May 21st is World Fish Migration Day. Yes, that’s a real thing, and groups around Cape Cod have already begun participating by doing what many of them do every spring – gathering to count and learn about river herring as they try to make their way from the ocean up to fresh water to spawn. There are some eighty herring runs in Massachusetts, with a number on Cape Cod. Biologists estimate that one hundred fifty years ago, each run may have supported a million fish. Today, it’s a fraction of that