Frozen sardines have been a “go-to” bait on Canada’s west coast for decades. They are usually fished whole with a sardine-cap to give them the right roll and action. Those sardines are native and usually under 6″ in length.
Some anglers like our good friend Michael Kumar and Bill Selby have experimented using larger sardines that are imported. They thaw them, cut them to the right shape to fit the “herring-heads” and use a brine mixture to toughen-them up so that they maintain their shape when trolled. They have had excellent success using them and saved quite a bit of money then buying prepared ones.