I know it's listed in the Updates as a known bug and to leave one fish. This doesn't work and in fact the fish never change from realm to realm. They follow your wizard. If you catch all but 3 fish in a spot, all are for example. Change realms, the same 3 fish will be in the same spots in any realm. Realms change nothing except who is standing in the area. So basically until you fish all the fish out of an area and wait for respawning, the fish always remain the same. This makes catching rare and epic fish extremely cost intensive on energy. If I check a hole for fish, and come back hours later the fish should change, they appear to be locked to your wizard until you clear that pool and allow it respawn.
for example. Change realms, the same 3 