Farm raised fish are grown in huge pens or other enclosed areas under conditions that are quite different from their wild counterparts. Deliberate sex reversal of farmed fish is common in order to increase production. The impact on human health of the hormonal treatments used to effect fish sex changes has not been determined. The pesticides, antibiotics and other drugs used in aquaculture are also not well studied and are close to unregulated in some countries that supply U.S. consumers with fish. Plant-based feeds are partially replacing fish-based feeds in aquaculture in order to reduce costs. The result is a less favorable omega-3/omega-6 profile of farmed fish compared to wild caught fish. Now a new study has reported that some commercial fish feeds are estrogenic.

Deliberate sex reversal of farmed fish is common

Deliberate sex reversal is a common practice in aquaculture of some popular fish such as tilapia and halibut. The primary purpose is produce fish of the sex that is larger at maturity or otherwise has more desirable commercial characteristics. Sex change is accomplished using a combination of testosterone and other androgens, estrogens, and aromatase inhibitors such as Femara at various life stages of the fish. Efforts are made to keep the hormones to a low enough level to avoid obvious deformities in the fish or compromise their survival. Although use of such hormones does not necessarily mean that the fish themselves have high hormone levels when slaughtered, the safety of this practice has not been established.

Other common aquaculture treatments also have unknown human health effects

Sex reversal is just one of the practices used to farm fish that have not been well studied to determine the potential influence on human health. The majority of the fish sold in the U.S. are produced using aquaculture and much of them are imported from Asia where the safety standards can be remarkably lax or else simply unenforced. Certain practices are fairly widespread, depending on the type of fish or shellfish and the geographic location. This includes the use of antibiotics, pesticides, veterinary drugs, and other treatments to maintain the health of the fish, which are confined at far higher densities than those found in open waters.

In addition, farmed fish may be penned in coastal waters that are heavily polluted with industrial and agricultural chemicals and wastes. As a result, farmed fish tend to have much higher levels of natural and man-made toxic substances (e.g., antibiotics, antimicrobials, and heavy metals) than wild fish. In fact, some aquaculture practices have themselves caused environmental degradation. For example, there have been problems with use of groundwater, removal and disposal of dead fish, and handling of medicated feed.

Plant-based feeds are replacing fish-based feeds in part

The use of fish oil and fish meal (which are designed to reflect the actual diets of most fish) is considered an impediment to the growth of the aquaculture industry because of limited supply. Also, such feeds can contain unacceptable levels of heavy metals and other contaminants. As a result, fish-based feeds are being replaced, in part, with plant-based feeds in order to reduce costs. These plant-based feeds typically incorporate soybean meal, soybean oil, and corn meal, as well as grains and agricultural by-products. The proportion of plant-based feed used is determined by the maximum percentage a given type of fish can tolerate without reducing production.

However, increased fish consumption has been shown to be the only realistic way to increase dietary levels of the beneficial marine omega-3 fatty acids docosahexaenoic acid and eicosapentaenoic acid. Replacing fish-based feeds with plant-based feeds in the diets of farm-raised fish increases the proportion of omega-6 fats in their flesh. Studies of fish sold in supermarkets have found that farm-raised fish have lower omega-3 levels than their wild caught counterparts. For example, one study reported that the omega-3 to omega-6 ratio was approximately 10 to 1 in wild salmon compared to 3 to 4 in farmed salmon.

Latest research finds that many fish feeds are estrogenic

The study referenced at the beginning of this news article was designed to investigate the possible estrogenic and thyrogenic activities of 32 commercial fish feeds used in aquaculture. Normal endocrine system functioning is vital for the development and reproduction of fish. One of the mechanisms of endocrine disruption involves modifications in the functioning of the estrogen receptor (ER). However, some endocrine disruptors interfere with thyroid function, which is also crucial to various fish and human biological functions. Feed used in fish farming can be a source of hormones or persistent pollutants that are potential endocrine disruptors. To conduct the study, the authors developed a new ER-specific gene assay using sea bass ERα. Potential thyroidal disruption was also screened using a cell line containing avian thyroid receptor α.

A total of 11 of the 32 feeds were found to be able to activate sea bass ERα. This was probably partially a result of the soy isoflavone content of the feeds. Eighteen of the feeds were able to activate avian thyroid receptor α. The authors conclude that, given that maintaining proper functioning of the endocrine system is critical for the development and reproduction of fish, any estrogenic or thyrogenic activity caused by feedstuffs should be taken into account.

Final comment

Although aquaculture produces sustainable fish, wild caught fish are a better choice for breast cancer patients, survivors and those at high risk for breast cancer.

Please see our articles on salmon, mackerel, herring, sardines and halibut for more information.