Fish oils contain about 12%DHA and 18%EPA. So figure out how much fat the fish you are eating has and then calculate from that the DHA and EPA content. Typically researchers use 3g doses in the studies. So that would mean that 10g of fish oil would give a total of 1.2gDHA and 1.8gEPA or 3 total grams.
As far as heat, extreme temps will degrade the omega 3's but as I understand it, and Im no expert of food processing, the heat treatment for canned salmon doesnt do much harm to the omega 3 content.
And to admit a mistake, from a prior posting, I recently learned that cod liver oil may contain a good amount of DHA and EPA. Although I really dont know why ANYONE would use that stuff (b/c the taste is BAD)!