How to Make Sustainable Seafood Choices at the Fish Market

Most of the mercury people acquire gets into the environment from burning coal, but we usually get it into our bodies through eating seafood. Most animals we eat are killed when they are young (six weeks for a chicken) and have not accumulated that much mercury. By contrast, the large bluefin tuna we catch are 10 years old. Contaminants like mercury, pesticides, PCBs and other metals and toxic chemicals aren’t just passed along in the food chain; they accumulate and concentrate toward the top. Think of the ocean food chain as a simple pyramid, with, say, a shark at the top, a large number of herring in the middle and a vast horde of planktonic plants and animals at the base. (In real life, it’s more complicated, of course, with more steps.) The plant plankton absorb minute quantities of contaminants as they turn nonliving components of seawater into living cells. Think of the total of all the contaminants in all the plankton along the pyramid’s base, and imagine it all concentrating into fewer herring and ultimately in the one big old shark. Basically, that’s what happens.

The higher on the pyramid you eat, the more likely you’ll be getting a larger portion of concentrated contaminants. Dining on plankton-eating herring is better than eating the shark that ate all those herring. Herring, anchovies, Atlantic mackerel, clams and oysters (small plankton-eaters) have among the lowest mercury concentrations; sharks and tunas (big fish-eaters) have among the highest. And even with farmed fish, smaller is better. Big, carnivorous fish must be fed smaller fish that have been caught in the ocean. Many of those nutrient-rich smaller fish that are turned into fishmeal—like herring and sardines—are healthy for people and would be better used as human food. (Is your fish toxic? Find out here.)