I've done a ton of research on mercury for an article I wrote and some subsequent flame wars with the "sky is falling" crowd. You can do some of the same research and get most of your questions answered quickly...however, I can give you the short version:

Organic mercury (the only kind you'll ingest from fish) is both man-caused and natural. It's been around probably forever as a result of a microorganism that converts metallic mercury into organic mercury.It's in the air, water (primarily) and just about every living thing, if not all living things. Currently, over half the organic mercury created each year comes from natural sources. The bulk of the remainder comes from Asia, China and India specifically.

In North America, it seems to be declining based on samples taken from animals killed in the 1800s and preserved.

While all fish likely have some mercury in them, older and larger fish are likely to have more than younger/smaller fish. So if you're concerned, eat younger/smaller fish.

Certain predator fish contain more mercury than others...such as some tunas, sharks and the like.

There's a lot of contradictory discussion on the health aspects of eating fish with high mercury content. However, my research turned up only two situations where ingesting mercury caused serious health problems. In the first, hungry villagers in Iran or Iraq ate seed wheat that had been treated with mercury fungicide. In the other, Japanese people who ate fish from a bay that was an industrial dumping ground were affected, as were their children.

There also is evidence to support that eating fish with a high-ish mercury content does no measurable harm. The Seychelle Islanders eat a lot of the tuna/shark/swordfish species that are high in mercury without any ill effects. There have been several other studies with similar results.

Also, some testing has shown hand-eye coordination improvement in older people who have a diet high in mercury...don't know how high is high in this case, though. The Lancet (the British equivalent of the AMA Journal) just published an article that downplayed the risks of eating fish for pregnant women. The article stated that there were improvements in the mental abilities of the babies they carried when fish were eaten...which may or may not be a direct link to mercury...I just read the article about the study and not the study itself.

You can find more information (way more information) in a Google search, but you have to skim through a lot of misinformation to find it. The sources I used were the American Medical Association, the British version of the AMA, and the federal Center for Disease Control. The CDC has some fairly intensive studies.