Scrape out and discard the seeds from the halved cucumber with a teaspoon and slice the halves into 1/2" cubes. Place these cubes in a small bowl and sprinkle with salt to draw out their excess moisture. Let the cubes stand 15 minutes, then drain and pat them dry with paper towels.
Cook the macaroni in rapidly boiling salted water for 20 minutes, following the directions on the package. Meanwhile, wash the herring fillets in cold water, pat them dry with paper towels and cut them into 1-1/2" lengths. Drain the cooked macaroni in a colander, run cold water over it to cool it, then spread out on paper towels to get rid of any excess moisture.
Place the cut-up herring in a salad bowl along with the cucumber, macaroni and sliced raw mushrooms. In a separate bowl, beat together the vinegar, olive oil, salt and pepper with a small wire whisk and pour this mixture over the ingredients in the salad bowl. With 2 wooden spoons, mix the salad thoroughly but gently. Refrigerate the salad for 1 to 2 hours before serving.
DRESSING: For a richer dressing, preferred by the Danes, whip the mayonnaise, sour cream and curry powder together in a small bowl and add to the salad just before serving.