Fish stock is prepared with fish bones (and heads), water, and often spices and finely chopped mirepoix, a combination of onions, carrots and celery. No specific type of fish must be used, but often leaner cuts are preferred to oily fish. The ingredients are combined with the water and simmered to integrate the flavors. It is used as a base for soups and stews, but has fish allergen properties so should be avoided by individuals with fish allergies.