The proportion of a fish stock that is killed by fishing activity is often calculated as the catch divided by the estimated stock biomass. However, stock biomass is notoriously difficult to estimate reliably, and moreover, the catch may be uncertain or misreported and does not include losses due to discarding. In all too many fisheries, these difficulties have lead to underestimates of total fishing mortality and the commercial demise of the fishery. No-take marine reserves eliminate fishing mortality from within their boundaries and, for species that exhibit seasonal migratory behaviour, comparison of reserves with fished areas can provide direct estimates of the proportion killed by fishing. For an important exploited species in New Zealand, seasonal changes in density of sub-legal fish at three marine reserves were similar in both reserve and adjacent non-reserve areas. However, this result did not hold for legal-size fish, and the difference in seasonal change between reserved and non-reserved areas was used to obtain direct estimates of the total localized fishing mortality in the non-reserve area over 6-month periods. Estimates of the percentage of legal-size fish killed by fishing ranged from 70 to 96%. These results demonstrate an unanticipated practical benefit from marine reserves that goes beyond their ecological role.