Pre-heat oven to 400°F.
Place the butter in large a baking dish and melt in the oven, about 2 minutes. Remove it from the oven and place the Mahi-Mahi portions in the baking dish, turning once to coat with the butter.
Sprinkle them with the lime juice and season with salt and pepper.
In a separate bowl, combine the Panko breadcrumbs and paprika and season the portions with the mixture.
Bake the fish, uncovered, for about 20 minutes, or until the top is lightly browned and the fish flakes easily when pierced with a knife.
Remove from the oven and serve warm with your favorite side.