I do not think the sale of fish is aided by masquerading one fish as another. More than that, the Meat and Canned Foods Act specifically provides in section 18, subsection 2, paragraph (b):
A true and correct description of the contents of the can, including the vernacular name, and in the case of fish the minimum weight in avoirdupois of the contents and in the case of shellfish the minimum weight in avoirdupois of the dry meat in the can, plainly printed in a conspicuous manner, and the name of the place where the same was packed.
If mackerel could be sold for a short time at better prices by labelling it as tuna it might aid the man who is canning it, but it would certainly harm the trade. Large tuna used to be called horse mackerel in the United States and they marketed horse mackerel as chicken of the sea, but nevertheless it is a tuna. I do not think the fishing industry of Canada would be very happy if certain canners were allowed to sell fish under names which are not commonly accepted or under other names. In any case, the law would prohibit it here in Canada.
Subtopic: DEPARTMENT OF FISHERIES