1.Segment oranges over a small bowl; reserve ¼ cup orange juice for the dressing.
2.Make orange and dill dressing by placing vinegar, capers, dill and reserved orange juice in a screw-top jar; shake well.
3.Combine salmon and half the dressing in a medium bowl; stand for 5 minutes.
4.Place salmon mixture in a large serving bowl with remaining dressing, watercress and orange segments; toss gently to combine. Season to taste.