Heat the oil in a large saucepan or Dutch oven over medium-high heat. Add the fennel, celery, and garlic. Cook, stirring occasionally, until slightly tender, 3 to 4 minutes.
Add the wine and bring to a boil. Add the cod, shrimp, and tomatoes and cook, covered, for 3 minutes. Add the mussels and cook, covered, until the cod and shrimp are cooked through and the mussels have opened, 3 to 5 minutes.
Stir in the vinegar, 1/2 teaspoon salt, and 1/2 teaspoon pepper. Sprinkle with the tarragon before serving.
Substitution: This light French seafood stew works well with a variety of fish. Sea bass, flounder, and halibut are tasty alternatives to cod, so buy what looks best at the market.