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 and ½ teaspoon each salt and pepper. Sprinkle with the tarragon before serving.