Canon Law 1251 says:
Abstinence from meat, or from some other food as determined by the Episcopal Conference, is to be observed on all Fridays [...] (emphasis mine).
In other words it is to be determined by the bishops exactly what constitutes abstinence. The bishops of Quebec can simply state that beaver meat is OK to eat on days of abstinence. They don't have to declare it 'fish'. I would imagine that this is common practice in areas where fish is very hard to come by and meat is a staple food. In areas where meat is a rarity, giving it up would be little sacrifice and the rules might again be changed. As this page puts it, "when we abstain, it's not because the food is impure; we're voluntarily giving up something good, for our spiritual benefit". The exact nature of what is given up is of secondary importance.