Nice find the_chemist.
"Even the least contaminated farmed salmon, from Chile and Washington state, had significantly higher contaminant loads of PCBs, dioxins, and dieldrin than wild salmon."

That study mentions they did not test farmed pacific salmon. Here is a comparison:
http://pubs.acs.org/doi/abs/10.1021/es060409%2B?journalCode=esthag

"Atlantic salmon contained higher PCB concentrations (means, 28−38 ng/g) than farmed coho or chinook salmon, and levels in these latter species were similar to those in wild counterparts (means, 2.8−13.7 ng/g"

"Safe to eat"...