A formula was derived for quantification of benefit-risk ratio (hazard quotient) for the intake of a product containing essential polyunsaturated fatty acids vs. heavy metals. The quotient was used in a three year case study of the contents of essential fatty acids and heavy metals in Siberian grayling, the main food fish from the middle section of the Yenisei River. As found, in general the fish intake was potentially very beneficial for human health, except on a few occasions, when the risk overweighed the benefit because of high contents of chromium in the fish muscle tissue. The data demonstrated the necessity for regular monitoring of the hazard quotients for food fish in wild conditions, based at least on monthly sampling frequency.