Apparently, some time ago, the Vatican gave permission for the poor of Venezuela to eat capybara on Fridays in Lent. The large rodent being, I assume, sufficiently non-meaty to count as a penitential meal. Wikipedia has confirmed such reports, but I'm still looking for proof that water-dwelling mammals in general are acceptable! Beavers seem quite capybara-ish.
Before you get yer dander up, the whole "fish Fridays" thing is not some ploy to subsidize the fishing industry. It's an opportunity to forgo celebratory foods in memory of the Passion and in solidarity with the poor. Meat has traditionally been an expensive food, and to give it up is a sign of this remembrance. I cannot think of a better way to express poverty than a good rodent chili.