The proof is in: Farmed salmon contains significantly higher levels of contaminants like PCBs and dioxins, according to a global scientific study published in Science magazine today

Farmed salmon should be consumed in limited
amounts and consumers should be aware that consumption of even one or two
meals of farmed salmon per month may exceed acceptable levels of dangerous
contaminants, according to the findings of an unprecedented global study
published in the US-based journal, Science today.
http://www.newswire.ca/en/releases/archive/January2004/08/c8561.html