In a 6-quart Dutch oven, heat the oil over medium-high heat. Add the sausage. Stir, breaking up the sausage into small pieces with a wooden spoon, until cooked through, about 4 minutes. Add to the chicken to the skillet and season with 1/4 teaspoon of salt and 1/4 teaspoon of pepper. Cook until no longer pink on the outside, 1 to 2 minutes. Using a slotted spoon, transfer the chicken and sausage to a medium bowl.
Add the fennel, bell pepper, onion, and garlic to the pan. Season with 1/4 teaspoon of salt and 1/4 teaspoon of pepper. Cook until almost tender, about 5 minutes. Add the clam juice, the tomatoes with juices, bay leaves, paprika, saffron, and 1/8 teaspoon cayenne. Add the spaghetti and cook, uncovered, until almost tender, stirring occasionally, about 9 minutes. Return the chicken and sausage to the pot. Bring the sauce to a simmer. Add 1/4 teaspoon salt and 1/4 teaspoon pepper. Add clams, shrimp and halibut. Cover and cook until the clams open, 4 to 5 minutes (discard any unopened clams). Remove the cover, reduce heat to medium-low, and simmer gently until fish, shrimp, and chicken are just tender, 4 to 5 minutes. Remove the bay leaves and discard. Season with salt and pepper, to taste. Mix in the parsley and serve.