If you're playing on the current 1.14.1 version of Java-Edition Minecraft, you may have noticed a lack of aquatic mobs lately. Or you may not, it depends if you like to catch fish in buckets or not.
An old bug from 1.13 days whereby fish caught in buckets and then re-released into water count against the passive aquatic mob cap has resurfaced. Normally, mobs like fish, dolphins, and squid spawn randomly in water blocks at the appropriate near-sea-level depths in appropriate ocean and river biomes, but then despawn if the player moves far enough away or the chunk gets unloaded. Fish caught in buckets and then placed back into water will not despawn over time or if no player is nearby, enabling players to stock their Koi ponds and fishtanks without necessarily needing to collect a bunch of nametags. However, they're supposed to no longer count against the aquatic mob spawn cap, but this bug means that they do. Thus, if there are more than 16 fish-from-buckets in loaded chunks, no further fish, squid, or dolphins will spawn anywhere in the world. Turtles, however, will continue to spawn with newly-rendered chunks as they spawn with the world seed rather than randomly, and do not naturally despawn.
There are two temporary solutions to this problem.
First, if all the fish-from-buckets in your world are in an easy-to-access aquarium or fish pond someplace, you can temporarily re-catch the fish in buckets until the next bugfix release comes out. Fish in buckets don't count against the mob cap until they're put back in water, so if you stick the bucket in a chest somewhere, that should fix the problem provided you can re-bucket all or nearly all such fish on your world map, or at least those in loaded chunks.
Second, there is apparently a datapack someone made when this buck first appeared back in 1.13 days, you can apparently apply it to your world and it will edit the data files for all those fish-from-buckets so they remain persistent but no longer count against the mob cap. You can download the datapack HERE from the official Mojang bug tracker website.
Hopefully the Mojangsters will fix the problem in the next 1.14.2 bugfix release and I can put all the fish back in our aquarium.