We’ve all heard news reports about the benefits of eating fish, and replacing some red meat meals with fish as an alternative source of protein. Fish contains a lot of minerals, including iodine and selenium, which are beneficial for the human body. Most fish species are lean and therefore fit perfectly in a healthy diet. Fattier fish species, such as salmon, offer other benefits: they are full of unsaturated omega-3 fatty acids, which are important for optimal functioning of cells and organs, and for good development of the central nervous system. Unfortunately, these important health benefits are leading to increased demand for these species, which now threatens global aquatic biodiversity.

Fatty acids and fish

So why is it important, from a health standpoint, for humans to eat fatty acid-rich foods? There are two essential fatty acids: the omega-3 fatty acid alpha-linolenic acid (ALA) and the omega-6 counterpart linoleic acid (LA); our body needs both of them for good health, but we don’t synthesize them ourselves. Hence, we must obtain them through our diet. ALA is found in many leafy vegetables, nuts, and fish; LA is present in all major plant and vegetable oils. So what is the big difference between these essential fatty acids? Omega-6 fatty acid LA is converted into Prostaglandin E1 (PGE1), while omega-3 fatty acid ALA is converted into Prostaglandin E3 (PGE3). PGE1 and PGE3 can be referred to as “good” prostaglandins, since both are anti-inflammatory. Omega-6 arachidonic acid, found in dairy products, meat, and breast milk, is converted into Prostaglandin E2 (PGE2), which promotes immune inflammation through the differentiation and proliferation of specific types of lymphocytes (respectively Th1 and Th17 cells) []. The functions of the three prostaglandins of the E-series, although different, all reflect a balance between effects upon interaction with activating and inhibiting proteins within cells [].

The human body converts ALA to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), two other essential omega-3 fatty acids that promote the “good” PGE3 pathway. This conversion, however, is not efficient and will only happen if certain enzymes are present and active. Therefore it is essential for people who consume little ALA to also get EPA and DHA in their diet. The best source of these two omega-3 fatty acids is fatty fish.

Omega-3 fatty acids are significant structural components of the phospholipid membranes that surround cells in tissues throughout the human body, and are especially rich in the retina, brain, and spermatozoa []; their presence is necessary to maintain optimal health. Omega-3 fatty acids may also reduce the risks of heart and vascular diseases through a complex cascade of reactions. Recent findings, however, put this correlation under debate, suggesting these reactions are more likely to be mediated by the full range of nutrients present in fish []. Interestingly, in contrast to omega-6 fatty acids, omega-3 fatty acids do not decrease the concentrations of HDL (known as “good cholesterol”). Until the early 1900s, the consumption of omega-3 and omega-6 was more or less equal, but today we consume 10 times more omega-6 than omega-3 []. It is therefore also important not to forget about the direct impact of LA-rich (omega-6) diets, which can reduce ALA’s (omega-3) health benefits and make ALA’s conversion to EPA and DHA less efficient.

In both human consumption and the aquaculture industry, fish oil is a key source of omega-3. Its use has become controversial since it is extracted unsustainably from world oceans, yielding around one million tons of fish oil annually worldwide []. Fish, however, do not themselves produce EPA and DHA; these fats make up half the weight of certain species of phytoplankton and become increasingly concentrated in marine organisms higher on the food chain. These phytoplankton, which are single-cell photosynthesizing algae, are easy to farm and may therefore play an important role in future, more sustainable directions for production of omega-3 supplements.

The nutritional-toxicological conflict

For 450 million years, fish have been living in seas and oceans all over the world, threatened only by natural enemies including larger fish, sea mammals, and birds of prey. Much worse are the threats they have faced in the last few decades. The water they live in has become increasingly polluted. Since the industrial revolution, oceans, seas, lakes, and rivers have become contaminated with many persistent chemical contaminants. Due to the accumulation of toxins in animals that are higher in the food chain, contaminants are present in fish at concentrations that could form a health risk for humans. It seems that the consumption of seafood has become a conflict between its nutritional qualities and the toxic risks []. For example, the recommended intake of EPA and DHA can be reached when consuming fatty fish species twice a week, but the intake of toxic dioxin-like compounds will reach its threshold when consuming fatty fish three times a week or more.

Gone forever: the sixth extinction crisis

The Earth has a rich evolutionary history, and has witnessed the rise and fall of many species. The number of species today is thought to be the largest in the history of life. Human behavior, however, has started to cause a massive decline in biodiversity, a loss that some fear is catastrophic enough to deserve the title of the sixth extinction crisis. This crisis, like the five previous mass extinctions, is characterized by huge loss of biodiversity.

One of the greatest current threats to biodiversity is the overexploitation of marine stocks. More than 70 to 75% of all commercial fishing grounds are severely overfished. It is estimated that 0.97 to 2.7 trillion wild fish are caught globally each year [], adding up to 77.4 million tonnes of fish species [].

Figure 1. Fisheries in a given marine ecosystem deplete the large predator species, turn to increasingly smaller fish species, and will eventually be left with only previously dismissed small fish and invertebrates. This concept was entitled fishing down the food web []. Image by Hans Hillewaert.

Time is running out for quite a lot of fish species, as we are “fishing down the food web” as depicted in Figure 1: so many have already been caught that it is almost too late to save them from complete destruction. Bluefin tuna (Thunnus thynnus) is one of the most striking examples. The populations of these tuna have been overexploited during the most recent decades, demonstrated by both a reduced biomass and a reduced geographic range. The east Atlantic and Mediterranean population is at risk of collapse, as there has been a 90% decline in adult biomass within three generations [], leading the International Union for Conservation of Nature (IUCN) to categorize this population as Critically Endangered.

Can we save the bluefin tuna and others?

There are many ways to prevent the impending loss of aquatic biodiversity, including the implementation of more sustainable fisheries and a global reduction in fish consumption. Since eating fish implies a precious balance between its nutritional benefits and the toxins present in fish, a moderate consumption may be beneficial for our health, while it will also be advantageous for environmental reasons.

Let’s end with some good news: on Monday November 19, the International Commission for the Conservation of Atlantic Tunas (ICCAT) for the first time followed scientific advice not to increase the fishing quotas for Atlantic bluefin tuna to unsustainable levels. If future policy makers continue to heed scientific advice, the bluefin tuna population may fully recover and continue playing its critical role in the marine ecosystem.

Danny Haelewaters works at the Farlow Herbarium of the Harvard University Herbaria as a PhD student. He also writes popular science articles for SciLogs.com.


[] Yao C, Sakata D, Esaki Y, Li Y, Matsuoka T, Kuroiwa K, Sugimoto Y & S Narumiya 2009. Prostaglandin E2-EP4 signaling promotes immune inflammation through Th1 cell differentiation and Th17 cell expansion. Nature Medicine 15 (6): 633-640.

[] Iyú D, Glenn JR, White AE, Johnson A, Heptinstall S & Fox SC 2012. The role of prostanoid receptors in mediating the effects of PGE3 on human platelet function. Thrombosis and Haemostasis 107: 797–799.

[] Connor WE 2000. Importance of n−3 fatty acids in health and disease. American Journal of Clinical Nutrition 71 (1): 1715-1755.

[] Chowdhury R, Stevens S, Gorman D, Pan A, Warnakula S, Chowdhury S, Ward H, Johnson L, Crowe F, Hu FB & OH Franco 2012. Association between fish consumption, long chain omega 3 fatty acids, and risk of cerebrovascular disease: systematic review and meta-analysis. British Medical Journal 345:e6698. <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3484317/pdf/bmj.e6698.pdf>

[] Kris-Etherton PM, Shaffer Taylor D, Yu-Poth S, Huth P, Moriarty K, Fishell V, Hargrove RL, Zhao G & TD Etherton 2000. Polyunsaturated fatty acids in the food chain in the United States. American Journal of Clinical Nutrition 71 (1): 179S-188S.

[] De Silva S, Francis DS & AGJ Tacon 2011. Fish oil in aquaculture : in retrospect. In: Turchini GM, Ng WK & DR Tocher (eds.) Fish oil replacement and alternative lipid sources in aquaculture feeds, CRC Press, Boca Raton, Florida: 1-20.

[] Sioen I 2007. The Nutritional-Toxicological Conflict related to Seafood Consumption. Phd Thesis, Ghent University (Belgium).

[] Mood A 2010. Worse things happen at sea: the welfare of wild-caught fish. Fishcount.org.uk, 2010: 1-139. <http://www.fishcount.org.uk/published/standard/fishcountfullrptSR.pdf>

[] FAO 2009. Capture Production 1950-2007 (Release Date: February 2009). Food and Agriculture Organization of the United Nations, Rome (Italy). <>

[] Pauly D, Christensen V, Dalsgaard J, Froese R & Torres F 1998. Fishing down marine food webs. Science 279: 860-863.

[] MacKenzie BR, Mosegaard H & AA Rosenberg 2009. Impending collapse of bluefin tuna in the northeast Atlantic and Mediterranean. Conservation Letters 2: 25–34.

Further Reading

Woodard C 2007. Oceans in Crisis. Can the loss of ocean biodiversity be halted? CQ Global Researcher 1 (10): 237-264. <http://www.cqpress.com/docs/AffiliationsPDFs/oceans.pdf>

EJF 2005. What’s the catch?: Reducing bycatch in EU distant water fisheries. Environmental Justice Foundation, London (UK). <http://www.imcsnet.org/imcs/docs/whats_the_catch_%20reducing_bycatch.pdf>

World Wide Fund for Nature 2012. Paving the way for recovery of bluefin tuna – an example for EU fisheries reform. November 21, 2012. <http://www.wwf.eu/?206799/Paving-the-way-for-recovery-of-bluefin-tuna—an-example-for-EU-fisheries-reform>

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