Soak hickory chips in water to cover 30 minutes; drain. Wrap chips in heavy-duty aluminum foil; make several holes in foil. Set aside.
Brush fillets with oil; sprinkle with salt and pepper.
Light gas grill; place foil-wrapped chips directly on hot coals on one side. Coat grill rack on opposite side with cooking spray, and place on grill.
Let grill preheat 10 to 15 minutes; turn off burner opposite wood chips. Arrange fillets on rack opposite wood chips, and grill, covered with lid, over medium heat (300 to 35
minutes on each side or until fish flakes with a fork.
Wrap flour tortillas in heavy-duty aluminum foil.
Place on grill during last 5 minutes of cooking time to warm.
Serve immediately with Mango-Avocado Relish; garnish, if desired.