Preheat oven to 250 degrees. Line a large rimmed baking sheet with foil; fit another with a wire rack. In a large bowl, whisk together egg whites, 2 teaspoons salt, and 1/4 teaspoon pepper; add fish, and toss to coat. In another large bowl, combine cornmeal and flour; season with salt and pepper.
Working in batches, lift fish from egg-white mixture (letting excess drip off), and dredge in cornmeal mixture, patting to adhere; transfer to foil-lined sheet.
In a large skillet, heat oil over medium-high. Working in batches, cook fish until golden brown and opaque throughout, 4 to 6 minutes, turning once (if fish is browning too quickly, reduce heat). Transfer to wire rack, and keep warm in oven while remaining fish cooks. Serve with ketchup.
Slicing fillets lengthwise then crosswise into same-size strips allows the fish sticks to cook evenly.