Preheat the oven to 375 degrees F. Bring a large pot of salted water to a boil.
Cook the salmon fillets as desired. Meanwhile, cook large pasta shells as directed on package.
In a medium bowl, combine the salmon, ricotta, spinach, and 1 cup of the mozzarella. Mash together with a large fork until thoroughly combined.
In a 13-by-9-inch baking dish, spread ½ cup of the sauce on the bottom. Fill each shell with about 1 tablespoon of the ricotta mixture. Layer the filled shells in the dish. Pour the remaining sauce over the shells. Sprinkle with the remaining mozzarella. Cover with foil. Bake until hot and bubbling, 35 minutes.