The effectiveness of management practices associated with a human–environmental system in the Blue Economy (BE) of the Ganges Delta depends on whether stakeholders internalize the intricacies of this system. This is because mental models (MM) influence decision-making and determine human action in such systems. However, additional research needs to be conducted on whether mathematically aggregated collective MMs of human–environmental systems in the Blue Economy accurately capture and depict reality. Therefore, the research questions that this chapter seeks to answer are: How and why do stakeholder MMs about human–environmental systems (i.e., polders) and management practices (i.e., tidal river management) associated with such a system vary depending on the type of concept (technocratically derived/locally developed) under consideration and according to the type of stakeholder? What does such variation tell us about the association between mental models and institutional fit? These questions are answered by analyzing data collected from a sample of polders (some of which are characterized by tidal river management) in the Ganges delta of Bangladesh. Data were analyzed using matrix algebra, graph theory and GIS techniques. Based on this analysis, this research argues that mental models based on incomplete understanding or erroneous assumptions about human–environmental systems and associated management practices may influence rulemaking leading to institutional misfits in the Blue Economy.