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Reservoir food web with an emphasis on culture-based fisheries
(Australian Center of International Agricultural Research, 2026) Silva, S.U.; Jayasinghe , A.D.; Deepananda , K.H.M.A.; Jones , C.M.
In Sri Lanka, reservoir ecosystems are abundant and significantly contribute to CBF, enhancing inland fish production, food security and economic benefits. Although, studies on reservoir food webs and species interactions are scarce and the influence of those interactions on CBF is still unclear. On top of that, there is no published information on using stable isotope (SI) analysis as a tool to assess trophic interactions in reservoir food webs in Sri Lanka. This study aimed to use SI analysis to examine the trophic relationships between species utilised in CBF and native species in reservoir ecosystems in Sri Lanka. Samples were collected from 6 reservoirs by visiting once every 3 months from May 2023 to June 2024 period. Samples included basal resources (plankton, detritus, aquatic plants) and consumers (culture-based fish species, macrobenthos and indigenous fish species). Tissue samples of all species were dried, powdered and sent to James Cook University, Australia, to determine δ¹³C and δ¹⁵N isotope ratios. Multivariate statistical analyses and Bayesian mixing models provided insights into species interactions supporting the development of a conceptual food web. The dietary proportions for CBF species, estimated using the SIMMR model, indicated that plankton (50.1%) was the dominant food source, emphasising the importance of pelagic nutrient pathways. Detritus (29.4%) served as the primary benthic resource. The trophic position analysis calculated using freshwater mollusks as baselines, revealed that M. rosenbergii (3.49), Catla catla (3.06), Hypophthalmichthys molitrix (3.08), and Oreochromis niloticus (2.88) occupied mid-to-high trophic levels. Labeo rohita (2.36) and Cirrhinus mrigala (2.00) were positioned at lower trophic levels, relying more on macrophytes and macrophytic detritus. Predators such as Anguilla bicolor (4.3) and Channa spp. (3.93) occupied the top trophic levels, with potential predation on highly valued GFP and other CBF species. These findings on trophic relationships provides a baseline for better management of CBF in reservoirs in Sri Lanka.
Giant freshwater prawn and finfish distribution in microhabitats in irrigation reservoirs of Sri Lanka
(Australian Center for International Agricultural Research, 2026) Perera, D.; Jayasinghe, A.D.; Deepananda, K.H.M.A.; Wijenayake , W.M.H.K.
This investigation examined spatial distribution patterns of GFP (M. rosenbergii) and associated ichthyofauna across distinct microhabitats within irrigation reservoirs utilised for CBF in Sri Lanka. Methodology employed stratified random sampling across 185 sampling stations distributed among 5 reservoirs. Limnological parameters were quantified in situ, and fish and prawn populations were sampled via overnight deployments of multi-mesh gillnets (25–100 mm mesh sizes). Statistical analysis through PERMANOVA revealed significant spatial heterogeneity in species assemblages across microhabitat types (p<0.05). Wood log (WL) habitats exhibited significantly different community structures compared to gravel and rocky substrates (GR), vegetation-associated zones (VA), and pits and channels (PC). Notably, open water (OP) microhabitats demonstrated no statistically significant differences in species composition when compared with other microhabitat types. Principal species contributing to microhabitat-specific community differentiation included members of the family Channidae and various size classes of cyprinid species. M. rosenbergii was identified as a significant contributor to dissimilarities between GR and VA habitats. These findings demonstrate the ecological importance of microhabitat heterogeneity in structuring aquatic communities and provide empirical evidence to be considered in the management of CBF in tropical reservoir ecosystems.
Production trends and yield predictive models for culture-based fisheries in irrigation reservoirs of Sri Lanka
(Australian Center for International Agricultural Research, 2026) Adikari, A.M.A.N.; De Croos , M.D.S.T.; Amarasinghe , U.S.; Perera , H.A.C.C.
This study aims to develop empirical yield predictive models for CBF in perennial irrigation reservoirs of Sri Lanka, focusing on the GFP (M. rosenbergii), alongside carp and tilapia species. Conducted across 39 reservoirs, the research involved the collection of extensive data on species stocking densities, fish yields and various limnological parameters. Results indicate that stocking densities of carp and GFP significantly influenced yields, with optimal stocking densities identified as over 900 fingerlings ha-¹ yr-¹ for carp and approximately 1,000 post larvae ha-¹ yr-¹ for GFP in major reservoirs. Notably, the study found a significant relationship between fishing intensity and carp yield, although no such correlation was observed for GFP. Limnological assessments revealed that chlorophyll-a concentrations positively correlated with carp yields, while other parameters exhibited limited variability. TSI indicated that most reservoirs were classified as eutrophic. PCA highlighted key variables influencing productivity, with significant relationships established for carp yield. The study underscores the critical role of stocking strategies and limnological factors in optimising fish production, providing valuable insights for sustainable aquaculture development in Sri Lanka’s inland water bodies. The predictive models developed offer a framework for enhancing CBF, thereby contributing to improved livelihoods in rural communities reliant on freshwater fish as a vital protein source.
The culture-based fishery of giant freshwater prawn in Sri Lankan perennial reservoirs: evaluation of past stocking and production data
(Australian Center for International Agricultural Research, 2026) Digamadulla, D.S.; Amarasinghe , U.S.; Wijenayake , W.M.H.K.; Jayasinghe-Mudalige , U.K.; De Croos , M.D.S.T.
The present study has been articulated as a system approach to sustainably develop the high valued giant freshwater prawn (FWP) CBF in Sri Lankan perennial reservoirs. The approach comprised of (a) appraising past stocking practices; (b) assessing reservoir capacity and CBF performance with multiple strategic scientific studies; (c) investigating possible measures to increase the production and recapture; (d) identifying the market driving factors; and (e) understanding the constraints in the management of a socioecological system. Based on the scientific evidence, recommendations for better approaches for future FWP CBF have been suggested.
Analysing the agrarian reservoir environment to optimise culture-based fisheries in Sri Lanka
(Australian Center for International Agricultural Research, 2026) Jayasinghe, A.D.
Compared to the long agrarian history of Sri Lanka, inland fisheries in agrarian reservoirs are a recent development based mainly on introductions of exotic species. The aim of this short review is to analyse published information on agrarian reservoir environment and its suitability for fisheries production. In global scale, total fisheries production keeps rising due to both marine and inland aquaculture inputs, and a similar trend can be found in Sri Lanka as well. The development of culture-based fisheries (CBF) in agrarian reservoirs was not efficient at first, due to lack of knowledge on the limnological properties of the reservoirs. Since then, several scientists have formulated relationships between fish yield and the reservoir nutrient levels and other factors deciding primary production. Lately, indigenous riverine giant freshwater prawns (GFP) were introduced to the reservoir systems as a culturable species to diversify the catch and boost the income of rural fishing communities. Despite the continued stocking, the catch of GFP in return was not comparable. As a solution, proper scientific investigations on harvesting techniques, assessment of existing biomass, GFP distribution in microhabitats and their interactions with the agrarian reservoir environment and co-existing species, were subject to research.