It is highly advised  to consume 140 g to 300 g oily  fish per week because they contain high levels of omega-3 fatty acids, proteins, vitamin B complex, water soluble vitamins (A,D,E), phosphate, sulfur,  natrium,  magnesium and microelements such as zinc,copper, manganese, iodine. On the other hand, we should remember that fish not only contain good elements but also bad(they accumulate heavy metals and water pollutants).

                     /"MercuryFoodChain-01" by Bretwood Higman, Ground Truth Trekking/

Mercury occurs naturally in the environment and accumulates in streams, seas, oceans. In the water it changes into methylmercury- highly toxic organic compound that bioaccumulate in fish and humans. Methylmercury toxicity results in damage to developing nervous system(unborn baby or young child), paresthesia, depression, blurred vision, lack of coordination, imapired speech, hearing, walking, muscle weakness.  Although anyone can be harmed by high levels of methylmercury, at risk are especially pregnant, nursing women, children under age of 6 and people with impaired kidney function. Food and drug administration (FDA US) emphasize that shark, swordfish, king mackerel and tilefish contain high levels of mercury and should be totally avoided by those people.

             /source: https://en.wikipedia.org/wiki/Mercury_in_fish /

Another 'issue fish' should be raised up-  farmed salmon from Northern Europe, North America and Chile which is so easily available and cheap is significantly higher in organochlorine contaminants than the wild one.
In the study Global Assessment of Organic Contaminants in Farmed Salmon by R. Hites, J.Foran,D. Carpenter,M.Hamilton, B. Knuth and S. Schwager, 14 organochlorine pollutants were measured in 700 wild and farmed salmons collected from around world. It revealed that 13 of these were significantly more concentrated in the farmed salmon. Especially 6 of these:  polychlorinated biphenyls, dioxins, dieldrin, cis-nonachlor, total DDT and mirex . These chemicals are cancerogenic, toxic for our ogranisms and lead to autoimmunologic diseases, nervous system diseases, can cause infertility and can harm fetus. Although, farmed salmon is 3 times higher in omega 3 fatty acids, it is also higher in toxins,preservatives and food colouring. That is why it is recommended not to eat it too often(some sources advise 230 g per 2 months).


In my opinion, if you have possibility to eat wild one, do not hesitate and just resign from farmed salmon. But do not resign from fish at all! They contain many nutrients (especially omega 3 fatty acids) that are necessary for our organisms.

REMEMBER:

1. Fish high in methylmercury :shark, swordfish, king mackerel, tilefish. DO NOT GIVE IT to children,pregnant and nursing women!
2. Farmed salmon- high in cancerogenic organochlorine contaminants-do not eat it more than 230 g per 2 months or just choose wild one!