Environmental cues driving intra-reservoir and directional migration of Macrobrachium rosenbergii: Insights from a culture-based fishery in  Sri Lanka

The culture-based fishery (CBF) for giant freshwater prawn (Macrobrachium rosenbergii) is increasingly important in Sri Lankan reservoirs, yet the spatial ecology of stocked populations remains critically understudied. This study examined spatial segregation and directional migration cues of M. rosenbergii in the Hurulu Wewa reservoir between June 2020 and April 2021. The reservoir was stratified into three hydrological zones: lacustrine (deep), transitional and upstream lotic (shallow). Monthly sex-specific catch data were analysed alongside rainfall, outflow, depth and storage capacity using generalised additive models (GAMs). Sex ratios differed significantly among strata (p = 0.011), with pronounced male bias in upstream and transitional zones compared to the lacustrine zone. This spatial structuring persisted monthly (p = 0.429), indicating stable habitat segregation. GAMs revealed strong non-linear associations between depth, storage capacity and prawn distribution, with rainfall–outflow interactions influencing hydrological gradients. Increased male aggregation in upstream zones during hydrological transitions suggests flow-mediated directional movement. We conclude that hydrological gradients serve as primary ecological cues structuring M. rosenbergii distribution in CBF systems. These findings provide the first quantitative evidence of directional migration cues for this species in reservoirs and support stratum-specific stocking and harvest strategies to enhance fishery sustainability.