I am curious. Is it because the fish are smaller...? Is it because of measurement error?
different toxins show up in different species (generally speaking) depending on diet, migratory patterns, length of life.
The main reason salmon don't bio accumlate is age. They aren't around long enough to bulid up significant levels. Wild salmon and sardines are great option. "Atlantic" salmon isn't a good option. While mercury isn't an issue, PCBs are.
In terms of other fish, the seychelles child develpment study showed pretty clearly fish is a good thing even if the mercury content is measurable. it appears that the high, high selenium content of fish is neuroprotective on it's own and protective because it binds with mercury, prohibiting mercury accumulation in the brain.
We're doing our kids no favors by avoiding fish during pregnancy and breastfeeding. The low DHA, low selenium is a double whammy, potentially increasing risk...
Wild Planet tuna is fantastic. I get it from amazon.com -so far they have the best prices I've found Both their albacore and skipjack are caught in locations and via methods that get a "best" rating from the Monterey Aquarium Seafood Watch Program. I also eat sardines, wild salmon and some halibut, though halibut is much much lower in fat overall and therefore lower in DHA.