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A sorting method for eliminating fish larvae without functional swimbladders
Aquocuhm-, 107 (1992) 81-88 Eisevier Science Publishers B.V.. Amsterdam
AQUA
81
20003
A sorting method for eliminating fish larvae without functional swimbladders B. Chatain
and D. Corrao
Eqwpe MEREA. Slarron Exp~rim~n~aled’Aqriaci‘f~ure IFRE.:.tfER. falavas-ies-Flats. France (Accepted 2 March 1992)
ABSTRACT Chat&n. B. am’ Corrao. D.. 1992. A sorting method for eliminating fish larvae withou: funclianal swmbladders. Aquac~rllsrr, 07: 81-88. We describe.a simple soning method for rcpararing cubured fish larvae with functional swimbladden from those wthoul based on density differences. The wiiolc population wyasfirst anacsthetired with MS 22’! and then the fish were separated: fish with a functional swimbladder float and those without sinl.. The efficiency of the separation method was tested at several anaesthetic doses (0.02 to 0.1 g Iwith sea bass (D~cen~rarchus/abrm) and sea bream (Sparus aurarus) larvae in the 6-34 mm (total length) range. The minimal :wiog size was for sea bass with an optimal anaesthetic dos: of 0.07 g MS 222 I- ‘_ There were “I,! enough data lo draw conclusions for sea bream. The method was satisfactory when applied in real conditions ID a large (90 000) population of sea bass iry with an eficiency ratio ofover 80%.
’)
I5 mm
IN? RODUCTION
The absence of swimbladder development is a major pathological problem in the larval rearing of many sea fish species. Fis!, with no functional swimbladder show delayed growth (AI Abdul Elah et al., 1983; Giavenni and Doimi, 1983; Chatain, 1987), lordotic skeletal malformations (Kitajima, 1978; Paperna, 1978; Chatain, 1982) and higher mortality rates when they are subjected to stress (Chatain and iIewavrin, 1989). In marine fish culture, normal swimbladder inflation generally varies from 0 to 25%. This weak rate is a considerable obstacle in intensive rearing conditions; thus there have been many investigations into the possible causes of swimbladder non-developmenr. Depending on the species studied, depleted swimbladder inflation rates have been linked to water quality (Chatain, 1982; Battaglene and Talbot, 1990), tank hydrodynamics (Dorosbev and Cornacchia, 1979: Iseda et al., Correspondence to: Dr. B. C’hatain, Equipe MEREA, Station Experiment& IFREMER, chemio de Maguclone. 34250 Palavas-tes-flats, France.
0044-8486/92/$05.00
d’Aquaculture
0 I992 Elsevier Science Publishers B.V. All rights reserved.
B CHATAIN AND D CORRr\O
82
1979; Kitajima et al., I98 I; Chat+ 1982; Battaglene and Talbot, 1990; Chatain and Ounais-Guschemann, 1990), lighting characteristics ( BattagJene and Talbot, 1990; Joassard, unpubl. results) and food quality (Kitajima, 1978; Watanabe et al., 1978, 1982, Chatain, I 48i; Comeillie, 1989). It is now clear that in some species the swimbladdii inflation process is triggered by air gulping at the surface (Kitajima et al., 198 I ). Chatain and Ounais-Guschemann ( 1990) recently reported a straightforward device composed of an air blower connected to a floating trap to eliminate oily surface film. This apparatus improved the initial inflation rate in Spares aurutus and Dicentrurchus labrax. When the initial inflation process does not depend on surface access. or when air gulping partially fails, then it is crucial to separate and eliminate fish larvae without functional swimbladdr .:s zs early as possible to avoid the abovementioned problems. We describe a sim,Ae method for sorting such fish any time during the rearing process. T!tis ntethod was tested on Dicentrurchars labrax and Sparus auratus, two essential aquaculture species. MATERIAL
AND
METHODS
Sorting method principle The sorting method is based on density differences between fish with or without functional swimbladders. The whole population is first anaesthetized which causes the fish to separate into two groups: fish with a functional swimbladder fioat and those without sink. Choice ofminimalsorting size The functional swimbladder in Dicentrarchw labrax and SpanIs auratw is characterized by the presence of gas inside the primordial vesicle (Chatain, 1986). This vesicle contains no gas in newly hatched larvae. Normal swimbladder development begins with the initial intlation: a gas bubble appears inside the bladder. When this initial inflation fails, swimbladder development stops at a stage that resemb!es pre-inflation. The organ is thus considered as being non-functional (Chatain, 1986), even though late inflation has been observed occasionally, irr 7-z :?a bream (Chatain, in prep. ). Normal inflation occurs at 5 mm (total length f in sea bass larvae and 4 mm in sea bream. The upper size limits for this initial inflation phenomenon are 6.5 and 5.0 mm for the;e species, respectively (Chatain, 1986). We therefore selected
lengths slightly higher than these two values for lowest size limits to test the sorting method. Experimental Test groups of 20 larvae were introduced into separate l-l beakers tilled with 35% sea water at 20°C. The sire range v’;fs 7-3 I mmfor sea bass larvae and 6-34 mm for sea bream larvae. We used MS 222 (Sandoz) as an anaes-
METHOD
FOR SoRtlNG
FISH LARYAE WITHOLT
FUNCTlON.~L
SWIMBLADDERS
83
thetic. This chemical is effective at low doses, with an immediate and reversible effect. It is highly soluble in sea water and harmless to fish and man. The molecule is not persistent in fish (Hun? c? al., 1968). Nine different doses, ranging from 0.02 to 0.1 g I-‘, were obtained by successive dilution from a 1 g I- ’ stock solution. Larvae were introduced into the beaker and the time required for all larvae to be anaesthetized was recorded for each group. Floating and sinking larvae were counted and placed in separate beakers containing fresh sea water. The time required far larvae to recover their normal behaviour was recorded. At the end of the experiment, larvae with or without a functional swimbladder were determined by counting the floating and sinking larvae under a profile projector. RESULTS
AND DISCUSSION
Anaesthesia and recovery times are shown in Fig. 1. For both species and all groups. no anaesthesia was observed at 0.02 g I-’ and generally-more than 10 min was required to anaesthetize the fish at the 0.03 g I-’ dose. At 0.040.06 g I-’ concentrations, anaesthesia was complete afte; 3-l 1min, whereas it was complete in l-3 min at doses of 0.07-0.1 g 1-l. For the smallest larvae (6 mm for sea bream and 7 mm for sea bass), recovery occurred after 4-5 min when the anaesthetic dose did not exceed G.06 g I-‘. At higher doses, recovery time was proportional to the dose, reaching 9 and 7 min, respectively, for bream and bass at 0.1 g I- ’ MS 222 concentration. For the other size classes and for all doses, complete recovery occurred after 2-6 min. Some fish with functional swimbladders (SB+ ) were co!!ect?d wirh fish having no functional swimbladder (SB- ) and vice versa. Sorting is perfect when only SB+ fish float and only SB- fish sink. The efficiency of the sorting method can thercfore be expressed by Ef (the ratio of SB+ fish in the floating group) and by E- (the ratio of SB- fish in the sinking group). In the 42 trials carried out with sea bass, E+ ranged from 60% to 100%and Eranged from 33% to 100%. In the 27 trials with sea bream, E+ and E- ranged from 0% to 100%. The interactions between the two ratios with fish size, anaesthetic dose and the proportion of SB+ fish in the population to be sorted were expressed as Spearman’s rank correlation coefficients (Table 1). For each species, both sorting method efficiency ratios were independent of the effective anaesthetic doses (0.03-o. I g 1-l ). For sea bass. the ratios were also independent of the percentage of SB+ 5sh in the population to be sorted (range O-90%) but dependent on larval size. Sorting was more effective with increased sizes. Both ratios (E+ and E- ) were maximal ( 100%) when fish length exceeded 15 mm, and ranged from 70- 100% for lo- ! 5 mm larvae and were more variable (30-l 00%) for the smallest larvae (Fig. 2). This discrepancy might be explained by the fact that the youngest larvae were morphologica!!y similar to
A
Fig.I. Time required for anaesthesia with MS 2;: and recovery in Dicenlrarcfms luhruv (A) and Sparus auroras (B) larvae. Groups arecharacterized by dlfferenc mean lengths. For sea bass: 7 mm ~0~;10mm(0);12mm(A);13mm(A):15mm(0~:21mm(~):31mm(*).Forscabream: newly hatched Larvae:flat body, high surface to mass ratio (about 1:0.6) and presence of a primordial tin. All of these characteristics promotc floating, even without an inflated swimbladder. This morphology progressively disappears as the larvae reach IO-15 mm and begin their metamorphosis. For sea bream two significant correlations were observed, i.e between the efficiency ratios E+ ?nd E- and the proportion of SB+ fish in the groups to be sorted (fig. 3 ). ‘ihe negative correlation for both ratios indicated that the efticiency for reccvery of SB+ or SB- fish decreases as the proportion of SB+ in the initial population increases. This conclusion must be considered with caution
!Az!rla of Spearman’s rank correkaion coefliciena !correlated for ties) txlween the wtingefliciency ratios (E), the total larval length (Tl ), the anaesthetic dosL and the percentage of fish with functional swimbladders in the group to be soned (SB+ ). *.**Results are significant at 5% and l%levels, respectively. E+. percentage of SB+ fish recovered from floating fish; E-. percemagc of SB- fish recovered from sinking fish Dose
T1
SB+
E+
Sea bass TI SBt E+ E-
-0.152 0.043 -0.013 -0.228
-0.546” 0.362’ 0.535**
-0.111 -0.169
0 16Q
Sea bream TI SB+ E+ E-
0.023 -0.002 - C.026 O.OC’
-0.151 0.003 0.393
-0.732” -0.597**
---
‘1.196 -
Fig. 2. Efiiciency rate (E) variations for a method IO separate fish wiih (SB+ ) and without (SB- ) functional swimbladders versus mean larval length for Diccntrarchus lobrax. Ef. pezxntage of SB+ fish recovered from floaring fish; E-. perceaage of SB- fish recovered from sinking lish.
since few high percentages of SB+ were tested and only for larvae around 9 mm. Further experiments are necessary to set out the limits ofthe method for this species. For this sorting method, we thus recommend MS 222 doses of 0.07 to 0. I g I-’ for both species, and larvae exceeding 15 mm for sea bass until the optimal sorting size for sea bream is determined.
rm-
2 *so Y
. t
)
) Spurus u~~r,w~~s.
Rg. 3. EfFciency rate (E) variations for a method lo separate fish with (SB+ and whom (SBfunclional swimbladders versus the ratio of SB+ fish in the group to be sorted for E+ of SB+ lish recovered from floating fish: Eof SB- fish recovered from slnkmglish.
pcrcenlage
APPLICATION
percentage
TEST
The sorting method was tested on a large scale during the transfer of about 90 000 sea bass fry from a iarvai rearing tank to a weaning tank. These fish .vcre 35 days old and weightd about 30 mg with a mean length of 15 mm. Larvae were kept in the dark and not fed for 12 h before the beginning of sorting. These two conditions improved the density differences between SB+ and SB- fish: keeping larvae in the dark for several hours results in maximum expansion of the swimbladder (Chatain, 1982) and starving decreases the weight of the larvae. thu: _cpb&Gig the buoyancy characteristics of an inflated swimbladaer. Larvae were taken out of their tanks and placed in IO1buckets at 500-700 fry per liter. Air stones were used to provide air until the anaesthetic (0.07 g I-‘) was added. The anaesthesia time varied from 1 to 4 min. After anaesthesia, the fry were divided into three groups: larvae with fully expanded swimbladders floated at the surface, those with only partially expanded bladders remained at mid-depth. .vhile those without functional swimbladders or with functional but poorly expanded bladders sank to the bottom of the btlcket. Fry floating at the surface and at mid-depth were gently transferred from the bucket to the nursery tank by carefully pouring the water so that larvae on the bottom remained there. The SB+ larvae revived in less than I min and recovered normal swimming behaviour in 4-6 min. Recovery time for the SB - group exceeded IO min. After sorting, the two groups were counted, and sub-samples of 100 fish were randomly chosen to determine E + and E - ra-
tios. Four unexperienced cperators were able to sort 91 230 larvae in 3 h: 18 246 floating larvae were separated, which was 80% of the SB+ fry (E+ ). Only 2% of SB t were collected within the 72 984 sunken iarrae (E- =98%). Mortality was recorded for 3 days after sorting and affected 1% of the SB+ fish and 90% of the SB- fish. The method was also adapted to 1-S g fry. Fish were handled by net and retrieved in a contaiuer filled with 600/w sea water with 0. I g I-’ anaesthetic added. Efficiency ratios Et and E- were maximum (100%) and retrieval did not induce mortality. This nieihod was ad.opted by private producers in 1984 and most hatcheries now use it for both sea bream and sea bass with steady efticicncy ratios.
REFERENCES Al Abdul Elah. K.. Akatsu, S. and Ten8. SK.. 1983. Effects of water temperature and water turbul-nce on the growth. survival and accurrcncc of initial swimbladder inflation in sobaity (.-lcmihomzmo CIIWNI~ larvae. Annu. Res. Reo. Kuwait Inst. Sci. Res.. S: 40-48. Bauaglene. SC. and Talbot. R.B.. 1990. Initial &mb!adder inflation in intensively reared Austrahan bass larvae. .Ilurqaoriu now~~arnieura (Stcindachner) (Perc~formes: Perctchlnyidae). Aquaculture. 86: 431-442. Chalain. B.. I982 Contribution g Y&rude de I’&evage larvaire de la doradc japonaise (Chr,w+ phrts nrajor). Th&se de 3eme cycle. Univ. Aix-Marseille II, 121 pp. Chatain. B.. 1986. La v&e natatoire chez Dlcewarchus Io6ru.v et Sparus CW~~~BS.I. Aspect5 niuIphdu8iqu;sdu d~velcppement. hquar~lture. 53: 303-31 I. (‘Batam. 8.. I987 La vessie natatoire chez Dicmrrard~~r~ labraa et Spr. UJ (IIOULIF. II. lnlluence des anomalies de dCveloppement sur la crowance de la law?. Aquacultwe, C’ 175-t 81. Chatam. B.. in preparation. Abnormal swimbladder development and lordosis in sea bass (Dicr~~rrarchus labras) and sea bream (Spurus awal~~~ )_ Submitted t? Aquaculture. Chatain. B. and Dewavrin, G.. 19PO !n”ti~,u dcl anomaiies de dCveloppement de la vessie !I*,,.&.c hrlr la mortalite de Dicrwmrchtrs lahrar ad tours du sewage. Aquaculture. 78: 5561. Chatain. B. and Ounais-Guschemann, N.. 1990. Improved rate of initial swmbladdcr inflation
:
I
in intensivel? reared .Sportrsaur&u5. Aquaculture. 84: 345-353. Corneillie. S.. 1989. lnvloed van w3-poly-onverzadigde vewxen en verlichtingssterkte op de owrlrving. groei en morfologische ontaikkeling van de larven van de zeebaars, D~cnilrurchars /ob~as (Linnaeus. 1758). Doctoral thesis. Catholic Univ. of Louven. Belgium. I83 pp. Doroshev. S.1. and Cornacchia. J.W.. 1979. Initial swimbladder inflation in the larvaeof Tilapia ~~ros.wf~d~ica and Jforone su.w~/~s ( Walbaum Aquaculture, 16: 57-66. Giavenni. R. and Doimi. M.. 1983. Formazione. difierenziamento istologico ed aspetti patolo-
(Peters )
).
gici della vescica natatoria in larvr di branrmo (D~awrarchtrs labrax L., 1758). Riv. Ital. Pwic. Ittiop. A. XVlll(2). 71-80. Hunn. B.. Schoeltger. R.A. and Williord. W.A.. 1968. Turnover and urinary excretion of iree and acctylated MS 222 by rainbow trout (.Sa/fuu gurdrwri). 1. Fish. Rcs. Board Can., 25: 25-31. Iseda. H.. Ishihara. M.. Sumida. S.. Owaki. M. and Tabata. S.. 1979. PI e\ention ofdeformation in theJuveniles of red sea bream. I’ugrtir wa,or, reared in ponds. Relationship between the initial wring conditions and the lordotic deformity. Bull. Kumamoto Pref. Fkh. Exp. Sm., l:9-17.
88
a. CHATAlN AND D CORRAO
Ritajins, C., 1973. Acquisition offectilized eggs and mass culture cijuveniles of red sea bream t’a@ws major. Spec. Rep. Nagasaki Pref. Inst. Fish., no. 5,92 pp. Kitajima, C., Tsukashima, Y., Fojita, S., Watanabe, T. and Yone, Y., 1981. Relationship between uninflated swimbladder and lordotic deformity in hatchery-reared red sea bream Pag.w major. Boll. Jpn. Sot. Sci. Fish., 47 ( IO): 1289-1294. Psperna, !., :978. Swimhladder and skeletal deformation in hatchery-bred Sparus ~uw.ux. J. Fish Biol., 12: 109-I 14. Watanabe, T., Oowa, F., Kitajima, C. and Fuji& S.. 1978. Nutritional quality of brine shrimp Arremh salinn. as a living feed from the viewpoint of essential fatty acids for fish. Bull. Jpn. Soc.Sci.Fish.,44(10): 1115-1121. Watanabe, T., Ohta, M.. : jtajima, C. and Fujita, S.. 1982. Improvement of dietary value of brine shrimp Arfemia salina for fish larvae by feeding them on 03 highly unsaturated fatty acids. Bull. Jpn. SIX.. Sci. Fish.,48 (12): 1775-17X2.
AQUA
81
20003
A sorting method for eliminating fish larvae without functional swimbladders B. Chatain
and D. Corrao
Eqwpe MEREA. Slarron Exp~rim~n~aled’Aqriaci‘f~ure IFRE.:.tfER. falavas-ies-Flats. France (Accepted 2 March 1992)
ABSTRACT Chat&n. B. am’ Corrao. D.. 1992. A sorting method for eliminating fish larvae withou: funclianal swmbladders. Aquac~rllsrr, 07: 81-88. We describe.a simple soning method for rcpararing cubured fish larvae with functional swimbladden from those wthoul based on density differences. The wiiolc population wyasfirst anacsthetired with MS 22’! and then the fish were separated: fish with a functional swimbladder float and those without sinl.. The efficiency of the separation method was tested at several anaesthetic doses (0.02 to 0.1 g Iwith sea bass (D~cen~rarchus/abrm) and sea bream (Sparus aurarus) larvae in the 6-34 mm (total length) range. The minimal :wiog size was for sea bass with an optimal anaesthetic dos: of 0.07 g MS 222 I- ‘_ There were “I,! enough data lo draw conclusions for sea bream. The method was satisfactory when applied in real conditions ID a large (90 000) population of sea bass iry with an eficiency ratio ofover 80%.
’)
I5 mm
IN? RODUCTION
The absence of swimbladder development is a major pathological problem in the larval rearing of many sea fish species. Fis!, with no functional swimbladder show delayed growth (AI Abdul Elah et al., 1983; Giavenni and Doimi, 1983; Chatain, 1987), lordotic skeletal malformations (Kitajima, 1978; Paperna, 1978; Chatain, 1982) and higher mortality rates when they are subjected to stress (Chatain and iIewavrin, 1989). In marine fish culture, normal swimbladder inflation generally varies from 0 to 25%. This weak rate is a considerable obstacle in intensive rearing conditions; thus there have been many investigations into the possible causes of swimbladder non-developmenr. Depending on the species studied, depleted swimbladder inflation rates have been linked to water quality (Chatain, 1982; Battaglene and Talbot, 1990), tank hydrodynamics (Dorosbev and Cornacchia, 1979: Iseda et al., Correspondence to: Dr. B. C’hatain, Equipe MEREA, Station Experiment& IFREMER, chemio de Maguclone. 34250 Palavas-tes-flats, France.
0044-8486/92/$05.00
d’Aquaculture
0 I992 Elsevier Science Publishers B.V. All rights reserved.
B CHATAIN AND D CORRr\O
82
1979; Kitajima et al., I98 I; Chat+ 1982; Battaglene and Talbot, 1990; Chatain and Ounais-Guschemann, 1990), lighting characteristics ( BattagJene and Talbot, 1990; Joassard, unpubl. results) and food quality (Kitajima, 1978; Watanabe et al., 1978, 1982, Chatain, I 48i; Comeillie, 1989). It is now clear that in some species the swimbladdii inflation process is triggered by air gulping at the surface (Kitajima et al., 198 I ). Chatain and Ounais-Guschemann ( 1990) recently reported a straightforward device composed of an air blower connected to a floating trap to eliminate oily surface film. This apparatus improved the initial inflation rate in Spares aurutus and Dicentrurchus labrax. When the initial inflation process does not depend on surface access. or when air gulping partially fails, then it is crucial to separate and eliminate fish larvae without functional swimbladdr .:s zs early as possible to avoid the abovementioned problems. We describe a sim,Ae method for sorting such fish any time during the rearing process. T!tis ntethod was tested on Dicentrurchars labrax and Sparus auratus, two essential aquaculture species. MATERIAL
AND
METHODS
Sorting method principle The sorting method is based on density differences between fish with or without functional swimbladders. The whole population is first anaesthetized which causes the fish to separate into two groups: fish with a functional swimbladder fioat and those without sink. Choice ofminimalsorting size The functional swimbladder in Dicentrarchw labrax and SpanIs auratw is characterized by the presence of gas inside the primordial vesicle (Chatain, 1986). This vesicle contains no gas in newly hatched larvae. Normal swimbladder development begins with the initial intlation: a gas bubble appears inside the bladder. When this initial inflation fails, swimbladder development stops at a stage that resemb!es pre-inflation. The organ is thus considered as being non-functional (Chatain, 1986), even though late inflation has been observed occasionally, irr 7-z :?a bream (Chatain, in prep. ). Normal inflation occurs at 5 mm (total length f in sea bass larvae and 4 mm in sea bream. The upper size limits for this initial inflation phenomenon are 6.5 and 5.0 mm for the;e species, respectively (Chatain, 1986). We therefore selected
lengths slightly higher than these two values for lowest size limits to test the sorting method. Experimental Test groups of 20 larvae were introduced into separate l-l beakers tilled with 35% sea water at 20°C. The sire range v’;fs 7-3 I mmfor sea bass larvae and 6-34 mm for sea bream larvae. We used MS 222 (Sandoz) as an anaes-
METHOD
FOR SoRtlNG
FISH LARYAE WITHOLT
FUNCTlON.~L
SWIMBLADDERS
83
thetic. This chemical is effective at low doses, with an immediate and reversible effect. It is highly soluble in sea water and harmless to fish and man. The molecule is not persistent in fish (Hun? c? al., 1968). Nine different doses, ranging from 0.02 to 0.1 g I-‘, were obtained by successive dilution from a 1 g I- ’ stock solution. Larvae were introduced into the beaker and the time required for all larvae to be anaesthetized was recorded for each group. Floating and sinking larvae were counted and placed in separate beakers containing fresh sea water. The time required far larvae to recover their normal behaviour was recorded. At the end of the experiment, larvae with or without a functional swimbladder were determined by counting the floating and sinking larvae under a profile projector. RESULTS
AND DISCUSSION
Anaesthesia and recovery times are shown in Fig. 1. For both species and all groups. no anaesthesia was observed at 0.02 g I-’ and generally-more than 10 min was required to anaesthetize the fish at the 0.03 g I-’ dose. At 0.040.06 g I-’ concentrations, anaesthesia was complete afte; 3-l 1min, whereas it was complete in l-3 min at doses of 0.07-0.1 g 1-l. For the smallest larvae (6 mm for sea bream and 7 mm for sea bass), recovery occurred after 4-5 min when the anaesthetic dose did not exceed G.06 g I-‘. At higher doses, recovery time was proportional to the dose, reaching 9 and 7 min, respectively, for bream and bass at 0.1 g I- ’ MS 222 concentration. For the other size classes and for all doses, complete recovery occurred after 2-6 min. Some fish with functional swimbladders (SB+ ) were co!!ect?d wirh fish having no functional swimbladder (SB- ) and vice versa. Sorting is perfect when only SB+ fish float and only SB- fish sink. The efficiency of the sorting method can thercfore be expressed by Ef (the ratio of SB+ fish in the floating group) and by E- (the ratio of SB- fish in the sinking group). In the 42 trials carried out with sea bass, E+ ranged from 60% to 100%and Eranged from 33% to 100%. In the 27 trials with sea bream, E+ and E- ranged from 0% to 100%. The interactions between the two ratios with fish size, anaesthetic dose and the proportion of SB+ fish in the population to be sorted were expressed as Spearman’s rank correlation coefficients (Table 1). For each species, both sorting method efficiency ratios were independent of the effective anaesthetic doses (0.03-o. I g 1-l ). For sea bass. the ratios were also independent of the percentage of SB+ 5sh in the population to be sorted (range O-90%) but dependent on larval size. Sorting was more effective with increased sizes. Both ratios (E+ and E- ) were maximal ( 100%) when fish length exceeded 15 mm, and ranged from 70- 100% for lo- ! 5 mm larvae and were more variable (30-l 00%) for the smallest larvae (Fig. 2). This discrepancy might be explained by the fact that the youngest larvae were morphologica!!y similar to
A
Fig.I. Time required for anaesthesia with MS 2;: and recovery in Dicenlrarcfms luhruv (A) and Sparus auroras (B) larvae. Groups arecharacterized by dlfferenc mean lengths. For sea bass: 7 mm ~0~;10mm(0);12mm(A);13mm(A):15mm(0~:21mm(~):31mm(*).Forscabream: newly hatched Larvae:flat body, high surface to mass ratio (about 1:0.6) and presence of a primordial tin. All of these characteristics promotc floating, even without an inflated swimbladder. This morphology progressively disappears as the larvae reach IO-15 mm and begin their metamorphosis. For sea bream two significant correlations were observed, i.e between the efficiency ratios E+ ?nd E- and the proportion of SB+ fish in the groups to be sorted (fig. 3 ). ‘ihe negative correlation for both ratios indicated that the efticiency for reccvery of SB+ or SB- fish decreases as the proportion of SB+ in the initial population increases. This conclusion must be considered with caution
!Az!rla of Spearman’s rank correkaion coefliciena !correlated for ties) txlween the wtingefliciency ratios (E), the total larval length (Tl ), the anaesthetic dosL and the percentage of fish with functional swimbladders in the group to be soned (SB+ ). *.**Results are significant at 5% and l%levels, respectively. E+. percentage of SB+ fish recovered from floating fish; E-. percemagc of SB- fish recovered from sinking fish Dose
T1
SB+
E+
Sea bass TI SBt E+ E-
-0.152 0.043 -0.013 -0.228
-0.546” 0.362’ 0.535**
-0.111 -0.169
0 16Q
Sea bream TI SB+ E+ E-
0.023 -0.002 - C.026 O.OC’
-0.151 0.003 0.393
-0.732” -0.597**
---
‘1.196 -
Fig. 2. Efiiciency rate (E) variations for a method IO separate fish wiih (SB+ ) and without (SB- ) functional swimbladders versus mean larval length for Diccntrarchus lobrax. Ef. pezxntage of SB+ fish recovered from floaring fish; E-. perceaage of SB- fish recovered from sinking lish.
since few high percentages of SB+ were tested and only for larvae around 9 mm. Further experiments are necessary to set out the limits ofthe method for this species. For this sorting method, we thus recommend MS 222 doses of 0.07 to 0. I g I-’ for both species, and larvae exceeding 15 mm for sea bass until the optimal sorting size for sea bream is determined.
rm-
2 *so Y
. t
)
) Spurus u~~r,w~~s.
Rg. 3. EfFciency rate (E) variations for a method lo separate fish with (SB+ and whom (SBfunclional swimbladders versus the ratio of SB+ fish in the group to be sorted for E+ of SB+ lish recovered from floating fish: Eof SB- fish recovered from slnkmglish.
pcrcenlage
APPLICATION
percentage
TEST
The sorting method was tested on a large scale during the transfer of about 90 000 sea bass fry from a iarvai rearing tank to a weaning tank. These fish .vcre 35 days old and weightd about 30 mg with a mean length of 15 mm. Larvae were kept in the dark and not fed for 12 h before the beginning of sorting. These two conditions improved the density differences between SB+ and SB- fish: keeping larvae in the dark for several hours results in maximum expansion of the swimbladder (Chatain, 1982) and starving decreases the weight of the larvae. thu: _cpb&Gig the buoyancy characteristics of an inflated swimbladaer. Larvae were taken out of their tanks and placed in IO1buckets at 500-700 fry per liter. Air stones were used to provide air until the anaesthetic (0.07 g I-‘) was added. The anaesthesia time varied from 1 to 4 min. After anaesthesia, the fry were divided into three groups: larvae with fully expanded swimbladders floated at the surface, those with only partially expanded bladders remained at mid-depth. .vhile those without functional swimbladders or with functional but poorly expanded bladders sank to the bottom of the btlcket. Fry floating at the surface and at mid-depth were gently transferred from the bucket to the nursery tank by carefully pouring the water so that larvae on the bottom remained there. The SB+ larvae revived in less than I min and recovered normal swimming behaviour in 4-6 min. Recovery time for the SB - group exceeded IO min. After sorting, the two groups were counted, and sub-samples of 100 fish were randomly chosen to determine E + and E - ra-
tios. Four unexperienced cperators were able to sort 91 230 larvae in 3 h: 18 246 floating larvae were separated, which was 80% of the SB+ fry (E+ ). Only 2% of SB t were collected within the 72 984 sunken iarrae (E- =98%). Mortality was recorded for 3 days after sorting and affected 1% of the SB+ fish and 90% of the SB- fish. The method was also adapted to 1-S g fry. Fish were handled by net and retrieved in a contaiuer filled with 600/w sea water with 0. I g I-’ anaesthetic added. Efficiency ratios Et and E- were maximum (100%) and retrieval did not induce mortality. This nieihod was ad.opted by private producers in 1984 and most hatcheries now use it for both sea bream and sea bass with steady efticicncy ratios.
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