They come for the sun, sea and rich plankton that occurs in the ocean waters around the Island. This is the season when North Atlantic right whales migrate north to Cape Cod from their wintering grounds off Florida and Georgia. Last month eight of the endangered marine mammals were spotted near the Vineyard and Nantucket. Six were seen from the air swimming between the two Islands; the other two were seen south of Nantucket. Then early this week, twenty whales were seen swimming in the Atlantic Ocean off South Beach in Edgartown.