mmmm. i'm really not 100% sure whether it's just canned salmon or if it's fresh caught salmon as well. here's some research i found;
And it's not just "canned" tuna. The New York Times did a feature in early 2008 on the absurd levels of mercury found when they sampled tuna sushi from 20 Manhattan stores and restaurants.
Nearly all fish and shellfish contain traces of methylmercury. However, larger fish that have lived longer have the highest levels of methylmercury because they've had more time to accumulate it. These large fish (swordfish, shark, king mackerel, tuna, and tilefish) pose the greatest risk. Other types of fish and shellfish may be eaten in the amounts recommended by FDA and EPA.
but seeing as though you know who caught the actual fish, i'm not sure whether tuna naturally has high mercury levels or it's just used as a preservative or something after being processed. hopefully someone else can answer that.
here are the top 4 types of fish with the lowest mercury levels;
I believe the reason these larger fish have higher levels of mercury is because of biomagnification (or something like that) where they eat fish that have mercury in it all the time so they acquire more mercury content that way. That is why the mercury is present in the fish in the first place, so it doesn't matter whether it's raw or canned, it still has it in there.