Started a server in pre-2 and shifted to pre-3 when it was released.
Fish occasionally have long periods in which they refuse to spawn in anywhere. Was mostly noticeable in pre-2 only in coral reef areas, though now in pre-3 all ocean areas seem to go many in-game days without spawning any fish at all. In the rare times they do spawn it is far less than originally showed up when the server was first started.
I'm having this same problem. I only noticed after reading this. I've seen one group of salmon since I updated to 1.13 pre-3.