[darksaints]

Smaller fish have negligible quantities of mercury and other pollutants, due to their short lifespan not allowing for bioaccumulation, as well as being much lower on the food chain. Herring, Anchovies, Smelt, Sardines, and even Mackerel or Trout, are all are excellent sources of EPA/DHA, and without any of the risks of larger fish.

[pmoriarty]

Another option is krill oil, which contains large amounts of omega-3's and is low in mercury because krill (tiny shrimp-like creatures) are at the bottom of the food chain and eat only algae, unlike large fish which eat other fish and thereby concentrate mercury in their flesh.[1][2]

Incidentally, it's also possible to get DHA supplements sourced completely from algae.

[1] - https://www.drnewtons.com/blog/avoid-dangers-mercury-pure-kr...

[2] - http://krill-oil.wellwise.org/krill-oil-side-effects/krill-o...