So...assuming that a skum (Bestiary, p. 253) either is disarmed of its trident or is too poor to have a weapon in the first place.
Do its claws (or perhaps its bite) function as a primary attack instead of a secondary one?
Both bite and claw will be primary attacks any time it is not attacking with a melee weapon in the same turn. Doing the latter always turns anything's natural attacks into secondary attacks.