Heat olive oil in a large pot over medium heat. Add onion, roasted red pepper, and garlic; cook and stir until fragrant, 3 to 4 minutes. Stir in tomatoes, salmon fillet, and merguez sausage; cook for 2 to 3 minutes.
Pour 2 1/2 cups broth and white wine into the pot. Season stew with cumin, salt, and pepper; bring to a boil. Reduce heat to low and stir in shrimp; simmer until opaque, about 3 minutes. Remove from heat and add mussels; cover and let stand until mussels open, about 5 minutes.
Bring remaining 3 1/2 cups broth to a boil in a separate pot. Stir in couscous. Remove from heat and let stand until couscous absorbs broth, about 5 minutes.
Serve couscous in shallow bowls; ladle stew on top.