A vast majority of fish that contain high amounts of mercury are usually bottom dwellers such as tuna, shark, and mackerel. Mercury builds from the lowest on the food chain in these environments and gets higher further up the chain. It is important to avoid eating too much fish with mercury in it as it can be dangerous and sometimes even lethal when consumed in excess.
Salmon however is not a fish that has a lot of mercury in it so it is safe to eat as much as you want and it is actually quite good for you. Pregnant women especially will want to avoid eating a lot of fish with mercury in it because of the fact that it can have adverse effects on the child.