We tested whether it is possible to replicate the effects of a size limit with a voluntary mechanism – a price premium for larger fish. We randomly offered fishers in Indonesia a bonus for catching certain species of fish above certain sizes. We observe clear differences in catch as a result. Instead of inducing fishers to catch more fish of every size, the premium caused a shift in the size distribution of catch towards larger fish. The price premium worked well for a large-sized species but had no effect on the catch of a small-sized species. In particular, we find that the length of Aprion virescens increased by 15% and the average length of all fish caught increased by 8% when fishers were offered the price premium. It does not appear that fishers respond to the premium by high-grading their catch, which would only affect supply and not extraction. The changes appear to be driven by fishers going to different locations and fishing at different depths. Our experiment is the first ever proof-of-concept of Payment for Ecosystem Services in a developing country fishery. Our results also suggest that prices can alter length–frequency distributions, confirming a possible source of bias in length-based stock assessment models.