Comparison of meat quality of tench, Tinca tinca, reared in extensive and semi-intensive culture systems

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1 Journal of Applied Ichthyology J. Appl. Ichthyol. 30 (Suppl. 1) (2014), Blackwell Verlag GmbH ISSN Received: June 10, 2013 Accepted: January 5, 2014 doi: /jai Comparison of meat quality of tench, Tinca tinca, reared in extensive and semi-intensive culture systems By D. Ljubojevic 1,M. Cirkovic 1, N. Novakov 2, N. Puvaca 2, N. Aleksic 3, J. Lujic 4 and R. Jovanovic 5 1 Scientific Veterinary Institute Novi Sad, Novi Sad, Republic of Serbia; 2 Faculty of Agriculture, University of Novi Sad, Novi Sad, Republic of Serbia; 3 Faculty of Veterinary Medicine, University of Belgrade, Belgrade, Republic of Serbia; 4 Faculty of Science, University of Novi Sad, Novi Sad, Republic of Serbia; 5 Institute for Science Application in Agriculture, Belgrade, Republic of Serbia Summary The aim of the study was to examine the production parameters and chemical composition of fillets in tench, Tinca tinca L., farmed in one of two systems: an extensive system based on only natural food (NF) available in the fish ponds; and a semi-intensive system based on natural food plus the addition of formulated feeds supplemented with various oils: fish (FO), rapeseed (RO), soybean (SO), or linseed oil (LO). Proper pond preparation resulted in a favourable amount and structure of natural food in all ponds. The rearing system had a significant influence on the tench yield, muscle lipid contents and fatty acid composition, and the supplemented feed influenced the fatty acid composition. The percentages of eicosapentaenoic (EPA) and docosahexaenoic (DHA) acid and total highly unsaturated n-3 fatty acids (n-3 HUFA) were higher in the fish oil and the natural food groups than in the others. The n-3/n-6 ratio in the five dietary treatments was notably variable, and in any case higher in fillets than in the diets. In conclusion, the present study suggests that RO, SO and LO represent effective lipid sources for tench fed formulated diets. On the other hand, tench that fed on naturally available pond foods had a high content of n-3 HUFA in their muscle lipids, but poor growth parameters and low yields. Introduction Highly unsaturated n-3 fatty acids (n-3 HUFA), especially eicosapentaenoic (EPA; 20:5, n-3) and docosahexaenoic (DHA; 22:6, n-3), are very important for the maintenance of physiological functions in humans as well as for the prevention of numerous diseases. Thus, the increase in consumption of food rich in n-3 HUFA, primarily fish being an excellent source of these acids, is highly recommended (Connor, 2000). Fish consumption in Serbia is much lower than advisable (Ruxton et al., 2007), approximately 5 kg per person year 1 (Baltic et al., 2009). For these reasons, it is important to enhance the awareness of its positive influence on health, promote fish meat and change the habits of consumers. These goals can be achieved by broadening the assortment in the fish market, with tench (Tinca tinca L.) being one of the species on offer. Tench is a benthos-eating omnivorous cyprinid species with increasing popularity in the European market. In recent years there has been an increased interest in the tench fish industry in Serbia; in the 1960s it was one of the most prevalent species, but almost disappeared due to loss of natural habitat and the introduction of allochthonous species (Lenhardt et al., 2011). A commercial diet that meets all needs of this species does not exist; the use of feed suitable for other fish species does not necessarily produce the best results (Quiros et al., 2003). As a result of the recent increase in interest, the nutritional needs of tench have been widely investigated. Experiments have been conducted under natural fishpond conditions (Jankowska et al., 2006) and in recirculating aquaculture systems with the aim to determine the source of lipids that have the best influence on the production performance and the chemical and fatty acid composition of the tench meat (Turchini et al., 2007; Zakez s et al., 2010). The cost of fishmeal (FM) and fish oil (FO) on the global market increases continuously, rendering their sustainability as questionable components in fish feed (Naylor et al., 2000). Neither FM nor FO is produced in Serbia; the import thereof raises production expenses and poses a serious problem for the fish farming industry. Thus alternative feed components are necessary to meet these requirements. Plankton and benthos contain high levels of n-3 polyunsaturated fatty acids (n-3 PUFA), including EPA and DHA (Mraz et al., 2012). It is hypothesized in our research that natural food in fishponds may be the source of essential fatty acids for the tench and that this species might be reared successfully on feed of plant origin. The aim of this study is the production possibilities, growth performance, and chemical and fatty acid composition of tench fillets from two farming systems: an extensive system and a semi-intensive system in which fish receive formulated feed supplemented with different oil sources: fish oil (FO), rapeseed oil (RO), soybean oil (SO), or linseed oil (LO). Materials and methods Experimental diets and feeding For experimental purposes 60 g kg 1 (77% of the total fat content) of fish oil (FO diet), rapeseed (RO diet), soybean U.S. Copyright Clearance Centre Code Statement: /2014/ $15.00/0

2 Meat quality of tench in different culture conditions 51 (SO diet), or linseed oil (LO diet) was added in a vacuum coater to the basic formulated feed. Ingredients used for the basic diet were: soybean meal, sunflower meal, corn, wheat flour, yeast, mineral and vitamin mix, and sithetis amino acid (methionine and lysine). The experimental diets contained 254 g kg 1 crude proteins and 77 g kg 1 crude fat (Table 1). Fish were fed manually twice a day, at 8.00 h and h. Feed provided was: % in April, 0.3 1% in May, 1 2% in June, 3% in July and August, and 2 3% in September, with respect to fish biomass and depending on the water temperature, its saturation with oxygen and on the amount of natural food accessible. Fish and facilities Tench larvae were imported from the Czech Republic (Nove Hrady) and stocked in the experimental fishpond in Mosorin ( N, E). In the first season the tench were farmed in a monoculture. The 2-year-old tench were farmed in a polyculture with the common carp (Cyprinus carpio L) (ratio 40 : 60). The survival rate in the second season was rather low (40%) and accompanied by a low weight gain, which may be explained by the food competition with the common carp. In the third year of farming, tench with an average body mass of 200 g were transferred to three fishponds at the same farm; to compare fish groups, each fishpond (1 ha area) was divided with nets into five equal units; each treatment was Table 1 Ingredients and chemical composition of experimental diets supplemented with fish oil (FO), rapeseed oil (RO), soybean oil (SO) or linseed oil (LO) Ingredients (g kg 1 dry diet) FO RO SO LO Soybean meal Fish oil Rapeseed oil Soybean oil Linseed oil Sunflower meal Brewery yeast Wheat flour Corn Methionin Lysine L Vitamin mix Mineral mix Chemical composition (g kg 1 dry diet) Dry matter (DM) Crude protein (CP) Crude fat (CF) Crude ash (CA) NFE Vitamin mix (mg kg 1 of diet): vit. B1, 15; vit. B2, 10; vit. B6, 20; vit. B12, 0.15; vit. K3, 15; inositol, 250; Ca-pantothenic acid, 80; nicotinic acid, 100; folic acid, 1; vit. H (biotin), 1; vit. E, 140; vit. C, 500; vit. A, IU; vit. D3, 6000 IU; choline chloride, 1800, and cellulose used as a carrier. 2 Mineral mix (mg kg 1 of diet): Cu 20, Fe 40, Mn 30, Se 0.4, Zn 125, and cellulose used as a carrier. 3 NFE, nitrogen-free extract, g kg 1 DM = 100 (CP + CF + CA). repeated three times. Water was supplied from a local well. In the first group the tench were fed with natural food only and the stocking density was 1200 individual ha 1. The remaining four groups were farmed at a higher density (2500 individual ha 1 ) and, apart from the natural food (NF group), were given feed supplemented with 6% (77% of the total dietary fat content) of various oils: fish oil (FO group), rapeseed oil (RO group), soybean (SO group), or linseed oil (LO group), which were the basic fat sources in the feed. The ponds were stocked in April and harvested in October, with the experiment lasting for 180 days. Certain agrotechnical procedures were employed: the fishponds were drained and dried completely in winter and the pond bed ploughed. Disinfection was completed with lime (1000 kg ha 1 ) followed by fertilisation with cow and sheep dung at 2000 kg ha 1. Fertilisation continued throughout the production period by adding organic fertiliser every second week; thus, the total amount of fertiliser was increased to 4000 kg ha 1 per season, as described by Cirkovic et al. (2012). Continuous aeration was provided with one Venturi aerator per fishpond. Water quality and collection of plankton and benthos Basic parameters of water quality were measured in the morning (9.00 h) every second week. Hydrochemical monitoring was performed with a handheld meter (WTW 315i, Germany). Water quality parameters were roughly the same in all experimental units. Dissolved oxygen content varied substantially in all experimental groups, ranging from 1.4 to 14.8 mg L 1. The ph varied from 7.04 to 8.62, and water temperature from 15 to 28.7 C. Fatty acid composition of natural food in the fishponds was analysed in both plankton and benthos 15 days prior to harvesting. Plankton was sampled with a mesh with apertures of 30 lm, from three places in each experimental unit in order to make composite samples. Benthos was collected with an excavator, and flushed with water through smallaperture sieves until only detritus and pond bottom fauna remained, which were picked out with tweezers and packed into glass bottles. Chemical analyses In each experimental unit dorsal muscles from three tench were sampled (nine fish from each feeding group) and analysed. Chemical composition of the fish and fish feed Water content in fish fillets and feed was measured after drying at C until their weight was constant for 24 h (SRPS ISO 1442:1997). The level of crude proteins was assessed with the Kjeldahl method (Manual book; KjeltecAuto 1030 Analyzer, Tecator, Sweden), and crude ash was determined after burning at C (SRPS ISO 936:1998). Crude fats from the fish muscles and feed were extracted with a Soxhlet extractor (SRPS ISO 1443:1997).

3 52 D. Ljubojevic et al. Lipid extraction and fatty acid analysis Fatty acid composition analysis was performed as described by Spiric et al. (2010). Briefly, the fatty acids (FA) in fish fillets, feeds and samples of natural food were determined following the extraction of total lipids by means of accelerated solvent extraction (ASE) on Dionex ASE 200. Homogenised samples mixed with diatomaceous earth were extracted with the mixture of n-hexane and isopropanol (60 : 40 v/v) in 33-ml extraction cells, at 100 C and under the nitrogen pressure of 10.3 MPa. Evaporated at 50 C (Dionex SE 500 evaporator; Dionex, Sunnyvale, CA) the extracts were then processed for further analysis. Fatty acid methyl esters were prepared by trans-esterification with trimethylsulfonium hydroxide. Methyl esters were separated on a polar cyanopropylaril column HP-88 (100 m column length, 0.25 mm diameter, 0.20 lm film thickness; Agilent J&W Scientific, Orangevale, CA, USA), in a programmed temperature range, on a capillary gas chromatograph (Shimadzu 2010, Kyoto, Japan), with a flame ionisation detector. The carrier gas was nitrogen, with a flow rate of 1.33 ml min 1 and split ratio 1 : 50. The identification of fatty acid methyl esters was based on their retention times compared with the standard, Supelco 37 Component FAME Mix (Supelco, Bellefonte, PA). The content of each fatty acid was expressed as a percentage of the total. Statistical analysis Statistical analysis of the data was performed with Statistica 10 (StatSoft Inc., Tulsa, OK). Single factor analysis of variance (ANOVA) was applied, and when statistically significant, differences were confirmed (P < 0.05) and Tukey s post hoc test was applied. Table 2 Fatty acid composition of experimental diets supplemented with fish oil (FO), rapeseed oil (RO), soybean oil (SO) or linseed oil (LO) Fatty acid (%) FO RO SO LO C14: C15: C16: C16: C18: C18:1cis C18:1 cis C18:2n C18:3n C18:3n C20: C20: C20: C20:3n C20:3n C22: C22: C24:1n C20:4n C24: C20:5n C22:6n SFA MUFA PUFA n n n-3/n SFA, saturated fatty acids; MUFA, monounsaturated fatty acids; PUFA, polyunsaturated fatty acids; n-3 PUFA, polyunsaturated fatty acids from the n-3 family; n-6 PUFA, polyunsaturated fatty acids from the n-6 family. Results Diet composition In all tested diets, palmitic acid was the dominant saturated fatty acid (SFA). The inclusion of vegetable oils (VO) resulted in the reduction of palmitic acid and consequently the total SFA by 1.4- (SO diet) to 1.8-fold (RO diet) compared to the FO diet. The total monounsaturated fatty acids (MUFA) almost doubled in the RO diets compared to the other diets, largely due to the high amount of oleic acid (OA). Among polyunsaturated fatty acids (PUFA), dominant were: linoleic acid (LA), which was 1.9-fold higher in the SO diet than in the RO diet and 2.6-fold higher than in the LO and FO diets; and a-linolenic acid (ALA), which was several-fold higher in the LO diet than in the other diets. The contents of EPA and DHA were detected in only the FO diet. The n-3/n-6 PUFA ratio was 2, 1.2, 0.3, and 0.2 for the LO, FO, RO and SO diets, respectively (Table 2). Production performance Total fish meat yield, average body mass at harvesting and average daily weight gain were significantly higher in fish fed with formulated feed than in fish fed only natural feed. However, there were no significant differences between groups fed feed supplements. At the end of the farming period in October, the average final body mass of tench fed with natural feed was only 300 g, whilst in the semiintensive systems it reached 420 g. The harvest weight was 288 kg ha 1 in the extensive and approx. 860 kg ha 1 in semi-intensive systems (Table 3). Thus, the addition of formulated feed resulted in a three-fold increase in harvest weight. Chemical composition of fish meat The chemical composition of tench fillets is given in Table 4. The experimental diets had no significant influence on the content of water, proteins, fats or ash in fillets, unlike the farmed system (P < 0.05). The moisture content in fillets was significantly higher in the extensive (82.4%) than in the semiintensive rearing system ( %). Lower water content was accompanied by higher fat content, which was significantly higher in the semi-intensive (3.3%) than in the extensive system (0.8%). The protein content was higher in the semi-intensive system than in the extensive system (18.9 vs %). Ash content was practically unchanged in all experimental groups (approx. 1%).

4 Meat quality of tench in different culture conditions 53 Table 3 Production performance of tench reared on natural food (NF) or fed experimental diets supplemented with fish oil (FO), rapeseed oil (RO), soybean oil (SO) or linseed oil (LO) NF FO RO SO LO IBW(g) FBW (g) b a a a a SR (%) SD (kg ha 1 ) HD (kg ha 1 ) WG (g per fish) b a a a a DGR (g day 1 ) b a a a a SGR (% day 1 ) b a a a a FCR (g g 1 ) All values are mean SD (n = 3). Groups with different letter indexes in the same row statistically significantly different (P < 0.05); IBW, initial body weight; FBW, final body weight; SR, survival rate = (Final fish number/initial fish number) 9 100; SD, Stocking density; HD, Harvesting density; WG, weight gain = [Final body weight (g) initial body weight (g)] [initial body weight (g)] 1 ; DGR, Daily growth rate = [final body weight (g) initial body weight (g)] 9 time 1 of rearing (days); SGR, specific growth rate = [ln final body weight (g) ln initial body weight (g)] 9 time 1 of rearing (days); FCR, feed conversion ratio = total food supply (g) 9 [final fish biomass (g) initial fish biomass (g)] 1. Table 4 Chemical composition in fillets of tench reared on natural food (NF) or fed experimental diets supplemented with fish oil (FO), rapeseed oil (RO), soybean oil (SO) or linseed oil (LO) Variable NF FO RO SO LO Moisture (g kg 1 ) a b b b b Crude protein (g kg 1 ) b a a a a Crude lipid (g kg 1 ) b a a a a Crude ash (g kg 1 ) a a a a a All values are mean SD (n = 9). Groups with different letter indexes in the same row are statistically significantly different (P < 0.05). Fatty acid composition of plankton and benthos In the total lipid of natural food samples the proportion of SFA, MUFA and PUFA ranged from 27.6 to 28.4%, , and % of total identified fatty acids (FA), respectively (Table 5). The content of total SFA as well as of palmitic acid, which was the most prevalent SFA in all groups, did not vary considerably among the experimental units. OA was the most prevalent MUFA in all units. Among n-6 PUFA, the most prevalent was arachidonic acid (AA), and relatively high percentages of LA and ALA were also detected. Analysis of the natural food (Table 5) disclosed that it was rich in EPA and DHA, with a high n-3/ n-6 ratio. Fatty acid composition of tench fillets The shares of all fatty acid groups analysed were determined significantly according to the farming system and diet applied. The FO group contained the highest levels of SFA ( ), whilst the lowest was measured in the NF group ( ); the most prominent differences concerned stearic acid, which was almost three times higher in the FO than in the NF group ( vs ). The content of MUFA in the fillets of tench from the RO group was significantly higher than in the other groups (37.4% vs %). The OA content in the RO group (28.4%) was significantly higher than in the other dietary groups (P < 0.05), while the content of LA in the fillets of fish from the SO group ( %) was significantly higher than that in the fish from the RO (12.9%), LO (7.7%), and FO (7.9%) (P < 0.05) groups. The contents of OA, LA and ALA in the tench fillets were lower than determined in the VO diets analysed, and ARA, EPA, docosapentaenoic acid (DPA) and DHA were several-fold higher than the content in the VO diets. Increased levels of the intermediates gamma-linolenic acid (18:3n-6), eicosadienoic acid (20:2n-6), dihomo-y-linolenic acid (20:3n-6), and eicosatetraenoic acid (20:4n-3) in fish fed VO diets were detected in comparison with the other diets tested (Table 6). Regarding AA, its content in fish fillets from the extensive farming system was incomparably higher (5.14%) than in those fed additional feed ( %). Very high percentages of EPA and DHA were detected in the FO ( ; , respectively) and NF groups ( ; , respectively). However, no statistically significant differences were noted in the contents of ARA, EPA, or DHA in the fillets of tench fed the diets supplemented with VO (P > 0.05; Table 6). The ratio of n-3/n-6 ranged from 2.6 (LO group) to 0.6 (SO group) (P < 0.05). Discussion There were no adverse effects of the diet on growth performance and fillet proximate composition of the tench after 180 days of feeding. Our findings are in agreement with previous studies on cyprinid species (goldfish, Carassius auratus;

5 54 D. Ljubojevic et al. Table 5 Fatty acid composition of natural food from experimental units in which tench reared on natural food (NF) or fed experimental diets supplemented with fish oil (FO), rapeseed oil (RO), soybean oil (SO) and linseed oil (LO) Fatty acid (%) NF FO RO SO LO 12: : : : : : : SFA : :1cis :1cis : MUFA :2n :3n :3n :2n :3n :3n :4n :5n :5n :6n PUFA n n n-3/n N-6/n PUFA/SFA USFA/SFA All values are mean SD (n = 3). Groups with different letter indexes in the same row are statistically significantly different (P < 0.05); SFA, saturated fatty acids; USFA, unsaturated fatty acids; MUFA, monounsaturated fatty acids; PUFA, polyunsaturated fatty acids; n-3 PUFA, polyunsaturated fatty acids from the n-3 family; n-6 PUFA, polyunsaturated fatty acids from the n-6 family. common carp, Cyprinus carpio; grass carp, Ctenopharyngodon idella; tench, Tinca tinca) in which VO did not affect the growth parameters or chemical composition of these fish over either short or long-term periods (Wiegand, 1993; Steffens et al., 1995; Pozernick and Wiegand, 1997; Du et al., 2008; Zakez s et al., 2010). The results obtained by Jankowska et al. (2006) suggest that the tench fed formulated diets deposit not only perivisceral and peritoneal fat but also muscle fat. Thus, tench fed pelleted feed in the current research deposited energy reserves in the muscles, apart from the peritoneal cavity, which led to higher fat contents in the muscle tissue when compared to the NF group. The content of SFA in tench fillets was higher than that in the VO diets tested; in the fillets of the tench fed diets supplemented with VO, the SFA increased sufficiently so that it was comparable to the values noted in the FO group. Accordingly, it seems that the quantitative and qualitative content of SFA in the experimental diets supplemented with VO was sufficient to maintain this group of fatty acids at a level comparable to that noted in the FO group fish. This is a noteworthy finding, since changes in the quantity of palmitic acid, the most abundant in SFA, plays a key role in the synthesis of phospholipids, can cause massive accumulation of free lipid droplets in enterocytes, and can alter fatty acid profiles in membranes (Olsen et al., 2003). It appears that tench make good use of OA as a source of metabolic energy. Lower levels of this FA in the muscle tissues, regardless of the dietary treatment, might mark highly active mitochondrial enzymes oxidizing FA (Torstensen et al., 2000). Other fatty acids that may have been used as energy sources include arachic acid (20:0), cetolic acid (22:1n-11) and nervonic acid (24:1n-9). This conclusion is based on the observation of lower contents of these FA in the fillet lipids than in respective dietary lipids of fish. The ratio of LA and ALA content in the muscles to that in the feeds was similar to that for OA. This can be related to the fact that mitochondrial b-oxidation is of specific importance to muscle tissues (Nanton et al., 2003). Higher contents of LA and ALA in the fillets of freshwater and marine fish species fed feeds with VO were also noted by Pozernick and Wiegand (1997), Regost et al. (2003), and Turchini et al. (2007). The intermediate gamma-linolenic (18:3n-6), eicosadienoic (20:2n-6), dihomo-c-linolenic (20:3n-6) and eiocosatrienoic (20:3n-3) acids were detected in tench fillets in all groups. Since vegetable oils are deprived of these fatty acids which are part of the biosynthetic pathways of n-6 and n-3 HUFA, this result

6 Meat quality of tench in different culture conditions 55 Table 6 Fatty acid composition (% of sum of all fatty acids) in fillets of tench reared on natural food (NF) or fed experimental diets supplemented with fish oil (FO), rapeseed oil (RO), soybean oil (SO) or linseed oil (LO) Fatty acid (%) NF FO RO SO LO 12: a a a a 14: b a a a a 15: c a b b b 16: ab a b a a 17: b a a a a 18: c a b b b 20: a a a a a SFA c a b b b 16: c c b a b 18:1cis c c a b d 18:1cis b a a a a 20: a b b b b 22:1n a b b 0 MUFA c c a b d 18:2n c cd b a d 18:3n c b b a b 18:3n b d c c a 20:2n b b b a b 20:3n b b a a a 20:3n a a b b a 20:4 n a a c c b 20:4n a b c c c 20:5n a a b b b 22:5n a b c c c 22:5n a b b b 22:6n a b c c c PUFA a b b a a n c e b a d n a b c c a n-3/n a a b c a N-6/n c c b a c PUFA/SFA a c bc b a USFA/SFA a c b b b All values are mean SD (n = 9). Groups with different letter indexes in the same row are statistically significantly different (P < 0.05); SFA, saturated fatty acids; USFA, unsaturated fatty acids; MUFA, monounsaturated fatty acids; PUFA, polyunsaturated fatty acids; n-3 PUFA, polyunsaturated fatty acids from the n-3 family; n-6 PUFA, polyunsaturated fatty acids from the n-6 family. features adaptive attempts to moderate HUFA deficiencies. When the content of ALA or LA in the diets was increased, eicosatrienoic acid 20:3n-3 or eicosadienoic acid 20:2n-6 content in muscle was also increased. A similar phenomenon was observed in Atlantic salmon (Tocher et al., 2002). A report by Du et al. (2008) showed that grass carp could synthesize n-3 HUFA from dietary ALA. In the present study, although VO diets did not contain HUFA, some n-6 and n-3 HUFA, including EPA and DHA, was also found in muscle tissues of tench fed with VO diets for 180 days, excluding essential fatty acids (EFA) deficiency and suggesting that tench was able to synthesize n-3 HUFA from the ALA in VO diets. In addition, in muscle tissue of VO groups, the presence of AA and n-6 docosapentaenoic acid (22:5n-6, DPAn-6) arising from LA supports this theory. Further, this can be explained by a phenomenon observed in several fish species: some fatty acids may be selectively deposited or used depending on their degree of insufficiency in feed; this often happens with DHA, which, because of its very important physiological role, accumulates in the body reaching considerably higher levels than in the diet (Ng et al., 2003; Regost et al., 2003; Turchini et al., 2007). The mechanism of the selective deposition of fatty acids may originate from the specifics of the fatty acyl transferases for DHA, or also from its lesser susceptibility to the b-oxidation process (Bell et al., 2001). However, in the FO group, n-3 PUFA and especially EPA and DHA decreased in fish tissues. The decrease in n-3 HUFA might also derive from peroxisomal, and then mitochondrial b-oxidation. DHA and EPA are major components of membrane phospholipids (Henderson and Tocher, 1987) and their contents in the body are normally relatively stable, which suggests that EPA and DHA oxidation is prevented as much as possible. In the present study, their relative decrease in the FO group might be considered as the result of a protection mechanism to avoid possible adverse effects of the accumulation of EPA and DHA. In our study, trends in levels of docosapentanoic acid (22:5n-3, DPA) and DHA in tench fillets relative to their respective dietary contents (DPA and DHA increased in fillets) suggested the bioconversion of EPA to DHA. A similar trend of the selective deposition was seen in n-6 FA in which improved retention was seen in AA. The changes in muscle fatty acid composition indicate selective utilization or retention of individual fatty acids.

7 56 D. Ljubojevic et al. Thus, the SFA, EPA, and the total n-3 PUFA were utilized to a higher extent by the fish fed the FO diet compared to the VO groups. DHA appeared to be less utilized in the FO group but accumulated in the muscle in the VO groups. On the contrary, OA, LA and ALA were found in higher concentrations in the muscle in the FO group but were also utilised in the RO, SO and LA groups, respectively. It has been shown previously when specific fatty acids are in abundance in the diets that they are selectively utilized for energy production via b-oxidation, and for desaturation and elongation. In contrast, when FA, and especially n-3 HUFA, are limited in the diet they are retained or deposited in the muscle or in other tissues (Bell et al., 2001; Torstensen et al., 2004). It is a natural consequence that the main repercussion of higher DHA and lower LA contents in tench fillets from all dietary groups was an increased value of the n-3/n-6 ratio in comparison to the values of this ratio in the diets. However, in the VO groups, it was significantly lower than that detected in the FO group. Fish species differ in their ability to synthesize long chain HUFA from the shorter precursors, EPA and DHA from ALA and AA from LA (Sargent et al., 2002). The metabolic effectiveness of this process in freshwater fish species is generally greater than it is in marine species (Steffens, 1997; Sargent et al., 2002). Fish able to synthesize long chain HUFA include the Cyprinidae, of which tench is one by classification, and able to synthesize HUFA like other Cyprinidae according to the present results. The abundance of LNA and LA provided by VO diets coupled with some n-3 HUFA from natural food seemed to meet the EFA needs of the tench under the natural atmospheric conditions of this study. This was probably due to the ability of tench to selectively retain DHA and possibly bioconvert LNA to EPA and DHA, EPA to DHA, and LA to AA. Therefore, results obtained in the current study suggest that tench have high tolerance to diets that differ noticeably in FA composition. Simopoulos (2002) suggested that the n-6/n-3 ratio should not exceed 4 in human diets, which was found in fish from all experiment groups. Given the results, the high price of FO and its tendency to oxidate and with consequential storage difficulties, it can be concluded that all of the assessed vegetable oils showed satisfactory effects and that the use of each is justifiable. When deciding on their use in tench feed the choice should be based on their cost and availability on the market. 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