El Niño is well known to affect marine fishery. The Southern Oscillation, however, may also have a strong effect on riverine fish production in flood-plains via its effect on river discharge. In years with high river discharges larger parts of the floodplain are innundated, increasing the surface area of suitable nursery grounds for young fish stock. In this way high river discharges are thought to have a positive effect on fish production, while low discharges are thought to have a negative effect. The collapse of the sábalo fishery in the Pilcomayo River in Bolivia, for instance, can be attributed to the 1990-1995 El Niño event and subsequent overexploitation of the fish stocks.