The effect of different growing conditions to proximate composition and fatty acid profiles of rainbow trouts (Oncorhynchus mykiss)

ABSTRACT

The experiment was carried out using escaped rainbow trout (weights between 268 and 310 g) caught in Seyhan Dam Lake, Adana, Turkey and three groups of rainbow trout (weights between 226 and 277 g) cultured in concrete freshwater ponds, in freshwater cages in Seyhan Dam Lake and in marine cages in Black Sea by private companies. Proximate composition and the fatty acid profiles of the fishes were determined. Analysis of the proximate composition of the present study revealed that the escaped rainbow trout flesh to have the highest water (74.23%) and the lowest lipid (4.47%) content while lowest level of moisture and highest levels of lipid was observed in farmed fish in ponds (68.69% and 7.78%). According to the results of fatty acid analysis, the highest saturated fatty acid (31.3%) and the lowest polyunsaturated fatty acids (PUFA) (26.51%) levels were in the escaped group obtained from wild, while PUFA levels were higher in the cultured groups. On the other hand, the levels of n3/n6 ratio were higher in escaped and marine cages groups (2.71 and 2.58) than in ponds and Dam Lake cages groups (1.75 and 1.33). According these results, rainbow trout from all groups studied are a desirable food item for human consumption.

KEYWORDS:

• Rainbow trout
• proximate composition
• fatty acid profiles
• growing conditions

1. Introduction

The nutritional and health benefits received from the consumption of fishery produce are one reason for the continuing rise for fish by consumers. A number of fatty acids cannot be synthesized by human body. These essential fatty acids must be consumed in the diet. The low fat nature of fish and the presence of long-chain n-3 fatty acids, eicosapentataneoic acid (EPA) and docosahexaenoic acid (DHA) provide additional health benefits (Belinda & Roberts 2000). Polyunsaturated fatty acids (PUFA) are essential for normal growth and development; they reduce the risk of coronary heart diseases and may play an important role for prevention or treatment of diseases such as arthritis and inflammation, autoimmune disease, type 2 diabetes, hypertension, kidney and skin disorders and cancer (Connor 1997; Schmidt et al. 2005; Gonzales et al. 2006; Nasopoulou et al. 2007).

Fish are the main contributors of n-3 PUFA for the human diet although there are some interspecific and intraspecific differences in fatty acid profiles. The fatty acid composition of fish differs depending on a variety of factors, including species, diet, environmental factors such as salinity, temperature, season, geographical location and whether they are farmed or wild (Belinda & Roberts 2000; Gonzales et al. 2006).

Rainbow trout (Oncorhynchus mykiss) is widely used as a farmed fish in many countries such as USA, Japan, The Russian Federation, Canada and in Europe because of rapid growth and high values as a food (Yasmin et al. 2004). It is also one of the economically important and the most commonly raised fish followed by European sea bass (Dicentrarchus labrax) and gilthead sea bream (Sparus aurata) in Turkey. It has been preferred by the consumers due to its quality, desirable aroma and moderate price (Celik et al. 2007).

Intensive production of rainbow trout in different culture conditions raised concerns of consumers over the quality of these fish on the market. Currently, it is cultured in concrete freshwater ponds in the highland areas, in freshwater cages mostly located in Dam Lakes and in marine condition of Black Sea. Escaped trout have also been caught and commercialized by anglers in the fish markets with others obtained from different culture systems and all the trout are traditionally marketed as whole fresh fish.

The nutritional values of rainbow trout grown in different intensive culture conditions and caught from lakes are questioned by the consumers. Therefore the objective of this study was to investigate proximate composition and fatty acid profile of muscle of rainbow trout (O. mykiss) grown in different culture conditions and the escaped ones.

2. Material and methods

The experiment was carried out using escaped rainbow trout caught in Seyhan Dam Lake and three groups of rainbow trout cultured in concrete freshwater ponds, in freshwater cages in Seyhan Dam Lake (province of Adana, Turkey) and in marine cages in Black Sea by private companies. Each group of fishes containing eight samples were collected in the same period of time. Specimens grown in ponds and cages had same commercial diet (ad libitum) containing approximately 45% protein and 19% fat.

2.1. Fish samples

Eight specimens of each group were transported in ice to laboratory and upon arrival they were weighed. Fish were beheaded, eviscerated, filleted and washed with tap water to remove adhering blood. Boneless trout fillets were homogenized by using a meat grinder. All chemical analyses were carried out in triplicates.

2.2. Proximate composition

Moisture, crude protein and ash contents were determined by the methods of AOAC (1995). Fish muscle samples were dried in oven at 105°C until constant weight for the moisture content and ash content determined by using a muffle furnace at 7 h 550°C. Crude protein was done (N × 6.25) by the Kjeldhal method. Lipid content was determined according to the method of Bligh and Dyer (1959).

2.3. Fatty acid analysis

Methyl esters were prepared by transmethylation using 2 M KOH in methanol and n-hexane according to the method described by Ichihara et al. (1996) with minor modification; 10 mg of extracted oil were dissolved in 2 ml hexane, followed by 4 ml of 2 M methanolic KOH. The tube was then vortexed for 2 min at room temperature. After centrifugation at 4000 rpm for 10 min, the hexane layer was taken for gas chromotography (GC) analyses. The fatty acid composition was analysed by a GC Clarous 500 with autosampler (Perkin–Elmer, USA) equipped with a flame ionization detector and a fused silica capillary SGE brand column (30 m · 0.32 mm ID · 0.25 lm BP20 0.25 UM, USA). The oven temperature was 140°C, held 5 min, raised to 200°C at the rate 4°C/min and held at 220°C at 1°C/min, while the injector and the detector temperatures were set at 220°C and 280°C, respectively. The sample size was 1 µl and the carrier gas was controlled at 16 psi. The split used was 1:100. Fatty acids were identified by comparing the retention times of fatty acid methyl esters (FAME) with the standard 37 component FAME mixture.

2.4. Statistical analysis

Data were subjected to one-way analysis of variance (ANOVA) SPSS 15.0 Windows software package. Differences between the means were tested by Duncan's multiple-range tests (Duncan 1955). The level of significance was chosen at p < .05 and the results are presented as mean ± standard deviation for triplicate samples.

3. Results and discussion

3.1. Proximate composition

The result of proximate analysis in the flesh of studied groups of rainbow trout is shown in Table 1. Moisture content in fish is usually inversely related to lipid content (Sargent et al. 1989). Analysis of the proximate composition of the present study showed the escaped rainbow trout muscle to have the highest water (74.23%) and the lowest lipid (4.47%) content while lower levels of moisture and higher levels of lipid were observed in farmed fish in ponds (68.79% and 7.78%), marine cages (5.90% and 69.63%) and Dam Lake cages (69.64% and 6.60%), respectively. The higher lipid content in farmed fish is expected when compared to their wild counterparts (Haard 1992). This could be the result of variety factors such as availability and type of food, dietary ingredients (commercial diets are usually high in fat content), possible periods of starvation encountered by wild fish and reduced activity of the cultured fish (Haard 1992; Orban et al. 2003; Gonzales 2006; Fuentes et al. 2010).

Table 1. Proximate composition (% of wet weight basis) of the muscle in rainbow trouts from the different growing conditions.

	Escaped fish	Ponds	Marine cages	Dam Lake cages
Moisture	74.23 ± 0.59^a	68.79 ± 0.73^b	69.63 ± 1.66^b	69.34 ± 0.55^ab
Ash	1.20 ± 0.07^b	1.37 ± 0.01^a	1.20 ± 0.05^b	1.31 ± 0.02^ab
Lipid	4.47 ± 0.71^b	7.78 ± 0.89^a	5.90 ± 1.17^ab	6.60 ± 0.85^ab
Protein	19.70 ± 0.41^b	21.60 ± 0.42^ab	22.41 ± 0.43^a	22.45 ± 0.65^a

Notes: Data are expressed as mean ± SD of triplicate, n = 3 (p < .05). Different superscripts in a row show significant differences between samples (p < .05).

These results corroborate the finding of some previous studies carried out on Sander lucioperca (Jankowska et al. 2003a, 2003b), yellow perch (Perca flavescens) (Gonzales et al. 2006), gilthead sea bream (S. aurata) (Grikorakis et al. 2002), sea bass (D. labrax) (Fuentes et al. 2010) and rainbow trout (O. mykiss) (Blanchet et al. 2005; Ehsani et al. 2013).

While marine (22.41%) and Dam Lake cage (22.45%) groups had the highest protein contents, the lowest level of protein was found in escaped fish group (19.70%). The protein values obtained for the present study are higher than the previously studied rainbow trout in Turkish waters (Gokoglu et al. 2002; Celik et al. 2007). Protein is considered to be stable component of fish in respect to diet and feeding level depending mainly on fish weight (Shearer 1994; Ehsani et al. 2013). It usually increases with size, remaining stable after a certain size of fish reached (Grikorakis et al. 2002). However, although fish sizes differ among the studied groups, a relation of protein levels with fish size was not seen in the present study probably due to differences in cultural and environmental conditions.

3.2. Fatty acids profiles

The muscle fatty acid profiles of the escaped and cultured rainbow trouts are listed in Table 2. The saturated fatty acid (SFA), monounsaturated fatty acid (MUFA) and PUFA levels showed significant differences between the studied groups (p < .05). Among the SFAs, most abundant fatty acids were palmitic acid (C16:0), stearic acid (C18:0) and myristic acid (C14:0), respectively. The results revealed that the highest SFA and the lowest PUFA levels were in the escaped group obtained from wild. However, the highest PUFA level was found in marine cage group and lowest SFA level was determined in pond group. The freshwater cultured groups (pond and Dam Lake cage) had the highest percentages of MUFA. Oleic acid (C18:1n9) was the predominant fatty acid in MUFAs for all groups. Interestingly, the freshwater cultured groups (pond and Dam Lake cage) showed similarity when the SFA, MUFA and PUFA values taken into account and the values of all three fatty acid groups of them differ from escaped and marine cage groups. DHA (C22:6n3), linolenic acid (C18:2n6) and EPA (C20:5n3) were the most abundant fatty acids in PUFAs for all groups, respectively. These results showed an agreement with the some studies which have been reported by Celik et al. (2007), Fallah et al. (2011), Zakipour et al. (2012) and Ehsani et al. (2013).

Table 2. Muscle fatty acid compositions (% of total fatty acids) in rainbow trouts from different growing conditions.

Fatty acids	Escaped fish	Ponds	Marine cages	Dam Lake cages
C 12:0	0.03 ± 0.01^a	0.03 ± 0.01^a	0.02 ± 0.01^a	0.02 ± 0.01^a
C 14:0	3.03 ± 0.28^b	2.76 ± 0.12^bc	3.26 ± 0.21^a	2.63 ± 0.12^c
C15:0	0.03 ± 0.01^a	0.03 ± 0.01^a	0.03 ± 0.01^a	0.03 ± 0.01^a
C16:0	18.54 ± 0.47^a	14.20 ± 0.67^c	16.05 ± 0.95^b	14.64 ± 0.49^c
C17:0	0.49 ± 0.05^a	0.26 ± 0.04^c	0.34 ± 0.02^b	0.23 ± 0.02^c
C18.0	6.16 ± 0.3^a	3.80 ± 0.49^c	4.1 ± 0.34^cb	4.4 ± 0.32^b
C20:0	0.22 ± 0.21^ab	0.37 ± 0.12^a	0.19 ± 0.01^b	0.29 ± 0.05^ab
C22:0	0.06 ± 0.04^d	2.82 ± 0.24^a	0.97 ± 0.17^c	2.50 ± 0.1^b
C23:0	0.18 ± 0.05^b	0.15 ± 0.01^b	0.29 ± 0.02^a	0.15 ± 0.02^b
C24:0	2.56 ± 0.23^a	1.78 ± 0.12^b	1.98 ± 0.23^b	1.77 ± 0.1^b
∑SFA	31.3 ± 0.93^a	26.2 ± 0.87^c	27.23 ± 1.04^b	26.66 ± 0.63^bc
C14:1n9	0.27 ± 0.02^a	0.11 ± 0.05^bc	0.13 ± 0.01^b	0.08 ± 0.01^c
C15:1	0.08 ± 0.01^a	0.04 ± 0.01^c	0.07 ± 0.01^b	0.04 ± 0.01^c
C16:1n7	0.36 ± 0.06^a	0.20 ± 0.06^b	0.18 ± 0.03^bc	0.16 ± 0.02^c
C16:1n9	8.07 ± 0.98^a	4.99 ± 0.57^b	4.93 ± 0.55^b	3.91 ± 0.29^b
C17:1n9	0.14 ± 0.03^b	0.11 ± 0.01^c	0.20 ± 0.01^a	0.11 ± 0.01^c
C18:1n9	25.63 ± 1.48^bc	27.38 ± 0.38^ab	24.14 ± 1.29^c	28.42 ± 0.27^a
C20:1n9	0.84 ± 0.05^c	4.34 ± 0.43^a	1.63 ± 0.34^b	4.02 ± 0.19^a
C20:1n11	0.20 ± 0.01^b	0.12 ± 0.01^d	0.24 ± 0.03^a	0.15 ± 0.01^c
C22:1n11	0.03 ± 0.01^c	0.05 ± 0.01^c	0.05 ± 0.01^c	0.07 ± 0.01^a
C22:1n9	0.19 ± 0.03^a	0.05 ± 0.01^c	0.12 ± 0.05^b	0.05 ± 0.01^c
∑MUFA	35.81 ± 2.28^b	37.39 ± 0.64^a	31.69 ± 1.98^c	37.01 ± 0.47^ab
C18:2n6	5.12 ± 0.41^d	10.76 ± 0.48.^b	9.14 ± 0.96^c	12.68 ± 0.38^a
C18:2n9	0.12 ± 0.01 ^d	0.12 ± 0.01^c	0.20 ± 0.03^b	0.26 ± 0.03^a
C18:3n6	0.25 ± 0.08^a	0.18 ± 0.04^b	0.20 ± 0.02^ab	0.18 ± 0.02^b
C20:2n6	0.26 ± 0.03^ab	0.25 ± 0.02^b	0.21 ± 0.03^c	0.29 ± 0.03^a
C20:4n3	1.04 ± 0.14^a	0.96 ± 0.08^ab	0.76 ± 0.11^c	0.80 ± 0.06^bc
C20:4n6	1.48 ± 0.16^a	0.55 ± 0.04^b	0.65 ± 0.04^b	0.56 ± 0.05^b
C18:3n3	2.51 ± 0.34^a	2.11 ± 0.15^bc	1.73 ± 0.17^c	2.18 ± 0.17^ab
C18:4n3	1.15 ± 0.13^a	0.64 ± 0.05^b	0.59 ± 0.05^b	0.53 ± 0.04^b
C20:3n3	0.18 ± 0.02^cb	0.20 ± 0.02^ab	0.15 ± 0.02^c	0.23 ± 0.03^a
C20:5n3 EPA	8.74 ± 0.52^a	3.65 ± 0.24^c	4.59 ± 0.33^b	3.14 ± 0.24^c
C22:6n3 DHA	5.66 ± 0.87^c	13.03 ± 0.56^b	18.49 ± 1.66^a	11.33 ± 0.55^b
∑PUFA	26.51 ± 2.04^c	32.45 ± 0.71^b	36.71 ± 2.47^a	32.18 ± 0.70^b
PUFA/SFA	0.85 ± 0.07^c	1.24 ± 0.07^b	1.35 ± 0.13^a	1.21 ± 0.05^b
∑n3	19.28 ± 1.82^bc	20.59 ± 0.54^b	26.31 ± 2.02^a	18.21 ± 075^c
∑n6	7.11 ± 0.54^d	11.74 ± 0.46^b	10.20 ± 0.95^c	13.71 ± 0.37^a
n3/n6	2.71 ± 0.29^a	1.75 ± 0.08^b	2.58 ± 0.26^a	1.33 ± 0.07^c
n6/n3	0.37 ± 0.04^c	0.57 ± 0.03^b	0.39 ± 0.04^c	0.75 ± 0.04^a
DHA/EPA	0.65 ± 0.08^c	3.57 ± 0.20^b	4.03 ± 0.18^a	3.61 ± 0.26^b

Notes: Data are expressed as mean  ±  SD of triplicate, n = 3 (p < .05). Different superscripts in a row show significant differences between samples (p < .05).

The higher levels of MUFA in the cultured fish (except marine condition) may be explained with the high content of monoenoic fatty acids in the feed of cultured fish as in some previous studies (Alasalvar et al. 2002; Fuentes et al. 2010; Ehsani et al. 2013). However, the lowest level of MUFA was in the marine cage group, although they were fed with a diet including similar contents probably due to marine conditions. PUFA/SFA value is important for human consumption, and minimum value was recommended as 0.45 by the UK Department of Health (HMSO 1994). Özogul and Özogul (2007) reported that PUFA/SFA values for freshwater fish from 0.78 to 1.56, Ehsani et al. (2013) were found PUFA/SFA values 1.32, 1.65 and 1.65 in three different size (low, medium and high weight) cultured rainbow trouts. The present study results agree with the results of Özogul and Özogul (2007) and Ehsani et al. (2013).

Among PUFAs, EPA and DHA are desirable properties in fish because they have an important role in nutrition for human health (Belinda & Roberts 2000). In the present study, DHA, linoleic acid and EPA levels were found as 5.66–18.49%, 5.12–12.68% and 3.14–8.74% in total fatty acids for all groups, respectively. Similar results for different species have also been reported in the literature (Luzia et al. 2003; Özogul & Özogul 2007). Nevertheless, cultured groups contained significantly higher proportions of linoleic acid and DHA than the escaped ones (p < .05). In this case, DHA/EPA level was higher in cultured groups (from 3.57 to 4.03) than the escaped group (0.65). Cultured groups DHA/EPA levels have similar results with some previous studies (Celik et al. 2007; Ehsani et al. 2013). In the other hand, the fatty acid profiles are affected by environmental conditions, size, genetic variation and dietary lipid (Saito et al. 1999; Belinda & Roberts 2000; Gonzales et al. 2006). According to the results of the present study, all groups are good resources of EPA and DHA; especially DHA is remarkably higher in marine cage group. The n3/n6 ratio is a good index for comparing relative nutritional value of fish oils (Pigott & Tucker 1987). However, there is no recommended intake in terms of n3/n6 ratios, but evidence in wild animals and estimated nutrient intake during human evolution suggest a diet ratio 1:1 (Simolopoulos 1989). According to the data of the present study, n3/n6 ratios were 2.72, 1.74, 2.58 and 1.33 for escaped fish, pond, marine cage and Dam Lake groups, respectively. The rainbow trout from all groups appear to be quite nutritious in terms of fatty acids and n3/n6 ratio. However freshwater groups showed a lower n3/n6 ratio than the others. In addition to this, n6/n3 value at maximum 4.0 was recommended by UK Department of Health (HMSO 1994). Values higher than the maximum value are harmful to health and may promote cardiovascular diseases (Moreira et al. 2001). Present study n6/n3 values are varied from 0.37 to 0.75 and these results are in agreement with the results of Valfre et al. (2003), Celik et al. (2007) and Ehsani et al. (2013).

4. Conclusion

This study has shown that rainbow trout from all groups studied are a desirable food item in the human diet when proximate composition and fatty acid profiles are considered. Especially DHA, PUFA levels in cultured groups had relatively better values. Also n3/n6 and n6/n3 ratios are acceptable ranges for human health in all groups. In conclusion, although different groups of rainbow trout obtained from different growing conditions show some differences in proximate composition and fatty acid values, rainbow trout samples of this study are beneficial food sources for the human consumption.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This work was supported in part by the Scientific and Technological Research Council of Turkey [grant number 111O774].