Gaffs are traditionally used to secure larger fish after they've been caught with a net or hook. They have a long, sturdy handle and a barbed hook at the end that's used to pierce the fish's flesh.
While it's possible to use a gaff to catch salmon directly, it's not a common or recommended practice. Gaffs are primarily used for securing fish that have already been caught, and directly using one to catch fish can be considered unethical and potentially harmful to the fish.