To add some information--athough there is some natural mercury and there may indeed have been some elevation of levels as you reference, particularly in the food-chain hierarchy, I think that the extremely high mercury levels in our time which are seriously contaminating the world's supply of fish, and are referenced along with PCB's and other toxins, are more the direct result of industrial byproducts.

Here's a clip from a CNN article on mercury:

http://www.cnn.com/2001/HEALTH/pare.../fish.pregnant/

(from 2001)
According to the EPA, states are primarily responsible for protecting their residents from contaminated noncommercially caught fish. Almost 68 percent of state-issued consumption advisories are a result of mercury contamination in fish and shellfish. Last December, the Clinton administration announced that mercury emissions from power plants pose a significant health hazard and proposed regulations to reduce them by 2004. An EPA official now says that decision is "under review" and that "no decisions have been made yet."

Mercury occurs naturally in the environment, but in the United States more mercury enters the air through industrial emissions, the biggest source being coal-fired power plants. Once there, the mercury enters waterways and accumulates in the muscle tissue of fish.

Fish and other seafood products are the main source of methylmercury toxicity in humans, and fetuses are particularly vulnerable.

Here's another article of interest:

http://www.oehha.org/fish/general/memerc.html

Where does methylmercury in fish come from?
Methylmercury in fish comes from mercury in the aquatic environment. Mercury, a metal, is widely found in nature in rock and soil, and is washed into surface waters during storms. Mercury evaporates from rock, soil, and water into the air, and then falls back to the earth in rain, often far from where it started. Human activities redistribute mercury and can increase its concentration in the aquatic environment. The coastal mountains in northern California are naturally rich in mercury in the form of cinnabar ore, which was processed to produce quicksilver, a liquid form of inorganic mercury. This mercury was taken to the Sierra Nevada, Klamath mountains, and other regions, where it was used in gold mining. Historic mining operations and the remaining tailings from abandoned mercury and gold mines have contributed to the release of large amounts of mercury into California's surface waters. Mercury can also be released into the environment from industrial sources, including the burning of fossil fuels and solid wastes, and disposal of mercury-containing products.

Once mercury gets into water, much of it settles to the bottom where bacteria in the mud or sand convert it to the organic form of methylmercury. Fish absorb methylmercury when they eat smaller aquatic organisms. Larger and older fish absorb more methylmercury as they eat other fish. In this way, the amount of methylmercury builds up as it passes through the food chain. Fish eliminate methylmercury slowly, and so it builds up in fish in much greater concentrations than in the surrounding water. Methylmercury generally reaches the highest levels in predatory fish at the top of the aquatic food chain.

This is another interesting article:

http://www.usgs.gov/themes/factshee.../#contamination

Mercury Contamination - Past, Present, and Future

In highly polluted areas where mercury has accumulated through industrial or mining activities, natural processes may bury, dilute, or erode the mercury deposits, resulting in declines in concentration. In many relatively pristine areas, however, mercury concentrations have actually increased because atmospheric deposition has increased. For instance, concentrations of mercury in feathers of fish-eating seabirds from the northeastern Atlantic Ocean have steadily increased for more than a century. In North American sediment cores, sediments deposited since industrialization have mercury concentrations about 3-5 times those found in older sediments. Some sites may have become methylmercury hot spots inadvertently through human activities. Lake acidification, addition of substances like sulfur that stimulate methylation, and mobilization of mercury in soils in newly flooded reservoirs or constructed wetlands have been shown to increase the likelihood that mercury will become a problem in fish. Although scientists from USGS and elsewhere are beginning to unravel the complex interactions between mercury and the environment, a lack of information on the sources, behavior, and effects of mercury in the environment has impeded identification of effective management responses to the Nation's growing mercury problem.