In general, fish from the top of the food chain has more mercury (and many other pollutants!) than other fish. So tuna generally has more mercury than shrimps, though it may depend on where the fish were caught.
There’s still no problem in consuming tuna or other fish at the top of the food chain. You should just limit your consumption.
Fish is a very good thing for your health. There comes a point where worrying about pollutants does more harm than the pollutants themselves.