Got this from a website.

Catholics are among the only Christians who freely don’t eat meat on Fridays during Lent. In fact, you know you are in a Catholic town when, only during Lent, every single restaurant advertises one item on their menu: fish! I have even noticed how major fast-food chains point out on their fliers the date of Ash Wednesday! Suddenly everyone cares about the liturgical seasons of the Church!

So why is it that the Church instructs Catholics to abstain from meat on Fridays (as well as Ash Wednesday and Good Friday), but gives the “thumbs-up” for Catholics to eat fish? Sounds fishy to me!

Why Catholics don’t eat meat on Fridays
First of all we must ask the question, “why Friday?” The USCCB gives a succinct explanation:

Catholic peoples from time immemorial have set apart Friday for special penitential observance by which they gladly suffer with Christ that they may one day be glorified with Him. This is the heart of the tradition of abstinence from meat on Friday where that tradition has been observed in the holy Catholic Church.

Since it is believed Jesus Christ suffered and died on the cross on a Friday, Christians from the very beginning have set aside that day to unite their sufferings to Jesus. This led the Church to recognize every Friday as a “Good Friday” where Christians can remember Christ’s passion by offering up a specific type of penance. For much of the Church’s history meat was singled out as a worthy sacrifice on account of its association with feasts and celebrations. In most ancient cultures meat was considered a delicacy and the “fattened calf” was not slaughtered unless there was something to celebrate. Since Fridays were thought of as a day of penance and mortification, eating meat on a Friday to “celebrate” the death of Christ didn’t seem right. (As an aside, some bishops have chosen to lift the ban when Saint Patrick’s Day falls on a Friday during Lent, as it is considered a “solemnity” for many Irish Catholics.)

MEDIEVAL FEAST

Read more:
“No red meat Lent” too hard? Here’s how the medievals did it

But why is fish not considered “meat”?
According to the USCCB, the laws of the Church classify the abstinence from “land animals.”

Abstinence laws consider that meat comes only from animals such as chickens, cows, sheep or pigs — all of which live on land. Birds are also considered meat.

Fish, on the other hand, are not in that same classification.

Fish are a different category of animal.  Salt and freshwater species of fish, amphibians, reptiles (cold-blooded animals) and shellfish are permitted.

In Latin the word used to describe what kind of “meat” is not permitted on Fridays is carnis, and specifically relates to “animal flesh” and never included fish as part of the definition. Additionally, fish in these cultures was not considered a “celebratory” meal and was more of a penance to eat.

Our current culture is much different as meat is generally considered the cheaper option on the menu and no longer has the cultural connection to celebrations. This is why many people are confused about the regulations, especially those who love to eat fish and do not consider it a penance.

In the end, the Church’s intention is to encourage the faithful to offer up a sacrifice to God that comes from the heart and unites one’s suffering to that of Christ on the cross. Meat is given as the very basic penance, while the purpose of the regulation should always be kept in mind. For example, it does not necessarily give a person the license to eat a lobster dinner every Friday in Lent. The whole point is to make a sacrifice that draws a person closer to Christ, who out of love for us made the ultimate sacrifice a person can make.

Interesting. When I read it, something else occurred to me.

When God caused the great Flood to eliminate sinful humans from the Earth, all those 'meat' animals died with them. However, the FISH creatures did not all die. So, if we put two and two together we find another reason why fish is allowed and meat is not. How come no saint figured this out.

Very interesting.  Thanks.  I never connected the land animals with the Flood, but the connection is definitely there.  Classification of stuff is somewhat arbitrary.  Is a fish in the same category as a cow?  Depends on your definitions of the categories.

Of course, a Prot attack against the Church I've seen was that the Pope allowed fish because some wealthy fish merchants paid him off.

Some more liturgically-oriented non-Catholics, such as Anglicans, often abstain from meat on Fridays, and I have heard of many African American families serving fish for dinner on Friday, not for any religious reason, but because "that's just what they do".  I asked an AA coworker about this once, and she said no, it's not a religious thing, it's just a black thing.  Go figure.

Interesting. When I read it, something else occurred to me.

When God caused the great Flood to eliminate sinful humans from the Earth, all those 'meat' animals died with them. However, the FISH creatures did not all die. So, if we put two and two together we find another reason why fish is allowed and meat is not. How come no saint figured this out.

Which bible states that all meat animals died?  Mine says Noah was instructed to take "of all clean beasts take seven and seven, the male and the female" that's 14 of each cows, sheep, horses, camels deer etc plus "but of the beasts that are unclean two and two, the male and the female.  Of the fowls also of the air seven and seven the male and the female" that too is 14 of each ducks,  turkeys, chickens, geese, etc.
Seems like they had plenty of meat available for eight people after they disembarked.

And here I thought protestant apologetics were frequently uneducated and low-IQ. (I was right!)
I think I just became dumber after reading that particular gem of Protestant propaganda -- it's that dumb.

Right.  I saw this somewhere on a video made by some Prots (though the Catholic Church wasn't the main subject).

You got me there Miseremini, I forgot about that. But surely Noah told his kids not to kill them as there were only two of each left to continue their KIND but plenty of fish. Then again maybe they were told they could kill a few dinosaurs. That would explain their extinction.

OK, another guess; because God shared loaves and FISH rather than loaves and chicken, fillet steak or even lamb, which He could have. Could it have anything to do with the Apostles who were fishermen and the Lord filled their nets?

And fish is certainly not an inferior flesh than meat. A friend of mine has salmon darns three times a week for dinner and I don't think he even goes to Mass on Sunday. Ask any doctor and he or she will tell you fish is better for health than meat. Maybe we are told to eat fish on Friday to keep us healthy?

"Darnes" (it takes an "e") --- learned a new word today.  At first I thought it was a typo.

Fish is a great food, great source of protein, but it goes bad quickly, and oftentimes, it smells bad when it's cooking.  I love salmon patties but I have to open the kitchen window while they're cooking, and let the house air out afterwards.  It's also hard to cook from scratch at home, so I normally either get frozen battered fillets and bake them, or go to a seafood restaurant.  Part of the genius of many a fast-food enterprise is to take something that is either difficult, or messy, or both, to make at home --- hamburgers, French fries, pizza, fried chicken, ice cream, and so on --- and serve it in a way that is easily accessible, fairly economical, and doesn't require home preparation.

And fish is certainly not an inferior flesh than meat. A friend of mine has salmon darns three times a week for dinner and I don't think he even goes to Mass on Sunday. Ask any doctor and he or she will tell you fish is better for health than meat. Maybe we are told to eat fish on Friday to keep us healthy?

I do think there is some symbolism with Noah, the Ark and fish surviving the Flood on their own.  Let's not forget that the holy name of "Maria" comes from the latin meaning "seas/oceans".  And God used the seas/oceans to purify the land, which was filled with sin.  So it stands to reason that water/fish would also purify/cleanse a body during a fast. 


Also, after the flood was the first time that God allowed mankind to eat flesh from animals, no?  Before the Flood, men who followed God's laws ate a vegan diet.  They did not kill animals for food.  They followed the 'Garden of Eden' rules, where God gave man "seeds" to be thy food. 

²⁹ Then God said, “I give you every seed-bearing plant on the face of the whole earth and every tree that has fruit with seed in it. They will be yours for food.  (Genesis 1:29)

It's not stated in the bible, but I think many saints talk about how after the Flood, God made all manner of changes to men and the earth. 
1) shortened man's lifespan from hundreds of years to only 1 hundred, at max.
2) due to man's weakened nature, God allowed man to eat animals for strength.  He also told Noah how to make wine, also for strength.
3) nature/weather and the animal kingdom were at odds with mankind, whereas before the Flood, were much more calm and docile.

I think the Church Fathers talk about how the evil men in the days of Noah fell away from God and started eating meat and killing animals.  This increased their aggression, they started wars and this caused all manner of chaos and suffering.

St Thomas also talks about how eating flesh meat and animal products can cause an increase of energy and possibilties for "carnal desires".  Whereas fish does not produce the same effects.  In other words, if you eat a lot of flesh meat and animal products (i.e. eggs, cheese, etc), this increases testosterone, which increases your libido.

Since the point of Lent is to reflect, pray, and subject one's body, then it stands to reason that this is why the Church limits intake of meat and animal products.  It's easier to mediate and reflect when you're not fighting temptations of the flesh.  Thus, eating fish (since we still need protein) is a great substitute for Lent and any time of fasting.

Re Prots disparaging Catholics, there's the Lutheran myth of the origin of lutefisk, a Norwegian "delicacy" that might be familiar to older folks here from Minnesota or Wisconsin:

Who do we have to blame for Lutefisk?
There are a bunch of stories that float around, but my favorite one says Lutefisk is St. Patrick’s fault. 
Once upon a time, Viking raiders were pillaging Ireland (as Vikings do), and St. Patrick thought he’d teach those pesky Vikings a lesson. 
So he attempted to poison the Vikings by covering their fish stores with lye.  That’ll teach ’em. 
When the Vikings discovered the fish, they dug in (didn’t die) and declared it a delicacy. 
It’s a good story, until I ruin it for you and tell you St. Patrick died centuries before the Vikings started plundering northern Europe.

As for fish during Lent in the Catholic parts of Europe post-1500s, it would have been stockfish (salt cod). Before refrigeration and efficient transportation, fresh fish wasn't usually available inland year-round anyway. Even on the coasts, setting your boats out in late winter weather probably didn't work too well either.