Mackerel
An oily sea fish with a robust, savoury flavour and firm, dark flesh.
Fresh or frozen whole mackerel and fillets can be found at fish counters, in Asian grocers and often in Turkish supermarkets; smoked mackerel from the chilled aisle is a different product and works for neither grilling nor braising. Asian grocers also carry frozen, pre-salted fillets of the kind commonly used in Korea.
Substitute:
Herring or large sardines are similarly oily and robust, though herring is a little milder and has finer bones. Lean white fish is no substitute, as it lacks the fat and full flavour the dish relies on.