Regional councils manage U.S. fisheries. Fishermen can participate in fisheries managed by multiple councils, and effort controls in one region could lead to effort leakage into another. Theoretical modeling demonstrates that positive, negative, and no leakage are possible. Using difference-in-differences, we test for leakage across regional boundaries for a catch share program in New England and find evidence that the New England groundfish sector program caused spillover into adjacent Mid-Atlantic fisheries. Aggregate Mid-Atlantic harvest volume increased among sector members after the policy change. We find leakage in individual fisheries with similar gear and high market substitutability with sector species.