Cook the potatoes with their skins in salted water for 20 minutes. Drain and peel immediately. Let cool down.
In the meantime, remove the pulp of the avocado and dice. Place in a bowl and drizzle with the lemon juice to prevent it from discoloring. Divide the trout fillets into bite-sized pieces and remove any bones. Mix with the avocado.
Stir the sour cream and milk until smooth and pour over. Add the capers and the chives cut into rolls. Finally, carefully fold in the sliced potatoes and the spices.