Eating Iowa fish can be safe and healthy
—by Steve Lekwa
The good news is that eating Iowa caught fish at reasonable levels of consumption is safe and healthy. There are still some concerns, however, and a recent program presented to the Ames Anglers group detailed some of those. They relate primarily to a process called bio-accumulation or bio-magnification where living organisms can’t remove certain chemicals from their tissues once they’re consumed. Thus, those chemicals build up over time. Most plants and animals are part of complex associations called food chains or food webs. A simple food chain would involve a plant using sunlight energy to process nutrients and minerals into plant tissue. Small amounts of some things the plant really doesn’t need or want get picked up in the process. The plants are then eaten by animals called primary consumers who are, in turn, eaten by secondary consumers, and so on up the chain to a larger creature at the top of the food chain. Good nutrients and a few harmful things originally found in the plant work their way up the food chain to become part of us whether we eat the plant directly or eat some kind of secondary consumer that originally ate the plant or smaller creatures that did.
Most animals filter and excrete toxic compounds through the actions of the liver and kidneys. Animals have difficulty filtering out a few naturally occurring elements like mercury or man-made chemicals like DDT or PCBs even though they are harmful, though. They then accumulate in body tissues; most often in fatty tissue. More of these toxic compounds accumulate the longer an animal lives. Animals farther up the food chain therefore get more concentrated doses of the bio-accumulated toxic compounds when they eat. A top predator such as a Northern Pike, Tuna, or a human being will be several steps up the food chain and may be getting doses of bio-accumulated compounds that are hundreds or even thousands of times higher than might be found in the general environment.
Toxins like mercury have always been in our environment at very low levels that most life forms could tolerate with no ill effects. Higher levels of mercury began to develop when humans started burning fossil fuels like coal. Mercury increased in the food chain through direct deposits from polluted air and through more rapid leaching out of some bedrock formations by the increased acidity of rain that fell from polluted air. Iowa is blessed with predominantly limestone bedrock formations and soils that help to buffer more acidic rain and lacks the metal-rich igneous bedrock formations found farther north in Minnesota and Canada.
The Iowa Department of Natural Resources tests fish from a variety of Iowa streams and lakes to determine if the fish they support are safe to eat. They publish consumption advisories that are available on their web site when certain species or sizes of fish are found to contain elevated levels of toxins. The advisory identifies the compound(s) that are concerned and typically recommends that no more than one meal per week be eaten containing fish of specific species or sizes identified. Top predators like Large Mouthed Bass, Walleye, and Northern Pike and bottom feeders like Carp and Channel Catfish are more likely to have higher levels of toxins. Pan fish like Bluegill, Crappies, and Yellow Perch are lower on the food chain and are nearly always safe to eat. Women who are, or wish to become pregnant, or who are breast feeding will want to be more cautious and not consume any fish from listed lakes or streams. This would apply to store-bought fish that are wild-caught larger top predators like tuna and salmon, too. Consider also that the DNR cannot test fish from every stream and lake in Iowa. Consumption advisories are listed only for the places where tests have been made. To be on the safe side, it would be wise to consider the advisories as a general summary of Iowa waters and fish. The lakes and stream segments where you fish may not be listed, but are likely similar to nearby ones that have been tested and show up in the advisories. A good rule of thumb is that smaller, younger fish and fish from lower on the food chain (plant eaters) tend to have less bio-accumulated toxins than predators and larger, older fish. Have fun catching those old lunkers, but know that the smaller ones are the best eating.