In a heavy pot, heat the oil. Add onion, garlic, and parsley. Stir over moderate heat. Do not brown.
Add the chopped, seeded tomatoes and tomato paste, wine, salt and pepper. Simmer for 10 minutes. Simmer for 10 minutes.
Cut the fish into portions. Add to pot together with the clean shrimp. Cook for 5 minutes.
Steam the clean cherrystone clams and mussels in a pot with a little water. Add to the stew with the strained juice.
Heat before serving.