With ongoing resource depletion and environmental degradation in the floodplains of Bangladesh, there is a clear need to include insights from traditional ecological knowledge (TEK) in developing and implementing plans for sustainable natural resource management. The knowledge, skills, and practices related to fishing in the floodplains of Bangladesh are almost exclusively a product of the local fishers' TEK. As fishers are deeply integrated with their landscapes, they develop and refine their ecological knowledge through multi-generational sharing of experience of and knowledge about fishing and socio-cultural learnings. This study documents the knowledge of floodplain fishers of Bangladesh and examines their decision-making about harvesting as a process based on heuristic rules developed through years of curiosity experimentation, adaptive learning, and empirical and inductive reasonings. Using a participatory ethnographic approach, ten major categories of TEK systems are examined: ethno-topography of floodplains, ethno-taxonomy and aquatic productivity, diurnal and nocturnal variation patterns of fish behavior, birds and other animals, actions for aggregating fish in leased waterbodies, importance of aquatic vegetation for growth and conservation of floodplain fisheries, breeding biology tuned to complex hydrological cycles, fishers' perception of fish ulcerative syndrome, landlocked Hilsa fish from the Hakaluki haor, and fishers' views on biodiversity. This research explores the cognitive mechanisms behind fishers' decision-making about harvesting. Experienced fishers argue that if the important mother fishery sites are protected and managed through community-based institutions, floodplains of the country would benefit immensely in terms of production and biodiversity conservation.