Mix trout, apple, celery, and chives in large bowl to blend. Whisk sour cream, mayonnaise, lemon juice, and 1 tablespoon horseradish in small bowl to blend. Fold horseradish mixture gently into trout mixture (do not break up trout). Season to taste with salt, pepper, and more horseradish if desired. (Can be made 1 day ahead. Cover and refrigerate.)