Introduction
Fish is vital in any healthy diet. They are a lean, low-calorie source of protein and are normally recommended by health experts around the world. However, scientists have discovered that some species of fish contain methylmercury or other chemicals that are harmful to the body at higher concentrations. The levels of these chemicals in specific species of fish vary from one region to another and for this reason, federal and local governments have issued fish consumption advisories on the kinds of fish identified to contain these chemicals. Even though fish consumption advisories are issued by various governments around the world, it is important to note that consumption limits are implemented for some fish in some places and not in others.
Objective
The aim of the paper is investigate why there are disparities in consumption limits in different regions. The paper will also investigate why consumption limits are enforced on certain species of fish but not others, and finally, the paper will present the dangers of exceeding these consumption limits.
Consumption Limit Areas & Types of Fish
Fish consumption limits have been enforced by various governments around the world. For instance, the Canadian government has issued advisories on the consumption of tuna, shark, swordfish, marlin and escolar while the US federal government has issued advisories on fish species such as shark, swordfish, tilefish and King mackerel (FDA, 2010). While these fish are found to contain higher levels of mercury in form of methylmercury in the specific countries, escolar belongs to a special category as it contains an indigestible type of fat. When ingested, this oil may go through the digestive tract undigested, and this may cause temporary effects such as nausea, diarrhea and cramps.
It is important to note that fish consumption regulations vary in a country or state, manily depending on the quantity of methylmercury in the water bodies. For instance, the state of Oregon advises persons living around Bonneville dam to limit their consumption of smallmouth bass to one meal a month due to fairly high levels of methylmercury and polychlorinated biphenyls (PCB), however, persons living around the Columbia River and Cooper Creek Reservoir are advised to limit their consumption of all types of fish caught in these waters. Still in Oregon, persons living around East Lake are advised to avoid eating brown trout 16″ or longer due to very high mercury levels in the lake. Generally, each area has its own legislation regarding fish consumption limits.
Consumption limits on Tuna
The consumption of tuna is regulated in most areas of the world due to high concentrations of methylmercury in the muscle tissues of the fish. This occurs because tuna is a predatory fish and piscivorous and is very high on the food chain due to its massive size. This fish species belongs to the family Scombridae, and genus Thunnus. There are a number of tuna species and all of then vary in the level of methylmercury in their muscle tissues, for instance, Yellowfin tuna, Thunnus albacores, contains lower amounts of methylmercury while Albacore (White tuna), Thunnus alalunga contains higher levels of the chemical (Burger and Gochfield, 2004). Albacore has been found to have a methylmercury concentration of 0.407 ppm while Yellowfin tuna has a concentration of 0.118 ppm. Although these concentrations are below the recommended total mercury concentration of 0.5 ppm, regular intake of canned albacore could lead to exposure to dangerously high levels of the chemical. Other fish species that contain high methyl mercury levels include swordfish, Xiphias gladius, (Family: Xiphiidae, genus Xiphias and genus: gladius, black marlin, Makaira indica (Family: Istiophoridae, Genus: Makaira, and species: indica), and King mackerel, Scomberomorus cavalla (Family: Scombridae, Genus: Scomberomorus, Species: cavalla). These three fish species have methylmercury levels of 0.995 ppm, 0.485 ppm, and 0.730 respectively. The Canadian advises its citizens to avoid the consumption of fish products with MeHg levels that are greater than 0.5 ppm.
Consumption limits are not enforced on all species of fish. Some fishes, especially those that are low in the food chain have insignificant methylmercury levels and pose no health risks even if consumed on a daily basis. These species include several species of catfish, such as Channel catfish, Ictalurus punctatus (Family: Ictaluridae, Genus: Ictalurus and Species: punctatus), European sardines, Sardina pilchardus (Family: Clupeidae, Genus: Sardina and Species: pilchardus), and the Atlantic Mackerel, Scomber scombrus, (Family: Scombridae, Genus: Scomber and Species: scombrus). These three species of fish have methylmercury concentrations of 0.025 ppm, 0.013 ppm, 0.050 respectively.
Health Risks of Methylmercury (MeHg)
Methylmercury is an organic form of mercury and is very poisonous at very high concentrations in the human body. It is the most common form of mercury in fish. When it enters the body through ingestion, methylmercury is easily absorbed into the bloodstream where it moves into all parts of the body including the brain (Crompton, 2007).
Methylmercury is found in almost all fish species, however, the levels are higher in some fish than in others. An explanation for this anomaly is that fish bioaccumulate methyl mercury in their body mainly by feeding on plants and other aquatic organisms that contain the chemical. Methylmercury then bioaccumulates when predators, feed on other organisms (and other fish) that already have some methylmercury in their body tissues (Health Canada, 2011). After a number of years, the fish predators will have greater levels of methylmercury in their body tissues (Pirrone and Mahaffey, 2005). Tuna is a known piscivorous fish (feed on other fish) that is high in the food chain and hence has higher levels of MeHg, and for this reason an advisory on its consumption is enforced in the US, Canada, and many other countries.
The presence of MeHg in our body at high levels can be very harmful, these include impairment of the central and peripheral nervous system which can lead to low cognitive and physical development and lack of coordination of body organs. In adults, exposure to excessive levels methylmercury can cause memory loss, loss of sight and hearing, intellectual impairment, and in severe instances, death.
Why are consumption limits enforced on some fish in some places and not in others
Fish consumption advisories differ from one country, or federal state, to another because fish consumption practices and the fish species consumed may differ between these areas. Consumption advisories take into consideration other regulatory mechanisms aimed at protecting citizens, such as the institution of maximum levels of exposure to harmful chemicals. These advisories also differ from region to region because methylmercury levels in different water ecosystems differ from place to place and this directly affects the methylmercury levels in fish. Besides, each state or country has its own regulatory mechanisms regarding the maximum exposure to harmful chemicals. There are consumption limits for some fish in some places and not in others because of the various levels of methylmercury that are found in different amounts from one place to another. In one area, there may be low exposures to methylmercury and therefore no consumption level warnings are required. In other areas the complete opposite may be in effect and cause a warning for the consumers.
Consumption Advice
Although consumption limits differ for different species of fish, these advisories are consistent with the levels of methylmercury present in certain fish species and in water ecosystems. For instance, the Canadian government advices its citizens to eat a maximum of 0.150 kg of weekly of one or a combination of the following species of fish: fresh/frozen tuna, shark, swordfish, escolar, marlin, and orange. However, expectant women are advised to have a maximum of 0.150 kg of these fish species monthly. Additionally, children between 5 and 11 years are allowed to eat a maximum of 0.125 kg per month while those between 1 to 4 years should eat not more that 0.075 kg. The US government, on the other hand, warns its citizens against eating Shark, Swordfish, King Mackerel, or Tilefish as they contain very high levels of methylmercury. It further advises citizens to consume a maximum of 0.340 kg, or two meals a week, of fish with low methylmercury levels per week such as catfish, shrimp, canned ‘light’ tuna and Pollock (Crompton, 2007). However, Albacore should be consumed up to a maximum of 0.170 kg a week, or a single meal per week. The consumption advisories in the two countries are almost similar.
Separate consumption limits for pregnant women and children are important because a developing fetus and young children are more vulnerable than adults to the harmful effects of methylmercury. However, this should not deter these two groups from eating fish since there are many more fish species that contain lower methylmercury levels such as salmon, pollock, and catfish. Besides, fish provide vital nutrients to the fetus and children that are vital to maintaining a healthy heart function. Fish also reduces the risk of a heart attacks, helps in fetal development, and a valuable source of vitamin D and other important trace minerals (Health Canada, 2011).
Conclusion
Despite the huge importance of nutrients present in fish, we must take absolute precaution while enjoying a meal consisting of canned or fresh fish due to the presence of methylmercury. Individuals are advised to check with government agencies regarding the level of methylmercury in fish. If such information cannot be obtained, the safest way to avoid exposure to high levels of the chemical is to limit fish consumption to one meal per week, which totals to 160 g of fish per week/ however, pregnant women and children between the ages of 1-11 should eat very little fish.
References
Burger, J., and Gochfield, M. (2004). Mercury in Canned Tuna: white versus light and temporal variation. Environmental Research, 96, 239-249.
Crompton, T. R. (2007). Toxicants in aqueous ecosystems. NY: Springer.
FDA (U S Food and Drug Administration). (2010). Mercury Levels in Commercial Fish and Shellfish (1990-2010). Web.
Health Canada. (2011). Mercury in Fish – Questions and Answers. Web.
Pirrone, N., and Mahaffey, K. R. (2005).Dynamics of mercury pollution on regional and global scales. NY: Springer Science.