This is a distant cousin of the mackerel we are used to in the UK. It hasn't got a lot to do wth Spain - various species of occur all over the world, but especially in the Far East (where they call it tenggiri) and the Caribbean (sierra). Ours is from the Caribbean. It's a nice-sized fish with firm flesh and excellent flavour, a little less oily than the mackerel you're used to. Fish guru Alan Davidson recommended serving Spanish mackerel with ginger sauce. Each pack contains two headed, cleaned and tailed fish.
