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Chinook Salmon (Oncorhynchus tshawytscha) is the largest of all Pacific salmon and ranges from the Monterey Bay area of California to the Chuckchi Sea of Alaska in North America. Although most Chinook are red fleshed in color, there are several runs that can exhibit other coloration, including several off the Washington Coast and in the Puget Sound region. These include a white salmon (Pearl), a motled salmon (Marbled) and a red salmon (Red). There is interest in determining the amount of lipid (fat) and omega-3 fatty acids in these fish. The health advantages of eating fish and fishery products have been well documented in the literature. For those interested in more extensive reviews on this topic, these following publications are recommended: Simopoulos et al. (1986); Kinsella (1987); Harris (1989); Stansby (1990); Nestel (1993); Nettleton (1995). There is general agreement among researchers that the lipid component of seafood, which is high in omega-3 fatty acids, provides numerous beneficial health effects.

Research has focused on the roles of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), important omega-3 fatty acids. Several researchers have shown EPA and DHA to reduce blood cholesterol, lower blood triglyceride levels, become incorporated into erythrocyte and other cell membranes, be readily taken up by heart muscle, and affect the electrical behavior of the heart (Dyerberg, 1982; Swanson and Kinsella, 1986; Harris, 1989; McLennan et al. 1990). EPA and DHA inhibit platelet formation and ischemic heart disease associated with clotting (Nelson et al. 1991; Silvermen et al. 1991). These biochemical studies have been correlated with epidemiological research in human populations. Studies in Greenland (Dyerberg et al. 1978) and Japan (Hirai et al. 1987) have confirmed that diets high in fish oils result in an antithrombic action in populations with high fish consumption.

Because of the effects of omega-3 fatty acids on the production of eicosanoids, the consumption of fish oils has been thought to have several other health benefits, especially in the area of inflammation and immune response (Nettleton, 1995). Inflammatory responses are mediated by prostaglandins and leukotrienes produced by neutrophils, macrophages and mast cells. Chromic inflammation is associated with several diseases such as rheumatoid arthritis, psoriasis and gout. Although there have been numerous studies showing beneficial effects of diets high in fish oil in alleviating a number of the symptoms of these diseases there also has been conflicting results in which little effect of EPA and DHA has been shown. Future research in this area will help to clarify the role of omega-3 fatty acids in mediating the immune response.

There has been considerable research regarding the effects of omega-3 fatty acids on early development of retina and brain cells (Neuringer and Conner, 1986; Carlson and Salem, 1991; Innis, 1992). Currently, it is thought that DHA is necessary for these functions in premature infants and may be necessary for full term infants (Carlson et al. 1994). The Japanese dietary allowances of important nutrients now recommends that pregnant women should eat sufficient seafood to obtain 0.5-1.0 g of DHA per day (Linko and Hayakawa, 1996).

There are several other reported nutritional benefits related to fish oils. These include partial remediation of diseases such as diabetes (Malasanos and Stacpoole, 1991), multiple sclerosis (Holman et al. 1989), and various types of cancer (Nettleton, 1995). However, because of the broad range of physiological effects of omega-3 fatty acids and the roles of genetics and life-style factors in these diseases it is difficult to definitively relate a cause-effect relationship.

The objective of this study is to determine differences in lipid content and fatty acids composition of three types of Chinook salmon (Pearl. Marbled, Red) captured off the Washington Coast. This will include the total lipid content in the muscle (loin) region and the belly flap. Omega-3 fatty acid analysis will also be determined for these fish and compared to other findings reported in the literature.

This study was conducted on troll-caught Chinook salmons captured off the coast of Washington State in the month of May, 2003. The fish were divided into groups depending on color of muscle tissue (Pearl, Marbled, Red). Each group was comprised of three individual fish that were gutted and frozen. Fish were delivered to OSU Seafood Laboratory, Astoria, Oregon on June 18^th 2003 and stored at –30℃ until tested. The total body weight, length and circumference were measured before sample preparation. Approximately 100 g of tissue were collected from the upper loin (muscle) and belly flap section and homogenized in a blender at low speed for 1 min. Three individual fish of each batch were used and samples were analyzed in duplicate.

The lipid analysis was done according to the modified AOAC Official Method 948.15 (Crude Fat in Seafood, Acid Hydrolysis method, 1995). Ground 3g samples were placed in 50 ml centrifuge tubes and mixed well with 10 ml of HCl. The samples were heated in 100° C water bath for 45 min and mixed by Vortex, and then heated another 45 min. The samples were cooled, and 5 ml of methanol was added. A 15 ml aliquot of diethyl ether and 15 ml of petroleum ether were added and vigorously shaken for 1 min. The samples were centrifuged for 10 min at 1200 RPM, and the ether-fat layer was transferred to clean tubes with two more additional extractions. Total ether-fat layer was transferred to the tubes, and ether was evaporated in a stream of dry N₂.

Fatty acids of extracted salmon oils were converted into fatty acid methyl esters (FAME) according to AOAC method (1995), and their composition was determined by Gas chromatography (GC). Aliquots of 25mg of oil (±0.1mg) were placed into a glass tube, and 1.5 ml 0.5N sodium hydroxide (NaOH) was added. The mixture was agitated and heated at 100° C for 5 min replacing air with N₂. The mixture was cooled, and 2 ml BF₃ in methanol was added and heated at 100 ° C for 30 min. The mixture was then cooled to 30-40° C and 1 ml isooctane was added; immediately followed by an addition of 5 ml of saturated Sodium Chloride (NaCl) solution, agitated and cooled to room temperature. The isooctane layer was transferred to a clean glass tube and capped. Another extraction with additional 1 ml isooctane was done, and combined isooctane extracts were concentrated to 1 ml in a stream of dry N₂, and then 1μL of the isooctane layer was injected into the GC system (AOAC 991.39, 1995).

A Hewlett-Packard 5890 SeriesⅡgas chromatograph (Palo Alto, CA), equipped with a flame-ionization detector, capillary column (EC-wax,30m×0.25mm i.d; split ratio, 100:1; Alltech, Deerfield, IL) was used for analyzing fatty acid methyl esters (FAME). Parameters of GC system was set as follows: injector and detector temperatures 250^oC and 270^oC, respectively. The column temperature was set at 50^oC first and gradually heated to 180^oC at a rate of 5^oC/min, then slowed to a rate of 0.8^oC/min until it reached 220^oC. Helium was used as a carrier gas. The fatty acid concentrations were calculated by comparison of their retention times with those of the reference standards (extra-standard). (Supelco 37 Component Fatty Acid Methyl Ester (FAME) Mix, Supelco, Park, Bellfonte, PA). Samples were analyzed in triplicate.

Table 1 shows a physical measurement of Chinook salmon used in this study.

Pearl salmon had the highest average weight and length compared to Marbled and Red salmon. The lipid content of the three types of Chinook salmon are shown in Table 2. The belly flap had significantly higher lipid content for all fish in each category. Overall lipid levels for analysis for all salmon varied from 0.83 to 8.18% for muscle tissue and 2.26 to 22.27% for the belly flap. Marbled salmon had the lowest average lipid content with 1.78% for muscle and 6.61% for the belly flap. Data showed lower levels of lipid than those reported in the literature such as US Department of Agriculture (USDA) Nutrition Tables (10.44%) and others (7%, 11.4%, 11.5%, 11.6% and 13.2%) (Wander and Patton, 1991; Standby, 1976; Sidewell, 1981; Gruger et al. 1964). However these studies reported the lipid content in the edible portion, which includes both muscle and belly flap. Salmon have their highest lipid content before they migrate uprivers to spawn. Therefore, fish captured at the river mouth generally have higher lipid levels than fish caught offshore by trollers before migration begins (Wander and Patton, 1991). The samples tested were ocean toll-caught in May, 2003 and are probably representative of the early season Chinook. The wide variation in lipid content among fish is not untypical of wild-caught salmon.

Table 3 describes the amount of omega-3 (n-3) and omega-6 (n-6) fatty acids found in tested fish. All salmon had low levels of omega-6 poly unsaturated fatty acids (PUFAs) compared to omega-3 PUFAs indicating a desirable ratio (reported as an average) for physiological functions in humans. The n-3/n-6 ratio was from 8-14:1 which is higher than the ratio reported by Wander and Patton (1991) and considerably higher than that found in farmed Atlantic salmon (3:1). Due to higher lipid content in the belly flaps, fatty acid analysis also showed higher total omega-3 PUFAs content (g/100g tissue) compared to muscle for all fish. The belly flap section of pearl salmon contained the highest levels of omega-3 PUFAs ranging from 0.98 to 1.57g/100g tissue. The USDA tables show 0.788g/100g of EPA and 0.567g/100g tissue of DHA in edible portions which includes the muscle and belly flap for Chinook salmon. The total lipid content for the USDA samples

were 10.44% and had a total omega-3 PUFA of 1.586g/100g. Because these fish had lower total lipid content than reported in USDA Tables, the data showed lower total omega-3 PUFAs per 100g of tissue. Wander and Patton (1991) reported 0.44 g/100g tissue for EPA and 0.55 g/100 g tissue for DHA with 7% lipid content in the edible portion (muscle and belly flap) of Chinook salmon caught off the Oregon coast.

Table 4 shows fatty acid composition in the three types of Chinook salmon and reported as mg/g oil. The predominant fatty acid was C16:0 (hexadecanoic acid, common name: palmitic acid) ranging from 103.47mg to 135.41mg/ g oil for muscle and 95.97mg to 141.35mg/100g oil in the belly flap. Although they do not specify the salmon species, the USDA Table also lists hexadecanoic acid as the highest level of saturated fatty acids at 98.40mg/g oil, similar to our data. DHA was the second highest-level fatty acid in Chinook salmon analyzed in this study ranging from 70.77mg/g oil to 101.44mg/g oil for muscle and 41.87mg/g oil to 55.17mg/g oil for the belly flap. By back-calculating USDA data for Chinook salmon the DHA and EPA data is 54.31 mg/g oil and 75.48 mg/g oil respectively. In our study, muscle contained higher amounts of total PUFA and DHA than in the belly flaps while a relatively similar amount of EPA was found the muscle and belly flaps. Data showed DHA levels ranging from 70.77mg to 101.44mg/g oil for muscle and 41.87mg to 55.17mg/g oil for belly flap. Although marbled salmon had lower total lipid levels, it contained the highest DHA as well as total omega-3 PUFAs per g oil (101.44mg/g oil, 138.16mg/g oil, respectively) in the muscle.

An important question is how these results compare to lipid content and omega-3s in other salmon. Part of the problem in comparing reported data in the literature are differences in total lipid content in species at different times of the year or in farmed fish. For example, farmed salmon may have less % DHA or EPA as part of the overall lipid content in the fish but higher total DHA and EPA per 100 g tissue because of the higher overall lipid content (Nettleton and Exler, 1992). Furthermore, several of the papers report their findings differently (for example g fatty acid/100 g muscle; % fatty acids, mg fatty acids/g oil). Others have separated out the belly flaps from the muscle tissue while some (including USDA tables) report a composite result. The three types of salmon tested were found to be good sources of omega-3 fatty acids. The overall lipid content was variable between the 9 fish tested having a lipid range of 0.83% to 8.18%. This is probably more reflective of the time of year captured. All three salmon have high percent levels of DHA and EPA and excellent n-3/n-6 ratios.

There are several differences as well as similarities with other salmon. By back-calculating reported results we can compare them with our findings. One of the more striking differences, is the reported values of DHA and EPA in the USDA tables for Chinook salmon. One presumes the data in the USDA Tables are average results from several reports to derive numbers that represent a typical Chinook salmon. The USDA Chinook table shows 10.44% lipid, 20% protein and a list of vitamins, minerals, and fatty acids. However, they report EPA (0.79 g/100g tissue) higher than DHA (0.57g/100g). All of the other reported data (including ours) has DHA in higher concentration that EPA. For example, pearl salmon has an average of 0.24 g/100/g tissue and 0.16 g/100g tissue respectively. (These lower values only reflect the lower average % lipid in tissue).

Wander and Patton (1991) reported on the lipid composition of Chinook salmon captured off the Oregon Coast. They did not separate into muscle tissue and belly flaps but made a composite of all muscle tissue. The Chinook salmon they measured were 7.0% fat and the n-3/n-6 ratios were 3:1 which are considerably lower than our results. The mg/100 g of tissue results were 0.55 for DHA and 0.44 for EPA which would be similar for our results if one combine muscle tissue and belly flap.

Bell et al. (1998) compared Scottish farmed Atlantic salmon with wild-caught salmon. The n-3/n-6 ratio was 10.3 for wild-caught while it ranged from 3.3 to 5.8 for salmon from different farms. This is due to the use of plant oils with high amounts of linoleic acid (18:2n-6) in the formulated diets. The amount of DHA was twice that of EPA for both the farmed and wild-caught salmon. The farmed salmon delivered more DHA and EPA per 100 g of tissue as these fish had higher overall fat content (10% for farmed fish, 3.5% for wild-caught).

Aursand and coworkers (1994) undertook an extensive study of the lipid distribution and composition of farmed Atlantic salmon from Norway. The lipid content of the muscle was 9.6% while the belly flap had 28.1% lipid. The n-3/n-6 ratio was 3.7:1 and 3.3:1 respectively, which is considerably lower than the wild-caught salmon tested in our study and that reported by Bell et al. (1998). Jonsson et al. (1997), however, showed a more favorable n-3/n-6 ratio of 9.1 to 10.9 but high amounts of oleic acid (18:1:n-9) in the muscle tissue.

There were few differences in the fatty acids content among the three types of Chinook salmon captured off the Washington Coast. DHA was found to be the omega-3 fatty acid in the highest amounts followed by EPA with the exception of the pearl salmon belly flap. All three salmon were shown to be excellent sources for omega-3 fatty acids for human consumption.

Aursand, M., Bleivik, B., Rainuzzo, J.R., Jorgensen, L., and Mohr, Viggo. 1994. Lipid distribution and composition of commercially farmed Atlantic salmon (Salmo salar). J. Sci. Food Agric. 64: 239-248.

Bell. G. McEnvoy, J., Webster, J.L., McGhee, F., Millar, R.M., and Sargent, J.R. 1998. Fresh lipid and carotenoid composition of Scottish farmed Atlantic salmon (Salmo salar). J. Agric. Food Chem. 46: 119-127.

Carlson, S.E., Werkman, S.E., Peeples, J.M., and Wilson, W.M. III. 1994. Growth and development of premature infants in relation to ω 3 and ω 6 fatty acid status. In: World Review of Nutrition and Dietetics, vol. 75, pp. 63-69. Karger, New York.

Dyerberg, J. 1982. Observations on populations in Greenland and Denmark. In: Nutritional Evaluation of Long-chain Fatty Acids. S.M. Barlow and M.E. Stansby, eds. pp. 245-261. Academic Press, New York.

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Hirai, A. Terano, T., Saito, H., Tamura, Y. and Yoshida, S. 1987. Clinical and epidemolgical studies of eicosapentaenoic acid in Japan. In: Proc. of the AOAC Short Course on Polyunsaturated Fatty Acids and Eicosanoids. W. E. M. Lands, ed., pp 9-24, American Oil Chemists Soc, Champaign, IL.

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