Last spring, there was a lot of heavy lifting at the Essex Dam, but no one seemed to mind. In fact, they were thrilled. For more than 30 years, migratory fish have passed over the dam on the Merrimack River in Lawrence, Massachusetts, by riding in large buckets on a chain-driven fish lift. Last spring, they showed up in record numbers, with nearly 450,000 river herring — that’s blueback herring and alewife — hoisted up and over to continue their journey upstream.