The 'Y' prefix is not totally exclusive to Canadian airports and nor - as Pinotage points out - do absolutely all such airports' codes start with 'Y'. However, if you click http://www.airlinequality.com/Interact/Q1_J00135.htm you will find a web-page where the same question as yours is asked and answers provided.
Oh dear! Sorry the above link did not work. I don't know if this is the result of yesterday's momentous changes to AnswerBank or because of some mistake on my part. But here's another attempt to provide it. If it fails this time, just note the URL/address and type it in yourself. Click http://www.airlinequality.com/Interact/Q1_J00135.htm