Friday, March 1, 2013

Farmed Fish Physiology

Eric Anderson                

               Modern aquaculture, known as the “Blue Revolution,” began after World War II after industrial sized aquaculture operations made farmed fish a commercially profitable product.  It has also been presented as a solution to reduce the pressure on collapsing wild fisheries, as the demand for fish increases.  But because profit drives everything, most fish farms do not produce an environmentally friendly product that can be a source of protein for developing countries.  Instead high-value carnivorous fish, such as salmon are artificially reared and fattened up in floating pens until they grow to market size.

Image credit: Patrick Pleul/DPA via ZUMA Press
                But I do not think the notion that these types of fish farms are unsustainable is news to anyone.  Salmon aquaculture has been criticized by several environmental organizations in the recent years.  So instead of using this blog to propagate how these farms are destroying the environment and ultimately hurting wild fish stocks, I would like to investigate the physiological differences between domesticated and wild salmon and the fillets that end up on your plate.

                Salmon farming started in the late 1960s.  However, even though it has not been around for very long, breeders have been able to significantly alter Atlantic salmon (Salmo salar) to the point where some suggest the domesticated variant belongs in a separate species: Salmo domesticus.  The rapid domestication of salmon is due to technological advances, better understanding of genetics, and antibiotics that were not available to early breeders of terrestrial livestock, such as cows, pigs, and chickens.

So what are the differences between the two “species”?

             The biggest difference between domesticated and wild salmon is the characteristic that matters most to aquaculturists – growth rate.  A study has shown that seventh-generation domesticated Atlantic salmon can weigh up to three times as much as wild salmon from the principal founder population after two years in the same farming conditions.  Not surprisingly, this disparity results from differences in the endocrinology between the two fish.

                  The researchers found that the domesticated fish had higher levels of pituitary and plasma growth hormone (GH) than wild fish and that these levels of GH were positively correlated with growth.  Interestingly though, the difference in insulin-like growth factor I (IGF-I) was not significant.  This may be an indication of genetic selection towards more active growth regulation in farmed fish, where the negative feedback inhibition of GH on the secretion of IGF-I is reduced.

                It is obvious why breeders selectively breed fast growing fish; they need to get their product on the market as soon as possible, while using the least amount of resources, in order to maximize their profit.  But another extremely important characteristic of a successful domesticated fish is a low feed conversion ratio.  This is the amount of food required in order to produce one kilogram of flesh.  Selective breeding of salmon has decreased this amount from 3.5 to 1.2 kg in the last 40 years.  Wild salmon, on the other hand, require as much as 10 kg of prey in order to produce 1 kg of flesh.  However, selective breeding is not the only thing responsible for this difference.  The protein and fat composition of the pellets used to feed farmed fish is being perfected so that this ratio can be as efficient as possible.

                If aquaculturists have been able to change the physiology of Atlantic salmon so drastically, how does it affect your meal?  A consumer survey found that wild caught Chinook salmon was significantly more enjoyable the farmed Chinook and Atlantic salmon.  A separate study associated farmed Atlantic salmon with having and increased intensity of bitter, oily, and metallic tastes compared to wild salmon.  Furthermore, these unpleasant tastes were exacerbated when stored at temperatures greater than -30 degrees Celsius for an extended period of time.  These tastes were found to be correlated with an increased concentration of free unsaturated fatty acids in the flesh and are a product of hydrolysis of neutral lipids during the frozen storage of the fillets.

                Another notable difference between wild and farmed salmon meat is the color of the fillets.  Wild salmon get their pink-orange color from pigments they consume in their diet.  The previously mentioned survey found that consumers identified the color of wild salmon as “healthier” than farmed salmon.  This difference would be even more apparent if farmers did not add synthetically derived caratanoids, such as anthaxanthin and canthaxanthin to their fish food.

A wild salmon fillet on the left and a farmed salmon fillet on the right. Image credit: Joanna Sciarrino

                The more serious difference in fillets that come from farmed salmon is the amount of pollutants contained in their flesh relative to wild fish.  The most notable of these contaminants are polychlorinated biphenyls (PCBs), which have been linked to cancer.  These pollutants are picked up by zooplankton in the ocean and when they are consumed by wild filter-feeding fish, such as sardines and herring, the PCBs get concentrated in their tissues.  These filter-feeding fish are then processed and turned into fishmeal for salmon farms.  Wild salmon do not consume as much prey containing PCBs compared to farmed fish, so they consequently have lower concentrations of these pollutants than farmed salmon.

Concentrations of various polutants contained in wild-caught Atlantic salmon (green), compared to farmed Atlantic salmon (red).  Image from Ronald A. Hites et al., 2004 <>

                There are several evident differences between farmed and wild salmon.  This post has only highlighted a select few of them, as I attempted to avoid the more common issues with salmon farming.  However, if you are interested in the environmental and ecological sustainability of aquaculture, I encourage you to  visit the following links:


Baker, R.T.M., A.-M. Pfeiffer, F.-J. Schoner, and L. Smith-Lemmon. 2002. Pigmenting efficacy of astaxanthin and canthaxanthin in fresh-water reared Atlantic salmon, Salmo salar. Animal Feed Science and Technology 99:97-106.

Einen, O. and A.J. Roem. 1997. Dietary protein/energy ratios for Atlantic salmon in relation to fish size: growth, feed utilization and slaughter quality. Aquaculture Nutrition 3:115-126

Fleming, I.A., T. Agustsson, B. Finstad, J.I. Johnson, and B.T. Bjornsson. 2002.  Effects of domestication on growth physiology and endocrinology of Atlantic salmon (Salmo salar). Canadian Journal of Fisheries and Aquatic Sciences 59:1323-1330.

Greenberg, Paul. Four Fish: The Future of the Last Wild Food. New York: Penguin, 2010. Print.

Gross, M.R. 1998. One species with two biologies: Atlantic salmon (Salmo salar) in the wild and in aquaculture. Canadian Journal of Fisheries and Aquatic Sciences 55:131-144.

Hites, R.A., J.A. Foran, D.O. Carpenter, M.C. Hamilton, B.A.Knuth, and S.J. Schwager. 2004. Global assessment of organic contaminants in farmed salmon. Science 303:226-229

Refsgaard, H.H.F., P.M.B. Brockhoff, and B. Jensen. 2000. Free polyunsaturated fatty acids cause taste deterioration of salmon during frozen storage. Journal of Agricultural and Food Chemistry 48:3280-3285.

Refsgaard, H.H.F., P.B. Brockhoff, and B. Jensen. 1998. Sensory and chemical changes in farmed Atlantic salmon (Salmo salar) during frozen storage. Journal of Agricultural and Food Chemistry 46:3473-3479.

Sylvia, G., M.T. Morrissey, T. Graham, and S. Garcia. 1995. Organoleptic qualities of farmed and wild salmon. Jorunal of Aquatic Food Product Technology 4:51-64

Wang, X., L.M. Olsen, K.I. Reitan, and Y. Olsen. 2012. Discharge of nutrient wastes from salmon farms: environmental effects, and potential for integrated multi-trophic aquaculture. Aquaculture Environment Interactions 2:267-283.

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