What has the North to do with slavery?

An item from New Orleans claims that a ship, named the Gibraltar, from Boston, has “lots of Negroes on board”.  The item says,  “Who are the owners of the vessel?  If we can ascertain their names, we will publish them, that the people of Boston, of Massachusetts, of New England, will know who are the men that dare, at this time of day, to engage in the nefarious business of transporting slaves for the accommodation of the vile traders in human flesh.”