The Fremennik shield is a members-only shield. To wield one the player must have started the Fremennik Trials quest. 30 Defence is required to wield the shield.
Bardur, located in the Waterbirth Island dungeon, will trade you sharks for Fremennik blades, Fremennik shields or Fremennik helms. However, it is usually more cost-effective to high-alch them, due to how out of the way Bardur is, and the value of sharks. Still, if you're there and got any of these as a drop, trading it for food can extend your trips.