Mercury is generally concentrated in the meat, not the fat. (It has an affinity for the thiol groups on certain amino acids in protein.) There's certainly other fat-soluble pollutants like PCBs to worry about, but I'd never heard of any fish oil products having significant mercury contamination. I'd love to see the info you've been looking at?
Worth noting that canned "sardines" can be any of a multitude of unrelated fish, so it's not a good differentiator. Although they tend to be small fish, which is probably a reasonable predictor for lower mercury content.
Good advice. Additionally, I’d recommend that with any supplement you evaluate the source you’re buying from. I typically use Nootropics Depot (avoiding linking so this doesn’t look like a full blown add - check my history though, not a shill) and they use 3rd party lab testing to back up claims.
Overlooked one major benefit of salmon though: it tastes great and I would rather eat salmon than krill oil caps ;)
To be fair, there’s low heavy metal contamination salmon on the market; a few sibling comments mentioned wild Alaskan salmon, but I also think farmed salmon tends to be lower in Hg content that the “average” wild salmon.
Flax is also really easy to add to a smoothie if you have a decent blender, my recipe: almond milk (any type of milk or yogurt), frozen berries, oats, flax seeds. Just blend it up and eat it for breakfast. Washing blenders after you finish is often easy as you can just add water and a drop of dish soap and then run it once more and then pour it out.
Some skimming of Wikipedia suggests that our total annual krill harvest is about 1/950th of the amount that predators eat of the main krill species per year.
Also usually the smaller the animal / the farther down the food chain, the faster they reproduce. And some more reading of Wikipedia suggests that krill can reproduce very quickly.
Instead, you should get your omega-3 from krill oil, which is low on the food chain does not concentrate Mercury is all around better