A parallel-bar barrier is probably the best option to prevent adult fish passage through spillways. Parallel-bar barriers are especially important for ponds stocked with grass carp. When water flows only a few inches deep through spillways, grass carp tend to leave ponds unless appropriate barriers exist. Properly constructed parallel-bar barriers do not rust out quickly and do not clog regularly with leaves and aquatic vegetation, unlike fish barriers made of poultry wire, net wire, hardware cloth or netting. When constructed and installed properly, parallel-bar barriers restrain grass carp larger than 8 inches, restrain adult game fish, prevent undesirable adult fish from entering ponds, last many decades and require little maintenance.
A box type parallel bar barrier over a pond’s overflow pipe protects the overflow pipe, the emergency spillway, and the fishery. It functions as a trash guard that prevents large debris from flowing into and plugging the pipe. It deters beavers from plugging the overflow pipe. By allowing the overflow pipe to function properly, it minimizes erosion in the emergency spillway caused by excessive water overflows. It prevents harvestable size fish from leaving the pond through the pipe. It does not allow grass carp over 8 inches long to escape through the pipe. It prevents adults of undesirable fish species from coming in through the pipe.