That is kind of what I have heard about small fish vs large fish.

What I think makes the difference is what the fish eats. The small herbivore fish eat algae and plankton and that is where they get the omega 3 from. They may pick up a small amount of toxins but they are limited to what they get in their environment. Then as you go up the food chain the big pescavore fish eat the little herbivore fish. They gain the omega 3's from the little fish but also gain the toxins. So the higher up the food chain the more concentrated the toxins as the heavy metals never leave the fish. So things like swordfish are the worst because they tend to be up towards the top of the food chain and have eaten fish that have eaten other fish. At least that is my understanding.

So if that is correct, it may not make a difference if it is a small sardine or a big sardine. They both would have had the same diet of the stuff they get the omega 3's from.

That is just the way I see it in my minds eye. It may not be correct. At any rate I am not to concerned about contamination in the small fish species like sardines. Someone closer to the ocean or who has studied it may have better information.

John