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Abstract

Duplicate groups of Atlantic salmon parr were fed diets containing either fish oil (FO), rapeseed oil (RO), linseed oil (LO) or linseed oil supplemented with arachidonic acid (20:4n-6; AA) (LOA) from October (week 0) to seawater transfer in March (week 19). From March to July (weeks 20-34) all fish were fed a fish oil-containing diet. Fatty acyl desaturation and elongation activity in isolated hepatocytes incubated with [1-14 C]18:3n-3 increased in all dietary groups, peaking in early March about one month prior to seawater transfer. Desaturation activities at their peak were significantly greater in fish fed the vegetable oils, particularly RO, compared to fish fed FO. Docosahexaenoic acid (22:6n-3:DHA) and AA in liver and gill polar lipids (PL) increased in all dietary groups during the freshwater phase whereas eicosapentaenoic acid (20:5n-3;EPA) increased greatly in all groups after seawater transfer. The AA/EPA ratio in tissue PL increased up to seawater transfer and then decreased after transfer. AA levels and the AA/EPA ratio in gill PL were generally higher in the LOA group. The levels of 18:3n-3 in muscle total lipid were increased significantly in the LO, LOA and, to a lesser extent, RO groups prior to transfer but were reduced to initial levels by the termination of the experiment (week 34). In contrast, 18:2n-6 in muscle total lipid was significantly increased after 18 weeks in fish fed the diets supplemented with RO and LO, and was significantly greater in the FO and RO groups at the termination of the experiment. Gill PGF production showed a large peak about two months after transfer to seawater. The production of total PGF post-transfer was significantly lower in fish previously fed the LOA diet. However, plasma chloride concentrations in fish subjected to a seawater challenge at 18 weeks were all lower in fish fed the diets with vegetable oils. This effect was significant in the case of fish receiving the diet with LOA, compared to those fed the diet containing FO. The present study showed that during parr-smolt transformation in Atlantic salmon there is a pre-adaptive increase in hepatocyte fatty acyl desaturation/elongation activities that is controlled primarily by environmental factors such as photoperiod and temperature but that can also be significantly modulated by diet. Feeding salmon parr diets supplemented with rapeseed or linseed oils prevented inhibition of the desaturase activities that is induced by feeding parr diets with fish oils and thus influenced the smoltification process by altering tissue PL fatty acid compositions and eicosanoid production. These effect, in turn, had a beneficial effect on the ability of the fish to osmoregulate and thus adapt to salinity changes.

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“…In a similar study on sea bass, Izquierdo et al (2003) reported that the replacement of 60% of fish oil with linseed oil (n-3 PUFA-rich), soybean oil (n-6 PUFA-rich), rapeseed oil (MUFA-rich) and a mixed-blend did not negatively impact on fish performance, when high lipid content (25%) diets were evaluated. As in many instances, this result is similar to a number of the other earlier studies with European sea bass and salmonids where the complete replacement of dietary fish oil with alternative vegetable oils has resulted in no affect on fish growth performance (Guillou et al, 1995;Tocher et al, 2000;Torstensen et al, 2000;Bell et al, 2001, Mourente et al, 2005bTurchini et al, 2011b;Eroldoğan et al, 2012;Eroldoğan et al, 2013).…”

UVB Radiation Variably Affects n‐3 Fatty Acids but Elevated Temperature Reduces n‐3 Fatty Acids in Juvenile Atlantic Salmon (Salmo salar)

“…It has been suggested that lipid metabolism in salmon may influence the process of parr-smolt transformation, ultimately affecting successful seawater adaptation [41]. For example, hepatocyte fatty acyl desaturation/elongation activities in juvenile Atlantic Salmon are primarily controlled by environmental factors such as temperature and photoperiod but diet also plays a role [42]. Thus, prior to smoltification the FA composition of parr salmon is influenced by, (1) diet, (2) their ability to modify that dietary FA input via catabolism and by desaturation and elongation pathways and, (3) the influence of water temperature and photoperiod (and possibly also the levels of UV to which they are exposed) on the latter processes.…”

“…Thus, prior to smoltification the FA composition of parr salmon is influenced by, (1) diet, (2) their ability to modify that dietary FA input via catabolism and by desaturation and elongation pathways and, (3) the influence of water temperature and photoperiod (and possibly also the levels of UV to which they are exposed) on the latter processes. During the period leading up to smoltification, tissue FA composition changes from a typical freshwater pattern, which is relatively low in long-chain polyunsaturated fatty acids (LC-PUFA), to a more typical marine pattern which is relatively rich in LC-PUFA such as 20:5n-3 and 22:6n-3 [42]. Thus, the elevation of n-3 LC-PUFA in salmonids is considered a key pre-adaptive response to seawater entry [43,44].…”

Fatty acid composition, osmolality, Na + , K + ‐ATPase activity, cortisol content and antioxidant status of rainbow trout ( Oncorhynchus mykiss ) in response to various dietary levels of eicosapentaenoic acid and docosahexaenoic acid

“…The demand for essential fatty acids varies in different species (Glencross, 2009). It has been reported that the requirements for polyunsaturated fatty acids (PUFA), especially docosahexaenoic acid (DHA, C22:6n3) and eicosapentaenoic acid (EPA, C20:5n3), in Salmonidae increase during the parr-smolt transition (smoltification) (Ackman & Takeuchi, 1986;Tocher et al, 2000). Furthermore, previous studies have revealed that the accumulation of enough n-3 PUFA, especially DHA and EPA, is a prerequisite for smoltification because Salmonidae require sufficient DHA and EPA to cope with a series of physiological responses during seawater acclimation (Bell et al, 1997;Li & Yamada, 1992;Sheridan, Allen, & Kerstetter, 1985).…”