Roasted garlic mackerel in olive oil. Why Mackerel? A mild, meaty smaller fish, Atlantic mackerel are an excellent source of protein and vitamin B-12. Ours swim in immense schools off the coast of northern Spain, and by choosing to eat these fish, we're taking the pressure off less abundant species like tuna. We're also supporting local fishing families who use traditional hook and line techniques, which means almost no bycatch.