Spot on Tom.
just thought i would add to the thread that any tank made for keeping mackerel alive should have a flow of fresh sea water. Found a site a while ago which said, i quote "The common mackerel has no swimbladder and uses a passive gill ventilation form of breathing. The fish swim mouth open and the gill chambers expanded, thus water flows through the gills. (Ramjet effect) This saves engery which is used by other fish to pump water through the gills when the mouth closes. Therefore it must move to live, if the fish is not swimming it will drown."
I think that sorts that out.
The site is here if any one is interested Mackerel
Alan