We design and conduct a lab-in-the-field experiment to test the effect of a conditional contract on the sustainability of an open access fishery, where unit prices are conditional on aggregate catch. The contract provides collective incentives to decrease extraction but maintain the individual incentives of extraction maximization. We conduct the experiment with two communities of artisanal fishermen differing in their market and technological restrictions. We find that the conditional contract, compared to a fixed price scheme, increases efficiency, the duration of the resource and the total yield. The contract has a greater effect upon groups from the less restricted community.