Inseason tools available for gauging Copper River sockeye salmon run strength are limited and fisheries management relies heavily on sonar data. But even in the Copper River fishery, fisheries managers examine other indicators in combination with sonar data to gauge run strength. Commercial harvest reports, for example, provide fisheries biologists with early clues about the strength of the arriving sockeye run. But biologists are careful in drawing conclusions based on early season harvest data because it is sensitive to variations in run timing. A large harvest on the opening day of the commercial fishing season might indicate that the run will be large or it might indicate that it is early. And many factors unrelated to run size can influence how many fish are caught during a commercial period including tides, the number of boats fishing, whether the fleet was clumped or randomly distributed and whether fishermen found fish in schools or spread out. Biologists consider these factors as well. Biologists also uncover clues to help them gauge run strength by collecting age composition data from commercially harvested sockeye. If, for example, the proportion of returning sockeye that are five years of age is similar to the proportion found in average runs in the past, that might suggest the run will be average.