| The Nujiang River originates from the southern foot of Tanggula Mountain on the Qinghai-Tibet Plateau and is located in the valley of Hengduan Mountains.It is an important biodiversity area in China.In order to identify fish diversity characteristics and habitat suitability in the Tibet section of the Nujiang River,eight surveys were conducted and fish and environmental factors data were collected in the Tibet section of the Nujiang River from 2017 to 2020.The survey sites were used as geographical units to systematically sort out fish composition,distribution,dominant species composition,ecological type structure and external disturbance in the Tibet section of the Nujiang River.By combiningαandβdiversity index with correlation analysis,dilution curve,GAM model,clustering,non-metric multi-dimensional scale ranking and other multivariate statistical analysis methods,the environmental impact mechanism of fish species diversity and the spatial distribution characteristics of fish communities in the plateau were investigated.The evaluation system of fish habitat suitability was constructed based on the three criteria of habitat suitability,biodiversity and important habitat suitability,and the index weight was calculated by analytic Hierarchy Process(AHP).The research can provide theoretical support and suggestions for fish diversity conservation and fishery development in Nujiang River,and provide important basis for formulating scientific and reasonable plateau basin protection strategies.The main research results are as follows:1、A total of 3686 fish with a total weight of 349746.55g were collected from the Xizang section of Nujiang River,belonging to 2 orders,6 families,9 genera and 23species,among which 6 species were endemic to the Nujiang River basin and 5 were exotic.There were 8 species in 3 genera of schizothorax,accounting for 34.78%of the total species,9 species in 2 genera of Cobitidae,accounting for 39.13%of the total species,4 species in 2 genera of Sisoridae,accounting for 17.39%of the total species.The distribution of fish species on the whole showed a decreasing trend with the increase of altitude.The dominant species were Schigothorax gongshanensis,Ptychobarbus kaznzkovi and Schizopygopsis thermalis,the dominant species accounted for 96.28%of the total fish biomass and 91.25%of the total biomass.The ecological types can be divided into benthic slow type,benthic gap type,riffle type and current middle type.The feeding types are omnivorous,which may be caused by the jet stream environment and the lack of food organisms in the plateau basin.The abundance biomass curve(ABC curve)shows that the basin is moderately disturbed on the whole,the reach between the upstream dike and Zuo Gong Yuqu River are in a serious state of disturbance.2、The mean value of Shannon diversity index(~′)varied from 0.369 to 0.908.The mean of Simpson index(C)varies from 0.222 to 0.496.The mean of Margalef richness index(D)ranged from 0.317 to 0.754.The mean of Pielou evenness index(J)ranged from 0.405 to 0.692.The species diversity index and richness estimation index showed that the fish community diversity in Xizang section of Nujiang River was low on the whole.The highest diversity area is located in the lowest elevation of Chawalong and the middle reaches of Luolong and Basu,and the lowest diversity area is located in the upper reaches of Nujiang River Naqu region.Correlation analysis of environmental factors showed that latitude and longitude were significantly correlated with altitude,water temperature and dissolved oxygen(p<0.05).The results of GAM model fitting showed that species richness had a nonlinear relationship with altitude and dissolved oxygen(degrees of freedom greater than 1),and the nonlinear relationship with dissolved oxygen was significant(p<0.05);And the linear relationship between water temperature,p H and flow velocity(degree of freedom is 1),and the linear relationship between water temperature is significant(p<0.05);There was a significant positive correlation between fish species richness and water temperature in the Xizang section of Nujiang River(p<0.05)and had little relationship with p H change.The main factors affecting species richness were water temperature,dissolved oxygen and altitude.Theαdiversity of freshwater fish species in the Xizang section of Nujiang River is mainly influenced by geographical location and physical and chemical properties of water.3、The Jaccard similarity index showed that the lower CWL and the NQS had the lowest species composition similarity(11.76%),while the lower reaches of ZG and BRX had the highest species composition similarity(70%).The results of cluster analysis,NMDS ranking and SIMPER similarity percentage analysis showed that,the fish communities at nine survey sites in Xizang section of the Nujiang River were divided into three groups.The high diversity area composed of the lower reaches of CWL and BS(average similarity within the group 49.39%);The moderately diverse areas of BB,LL,BRX and BRS(mean similarity within the group 71.68%);The low diversity area(mean similarity within group 63.13%)was composed of tributaries ZG,NQS and NQX.The fishes of Schizothorax,Cypoid family,provided the major contribution of intra-group similarity and inter-group heterogeneity,and occupied the absolute niche advantage in the Xizang section of Nujiang River.ANOSIM similarity analysis also verified the significant difference in fish composition among the cluster groups(R=0.969,p<0.05).4、Eight survey sites in the main stream of the Tibet section of the Nujiang River were used as habitat suitability evaluation objects to construct a habitat evaluation index system in Tibet section of the Nujiang River with three index layers and 18calculated parameter layers,Based on the root method of analytic Hierarchy Process(AHP),the weight coefficient of the calculation parameter layer was determined.The results showed that the habitat suitability index of the downstream CWL was 0.6322,the suitability grade was"suitable",and the protection priority was the highest.The habitat suitability index of NQS was 0.3973,the suitability level was"barely suitable",and the conservation priority was the lowest.The differences in suitability were mainly caused by hydrological and water quality conditions and sediment types.The habitat suitability of most survey sites in the Tibet section of Nujiang River was at the level of"suitable"or"generally suitable".Overall,the higher the habitat suitability,the higher the fish diversity and the more complex the community structure.In order to protect fish diversity and reduce intrareal disturbance in the Xizang section of Nujiang River,it is suggested to set up a protected area,strengthen fishery management in the basin,regulate the behavior of release of wild animals,and do a good job in ecological protection and restoration and planning research of basin development. |
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