Mercury Concentrations in Farmed Raised Atlantic Salmon (Salmo salar) and Wild Sockeye Salmon

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	Farm-raised salmon
Mean	0.085	0.005
Standard Error	0.010	0.0050

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 farm-raised 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.

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