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Sun J, Du W, Lucas MC, Ding C, Chen J, Tao J, He D. River fragmentation and barrier impacts on fishes have been greatly underestimated in the upper Mekong River. JOURNAL OF ENVIRONMENTAL MANAGEMENT 2023; 327:116817. [PMID: 36459786 DOI: 10.1016/j.jenvman.2022.116817] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/27/2022] [Revised: 11/09/2022] [Accepted: 11/14/2022] [Indexed: 06/17/2023]
Abstract
River barriers reduce river connectivity and lead to fragmentation of fish habitats, which can result in decline or even extinction of aquatic biota, including fish populations. In the Mekong basin, previous studies have mainly focused on the impacts of large dams but ignored the impacts of small-scale barriers, or drew conclusions from incomplete barrier databases, potentially leading to research biases. To test the completeness of existing databases and to evaluate the catchment-scale fragmentation level, a detailed investigation of river barriers for the whole Upper Mekong (Lancang catchment) was performed, by conducting visual interpretation of high-resolution remotely sensed images. Then, a complete catchment-scale barrier database was created for the first time. By comparing our barrier database with existing databases, this study indicates that 93.7% of river barriers were absent from the existing database, including 75% of dams and 99.5% of small barriers. Barrier density and dendritic connectivity index (DCID and DCIP) were used to measure channel fragmentation within the catchment. Overall, 50.5% of sub-catchments contained river barriers. The Middle region is the most fragmented area within the Lancang catchment, with a median [quartiles] barrier density of 5.34 [0.70-9.67] per 100 km, DCIP value of 49.50 [21.50-90.00] and DCID value of 38.50 [9.00-92.25]. Furthermore, since 2010, distribution ranges of two representative fish species Schizothorax lissolabiatus (a rheophilic cyprinid) and Bagarius yarrelli (a large catfish) have reduced by 19.2% and 32.8% respectively, probably due in part to the construction of river barriers. Our findings indicate that small-scale barriers, in particular weirs and also small dams are the main reason for habitat fragmentation in the Lancang and must be considered alongside large dams in water management and biodiversity conservation within the Mekong.
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Affiliation(s)
- Jingrui Sun
- Yunnan Key Laboratory of International Rivers and Transboundary Eco-security, Yunnan University, Kunming, 650091, China; Institute of International Rivers and Eco-security, Yunnan University, Kunming, 650091, China
| | - Weilong Du
- Yunnan Key Laboratory of International Rivers and Transboundary Eco-security, Yunnan University, Kunming, 650091, China; Institute of International Rivers and Eco-security, Yunnan University, Kunming, 650091, China
| | - Martyn C Lucas
- Department of Biosciences, University of Durham, Durham DH1 3LE, UK
| | - Chengzhi Ding
- Yunnan Key Laboratory of International Rivers and Transboundary Eco-security, Yunnan University, Kunming, 650091, China; Institute of International Rivers and Eco-security, Yunnan University, Kunming, 650091, China.
| | - Jinnan Chen
- Yunnan Key Laboratory of International Rivers and Transboundary Eco-security, Yunnan University, Kunming, 650091, China; Institute of International Rivers and Eco-security, Yunnan University, Kunming, 650091, China
| | - Juan Tao
- Yunnan Key Laboratory of International Rivers and Transboundary Eco-security, Yunnan University, Kunming, 650091, China; Institute of International Rivers and Eco-security, Yunnan University, Kunming, 650091, China
| | - Daming He
- Yunnan Key Laboratory of International Rivers and Transboundary Eco-security, Yunnan University, Kunming, 650091, China; Institute of International Rivers and Eco-security, Yunnan University, Kunming, 650091, China.
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Garcia de Leaniz C, O'Hanley JR. Operational methods for prioritizing the removal of river barriers: Synthesis and guidance. THE SCIENCE OF THE TOTAL ENVIRONMENT 2022; 848:157471. [PMID: 35868378 DOI: 10.1016/j.scitotenv.2022.157471] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/17/2022] [Revised: 06/30/2022] [Accepted: 07/14/2022] [Indexed: 06/15/2023]
Abstract
Barrier removal can be an efficient method to restore river continuity but resources available for defragmenting rivers are limited and a prioritization strategy is needed. We review methods for prioritizing barriers for removal and report on a survey asking practitioners which barrier prioritization methods they use. Opportunities for barrier removal depend to a large extent on barrier typology, as this dictates where barriers are normally located, their size, age, condition, and likely impacts. Crucially, river fragmentation depends chiefly on the number and location of barriers, not on barrier size, while the costs of barrier removal typically increase with barrier height. Acting on many small barriers will often be more cost-efficient than acting on fewer larger structures. Barriers are not randomly distributed and a small proportion of barriers have a disproportionately high impact on fragmentation, therefore targeting these 'fragmentizers' can result in substantial gains in connectivity. Barrier prioritization methods can be grouped into six main types depending on whether they are reactive or proactive, whether they are applied at local or larger spatial scales, and whether they employ an informal or a formal approach. While mathematical optimization sets the gold standard for barrier prioritization, a hybrid approach that explicitly considers uncertainties and opportunities is likely to be the most effective. The effectiveness of barrier removal can be compromised by inaccurate stream networks, erroneous barrier coordinates, and underestimation of barrier numbers. Such uncertainties can be overcome by ground truthing via river walkovers and predictive modelling, but the cost of collecting additional information must be weighed against the cost of inaction. To increase the success of barrier removal projects, we recommend that barriers considered for removal fulfill four conditions: (1) their removal will bring about a meaningful gain in connectivity; (2) they are cost-effective to remove; (3) they will not cause significant or lasting environmental damage, and (4) they are obsolete structures. Mapping barrier removal projects according to the three axes of opportunities, costs, and gains can help locate any 'low hanging fruit.'
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Affiliation(s)
- Carlos Garcia de Leaniz
- Department of Biosciences, Centre for Sustainable Aquatic Research (CSAR), Swansea University, Swansea, UK.
| | - Jesse R O'Hanley
- Kent Business School, University of Kent, Canterbury, UK; Durrell Institute of Conservation and Ecology, University of Kent, Canterbury, UK
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A national fish passage barrier inventory to support fish passage policy implementation and estimate river connectivity in New Zealand. ECOL INFORM 2022. [DOI: 10.1016/j.ecoinf.2022.101831] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
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Zapletal T, Šlapanský L, Grmela J. Potential Factors Limiting Brown Trout Catches by Anglers - A Case Study. ACTA UNIVERSITATIS AGRICULTURAE ET SILVICULTURAE MENDELIANAE BRUNENSIS 2021. [DOI: 10.11118/actaun.2021.031] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
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More than one million barriers fragment Europe's rivers. Nature 2020; 588:436-441. [PMID: 33328667 DOI: 10.1038/s41586-020-3005-2] [Citation(s) in RCA: 115] [Impact Index Per Article: 28.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/28/2020] [Accepted: 10/26/2020] [Indexed: 11/08/2022]
Abstract
Rivers support some of Earth's richest biodiversity1 and provide essential ecosystem services to society2, but they are often fragmented by barriers to free flow3. In Europe, attempts to quantify river connectivity have been hampered by the absence of a harmonized barrier database. Here we show that there are at least 1.2 million instream barriers in 36 European countries (with a mean density of 0.74 barriers per kilometre), 68 per cent of which are structures less than two metres in height that are often overlooked. Standardized walkover surveys along 2,715 kilometres of stream length for 147 rivers indicate that existing records underestimate barrier numbers by about 61 per cent. The highest barrier densities occur in the heavily modified rivers of central Europe and the lowest barrier densities occur in the most remote, sparsely populated alpine areas. Across Europe, the main predictors of barrier density are agricultural pressure, density of river-road crossings, extent of surface water and elevation. Relatively unfragmented rivers are still found in the Balkans, the Baltic states and parts of Scandinavia and southern Europe, but these require urgent protection from proposed dam developments. Our findings could inform the implementation of the EU Biodiversity Strategy, which aims to reconnect 25,000 kilometres of Europe's rivers by 2030, but achieving this will require a paradigm shift in river restoration that recognizes the widespread impacts caused by small barriers.
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