Reservoir food web with an emphasis on culture-based fisheries

Abstract

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.