John Metz
Dr. Ely
Bio 303
Fishing vs. Farming: A Look into the Impacts of Over Farming Atlantic Salmon
It has been documented that fish-farming, better known as aquaculture, has been
utilized for thousands of years, going all the way back to ancient China. Today
arguments against fish-farming, salmon farming in this case specifically, have arisen
because of environmental and economic concerns that are resultant of the recent rapid
growth of the industry. Environmentally it has been argued that large-scale aquaculture
can lower genetic variability, and disrupt ecosystems. On the economic level, traditional
fishermen have had increased competition due to the growth of aquaculture, hurting
their profits and causing a decline in the profession (White et al. 2004).
Thinking about the dawn of agriculture, and what it meant for humanity, is a good
start for understanding the rise of aquaculture. Agriculture was developed because it
was no longer efficient for humans to hunt and gather for food. The groups of people
were growing and there was not enough food to be hunted and gathered to support
these growing groups. Therefore the idea of growing one’s own food, which is
agriculture, arose and allowed the establishment of civilization. The recent growth of
aquaculture can be looked at along a similar path. The demand for seafood today is
greater than what fishermen can catch with out completely depleting the ocean, and
people are looking to aquaculture as a solution (White et al. 2004).
There are multiple types of aquaculture, and they all have their own issues, some
of which can be worse than others. Ranging from least environmental impact to most,
the following are various types of aquaculture: mollusk culture, closed container, inland
pond, flow-through area, coastal pond, and cages. Mollusk culture involves introducing
various members of the mollusk family into an ecosystem that is either lacking or
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depleted, and with proper control can actually be beneficial. The next four on the list
are similar in that they have some negative effects, but with proper control they can be
minimized. The one that is important to understand when talking about salmon farming
is the cage method. This method is the most environmentally damaging form of
aquaculture. It allows a decent chance for fish escape, which can cause harmful
impacts on the ecosystem, or problems with disease due to the introduction of lower
genetic variability. Cage farming also allows for direct release of waste from the farming
of fish into the environment. The use of cages is the usual method for salmon and other
carnivorous, highly sought after fish. This is because of their need for open space and
their larger size (White et al. 2004). An example of a caged salmon farm off the coast of
Norway can be seen in figure 1 (Schwartz 2000). It can be seen in this picture how the
cages are very exposed out in open water, and why problems such as escape are so
prevalent.
Figure 1:
According to Fiske et al. (2006) the frequency of farmed salmon being present in
an area is directly related to the amount of fish-farming in that area. They found this to
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be resultant of the fact that when smolts, which are young salmon, escape they tend to
remain in the area from which they escaped. However, adult salmon that escape tend
to spread out widely upon escape. To show that farmed salmon presence was directly
related to farming intensity in the area, Fiske et al. (2006) analyzed the occurrence of
farmed fish in fishermen’s catch relative to the amount of fish farming in the area. The
analysis was done in various parts of Norway and in the period 1989-2004. Salmon
samples were classified based on when they were caught, either summer or autumn,
and a morphological assessment that was done by the fishermen. The results for
number of farmed salmon relating to intensity of fish farming in the area can be found as
a correlation number in figure 2 (Fiske et al. 2006).
Figure 2: A plot of correlation constant vs. time
A correlation constant is a method for showing the similarity with regards to
linearity on a graph of two measured values. For this analysis the value of farmed fish
caught in an area was related to a value that represented the intensity of fish farming in
the area, thus producing a correlation constant. The closer a correlation constant is to
one, the more correlated the two things are. Fiske et al. (2006) hypothesized that
management of the farms was getting better when it came to stopping the escape of
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salmon, smolts in particular. If fewer smolts escaped then the correlation constant
would go down even if the number of adults escaping increased.
The results of Fiske et al. (2006) are important in showing the correlation
between the amount of fish farming in an area and the presence of escaped farmed fish
in the area. With this knowledge government agencies can begin to require aquaculture
companies to improve their methods in fish containment. White et al. (2004) present
several viable ideas for integrating aquaculture and lessening its environmental impact.
The authors insist that governments require companies to further research better
methods of fish farming, to make use of mollusk farming and to decrease the amount of
open water cage farming. It is important that the number of farmed fish is controlled
because of the possible negative impacts of too much interaction between wild and farm
raised fish. In a study done by Glover et al. (2013) a genetic analysis of wild and
farmed samples was performed and compared based on their allelic diversities. The
results can be seen in figure 3. When comparing the allelic diversity the authors found
that farmed fish had a much lower number of alleles. A decrease in allelic diversity can
be dangerous when it comes to the spread of disease. For example Glover et al. (2013)
analyzed the presence of Piscine Reovirus in the farmed salmon, which is a virus
associated with heart and skeletal muscle inflammation. The virus was more commonly
found in the farmed fish because of their similar parents and lack of genetic variability.
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Figure 3: Data showing the allelic diversity difference between wild and farmed salmon
Farmed salmon being on a different yearly cycle was another impact that was
looked at by Fiske et al. (2006). Their work included an analysis of farmed salmon
being present in the rivers during a different time of year than normal wild salmon would
be. They wanted to show that farmed fish were more common later in the fishing year
than were normal salmon. The results for the samples are found in a normalized plot in
figure 4 (Fiske et al. 2006). The results are presented as a percentage of the whole
sample collected. A weighted plot was also included in order to give a closer number to
what the expected population percentage in the river as a whole would be. This number
was found by weighting the percentage in the sample with the catches in the river (Fiske
et al. 2006).
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Figure 4: Plots of the percentage of farmed salmon in Norwegian rivers
Looking at the difference of the values used as the scale for the y-axes shows
that the samples showed a much larger presence of farmed salmon in the autumn
catch, rather than the summer catch. This result supports the idea that farmed salmon
return to the rivers later than do wild salmon. Fiske et al. (2006) hypothesized that the
decrease seen in the autumn percentage can be attributed to a longer summer season,
causing more fish to be caught sooner, and to fish farms improving their containment of
their salmon.
The information that Fiske et al. (2006) found about farmed fish entering the river
later than wild salmon supports work that was done by Glover et al. (2013) to show that
the stomach contents of Atlantic cod revealed that the cod were eating farmed salmon
out of season. Glover et al. (2013) began their study after they were notified by
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fishermen that cod that had been caught seemed to have different stomach contents
than usual. The fishermen noticed that the partially digested samples resembled
Atlantic salmon smolts. As it was out of season for salmon smolts, the fishermen
assumed the prey was from a local farm and thus reported their findings.
The authors designed the experiment so that they could answer multiple
questions. They wanted to first see if the contents were salmon, and then if the
contents were salmon they wanted to see if the salmon were wild or from a particular
farm. To answer these questions a genetic profile was determined for each salmon
using a highly polymorphic microsatellite panel. The test was also performed on
samples of wild salmon and samples from a local fish farm to provide comparison data.
The answer to the first question, which was whether the samples were indeed
salmon, was found to be yes the prey were Atlantic salmon. Glover et al. (2013)
determined that from the 17 microsatellite loci analyzed from the 37 samples of prey,
only two out of the 629 possible genotypes were missing, which is over a 99% similarity
between the samples of prey, and the known Atlantic salmon microsatellites.
Once the samples of prey were determined to be Atlantic salmon, an analysis
was done to determine whether the samples were wild or farmed fish. To determine
this, Glover et al. (2013) looked at the allelic diversity numbers found from their previous
testing, and also performed a genetic assignment test.
It can be seen in figure 3 that the number of alleles in the prey sample was very
similar to the number of alleles found from the farmed samples, and much lower than
the number of alleles found from the wild sample. Although this is not conclusive
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evidence to say that the prey samples definitely came from a farm, it supports the
hypothesis that they did (Glover et al. 2013).
The results from the genetic assignment testing can be found in figure 5.
Genetic assignment is a method for taking the genetic similarity between one target
sample and other groups of samples and determining which group each individual
target sample is most similar to. This test can also be done in a way as to determine
which group the sample definitely cannot be from. Part A of figure 5 is a graph of the
genetic assignment to the most similar group, while parts B and C are the exclusion
methods varying only in the threshold used for the test (Glover et al. 2013). The
authors proved genetic assignment to be a viable option by performing the test on every
known sample and checking themselves to see how right the answers were. They
concluded that 70% of the answers they got were exactly right and that most missassignment was due to one farmed fish being assigned to the wrong cage because of
how similar the cage samples are. On top of that it was found that none of the farmed
samples were miss-assigned to be wild, and only 3 out of 101wild samples were
incorrectly assigned to be from the farm.
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Figure 5: Results from genetic assignment and exclusion
The genetic assignment in part A of the prey samples shows that none of the
prey were matched as wild. From parts B and C the exclusion testing supports part A’s
finding in that a large portion of the prey samples are different enough from the wild
samples that they cannot be wild. Glover et al. (2013) went further to note that the only
reason prey samples were excluded from the one of the farm’s cages is because that
sample was so similar to one of the other cage’s samples.
The work done by Glover et al. (2013) could be very persuasive to show
governments that fish farming is causing an impact on the surrounding environment.
Cod do not usually eat salmon at the time the cod were caught because no salmon are
supposed to be around at that time of year. But because the cod were eating salmon,
something they normally eat wasn’t getting eaten. While having another food source
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could be seen as beneficial for the cod harvest, an introduction of new prey could cause
an imbalance in the trophic level for the environment, affecting the lower levels of the
food chain by decreasing the amount of predation on those species.
The escape of farmed salmon could be dangerous for ecosystems all throughout
the North Atlantic Ocean depending on where salmon spread out to. The migration
patterns of salmon in the North Atlantic Ocean were monitored by Hansen and
Jacobsen (2003) in order to determine how widely the salmon travel. They monitored
salmon migration by tagging samples of escaped farm salmon and wild salmon. It was
then recorded where the tagged fish were recaptured, and plotted on a map, which can
be seen in figure 6. All fish were tagged in open water north of the Faroe Islands so
that the fish would be recaptured once they had returned to a coastal origin.
Figure 6: Map of the distribution of tagged fish
The wide migration patterns of North Atlantic Salmon are exposed by Hansen
and Jacobsen (2003) in figure 6. The observation that Atlantic salmon are spread out
over so many ecosystems in conjunction with the tendency of adult farmed salmon that
escape to not return to their place of origin, creates a combination with dangerous
possibilities (Fiske et al. 2006).
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The fishermen were so diligent to report their findings of what they thought was
farmed salmon in the cod stomachs because fishermen and fish farms are in constant
competition. This competition also led the fishermen to be quite helpful in the other two
studies as well, in which they helped with the morphological assessment of caught fish,
and helped with the recapturing of the tagged fish. Fishermen around the world have
taken large hits to their profits because of the competitive prices presented by fish
farms. From 1990 to 2002 the price of yearly harvest for salmon fishermen fell from 559
million dollars to 130 million dollars, which is a 78 percent decrease. Such a massive
decline in profits has caused a decrease in Alaskan fishermen, from 10,487 to 6,567 in
a 12 year span (White et al. 2004). The rising competition from the fish farms has made
the profession of commercial fishing significantly harder.
Through the work of studies such as the ones presented in this paper, the true
impact of aquaculture on the environment is still being determined. Much of the danger
presented by the growth of aquaculture lies in the unknown. It is important though to
keep in mind the positive intentions of aquaculture. Just as agriculture was needed to
sustain populations when the gathering of vegetation was no longer feasible,
aquaculture could be the answer to an increased demand for seafood. From 1970 to
1998 worldwide fish consumption more than doubled, a dangerous increase considering
that 47 percent main ocean stocks are already fully exploited (White et al. 2004). The
conclusions that came from the tests done on Atlantic salmon can be applied to many
other farm-raised species as well. Hopefully with this knowledge governments and
companies will begin to see the necessity of proper control when it comes to large scale
aquaculture.
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Literature Cited
Fiske P, Lund R, Hansen L. 2006. Relationships between the frequency of farmed
Atlantic salmon, Salmo salar L., in wild salmon populations and fish farming
activity in Norway, 1989-2004. ICES 63: 1182-1189.
http://icesjms.oxfordjournals.org/content/63/7/1182.short
Glover K, Sorvik A, Karlsbakk E, Zhang Z, Skaala O. 2013. Molecular Genetic Analysis
of Stomach Contents Reveals Wild Atlantic Cod Feeding on Piscine Reovirus
(PRV) Infected Atlantic Salmon Originating from a Commercial Fish Farm. PLOS
One 8(4).
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0060924
Hansen L, Jacobsen J. 2003. Origin and migration of wild and escaped farmed Atlantic
salmon, Salmo salar L., in oceanic areas north of the Faroe Islands. ICES 60:
110-119. http://icesjms.oxfordjournals.org/content/60/1/110.full.pdf+html
Shwartz M. 2000 Do fish farms really add to the world's supply of fish? Stanford News
Service. http://news.stanford.edu/pr/00/fishfarms628.html
White K, O’Neill B, Tzankova Z. 2004. At a Crossroads: Will Aquaculture Fulfill the
Promise of the Blue Revolution? Sea Web.
http://www.seaweb.org/resources/documents/reports_crossroads.pdf