Heavy metals exist naturally in various concentrations in the earth's crust, soil, air, water and all biological matter and have spread widely as a result of anthropogenic activities such as cement production, the steel industry, rubbish and mining activities, among others. They can also be spread by natural events such as wind, soil erosion and volcanic activity.
All metals, whether essential or non-essential to an organism, can become toxic if they exceed the toxicity threshold.
Heavy metal intoxications occur in fish farming with varying levels of mortality and impact on physiology. In salmonids, they occur with acute mortalities in early stages of development (0.2 g - 1 g), where losses can reach values of 80% in severe cases. The events generally occur in spring-summer/autumn in both open flow and recirculating systems. In many cases, mortality patterns have been observed in association with high rainfall after periods of little or no rainfall (graph 1).