| The occurrence of fish parasitic diseases is closely related to ecological factors,and has certain seasonal dynamic changes.Brackish water aquaculture is a special habitat,but it is unclear how to explain the correlation and law formed in the salt-fresh water aquaculture environment.This study took grass carp parasites in salt-fresh water breeding ponds along the coast of Guangzhou as the research object,carried out research on the population dynamics of grass carp parasites and their correlation with ecological factors,and analyzed the relationship between the occurrence of parasitic diseases in cultured fish and aquaculture environment factors.Reveal the ecological mechanism of parasitic diseases,and provide a reference for the prevention and control of parasitic diseases in cultured fish.From April 2019 to November 2019,continues weekly sampling survey and analysis were conducted in the breeding ponds,and then the main results and conclusions are as follows:1.The parasites of grass carp in brackish water aquaculture ponds mainly include Trichodina,Gyrodactylus,Lernaea and Dactylogyrus,and they have obvious monthly dynamic changes.Different types of parasites have different peak periods.The peak infections of Gyrodactylus,Lernaea and Trichodina were from April to May,May to June and September to November,respectively.Dactylogyrus has a low abundance of infection throughout the year,and there is no peak of infection.2.The dynamic changes of ecological factors in brackish water grass carp culture ponds are as follows:In April,the p H value(8.5)and transparency(24 cm)were high,and gradually decreased with the increase of the cultivation time.The chemical oxygen demand was around 15 mg/L in April and rose to 23 mg/L in November.Dissolved oxygen is higher in May-June and lower in July-October.Ammonium nitrogen and nitrite fluctuate greatly.The salinity is higher(5‰)during the dry season(January-May and November-December),and lower(2‰)during the wet season(June-October),changes in pond salinity are mainly affected by salinity and rainfall lingdingyang estuary.The range of each ecological factor was as follows: salinity1-6%,water temperature 20.34-32.45℃,transparency 13.11-24 cm,dissolved oxygen1.13-6.23 mg/L,ammonia nitrogen 0.41-4.02 mg/L,molecular ammonia 0-0.43 mg/L,nitrite 0.17-1.86 mg/L,COD 8.67-23.5 4mg/L,p H 7.2-8.49.A total of 65 genera and135 specieswere found in phytoplankton.The main dominant species were Planktolyngbya subtilis,Microcysti minutissima,Pseudoanabaena,Chlorella pyrenoidosa,and Scenedesmus quadricauda,and a small amount of Chaetoceros was found from October to November.The biomass range of phytoplankton was20.69-65.14 mg/L,with an average value of 38.04±10.71 mg/L.A total of 32 genera and 56 species were found in zooplankton.The main dominant species were Difflugia avellana,Tintinnopsis sinensis,Strombidium viride,Polyarthra vulgaris and Moina brachiata and the biomass range of zooplankton was 8.06-55.88 mg/L with an average value of 22.82±13.29 mg/L.3.The correlation analysis results of grass carp parasites and ecological factor were as follows: the infection rate(infection intensity)of Gyrodactylus was negatively correlated with feeding quantity(P<0.01),water temperature(P<0.01),chemical oxygen demand(P<0.05),density and biomass of Phytoplankton(P<0.01),cyanobacteria density(P<0.05),chlorophyta biomass(P<0.01)and was positively correlated with dissolved oxygen(P<0.01),Protozoan biomass(P<0.05)and transparency(P<0.05).The infection rate(infection intensity)of Lernaea was positively correlated with dissolved oxygen(P<0.05),phytoplankton Shannon index(P<0.01),phytoplankton evenness index(P<0.01)and cryptophyta density(P<0.05)and was negatively correlated with molecular ammonia(P<0.01),ammonia nitrogen(P<0.01),zooplankton Shannon index(P<0.05),zooplankton evenness index(P<0.05)and diatom phylum biomass(P<0.05).the infection rate(infection intensity)of Trichodina was positively correlated with the feed amount(P<0.01),phytoplankton evenness index(P<0.05)and chemical oxygen demand(P<0.01).Similarly,the infection rate and infection intensity of Trichodina were positively correlated with the density and biomass of phytoplankton(P<0.01),cyanobacteria(P<0.01),green algae(P<0.01),dinoflagellate(P<0.01)and the phytoplankton evenness index(P<0.05)and were negatively correlated with dissolved oxygen(P<0.01),p H(P<0.01),zooplankton density and biomass(P<0.01),zooplankton richness index(P<0.01),cryptophyta biomass(P<0.01),Protozoan density and biomass(P<0.01).In summary,the infection intensity(infection rate)of grass carp Gyrodactylus,Trichodina and Lernaea is closely related to each ecological factor in brackish water aquaculture ponds.Among them,the feed amount and biomass of Chlorophyta are the key ecological factors that affect the intensity of infection(infection rate). |
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