How to Make It
Let scallops, salmon, and fish come to room temperature. Heat oil in a large Dutch oven over medium-high. Add scallops; cook until they develop a golden-brown crust on one side, about 2 minutes. Remove from pan, and set aside.
Add wine, 1/2 cup water, and clams to pan. Cover, increase heat to high, and cook until clams open, 5 to 7 minutes. Transfer clams to a large bowl (discard any unopened clams). Add mussels to pan; cover and cook until they open, 3 to 4 minutes. Turn off heat. Remove mussels; set aside. Pour cooking liquid through a fine-mesh sieve over a bowl; set aside.
Add reserved oil from anchovies to pan over medium-high. Add anchovies; cook until dissolved, about 2 minutes. Stir in fennel; cook until tender, 5 minutes. Add garlic and orange zest; cook 1 minute. Stir in reserved cooking liquid, tomatoes, and crushed red pepper. Reduce heat to medium; simmer, uncovered, until sauce has thickened slightly, 12 to 15 minutes.
Nestle fish and shrimp into broth. Cover and reduce heat to low. Cook until seafood is opaque, 6 to 8 minutes. Add mussels, clams, and scallops, cooked sides up. Heat until warmed through. Top with reserved fennel fronds.