2006) 495–506www.elsevier.com/locate/aqua-online
Aquaculture 255 (
Larval and juvenile rearing of common sole (Solea solea L.) in theNorthern Adriatic (Italy)
Renato Palazzi, Jacopo Richard ⁎, Gabriella Bozzato, Lorenzo Zanella
Veneto Agricoltura, Impianto Ittico Sperimentale “Pellestrina”, Strada dei Murazzi, Pellestrina, Venice, Italy
Received 21 October 2005; received in revised form 9 January 2006; accepted 19 January 2006
Abstract
Research on the rearing cycle of the common sole, Solea solea, was done at an experimental hatchery in the Lagoon ofVenice (Italy). The aim was to develop rearing schedules suitable for market production and document any technical problems.Larval metamorphosis studied on 9 groups of larvae reared at 18°C demonstrated high temporal variability. Caudalmetamorphosis and eye migration occurred between 9days after hatching (DAH) and 24DAH, and between 13DAH and25DAH, respectively. A larval rearing schedule based on live-food feeding was set at 18–19°C, which achieved an averagesurvival rate of 40% at 28DAH.
Three weaning trials comparing two commercial feeds were carried out on larvae about 30DAH. One of these feeds wassufficient in itself to complete juvenile weaning, reaching average survival rates of 85%, which are comparable to those obtained inthe control groups fed with live Artemia. Average survival rates of 43% were obtained with the second commercial feed. Bothcommercial feeds enabled superior juvenile growth on average to that in the control groups.
An on-growing trial in extensive conditions was done in an earthen pond of 370m2, stocking juveniles with an averageweight of 3.6g at a density of 1.5juveniles/m2. The trial started in mid-September and lasted until the following August, whenit was stopped because of high mortality due to viral encephalopathy and retinopathy infections. Growth was negligible duringwinter and began again in spring, reaching the maximum incremental rate between May and June, at temperatures of between20 and 25°C. The specific growth rate never exceeded a daily value of 2%, while the average final size reached after10months rearing was 12g.
A double replicate trial of intensive rearing was carried out starting with soles of 7g, reared at a density of 150juveniles/m2
in circular fibreglass tanks of 10m2 surface. This trial was also stopped during the following summer because of theconsiderable mortality due to viral infection. The soles had reached an average size of 54g in August, after 300-day rearing.
Sole can be bred and reared with good efficiency related to its survival rate, but the results of the growth trials, both intensiveand extensive, do not allow conclusions to be made on the growth performance in the experimented conditions. The healthproblems compromised the growth trials towards the middle of the favourable growing season. The trials highlight both the highsusceptibility of sole to viral encephalopathy and retinopathy infections, and the scarce tolerance of this species to temperatures ofabove 25°C, which caused the onset of frequent bacterial infections.© 2006 Elsevier B.V. All rights reserved.
Keywords: Solea; Larval rearing; Metamorphosis; Weaning; On-growing
⁎ Corresponding author. Fax: +39 041 2734014.E-mail address: [email protected] (J. Richard).
0044-8486/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.aquaculture.2006.01.042
496 R. Palazzi et al. / Aquaculture 255 (2006) 495–506
1. Introduction
The common sole (Solea solea L.) is a speciesappreciated by the market and is a candidate forrearing on a commercial scale, although no signifi-cant productions presently exist in Italy. Somestudies on different aspects of the biologicaldevelopment and husbandry of this species havebeen published, most of which were aimed atformulating a diet suitable for supporting high solelarvae survival through the critical weaning stage(Bromley, 1977; Mackie et al., 1980; Métailler et al.,1981; Cadena Roa et al., 1982; Métailler et al., 1983;Appelbaum, 1985; Day et al., 1997). There are fewdata on the growth performance of common sole andalmost none for the on-growing stage of the rearingcycle. Howell (1997), in an interesting analysis ofthe literature, pointed out that nutritional require-ments and bacterial pathologies represent the techni-cal difficulties which researchers must overcome inorder to encourage the farming of sole. Morerecently, Imsland et al. (2003) published a reviewof the culture potential of S. solea in comparisonwith Solea senegalensis. More information is avail-able on the commercial husbandry of S. senegalensis(Dinis et al., 1999; Imsland et al., 2003), but thisflatfish presents important differences in growthperformance and optimal thermal regime.
Our research focused on survival and growthperformances of common sole in mass culture condi-tions, starting with the fertilised eggs and continuing thetrials for 18months. The aim was to apply rearingtechniques commonly adopted in commercial fishfarming, adapting them to sole on the basis of theinformation reported in the scientific literature. The aimwas to propose suitable rearing methods for commonsole farming and also to verify the data, generallyobtained from small-scale trials, reported in theliterature.
Age (DAH) 0 1 2 3 4 5 6 7 8 9
Microalgae (n° cell. / ml)
Rotifers (n° / ml)
Artemia (430 µm sized) nauplii (n°•106/105 initial larvae/day)
(n°•106/105 initial larvae/day) Artemia metanauplii
Fig. 1. Feeding schedule adopted for larval rearing until 28days after hatchingreported throughout the day.
Several groups of larvae were also studied in order todescribe morphological and chronological developmentof the larvae through metamorphosis. Larval survivalwas assessed in a large-scale hatchery, supplyingtechnical feeding regimes based on live food andweaning diets available on the market. Juveniles werereared in extensive and intensive conditions, testinggrowth performances and tolerance to environmentalconditions in the northern Italian lagoons.
2. Material and methods
The research was performed in the Veneto Agricol-tura hatchery sited in the lagoon of Venice (NorthernAdriatic). All tanks utilised in the experiment, except theextensive pond, were integrated in recirculation systemswith a total volume of about 50m3 each and equippedwith mechanical–biological filters. Salinity ranged from33‰ to 35‰.
The trials were designed to collect data on survivaland growth rates of common sole through both the larvaland juvenile stages. Methods based on the current stateof knowledge were tested and adapted to make themsuitable for large-scale productions, according to thefollowing sequence:
• larval rearing from fertilised egg until 28days afterhatching (DAH), adopting a feeding regime based onlive food only;
• weaning trials performed on juveniles aged 30 and40DAH using commercial weaning diets;
• juvenile growth for about 12months in extensive andintensive stocking conditions.
2.1. Larval rearing
After a series of preliminary trials, a feeding regimewas obtained that guaranteed high survival rates even ingroups of larvae of different ages (Fig. 1). The efficiency
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
(DAH), microalgae and rotifers densities were maintained at the values
Table 2Chemical composition of pre-growing (Perla) and on-growing feed(Excel) supplied by Skretting S.p.A.
Skretting Perla Skretting Excel
% Protein 55 48% Lipid 18 20% N-free extract 5–6 8–10% Ash 10 10.3% Moisture 7–8 7–8Raw energy (kJ g−1) 19.7 19.5
Table 1Chemical composition of weaning diets
Aglonorse (SSF) Gemma (Trow France)
% Protein 58–60 55% Lipid 18–22 14% N-free extract 4 6% Ash 9–15 13.5% Moisture ≤10 10Raw energy (kJ g−1) 24 19.5
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of the feeding regime was tested during three larvalrearing trials starting from groups of 90,000–110,000fertilised eggs until 28DAH. They were performed at18–20°C in conical-based cylindrical tanks of 550-l capacity, mixing larvae obtained from egg spawn over3–4 subsequent days in order to reach a density of 150–200larvae/l. Until 28DAH, their diet was exclusivelybased on live food, consisting of rotifers enriched withcommercial integrators, Artemia nauplii at high HUFAcontent and Artemia metanauplii enriched with com-mercial integrators.
Other trials were carried out in conical-basedcylindrical tanks of 80-l capacity, where 10 groupsof larvae obtained from single spawnings, between7000 and 11,500 individuals, were reared to study thevariability of survival rates and metamorphosis timing.Temperatures were maintained at 18±0.5°C in orderto achieve comparable development times in thedifferent groups of larvae.
2.2. Weaning trials
The test consisted of a comparison between thesurvival rate of a control group fed only on liveenriched Artemia metanauplii, and experimentalgroups fed with two dry feeds (Table 1). The trialswere performed in flat-bottomed cylindrical tanks of0.95m2 surface, where the fish were stocked at adensity of 3157juveniles/m2.
The experimental program lasted 2years according tothe following scheme:
– first year: trial 1 followed by trial 2 started with 30-and 40-day-old larvae, respectively. The larvae wereweaned using the commercial feed “Aglonorse”produced by Norwegian Herring Oil and MealIndustry Research Institute (SSF, Fyllingsdalen,Norway). It was distributed by hand several timesper day from 0800 to 2200 h. The temperature rangedfrom 18°C to 27°C according to the season.
– second year: just one double replicate trial wasperformed (trials 3a and 3b) starting with 30-day-
old larvae and completing activities in springtime,before any unfavourable temperature occurred(>25°C). The same previously tested dry feed,this time marketed by EWOS (Norway), wascompared with the formulated diet “Gemma”,supplied by Trow France (France). The feedswere distributed from 1000 to 2400 h by beltfeeders. The temperature ranged from 18°C to27°C, but the values >25°C only occurred at theend of the trial.
Weaning was done according to the followingfeeding regime:
• pre-weaning period of 4–7days, during which thegroups of weaning larvae were fed on formulatedfeeds together with the usual Artemia ration;
• a weaning period of 17days during which theArtemia ration was progressively reduced until itwas completely suspended;
• a final period of 28–31days during which the fishwere only fed with formulated feeds. Unweanedindividuals starved. The duration of this period wasdefined in order to guarantee the negative selection ofunweaned individuals.
2.3. On-growing trials
Two growth rate trials were carried out in bothextensive and intensive culture conditions. The exten-sive rearing began in September by stocking soles of3.6g, at a density of 1.5 juveniles/m2, in an earthen pondof rectangular section and surface of about 370m2. Fishfed on natural diet only.
A double replicate growth rate trial was carried outin intensive culture conditions, rearing soles of 7g,aged 200DAH, at a density of 150juveniles/m2. Twocircular fibreglass tanks, of 10m2 surface and about9m3 capacity were used. Soles were fed ad libitumwith commercial extruded feed formulated for sea fishand supplied by Skretting SpA (Table 2).
0
10
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30
40
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70
80
surv
ival
per
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multiple spawnings groups single spawning groups
average
average
Fig. 2. Survival percentage at 28DAH in groups of larvae obtained from multiple or single spawning.
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In winter, the water in the tanks was thermo-conditioned to prevent temperatures falling below 10°C.Biometric data were recorded every 2weeks, but while inthe intensive condition 50 specimens could be caughtregularly, in the extensive condition only a few individualswere caught in several sampling operations. The growthperformances were studied computing the specific growthrate (SGR=(ln final weight− ln initial weight)⁎100/days).
3. Results and discussion
3.1. Larval rearing
The feeding regime proposed gave a survival rate ofabout 40% at the 28DAH stage. Fig. 2 shows howlarval survival in the single spawning groups resultedas highly variable in comparison to the groupsobtained by mixing different spawnings. The variabil-ity of survival in the single spawning groups could be
CAUDAL METAMORPHOSIS (18ºC)
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10121416182022242628
1 3 4 5 6 7 92 8
AG
E (
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YS)
GROUPS
Fig. 3. Representation of onset and duration of caudal metamorphosis (left) iinvolved in metamorphosis events. In the right histogram, the three shades of gmovement on left side of the head, passage through the midline of the head
related to fertilised egg quality. Nevertheless, averagesurvival appears to be similar in both 80-l and 550-l rearing tanks, demonstrating that the practice ofmixing several spawnings increases the probability ofobtaining standard results.
The trials also showed that survival at 28DAH is notsignificantly affected by the presence, in the same tank,of larvae hatched over 3–5 successive days. The basicexplanation for this is not only the absence of canniba-lism in this species, but it also suggests that food changesin the feeding regime were correctly balanced.
The study of metamorphosis at 18°C pointed outhigh variability in the timing of both eye migration andcaudal metamorphosis. Important differences in thebeginning and duration of the metamorphic phaseswere observed in specimens of the same group and alsobetween single spawned groups. Fig. 3 summarises themetamorphosis timing observed in 9 groups of larvaeobtained from single spawnings, reared at 18°C. The
1 3 4 5 6 7 92 8
AG
E (
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YS)
EYE MIGRATION (18ºC)
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GROUPS
n single spawned larvae at 18°C. The grey areas represent the periodsrey (from light to dark) correspond to the three phases of eye migration:, movement on the right side of the head.
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histogram simplifies the stages of the eye migrationcompared to the scheme scrupulously represented byFernández-Díaz et al. (2001) for S. senegalensis.Caudal metamorphosis took place in the period from9 to 24DAH, and required 8–14days to reachcompletion. The eye migration took place in the periodfrom 13 to 25DAH, requiring 8–11days to reachcompletion. The eye metamorphosis of group no.8 appears anomalous, with the conclusion of the eyemigration being delayed until 28DAH.
It should be mentioned that metamorphic eventsinvolving the head and tail of larvae can occur withoutreciprocal co-ordination. A larva can present earlymetamorphosis of the tail and late migration of the eye,or vice versa.
Fig. 4. Pre-larva at 0DAH. On the day of hatching, the larva presentsseveral incomplete organs and apparatus. This terminal stage of theembryonic period is also defined “pre-larval stage”. The eyes are notpigmented and still not functional. The anus is open but not the mouth.The yolk is very abundant and feeding is exclusively endogenous.
Fig. 5. Pre-larva at 1DAH. Most of the yolk has been resorbed. Eyedevelopment is in progress but the retina is not pigmented. The lowerjaw begins to appear.
The following photographs illustrate the main stagesof larval development at 18°C [Figs. 4–15].
3.2. Weaning trials
The results of the first and third trials demonstratedthat weaning techniques can produce excellent survivalrates, if the characteristics of the feed used suit therequirements of the sole, which is much moredemanding than sea bass or gilthead sea bream. Thebest results were produced by Norwegian Aglonorsefeed, which was also recommended by Howell (1997).According to this author and Day et al. (1997), theaddition of highly digestible proteins deriving fromhydrolysed fish protein concentrate is very important.The use of Aglonorse provided sole weaning survival
Fig. 6. Pre-larva at 2DAH. The mouth is open and the pre-larval stageends. At 18°C, the larvae complete this important event at differentmoments of the day. The specimen in this picture was photographed inthe morning.
Fig. 7. Larva at 3DAH. The mouth and eyes are completely functional.Internal feeding is still active but some larvae feed on live food.
Fig. 10. Larva at 16DAH. This larva is 1day older than the specimen inFig. 9, but eye migration has not yet started. Caudal metamorphosis,however, is underway: urostyle bends dorsally and cartilaginousstructures are forming along its ventral edge. These structures consistof basal plates that will support the caudal fin rays. The swim bladder,very evident in this specimen, is not present in all larvae and isresorbed no later than 30DAH. The development of muscle tissuesreveals fibre arrangement in myotomes.
Fig. 9. Larva at 15DAH. Caudal metamorphosis is underway: thecaudal region becomes asymmetric for proliferation of cartilaginoustissues in the ventral–caudal area of the primordial fin fold. Themuscle tissues of the trunk are developing along the sagittal plane. Eyemigration is beginning. Tissue transparency reveals that the left eye isshifting to the dorsal midline of the head.
Fig. 8. Larva at 9DAH. The larva grows stronger and muscle organisationin myotomes begins to be appreciable. The specimen in the picture hasabundant food in the gut. An early proliferation of cartilaginous structurescan be observed along the lower caudal tip of the notochord. Thismorphological evolution generally appears 1 or 2days later.
Fig. 11. Larva at 18DAH. Urostyle bending goes ahead while thecaudal fin begins to assume its typical shape and caudal rays can beobserved. The whole trunk becomes stronger maintaining a flattenedshape along the sagittal plane.
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Fig. 14. Juvenile at 23DAH. The sole has concluded its larval stage tobegin juvenile life, according to Boulhic and Gabaudan (1992). Botheyes are on the right side of the body and the sole assumes the benthicbehaviour typical of flatfishes.
Fig. 12. Larva at 20DAH. In a lateral view from the right side, the lefteye is appearing round the midline. The urostyle gets shorter andcaudal peduncle appears stronger. The dorsal and ventral fins begin toassume their definitive look.
Fig. 13. Larva at 19DAH. The picture shows the most representativestage of metamorphosis, when the left eye reaches the dorsal midlineof the head. The larvae begin to change their swimming from verticalto benthic. Transparency of the body begins to reduce as skinpigmentation intensifies.
Fig. 15. Juvenile at 28DAH. Appearance of the juvenile at the start ofthe weaning stage.
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rates comparable with those of the control groups.Acceptable final survivals of between 33% and 54%were also obtained using the Gemma feed. All weanedindividuals registered weight increases similar to orabove those of the controls, with a distinctly positiveaverage result for Aglonorse feed. The trial results arerepresented in Figs. 16 and 17.
The second trial is not comparable with the othersbecause it took place during summer and was thereforeaffected by unfavourable temperatures. During the first10days the temperature rose to 30°C. This caused stressto the soles, which were affected by bacterial diseaseswhenever exposed to a thermal regime of above 25°C.We could activate a small cooler to reduce the water
temperature to 25°C, but this condition did not provesuitable for weaning.
Trials 1 and 3 (both replicates) gave comparableresults for the Aglonorse feed, which produced survivalsvarying from 79% to 89% of the initial groups. Fig. 16shows a moderate reduction in survival 7–10days afterthe end of feeding with live food. This mortalityinvolved the loss of unweaned soles and lasted about 5–7days, suggesting that juveniles aged 50DAH starved in12–15days at these temperatures.
The Gemma feed produced fewer and more variablesurvivals, suggesting that its composition probablymade the weaning practice more critical. Nevertheless,we consider that the Gemma feed satisfied thenutritional requirements of the sole, as the relatedSGR values (Fig. 17) showed a growth performancecomparable with the other experimental groups.
3rd weaning trial (1 replicate)st
28 34 40 46 52 58 64 70 76 82
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Control Aglonorse Gemma Temperature
3rd weaning trial (2 replicate)nd
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Control Aglonorse Temperature
weaningtime
weaningtime
weaned groups fedwith dry feed only
weaned groups fedwith dry feed only
Fig. 16. Survival rate (%) of the sole (DAH: days after hatching) compared against temperature fluctuations (°C) recorded in four weaning trials.
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3.3. On-growing trials
During the intensive rearing trial in winter, thetemperature was maintained at 10°C. Over this pe-riod, the fish ate regularly and a modest but con-
Fig. 17. Specific growth rate (SGR) obtained during the 1st and 3rdtrials.
stant weight gain was recorded (Fig. 18A), while thegrowth rate increased from May when the temperaturerose above 18°C. In both reared groups, soles mainlysuffered from bacterial diseases caused by bacteria ofthe Vibrio group. These events interfered with theregular growth of fish, causing significant mortality(Fig. 18B).
In June, when the temperature rose above 25°C,there was evidence of the first typical symptoms of viralencephalopathy and retinopathy infection in one of thetwo tanks. Virus isolation and study of the pathologicalcourse in sole were performed by Borghesan et al.(2003). The affected group of soles was eliminated toavoid the spread of infection. Despite this, the viralsymptoms subsequently also spread to the second tank,causing high mortality and the almost total death of thereared group of soles in a short time.
As Fig. 18A shows, the trial was interrupted during astage of rapid growth of the soles. Moreover, the growthperformances were reduced due to the spread of diseaseamong the cultured fish, which showed a significant
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diminution in appetite. We consider that the averagefinal weight of 54g, reached in August, could easilyhave been higher in healthy fish.
The extensive rearing trial involved water tempera-ture fluctuations from a minimum of 3°C to a maximumof 29°C. There was no evidence of relevant mortalitiesin winter, despite the low temperatures. Fish growthagain became appreciable in March, reaching amaximum in May and June. During this period, thewater temperature ranged between 20 and 25°C and thefish displayed no evidence of pathologies. Low stockingdensity and the benthic behaviour of the fish madesampling operations difficult, so average weights wereoften computed on few specimens.
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oct nov dec jan feb mar
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(ºC
)
Temp. (ºC) Re
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B
A
Fig. 18. Temperature fluctuation (°C), average weight (g; A) and survivalindicate the standard deviation related to average weight.
From July onwards the water temperature wasalmost always above 25°C and, as in the intensiverearing, a serious viral encephalopathy and retinopathyinfection was recorded, causing the death of almost allthe fish in August. The growth data are represented inFig. 19.
4. Discussion and conclusions
Common sole rearing is not a relevant commercialactivity at present, although Mediterranean aquacul-ture needs to diversify its productions. We attemptedto perform experimental trials to verify specifictechnical information in the literature, applying them
5
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plicate 2 Temp. (ºC)
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(%; B) registered during the intensive rearing trial. The vertical lines
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Oct JanDecNov AprMarFeb JulJunMay
Fig. 19. Temperature fluctuation (°C) and average weight (g) registered during the extensive rearing trial. The vertical lines indicate the standarddeviation related to average weight.
Table 3Specific growth rate (SGR) at different temperature ranges during theon-growing trial (1st replicate)
Period of time(days)
Temperature(°C)
Averagetemperature (°C)
Sizerange (g)
SGR
169 10–15 11.6 7.3–17.5 0.528 15–20 18.7 17.5–19.5 0.415 20–25 22.8 19.5–22.1 0.869 25–29 26.8 22.1–53.9 1.3
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to rearing techniques suitable for adoption in large-scale farming. The species has demonstrated to beadaptable to hatchery practices, especially during theinitial larval stages, when feeding is based on livefood. Broodstocks spawned eggs progressively overseveral days, so making it difficult to obtain highnumbers of fertilised eggs daily. This was the firstproblem to overcome, because commercial-scaleproductions need to manage larva stocks composedof several tens of thousands of specimens, accordingto tank size. In this study we could rear a densebatch by mixing larvae differing by up to 4days inage, but in other trials we obtained similar resultsmixing larvae differing by up to 6days in age. Thiswas possible due the absence of cannibalism but, atthe same time, the presence of individuals at differentdevelopment stages obliged us to change live foodsgradually in order to guarantee that younger larvaecould feed efficiently.
We managed live foods following the traditionaltechnique adopted for sea bream rearing, althoughHowell (1997) attests that sole do not demand nutrientintegrators in live foods. On the other hand, severalabnormalities in larval development are described inSolea senegalensis (Lagardère et al., 1993; Gavaia etal., 2002) and nutritional deficiencies are consideredto be among the probable causes. These anomalousmorphologies are sometimes not easily detectable inlarvae and juveniles and we preferred to feed larvaewith enriched plankton to prevent any complications.
Metamorphosis occurred, at 18°C, from 10 to25DAH, but the events of this delicate process canvary their starting time and duration. Survival of thelarvae at 28DAH, when metamorphosis was always
complete, was roughly equal to 40–45%. Highersurvival rates have been reported by other researchers(Howell, 1997), but the data have often been obtained ona small batch of larvae under laboratory conditions. Ourtrials also demonstrate that survival can vary widelybetween groups of larvae obtained from single spawn-ings, even though the data collected confirm that anaverage survival of 40% can be considered a sustainabletarget.
In the past, weaning was considered the majorobstacle to sole rearing and several attempts were madeto obtain suitable weaning diets (Bromley, 1977;Métailler et al., 1981; Appelbaum, 1985; Cañavateand Fernández-Díaz, 1999), focusing on the chemicalsubstances acting as feeding stimulants (Mackie et al.,1980; Cadena Roa et al., 1982; Métailler et al., 1983).Diet palatability and digestibility both appear to becritical aspects in sole weaning, but the latter is the moredifficult to solve in our opinion. Very attractive weaningdiets have recently been developed by feed producers inorder to reduce the consumption of artemia cysts ineuryhaline fish hatcheries and we have tested some ofthese in the past. The weaning trials did not produce any
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positive results despite the fact that the fish started tofeed on weaning diets (unpublished). The capability ofthe sole to digest dietary proteins is conditioned by theprogressive development of the gastrointestinal appara-tus (Boulhic and Gabaudan, 1992). The proteolyticdigestive enzymes have been detected in S. solea laterand at lower concentrations than in S. senegalensis(Clark et al., 1986; Ribeiro et al., 1999), explaining thespecific nutritional requirements of the common sole.
The trials reported here confirm the conclusionspresented by Day et al. (1997) and Howell (1997) aboutthe effect of dietary hydrolysed fish protein concentrate(HFPC) on growth and survival of juvenile sole. Howell(1997) reported that the SSF diet for sole, the same onewe used, is formulated with 6% of betaine as feedingattractant and HFPC up to 30% of the total proteincontent. Our weaning trials were performed at an earlierstage of juvenile development (30–54DAH) than thestudies of Howell (1997) and Day et al. (1997),nevertheless survival varied from 80% to 90%, attestingto the diet digestibility also at this age.
Weaning trials performed by Harman et al. (2001) onthe Senegal sole comparing the SSF diet with anotherweaning diet confirmed the former as the best.
During the 2years of this research, beside theexperimental trials reported, we applied the samemethods to run little fingerling productions, obtainingmore than 100,000 weaned juveniles per year. We hadno reliable information on the age at which commonsole become able to efficiently digest commercial feedsfor sea-fish. We gradually replaced the SSF diet withanother commercial diet (Perla—Skretting SpA) beforethe fish reached a mean weight of 1g.
The on-growing trials showed that common sole canbe reared efficiently in intensive conditions, with growthrates above those in extensive conditions. Unfortunately,the occurrence of viral pathologies affected our results,reducing both the appetite and survival of the experi-mental groups. It is interesting to note that we recordedthe maximum growth rates when the temperature roseabove 25°C despite the high mortality. The datareported in Table 3 allow a comparison of the SGRrecorded at different temperature ranges in the firstreplicate of the intensive trial. The temperature favour-able to sole growth (20–25°C) occurred for only15days, which could be too short a time to appreciatethe potential growth performance. Although the follow-ing stage of rearing at 25–29°C was affected by heavymortality, the SGR resulted as the highest in the trial andis comparable to the values reported for S. senegalensisby Dias et al. (2004). We considered the possibility thatthe virosis might select the largest individuals in the
population, so invalidating the recorded SGR. Werejected this hypothesis after end of the trial, giventhat the average size of the soles decreased with theprogressive loss of the survivors. This would suggestthat resistance to the viral pathology, among soles of thesame age, is not positively related to the size ofspecimens.
According to Howell (1997), common sole resultedas being very sensitive to bacterial diseases, butBaynes and Howell (1993; cited in Howell, 1997)also suggest that diet can strongly affect the resistanceof this species to bacterial pathologies. Given the goodresults obtained in weaning trials with the specific dietproduced by SSF-Ewos, we feel that the main obstacleto sole farming is currently the lack of a balanced on-growing feed for sole nutritional requirements. Al-though little information is available on this for S.solea, studies on S. senegalensis show that specificdiets can improve the growth performance (Coutteau etal., 2001). Moreover, unlike sea bass and sea bream,the growth performance of S. senegalensis is notpositively affected if fed diets with a high fat/carbohydrate ratio (Dias et al., 2004).
Our overall experience suggests that common soleS. solea is suitable for commercial rearing although itappears to be less tolerant to a farming environmentthan traditional euryhaline species. Spawning andlarval rearing can presently be performed efficientlyusing specific commercially available weaning diets,while the on-growing stage could be improved bythe availability of specifically formulated commercialdiets. Low tolerance to high temperature will preventthe farming of this species wherever waters arewarmer than 25°C in summer. Further investigationsinto nutritional requirements are necessary to under-stand the real growth performance of the sole and itssusceptibility to disease. The availability of a specificon-growing diet could have a positive effect on thegrowth rate by enhancing feed digestibility andimproving the health status of the fish.
Acknowledgements
We would like to give special thanks to SandroMarangon and Giovanni Salvagno for their essentialcontribution to all the rearing and experimental practices.We are also grateful to Dr. Antonia Francescon, Dr.Alvise Barbaro and Dr. Daniela Bertotto who managedthe sole spawnings making the fertilised eggs available.
This study has been funded by the Italian Ministry forAgriculture and Forestry Policies (Project no. 6.C.8 ofthe 6° Plan for Fishery and Aquaculture).
506 R. Palazzi et al. / Aquaculture 255 (2006) 495–506
References
Appelbaum, S., 1985. Rearing of the Dover sole, Solea solea (L),through its larval stages using artificial diets. Aquaculture 49,209–221.
Borghesan, F., Palazzi, R., Zanella, L., Vascellari, M., Mutinelli, F.,Montesi, F., Manfrin, A., Selli, L., Bovo, G., 2003. Viralencephalopathy and retinopathy infection in reared common sole(Solea solea). (Abstract). 11th International Conference “Diseasesof fish and shellfish” of the European Association of FishPathologists (EAFP), 21st–26th September 2003, St Julians(Malta).
Boulhic, M., Gabaudan, J., 1992. Histological study of theorganogenesis of the digestive system and swim bladder of theDover sole, Solea solea (Linnaeus 1758). Aquaculture 102,373–396.
Bromley, P.J., 1977. Methods of weaning juveniles hatchery rearedsole (Solea solea (L.)) from live food to prepared diets.Aquaculture 12, 337–347.
Cadena Roa, M., Huelvan, C., Le Borgne, Y., Métaller, R., 1982. Useof rehydratable extruded pellets and attractive substances for theweaning of sole (Solea vulgaris). J. World Maric. Soc. 13,246–253.
Cañavate, J.P., Fernández-Díaz,C., 1999. Influenceof co-feeding larvaewith live and inert diets on weaning the sole Solea senegalensisonto commercial dry feeds. Aquaculture 174, 255–263.
Clark, J., Murray, K.R., Starck, J.R., 1986. Protease development inDover sole (Solea solea L.). Aquaculture 53, 253–262.
Coutteau, P., Robles, R., Spruyt, W., 2001. Ongrowing feed forSenegal sole (Solea senegalensis Kaup). Abstracts of Contribu-tions Presented at the International Conference AquacultureEurope 2001. Special Publication no. 29. European AquacultureSociety, pp. 58–59.
Day, O.J., Howell, B.R., Jones, D.A., 1997. The effect of dietaryhydrolysed fish protein concentrate on the survival and growth ofjuvenile Dover sole, Solea solea (L.), during and after weaning.Aquac. Res. 28, 911–921.
Dias, J., Rueda-Jasso, R., Panserat, S., da Conceição, L.E.C., Gomes,E.F., Dinis, M.T., 2004. Effect of dietary carbohydrate-to-lipidratios on growth, lipid deposition and metabolic hepatic enzymesin juveniles Senegalese sole (Solea senegalensis., Kraup). Aquac.Res. 35, 1122–1130.
Dinis, M.T., Ribeiro, L., Soares, F., Sarasquete, C., 1999. A review onthe cultivation potential of Solea senegalensis in Spain and inPortugal. Aquaculture 176, 27–38.
Fernández-Díaz, C., Yúfera, M., Cañavate, J.P., Moyano, F.J.,Alarcón, F.J., Diaz, M., 2001. Growth and physiological changesduring metamorphosis of Senegal sole reared in the laboratory. J.Fish Biol. 58, 1086–1097.
Gavaia, P.J., Dinis, M.T., Cancela, M.L., 2002. Osteologicaldevelopment and abnormalities of the vertebral column and caudalskeleton in larval and juvenile stages of hatchery-reared Senegalsole (Solea senegalensis). Aquaculture 211, 305–323.
Harman, L.A., Conceição, L., Hardy, C., 2001. The effects (growthand survival) of weaning Senegal sole (Solea senegalensis) post-larvae with natural and artificial diets. 18th Annual Meeting of theAquaculture Association of Canada “Aquaculture Canada 2001”,May 6–9 2001. Nova Scotia, Halifax. Short communication.
Howell, B.R., 1997. A re-appraisal of the potential of the sole Soleasolea (L.) for commercial cultivation. Aquaculture 155, 355–365.
Imsland, A.K., Foss, A., Conceição, L.E.I., Dinis, M.T., Delbare, D.,Schram, E., Kamstra, A., Rema, P., White, P., 2003. A review ofthe culture potential of Solea solea and S. senegalensis. Rev. FishBiol. Fish. 13, 379–407.
Lagardère, F., Boulhic, M., Bürgin, T., 1993. Anomalies in thecephalic area of laboratory-reared larvae and juveniles of thecommon sole, Solea solea: oral jaw apparatus, dermal papillae andpigmentation. Environ. Biol. Fish. 36, 35–46.
Mackie, A.M., Adron, J.W., Grant, P.T., 1980. Chemical nature of thefeeding stimulants for the juvenile Dover sole Solea solea (L.). J.Fish Biol. 16, 701–708.
Métailler, R., Cadena Roa, M., Person-Le Ruyet, J., 1983. Attractivechemical substances for the weaning of Dover sole (Soleavulgaris): qualitative and quantitative approach. J. World Maric.Soc. 14, 679–684.
Métailler, R., Menu, B., Morinière, P., 1981. Weaning of Dover sole(Solea vulgaris) using artificial diets. J. World Maric. Soc. 12,111–116.
Ribeiro, L., Zambonino-Infante, J.L., Cahu, C., Dinis, M.T., 1999.Development of digestive enzymes in larvae of Solea senegalensis,Kraup 1858. Aquaculture 179, 465–473.
