This is an odd little gem of a recipe, not at all untypical of the way Ecuadoreans use plantains. Corvina is a saltwater fish very common in Latin America. Sea bass is a good substitute, though most any white, firm fleshed fish would do. There is a recipe for making achiote from annatto seeds by Rita L -- Recipe #109238. After Kathy's review, I did go over the recipe and make some corrections.