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Monounsaturated fatty acids (MUFA)-rich and n-6 polyunsaturated fatty acid (n-6 PUFA)-rich vegetable oils are increasingly used as fish oil replacers for aquafeed formulation. The present study investigated the fatty acid metabolism in juvenile European sea bass (Dicentrarchus labrax, 38.4 g) fed diets containing fish oil (FO, as the control treatment) or two different vegetable oils (the MUFA-rich canola/rapeseed oil, CO; and the n-6 PUFA-rich cottonseed oil, CSO) tested individually or as a 50/50 blend (CO/CSO). The whole-body fatty acid balance method was used to deduce the apparent in vivo fatty acid metabolism. No effect on growth performance and feed utilization was recorded. However, it should be noted that the fish meal content of the experimental diets was relatively high, and thus the requirement for n-3 long-chain polyunsaturated fatty acid (n-3 LC-PUFA) may have likely been fulfilled even if dietary fish oil was fully replaced by vegetable oils. Overall, relatively little apparent in vivo fatty acid bioconversion was recorded, whilst the apparent in vivo β-oxidation of dietary fatty acid was largely affected by the dietary lipid source, with higher rate of β-oxidation for those fatty acids which were provided in dietary surplus. The deposition of 20:5n-3 and 22:6n-3, as % of the dietary intake, was greatest for the fish fed on the CSO diet. It has been shown that European sea bass seems to be able to efficiently use n-6 PUFA for energy substrate, and this may help in minimizing the β-oxidation of the health benefiting n-3 LC-PUFA and thus increase their deposition into fish tissues.

The purpose of this study was to investigate the effect of freshwater rearing on the fatty acid profiles of the whole body and muscle tissue of the European sea bass (Dicentrarchus labrax). Half of initial fish were gradually acclimated to freshwater (FW) kept at the same temperature to salt water and grown in same conditions as their counterparts in saltwater (SW). The decrease in salinity caused an increase in the percentages of 18:1n -9, 24:1n -9, 18:3n -3, 18:2n -6 and decrease in the percentages of 14:0, 15:0, 20:0, 21:0, 20:5n -3 and 22:6n -3 both in the whole body and in the muscle tissue fatty acid profiles. The lipids of FW-reared fish contained significantly (P \ 0.01) higher percentages of 18:2n -6 and 18:3n -6 than that of SW-reared fish. However, percentages of 20:5n -3 and 22:6n -3 fatty acids decreased significantly (P \ 0.05) compared with those of salt water-reared European sea bass. There was a clear trend of decrement in the percentages of n -3 PUFA fatty acids due to the decrease in water salinity. However, the percentages of n -6 PUFA fatty acids were also increased with the decrease in water salinity. We concluded that the FW acclimation is followed by changes in certain lipid classes of sea bass muscle tissue and whole body samples. n -3/n -6 PUFA ratios were characteristic to previously reported ratios for both FW-and SW-reared European sea bass. In addition, EPA/DHA ratios were basically similar for the fish reared in both SW and FW indicating the equal nutritional value of the final products in terms of providing PUFA's for human nutrition.

Triplicate groups of 20 European sea bass (35 g) were fed five diets in which the added lipid was 100% fish oil (FO), 40% (CSO40), 60% (CSO60), 80% (CSO80) and 100% (CSO100) refined cottonseed oil (CSO), for a period of 120 days. Overall fish growth, feed conversion ratio and protein utilization were unaffected by dietary treatment, but hepatosomatic and visceral fat indexes increased with increasing dietary CSO. Fillet fatty acid composition of total lipids reflected the fatty acids in the test diets. The monounsaturated fatty acids were significantly higher in fillet of fish fed diet FO, CSO40 and CSO60 compared to other treatments while saturated and polyunsaturated fatty acids (PUFA) were not affected by the dietary treatment. Some fatty acids (18:0, 18:1n-9, 20:5n-3 and 22:6n-3) were present in higher concentration in fillet lipid than in the CSO100 dietary lipid indicating accumulation in fillet relative to test diets. Retention of n-3 LC-PUFA within the fillet was increasingly inefficient among fish fed increasing levels of FO. Thus, this study suggests that CSO can be considered as a relatively effective substitute for fish oil in European sea bass (35 g) in terms of growth performances and feed efficiency as far as fish meal is present in the diet.Keywords: Dicentrarchus labrax, linoleic acid, fish oil replacement, aquafeed, vegetable oils. Avrupa Deniz Levreği Yem Formulasyonunda Pamuk Tohumu Yağının Potansiyel olarak Balık Yağına İkamesi ÖzetYirmi adet Avrupa deniz levreği bireyi (35 g) 120 gün boyunca %100 balık yağı (BY), %40 Pamuk Toplama Yağı(PTY40), %60 (PTY60), %80 (PTY80) ve %100 (PTY100) oranlarında rafine edilmiş pamuk tohumu yağı (PTY) ilave edilmiş beş yemle beslenmiştir. Balık büyümesi, yem çevirim oranı ve protein değerlendirme yem uygulamasıyla değişmezken, hepatosomatik ve visseral yağ indeksi yemdeki PTY'nin seviyesiyle artmıştır. Fileto toplam lipit yağ asidi kompozisyonu test yemlerinin yağ asidi kompozisyonuna yansımıştır. BY, PTY40 ve PTY60 ile beslenen bireylerin fileto tekli doymamış yağ asidi miktarı diğer gruplara göre daha yüksek iken doymuş (DYA) ve çoklu doymamış yağ asitleri (ÇDYA) miktarı etkilenmemiştir. Test yemleri göz önüne alındığında, PTY100 grubu bireylerinin fileto yağlarında bazı yağ asitleri (18:0, 18:1n-9, 20:5n-3 ve 22:6n-3) yüksek oranda birikmiştir. Filetodaki uzun zincirli n-3 ÇDYA'lerin birikimi artan balık yağı oranı ile beslenen balıklarda yetersizdir. Bu çalışma, yemlerde balık unu kullanıldığı sürece, büyüme performansı ve yem değerlendirme açısından Avrupa deniz levreği (35 g) için PTY'nin balık yağını ikame edilebileceğini önermektedir.Anahtar Kelimeler: Dicentrarchus labrax, linolenik asit, balık yağı değişimi, yem, bitkisel yağlar.