When making an underwater area closed off by fences, underwater creatures such as axolotls, cod, salmon, turtles, tropical fish, and dolphins will hug the fences and freeze in place. This is probably due to their pathfinding where they want to leave the area, and my guess is they think that a waterlogged fence is the same as a block of water. The only issue with this however is that the animals can't squeeze through fences (unlike some other cases where cod can go through trapdoors etc.).

These animals remain in place until you break the fence that they are stuck to and/or push them out of the way completely. It only takes a short amount of time (5-10 minutes) for them to return to the fences again.