• Many fish have relatively low levels of mercury.Dry fish on a dish in the form of a fish image by terex from Fotolia.com
  
  Historically, fish have been the staple diet for many civilizations. Fish provide lean protein and omega three fatty acids, which are essential for human health. However, human development has contributed to increasing levels of mercury found in fish. Excessive consumption of mercury, especially by pregnant women, can lead to serious health conditions. It is easy to understand why some fish have higher levels of mercury. Focusing on fish that come from conditions that favor lower levels will help you make safe decisions on which fish you should consume.
  
  

Location

• Location is probably the single most important factor that contributes to the level of mercury in fish. Lower levels of mercury is released in areas near the poles. Much of this is due to the fact that there are less humans in cold climates. Also, mercury in sea water travels toward the equator, creating a higher concentration in the tropical zone. Tropical fish, such as tuna, have higher levels. Freshwater fish are also susceptible to mercury. Freshwater fish located in areas with little pollution are have much less mercury in their tissues. Industry can lead to pollution by dumping chemicals in the water or releasing toxins over the ground, which end up in the rivers due to runoff.
  
  

Size of Fish

• Generally, the larger the fish, the more mercury it contains. This is due to the fact that larger fish are older, and have had more time for mercury to accumulate in their bodies. Smaller fish usually have much lower mercury levels. This is not always the case, and many times some fast-growing large species are low in mercury. Size is also not as important if the fish are in a lower mercury environment to begin with. Salmon are generally safe as they have a short lifespan and are mostly caught in the Pacific Northwest.
  
  

Position on the Food Chain

• The eating habits of the fish also affect the mercury levels. Fish that are lower on the food chain tend to have less mercury. As fish eat other fish, they absorb the mercury that was present in its prey. This can create large concentrations if a fish eats other carnivorous fish. Swordfish and shark are notorious carnivores and have the highest levels of mercury of all fish. The fish with the lowest mercury concentrations usually eat plankton or insects. They are not exposed to mercury through their diet, and only absorb mercury through their gills when filtering water.