Mercury Concentrations in Farmed Raised Atlantic Salmon (Salmo salar) and Wild
Sockeye Salmon (Oncorhynchus nerka)
Dylan Gunderson, Sequoi Zozula and Tran Bui
Department of Biological Sciences
Saddleback College
Mission Viejo, CA 92692
Salmon is a good source of nutrients beneficial to human health. Recently, the Food
and Drug Association (FDA) and US Environment and Protection Agency (EPA) have
given warnings about fish contamination by dangerous substances such as mercury.
Mercury can negatively affect nervous system development in fetuses if high levels of
mercury are consumed by pregnant women. One of the many choices in diet that can effect
mercury consumption is the choice to consume farm raised or wild salmon. Logic would
dictate that these salmon could contain different concentrations of mercury due to a
variation in environment, growth rate and diet. Salmo salar and Oncorhynchus nerka are
the two species of salmon used in this study, representing the most common species of
farm-raised salmon and one of the most common species of wild salmon respectively. It is
hypothesized that there is a significantly higher mercury concentration in the muscle tissue
of wild salmon, Oncorhynchus nerka, than the mercury concentration in farm raised
salmon, Salmo salar. The tissue samples of each species are prepared to isolate the mercury
from the muscle tissue and determine the mercury concentration content in each sample
(ppm). The average mercury concentration in wild caught Sockeye salmon is 0.085 ppm ±
0.010 (± S.E.M., N=5) and the average mercury concentration in farm raised Atlantic
salmon is 0.005 ppm ± 0.0050 (± S.E.M., N=5). The mercury concentration in wild Sockeye
salmon is significantly greater than the mercury concentration in farm raised Atlantic
salmon (one-tailed unpaired t-test, p-value = 0.00019).
Introduction
Salmon is a good source of nutrients; known to contain high levels of proteins, vitamins
and minerals. OMG-3 fatty acids in fish reduce the chance of heart disease (Dariush and Eric,
2006; Harvard, 2013). However, in a research conducting by Food and Drug Association (FDA)
and the Environment Protection Agency (EPA), has suggested that some commercial fish and
shellfish have been contaminated by toxic substances such as mercury which can cause cancer
and health problems associated with the nervous system during child development (EPA, 2004;
“Mercury Levels Lower,” 2008). Therefore, consumers are faced with the problem of discerning
how to balance the benefits and drawbacks of consuming fish.
One important factor considered when choosing fish is whether the fish is wild caught or
farm raised. Most of the salmon available for consumption today is farm raised (“Farmed
Salmon vs Wild Salmon,” n.d.). But it is important to recognize which type has a lower
concentration of mercury and is safer for consumption. Farm raised salmon is generally cheaper
and more available than wild salmon but have a higher affinity for pollution, chemical
contamination, and disease (Tom and Olin, 2010; “Farmed Salmon vs Wild Salmon,” n.d.). On
the other hand, wild salmon may have higher mercury levels than farm raised salmon. One study
suggested that wild salmon has a mercury concentration that is three times higher than the
mercury concentration in farm raised salmon due to the rapid growth cycles of farm raised fish,
diet or lifestyle differences between farm raised and wild salmon (“Mercury Levels,” 2008). This
experiment will compare the mercury concentration levels in the muscle tissue of wild sockeye
salmon, Oncorhynchus nerka, and farm raised Atlantic salmon, Salmo salar. It is expected that
the mercury concentration in the muscle tissue of wild Sockeye salmon will be significantly
greater than the mercury concentration in farm raised Atlantic salmon.
Materials and Methods
A Sensafe mercury test strip kit, Mercury Check, was purchased from Industrial Test
Systems Inc. one week before data collection. These test strips would be applied to test the
mercury level in the sample after the fish tissue was digested and the resulting solution was
neutralized. The fish samples were purchased from the Seafood Section of Ralphs in Mission
Viejo, California on the day of the experimentation session. The samples comprised of the tail
muscle tissue of five wild Sockeye salmon (Oncorhynchus nerka) with a combined weight of
0.79 lb and five farm raised Atlantic salmon (Salmo salar) tail muscle with a combined weight of
1.28 lb. All other materials used during the experimentation process were provided by Professor
Teh and the Saddleback Chemistry Department.
The experimentation took place in room 254 of the Science and Mathematics building at
Saddleback College in Mission Viejo, California. The first step of the experimentation process
was the preparation of a bromide-bromate solution. This solution was made by mixing 0.556
grams of KBrO3, 2.380 grams of KBr and 100.0 mL of water. All three components of the
solution were mixed with a stir bar until full dissolution for two minutes. The solution was stored
in a small clean container which was capped and labeled until further use.
Each of the muscle tissue samples were patted dry and 0.25 grams of muscle tissue was
removed from the upper portion of each of the salmon tails. These ten samples were placed in ten
separate clean glass test tubes, which were labeled accordingly. Five milliliters of 3:1
H2SO4:HNO3 were measured in a 10 mL graduated cylinder and added to each of the test tubes.
The solution was allowed to sit at room temperature for half an hour to allow for chemical
digestion. Simultaneously, a hot water bath was heated to a temperature of 80 degrees Celsius.
After this digestion period, the ten solutions were placed in the hot water bath for forty
minutes. In the first five minutes of the heating process, clean glass stirring rods were used to
carefully grind the fish tissue inside the test tubes into small granules to increase the surface area
of fish tissue exposed to the second digestion treatment. After this second treatment, all the
samples were largely liquefied.
All the solutions were removed from the hot water bath and allowed to cool for ten
minutes. Then, 10 mL of HCl, 3 mL of BrCl and 4 mL of deionized water were added to each of
the samples. Each sample solution was mixed with a clean glass stirring rod. The hot water bath
was cooled to a temperature of 60 degrees Celsius and the samples were placed into the hot
water bath for the third digestion period. The sample solutions were heated for sixty minutes
before being removed from the hot water bath. The completion of this third digestion treatment
signified that the digestion of the samples was complete.
Each of the fully digested sample solutions were extremely acidic, with a pH around zero.
The mercury test strips only function in a moderate pH range, therefore, each solution was
neutralized to a pH between 6.5 and 8.5 in preparation for the mercury test strips which only
function within this pH range. This neutralization was achieved by titrating the solutions with 1N
NaOH, 1N HCl and .1N HCl. Each sample solution was poured into a separate, clean 250 mL
beaker. Deionized water was added until the total solution consisted of 100 mL. A pipet was
used to add about 72 mL of 1N NaOH to each of the sample solutions before 1N HCl, 1N NaOH
and .1N HCl were added drop by drop until the solution had a pH within the specified range.
A Sensafe Mercury Check test strip was applied to each solution to test for the mercury
concentration of the samples in a range of 0-1000 ppb. The Mercury Check test strip was dipped
into the sample solution for thirty seconds before it was removed and allowed to stand for two
minutes. Then, the color of the test strip, enabled by the Diphenylcarbozone/ Diphenylcarbazide
indicator, was compared to the provided color spectrum to determine the mercury concentration
of the samples.
Results between farm-raised Salmo salar and wild Oncorhynchus nerka sample groups
were compared using a one-tailed unpaired t-test. Differences were considered to be significant if
they were greater than 0.05. The data were expressed as means ± SEM.
Results
Mercury concentration in wild Sockeye salmon and farm raised Atlantic salmon was
collected and analyzed by Microsoft Excel 2007. The average of mercury concentration
measured in wild caught Sockeye salmon was 0.085 ppm (± S.E.M., N = 5). The mercury
concentration of farm raised Atlantic salmon was 0.0050 ppm (± S.E.M., N=5). These data are
shown in Table one.
Table one: The average mercury concentration of wild Sockeye salmon (Oncorhynchus
nerka) and farm raised Atlantic salmon (Salmo Salar) (± S.E.M).
Data
collection
Wild
salmon
Farmraised
salmon
0.005
Mean
0.085
Standard
0.010
0.0050
Error
A one-tailed unpaired t-test suggested that there was a significantly higher mercury
concentration in wild Sockeye salmon (Oncorhynchus nerka) in comparison to farm raised
Atlantic salmon (Salmo salar) (p-value = 0.00019). These data are shown in figure one.
Figure one. Average mercury concentration of wild Sockeye salmon (Oncorhynchus
nerka) and farm raised Atlantic salmon (Salmo Salar). Mercury concentration in the wild salmon
is significantly greater than the mercury concentration in farm raised salmon (p-value =
0.00019, one- tailed unpaired t-test). Error bars are ± S.E.M.
Discussion
The concentration of mercury in wild Sockeye salmon (Oncorhynchus nerka) is
suggested to be significantly greater than the concentration of mercury in farm raised Atlantic
salmon (Salmo salar). There are many factors that could have caused this variation.
Farmed salmon are raised in a controlled environment where their diet, health and
location are strictly monitored (Tom & Olin, 2010; “Farmed Salmon,” n.d.). Wild salmon
develop in various natural environments. Their diet and health are affected by their contact with
marine and freshwater ecosystems rather than human involvement (Mazurek & Elliot, 2004). It
is largely this variation of living conditions which causes the significant difference in mercury
concentration between wild and farm-raised salmon.
Mercury enters the atmosphere from natural sources such as volcanoes and human
sources such as waste incineration (Mozaffarian & Rimm, 2006). From the atmosphere,
inorganic mercury returns to water reservoirs with precipitation and is transformed by microbial
activity into organic methylmercury (Mozaffarian & Rimm, 2006; “What you Need to Know,”
2004). It is this methylmercury which accumulates in fish through bioaccumulation.
Methylmercury is the dangerous substance that can harm fetuses. The level of methylmercury in
fish is directly related to the diet of the fish (“What you Need to Know,” 2004).
Farm raised salmon diets consist of pellets containing protein from wild open ocean fish
such as anchovies where changes in feed have lowered the contaminant levels in these fish
(“Farmed Salmon,” 2010; Tom & Olin, 2010). Wild salmon consume zooplankton and fish as
their main sources of nutrients (Tom & Olin, 2010). Low levels of bioaccumulation occur in both
types of salmon, though the bioaccumulation in wild salmon is slightly more pronounced.
Another possible source of mercury differentiation could have a basis in the high growth rate of
farmed salmon in comparison to the lower growth rate of wild salmon. In another experiment,
the mercury levels in wild salmon were three times higher than those in the farm raised salmon
(“Mercury Levels Lower,” 2008). These results are similar to the results of this experiment. The
FDA also tested the mercury levels in commercial salmon. They calculated a mean mercury
concentration level of 0.022 ppm in salmon that is between the mean mercury concentration
values obtained from this experiment (“Mercury Levels in Commercial Fish,” 1990-2009). The
mercury concentrations in both wild and farm-raised salmon are classified in the fish group with
the lowest level of mercury contamination (“Protect Yourself and Your Family,” n.d.).
The recognition that many wild salmon have a higher mercury concentration than farmraised salmon can assist consumers in their selection of commercial salmon. These data suggest
that if high risk consumers, such as pregnant women and young children, choose to consume
small portions of fish, they should select farmed salmon more often than wild salmon.
Acknowledgement
We would like to thank Professor Teh for his support and instructions and the Chemistry
Department for providing the solutions utilized in this experiment.
Literature Cited
Farmed Salmon vs Wild Salmon. (n.d.). Washington State Department of Health.
Feil, D. P. (2006). Mercury Determination in Fish by Cold Vapor Atomic Fluorescence
Spectroscopy. Food Quality, 73-75.
Fish: Friend of Foe? (2013). Harvard School of Public Health.
Mazurek, R. & Elliot, M. (2004). Farmed Salmon. Monterey Bay Aquarium.
Mercury Levels in Commercial Fish and Shellfish. (1990-2010). Food and Drugs Association.
Mercury Levels Lower in Farmed Salmon than in Wild Salmon. (2008). Journal of
Environmental Health, 71(2), 60.
Method 245.7: Mercury in Water by Cold Vapor Atomic Fluorescence Spectrometry. (2005). US
Environmental Protection Agency, 2, 15-26.
Mozaffarian, D. & Rimm, E. B. (2006). Fish Intake, Contaminants, and Human Health:
Evaluating the Risks and the Benefits. JAMA, 296 (15), 1885-1899.
Protect Yourself and Your Family: Consumer Guide to Mercury in Fish. (n.d.). Natural
Resources Defense Council.
Tom, P. D. & Olin, P. G. (2010). Farmed or Wild? Both Types of Salmon Taste Good and are
Good for You. Global Aquaculture Advocate, 58-60.
What You Need to Know about Mercury in Fish and Shellfish. (2004). United States
Environmental Protection Agency.
Review Form
Department of Biological Sciences
Saddleback College, Mission Viejo, CA 92692
Author (s): Dylan Gunderson, Sequoi Zozula and Tran Bui
Title: Mercury Concentrations in Farmed Raised Atlantic Salmon (Salmo salar) and Wild
Sockeye Salmon (Oncorhynchus nerka)
Summary
Summarize the paper succinctly and dispassionately. Do not criticize here, just show that you understood the paper.
Salmon, a commonly eaten nutritious fish, has been shown to contain small concentrations
of toxic substances such as mercury, that when consumed frequently can accumulate in the
body and cause cancer or damage the nervous system during fetal and child development.
Studies have found there may be a difference in mercury concentrations between farm
raised salmon and wild caught salmon. This paper hypothesizes that the wild caught
Sockeye salmon will contain a greater concentration of mercury inside its tissue when
compared to the farm raised Atlantic salmon. This was tested by obtaining 5 different
samples of both farm raised and wild caught salmon. Each sample was broken down
chemicaly and mechanically in order to release the mercury contained inside the tissues. A
test strip was dipped into the digested solution samples and compared to a spectrum chart
to determine the concentration of mercury in each sample. The results found that the
mercury content of wild caught salmon was greater than the farm raised salmon,
supporting the hypothesis. This supports past research, asd is likely due to diet differences
between both species, with the farm raised fish having a more controlled diet. This study
suggests that farm raised salmon is a safer choice than wild caught salmon when
considering mercury concentration as a deciding factor.
General Comments
Generally explain the paper’s strengths and weaknesses and whether they are serious, or important to our
current state of knowledge.
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Good use of sources to support arguments.
Your methods are very thorough. You may actually consider omitting some of your
steps, particularly pertaining to what should be considered common lab practice.
Some sentences can be merged with following sentence to make paper flow better,
noted in pink highlight followed by yellow highlight.
Use of the word suggest in regard to past research used to support your argument
sounds a little off, and may weaken your position in some readers’ eyes. Suggest
using the word “found” or other similar word
o i.e. “the study by the FDA found that…” versus “The study by the FDA
suggests that…”
Clearly thought out, thoroughly researched, written well.
Technical Criticism
Review technical issues, organization and clarity. Provide a table of typographical errors, grammatical errors,
and minor textual problems. It's not the reviewer's job to copy Edit the paper, mark the manuscript.
This paper was a final version
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See comments in paper above.
o Pink highlights suggest omitting, merging information with the following
sentence.
o Yellow highlights suggest rewording of sentence
o Blue highlights are grammatical discrepancies, suggest new word to make it
sound better
o Place table caption below table.
o Fix labeling of x-axis figure. You have wild salmon identified, but not the farm
raised salmon.
o Make sure that when you cite sources in your work you include the author.
Corporations, organizations, and government agencies can be considered the
authors in this case.
o Make sure Works Cited page includes authors, including government
agencies, organizations, and corporations.
o Titrations in N? did you mean M?
Recommendation
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This paper was a rough draft
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