The future of fishing in Southeast Asia's largest lake: Quantifying fishing effort and harvest in Cambodia’s Tonle Sap
Loading...
Authors
Everest, Elizabeth
Issue Date
2025
Type
Thesis
Language
en_US
Keywords
Catch per unit effort , Fish Catch , Fisheries , Fishing Effort , Species Assemblage , Tonle Sap
Alternative Title
Abstract
Despite their importance for food security and livelihoods, many inland fisheries lack the spatiotemporal catch and effort data required for sustainable management. The Lower Mekong Basin supports one of the world’s most productive and biodiverse inland fisheries, but contemporary data on fishing effort and catch are largely absent. This study uses high-resolution (<80 cm) satellite imagery and unmanned aircraft systems (UAS) to map, measure, and standardize fishing-effort metrics for large, stationary arrow-shaped traps, one for the most abundant fishing gear in Cambodia’s Tonle Sap Lake. We also develop a generalized framework for using aerial technologies to assess fishing effort in data-limited, subsistence fisheries. Combining seasonal arrow trap counts from fisher reports and remote sensing with species-specific biomass data, we analyze spatiotemporal patterns in catch and effort and provide the first estimates of annual fish harvest for the Siem Reap arrow trap fishery since 1997-1998. Using satellite imagery covering 50 km of shoreline in Siem Reap Province, we mapped 1,050 traps in 2021, 1,288 in 2022, and 737 in 2023. Mean trap length was 178 ± 103 m (range: 10–700 m). Arrow trap netting density averaged 1,550 ± 1,539 m/km² (range: 1–7,290 m/km²), indicating significant spatial variation, with highest effort in Pouk district and lowest in Bakong district (p = 1.2×10⁻5). UAS imagery provided fine-scale seasonal data in 2023 and filled gaps where satellite coverage or contrast was limited. Aerial survey results corresponded with survey data, confirming reliability of remote sensing approaches while also revealing localized challenges with illegal, unreported, and unregulated fishing. These methods form a reproducible monitoring framework that assesses trap abundance, spatial distribution, and compliance with existing fishing regulations. This framework can be applied to support scalable, remote assessments of fishing effort and track long-term trends in fishing pressure in Cambodia and other marine and inland fisheries worldwide. By combining seasonal arrow trap counts from remote sensing and fisher reports with species-specific biomass data, we also analyzed spatiotemporal patterns in catch and effort. Arrow trap fishing effort and harvest in Siem Reap Province have increased approximately threefold since the last published assessment, likely driven by a shift from more vulnerable large-bodied species to small-bodied taxa and by changes in fisheries governance. More than 1,200 traps operated in 2023, producing approximately 3.0 × 10⁶ kg of harvest, compared to 327 traps and just over 920,000 kg reported in 1997-1998. Arrow trap catch was dominated by ten species, which together accounted for 51% of the total biomass. Analysis revealed strong seasonal and spatial variation in catch-per-unit-effort (CPUE) and species assemblage. Species assemblage differed significantly among fishing districts and seasons, with northern districts dominated by floodplain-resident species and the southernmost district characterized by species adapted to open-water and riverine habitats. Species richness and diversity were highest after peak inundation in December, whereas low- and rising-water periods supported fewer species. CPUE was highly variable (mean = 6.85 ± 9.23 kg/trap-day) and differed significantly by season and district (p = 0.001), peaking in March and December and declining in June and September, except in the southernmost district where catches remained consistently high. These results provide a contemporary baseline for evaluating long-term trends in catch, yield, community composition, and fishing pressure in the Tonle Sap Lake. More broadly, this study highlights the potential of remote-sensing approaches to enhance monitoring of illegal, unreported, and unregulated fishing and support sustainable fisheries management worldwide.