Salmon are a type of fish.
Like all types of fish, each salmon gives one fish fillet when caught.
Salmon can only be caught with blue and purple lures in fishing spot #8 (the rocky area next to the fishing boat).
Salmon can be caught in small circles and therefore are harder to catch.
See Fishing List for more detailed information.
Salmon in the fishing book
The salmon is on page 4 of the fishing book:
After a long journey up the river, this traveler is ready to take it easy.