Study on the Feeding Intensity of Grass Carp at Different Physiological Stages on Water Hyacinth ( Eichhornia crassipe )

Zhenhai YU Yongan ZHU Hong LU Fajun YAN Fei HE

Abstract    [Objectives]  This study was conducted to investigate the feeding intensity of different sizes of grass carp on water hyacinth.

[Methods] Grass carp of 10 sizes，  i.e. ， 50， 100， 150， 200， 250， 300， 350， 400， 450 and 500 g， were put in cages. Thirty grass carp were stocked in each cage， and only water hyacinth was fed. The grass carp were caught after 120 d， and the food intake and growth of the grass carp were analyzed.

[Results]  The body weight of all sizes of grass carp increased significantly， and the overall length of grass carp increased significantly. Compared with feeding feed， the body length and body weight in the experimental treatments were lower， and they were more slender; and the feed coefficient of small-sized grass carp was slightly larger than that of large-sized grass carp. When the stocking size was above 414.7 g， the net weight gain of grass carp increased rapidly， and the total food intake also increased.

[Conclusions] This study provides a theoretical basis for achieving an ideal purification effect of water hyacinth on aquaculture water by maintaining the growth of grass carp and the biomass and coverage of water hyacinth within suitable ranges.

Key words   Grass carp;  Eichhornia crassipe ; Feed coefficient; Stocking physiological stage

Water hyacinth （ Eichhornia crassipe  （Mart.） Solms.） can quickly and massively absorb nutrients in ponds， and use eutrophic water to meet its own needs for rapid reproduction  [1] . Water hyacinth has a very strong nitrogen and phosphorus absorption capacity  [2-3] ， and it is obviously superior to other aquatic plants in the purification effect of polluted water bodies  [4] . Water hyacinth’s stems and leaves contain crude protein 10.38， crude fat  1.76 ， crude fiber  24.76 ， carbohydrates 46.65， calcium 2.83， and phosphorus 0.61  [5] . The stems and leaves are soft and palatable. It has a long history as a feed. Herbivorous fish， especially grass carp and white bream， can not only inhibit the growth of water hyacinth， but also purify polluted water  [6] . However， water hyacinth multiply rapidly and cover the aquaculture water surface， destroying the biodiversity of water bodies and affecting the yield and quality of aquatic products  [7] . How to use the feeding effect of herbivorous fish to keep the biomass of water hyacinth in a dynamic balance and give full play to the maximum value of water hyacinth resource utilization is of great significance， while few studies have been conducted on the feeding situation of herbivorous fish on water hyacinth  [8] . In this study， the feeding intensity of grass carp on water hyacinth was investigated to study the relationship between the growth of grass carp and the feeding on water hyacinth. This study will help to further find out the growth and the feed coefficient of grass carp and the water hyacinth biomass required by water hyacinth， and provide scientific and technical support for controlling the crazy growth of water hyacinth， maintaining a  suitable  biomass， purifying the pond water body， and repairing pond water environments.

Materials and Methods 

Experimental materials

The size of the test cages was 4 m×3 m×2 m in length×width×height， and the meshes were 1 and 3 cm. The number was 3×6. The grass carp fingerlings and water hyacinth were all from local farms. Grass carp fingerlings were fed with full-price compound pellet feed before the experiment.

Experimental methods 

Water quality test method

Tests of total nitrogen （TN）， total phosphorus （TP）， ammonia nitrogen （NH +4-N）， nitrite nitrogen （NO -2-N）， phosphate phosphorus （PO  3- 4-N） and permanganate index （COD Mn ） were carried out according to the methods in  reference  [8] .

Amino acid content and fatty acid analysis

Water hyacinth was sent to Jinan Jinyu Biotechnology Co.， Ltd. for the analysis of its amino acid contents and fatty acid contents.

Experimental design

In order to keep the water quality stable， the experiment was carried out in a collapsed pit pond of Yutai Coal Mine in Jining， Shandong Province. In the collapsed pit pond of the coal mine with an area of about 15.33 hm 2， 15 cages were arranged， and the cages were fixed in the floating cage frame. Ten gradient test groups of grass carp body weight were set up according to the size of grass carp： 50， 100， 150， 200， 250， 300， 350， 400， 450， 500 g/carp， in 3 replicates. Each cage was stocked with 30 grass carp， which were fed once a day， and the number of water hyacinth input was recorded. The breeding period was from June 22， 2017 to October 20， 2017， that is， 120 d. The whole length， body weight， survival rate and other indices of the experimental fish were measured before and after stocking.

Mathematical statistical analysis

Statistical analysis and chart processing of data were performed using SPSS 15.0 （Statistical Package for Social Science） and Excel 2003 software  [9] . Analysis of variance （Analysis-of- Variance，  ANOVA for short） was used to test the homogeneity of variance for the data of the experimental groups. Then， paired samples  t -test was adopted for the analysis of the significance of differences， so as to observe the effect of the growth test.

Results and Analysis 

Detection and analysis of water quality in the collapsed pit pond of coal mine

After testing and analysis， the water quality indices in the collapsed pit pond were as follows： total nitrogen 1.319 mg N/L， total phosphorus 0.053 mg P/L， ammonia nitrogen 0.97 mg N/L， nitrite 0.012 mg N/L， phosphate 0.032 mg P/L， and permanganate index 5.7 mg/L. To sum up， the water quality in the collapsed pit pond of coal mine met GB 11607-1989 Water quality standard for fisheries and NY5361-2010 Pollution-free food： freshwater aquaculture production area environmental conditions.

Analysis of nutritional components of water hyacinth 

Analysis of conventional nutrient composition of water hyacinth

The results of the test bait water hyacinth were as follows： crude protein content 11.66%， crude fat content 0.91%， and crude ash content 22.03%.

Analysis of amino acid contents and fatty acid contents of  water  hyacinth

The contents of amino acids and fatty acids are shown in Table 1. Except that tryptophan was not detected due to acid hydrolysis， a total of 17 kinds of amino acids were detected in water hyacinth， and the content of essential amino acids accounted for 30.86%.

Fifteen kinds of fatty acids were detected in water hyacinth， including 6 kinds of saturated fatty acids SFA （39.72%）， 3 kinds of monounsaturated fatty acids MUFA （9.68%）， and 6 kinds of polyunsaturated fatty acids PUFA （50.59%）.

Through experimental analysis， water hyacinth contained 17 kinds of amino acids， of which the content of essential amino acids accounted for 30.86%. Fifteen kinds of fatty acids were detected， including 6 kinds of saturated fatty acids SFA （39.72%）， 3 kinds of monounsaturated fatty acids MUFA （9.68%）， and 6 kinds of polyunsaturated fatty acids PUFA （50.59%）.

Analysis of growth indices before and after feeding water hyacinth 

Grass carp growth

T test was adopted for the full length of grass carp before and after the experiment through two-sample   equal  variance analysis. After feeding water hyacinth，  P =0.008 6，   P <0.01， that is， the difference in the growth of grass carp was extremely significant， and the increase in total length was extreme significant. Similarly， for the body weight of grass carp，  P = 0.045 8，   P <0.05， that is， the body weight of grass carp growth was significantly different， and the body weight increased significantly （Table 2）.

Changes in total length and body weight of grass carp

The full length and body weight before and after stocking in Table 2 of the experimental results were subjected to simulated regression：

The simulation formula of the full length and body weight of grass carp fingerlings fed with feed：

y=7.951e  0.126 6x （R 2=0.984 5）  （1）

The simulation formula of the full length and body weight of grass carp fingerlings fed with water hyacinth：

y=7.281 5e  0.111 8x （R 2=0.992 9）  （2）

As can be seen from Fig.1， when comparing with the grass carp fed with single water hyacinth and the grass carp fed with the full-price compound feed， the body shape of grass carp was significantly different. The grass carp with the same body length fed with single water hyacinth had lower body weight and was slender than the grass carp fed the compound feed. The difference in body weight became more pronounced with the increase of body length.

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Analysis of different sizes of grass carp with net weight gain

Simulation regression was performed between stocking size and net weight gain of grass carp in the experimental groups：

Polynomial simulation regression equation：  y=0.013 2x 2-3.390 8x+300.36 （R 2=0.992 1）  （3）

Linear simulation regression equation：  y=3.879x-444.3 （R 2=0.847 1）  （4）

To find the intersections of the equations， then （3） was subtracted （4）：

0.013 2x 2-7.269 8x+744.66=0

x1=136.031 095 5， x2=414.713 287

They were substituted into （3）， obtaining：  Y1=83.364 619 44，  y2=1 164.372 84.

The coordinates of the intersection points were： （136.03，   83.36）;   （414.71， 1 164.37）.

As can be seen from Fig. 2， equation （3） represented the actual net weight gain simulation curve of various experimental groups， and equation （4） represented the average net weight gain simulation line of various experimental groups. It indicated that the general trend in this experiment was that the stocking size was positively correlated with net weight gain.

The intersection points of the two simulated lines：

When the stocking size was less than or equal to 136.03， the net weight gain was less than or equal to 83.36 g， which might be due to the fact that grass carp fry still fed on protozoa， rotifers， cladocerans and copepods， and the biomass of water hyacinth ingested was low.

When 83.3 6 g<stocking size<414.71 g， the biomass of water hyacinth ingested by grass carp increased gradually and slowly， but the net weight gain was lower than the average straight line of the net weight gain of various experimental groups.

When the stocking size was greater than or equal to 414.71 g， the net weight gain was greater than or equal to 1 164.37 g， and intake of water hyacinth by grass carp exceeded the average value of linear equation （4）， and with the increase of the stocking size and the increase of net weight gain， the biomass of grass carp ingested by water hyacinth increased rapidly.

Analysis of different sizes of grass carp and feed coefficient

Simulation regression was performed between stocking size and feed coefficient of grass carp in the experimental groups：

Exponential simulation regression equation：

y=120.5e  -0.000 6x  （R 2=0.999 1）  （5）

It can be seen from Fig. 3 that the stocking size of grass carp was inversely correlated with the feed coefficient. With the increase of the stocking size of the fish， the feed coefficient decreased slightly， but the net weight gain increased faster， and the biomass of water hyacinth ingested correspondingly increased.

Conclusions and Discussion 

Effects of different sizes of grass carp on the intake of water hyacinth

The nutrients of water hyacinth can meet the survival needs of grass carp and can be used as green fodder for herbivorous fish  [10] . The significant increase in the body weight of the experimental grass carp indicates that grass carp feed on water hyacinth for their own growth， which is consistent with the studies of Wang and Yu  et al.   [6，11] . However， using water hyacinth as single bait affected the growth rate of grass carp. Compared with the grass carp fed with the compound feed， the weight growth lagged behind  significantly. Therefore， when the size of grass carp is less than 83.36 g，  the culture of grass carp can be mainly based on compound feed. When the body weight of grass carp is greater than 83.36 g， the food intake of water hyacinth will gradually increase， and when the average size of grass carp is more than 414.71 g， the effect of feeding water hyacinth will be better， and the fish will gain weight faster. The experiment showed that with the increase of the stocking size of the fish species， the feed coefficient decreased slightly， but the net weight gain increased faster， and the total amount of water hyacinth ingested increased accordingly. It might be due to the ingestion of a large amount of water hyacinth roots to a large  amount  of whole water hyacinth by grass carp， and might be related to the absorption and utilization of grass carp and the reduction of energy conversion in the pond  [12-13] . As the size of grass carp increases， the oral fissure of grass carp increases， the chewing function of pharyngeal teeth is enhanced， and the palatability of eating water hyacinth is also enhanced， so the amount of compound feed can be reduced.

Effects of feeding water hyacinth on the growth of grass carp

Grass carp feeding on water hyacinth had a significant increase in body weight and an extremely significant increase in total length， which indicates that the biomass of water hyacinth ingested by grass carp increased with the size of the fish. The daily intake of water hyacinth biomass was also in a positive correlation with the size of grass carp. However， the grass carp that ingested water hyacinth alone became slender， which might be due to the palatability， rough feed and incomplete nutritional value of water hyacinth  [8] . For different sizes of grass carp， as the size of grass carp increased， they changed from eating a lot of roots of water hyacinth to a lot of rhizomes and leaves， that is， whole water hyacinth plants， which is consistent with the result obtained by Wang  et al.   [6]  that grass carp with a body weight of （150 ± 5） g could eat a lot of water hyacinth roots and reduce the growth rate of water hyacinth. With the increase in the biomass of water hyacinth ingested by grass carp， it is not clear whether it can make up for the singleness and deficiency of nutrition， and catch up with and exceed the fatness of the fed feed， which needs to be confirmed by further  research.

To sum up， when using water hyacinth to restore the ecological environment of ponds， and using the biological method—grass carp to control the biomass of water hyacinth  [14-15] ， it is necessary to focus on the stocking size of grass carp. Only suitable size of grass carp can more effectively inhibit the growth of water  hyacinth.  The results of this study showed that when the body weight of grass carp was greater than 83.36 g， the food intake of grass carp to water hyacinth began to increase， and the food intake to water hyacinth increased gradually with the increase of the size of grass carp， that is， they were positively correlated. Therefore， in aquaculture ponds， if the biomass ingested by grass carp exceeds the growth biomass of water hyacinth， the growth of water hyacinth will be inhibited， thereby affecting the purification effect of water hyacinth on aquaculture water. However， if the biomass ingested by grass carp is lower than the biomass of water hyacinth growth， it will lead to the uncontrolled growth of water hyacinth， affecting the growth of grass carp， and salvaging excessive water hyacinth will be time-consuming and labor-intensive， increasing the cost of farming. Studies have shown that the coverage rate of water hyacinth at 15%-20% in the water body in aquaculture ponds can achieve the best effect on water purification  [16] . Therefore， how to achieve an ideal purification effect of water hyacinth on aquaculture water by maintaining the growth of grass carp and the biomass and coverage rate of water hyacinth in aquaculture ponds within suitable ranges needs further research.

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