ABSTRACT: The population ecology of the tropical bêche-de-mer sea cucumber Stichopus herrmanni (curryfish) was investigated on One Tree Reef, a no-take protected area in the Great Barrier Reef Marine Park. The size class frequency and density of this species at several sites were determined over 2 years: 2009 and 2011. There was a spatial separation of populations that differed in size and density, but these parameters did not change over the 2 yr of the study, indicating stable population metrics. The spatially heterogeneous population pattern has relevance for fisheries management, as current size limits protect animals with low fecundity that occur in shallow habitat, but make it legal to remove 85% of large, fecund animals in deeper areas. Data for 4 S. herrmanni populations were used to address 2 theories on the potential drivers of population structure: (1) adult migration and (2) phenotypic plasticity in growth with respect to habitat conditions. While connectivity through adult migration appears possible, the size structure and location of some populations indicate that population features are determined by post-recruitment growth in the habitat. The latter likely plays a major role in population dynamics and terminal growth of S. herrmanni. There was no day-night difference in density at fixed transects, indicating that data obtained in daytime surveys was representative. A frequency distribution profile of density data from manta tows is presented as an alternative to using mean density as an assessment indicator in sea cucumber fisheries. S. herrmanni showed a noticeable affinity for reef features, an important finding for improved resolution of spatial planning in management.