[grey_whiskers]

For example, a premium fresh tuna steak will generally have much more mercury than cheap canned chunk light tuna.

Sources, please?
You see, I've been a canned tuna addict since childhood,
and always wondered why I didn't end up like the Mad Hatter
in Alice in Wonderland from all the mercury...

[PAR35]

"Source please"

FDA website - scroll to the table at the bottom of the page here: http://www.fda.gov/fdac/reprints/mercury.html (unfortunatley, this doesn't break it down by type of canned tuna. Cheap imported chunk light is safer than domestic premium, but that isn't reflected on this site).

For a reference to white vs. light, see this web site:
"Note: Albacore "White" tuna contains more mercury. Limit 6 oz (~1 meal) of albacore tuna per week."

http://www.healthcastle.com/tuna-safe-eat.shtml

The most complete breakdown by type is on this liberal web site: (Note canned White and fresh ahi and bluefin listed as high, canned light listed as lower as is fresh albacore)
http://www.nrdc.org/health/effects/mercury/guide.asp

Glad to be of service.