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Internatiooal Council tor theExploration ot the Sea

Paper C.M. 1990/F: 18Mariculture Committee

•Development and behaviour of larvae and juveniles

of turbot, Scophthalmus maximus L.in hatchery tanks

by

O. FUKUHARA1l, H. ROSENTHAL2l, U. wln3) AND G. QUANTZ3)

1) Nansei Regional Fisheries Research LaboratoryOhno-eho, Saeki-gun

Hiroshima-ken, 739-04, Japan

2) Department ot Fishery BiologyInstitute for Marine Seienee

University of KielDüsternbrooker Weg 202300 Kiel 1, Germany

3) Butt G.m.b.H.Bülker Huk

2307 Strande, Germany

. ~ , " ABSTRACT

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Larval behaviour and morphological development were studied in turbot(Scophthalmus maximus L.) reared in a pilot-scale hatchery. Early develop­mental stages were described and iIIustrated In detail with special reference todevelopment of morphological characteristics. pigmentation, tin developmentand metamorphosis. Volk absorption was completed wilhin 4 days afterhatchlng (average temperaturea17.8°C. range-17A- 18.0·C). Larvae coulddigest prey prior to complete yolk absorption. The alimentary canal changedfrom a straight tube to a convoluted shape at 5 mm larval length. Locomotoryactivity of yolk-sac larvae was determined tor fed and unfed larvae and partlytollowed up to metamorphosis. While total swimming activity gradually in­creased to a maximum at day 5 after hatching (1 day after yolk absorption).locomotory activily rapidly declined in unled larvae thereatter. Distributionpatterns In large-seale rearing tanks were tollowed tor early hatched and firstteeding tarvae over several days at various day times. Patchiness in the rear­ing tanks was trequently observed and changed with age. Ossifieation started ata larvallength 01 6.2 mm standard length. Asymmetry 01 eyes began at 7.5 mmSL. The smallestlarvae that completed eye migration were 16.2 mm SL Al thesame size a COnstant ratio between body height and length was tound. Larvaebegan 10 settle at the tank bottom before complete eye migration. Distributionof metamorphosed juveniles was quantitatively followed by video and flash­light pictures in weaning tanks (tish size 16 to 32 mm SL, stocking densityabout 400/m2l, demonstrating short-term variability in distribution. indi­vidual distances and dilferences In fish distribution kept in adjacent tanks.

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INTRODUCTION

For more than a decade turbot has been recognized as one of the mostpromising candidates for mariculture development in Europe (Purdomet al. 1972; Ruyet et al. , 1981; Jones, 1972), and efforts to establishappropriate rearing techniques for mass production of juveniles ofthis flatfish are increasing. Numerous studies have concentrated onthe development of feeding strategies tor rearing early larval stages(Scott & Middleton, 1979; Kuhlmann et al., 1981; Bromley & Howell, •1983; Witt et al., 1984) and on the design and operation of advancedculture systems (Poxton et al, 1982; Witt & Quantz, 1988). Ourknowledge of the biological requirements of the early Iife historystages of this species, however, is still fragmentary, despite thefact that various studies have already described the development ofsome external morphological characteristics (Jones, 1972; AI-Mag­hazachi & Gibson, 1984), the organogenesis of embryonie and larvalstages Naeve, 1984a, 1984b, 1986; Cousin & Laurenein, 1984), andsome environmental conditions required tor development and growth(Kuhlmann & Quantz, 1980; Quantz, 1985).

Despite its common occurrence in the North Sea and the Baltic and itscommercial importance, Iittle is known on the variability of develop­mental characteristics of turbot under commercial culture con­ditions. The objective of this study was, theretore, to describe thelarval and post-larval development of turbot under largely controlledconditions in a pilot-scale production unit, including the' developmentof pigmentation and the formation of the digestive tracl. The datapresented here are based on specimens collected at intervals fromvarious culture tanks, The accompanying studies on the behaviour of •larvae and post-Iarvae included quantifying descriptions ot activitypatterns and observations on the distribution of fish in circular andrectangular tanks in relation to age (size) and various operationalconditions at the commercial farm site •

MATERIALS AND METHODS

Viable eggs and milt were collected from the brood stock which wasmaintained under a controlled light regime in circular tanks of about3 m diameter. Eggs were tertilized, washed, disinfected and in­cubated at the BUTT pilot-scale hatchery in Strande near Kiel, in May1988. For turther rearing, eggs were transferred to prepared rearing

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3tanks prior to hatching. Tanks used are made of dark green glass­fiber (diameter 2,0 m, water depth 0.6 m). Ambient seawater wasfiltered through 5 ~ sieves, and salinity was adjusted to 14 pptduring the entire experiment. Illumination was supplied by two 36­Watt f10urescent tamps (Osram "White de Luxe"). These lamps wereinstalled at a distance of 0.45 m above the water surtace. Rotifers(Brachionus plicalilis) reared on dry bakers yeast and culturedmarine algae (Nannochloris spec.) were employed as first food andoffered twice daily. At day 12 freshly hatched Arlemia nauplii wereadded for 2 to 3 days, thereafter ongrown Artemia were used until aweaning size of 12 mm TL was reached. Mean temperature during the

• ongrowing experiment was 17.8°C (range 17.6-18.0). Tank water wascirculated by genUe aeration with three airstones which were tixedat the outer periphery of the tanks. Rearing methods of larval turbotand the environmental conditions in the rearing units were similar tothose reported by Kuhlmann et al. (1981).

Newly hatched and feeding larvae were collected randomly fromrearing tanks at irregular intervals, anesthetized in MS 222 andpreserved in a 5% formalin-seawater mixture (17%. salinity). Thesesampies were used to follow the larval development to determineadditional morphametrie characteristics (e.g. fin ray number, pig­mentation patterns) while a 10% formalin-seawater solution wasused to preserve metamorphosed juveniles. Fish were stained in Ali­zarin to study fin development.

Morphometric measurements were undertaken on anesthetized livelarvae and juveniles to determine the following body characteristics:preanal length (PL, measured from the tip of the snout to the anus);total length (TL, from the tip of the snout to the marginal end of thecaudal tin); standard length (SL, measured fram the tip of the snoutto the end of the notochord (in larvae), or to the end of the caudal finbase in metamorphosed fish); body height (BH, the vertical line fromthe anus to the edge of the dorsal fin).

Behaviour of early larval stages was studied by direct observationsand from recordings. made of the time spent moving or resting inteeding and starving larvae. These observations were performed in a20 L aquarium. Later stages (metamorphosed juveniles) were ob­served in regular intervals (photographed every 5 minute) in the largerearing tanks operated by the hatchery.

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RESULTS

Larval teeth occurred when the fish reached a size of about 7.7 mmSL, and separated nares were distinguished in larvae from body sizeof 8.1 mm SL onward. The lateral line became visible at a size of 17mm SL (Figure 1J). It should be noted, however, that in Figure 1 thedevelopment of the swim bladder is not shown, although inflatedswim bladders were seen in some larvae as small as 4 mm SL, whileothers never inffated theirs, or at a mueh later. date. Reasons for thisdifferenee in swim bladder development are unknown. eAsymmetry cf eye loeation started to occur in larvae larger than 7.5mm SL (Figure 1 H). Usually the right eye moved to the left side asthe larvae grew. Developmental sequenees of eye migration are shownbesides others also in Figure 2. The stages of eye migration weredetermined in the same sequence and using the same definitions asdescribed for Limanda yokohamae by Fukuhara (1988). Completion ofeye migration was reaehed when half cf the pupil has passed theridge of the head. The smallest and largest larvae that had completedeye migration (stage K) were 16.2 mm and 19.8 mm in SL, respee­tively; therefore, metamorphosis was assumed to be completed forall fish at least at the latter size (Fig. 2).

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Pigmentation

New/y hatched turbotlarvae showed several tiny melanophores on thesnout, the cephalic area, the trunk and the caudal region. MeJano­phores were also distributed along the gut, on the posterior sectionof the yolk, along the larval fin-fold on dorsal side and around thecaudal peduncle. No eye pigmentation occurred in freshly hatchedlarvae.

The melanophores discerned at hatching became larger and moreintense in pigment color as the larvae grew (Fig 1B, 1Cl. Melano­phores were particularly dense in dorso-Jateral and ventro-Iateralrows (Fig. 1Cl. Eye pigmentation also progressed as larvae grew.Smaller melanophores also occurred in increasing number in the gutregion, on the jaw and on the larval ventral fin-fold (Fig. 1 0).Melanophores also developed around the caudaf peduncle and becamemore dense and wider in their area coverage on the fin-fold itself.Thereafter, body pigmentation progressed continuously and evenlyover the entire body.

Fln development

Newly hatched larvae possessed an extended primordial fin-fold (Fig1A). ;Fan-shaped pectoral fins without rays developed one day afterhatching at 16.5°C (Fig.1 B). The primordial fin-fold started to dis­appear first in the region of the anus at a larval size of about 6.0 mmSL. At this stage of development the dorsal and anal fins appeared.The uniform fin-fold, however, did not change its overall shape untilthe larvae showed the early stages of the cartiJaginous hypuralelements. At that stage most of the larvae measured 6.2 mm in SL(Figure 1F).

At a standard Jength of 8.2 mm the buds of the ventral fin appeared.The first segmented fin rays were noted in the caudaJ fin at a larvalsize as small as 7.1 mm SL. Fin segmentation occurred in other finsin the following sequence: dorsal, anal, ventral and pectoral fins. Thenumber of fin rays in those metamorphosed larvae which had reached17 mm SL, were: 62 (dorsal), 48 (anal), 18 (caudal), 13 (pectoral), and6 (ventral fin). At a larval size of 20 mm SL, the respective fin raycounts yielded the following numbers: 57, 45, 18, 8, and 6. Theseaverage values (n - 10) show some of the variabiJity in thesemeristic characters.

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Development of the digestive tract

The various stages of development of the digestive tract from yolk-sac larvae to metamorphosed fish are depicted in Figure 3. In earlylarvae (about 3.0 mm SL) the gut is seen as a straight tube (Fig.3,Stage A). The alimentary canal is twisted and convoluted between alarval length of 3.0 and 4.5 mm (stages B to 0). The convoluted shapewas rounded at a size of 5.0 mm SL (stage E). All larvae larger than6.0 mm SL already possessed a growing number of pyloric. caeca(stage F). Turbot larvae are vigorous and almost continuous feeders. •The amount of live food organisms required by an individual larva togrow from 15.4 mm to 19.1 mm SL was estimated from counts on 16individuals. The figure obtained was about 900 Artemia nauplii.

Larval behaviour and initial growth

At high salinities. newly hatched larvae stayed almost motionless inthe water column, drifting mainly with the current and turbulencethrough the culture tank. Occasionally, larvae showed sudden andshort swimming bursts. These locomotory phases were recorded aswriggling. abrupt swimming or darting motions (Rosenthai & Hempel.1970; Jones, 1972). Aetive and more consistent locomotory patternsdeveloped rapidly in yolk-sac larvae when held in lower salinitieswhere neutral bouyaney was laeking and larvae tried to eompensate aagainst sinking by frequent vertieal swimming aetivity. This be­haviour beeame even more pronounced after the mouth opened and theeye pigmentation started. At that time the larvae switched graduallyfrom vertieal to more horizontal movements, and swimming aetivityreaehed its maximum level about 5 days after hatehing for both fedand unfed larvae (Table 1).

Aiming at prey was frequently observed in larvae offered live food.The intestine was filled with prey in many larvae already at day 3 inthe large tanks while only 40% of the larvae ingested rotifers inglass jars at day 5 after hatehing. The intensity of loeomotoryaetivity levelled off in feeding larvae after day 5 whereas starvinglarvae beeame less active shortly after that date and were almostinactive on the bottom 7 days post hatching. (Figure 4). Feedinglarvae showed a, steady initial length inerement during the tran­sitional phase from endogenous to exogenous energy supply whilestarving larvae continued growth only as lang as fraetions of the yolksae were still present (3.5 mm SL, 3.77 mm TL). Maximum growth

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7rate in yolk sac larvae was similar to that obtained in an earlierexperiment (Quantz, 1985).

In the rearing tanks, patchiness of' larval distribution was oftenobserved. The patterns of larval distribution changed frequently andsometimes rapidly. Several situations observed at different times oftha day are depicted in Figure 8. Often a large fraction of tha larvalpopulation was distributed around the perimeter of the tank. Theconcentrated clouds of larvae seldom dispersed to central parts ofthe tank but often moved slowly along the tank wall, thereby oftentouching the wall. Few larvae concentrated in other tank areas for noobvious reasons. Occasionally Artemia nauplii supplied randomly tothe rearing tanks congregated in places were few turbot larvaeoccurred.

Close to metamorphosis when fish reached the stage H (see Fig.1)behavioural patterns changed in a similar manner as described forsole larvae by. Rosenthai (1966). During the phase of eye migrationlarvae. tended either to swim in the middle layer of the water columnor .near or at the bottom of the tank, frequently touching the bottomsurface. Specimens of. 8.0 mm SL were the smallest and youngestsettling at the bottom, all of which belonging to stage H.

Behaviour of metamorphosed juveniles

ßehaviour of fish frequently resting at the bottom was monitored atone day in regular intervah (photographed every 5 minute). The pic­ture was devided into 4 sections (reflecting different parts of thetank) and number of fish contained therein (swimming and/orresting) were counted. Figures 9 and 10 show the results obtained. Itis obvious that the deviations from the calculated mean stockingdensity (zero Hne) deviate substantially in various sections of thetank and these deviations are very different in the two tanksobserved, although it can be assumed that both tanks were operatedin similar manner with regard to inflow-outlet arrangement, waterflow rate, aeration and position of the feeding devices. Differences inposition of the tanks in the culture room and consequently the slightdifferences in light intensity, however, may have been of importance.Tank 1 (Figure 9) shows that movements between sections are quiteintensive and numbers of fish occupying sections 1 and 3 are alwaysslightly higher than in section 2 and 4. In Tank 2, however (Figure10) the uneven distribution in the various quarters of the Ewos-tankis quite striking. There seems to be a distinct preference for section 1

8and densities in seetions 2 and 3 are often mueh lower than would beexpeeted if fish would distribute themselves more evenly in the tank.The density in seetion 2 was always c10se to the theoretieallyealeulated average density. Distanees between those individualsresting at the bottom vary greatly. Although the majority of thejuvenile fish seem to keep a eertain distanee between individuals(Figure 11) this pattern does obviously not influence the overalldistribution in the tanks. lt may be possible that the "miero lightelimate" over the tanks and current patterns caused by minordifferenees in inlet configuration as weIl as by aeration may be theeause of the observed differences in fish distribution.

Allometric growth and meristic characters

Proportional dimensions of body height and preanal length in relationto total length are shwon in Figure 6 for speeimens ranging fromyolk-sae stage to metamorphosed fish. 80th morphometrie eharae­teristics varied largely during the pre-Iarval and post-larval de­velopment, in particular between 4 mm and 13 mm SL. Thereafter, theproportionality of these body dimensions reaehed a eonstant ratio,indicating that most of the fairly structural development during thesymmetrie stages of the life eyele were not yet completed. Com­pletion was reaehed at or near 15 mm SL (19 mm TL). Under the givenset of environmental eireumstanees the onset of was reached at thatsize.

Standard length was used to deseribe growth in relation to thedevelopment of other morphological eharaeteristics of the larvae.Some morphometric data are plotted against total length in Figure 7.

DISCUSSION

Changing the energy souree from endogenous to exogenous soureeswith the initial feeding of turbot larvae has to begin within 60 and90 hrs after hatching (at 18°C) in order to aehieve more than 40%survival (Jones,1972). Quantz (1985) found that for turbot larvaereared at 15°, 18°, and 21°C, a suitrable prey density should beestablished within 90, 75, and 60 hours after hatehing, repseetively.Although part of the oil globule is still visible during first fooduptake, an' energy deficiency might already oeeur, when no food isavailable at that time. Swimming aetivity of unfed larvae reaehed amaximum about one day prior to eomplete yolk absorption. Cumulativeactive swimming time per observational interval declined sharply120 hrs after hatching (Fig.5),· and first intake of food was seen

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9before yolk absorption was complete. Hence larval turbot might bevulnerable to starvation or shortage of prey already during the final

. phase of yolk utilization and this may be expressed in behaviouralresponses.

It is important to determine the size range at metamorphosis forcomparison with the development of various organs as weil as forfeeding . and transferring procedures. Metamorphosis and morpholo­gical transformationfrom post-Iarvae to juvenile are associatedwith functional development of various organs which are related tothe development of larval behaviours. In previous studies, size rangesof metamorphosed fish varied widely (Jo nes,1972). 23-30 mm(Jones, 1973), 27-29mm (Jones et al., 1974) and 38-45 mm (AI-Mag­hazachi & Gibson,1984).

The present study identified the range of fish lengths at which turbotlarvae began metamorphosis. under the given experimental conditions(around 19.8 mm SL). In addition the allometric growth of preanallength and of body height also attained a fairly constant ratio aftermetamorphosis was reached. It has been considered as an indicatorof the status of morphological and functional development in f1atfish(Fukuhara, 1986;1988) . An example of this is the development of thedigestive system of teleost fishes after the change from post-Iarvaeto juvenile (Tanaka,1973).

The developing larvae tended to settle on the bottom of the tank soonafter eye migration began., a behaviour which has been observed forother f1atfish under laboratory conditions (Seikai,1985; Fukuhara,1988). Although the number of observations was low there was a dis­cernible size difference in eye-migrating larvae between the surfaceand the bottom (Fig.6). According to growth rates of starry f1ounder,Platichthys ste/latus, (Policansky & Sieswerda,1979) and Limandayokohamae (Fukuhara,1988), retardation or decrease in growth wasonly seen during metamorphosis. A similar phenomenon has beenrecorded for the turbot by Kuhiman et al. (1981). However, meta­morphosis cannot be regarded as a critical phase which leads toincreased mortality. Hence the size difference between the fish onand off the bottom presumably resulted from a slowing of growthimmediately after metamorphosis indicating that metamorphosis isthe critical phase for survival in the sea as weil as in the tank.

The observations on behaviour of larval and metamorphosed fishunderline the importance of external and operational characteristicswithin the culture system and their effect on fish distribution.Behavioural responses to even small envirorimental changes areimportant factors and nead to be considered in system design and

10operation. These interactions have not yet been weil documented invarious culture systems and warrant further investigations.

ACKNOWLEDGEMENTS

This study was carried out under the auspices of German-Japanese agreement ofscientific and technical cooperation in the field of marine research. The support obtainedwithin this programme is gratefully acknowledged.

LITERATURE CITED

AI-Maghazaehl, S.J., Gibson, R. 1984. The developmental stages of larval turbot.Scophthalmus maximus (L.). J. exp. mar, Biol. Ecol. 82: 35-51.

Bromley, P.J., Howell,B.R. 1983. Factors influencing the survival and growth ofturbot (arvae, Scophthalmus maximus L., during the change from live to compound feeds.Aquacullure 31: 31-40.

Cousin. J.C.B., Laurenein, F.B. 1985.Morphogenesis of the digestive system andswim bladder of the turbot, Scophthalmus max;mus L. Aquacullure 47: 305-319.

Fukuhara, O. 1986. Morphological and functional development of the Japanese f10underin early Iife stages. Bull. Japanese Soc. sei. Fish. 52: 81·91.

Fukuhara, O. 1988. Morphological and functional development of larval and juvenileLimanda yokohamae (Pisces: Pleuronectidae) reared in the laboratory. Mar. Biol. 99:271·281.

Jones, A. 1972. Studies on egg development and larval rearing of turbot, Scophthalmusmax;mus L., and brill, Scophthalmus rhombus L., in the laboratory. J. mar. bio. Assoe.U.K. 52: 965-986.

Jones, A. 1973. Observations on the growth of turbot larvae Scophthalmus max;mus L.reared in the laboratory. Aquaculture 2: 149-155.

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Jones, A., Alderson, R., Howell, B.R. 1974. Progress towards the development of a •successful rearing technique for larvae of the turbot, Scophthalmus maximus L. In:Blaxter, J.H.S. (Ed.) "The early life history of fish." pp.731·737. (Springer Verlag,Berlin)

Kuhlmann, 0., Quantz, G. 1980. Some effects of temperature and salinity on theembryonie development and incubation time of the tUrbot, Scopthalmus max;mus L.,!rom the Ballie Sea. Meeresforschung. Vol.28, pp.172·178.

Kuhlmann,D., Quantz, G., Wilt, U. 1981. Rearing of turbot larvae, Scopthalmusmax;mus L.. on cultured foad organisms and post-metamorphosis growth on natural andartifieial food. Aquaculture. Vol.23,pp 183-196.

Neave, D.A. 1984a, The development of the retinomotor reactions in larval plaice.Pleuronectes platessa L.. and turbot. Scopthalmus maximus L. J. Exp. Mar. Biol. Ecol.,Vol. 76, pp. 167·175.

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11Neave, D. A. 1984b. The development 01 vlsual aeuity in larval plaiee, Pleuroneetesplatessa L., and turbot, Scopthalmus maximus L. J. Exp. Mar. Biol. Eeol., Vol. 78. pp.167·175.

Neave, D.A. 1986. The development of the lateral line system In plaiee, Pleuronectesplatessa L.. and turbot , Scopthalmus maximus L. J. Exp. Mar. Biol. Ecol., Vol. 66, pp.683-693.

Polieansky,D., SIeswerda, P. 1979. Early lile history of the starry flounder,reared through metamorphosis in the laboratory. Trans. Am. Fish. Soc.• Vol. 108, pp.326·327.

Poxton, M. G., Murray, K.R., Unfoot, B.T. 1982 • The growth of turbot,Scopthalmus maximus L., in reeireulating systems. Aquaeult. Eng.• Vol. 1, pp. 23-34.

Purdom, C. E., Jones, A., L1neoln, R.F. 1987. Cultivation trials with turbot,Scopthalmus maximus L. Aquaeulture. Vol. 1, pp. 213·230.

Ouantz, G. 1985. Use 01 endogenous energy sources by larval turbot. Seopthalmusmaximus L. Trans. Am. Fish.Soc., Vol. 114, pp. 558-563.

Ouantz, G., Jäger, T., Wllt, U. 1988. SteinbuUzueht an der Kieler Förde. pp. 41·49. In: RosenthaI, H., Saint·Paul. U., Hilge, V. (Eds.). 'Perspektiven der DeutschenAquakultur". German Section of the European Aquaculture Society and EuropeanAssociation of Fish Pathologists. Hamburg, Biologische Anstalt Helgoland. 174 pp.

Rosenthai, H., Hempel, G. 1970. Experimental studies in feeding and foodrequirements 01 herring larvae. C/upea harengus L. In. Marine Food Chains, edited by J.H. Steel, Unive~sity 01 Calilornia Press, Berkeley, pp. 344-364.

Ruyet , J. P·L., L'Elchat, 0., Nedelee, G. 1981. Research on rearing turbot,Scopthalmus maximus L.,: results and perspeetives. J. World Marieul. Soe., Vol. 12, pp.143·152.

Seott, A. P., Middleton, C. 1979. Unieellular algae as a food for turbot,Scopthalmus maximus L., larvae- the importanee of dietary long-chain polyunsaturatedfauy acids. Aquaeulture. Vol. 18, pp. 227·240.

Seikai, T., 1985. Metamorphosing and settling processes of f1atfish larvae analysedfrom rearing experiments. Bull. Japan. Soc. Fish. OCeanogr.• Vol. 47. pp. 81-84.

Tanaka, M., 1973. Studies on the structure and lunetion of the digestive system ofteleost larvae. Ph. D. Thesis, Kyoto University, Japan, pp. 136.

Witt U., Ouantz, G., KUhlmann, 0., Kattner, G., 1984. Survival and growth ofturbot larvae Scophthalmus maximus L. reared on different food organisms with specialregard to long-chain polyunsaturated fauy acids. Aquacultural Engineering 3: 177·190.

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Figure 1: Mass rearing trials with Scophthalmus maximus. Larval development fromyolk-sac larvae to metamorphosis: A·C =pre-Iarvae; D·G =post·larvae; J[.J =metamorphosing larvae; K = juvenile. Standard lengthltotal length (mm): A =2.59/2.75; ß =3.1213.36; C =3.4413.62; D =5.6615.93; E =5.9/6.3; F =6.216.6; G =8.218.8;11 =7.819.3; I =14.7/19.2; J =17.0121.7; K =18.5124.0

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•Figure 4. Growth, development of some rnorphological events. and ofactivity patternsduring rearing Scophthalmus maximus larvae. Comparison between fed and unfedlarvae. Dots =fed larvae; open circles =unfed larvae; solid line =standard length;broken line = locornotory activity. Water temperature = 15.2 - 17.0·C. Each pointrepresents the rnean of 10 to 20 determinations

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Figure 5. Rearing of juvenile Scophthalmus maximus in shallow tanks. Relationbetween standard length of rnetamorphosed fish and tank level at which fish wereactive, Closed circles = bottorn dwellers; c10sed circles = specirnens swirnrning nearsurface or mid-water; n =total number of specimens observed

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iii.5 ·100c:.!!..-;:.. ·200Cl

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Observation Invertat (minutes)

Figure 9: Rearing juvenile turbot in circular tanks. Changes in flsh distribution at thetank bottom in four sections of the tank (see insert) expressed as deviations (n1m2)from the ca1culated average stocking density (about 400 individulas per m2). Tank 1

20

N.E 200S.

?:'..c:..c 100...c::;;;u·0

üiE 0gc:

~"> ·100..c

c;-E! 100

~..c 0..Q

tJ>C:;;; ·100u0

iiiE ·200,gc~ -300;;;'>..Q

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17ObserveUons IInlervelln mlnutes.\

Dala from "ButlTk2.Dens.(Secl-4)"

B

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Oberaavtlons (Intervalsln mlnutes)

•

•Figure 10: Rearing juvenile turbot in circular tanks. Changes in lish distribution atthe tank bottom in four seclions of the tank (see insert) expressed as deviations (n/m2)(rom the calculated average stocking density (about 400 individulas per m2l•Tank 2

21

:c 80~

•

'0 60..CI)

.D

; 40.s

11 TotalMeasure

5 10 15202530354045505560 65 70 75 80 85

Distance between Indivlduals (mm)

Figure 9. Rearing of turbot (Scophthalmus maximus) juveniles in circular tanks.Frequencies of observed distances (mm) between bottom settled and frea swimmingfish.

Table 1: Mass rearing of Scophthalmus maximus larvae. Growth andlocomotory activity (time active in 8ec per min) in red and starvedlarvvae. n =number of determinations; SL =standard length; sn =standard deviation

age (days n SL sn n time active snafter hatching) sec---------------------------------------------------

0 21 3.01 ±O.07 21 14.5 ±3.13

• 1 19 3.15 ±O.ll ID 16.1 ±5.062 10 3.34 ±O.04 a> 19.1 ±6.843 11 3.45 ±l.OO a> 39.4 ±7.974 10 3.59 ±O.ll 10 57.4 ±5.724 (starved) 11 3.48 ±O.04 10 57.5 ±3.275 16 3.66 ±O.07 10 59.5 ±O.935 (starved) 15 3.49 ±O.08 10 58.9 ±O.986 a> 3.71 ±O.08 10 59.3 ±1.196 (starved) 18 3.50 ±O.09 10 50.7 ±10.997 10 3.73 ±O.09 10 59.3 ±O.997 (starved) 16 3.34 ±O.10 10 08 ID 3.74 ±O.12 10 59.3 ±l.OO8 (starved) 17 3.41 ±O.l2 10 09 16 3.85 ±O.07 10 58.7 ±2.38

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