Mix together the pepper, cay-enne, salt, and herbs in a bowl. Center the fillets on the planks, skin side down. Drive a few nails part way into each fillet to hold it in place. Brush with oil and then sprinkle with the cayenne and herb mixture. Cover the ends of boards with foil to catch drippings and to keep boards clean. Stand the planked fish at an angle in front of fire, tail end down. Rotate the plank to provide even cooking. Cook until fish flakes in thickest part, about 20 minutes.