Fish, dolphins and squid stop spawning in the ocean biomes after placing ~16 Bucket of Tropical Fish in a body of water. Drowns and turtles appear to be spawning though. I also tested this in the latest snapshot (18w31a) and did the same thing. Tested in a newly generated world on version 13.1 and the snapshot as well.
I noticed this on my main world that was on versions 1.12.2 update to 13.1. Attached is the log file and screenshots.
Environment
OS - Windows 10 Java edition
Java - Version 8 Update 181
Memory - 16GB
Display - NVIDIA GeForce GTX 960M
Seed - 3232323232
So... 16 released fish in a bucket in a freshly generated world that stops all ocean mobs from spawning is as intended. Sound like a bug to me. Very frustrating and should not be a feature if that is the case.