Fish can have extremely high levels of chemical residues, as much as 9 million times that of the water in which they live. Mercury and other toxins accumulate in greater concentrations as you go up the food chain. Avoid large fish such as Atlantic Tuna, Orange Roughy, and Chilean Seabass. Pass on any farmed fish. Aquaculture is a dirty industry. The sea floor under the enclosed fish pens becomes covered with fish excrement and uneaten food, creating a dead zone where nothing can live or grow. There are also concerns about contamination with PCB, pesticides and antibiotics. Healthier choices are small, wild-caught, cold-water fish such as Pacific Salmon, Trout, Mackerel, Sardines, Herring, and Small Halibut. They provide good sources of omega-3 fatty acids, vitamin D and protein.