These can be found in a couple of small lakes in the Forbidden Lands. They can be killed with arrows when at the surface, and grabbed onto underwater.
Eels can be found in the same places as fish, and can be interacted with in the same ways.
Salmon is a rare fish type to be encountered in the game, there are two salmon found in the lake near the Kuromori's area. The fish is about as large as Wander, and the player can grab on its fin and go for a ride around the lake.