Why is mercury in fish?

Is Mercury in Pisces (fish) bad?

Negative Traits of the Mercury in Pisces:

When learning new information, the Mercury in Pisces tends to wade through minor facts to extract the bigger picture. ... While this helps them to process information easier, it also makes them a bit forgetful or disorganized at times.

I know this is not what you meant, but you did post this Anonymous...

Factories,  power plants, etc pollute the water, fish live in the water and absorb the mercury and other contaminants from the food they eat. Unlike some contaminants mercury stays in the flesh of fish and is not expelled. 

Usually the bigger the fish the larger the concentration of mercury

OIKO is correct that fish bio-accumulate mercury. I would disagree that the load of mercury in tuna is "huge", but it can be of concern, particularly if you're pregnant or nursing.

The main way mercury gets into the oceans and the environment is from coal-fired power plants which emit some of the element, and waste incinerators. 

There seems to be mercury in almost everything, including major-brand baby food. Fish tend to "bio-accumulate" it. There is a little bit in phytoplankton, a bit more in the zooplankton that eat them, more yet in the small fishes that eat the zooplankton, even more in the fishes that eat the smaller fishes. By the time you get to the tuna and other major fish-predators, the load of mercury is huge.

Because Venus was tooo big... and Earth was occupied 

irresponsible companies can dump it in oceans, and traces amounts can be ingested by fish