An exhaustive review of culverts, bridges, and dams in Massachusetts has found that many cause major problems for fish and other aquatic wildlife. The ‘‘critical linkages’’ survey took years to complete and the results are now being made public. The state Department of Transportation, University of Massachusetts, and the Nature Conservancy collaborated on the project. Researchers who analyzed thousands of road-stream crossings found that outdated or poorly designed culverts and other structures can impede the normal movement of animals. The structures can also pose dangers for humans by increasing the risk of flooding, or by forcing large animals on to roadways. The researchers said improvements to just 10 percent of the crossings would have a significant positive impact on wildlife.