Does farm raised salmon have mercury?

Whether a salmon has levels of Mercury in it is governed by where it has been swimming and what has been in its food chain, rather than whether it was farm raised or wild.

A farm raised salmon, raised in aquaculture rearing pens in the waters of the Puget Sound in Washington state would have a similar level of mercury as wild salmon that spent their lives in the same area.

Salmon live in a four year life cycle; being laid as eggs in rivers where they hatch and grow to an inch or so, then they swim down river and stay in relatively safe harbors and coves until they are large enough for their school to move to deeper waters. This is when they migrate out to the ocean, where they stay until they are about 4 years old. At 4 years old, they return to the rivers they were originally hatched in, spawn and die.

Because of this, an argument could be raised that mercury levels in wild fish should be lower than in farm raised fish, as ocean mercury levels are significantly less than that in inland waterways where foundries and mills have deposited for hundreds of years.