Modeling trophic structure and ecosystem functioning of the small fish-dominated largest lake of Bangladesh

Ecosystem modelling is becoming increasingly important, as it reveals how energy flows and species interactions shape ecosystem stability, resilience, and the sustainability of fisheries under natural and anthropogenic pressures. However, model-based ecosystem studies are scarce for the Kaptai Lake (KL), the largest artificial freshwater lake of Bangladesh. Therefore, a mass-balanced trophic model was built to illustrate the trophic structure and ecological features of KL. The model outputs indicated that Detritus along with Phytoplankton served as the primary sources of energy. 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, while 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 TPP/TR as well as TPP/TB ratios of 2.034 and 71.764, respectively, and a Finn Cycling Index (FCI) of 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 these, the ever-increasing trend of Clupeid and declination of large fish populations indicates that the KL ecosystem may encounter severe consequences in the future, including "fishing down the food web" and irreversible regime shifts. Consequently, ecosystem-based management interventions have been proposed that will help to restore food web integrity, recover vulnerable species such as carps and apex predators, and ensure long-term sustainability of fisheries in this lake.