Rubber tapper communities along the large white-water rivers of central-western Brazilian Amazonia rely heavily on local fish stocks all year round. This subsistence fishing faces severe competition from commercial fishing boats from urban centres, which in some areas have over-fished the economically most important fish species. In the Juruá River and its oxbow lakes, two species of freshwater crocodilians - the black and the spectacled caiman - indirectly benefit subsistence fishermen by disrupting commercial fishing operations. The gill-nets used by commercial fishing boats are often damaged beyond immediate repair by the caimans as they make easy pickings of the catch. This appears to deter over-harvesting of important fish stocks in many sections of the river. In contrast, subsistence fishermen use fishing gear that is rarely damaged by caimans. It would seem that caimans at their present numbers offer protection to local fish stocks by reducing the efficiency and financial viability of commercial fisheries, which would otherwise rapidly reduce fish yields for local rubber tappers.