Nekton Resource And Community Diversity In The Min River Estuary And Its Adjacent Waters,fujian Province,China

The Min River is the largest river in Fujian Province.The Min River Estuary and its adjacent waters is one of the high fishery productivity water zoons in the Province.To date,only a few studies associated with fishery resources and nekton communities have been conducted in the Min River Estuary and its adjacent waters.The purpose of this study is to enrich such data in the the Min River Estuary and its adjacent waters,and to provide important references for sustainable use and protection of fishery resources in the future.Based on the fishery resource seasonal surveys in 11 sampling stations(3 near the mouth of the Min River and 8 offshore)by bottom trawling in 2017 and 2018,we aimed to obtain the fishery resource density data in the Min River Estuary and its adjacent waters,and to assess the current statuses of 20 single-species stocks including fishes and shrimps.We also analyzed the nekton species composition,community diversity,and their spatal and temporal patterns.The main results were summarized as below:(1)Compared with data 10 years ago,the fishery resource density(both weight density and individual density)in the Min River Estuary and its adjacent waters showed a declining trend.Catches were mainly dominant by small-and medium-sized species.There were no significant spatal and temporal differences in fishery resource density(P>0.07),however,the resource densities of various taxon(including fishes,crustaceans,cephalopods)were different;the highest in fishes,followed by crustaceans and cephalopods.Among four seasons,the highest fishery resource density was found in Spring,followed by Summer,low in Autumn and Winter;fish densities were usually the highest,followed by crustaceans and cephalopods.The estimated average fishery weight density and individual density were 678.16±670.45 kg/km2(N=8)and 159265.60±162767.47 ind./km2(N=8),respectively.The estimated average fish weight density and individual density were 498.34±623.23 kg/km2(N=8)and 94736.66 ± 160094.37 ind./km2(N=8),respectively.The estimated average crustacean weight density and individual density were 160.42 ± 79.93 kg/km2(N=8)and 17520.73±13971.16 ind./km2(N=8),respectively.The estimated average cephalopod weight density and individual density were 19.40±14.56 kg/km2(N=8)and 2166.63±3313.15 ind./km2(N=8),respectively.The estimated average fishery weight density and individual density in Spring were 1558.67 ± 837.68 kg/km2(N=2)and 322001.53 ±304388.72 ind./km2(N=2),respectively.The estimated average fishery weight density and individual density in Summer were 538.73 ± 66.34 kg/km2(N=2)and 78094.30 ±20497.96 ind./km2(N=2),respectively.The estimated average fishery weight density and individual density in Aautumn were 382.48±100.45 kg/km2(N=2)and 34565.28±23515.43 ind./km(N=2),respectively.The estimated average fishery weight density and individual density in Winter were 232.76±35.46 kg/km2(N=2)and 23034.99 ±6713.04 ind./km2(N=2),respectively.(2)The length-based Bayesian estimation method(LBB)was used to assess 20 single-species stocks including 8 fishes and 12 shrimps for the first time in the Min River Estuary and its adjacent waters.Results revealed that 8 species(40.00%)were overexploited,5(25.00%)were fully exploited,7(35.00%)were non-fully exploited.Eight overexploited species were Coilia grayii,Collichthys lucidus,Johnius trewavasae,Odontamblyopus lacepedii,Metapenaeus ensis,Alpehus japonicus,Palaemon annandalei and Palaemon carinicouda,most of them were commercial species.(3)245 nekton species were identified in 2017 and 2018 in the Min River Estuary and its adjacent waters,belong to 4 classes,19 orders,72 families,and 144 genera.There were 141 fish species(included 6 Chondrichthyes species and 135 Actinopterygii species),accounting for 57.55%of total species of nekton;Perciformes had the speciesrichest,i.e.,69 species,accounting for 48.94%of total species of fishes.87 crustacean species accounted for 35.51%of total species of nekton;Decapoda had the speciesrichest,i.e.,75 species,accounting for 86.21%of total species of crustaceans.17 cephalopod species,accounting for 6.94%of total species of nekton;Octopoda had the species-richest,i.e.,8 species,accounting for 47.06%of total species of cephalopods.Compared with other sea areas in China,the number of nekton species in the Min River Estuary and its adjacent waters are higher than other sea areas in Fujian Province,such as Sandu Bay and Dongshan Bay,and higher than the Yellow Sea and The East China Sea.The number of fishes in the Min River Estuary were higher than that in the Yangze River Estuary and the Pearl River Estuary.In summry,the number of nekton species in the Min River Estuary and its adjacent waters was generally abundant.(4)Based on the criterion for dominant species that the relative importance index(IRI)greater than 500,there were 21 dominant species in the Min River Estuary and its adjacent waters.Thirteen fish species were Coilia mystus,Thryssa kammalensis,Arius maculatus,Harpadon nehereus,Decapterus maruadsi,Trachurus japonicu,Evynnis cardinali,Polydactylus sextarius,C.lucidus,Pennahia argentata,Pennahia macrocephalus,O.lacepedii,and C.hexanema.Seven crustacean species were Oratosquilla oratoria,Trachypenaeus curvirostris,Parapenaeopsis hardwickii,Portunus trituberculatus,Portunus sanguinolentus,Portunus hastatoides and Charybdis japonica.One cephalopod species was Octopus ocellatus.The composition of dominant species was different seasonally.Compared with data 10 years ago,the composition of dominant species in the Min River Estuary and its adjacent waters had changed largely in the last decade.The original dominant species(such as Wak tingi,Pampus cinereus,Dasyatis navarrae,Setipinna tenuifilis,etc.)did not appear or were general species(10 ?IRI<100)and even rare species(IR1<1)in this study.Still dominant species were all small-and medium-sized species with short life span,and showed a trend of size reduction(such as C.lucidus,H.nehereus,C.mystus,T.kammalensis).(5)The distribution of species individual numbers was relatively even in the Min River Estuary and its adjacent waters,however,the ecological environment was not very healthy,species richness was not good,and the stability of community was weak.Diversity indexes were all low in Spring,indicating the anti-interference ability was the weakest.Spring(May)is the first month of the national fishing moratorium,the low diversity indexes are likely associated with the 8-month exploitation(from September to April of the next year).In terms of weight and individual,Shannon-Wiener indexes(H')did not exceed 2.60.The seasonal average of Pielou indexes(J')were all higher than 0.5.The seasonal average Margalef indexes(D)were higher than 2.00,most were 2.00-3.00.(6)Community structure of nekton in the Min River Estuary and its adjacent waters showed obvious seasonal clustering,while the cluster groups and characteristic species in the same season but different years differed.Dominant species plaid a major role in community clustering.Spatially,there were significant differences in clustering groups near the mouth of Min River and near offshore waters.The characteristic species that caused the seasonal difference were C.mystus,H.nehereus,P.argentata and O.oratoria,Nekton community clusters in different seasons were generally divided into 2-4 groups,characteristic species that caused the difference were C.mystus,H.nehereus,D.maruadsi,T.japonicus,E.cardinalis,P.argentata,C.hexanema,O.oratoria,P.trituberculatus,P.hastatoides and Eriochier leptognathus.Among characteristic species,Eriocheir sinensis is only found near the mouth of the Min River.This species mainly lives in brackish waters with low salinity.Except for E.sinensis,all other characteristic species belong to broad salinity species,all were also dominant species in this study.(7)Fish community in the Min River Estuary and its adjacent waters was under grossly disturbance,the taxonomic relatedness of fishes was closer,ecosystem antiinterference ability and restoration ability was weak.There were 141 fish species in this study,belonged to 2 classes,14 orders,50 families,and 95 genera,the average taxonomic distinctness(?+)was 61.61,variation in taxonomic distinctness(?+)was 134.47.Compared with the taxonomic diversity of fishes in other domestic waters,the taxonomic relatedness of fishes in the Min River Estuary and its adjacent waters was closer,and the taxonomic tree was unevenness,most species are from very few orders.Abundance Biomass Comparison curves results showed that fish communities in the Min River Estuary and its adjacent waters were dominated by r-selected species(i.e.,small species that grow fast and have early sexual maturity),the communities were severely disturbed and under an unstable status.This study revealed that the fishery resource density in the Min River Estuary and its adjacent waters showed a declining trend,and some economic species were overfished.The number of nekton species was relatively rich,but the composition of dominant species had changed compared with 10 years ago.The previous dominant species 10 years ago did not appear or were general or rare species in this study.Dominant species were all small-and medium-sized species with short life cycles,and there was already a trend towards miniaturization.The nekton community diversity in the Min River Estuary and its adjacent waters was lower,the stability of nekton community was weak,and ecosystem anti-interference ability and restoration ability was weak.Therefore,the current status of fishery resources in the Min River Estuary and its adjacent waters should attract the attention of fishery authorities at all levels,analyze the causes of resource decline from all views,and formulate effective protection measures from the perspectives of maintaining the health and stability of ecosystems and sustainable use of fishery resources.