If a fish swallows a hook, it can be harmful and may cause serious internal damage. The hook can get stuck in the fish's throat, stomach, or intestines, leading to injury or even death. It's important to handle this situation with care. Try to gently remove the hook, but if it's deeply lodged or causing bleeding, it's best to leave it in place. Cutting the line and releasing the fish back into the water is another option. However, keep in mind that leaving a hook in a fish can affect its health and ability to feed. Preventing this scenario is crucial, so use barbless hooks, practice proper catch and release techniques, and consider using alternative fishing methods like fly fishing, which reduces the chances of deep hooking.