All images from the same cavern system, right next to each other & right below/beside an actual ocean.

This isn't difficult to find in 1.16.1 with Forge 32.0.90+. Perhaps the mod could limit fish spawning to levels near y=63 to limit this effect?

What biome mod are you using

This is with Biomes O Plenty. No other biome additions.

Fish spawns are controlled at a biome level so it could very well be bop since 1.16 did introduce a new fish spawn category. But again I'm not sure, take out bop and test it in only vanilla biomes.

Roger that, will do.