We prefer wild salmon not only for ecological reasons but for its flavor, which is more mild than that of farm raised. Although limited varieties are available year-round, peak season begins in the spring, when the fish are caught en route to their spawning rivers.
Roast tomatoes and prepare oil:
Put oven rack in middle position and preheat oven to 250°F.
Toss tomatoes with sugar, 1/2 teaspoon salt, and 1/4 teaspoon pepper and arrange, cut sides down, in a small shallow baking pan. Heat oil in a 9- to 10-inch heavy skillet over moderate heat until hot but not smoking, then cook garlic, stirring occasionally, until pale golden, 1 to 2 minutes. Stir in basil and whole oregano leaves, then pour oil over tomatoes. Roast tomatoes until very tender but not falling apart, 2 1/4 to 2 1/2 hours.
Transfer tomatoes with a spatula to a large plate, then pour oil through a fine-mesh sieve into a small bowl or measuring cup, discarding solids. Stir in chopped oregano, zest, juice, and remaining 1/4 teaspoon salt and pepper.
Heat 2 teaspoons olive oil in a 3-quart heavy saucepan over moderate heat until hot but not smoking, then toast couscous, stirring occasionally, until fragrant and pale golden, 3 to 5 minutes. Add broth, water, and salt and simmer, covered, until liquid is absorbed and couscous is al dente, 10 to 12 minutes. Remove from heat and let stand, covered, 10 minutes, then stir in 2 1/2 tablespoons lemon oregano oil. Season with salt.
Roast salmon while couscous stands:
Put oven rack in upper third of oven and preheat oven to 500°F. Line a 17- by 12-inch shallow baking pan with foil.
Arrange salmon, skin sides down, in baking pan, then drizzle with olive oil, rubbing it over tops of fillets, and sprinkle with salt. Roast salmon until just cooked through, 12 to 14 minutes.
Divide couscous among 6 plates. Lift salmon flesh from skin with a slotted spatula and transfer a fillet to each bed of couscous. Put 2 tomato halves on each plate, then sprinkle salmon with olives and drizzle with some lemon oregano oil.
Tomatoes can be roasted 3 days ahead and chilled in oil in an airtight container. Bring to room temperature before proceeding.