Can methylmercury be removed from fish?

No.  Mercury has always existed in small amounts in fish and other foods.  Some natural sources of mercury include volcanoes, cracks in the earth's crust, forest fires, and others.  In fact, according to research conducted by Alaska public health officials, eight 550-year-old Alaskan mummies (four adults and four infants) that were tested contained levels of mercury twice as high as pregnant Alaskan women today.  The researchers concluded that the remains "provide evidence that humans have always been exposed to naturally occurring mercury through fish and marine mammals in their diets."