As a rule, it’s best to dry-cure fish with a low fat content, such as cod, and brine-cure fish with a high fat content, such as mackerel. But the methods are usually interchangeable, especially if you are after a rather light cure to be followed by refrigeration or freezing. Unfortunately, there are infinite variations on the basic methods of salt-curing fish in preparation for smoking; instead of rhyme and reason, we have purely personal and regional variations, which are often staunchly defended. I’ll try to keep it simple.