IIRC, the fish from that quest respawn every few minutes or so. If you use a good Detect Life spell, you should be able to locate some of the others.
If your talking about the quest in weye, with the rumare slaughterfish, they do not respawn. I got up to 6 scales, left for an in game week pretty much, came back, and they hadnt respawned. I had to resort to using console commands to finish the quest, because there were only 6 fish in the entire lake.