.The feed was put 4 times daily, and the feeding amount was 5%-7% of body weight.For the experimental group, the carbon source was added once a week.After the carbon source was added, the aeration equipment was turned on in time.The amount of carbon source added was calculated according to the following formula:Δ=20×

H

×

S

×

C

NH-Nwhere Δdenotes the amount of carbon source added, expressed in g;

H

is the water depth, expressed in m;

S

is the area of the pond, expressed in m;

C

NH-Ndenotes the ammonia nitrogen content of the aquaculture water, expressed in mg/L.The experiment was carried out from June 7to August 5, 2020, with a total of 60 d.The experimental group was added with carbon source 7 times.

2.3 Data monitoring

Dissolved oxygen(DO)was monitored by iodometry, once a day, ammonia nitrogen and nitrite were monitored by spectrophotometry, once a week, and the monitoring time was about 10:00 in the morning.Weight gain rate(WGR), specific growth rate(SCR), feed conversion rate(FCR), and survival rate(SR)were calculated according to the following formulas:

WGR

=[(

W

-

W

)

/W

]×100%

SCR

=[(ln

W

-ln

W

)

/T

]×100%

FCR

=

W

/(

W

+

W

-

W

)

SR

=

N

/N

×100%where

W

and

W

are the average body weight(g)of

C.

carpio

var.Furui No.2 at the beginning and end of the experiment, respectively;

W

denotes the weight(g)of dead

C.

carpio

var.Furui No.2 in each group;

T

is the experimental time(d);

N

and

N

are the quantity of

C.

carpio

var.Furui No.2 at the beginning and end of the experiment, respectively;

W

is the feeding amount(g)of each group.

2.4 Statistical analysis

The statistical analysis was performed with the aid of Excel software.

3 Results

3.1 Growth and feed utilization situations

The growth and feed utilization situations of the experimental group and control group were listed in Table 1.From Table 1, it can be known that at the end of the experiment, the average weight of

C.

carpio

,

H.

molitrix

, and

H.

nobilis

in the experimental group was 20.75%, 17.04%, and 23.23% higher than that of the control group, respectively; the survival rate was 8.28%, 9.79%, and 4.42% higher than the control group, respectively; the weight gain rate was higher than the control group by 20.76%, 17.12%, 23.36%, respectively; the specific growth rate was higher than the control group by 4.08%, 2.80%, 3.94%, respectively, and there were significant differences with the control group(

P

<0.05).The feed conversion rate of the experimental group was 10.06% lower than that of the control group, and there were significant differences(

P

<0.05).

Table 1 Growth and feed utilization situations of experimental group and control group

3.2 Changes in ammonia nitrogen and nitrite

The changes in ammonia nitrogen are illustrated in Fig.1.In the first two weeks, the ammonia nitrogen of the experimental group was at a steady state of 0.36-0.37 mg/L, but it rose slightly after the second week, until it rose to 0.81 mg/L at the end of the experiment.In the first week, the control group was in a balanced state of about 0.37 mg/L, and it continued to rise steadily from the second to the third week.After the third week, the rise was greater, and then it rose slightly, until the end of the experiment, it rose to 1.18 mg/L.In the first week after adding the carbon source, the ammonia nitrogen of the experimental group was always lower than that of the control group, and after 3 weeks to the end of the experiment there were significant differences between the two groups(

P

<0.05).

Fig.1 Changes in ammonia nitrogen

The changes in nitrite are shown in Fig.2.In the first two weeks, there was no significant change in nitrite in the experimental group, it was in the range of 0.01-0.02 mg/L.After the second week, it rose slightly to 0.04 mg/L at the end of the third week and then gradually declined to 0.03 mg/L at the end of the fourth week, and then quickly rose to 0.09 mg/L at the end of the experiment.In the first 2 weeks, the nitrite in the control group has been increasing slightly and slowly, and it began to increase rapidly in the third week, until the 0.10 mg/L at the end of the fourth week, lasting until the end of the fifth week, and then rapidly rising to 0.13 mg/L at the end of the experiment.

Fig.2 Changes in nitrite

After the start of the experiment, the nitrite of the experimental group has been lower than that of the control group.There was no significant difference between the two groups in the first 3 weeks(

P

<0.05), and there were significant differences between the two groups after 3 weeks until the end of the experiment(

P

>0.05).

4 Discussions

4.1 Effects of adding carbon sources in the aquaculture system on the growth and feed utilization of cultured fishes

The core of biological flocs technology is to convert waste and even harmful substances from aquaculture waters into bacteria proteins that can be used by fishes.The heterotrophic microorganisms in the water body absorb ammonia nitrogen, nitrite nitrogen and other wastes and convert them into their own bacterial proteins to multiply.Through flocculation, they combine single-celled algae, dross, protozoa, organic debris,

etc.

in the water body to form biological flocculation particles that can be used by fishes.The biological flocs technology solves the retention of dross and residual bait in the aquaculture water body, increases the utilization rate of feed, and also removes ammonia nitrogen and nitrite in the aquaculture water body, and purify the water quality.According to findings of Luo Wen

et

al.

, biological flocs technology promotes the growth, increases the weight gain rate, specific growth rate and protein efficiency of Pengze

Carassius

auratus

, and significantly reduces the feed coefficient.In this experiment, we found that the weight gain rate, specific growth rate and survival rate of

C.

carpio

var.Furui No.2,

H.

molitrix

, and

H.

nobilis

in the experimental group were significantly higher than those in the control group(

P

>0.05).

C.

carpio

is an ingesting fish and can actively ingest biological floc particles in aquaculture water.With their growth, the size of the feed ingestion continues to increase, and

C.

carpio

larger than summerling can ingest floc particles of all sizes.

H.

molitrix

, and

H.

nobilis

are filtering fishes, and their flocs ingestive behavior is passive, and floc particles they ingest are increasing with their growth.The average feed conversion rate of the experimental pond in the experimental group was reduced by 10.06% compared with the control group, indicating that the biological flocs in the experimental group were better utilized by the cultured fishes.According to some studies, many beneficial microorganisms in biological floc nutrition can supplement the nutritional deficiencies in artificial feed, make the nutrition of farmed fish more balanced, and promote the rapid growth of cultured fishes.

4.2 Effects of adding carbon sources to the aquaculture system on the quality of aquaculture water

An important significance of biological flocs to aquaculture lies in its rapid heterotrophic transformation of ammonia nitrogen, reducing the accumulation of toxic and harmful substances such as ammonia nitrogen and nitrite.This is mainly due to the addition of carbon sources in the aquaculture system to promote the proliferation of heterotrophic bacteria, thereby promoting the absorption of inorganic nitrogen.According to some studies, heterotrophic bacteria NP1 has an efficient heterotrophic nitrification ability, and its removal rate of ammonia nitrogen is as high as 99.12%.Heterotrophic bacteria can convert the ammonia nitrogen in the aquaculture water into biological flocculation particles that can be directly used by the cultured animals, thereby achieving the purpose of reducing the ammonia nitrogen and improving the quality of the aquaculture water.In this experiment, there was no significant changes in ammonia nitrogen and nitrite in the experimental group and the control group within the first week.One week later, the ammonia nitrogen and nitrite in the experimental group were lower than those in the control group.As shown in Fig.1, after 3 weeks to the end of the experiment, there were significant differences in ammonia nitrogen between the two groups(

P

<0.05).As shown in Fig.2, the difference in nitrite between the two groups was not significant(

P

>0.05)in the second to third weeks.From the start to the end of the experiment after 3 weeks, there was a significant difference in nitrite between the two groups(

P

<0.05), thus it can be inferred after 3 weeks, a large number of biological flocs have been formed, and the effect of regulating water quality is significant.In the

C.

idella

aquaculture system, taking food-grade glucose as the carbon source, Rao Yi

et

al.

studied the effects of adding carbon sources on water quality, and the results showed that the ammonia nitrogen and nitrite content of the experimental group and the control group were not significantly different after adding carbon source for 0-18 d, and the ammonia nitrogen and nitrite content of the experimental group decreased significantly after 18 d, which was significantly lower than that of the control group, indicating that stable biological flocs were formed after 18 d.Taking anhydrous glucose as the carbon source in the

C.

auratus

aquaculture system, Luo Wen

et

al.

studied the effects of adding carbon source on the quality of aquaculture water; the results showed that the three-state nitrogen of the experimental group was relatively stable during the first 30 d of aquaculture, which can be inferred that stable biological flocs may have formed and played a role.In this experiment, we used molasses as the carbon source, and the stable biological flocs were formed and took effect after 3 weeks, which are similar to conclusions of Rao Yi, but greatly different from findings of Luo Wen

et

al.

, which may be related to factors such as water temperature, water quality, carbon source addition, and carbon source type during the experiment.Therefore, related issues need to be further studied.