Fish, Dolphins, Squid not spawning

Does making a backup world and killing all entities make them spawn?

How do I kill all entities?

You use /kill @e[type=!player]. I highly recommend to make a backup world as the command will kill all entities that are not players. Hopefully this solves your issue.

It did not solve the issue. What now?

Wait I restarted the server and now fish are spawning again. What are the 42 entities I just killed, and why did killing them resolve the problem?

They were most likely aquatic mobs and the aquatic mob cap was full. Are dolphins and squid spawning now too?

Squid, yes, but I don't see any dolphins yet, though am in deep cold ocean, so I'm assuming the dolphins will be back when I go to a different area. Could the mobs counting towards the mob cap have been people's bucket caught fish?

Confirm dolphins are spawning. Still not sure what could cause this though. I was the only one logged in, and there were ZERO aquatic mobs near me when I noticed the problem. 

You had to kill the aqatic mobs after the fix from MC-134702.