The fish feed- the fat that is used in the fish feed have been found to have cancer-causing polychlorinated biphenyls (PCBs). This is predominantly in farm-raised salmon and exists at 16 times the rate of wild salmon.
Canthaxanthin - this is found in farmed salmon and is a synthetic pigment that is used to add a pink colour to the flesh of salmon. Wild salmon gets their colour naturally from eating krill and excreting adrenalin (however you will even notice that the colour is a different pink between the two)
Polybrominated Diphenyl - researchers have found high levels of polybrominated diphenyl (a chemical used as a flame retardant) in high levels in farmed fish. PBDEs are endocrine disruptors.
Just be sure to make sure you fish is actually wild caught. Choose The Canadian Way for salmon and ALWAYS ask if you cold water fish is wild caught. You seriously DO NOT want to be eating farmed fish. Remember that you vote with your dollar too! Picture credit