Place the salmon in a deep-sided nonstick frying pan. Pour over the wine and add the bay leaves, lemon rind and pepper. Bring to the boil, then reduce the heat, cover and poach the salmon for 5–6 minutes or until just cooked – it should still be very slightly translucent in the centre.
Meanwhile, for the tarragon mayonnaise stir together the mayonnaise, yogurt, grated lemon rind and tarragon. Season with pepper and spoon the mixture into a serving bowl.
When the salmon is cooked, drain off most of the cooking liquid into a measuring jug and add enough boiling water to make 1 1/2 cups. Cover the pan with a lid to keep the salmon warm, off the heat.
Pour the diluted fish stock over the couscous in a bowl and leave for 3–4 minutes for the liquid to be absorbed. Fluff up the couscous with a fork and stir in the chopped tomatoes, spring onions and rocket. Drizzle over the olive oil and lemon juice, and stir to blend everything together. Season with more pepper.
Serve the warm salmon with the couscous salad and the tarragon mayonnaise.