River Health Asessment Based On A Fish-Based Index Of Biotic Integrity In The Upper And Middle Reaches Of The Lancang River

The Lancang-Mekong River is one of the most important international rivers,maintaining important global aquatic biodiversity.Meanwhile,the Lancang River is also an important construction area base for hydropower development in China.Recently,with the expansion of regional population and the demand of economic development,a series of problems of the river itself and the surrounding environment have been triggered,such as the reduction of riparian vegetation,the invasion of exotic species,the change of land use types and so on.In this study,the upper and middle Lancang River was choosed as the study area,and the fish was considered as biological indicator.This study analyzed fish community structure and spatial distribution characteristics and constructed a fish-based index of biotic integrity（F-IBI）to evaluate the river health on the main stream and tributaries of the upper and middle reaches of the Lancang River respectively.The outcomes would provided a scientific basis for understanding the health status and support the management and protection of fisheries.The main findings are as follows:（1）River health assessment based on F-IBI for main stream in the upper and middle reaches of the Lancang RiverTwo field surveys（September-October 2019,and May-June 2020）collected1,533 fishes,belonging to 3 orders,10 families,31 genera and 36 species,which including 19 indigenous species,17 exotic species,and 21 migratory fish species.The omnivorous fish species（20 species）,mainly including Abbotina rivularis,Pseudorasbora parva,Hemiculter leucisculus,and fishes of Cobitidae,are accounted for the largest part of all fishes.Fish species in the upper and middle reaches of the Lancang River varied greatly among different sampling sites.The fish community structure showed a certain spatial differentiation pattern:the richness and abundance in the middle reaches are overall higher than that in the upper reaches.The canonical correspondence analysis（CCA）showed that dissolved oxygen,water temperature and p H are environmental factors that significantly affected the spatial distribution of fish communities.The reference sites and impaired sites were screened by water quality and habitat quality scores（HQS）,and eight core indicators were obtained to construct the F-IBI:The number of exotic species,number of migratory species,percentage of indigenous species,percentage of migratory species individuals,percentage of herbivorous species individuals,percentage of omnivorous species individuals,number of viscous egg-laying species,and percentage of tolerant species.The F-IBI evaluation results showed that the overall health status of the upper and middle reaches of the Lancang River was good.3 sites with excellent health status,accounting for 18.75%of the total number of sites（16 sites）;1 site with good health status,accounting for 6.3%;4sites with fine health status,accounting for 25%;4 sites with fair health status,accounting for 25%;2 sites with poor health status,accounting for 12.5%;2 sites with extremely poor health status,accounting for 12.5%.The longitudinal distribution pattern indicated that the health of the river gradually deteriorated from the upstream to the middle reaches,with the increase of human activities.F-IBI scores were significantly and positively correlated with the HQS,and are significantly and negatively correlated with NH₃^-N,COD_Mn,and conductivity.In conclusion,it was practicable to use the F-IBI to assess the main stream river health in the upper and middle reaches of the Lancang River.Invasion of exotic species,fishing,dam construction and grazing were the main threats of the main stream river health in the upper and middle of the Lancang River.The contribution rank of the extrinsic factors to river health degradation was orderly listed as below:fishing,invasion of exotic species,dam construction,grazing,tourism development,water pollution,river bank solidification,sand mining,sewage outlet and natural disasters.（2）River health assessment based on F-IBI for tributaries in the upper and middle reaches of the Lancang RiverTwo field surveys（September to October 2019,May to June 2020）collected 437fish individuals,belonging to 4 orders,8 families,24 genera and 26 species,including9 indigenous species,17 exotic species,and 9 migratory fish species.The omnivorous fish species（13 species）,mainly including P.parva,H.leucisculus,C.auratus,C.auratus and fishes of Cobitidae,accounted for the largest part of all fishes.Fish species varied greatly between different sampling sites.The fish community structure showed a certain spatial differentiation pattern:richness and abundance in the middle reaches were higher than that in the upper reaches.The CCA analysis showed that latitude was the environmental factor significantly affecting the spatial distribution of fish communities.The reference sites and impaired sites were screened by water quality and the HQS.Seven core indicators were obtained to construct the F-IBI:number of invasive species,percentage of indigenous species,number of omnivorous species,percentage of omnivorous species individuals,number of viscous egg-laying,percentage of sensitive species and percentage of tolerant species.The F-IBI evaluation results showed that the overall health status of tributaries was good.2 sites with excellent health status,accounting for 12.5%of the total number of sites（16 sites）;2 sites with good health status,accounting for 12.5%;4 sites with fine health status,accounting for 25%;3 sites with fine health status,accounting for 18.6%;4 sites with poor health status,accounting for 25%;1 site with extremely poor health,accounting for 6.3%.The longitudinal distribution pattern indicated that the health status of the upstream sites was better,and the health status of the middle stream sites was poor.F-IBI scores were significantly and positively correlated with the HQS,and were significantly and negatively correlated with total phosphorus and conductivity.In conclusion,the constructed F-IBI evaluation system can well assess the health status of tributaries.Invasion of exotic species,water pollution,river bank solidification and sand mining were indentified as main threat factors.The contribution rank of the extrinsic factors to rivers were listed as below in order:invasion of exotic species,water pollution,river bank solidification,sand mining,dam construction,fishing,tourism development,grazing,sewage outlet,mines,and natural disasters.