First cook potatoes with skin for 15-20 minutes in boiling water, depending on their size. Drain, quench and allow to cool.
Clean, wash and roughly grate the zucchini. Wash dill, shake dry, pluck off flags and cut finely. Stir yoghurt, dill, lemon juice and vinegar until smooth. Season dressing with salt and pepper.
Peel the potatoes and cut into cubes. Mix with zucchini and the yoghurt dressing. Let the salad steep a little.
Pluck mackerel fillets into bite-sized pieces. Season salad again. Fold the mackerel loosely into the salad.