The area around the Unfrozen lake is safe to fish at. Predators don't often come to this particular lake as they prefer the colder atmosphere around the other lakes.

Byleth will be able to find animals like hares and reindeer that are colored like the northern lights. They come to the lake to drink before leaving again, but some braver ones will come investigate him. They're normal animals that have absorbed the energy of the Northern lights and nothing too dangerous.

Happy fishing!