There were once between 60 and 400 million beavers (Castor canadensis) occupying the rivers and streams of North America, from the great white north to the deserts of northern Mexico. Then the Europeans came. With them came disease along with an insatiable desire for beaver pelts and for beaver castoreum, a urine-like secretion often used in perfume and cologne. Combined with the once-sustainable hunting of beaver by indigenous North Americans for their meat, the beaver population rapidly declined. (The species is now rebounding, thanks to trapping regulations, and now includes some 6 to 12 million individuals)
In addition to disease, the European settlers also brought Catholicism with them, and successfully converted a large proportion of the indigenous population. And the native Americans and Canadians loved their beaver meat.
So in the 17th century, the Bishop of Quebec approached his superiors in the Church and asked whether his flock would be permitted to eat beaver meat on Fridays during Lent, despite the fact that meat-eating was forbidden. Since the semi-aquatic rodent was a skilled swimmer, the Church declared that the beaver was a fish. Being a fish, beaver barbeques were permitted throughout Lent. Problem solved!
The Church, by the way, also classified another semi-aquatic rodent, the capybara, as a fish for dietary purposes. The critter, the largest rodent in the world, is commonly eaten during Lent in Venezuela. “It’s delicious,” one restaurant owner told the New York Sun in 2005. “I know it’s a rat, but it tastes really good.” (source)