I caught a couple of those mackerel at Sekiu also. It was my impression that they indeed were Jack Mackerel, Trachurus Symmetricus. True Spanish Mackerel are a whole different fish and are only found in the Atlantic. Here in the Pacific we have the good old Sierra mackerel which is almost a dead ringer for the Spanish, yellow spots and all. Sierra, however, are not found much farther North than Baja as far as I know.
I released the mackerel I caught. I've since been told they smoke up pretty good ( not as good as coho I bet! ). Anyone out there ever eaten one of those?