Season the salmon steaks with salt and freshly ground black pepper.
Heat one tablespoon of the olive oil and all of the butter in a non-reactive frying pan, add the salmon steaks and fry for 2-3 minutes on each side, or until golden-brown, turning the salmon carefully. Set aside on a warm plate to rest. Reserve the pan and any juices.
Heat two tablespoons of the oil in a separate pan, add the potatoes and fry, stirring occasionally, until tender and golden-brown on all sides, about 15 minutes.
Meanwhile, add the capers and shrimps to the pan the salmon was cooked in and cook over a medium heat for 1-2 minutes.
Add the vinegar and the remaining olive oil and stir well to combine.
Add the chopped parsley and the fried potatoes and stir well to coat. Continue to cook for 1-2 minutes, then season, to taste, with salt and freshly ground black pepper.
To serve, place a salmon steak into the centre of each of four serving plates. Place a portion of the watercress leaves at the horseshoe end of the salmon steak. Spoon a portion of the potato and shrimp mixture over each piece of salmon. Drizzle the pan juices around the edge of the plates.