We know that Jewish dietary laws and traditions called for fasting and abstinence on several occasions.
We also know that from the earliest times of the church that members engaged in fasting and abstinence. Indeed, their diet was much more rigorous than ours today. Indeed, the diet of the majority of the people was much more rigorous and limited than ours today.
How many “common” people ate meat regularly? Few, very few. Meat in the majority of the countries where Catholicism/ Christianity spread was not readily available, and it spoiled easily (spices were of little help and were additionally very costly; refrigeration was a 20th century innovation, canned meat a 19th century one).
As people’s diets became better, as the standard of living became higher, as people became less nomadic, towns developed, fortified cities, and finally standing armies and legal, formal governments, fasting and abstinence as a near-daily part of life eased. Still, the underlying penitential aspect remained for Catholics/ Christians, and Jews, as it does today.
Fish was nearly always, in the Mediterrean regions and areas where Christianity spread first, available. Unlike game (which was rigorously protected in feudal societies, to the point of making poaching punishable by death), salt or freshwater fish and other marine specialties like clams, mussels, crabs, etc. were unregulated, cheap, and available. So it wasn’t “special” in the way meat and dairy products, and other luxuries, were. And as time went on, it was seen as common, “poor man’s food”. People craved luxuries and dainties and uncommon food, so eating fish was a sign of pennance, temperance, et. al.
Was there a problem with a fishing industry at some time? Maybe. But I have a cookbook with recipes from the 13th century–the beginning of the High Middle Ages–which lists fish as a penitential food and additionally milk, eggs, butter, etc. And it is known as a long-standing tradition–not something that’s a Johnny-come-lately or a “rescue of the fleets” tradition.
I can easily see a combination of a Pope wanting to both help out a deserving industry AND reinforce traditions, customs and PENITENTIAL PRACTICE among Catholics by stressing that fish was a “good choice” for one’s Lenten meals or other penitential days (again, we don’t realize just how little we do today compared with even 500 years ago). I can as easily see some anti-Catholic twisting and turning to put the worst possible interpretation (private interpretation, ha-ha) on it in another failed attempt to “do down” Catholics. God forgive those who act out of hate, fear, and anger.