Longer living fish usually have more mercury in them. Tuna steak has higher mercury than canned tuna though, which is pretty safe. I eat the oil based canned tuna right out of the can. I love it. I eat one - two cans a week. But I don't work out heavy and on a routine basis right now. I eat it for the omega-3 fatty acids. The omega-3 fatty acids of tuna help improve cardiovascular health
Flax seeds are very healthy and so is the oil that is obtained from them. Are you thinking about consuming it for the EFA's? Because, if that's the case, you could also consume chia seed oil. One disadvantage of flaxseed oil might be that it contains phytoestrogens. Chia seeds contain nutrients very similar to flax seeds, without the phytoestrogens.