Read the following scenario and answer the question below. Mercury is a heavy metal that occurs naturally in minerals and rocks. Release of mercury from natural sources has been relatively constant over time. However, anthropogenic release of mercury has increased significantly in recent years primarily due to increased fossil fuel mining and combustion. Mercury that is released into the air eventually ends up in soils or surface water. Once in water, microbes convert mercury into methyl mercury, which is absorbed quickly and concentrated in the tissues of predatory fish such as shark and swordfish. Large fish typically concentrate more mercury than small fish. People who eat fish with high methyl mercury concentrations can show tremors, deafness, muscle incoordination, and attention deficits. Pregnant women and children are especially sensitive to methyl mercury's toxic effects. Learning disabilities and developmental delays are common in children who have been exposed to significant levels of methyl mercury. Pregnant women and children are warned not to eat certain kinds of fish that have high mercury content. Why do some fish tend to have higher mercury levels than other fish?