Medieval Europeans had a fish mystery. Roman Catholics could eat only 'cold' foods on fast days, not flesh. Because fish came from water, it was deemed 'cold' and so was permitted (as were waterfowl and whales). Salted, dried cod became popular. No one had more salt cod than the Basques. Where were they getting it? Scandinavians and British, who had fished cod for centuries, never saw Basque boats in the North Sea. But when explorer Jacques Cartier 'discovered' the mouth of the St. Lawrence River in what is now Canada and claimed it for France, he noted the presence of a thousand Basque boats fishing for cod. The Basques had kept their secret, but hadn't claimed any of the New World for themselves.
Source: 'Cod,' by Mark Kurlansky (Walker and Co., 1997)