Why is fish allowed to be eaten when one is fasting from meat?

Pieter Bruegel the Elder: The Fight Between Carnival and Lent (1559)

I always assumed the exemption that allowed you to eat fish on fast days and during Lent was some sort of excuse because people want to eat some sort of animal protein, fructus maris as one might say in Latin (or fruit de la mer in French). The theory about you omnivores not wanting to be too strict vegetarians is probably mostly true, but here is the stated Biblical based reason for the exemption on fish, at least according to Atlas Obscura: While pigs, cows, sheep and other land-beasts had had to shelter from the Flood on Noah’s ark, fish were exempt, and therefore permissible.

I am not sure why that justifies eating fish, but there is a logic to it. I am not going to argue since I really like fish (and it does give a temporary reprieve for cows). Yes, I know you are supposed to sacrifice for Lent, but there is a long tradition of culinary creativity within the rules.  

What people will do to get out of eating the same old thing...

But this rule is the reason you can get those weird interpretations that allow the eating of certain aquatic animals like capybaras, beavers, and alligators as "fish." Since those animals are aquatic based (at least in part), they are potentially on the menu (I am not sure if water buffalo count). Scientific American's explanation about eating beavers missed the nuance of the Noah's Ark explanation. The barnacle goose exemption was just a wee bit off on the actual biology, and sounds like something a few of the lesser Maesters may have bought into in Oldtown, but there you have it.

Mental Floss: Why isn't Fish Considered Meat During Lent?  Hmmm. I like the Atlas Obscura explanation better.