NOBLEBORO, Maine - Chuck Reinhardt stood in awe as he gazed down at the multitudes of silver-sided fish crowded into a waterway no more than 5 feet wide. After three to four years at sea, thousands of alewives were following their instincts on a journey back to the freshwater where they were born. The fish crammed themselves into a manmade brook on the final leg of their trip to Damariscotta Lake, reminding Reinhardt of a familiar scene before he moved to Maine. "It looks like the Long Island Expressway," he said. Each spring, tens of thousands of alewives pass through...