Supplementary File

Ecosystem modelling is becoming increasingly important, as it reveals how energy flows and species interactions shape ecosystem stability, resilience, and the sustainability of fisheries. However, model-based ecosystem studies are scarce for the Kaptai Lake (KL) of Bangladesh. Therefore, a mass-balanced trophic model was built to illustrate the trophic structure and ecosystem features of KL. The model outputs indicated that the apex predator, Catfish (TL-3.364), occupied the top trophic niches, while the overabundant (B: 3.264 t/km²) Clupeid (TL-2.56) dominated the lower trophic level in the food web. The higher values of ecotrophic efficiency (>0.5) for most of the groups indicate heavy fishing pressure. The ecosystem is Phytoplankton-based, where two core food chains that make up the majority of the food web are the Detritus (35.12%) and primary production (64.88%). The mixed trophic impact plot revealed a substantial positive effect of Detritus and Phytoplankton on majority of the fish groups. Clupeids displayed negative effect on most of the fish groups, indicating a lack of major predators of this group. In the KL ecosystem, Catfish and Phytoplankton were the key predator and producer groups respectively. The network analysis indicates that the KL is a developing ecosystem with moderate system maturity, as evidenced by total primary production and respiration ratio (2.034), total primary production and biomass ratio (71.764), and Finn Cycling Index (5.627%). The low connectance index (0.426) indicates that the food web is still linear rather than a web-like structure and thereby vulnerable to external influences. Besides, the ever-increasing trend of Clupeid and declination of large fish populations indicates that the KL ecosystem may encounter severe consequences in the future. Consequently, ecosystem-based management interventions have been proposed that will help to restore food web integrity, recover vulnerable species and ensure long-term sustainability of fisheries in this lake.