I’ve been worried about the mercury in tuna. I didn’t know that selenium protects against mercury damage. See this paper, for example — see Table 1. Moreover, the same fish that contain mercury contain protective amounts of selenium. “It appears that selenium levels in fish are high enough to give protection against mercury toxicity,” concluded a review article. These important facts were missing from all discussions of the dangers of mercury in fish that I had seen, such as this one.
I already take selenium supplements because selenium protects against cancer. An especially convincing bit of evidence for this effect is a map of United States county-by-county colorectal cancer rates. There is a sharp separation of high- and low-rate areas along the Ohio and Mississippi Rivers. Attempts to explain the difference eventually decided it was due to selenium in the soil of the low-rate areas.