http://www.slavenorth.com/profits.htm
The effects of the New England slave trade were momentous. It was one of the foundations of New England's economic structure; it created a wealthy class of slave-trading merchants, while the profits derived from this commerce stimulated cultural development and philanthropy. --Lorenzo Johnston Greene, “The Negro in Colonial New England, 1620-1776,” p.319.

Whether it was officially encouraged, as in New York and New Jersey, or not, as in Pennsylvania, the slave trade flourished in colonial Northern ports. But New England was by far the leading slave merchant of the American colonies.
if it wasnt for yankees, slaves wouldnt be as prevalent in the south as they were. The south seceded so they wouldnt be subjugated and have their land owned and operated by northern financiers. It didnt matter, the yankees started the war to take control of southern assets. What the south feared came true. It lasted until the 1970's when the atlantic states finally started to gain independence again.