Restoration of the fatty acid composition of red seabream (Pagrus auratus) using a fish oil finishing diet after grow-out on plant oil based diets B.D. GLENCROSS 1 , W.E. HAWKINS 2 & J.G. CURNOW 1 1 Department of Fisheries, Research Division, North Beach, WA, Australia; 2 Department of Agriculture, Pulses and Oilseeds Program, Baron Hay Court, South Perth WA, Australia Abstract Thisstudyevaluatedthepotentialformanipulatingthefatty acid composition of juvenile red seabream, Pagrus auratus. Priortothestartofthestudy,threegroupsoffishhadbeen rearedfor3monthsonafishoilbaseddietordietswherethe added fish oil had been replaced with either canola or soy- bean oil. In the present study, fish that had previously been fed either the canola or soybean oil diets were fed a fish oil based diet. Three additional treatments included fish being maintained on their original diets of fish oil, canola oil or soybeanoil.Fishwerefedtheirrespectivedietstwicedailyto apparent satiety for 32 days. Samples of fish from each treatment were collected after 0, 1, 2, 4, 8, 16 and 32 days. Compositionandgrowthofthefishweredeterminedateach sample point. Most treatments showed no differences in growth performance, although fish fed a fish oil diet after previouslybeingfedasoybeanoildietshowedslightlybetter growth. No significant differences among treatments were observed in proximate composition of the fish, although there was a significant increase in total fat and individual fatty acid (g kg )1 live-weight) content of the fish from all treatments over the period of the study. No significant changes in the relative fatty acid composition (% of total fatty acids) over time were observed in the three treatments where fish were maintained on their original diets. In con- trast, fish that were previously fed either the canola or soy- beanoildietsandwerethenfedafishoildiethadsignificant changes in both the relative (% of total fatty acids) and absolute (g kg )1 live-weight)fattyacidcontent.Keychanges observed included a decrease in the relative levels of poly- unsaturated fatty acids (PUFA) such as 18 : 2n ) 6 and 18:3n ) 3. Increases in the relative levels of the long-chain polyunsaturated fatty acids (lcPUFA) 20 : 5n ) 3 and 22:6n ) 3werealsoobservedinbothtreatments.Therates of absolute (g kg )1 live-weight) change/accumulation of these fatty acids followed an exponential equation that dif- fered for each fatty acid in each treatment. Examination of the retention efficiency of specific fatty acids also showed markeddifferencesbetweenfattyacidswithintreatmentsand also differences between treatments. Biologically important fatty acids such as 20 : 5n ) 3 and 22 : 6n ) 3 had only moderate retention efficiencies and these were unaffected by treatment. In contrast, the retention efficiencies of 18:2n ) 6and18:3n ) 3 suggested selective retention of these fatty acids when fed fish oil diets, but moderate cata- bolism when fed the plant oil diets. There were also high retentionefficienciesofmostsaturatedandmonounsaturated fatty acids suggestive of active retention and/or active syn- thesisofthesefattyacidsbythefish.Theresultsofthisstudy, particularly the increases in lcPUFA, support the usefulness of a fish oil based finisher diet for fish raised predominantly on plant oil based diets. KEY WORDS KEY WORDS: canola oil, fatty acid, fish oil replacement, snapper, seabream, soybean oil Received 3 December 2002; accepted 24 June 2003 Correspondence: Brett D. Glencross, Department of Fisheries, Research Division, P.O. Box 20, North Beach, WA 6020, Australia. Introduction Most studies examining the use of plant oils in the diets of fin-fish have shown that these lipid resources have good nutritional value. Soybean oils were shown to be useful as lipidresourcesindietsforgiltheadseabream, Sparus auratus (Kalogeropoulos et al. 1992) and salmonids (Greene and Selivonchek 1990; Grisdale-Helland et al. 2002). Studies examining the influence of canola oil on the growth and Aquaculture Nutrition 2003 9 ; 409^418 .............................................................................................. .............................................................................................. Ó 2003BlackwellPublishingLtd 409