| Grass carp(Ctenopharyngodon idellus)is a freshwater farmed fish with the highest yield in China.It is a typical herbivorous fish,which can feed on water hyacinth(Eichhornia crassipes).However,grass carp tend to excrete a lot of feces after feeding due to the high content of cellulose in the dry matter of water hyacinth.These feces not only causes eutrophication of the water body but also affects the growth of fish.To solve the fecal pollution problem caused by grass carp,we have some scientific questions:(1)Can the feces generated by grass carp fed with water hyacinth be useful?(2)Can the snail(P.canaliculate)eat grass carp excrement?(3)What are the differences between snail fed with feces and snail fed with other feeds? Does the snail fed with feces have security risk?(4)Can this feeding strategy contribute to the reduction of N and P in the water.This study analyzed the nutritional differences between grass carp feces and duckweed(Lemna minor),clarifying the utilization value of grass carp feces.In this study,the snail was fed with grass carp feces as the experiment group,and the snail was respectively fed with hyacinth and duckweed,serving as a control group and reference group.After 35-day feeding,the growth status,survival rate,morphological indicators,digestive enzyme activity,muscle nutrition,and heavy metal content were measured to evaluate the feasibility and safety of this feeding strategy.The main results and conclusions are as follows:1.Analysis of nutrient composition of grass carp fecesThe determination results of nutritional components showed that the TP in grass carp feces is 37.6% lower than that of water hyacinth,and nitrogen-free extract of grass carp feces is similar to that of duckweed,whereas is 45.5% lower than that of water hyacinth.But the crude fat content in grass carp fercesis 4.5-fold higher than that of water hyacinth.In addition,the difference of crude protein,crude fiber,dry matter,ash and TN among grass carp feces,water hyacinth,and duckweed is about 10%,which is not statistically significant.Therefore,it can be preliminarily judged that the nutritional components grass carp feces are similar to that of water hyacinth,suggesting that grass carp feces can be used as a feed for snails.2.The growth,digestive enzyme activity and body composition of P.canaliculateAfter 35-day feeding,the specific growth rate(SGR)and food conversion rate(FCR)of snails fed on grass carp feces were significantly lower than that of snails fed on water hyacinth and duckweed(P < 0.01),but feeding rate(FR)was significantly higher than that of the latter two(P < 0.01).Besides,there was no significant difference in survival rate(SR)(P > 0.05).After 35-day feeding,the average body weight of snails fedon grass fish feces had an increase of 23.93% from(7.48 ± 0.26)g to(9.27 ± 0.36)g.Although the body weight significantly increased,the growth rate is relatively slow.After 35-day feeding,there was a siginificantly different in shell height/shell width and gastric amylase activity among different groups(P < 0.05).However,the significant difference wasn't observed in the longevity shell width/shell width,liver-to-body ratio,body-to-body ratio,lipase activity,crude protein,crude fat,crude ash,nitrogen-free extract,dry matter and other indicators among different groups(P > 0.05).And the protein content of each group was more than 10%.Only in the aspects of shell height/shell width and gastric amylase activity,it was significantly different between the grass fish feces group and the water hyacinth group.It showed that the ingestion of grass carp feces did not have a significant effect on the morphology,enzyme activity and body composition of snail.3 Muscle amino acid composition and content of P.canaliculateAfter 35-day feeding,17 amino acids were detected in the muscles of snail,including seven essential amino acids(EAA),two semi-essential amino acids(HEAA),and six non-essential amino acids(NEAA).The arginine(Arg)content of the grass fish feces group was significantly higher than that of the other two groups(P < 0.05).There was no significant difference in amino acid content among these groups(P > 0.05),and the content was basically in the same order.There were not significantly different in the essential amino acid index(EAAI),the ratio of branched-chain amino acids to aromatic amino acids(F value),and the amino acid score(AAS)and chemical score(CS)among these groups(P > 0.05).It showed that the ingestion of grass carp feces did not have a significant effect on the amino acid composition and ratio in the snail.4.Muscle fatty acid composition and content of P.canaliculateAfter 35-day feeding,30 fatty acids of muscle were detected in each group,including 12 saturated fatty acids(SFA),six monounsaturated fatty acids(MUFA)and 12 polyunsaturated fatty acids(PUFA).The results showed that the fatty acid content and rank of each group are basically the same.In addition,the total saturated fatty acids(?SFA),monounsaturated fatty acids(?MUFA),polyunsaturated fatty acids(?PUFA)and EPA + DHA were not significantly different from the other two groups(P > 0.05).It showed that the ingestion of grass carp excrement did not have a significant effect on the fatty acid composition and ratioin the snail.5.Determination of heavy metal content in P.canaliculateAfter 35 day feeding,the average heavy metal pollution index(PI)was used to evaluate the pollution of snail muscles by heavy metals.The results showed that the muscles of snails in each group were not contaminated with heavy metals.In addition,the content of heavy metals in muscles of each group is lower than the national standard for pollutants in food(GB 2762-2017),indicating that muscles can be eaten and used as feed.6.Enrichment of nitrogen and phosphorus by P.canaliculateAfter 35-day feeding,the nitrogen enrichment in grass carp feces group,water hyacinth group and duckweed group was 21 g/kg,16 g/kg,23 g/kg,respectively.And the phosphorus rich was 0.7 g/kg,1.23 g/kg,1.68 g/kg,respectively.Besides,there was no significant difference in nitrogen enrichment and phosphorus rich among three groups(P > 0.05).It showed that the feeding the grass carp feces can directly or indirectly reduce the nitrogen and phosphorus content in the aquaculture water and product certain ecological benefits,which is similar with that of feeding the water hyacinth and duckweed.In summary,the main purpose of this research is to study the grass carp farming and two invasive species,water hyacinth and snail.In this study,the grass carp was fed with water hyacinth,and the snail,as an invasive alien species,was fed with excrements produced by grass carp in order to fully utilize these excrements.In addition,a novel breeding system was conducted as following: water supply water nutrient growth nutrients ? feed grass carp with the water hyacinth ? grass carp excrete the feces ? feed snail with grass carp feces to improve the quality of water hyacinth.The contents of nitrogen,phosphorus,and other nutrients were reduced in the water as the growth of grass carp and snail,since grass carp and snail can absorb these nutrients.Eventually,the eutrophication of water can be relieved.Overall,our results can provide a new ideas to make full use of invasive species such as water hyacinth and snail. |
PDF Full Text Request