In a large pot, heat the oil. Add the onion, celery and carrot and cook over moderately high heat until softened, 5 minutes. Add the garlic, orange zest, thyme sprigs, fennel and coriander seeds and saffron and cook over moderate heat, stirring, until fragrant, 5 minutes. Stir in the tomato paste and cook over high heat until glossy, 1 minute. Add the tomatoes, clam juice, wine and port and bring to a boil. Add the grouper and shrimp, cover partially and simmer for 45 minutes. Discard the zest and thyme sprigs.