ABSTRACT: The influence of density on population dynamics is a fundamental concept of ecology; however, observations of marine populations affected by density effects occur are rare. Recently, extremely high abundances of Atlantic sea scallops Placopecten magellanicus have persisted over a wide range of their distribution, including 2 adjacent areas on Browns Bank, Canada, that were closed to fishing from 2014 to 2016. We hypothesized that the closures, named C2 and C3, would work as expected, i.e. scallop abundance would decrease through time due to natural mortality, while yield would substantially increase due to growth. To test this, we compared the density (number of scallops per unit area), crowding levels (number of scallops around an individual scallop), and shell growth of scallops through the closures. Despite similar scallop densities in each area, crowding levels were significantly higher within area C2 than in C3. This contrasting result suggests the intensity of scallop aggregation was significantly higher in area C2. This area also had individuals with reduced shell height growth in the final year of the closure. Combined, the results show yield was lost between Years 2 and 3 in area C2 while yield increased through time in area C3. The different levels of crowding between the 2 areas may explain the different growth patterns; factors causing discrepancies in scallop growth can be related to aggregation intensity. Spatial closures to increase fisheryyield are commonly thought of as ‘money in the bank’, but divergences from typical patterns, in this case growth, suggest considering these areas as shorter term ‘windows of opportunity’ will help their management.