Bio-magnification is a cumulative increase in the concentrations of a persistent substance (e.g. pesticides, metals, etc.) as it moves up the food chain. This occurs when agricultural, industrial, or human waste pours into the ocean directly or via rivers, sewage, etc. Some of the most dangerous of these toxins settle in the sediment of the sea floor and are consumed by bottom feeders. As they are consumed so are the chemicals they have absorbed. With each step up the food chain, there is an increase in the concentration of these chemicals. This affects humans because a large percentage of the fish we consume are higher on the food chain and therefore likely to carry a high concentration of potentially harmful chemicals. To read more on bio-magnification and check out an interactive animation go to NOAA.