The Glass Eel by J.J. Viertel

On Caterpillar Island off the coast of Maine, Jeanette King makes a living selling crabmeat to expensive restaurants on the east coast. One evening, she comes upon her ex-husband Simon's boat, which seems to have been involved in a dubious activity--illegally catching eels. When someone Jeanette knew well is murdered, she decides to get to the bottom of what's really going on. With the help of  local activist Jesse Ed Davis and cop Toland Bates (whom Jeanette has known since he was a child), she attempts to make things right and see justice served. Readalikes include The Trouble Up North and The Distant Dead.