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.
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| 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”?
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.
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| 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.
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|
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 <http://www.sciencemag.org/content/303/5655/226.full.pdf>
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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:
REFERENCES
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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