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‘Finishing’ feeds for carnivorous fish and the fatty acid dilution model

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Page 1: ‘Finishing’ feeds for carnivorous fish and the fatty acid dilution model

SHORT COMMUNICATION

‘Finishing’ feeds for carnivorous fish and the fatty acid

dilution model

Malcolm JoblingNFH, University of Troms�,Troms�, Norway

Correspondence:M Jobling, NFH, University of Troms�, 9037 Troms�, Norway. E-mail: [email protected]

Human nutritionists are recommending a higher in-take of n-3 highly unsaturated fatty acids (n-3 HU-FAs), especially at the expense of saturates and n-6fatty acids, due to the bene¢cial e¡ects of n-3 HUFAsfor human health (Connor 2000; Williams 2000).Fishes are the major source of n-3 HUFAs in the hu-man diet, and the promotion and marketing of ¢sh isoften based upon their high concentrations of thesefatty acids.Marine ¢sh oils have beenwidely used as the main

lipid source in feeds for farmed ¢sh because theyhavebeen readily available, relatively cheap and a goodsource of the n-3 HUFAs. The demand for ¢sh oilshas increased over recent years although productionhas remained static, and the current trend is towardsthe replacement of ¢sh oils by vegetable oils in feedsfor farmed ¢sh. Most vegetable oils are rich in unsat-urated18C fatty acids (18:1n-9,18:2 n-6,18:3 n-3) butare poor sources of n-3 HUFAs, so the £esh of farmed¢sh given feeds containing high concentrations ofvegetable oils may contain a limited amount of n-3HUFAs. One way to boost the n-3 HUFA concentra-tion of farmed ¢sh may be to use ‘¢nishing’ feeds con-taining ¢sh oil. The fatty acid composition of the ¢sh£esh could thus be altered to meet the consumer ex-pectation of a product that is rich in n-3 HUFAs andlow in n-6 fatty acids (e.g. Jobling, Larsen, Andreas-sen, Olsen & Sigholt 2002; Bell, McGhee, Campbell& Sargent 2003a; Bell, Tocher, Henderson, Dick &Crampton 2003b; Glencross, Hawkins & Curnow2003; Robin, Regost, Arzel & Kaushik 2003).Jobling et al. (2002) recently described the manipu-

lation of fatty acid compositions of tissues (¢llet, vis-cera and carcass) of Atlantic salmon, Salmo salar L.,post smolt using a ¢sh oil-based ‘¢nishing’ feed given

to ¢sh that had been primed with feeds containingvegetable oils. Jobling (2003; 2004) later assessed thechanges in tissue pro¢les of the18C fatty acids usinga dilution model (Robin et al. 2003). In the dilutionmodel it is assumed that the initial fatty acid contentbecomes diluted as the ¢sh growand deposit increas-ing amounts of fat, and that dietary fatty acids aredeposited in the tissues without in£uencing the me-tabolism or turnover of existing fatty acid stores.There is, however, considerable potential for metabol-ism, turnover and modi¢cation of fatty acids viachain elongation and desaturation pathways (Sar-gent,Tocher & Bell 2002). Given this, it is perhaps sur-prising that the dilution model gave accuratepredictions of the changes in tissue 18C fatty acidsfollowing the change from feeds containingvegetableoils to those with ¢sh oil (Jobling 2003; 2004), andmore work is needed to con¢rm or refute these¢ndings.Results of two recent studies on ‘¢nishing’ feeds

given to carnivorous ¢sh species (Bell et al. 2003b;Glencross et al.2003) can be used to evaluate the abil-ity of the dilution model to predict the changes in thepercentages of 18C fatty acids that occur following achange in feed oils from vegetable oils to ¢sh oil. Theauthors (Bell et al. 2003b; Glencross et al. 2003) didnot undertake such evaluations, and I have re-analysed the data using the dilution model. Before Ipresent the results of these analyses I will providebackground information about the dilution model,the method of analysis and the way in which theresults of such an analysis may be interpreted.The dilutionmodel (Robin et al.2003) describes the

change in the percentage of a given fatty acid in atissue when increasing quantities of fatty acids are

Aquaculture Research, 2004, 35, 706^709 doi:10.1111/j.1365-2109.2004.01067.x

706 r 2004 Blackwell Publishing Ltd

Page 2: ‘Finishing’ feeds for carnivorous fish and the fatty acid dilution model

added. In other words, when there is a change to anew type of feed, the fattyacid pro¢les of the‘test’ ¢shchange over time as more fatty acids are deposited inthe tissues.The fattyacid pro¢les of the‘test’¢sh grad-ually come to resemble those of ¢sh that have beenfed this feed for prolonged periods (the latter are a‘ref-erence’group that provide the basis for comparison).Themathematical description of this dilution e¡ect is

PT ¼ PR þ ½ðPI � PRÞ=ðQT=QIÞ�

PT is the percentage of a fattyacid in the tissues of the‘test’ ¢sh at time T, PI is the initial percentage and PRis the percentage of the fatty acid in the ‘reference’group. QI is the initial total amount of fatty acids (ortotal fat) present andQT is the amount present at timeT. PT values are predicted percentages of fattyacids inthe tissues at given times, and these can be comparedwith the values observed when tissue fatty acid com-positions are analysed directly. Such a comparison isshown in Fig.1, which provides a summary of the re-sults obtained when ¢sh oil-based ‘¢nishing’ feedswere fed to post smolt Atlantic salmon (Jobling 2003;2004). The plot shows a high degree of concordancebetween the tissue (¢llet, viscera and carcass) per-centages of18C fatty acids (18:1n-9,18:2 n-6 and18:3n-3) predicted using the dilution model (PT) and thevalues obtained from fatty acid analyses of the sametissues. This resulted in a regression very similar to a‘line of equality’ (regression line that passes through

the origin and has a slope of 1), leading to the inter-pretation that the dilution model provided an excel-lent prediction of the changes in the tissue pro¢les ofthe18C fatty acids. This led to the conclusion that di-lution was the major contributor to the observedchanges in the tissue percentages of these fatty acids,and that fatty acid metabolism, turnover and modi¢-cation played a very minor role in in£uencing thechanges.In a study examining the e¡ects of ‘¢nishing’ feed

on the fatty acid composition of red seabream, Pagrusauratus (Forster), Glencross et al. (2003) gave their ¢sh(ca. 15 g) feeds containing ¢sh oil, soya oil or canolaoil for 3 months. After 3 months the ¢sh weighed ca.60 g, and their fattyacid pro¢les re£ected those of thefeed oils. The seabream were then given a feed con-taining ¢sh oil for 32 days. After 32 days on the ¢shoil feed, the ¢shweighed ca.90 g. Percentage body fatincreased during the grow-out period so the ratio of¢nal to initial body fat (QT/QI, 1.91^1.97) was higherthan the equivalent ratio for ¢sh body masses. Fishwere sampled, and fatty acids of the ¢sh primed withsoya and canola oils were examined to assess the in-£uence of the dietary switch on the rate of modi¢ca-tion of the fatty acid pro¢le. Fish given ¢sh oilthroughout the trial represented the ‘reference’group. Analysis of these data using the dilution

Predicted = 0.973 Observed + 0.128(R2 = 0.994; N = 36)

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% Observed after 98 days of feeding

% P

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icte

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dilu

tio

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od

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Figure 1 Predicted (using the dilution model) and ob-served percentages of 18C unsaturated fatty acids (18:1n-9,18:2 n-6 and18:3 n-3) in the tissues (¢llet, viscera andcarcass) of Atlantic salmon, Salmo salar, 98 days after achange from feeds containing vegetable oil (rapeseed andlinseed) to ones containing ¢sh oil (sandeel, Ammodytesspp.). The observed values are the means for six ¢sh persample. The line is the plotted regression (data fromJobling 2003; 2004).

Predicted = 1.036 Observed - 0.814

(R2 = 0.996; N = 6)

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Observed % fatty acid

Pre

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% f

atty

aci

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Canola 18:1 Canola 18:2 Canola 18:3

Soya 18:1 Soya 18:2 Soya 18:3

Figure 2 Predicted (using the dilution model) andobserved percentages of 18C unsaturated fatty acids (18:1n-9,18:2 n-6 and18:3 n-3) in red seabream, Pagrus auratus,32 days after a change from feeds containing vegetable(soya or canola oil) to one containing ¢sh oil.The observedvalues are means for three replicate tanks per treatment.The line is the plotted regression (calculated from data inGlencross et al. 2003).

Aquaculture Research, 2004, 35, 706^709 ‘Finishing’ feeds for carnivorous ¢sh M Jobling

r 2004 Blackwell Publishing Ltd, Aquaculture Research, 35, 706^709 707

Page 3: ‘Finishing’ feeds for carnivorous fish and the fatty acid dilution model

model revealed that the model gave a good predictionof the changes in the percentages of 18C fatty acidsfollowing the switch from the feeds containing vege-table oils to the ‘¢nishing’ feed with ¢sh oil (Fig. 2).There was a high degree of concordance betweenpredicted and observed values, and the plotted re-gressionwas close to a‘line of equality’.The inferenceis that dilution was the major contributor to thechanges in the percentages of the 18C fatty acids,and that fatty acid metabolism, turnover and modi¢-cation played little role in in£uencing the changes.The study carried out by Bell et al. (2003b) on

Atlantic salmon gave a di¡erent result. Salmon postsmolt (ca.120 g) were given feeds containing ¢sh oil,linseed oil or rapeseed oil for 50 weeks. After 50weeks ¢sh weighed ca. 2.3 kg, had ca. 7^8.5% ¢lletfat, and ¢llet fatty acid pro¢les re£ected those of thefeed oils. The salmonwere then fed a diet containing¢sh oil for 20 weeks. After 20 weeks on the ¢sh oilfeed, the salmonweighed ca.3.5 kg, and had ca.9.9%¢llet fat. Fillet fatty acids of the ¢sh primed with lin-seed and rapeseed oils were examined to assess thein£uence of the dietary switch on modi¢cation ofthe fatty acid pro¢le. Fish given ¢sh oil throughoutthe 70 weeks trial represented the ‘reference’ group.Analysis of these data using the dilution model re-vealed that the model tended to underestimate thechange in the percentages of18C fattyacids followingthe switch from the feeds containing vegetable oils tothe ‘¢nishing’ feed with ¢sh oil. The slope of theplotted regressionwas steeper than in a‘line of equal-ity’ (Fig.3), implying that the changes in the percent-ages of the ¢llet 18C fatty acids were the result ofa combination of dilution, preferential metabolismand fatty acid turnover. Even though the changes inthe ¢llet pro¢les of the 18C fatty acids were morerapid than predicted using the dilution model theconcentrations of these fatty acids were still mark-edly higher than in the ¢llets of the ‘reference’ ¢shafter 20 weeks of feeding the ‘¢nishing’ feed (Table1).

Analysis of data using the dilution model coupledto ‘line of equality’ plots seems to provide a usefulmeans for the evaluation of the factors involved inthe changes of tissue fatty acid pro¢les following achange in feed oils. A high degree of agreement be-tween predicted and observed values, and a good ¢tto a ‘line of equality’ (Figs 1 and 2), implies that themodi¢cations in fatty acid pro¢le are largely the re-sult of the dilution that accrues as more fat is depos-ited. On the other hand, lower concordance betweenobserved and predicted values, and a regression linewith a slope steeper than 1 (Fig. 3), implicates fattyacid metabolismand turnover as factors contributingto the changes in tissue fatty acid pro¢les following aswitch in the type of feed oil.

Table 1 Initial (PI) and ¢nal (PF) percentages of 18C fatty acids in the ¢llets of Atlantic salmon, Salmo salar, given feeds con-taining vegetable oil (linseed or rapeseed) for 50 weeks and then fed for 20 weeks on a‘¢nishing’ feed containing ¢sh oil

Fatty acid PR fish oil

PI ! PF

Linseed ! fish oil Rapeseed ! fish oil

18:1 n-9 16.0 � 0.5 16.7 � 0.5 ! 16.1 � 0.4 41.5 � 0.3 ! 23.6 � 2.2

18:2 n-6 4.8 � 0.2 11.7 � 0.1 ! 7.2 � 0.7 14.6 � 0.1 ! 7.7 � 1.0

18:3 n-3 2.0 � 0.3 37.4 � 0.5 ! 13.3 � 3.3 6.8 � 0.3 ! 3.2 � 0.5

Data for a ‘reference’group of salmon (PR) given the feed containing ¢sh oil throughout the trial are shown for comparison.Values givenare means � SD (data from Bell et al. 2003b).

Predicted = 1.197 Observed - 0.118

(R2 = 0.965; N = 6)

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Rape 18:1 Rape 18:2 Rape 18:3

Linseed 18:1 Linseed 18:2 Linseed 18:3

Figure 3 Predicted (using the dilution model) and ob-served percentages of 18C unsaturated fatty acids (18:1n-9,18:2 n-6 and18:3 n-3) in the ¢llets of Atlantic salmon,Salmo salar,20weeks after a change from feeds containingvegetable oils (linseed or rapeseed oil) to one containing¢sh oil. The observed values are the means � SD for 15¢sh per treatment. The solid line is the plotted regression,and the dashed line shows the‘line of equality’ (calculatedfrom data in Bell et al. 2003b).

‘Finishing’ feeds for carnivorous ¢sh M Jobling Aquaculture Research, 2004, 35, 706^709

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Page 4: ‘Finishing’ feeds for carnivorous fish and the fatty acid dilution model

In addition, the predictions given using the dilu-tion model may also be su⁄ciently robust to assistproducers in obtaining increased control over thefatty acid compositions of farmed, carnivorous ¢shwhen they wish to manipulate the composition ofthe ¢nal product using ‘¢nishing’ feeds that contain¢sh oils.

References

Bell J.G., McGhee F., Campbell P.J. & Sargent J.R. (2003a) Ra-peseed oil as an alternative to marine ¢sh oil in diets ofpost-smolt Atlantic salmon (Salmo salar): changes in £eshfatty acids composition and e¡ectiveness of subsequent¢sh oil ‘wash out’. Aquaculture 218,515^528.

Bell J.G.,Tocher D.R., Henderson R.J., Dick J.R. & CramptonV.O. (2003b) Altered fatty acid compositions in Atlanticsalmon (Salmo salar) fed diets containing linseed and ra-peseed oils can be partially restored by a subsequent ¢shoil ¢nishing diet. Journal of Nutrition133, 2793^2801.

Connor W.E. (2000) The importance of n-3 fatty acids inhealth and disease. American Journal of Clinical Nutrition71,171S^175S.

Glencross B.D., HawkinsW.E. & Curnow J.G. (2003) Restora-tion of the fatty acid composition of red seabream (Pagrus

auratus) using a ¢sh oil ¢nishing diet after grow-out onplant oil based diets. Aquaculture Nutrition 9, 409^418.

Jobling M. (2003) Do changes in Atlantic salmon, Salmo sal-ar L., ¢llet fatty acids following a dietary switch representwash-out or dilution? Test of a dilution model and itsapplication. Aquaculture Research 34,1215^1221.

Jobling M. (2004) Are modi¢cations in tissue fatty acid pro-¢les following a change in diet the result of dilution? Testof a simple dilution model. Aquaculture 232,551^562.

Jobling M., Larsen A.V., Andreassen B., Olsen R.L. & SigholtT. (2002) In£uence of a dietary shift on temporal changesin fat deposition and fatty acid composition of Atlanticsalmon post-smolt during the early phase of seawaterrearing. Aquaculture Research 33,875^889.

Robin J.H., Regost C., Arzel J. & Kaushik S.J. (2003) Fatty acidpro¢le of ¢sh following a change in dietary fatty acidsource: model of fatty acid composition with a dilutionhypothesis. Aquaculture 225, 283^293.

Sargent J.R.,Tocher D.R. & Bell J.G. (2002) The lipids. In: FishNutrition, 3rd edn (ed. by J.E. Halver & R.W. Hardy), pp.181^257. Academic Press, San Diego, CA, USA.

Williams C.M. (2000) Dietary fatty acids and human health.Annales Zootechnie 49,165^180.

Keywords: ¢sh nutrition, ¢sh oils, vegetable oils,salmonids, ¢llet composition, dietary lipids

Aquaculture Research, 2004, 35, 706^709 ‘Finishing’ feeds for carnivorous ¢sh M Jobling

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