I'm over the Nether ceiling testing some stuff... This shows up second in the profiler.
Why should it addFishSpawn here?
SteveKunG commented
All dimensions are already stay loaded, if there are structures around spawn chunk which allows fishes to spawn. addFishSpawn method would be executed. I've move addFishSpawn method to SpawnHelper class instead of ChunkGenerator. Hopefully it solves minor performance issue.