Influence of limnological characteristics on culture-based fisheries in irrigation reservoirs of Sri Lanka

Abstract

This study investigates the limnological variations in perennial reservoirs in Sri Lanka across monsoon seasons and their impact on nutrient dynamics, primary production and CBF harvest. Surface water and bottom sediment were sampled from 3 random locations of the limnetic zone of 48 reservoirs on a bimonthly basis from February 2023 to April 2024. Significant seasonal shifts in physicochemical and hydrological factors were observed, influencing nutrient availability, DO levels and overall reservoir productivity. The study highlights the role of sediments as both a nutrient sink and a source, with phosphorus and nitrogen dynamics varying between wet and dry seasons. Fish production was predominantly shaped by limnological conditions, rather than stocking densities alone. GFP yields were significantly correlated with reservoir elevation, alkalinity and sediment phosphorus content. Similarly, finfish harvest, particularly for GIFT tilapia, catla, rohu and mrigal, was strongly influenced by key water quality parameters such as alkalinity, hardness and chlorophyll-a levels. The findings suggest that optimal fish yields require balanced water chemistry and reservoir morphometry, rather than increased stocking efforts. This study underscores the necessity of integrating seasonal limnological variations into fisheries management strategies to enhance fisheries production and maintain ecological stability in irrigation reservoirs. Future research should focus on the long-term impacts of climate change and anthropogenic activities on reservoir limnology and sustainability of fisheries.