Scale and wash mackerels, drain. Sprinkle 1 teaspoon salt over each fish on both sides, let stand 10 minutes. Brush grill rack with oil and heat rack over medium heat. Place fish on rack and grill 5 to 7 minutes or until brown. Turn fish over and grill another 4 to 6 minutes or until brown. Serve with grated daikon and sprinkle lemon juice and soy sauce if desired.