Fish can spawn in underwater caves, even below y=0

Applies to salmon too, seen at y = -35 with broken AI

This is an opinion and not a bug.

i think that it is nice that fish are in the caves it adds a way to get food when underground but I have found fish spawning in singular water source blocks below 
y-0 

his is a single water source block (source out of view) as you would find in the old caves but at least 10 fish have spawned in it. another example 

 
fish can be seen getting to the bottom of the water and a lot of bonemeal and raw cod can be found. i am happy to be corrected if this was intensional. 

If this is an opinion and not a bug, then I think I can offer my opinion: It makes no sense, and at times in which the fish spawn high up, they are likely to drop onto the cave floor and die loudly and often.

Edit: Affects version 21w06b as well.

The description might look like this is an opinion, but i have a few other reasons as to why this should be considered a bug.

Before on 1.17 the chance of seeing fish on caves was very low, because you would only find them spawning on underwater caves below oceans, and also the caves weren't big enough to create the massive waterfalls we currently have.

Right now fish is able to spawn basically everywhere where there is water below an ocean or river, and can lead to unnecessary clutter of entities in caves which die just from falling from waterfalls, on magma blocks or killed by axolotls, leaving items in the ground as can be seen in MC-236879 or in some duplicates of this report.

What i think should be the possible solution to this, would be to simply don't make fish spawn in complete darkness, so the opposite condition to the glow squid spawn behavior (as you don't see regular squids in caves anyways).

I think this may be worth marking for review again.