Preheat electric oven to 450°F. Cut the 12 oz of white fish into 24 thin, 4-inch strips. Cut the remaining 4 fish fillet into 4 thin strips each. Lay 1/2 sheet of nori on a flat surface. Place 6 strips of white fish, side by side, crosswise on nori. Using the side of a knife, lightly tap fish strips on nori. Turn nori over, spread with 1 tablespoon mayonnaise and place 1/4 of the crab, 2 shrimp, 1 strip each of the salmon, white fish, ahi, hamachi, and 1 slice of avocado on nori. Roll ingredients inside nori, with white fish on the outside. Make 3 more rolls. Cut each roll crosswise into 4 pieces. Place pieces, cut side up, in a baking pan. Bake for 5 minutes. Combine the remaining 1/2 cup mayonnaise and tobiko; place a heaping teaspoon on each piece of sushi. Bake for 3 more minutes, or until sauce turns golden brown. Make 4 servings.