Jackfish are a large family of sleek predators, native to warm and temperate waters around the world. Several varieties are found along the Eastern Seaboard as far north as Nova Scotia, though they become less common as you move away from warmer Southern waters. They're enjoyed as game fish throughout their range, and in some areas they're the subject of a limited commercial harvest. The flesh of jacks is well flavored and firmer than most fish, which makes them suitable for grilling.
Rinse your jack fillets or fillet pieces, and pat them dry with paper towels. Check the fillets with your fingertips for bones. If you find any, remove them with a pair of tweezers.
Sprinkle the fillets with lemon juice and season them with salt and pepper, and then brush them lightly with vegetable oil. This will help prevent them from sticking to the grill.
Preheat your grill to a moderately high temperature, approximately 400 degrees F. Place the fillets on the grill with the skin side up, so the flesh can take on grill marks.
Grill the jack fillet flesh side down for 3 to 4 minutes, until it is well marked by the grill. Turn, and cook for another 5 to 6 minutes, depending on the thickness of the fillets, until the jack is no longer translucent in the thickest part of the fillet.
Remove the fillet portions from the grill and serve them hot, with your choice of condiments or side dishes.