Migration Ecology of Select Mekong Fish Species in the Fish Pass of Xayaburi Hydroelectric Power Plant, Northern Lao PDR

River development for hydropower, irrigation, navigation and flood control has expanded rapidly worldwide, fundamentally reshaping riverine ecosystems. Although such developments deliver critical economic and social benefits, energy generation, water supply, and agricultural productivity, they simultaneously impose severe ecological consequences. Alterations to hydrological regimes, sediment transport, water quality and habitat connectivity have particularly affected migratory fish, which depend on longitudinal river connectivity to complete their life cycles. Large dams fragment river networks, disrupt migration routes and reduce access to essential spawning and feeding habitats, thereby threatening fisheries productivity and biodiversity.
To mitigate these impacts, fish passage structures, such as fish ladders, are increasingly incorporated into hydropower projects. These facilities aim to restore ecological connectivity and enable fish migration across barriers. However, their effectiveness, particularly in tropical river systems, remains poorly understood. Most empirical evaluations have focused on temperate regions with established monitoring frameworks, leaving significant knowledge gaps in tropical rivers such as the Mekong River. This gap is critical given that tropical rivers, though among the most biodiverse and productive on Earth, are also the most vulnerable to hydropower expansion.
The Mekong River, the world’s twelfth longest river system, supports an estimated 1,200–2,000 fish species, approximately one-fifth of which are long-distance migrants. These associated fisheries are central to regional livelihoods, providing up to 80% of animal protein in riparian communities. Yet the proliferation of dams—two operational on the mainstream and more than 120 on tributaries—poses a serious threat to ecological integrity and fish migration.
This thesis examines the potential application of Passive Integrated Transponder (PIT) tagging for evaluating fish passage performance in tropical river systems. Research was undertaken at the Xayaburi Hydropower Plant (XHPP) in Lao PDR—a run-of-river facility with a capacity of 1,285 MW and a maximum head of 39 metres—equipped with a large-scale fish ladder. The study aimed to (1) develop and validate a PIT antenna system suitable for tropical field conditions; (2) assess the biological impacts of PIT tag implantation on native Mekong species; and (3) investigate migration dynamics and environmental drivers using multi-year monitoring data.
An optimized antenna system was designed, constructed and field-tested at XHPP. The final configuration achieved a 99.6% detection rate for 23-millimetre PIT tags under operational flow conditions. Controlled laboratory experiments demonstrated that PIT tagging produced negligible mortality and no adverse behavioural effects across four representative Mekong species, confirming its suitability for tropical fish. Analysis of four years of detection data revealed species-specific migration patterns: Hemibagrus filamentus (local name in Lao: Pa Kod-rueng), Hypsibarbus lagleri (local name in Lao: Pa Pak), and Barbonymus schwanenfeldii (local name in Lao: Pa Vien-fai) ascended the ladder in response to increasing discharge, whereas Sikukia gudgeri (local name in Lao: Pa Mang) and Puntioplites falcifer (local name in Lao: Pa Sa-kang) migrated independently of flow variation. Some species exhibited inverse relationships with temperature, highlighting the complex cues underlying tropical fish movement. Notably, H. lagleri exhibited both migratory and resident behaviours, underscoring the need for passage designs accommodating diverse life histories.
This thesis advances understanding of fish passage monitoring in tropical rivers by refining PIT technology for large, hydraulically dynamic systems and providing the first long-term dataset of Mekong fish movements through a mainstream hydropower facility. The findings offer practical insights into antenna engineering, tagging methodology and environmental response analysis, contributing directly to the design and evaluation of effective fish passage solutions in the Mekong and other tropical basins.