Unusual mob spawning in oceans

I loaded in the 20w45a snapshot, and about an hour later I found an unusually large amount of fish spawning in a single chunk, motionless, which appeared to be the cause of lag on the world. I tried killing them with /kill @e, which registered to the game's log, but didn't completely solve the issue, as they were still visually there, and the framerate didn't go up at all.