Studies On Fish Community Diversity And Community Dynamics Based On Multivariate Analysis In The Yangtze River Estuary

Estuaries are critical ecotones where fresh waters and marine environment meet, theenvironmental gradients in estuaries are very steep, and biological productivity areespecially high, estuaries are also viewed as eco-crisis regions. The Yangtze Riverestuary is the most productive fishery ecological system in China and one of thebiggest estuaries in the globe. Estuarine fish assemblage takes a key role in structuringthe system, i.g. the stability of the system has been influenced directly and indirectlyby fishes via food chain and web. Recently decades, fish community of the systemchanges significantly because of human impacts and natural disturbances such asoverfishings, pollutants, climate changes and so on, which implying that it isnecessary to know community diversity, to test key structuring factors, to examine thetemporal and spatial variations of community structure and to analyze the historicalsuccessions of the community. So, our work could provide valuable information onassessing the fish resources and maintaining the sustainable development of theYangtze River estuary ecosystem.Firstly, fish and environmental data using bottom trawling carried across the entiresalinity gradient in the Yangtze River estuary from spring (May)2010to winter2011(February)were used, and relations in environmental variables (seven abioticinfluences and three biotic factors) and fish community, seasonal and spatial changesin fish community were firstly analyzed based on multivariate analysis. The resultsindicated that62species belonging to29families were collected, the most speciosefamilies were Enguridae (7species), Gobiidae (7species), Cyprinidae (7species),Sciaenidae (6species), Tetraodontidae (5species) and Cynoglossidae (5species),while19single family species were also captured. It is clearly that marine migrantand estuarine species dominated the fish assemblages either in total abundances or innumbers of species. Total catches were dominated by few species, for example, interms of Index of Relative importance IRI, six species (Collichthys lucidus (51.9%),Chaeturichthys stigmatias (20.9%), Coilia ectenes (8.5%), Harpadon nehereus(4.2%), Coilia mystus (4.1%), and Cynoglossus joyneri (3.1%)) were defined asdominant taxa. The increasing trends in species richness and abundance coupling with decreasing patterns in△+from riverine zone to the adjacent sea reflect that thenumbers of species and individuals are smaller, also they tend to be related moreclosely than other zones. The community structures showed spatial variation, nonmulti-dimension scaling analysis NMDS highlighted that there is a obvious boundarybetween riverine zone and other zones, and fish assemblages in this zone could bedefined as freshwater community, SIIMPER analysis further indicated that diagnosticspecies were freshwater species (Tachysurus nitidus), anadromous (C. ectenes) andsemi-anadromous (C. mystus). However, there is no clear spatial patterns among otherzones. Both Community diversity and community structure were relatively stableduring the surveys, and community structure in winter versus that in both spring andsummer did differ significantly. Detrended canonical correspondence analysis DCCAshowed that among the seven abiotic factors (temperature, salinity, depth, silt, sand,ph, grain size) and three biotic (crustacean biomass, benthos biomass, and chlorophyll)influences analyzed, salinity, temperature, and crustacean biomass were the threemost important factors driving the community, thus, spatial and temporal variationswere mainly structured by salinity and temperature, respectively. Results of G-tests ofindependence tests indicated that spatial separations occurred among the Gobiidae orCynoglossus whatever season, suggesting that one species is abundant in some sites,while the other taxon may be abundant in other sites.Secondly, fish and environmental factors using setnet surveyed monthly from May2010to April2011were analyzed. The results showed that56species were collectedduring the study period. In terms of species, marine migrants and estuarine speciesdominated the community, while, based on abundance, the marine migrant was themost abundant guild. Total IRI were totally represented by several small-size taxa,thus, six species (H. nehereus (34.7%), C. mystus (27.5%),C. lucidus (20.0%), C.ectenes (3.1%), Syngnathus acus (2.6%),and Stolephorus commersonii (2.2%))accounting for>90.0%of the IRI. Clearly temporal differences in communitydiversity were observed. NMDS and Cluster analysis showed community structures intwo continued months are similar, however, that in December2010differs from thatin January2011, these two techniques also illustrated a clear division into four distinctgroups of samples (GroupⅠ: samples capturing from January to April; GroupⅡ:samples capturing from May to the first half month of August; Group Ⅲ: samplescapturing from the second half month of November to December; Group Ⅳ: samplessampling between the second half month of August and the first half month of November), Results of G-tests of independence tests indicated that spatial separationsoccurred among the Enguridae during the study period, suggesting that one species isabundant in some months, while the other taxon may be abundant in other months. Itis clearly that abundance of dominant species differed among months. In order tofurther protect C. ectenes, we suggest that November should also be fishing closingtime. Multi regression demonstrated that temperature was the most importantinfluencing the monthly mean abundance across the study period. When comparedwith seasonal samplings via bottom trawlings, a stronger temporal patterns incommunity structure and diversity of the monthly samplings based setnet could befound.Finally the long-term changes in taxonomic diversity (average taxnomicdistinctness and variations in taxonomic distinctness) of fish community of theYangtze River estuary are examined. Fish species lists recorded between threedifferent periods (1985~1986,2004,2010~2011) and12surveys were used toestimate the∧~+and△+of per survey, the results showed that△+in2004and2010~2011(except spring survey) fell below the95%probability funnels for△+caculating the full list of fish species from the Yangtze River estuary, morever, thedecreasing trends in△+were also observed. However, patterns in∧~+was differentfrom△+, as all∧~+distributed in the95%probability funnels for∧~+calculating thefull list of fish species, mean∧~+didn’t differ significance between different periods.Declines in△+of the Yangtze River estuary was related to the overfishing, pollutantsand hydroprojects, suggesting that relations among species were more closely inrecent years than that in1985~1986, which implying that the niche similarity betweenspecies increases, and may cause the loss of stability or resilience of the community.