Eel

The  is one of sixteen(16) different 'fish' that can be caught, and either sold to the Lighthouse Keeper, processed for cooking, or - in it's particular case, once studied - processed into a powder for use in alchemy. The difficulty in catching it is relatively low, thus it's thought to be a relatively low-tier fish.

Fishing
The odds of catching a fish are affected primarily by the other fish in the area at that time. More competition means your chance of catching a particular fish over another is lowered. Here is a table of this fish's preferences by the bait used, the place you choose to fish, where you choose to cast, the rod you use, and the time of day.

Alchemy
Угорь