Heat the oil over medium-high heat in a 6-quart Dutch oven. Add the sausages and stir, breaking up the sausages into small pieces with a wooden spoon, until cooked through, about 4 minutes. Add the chicken, 1/4 teaspoon salt and pepper. Cook until the chicken is no longer pink on the outside, 1 to 2 minutes. Transfer the chicken and sausage to a medium bowl, using a slotted spoon.

Add the garlic, fennel, onion, bell pepper and 1/2 teaspoon salt and pepper to the pan. Cook until almost tender, about 5 minutes. Add the clam juice, tomatoes with juices, paprika, saffron, cayenne and bay leaves. Add the spaghetti and cook, uncovered, until almost tender, stirring occasionally, about 9 minutes. Return the chicken and sausage to the pan. Bring the sauce to a simmer. Add 1/4 teaspoon salt and pepper. Add the halibut, clams and shrimp. 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. Mix in the parsley and serve.