People from Rasht are known in Iran as “fish-head eaters” because they love fish so much they eat even the fish heads. This soup, beside being delicious, is also an inexpensive way, when combined with a pot of rice, to feed six people. In Rasht they use the white fish of the Caspian, which is similar to a striped bass/rockfish or branzino. But salmon heads also work well for this soup. If using salmon heads, just one or two will be enough.