Mercury in tuna is primarily methyl mercury while "silver fillings" are inorganic and elemental. Methyl mercury is far more toxic than inorganic or elemental, but is eliminated from the body at a faster rate. The good thing about ocean fish is that while they are high in mercury, they also contain lots of selenium.
Selenium is an essential element that can be toxic at high levels. Mercury and selenium have a high affinity for each other, and it is believed that the toxic effects of mercury are reduced/eliminated when bound to selenium. This has been shown but not proven. There are patents on toothpaste containing selenium for this very reason.
There is also some interesting information on high supplemental levels of Iodine binding to mercury, but possibly retaining it rather than eliminating it. This is something we are actively looking into at our lab. We developed/run a test for iodine, bromine, selenium, arsenic and mercury.