Combine shrimp shells and 3 cups water in a medium saucepan.
Bring to a simmer over medium.
Remove from heat, and let stand 15 minutes.
Pour mixture through a fine wire-mesh strainer into a bowl; discard solids.
Set stock aside.
Toss together fish pieces and salt in a large bowl.
Cover and chill until ready to use, up to 1 hour.
Heat oil in a large Dutch oven over medium.
Add flour; cook, whisking constantly, until smooth and lightly browned, about 1 minute.
Add onion, bell pepper, celery, and garlic.
Cook, stirring often and scraping bottom of Dutch oven or large stock pot to prevent scorching, until softened, 8 to 10 minutes.
Stir in shrimp stock, tomatoes with juices, okra, Worcestershire, thyme, bay leaves, Creole seasoning, and black pepper.
Bring to a simmer over medium-high heat.
Reduce heat to medium-low; cover and simmer until okra is almost tender, about 15 minutes.
Stir in fish and shrimp, and reduce heat to low.
Cover and cook until fish and shrimp are cooked through, 5 to 8 minutes.
Remove bay leaves.
Season to taste with salt and pepper.
Serve with black rice; garnish servings with scallions, and drizzle with olive oil.