The fishing locations are spread around the sea and each location will have different types of fish. The fish ponds will have a random spawn. They are not programmed to show in a specific place all the time. All the types and locations are randomly generated by the system. In each location, there’s a limited amount of fish you can catch.
When all the fish are caught from one location, it will vanish and spawn a new fishing location.
Also, the fishing locations have a lifetime of 24 hours. When those hours are over, the pond will vanish and spawn a new fishing location. If a boat is fishing during the reset, the fishing location will vanish as soon as no boat is at this location anymore.