In this study, a length-structured population model was implemented to describe changes in the abundance of Panopea globosa over time, given parameters for growth and natural mortality rate. The following modules described population dynamics: stochastic individual model growth, survival, selectivity, recruitment, and estimates of initial numbers. In addition, the harvest rate, total biomass, and vulnerable biomass (VB) were calculated to advise management quantities. Spatial serial depletion patterns were detected for P. globosa and evident from the expansion of the fishing grounds over time. These patterns of geographic expansion are common in developing fisheries and are a problem for stock assessment. The densest patches available are usually harvested or even depleted, with fishers switching to new beds and potentially rotating back over time. If the changes in the harvestable area are ignored, the increases in VB and recruitment may mask the population dynamics of several beds, including those that are depleted and have low yield.