Marine reserves deliver impressive increases in the abundance and size of exploited species on protected reefs, but larval dispersal makes it difficult to estimate their wider benefits. Australia’s Great Barrier Reef (GBR) contains an extensive network of marine reserves. By combining GBR-wide fish surveys, larval dispersal models, and commercial fishery catch data, we calculate the system-wide ecological and economic contributions of these reserves for coral groupers ( Plectropomus spp.), the region’s most important line fishery. Despite covering only 30% of reef habitat, the GBR’s marine reserve network contains half of the species’ biomass and generates most of its reproductive output (55%), half of the system’s larval settlement (50%), and almost half of the total fishery yield (47%).