a male spawning trout or salmon develops a hook like protrusion on the mandible. The kype is particularly striking in salmon.

large hook shape on lower jaw of spawning male trout.

Salmon hooked jaw: A male spawning trout or salmon develops a hook like protrusion on the mandible.

is that part of a fish's jaw that protrudes out, down (upper jaw), and upward (lower jaw). The kype of a male fish is usually more pronounced than that of the female.