The vulnerability of spawning aggregations to exploitation varies among fisheries as a result of differences in the population-density changes associated with this behaviour. However, vulnerability to fishing is also influenced by technology, environmental factors, and fish and fisher behaviours. Focusing on a fishery for the rabbitfish Siganus sutor at Praslin Island, Seychelles, we examined how catch rate varied across spawning and non-spawning habitats in relation to in situ population-density changes and other factors known to influence catchability. Catch rates in spawning habitat were disproportionate to density changes, being only fourfold greater than catch rates in non-spawning habitat, despite the fact that spawning-aggregation formation involved nine- to thirteen-fold increases in population density. Catch rates in spawning habitat were also highly variable across the spawning season (0–23.4 fish trap-hour−1). Current strength was of similar importance to density as a catch-rate predictor, with the highest catch rates in spawning habitats confined to months with the strongest currents. Therefore, in addition to density-dependent catchability, other factors that influence catch rates must be examined to avoid overestimation of the vulnerability of populations to aggregation fishing. The dynamics of catchability at spawning sites can limit the ability of fishers to predict and maximise returns based on increases in fish density.