Aquaculture 229 (2004) 361-376

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Aquaculture 229 (2004) 361 – 376 www.elsevier.com/locate/aqua-online First feeding of winter flounder (Pseudopleuronectes americanus) larvae: use of Brachionus plicatilis acclimated at low temperature as live prey ´ Laurence Mercier a, Celine Audet a,*, Joel de la Noue b, ¨ ¨ a c Brigitte Parent , Christopher C. Parrish , Neil W. Ross d ´ ´ ` ´ Institut des Sciences de la Mer de Rimouski, Universite du Quebec a Rimouski, 310 allee des Ursulines, Rimouski, Quebec, Canada G5L 3A1 b ´ GREREBA, Uni
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  First feeding of winter flounder (  Pseudopleuronectesamericanus ) larvae: use of  Brachionus plicatilis acclimated at low temperature as live prey Laurence Mercier  a  , Ce´line Audet  a, *, Joe¨l de la Nou¨e  b ,Brigitte Parent  a  , Christopher C. Parrish c , Neil W. Ross d a   Institut des Sciences de la Mer de Rimouski, Universite´ du Que´ bec a` Rimouski, 310 alle´ e des Ursulines, Rimouski, Quebec, Canada G5L 3A1  b GREREBA, Universite´ Laval, Quebec, Quebec, Canada G1K 7P4 c Ocean Sciences Centre, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada A1C 5S7  d  Institute for Marine Biosciences, National Research Council Canada, 1411 Oxford Street, Halifax, Nova Scotia, Canada B3H 3Z1 Received 23 September 2002; received in revised form 13 May 2003; accepted 13 May 2003 Abstract  Brachionus plicatilis are used as live prey for rearing winter flounder larvae at first feeding. Thisrotifer is grown between 20 and 25 j C (its optimal growth temperature) and then introduced intothe 10 j C water in which larvae are reared. The rapid thermal difference between the two media isthought to reduce B. plicatilis quality and consequently affect larval rearing efficiency. In order tooptimize larval rearing, a study was conducted to compare the effects of two different diets onlarval growth performance and nutritional condition: (1) larvae fed B. plicatilis reared at 24 j C and(2) larvae fed B. plicatilis reared at 24 j C but acclimated overnight at 10 j C. Comparisons wereundertaken using morphometric measurements, nucleic acid (RNA/DNA) ratios, total proteincontent, trypsin activity, and triacylglycerol/sterol ratios. Fatty acid composition of larvae was alsostudied with a focus on the levels and ratios of three essential fatty acids (docosahexaenoic acid or DHA, eicosapentaenoic acid or EPA, and arachidonic acid or AA). Unique information regardingtrypsin activity and lipid components (phospholipid, triacylglycerol, and sterol) of winter flounder larvae is provided in this study. Trypsin activity was detected very early in larvae and was not affected by acclimation of prey. Phospholipid, triacylglycerol, and sterol composition showed nosignificant difference between the two diets tested and was characterized by high phospholipid 0044-8486/$ - see front matter  D 2004 Elsevier B.V. All rights reserved.doi:10.1016/S0044-8486(03)00399-5* Corresponding author. Tel.: +1-418-723-1986x1744; fax: +1-418-724-1842.  E-mail address: celine _ audet@uqar.qc.ca (C. Audet).www.elsevier.com/locate/aqua-onlineAquaculture 229 (2004) 361–376  content as well as low triacylglycerol and sterol contents. EPA, DHA, AA, DHA/EPA, and DHA/ AA decreased from days 12 to 26. Interestingly, DHA content was significantly higher in larvae fedacclimated rotifers and AA content decreased significantly in 26-day-old larvae fed acclimated B. plicatilis . Overall results indicate that overnight acclimation of  B. plicatilis at 10 j C is not important for optimizing the rearing of winter flounder larvae although it does influence the fattyacid composition. D 2004 Elsevier B.V. All rights reserved.  Keywords: Pseudopleuronectes americanus ; Larval fish; Feeding; Nutritional condition; Brachionus plicatilis ;Acclimation 1. Introduction Winter flounder are common inshore finfish ranging from Labrador to Georgia(Scott and Scott, 1988). It inhabits waters varying between 0 and 25 j C, with a salinity rangeof 4–30 x (Pearcy, 1962),and can also withstand temperatures below À 1 j C becauseof antifreeze plasma proteins(Duman and DeVries, 1974; Fletcher and Smith, 1980).Since the 1970s, winter flounder has been identified as a promising candidate for coldwater marine aquaculture due to its tolerance to a wide range of temperature andsalinity, its adaptation to cage environments(Litvak, 1994, 1996),and its good market   price (US$2.27 lb À 1 for fillets in New York’s market, June 12, 2001). Winter flounder spawn many eggs (500,000 in average; up to 3.5 million)(Scott and Scott, 1988)at onetime. Eggs are small (diameter 740–850 A m)(Smigielski and Arnold, 1972),light (dry weight 35–56 A g)(Buckley et al., 1991),and sticky. They give rise to larvae with limited energy reserves. The yolk sac is completely absorbed 4–13 days after hatchingat, respectively, 10–2 j C, and larvae begin to feed at, or shortly after, yolk sacabsorption(Buckley, 1982). Production of larvae has been successfully achieved sincethe 1990s(Litvak, 1994, 1996, 1999; Batt, 1998)but rearing has not yet beenoptimized.Larvae are fed two types of live prey that are commonly used in most marine fishhatcheries but are not present in the natural environment of winter flounder. The rotifer   Brachionus plicatilis is given at larval first feeding and the branchiopod Artemia salina is used when larvae are bigger. The optimal growth temperature of  B. plicatilis is between 20 and 25 j C(Pourriot, 1986).When it is given to larvae, it suffers a thermal shock that is thought to reduce their vigor and nutritional value. Its availability in thewater column is also assumed to be reduced, as was generally demonstrated byFielder et al. (2000)following the transfer of  B. plicatilis into beakers at different water temperatures. If prey quality and availability are reduced, the survival and growth of larvae are threatened. As a general rule,Fielder et al. (2000)have recommended B. plicatilis acclimation to the conditions of larval rearing, 6 h before transferring theminto rearing tanks. Recent studies on winter flounder larvae have attempted to replace  B. plicatilis with an artificial diet (microencapsulated particles) but larvae show aninability to digest the particles until their gut loop development is completed(BenKhemis et al., 2000). At that stage, they measure between 5.5 and 6.3 mm (standard  L. Mercier et al. / Aquaculture 229 (2004) 361–376  362  length). Therefore, for rearing winter flounder larvae, artificial diets can only be usedas a replacement for  A. salina , with larval mass production remaining dependent on B. plicatilis .First feeding is recognized as a critical period for marine fish larvae and is oftenassociated with high mortality. During this stage, larvae are vulnerable and their nutritional requirements have to be met qualitatively and quantitatively. To rapidly andreliably assess the nutritional efficiency of a diet, growth performance and nutritionalcondition of larvae can be estimated and several biochemical components have been proposed as indicators. The nucleic acid (RNA/DNA) ratio, an indicator of proteinsynthesis potential, hasbeen shown to be a good index of growth and nutritionalcondition of fish larvae(Buckley, 1979, 1980, 1984; Richard et al., 1991).The use of  RNA concentration wasalso validated as an indirect measure of growth in young-of-the-year winter flounder (Kuropat et al., 2002).Some proteolytic enzymes havebeen  proposed as well as indicators of nutritional condition(Ueberscha¨r, 1993),andtrypsin-like activity has been used for evaluation of the nutritional status of herringand turbot larvae(Ueberscha¨r, 1988).As well, different lipid components were found to be reliable nutritional indicators(Fraser, 1989; Ha˚kanson, 1989a,b)and thetriacylglycerol/cholesterol ratio has been proposed as an index of the physiologicalcondition of fish larvae(Fraser, 1989; Ha˚kanson, 1989a).Total fatty acid levels arenot implicated as a nutritional indicator but polyunsaturated fatty acids have beenshown to be important for growth and survival of fish larvae in addition to their energetic role. Docosahexaenoic acid (DHA; 22:6 n À 3), eicosapentaenoic acid (EPA;20:5 n À 3), and arachidonic acid (AA; 20:4 n À 6), which are considered to beessential fatty acids, are involved in maintaining cell membrane structure and function(Sargent et al., 1999).The aim of this study was to test whether rearing of winter flounder larvae at first feeding could be optimized by the use of  B. plicatilis acclimated overnight at the larvalrearing temperature (10 j C). Winter flounder larvae fed acclimated B. plicatilis werereared in parallel to ones fed nonacclimated B. plicatilis . Larval growth performance andnutritional condition of both dietary treatments were compared using morphometricmeasurements, nucleic acid (RNA/DNA) ratios, total protein content, trypsin activity,and triacylglycerol/sterol ratios. Fatty acid composition of larvae fed each dietarytreatment was also examined, with special emphasis on the contents of three essentialfatty acids (DHA, EPA, and AA). 2. Materials and methods 2.1. Chlorella sp. culture The nutritive value of many microalgae species is recognized for the growth of  B. plicatilis (Coves et al., 1986; Pourriot, 1986).For this study, a Chlorophyceae( Chlorella sp.) from the St. Lawrence estuary (Canada) has been chosen, isolatedon agar medium, and cultivated semicontinuously in an 85-l Plexiglass cylinder. Chlorella sp. was grown at 18 j C, under 87.5 A M s À 1 m À 2 intensity and with  L. Mercier et al. / Aquaculture 229 (2004) 361–376  363  aeration. F/2 medium(Guillard and Ryther, 1962)as well as filtered (10 A m) and UV-treated seawater were used as media. The culture density was kept at about 15 Â 10 6 cells ml À 1 . 2.2. B. plicatilis rearing and acclimation B. plicatilis were purchased from Aquatic Research Organisms (USA) and werereared semicontinuously in 18.9-l plastic carboys, with filtered seawater (10 A m, 24 F 1 j C, 27 F 1 x ). Oxygen saturation and water circulation were maintained with anairstone at the bottom of each carboy. Four batches were raised simultaneously. Theywere fed with the Chlorella sp. suspension once a day and supplemented with theformulated diet ‘‘Culture Selco’’ (Inve Aquaculture, USA) four times a day. A measuredquantity of  B. plicatilis was removed from the carboys at the end of each day and placedovernight in a temperature-controlled room, set at 10 j C, for acclimation. Fatty acidcomposition in acclimated B. plicatilis and nonacclimated B. plicatilis is presented inTable 1. 2.3. Larvae rearing  One hundred winter flounder (  Pseudopleuronectes americanus ) were captured in aweir trap from the St. Lawrence estuary (Canada) during the reproductive season. Theywere transferred into two 200-l epoxy-covered wood tanks supplied with ambient filteredseawater at a flow of 5 l min À 1 (30 A m, 4 F 1 j C, 24 F 1 x ). When the gonadsmatured, artificial fertilization was performed as described byBen Khemis et al. (2000).The fertilized eggs were incubated in baskets within a trough, supplied with runningfiltered seawater at a flow rate of 4 l min À 1 (30 A m, 7 F 1 j C, 28 F 1 x ), and kept indarkness until hatching. Ten days after egg fertilization, hatching occurred and this daywas termed ‘‘day 0.’’ Larvae were randomly transferred into four 55-l cylindro-conical polyethylene tanks with a final density of 250 larvae l À 1 in each. Tanks were suppliedwith filtered seawater (10 A m, 10 F 1 j C, 28 F 1 x ) and maintained under a photope-riod 12 L:12 D. The light intensity at the water surface was approximately 7.1 A M s À 1 m À 2 . The sides and bottom of tanks were black and white, respectively. At the mouthopening, two diets were tested in duplicate: (1) B. plicatilis reared at 24 j C, and (2) B. plicatilis reared at 24 j C but acclimated overnight at 10 j C. Larvae were fed everymorning, at the time of illumination, after having added 100 ml of an algae pastesolution (1 g diluted in 450 ml of seawater; Innovative Aquaculture Products, Canada)and 500 ml of the cultivated Chlorella sp. suspension to each tank. This was done to prevent ‘‘larval wall syndrome,’’ to give a better optical environment for larvae, and toenhance water quality(Naas et al., 1992; Herna´ndez-Cruz et al., 1994; Tamaru et al.,1994). Rotifers were added in excess (3–5 prey ml À 1 ) according toLaurence (1977)andtheir density in tanks was checked every afternoon. A few larvae were also pipetted fromeach tank everyday and observed under a binocular microscope to ensure food uptake.During the feeding period, the water system was closed to prevent the loss of prey downthe drain. At night, the system was opened and 95% of the water was replaced accordingtoSprague’s (1973)table.  L. Mercier et al. / Aquaculture 229 (2004) 361–376  364
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