Critical dissolved oxygen level to Penaeus setiferus and Penaeus schmitti postlarvae (PL10–18) exposed to salinity changes

ELSEWIER

Aquaculture 152 (1997) 259-272

Critical dissolved oxygen level to Penaeus setiferus and Penaeus schmitti postlarvae ( PL 10_18)exposed to salinity changes Carlos Rosas a**, Adolf0 Sinchez a, Eugenio Diaz-Iglesia Roberto Brito b, Evenor Martinez ‘, Luis A. Soto d

b,

aLaboratorio de Ecojisiologia, Departamento de Biologia, Facultad de Ciencias, UNAM, Mixico 04510. D.F. Mgxico b Centro de Inuestigaciones Marinas, Uniuersidad de la Habana, Ave. la, 2808, Miramar, LQ Habana, Cuba ’ Uniuersidad National Aut6noma de Nicaragua, Le&, Nicaragua d Laboratorio de Ecologia de1 Bentos, Institato de Ciencias de1 Mary Limnologia, UNAM, Mexico 04510, D. F. Mkxico

Accepted 5 November 1996

Abstract Dissolved oxygen is the most important limiting factor in the intensive cultivation of shrimp species. The critical oxygen level and its effects upon the energy metabolism of postlarvae (PL ,,,_,s) of Penueus setifems and Penueus schmitti exposed to diverse salinities were estimated. In both species the critical oxygen level (COL; estimated as the point of inflection of the curve obtained from the relation between the oxygen consumption and the oxygen concentration) was affected by salinity concentration. In P. schmitti, the COL was 5 mg 1-l for salinities of 38, 30, 20 and 15%0, and 4.5 mg 1-l for animals kept at 25%0 salinity. In P. setiferus, the COL was 5.0 mg 1-’ in 37, 30, 25, 20 and 10%~ salinities and 4.5 mg 1-l in 15, 5 and 1%0 salinities. The energy deficit (END; deficit of metabolic energy) caused by the metabolic oxygen critical concentration was in P. schmitti 13.9% (38%0 salinity) and 26.3% (30%0 salinity), with intermediate values of 17.2, 22.7 and 24.7% in 25, 20 and 15%0 salinities, respectively. For P. set&mu, the END was between 9.1% cl%‘00salinity) and 25.1% (30%0 salinity), with intermediate values for the remaining salinities. Based on these data, the optimum salinity for P. setiferus postlarvae was between 5%0 and 15%0, and was 25%0 for P. schmitti postlarvae. At these salinities the tolerance for the decrease in the oxygen concentration was greater than in the rest of the salinities. The high COL obtained for the postlarvae of both species indicates that the culture conditions must be

* Corresponding author. 00448486/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOO44-8486(96)01516-5

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carefully maintained at this developmental stage to obtain the maximum metabolic performance of the shrimp. 0 1997 Elsevier Science B.V. Keywords:

Dissolved oxygen; Peruzeus setiferus; Penaeus schmitti; Salinity

1. Introduction Oxygen concentration is the major limiting water quality variable in intensive shrimp culture (Boyd, 1989; Hopkins et al., 1993). To avoid problems with low dissolved oxygen, many producers use mechanical aeration. The efficacy of aeration has been investigated in relation to salinity (Ruttanagosrigit et al., 1991), shrimp stocking rates (Hopkins et al., 1991) and orientation of ponds to the wind (Garcia and Brune, 1991). Nevertheless, the majority of these systems have been utilized without previously establishing the minimum concentration of oxygen that limits the species being cultivated. Consequently, shrimp farmers often maintain an oxygen concentration that exceeds requirements. This increases the cost of production. In general, aquatic organisms are oxygen regulators or oxygen conformers, depending upon their ability to regulate metabolism as a function of oxygen concentration. For oxygen regulators, this ability and their behaviour will be limited to the concentration of oxygen beneath which the respiratory metabolism is dependent upon the oxygen concentration (Vemberg, 1983). This point has been defined as the incipient limiting oxygen level (Fry, 1947) and is referred to here as the critical oxygen level or P,. For cultivated species, such as shrimp, this can affect growth and therefore production. Oxygen consumption has been considered as a direct indicator of the metabolic scope (MS; the quantity of energy in the organism that is available to bring about biochemical, physiological and ecological functions) (Vemberg, 1983). Analysis of the effects of oxygen concentration on oxygen consumption permits a more precise estimation of the minimum oxygen concentration needed to maximize the MS. No such studies exist in which the critical oxygen level (COL) for Penueus shrimp is reported. There have been studies that have reported COLs of between 4.5 and 5 mg l- ’ for Penueus juponicus (Egusa, 1961) and between 4 and 4.3 mg 1-l for Penaeus monodon (Liao and Chien, 1994). In other studies, the effect of oxygen concentration upon growth rate and survival of shrimp has been analysed. Seidman and Lawrence (1985) reported that, below 2 mg 0, l-‘, growth of juvenile Penaeus vannamei and P. monodon was significantly reduced. Allan et al. (1990) reported that the ammonium toxicity for P. monodon increased 60% when the animals were maintained in 2.3 mg 1-l of dissolved oxygen. Allan and Maguire (1991) found that exposures of between 0.5 and 1.1 mg 0, l- ’ for an interval of between 4 and 12 h did not affect the subsequent growth and survival of P. monodon maintained at saturation levels. The postlarvae of shrimp are produced or captured in salinities of between 30 and 40%0 but are later placed in lower salinities where their growth is greater (AQUACOP, 1986). Depending upon the water changes and the characteristics within each tank, the salinity can increase or decrease and can produce changes in the behaviour of the organisms. Although species such as P. uannamei, Penaeus setiferus and Penaeus schmitti can tolerate wide ranges of salinity, a salinity of between 15 and 25%0 has been

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recommended as ideal for the growth of postlarvae of these species (Castille and Lawrence, 1981; Boyd, 1989). P. schmitfi and P. setiferus are species that have been successfully cultivated in the tropical Atlantic (Hopkins et al., 1993; Sandifer et al., 1993; Femandez et al., 1994; Jaime et al., 1994). The objective of this study was to determine the COLs and their effects on the energy metabolism of postlarvae (PL 10_18) of P. setiferus and P. schmitti exposed to different salinities.

2. Material and methods Postlarvae (PL,,_,s) of P. setijkrus and P. schmitti were raised in the laboratory under optimum feeding conditions (Alfonso et al., 1988; Gallardo et al., 1995). The experiments with P. schmitti were carried out during March 1994 at the Centro de Production de Postlarvas de Tunas de Zaza, Cuba. The postlarvae were obtained from larval rearing tanks before being transferred to grow-out ponds. The experiments with P. setiferus postlarvae were carried out during July 1994 at the UNAM-INP Shrimp Program of the Regional Center for Fishing Research located at Lerma, Campeche, Mexico. Two hundred PL,, of P. schmitti (2.3 f 0.1 mg per animal) or P. setiferus (2.4 f 0.05 mg per animal) were placed in 24 1 containers at a density of 25 PL 1-l. The postlarvae of both species were maintained at the same temperature to which they had been exposed in the larval rearing tanks. The postlarvae of P. schmitti were maintained at 25 + 1°C 38%~, salinity, pH 8.1, and oxygen concentration greater than 5 mg 1-l. The postlarvae of P. setiferus were maintained at 28 f 1°C 37%0 salinity, pH 8.0, and oxygen concentration greater than 5.0 mg l- ‘. In both experiments, natural sea water was filtered through sand filters and cartridges (5 pm) and sterilized with ultraviolet light. The animals remained in the above conditions for 24 h before each experiment. 2.1. Experimental

design

Except for the first water change (38%0 to 30%~ salinity for the P. schmitti, and 37%0 to 30%0 salinity for the P. setiferus), the postlarvae from both species were exposed to salinity changes at intervals of 5%0 per day. The dilutions were made with distilled water at the same temperature used for each species. During this time, the animals were fed ad libitum with a balanced diet of more than 50% protein (Garcia et al., 1992; Jaime and Garcia, 1992; Gaxiola, 1994). The survival rates were checked daily. 2.2. Oxygen consumption The decrease in the oxygen concentration produced by one postlarva in a closed respirometer of 600 ~1 was recorded with a Stratkelvin RC 200 recorder. After 12 h of fasting and in the absence of food, postlarvae were transferred to the respirometer and the oxygen consumption measured every minute from saturation level down to 0.5 mg 0, 1-l. Before measurement, the postlarvae were acclimatized for 5 min in the respirometry chambers in order to reduce the effects of handling (Rosas et al., 1995a,b). After acclimation, the water in the chamber was carefully renewed; the chamber was

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C. Rosas et al. / Aquaculture 152 (1997) 259-2 72

then sealed with an RC 1302 electrode with a low intrinsic oxygen consumption. Oxygen consumption from individual measurements on a minimum of 10 postlarvae (replicate) for each salinity was measured at 25 f 0.5”C for P. schmitti and at 28 f 05°C for P. setiferus. Individual oxygen consumption was calculated taking into account the oxygen consumption of the electrode in natural sea water. Data were expressed as pg 0, hh’ mgg ’ (wet weight). The COL (mg 1-l ) was determined by the point of the inflection of the curve obtained from the relation between the oxygen consumption (pg 0, h- ’ mg- ’ PL) and the concentration of dissolved oxygen (mg 0, 1-l ). A two-phase linear regression was used to obtain the point of intersection of two lines which was used to indicate the COL. The maximum metabolic scope (MS,,, > and the critical MS level (MS,) were calculated based upon the data of oxygen consumption obtained above (maximum) and in the first point below the COL. That point was called metabolic critical oxygen concentration (MCOC). The oxygen consumption level in MCOC was labelled critical oxygen consumption (COC). The MS values were obtained from the conversion of oxygen consumption into energy units, using the factor of 14.06 J mgg ’ of oxygen consumed (Lucas, 1993). The difference between MS,,, and MS, was called the energy deficit (END) and was used as an indicator of the MS lost provoked by a P, oxygen concentration. Thus, the END was calculated with the goal of obtaining an index for the quantity of energy that could be lost after a determined amount of oxygen had been consumed as a consequence of a decrease in its concentration. END values for each salinity and species were obtained using the equation: END=

MS, - MS,,, MSnXW

Significant differences in the oxygen consumption by the postlarvae in each experimental salinity were assessed by ANOVA, using arcsine transformation before processing percentage data (Zar, 1974). A Duncan multiple range test was used to separate means where significant differences were found (Zar, 1974)

3. Results In general, the oxygen consumption of the postlarvae of both species remained constant at high oxygen concentrations (4.5-6 mg 0, l- ’ ) in all experimental salinities. At lower oxygen concentrations, the oxygen consumption decreased as a function of the concentration of dissolved oxygen (Figs. 1 and 2). The maximum oxygen consumption for P. schmitti (Fig. 1) was obtained in the animals kept at 38%0 salinities, with averages of 4.99 pg 0, hh’ mgg’ PL for oxygen concentrations between 5 and 6 mg 0, l- ‘. These values were statistically different from the maximum values obtained at the other salinities (Table 1). The COL was 5 mg 1-l for salinities of 38, 30, 20 and 15%0, and 4.5 mg 1-l for animals kept at 25%0 salinity (Fig. 1). The r2 values obtained from the linear equations of two-phase regression (Table 2) indicate that the points of intersection adequately describe the COL obtained in each salinity for both species.

C. Rosas et al./Aquaculture

0

0.5

1

1.5

2

2.5

Dissolved

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152 (1997) 259-272

3

3.5

Oxygen,

4

5

5.5

6

1 5

5.5

1 6

4.5

mgn

1.2-

* 0 0

I 0.5

1

1 2

1.5

’ 2.5

Dissolved

3

1 3.5

Oxygen,

1 4

4.5

mg/l

1.6pGzG&q

1.4-

E” ;- 1.2 9 5

-

-_ , :

1 ,I’

.Z

,’ ----

__

;

/,’

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z 8 0.6 &

0m’ 1 010.4 E 0.2 -

01 0

,,’

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’ 1.5

i I 2

1 2.5

Dissolved

1 3

3.5

Oxygen,

1 4

4.5

1

1

5

5.5

6

mg/l

Fig. 1. Oxygen consumption of Penaeus schmifti postlarvae in relation to dissolved oxygen and salinity. The arrow indicates the point of intersection between the two-phase linear correlations, corresponding with the critical oxygen level.

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3-

0

/ 0.5

0

I 1

I 1.5

I 2

1 2.5

Dissolved

I 3 Oxygen.

I 3.5

4

4.5

I 5

4

4.5

5

; 5.5

mg/l

0 0

0.5

1

1.5

2

2.5

Dissolved

3 Oxygen,

3.5

5.5

mg/l

4: IJc15o/oo

,

I

F >32

s I-x= E2: z ti

I

: :

5

,’ 0

0

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1 1.5

:

: :

f’r /’ ’ 2

I 2.5

Dissolved

I 3 Oxygen.

I 3.5

I 4

;

i

/ 4.5

/ 5

/ 5.5

mg/l

Fig. 2. Oxygen consumption of Penaeus setijenrs postlarvae in relation to dissolved oxygen and salinity. The arrow indicates the point of intersection between the two-phase linear correlations, corresponding with the critical oxygen level.

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0

0

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152 (1997) 259-272

i

0.5

1

1.5

2

2.5

Dissolved

3

3.5

Oxygen.

4

4.5

5

5.5

mgj

Fig. 2 (continued).

For P. setiferus (Fig. 2), the maximum oxygen consumption (at an oxygen level of 4.5-5.5 mg I-‘) was obtained in postlarvae held at 11%0 salinity, with an average value of 6.45 mg 0, h- ’ mg- ’ PL. In relation to salinity, oxygen consumption values could be divided into two groups: one group consisting of those animals kept at 37, 30, 25 and 20%0 salinity, the other group consisting of animals kept at 15, 10 and 5%0 salinity. The maximum average oxygen consumption of the first group (20-37%0: 2.41 l,r,g 0, h-’ - ’ PL) was 25% less than the maximum average obtained in the second group ;5!15%0: 3.19 kg 0, h-’ mgg’ PL). The maximum COL in this species was 5 mg l- ’ (20-37%0 and 10%~ salinities); the minimum COL was 4.5 mg 1-l in animals kept at 1, 5 and 15%0 salinities (Table 3). For P. schmitti, the COC decreased between 38 and 30%0 salinity and remained constant until 20%0 salinity (Fig. 3). In 15%0 salinity, the COC was significantly lower than values obtained between 20 and 38%0 salinity (Table 1). For P. setiferus, the COC

Table 1 Effect of salinity, critical oxygen level (COL) and metabolic critical oxygen concentration (MCOC) on oxygen 1, critical oxygen consumption (COC), metabolic scope (MS maximum, MS,,, and consumption maximum WO,,,, MS critical, MS,) and energy deficit (END) by P. schmitti postlarvae (PL 1o_18) Salinity

COL (mg 1-l)

MCOC (mg 1-l)

(Wo)

V0,ma.x (pg hK’

MS,, (J day-’

COC (pg h-’

MS, (J day-

mg-‘)

In-l)

w-l)

mg-‘)

A

38

5.0

4.5

30 25

5.0 4.5

4.5 4.0

20 15

5.0 5.0

4.5 4.5

2.16’+0.10 1.19 bf0.06 1.05 ’ + 0.04 1.38 d f 0.09 1.02 ‘f 0.08

0.73”~kO.Oh 0.39 b+0.03 0.36 ‘f 0.01 0.46 d + 0.03 0.33 ‘f 0.03

1.87”+0.16 0.86 hf0.04 I .03 ’ + 0.06 1.07 ’ f 0.06 0.75 ‘f 0.05

END

’

B

C=A-B (.I day-’ mg-‘)

0.63”*0.06 0.29 ‘k0.02 0.29 ’ f 0.02 0.36 ’ + 0.03 0.25 d f 0.01

0.09”f0.004 0.10 =fO.O08 0.06 b + 0.00 0.10 = * 0.008 0.08 a i 0.008

Values are mean f SE. VO,,,, and MS_ obtained above COL. MS, obtained in P, concentrations. Values with the same superscript indicates no statistical differences (P < 0.05)

C/A (%o)

13.9 26.3

11.2 22.7 24.7

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Table 2 Two-phase linear regression between oxygen consumption ( y, mg h-’ mg- ’ wet weight) and oxygen concentration (x, mg-’ l- ‘1 of P. schmitti and P. setifenrs postlarvae exposed to salinity (%o) changes b

a

P. schmitti 38%0 1 2 30%0 1 2 25%0 1 2 20%0 1 2 15%0 1 2 P. setiferus 37%0 1 2 30%0 1 2 25%~ 1 2 20%0 1 2 15%0 1 2 lO%O 1 2 5%0 1 2 I%0 1 2

2.10 -0.19

0.002 0.43

r2

SE

0.98 0.94

0.02 0.016

1.23 -0.19

-0.008 2.55

0.93 0.89

0.007 0.04

0.95 - 0.30

0.01 0.26

0.95 0.95

0.04 0.008

0.001 0.09

0.96 0.92

0.04 0.02

0.96 0.98

0.04 0.002

0.99 0.92

0.0002 0.02

1.35 0.57 1.25 -0.18

2.51 - 0.37

-0.04 0.21

0.001 0.53

2.79 -0.17

- 0.06 0.46

0.99 0.98

0.0002 0.0001

1.70 -0.13

0.12 0.40

0.98 0.91

0.00001 0.003

2.95 - 0.25

-0.11 0.46

0.99 0.99

O.WOOl 0.0001

- 0.008 0.54

0.96 0.97

0.008 0.0001

3.15 -0.53

0.04 0.72

0.99 0.98

0.0001 0.0006

2.84 1.60

0.45 0.27

0.94 0.92

0.02 0.0008

6.52 3.83

0.008 0.63

0.97 0.98

0.009 0.0003

3.18 0.51

y = a + bx. 1, regression above MCOC values; 2, regression below MCOC values.

remained constant at salinities between 37 and 20%0 (Fig. 3). Between 20 and 15%0 salinity, the COC of P. s&ferns increased by 43%. A maximum COC value was obtained in postlarvae exposed to 1%0 salinity (Table 3).

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Table 3 Effect of salinity, critical oxygen level (COL) and metabolic critical oxygen concentration (MCOC) consumptton maxtmum WOZ,max ), critical oxygen consumption (COCJ, metabolic scope (MS maximum, critical, MS,) and energy deficit (END) on P. setiferus postlarvae (PL ,n_,8) Salinity

COL (mg I-‘)

MCOC (mg 1-l)

(%I

“Oz.max (Pg hh’

MS,,,. (J day- ’

mg-‘)

mg- ‘)

COC

MS, (1 day-

(pg h- ’ mg-‘)

on oxygen MS,,,, and

END

’

mg-‘)

A

B

C=A-B

C/A

day-’ mg-‘1

(a)

(J

37 30 25 20 1s I0 5

I

5.2 5.0 5.0 5.0 45 5.0 4.5 4.0

4.5 4.5 4.5 4.5 5.0 4.5 5.0 3.5

2.52 2.40 2.41 2.33 3.15 3.52 3.15 6.45

il * 0.22 = & 0.19 a + 0.18 a k 0.15 h f 0.19 ’ + 0.27 h * 0.24 ’ * 0.4 I

0.85 0.81 0.81 0.78 1.06 1.19 1.06 2.20

=+ “& “* ‘k hf ‘f h+ ’f

0.06 0.05 0.03 0.06 0.05 0.06 0.06 0.09

2.28 1.90 1.96 1.84 2.52 2.92 2.73 5.98

’+ h* h& ’+ ’i ’+ ’f ‘I +

0.16 o.t3 O.i6 0.16 0. I6 0.25 0.13 0.50

0.77 0.63 0.66 0.62 1.08 0.98 0.92 2.01

a+ h* h& h* ’k + ‘k df

0.04 0.04 0.05 0.05 0.06 0.08 0.07 0.10

0.08 a It 0.21 h + 0.15 c * 0.16 ’ + 0.21 h * 0.2 1 h * 0. I6 ’ + 0.20 h +

0.004 0.008 0.008 0.004 0.008 0.008 0.008 0.01

9.4 25.9 18.6 20.3 19.6 17.5 14.9 9.1

Values are mean k SE. obtained above COL. MS, obtained in P, concentrations. vo 2.rna.xand MS,,, Values with the same superscript indicates no statistical differences f P < 0.05).

For levels between 37 and 5%0 and between 38 and 15%0 salinity, the survival rate remained between 85% and 100% and 90% and 100% for P. sctiferus and P. schmitti, respectively. A considerable decrease in the survival rate of the P. setiferus postlarvae was registered in those animals kept at 1%0 salinity (P < 0.05; Fig. 4). For both species the MS was affected by the salinity as well as by the oxygen concentration (Tables 1 and 3).

*.

0

0

!“‘l’,‘,“l” 5

Esetiferus

10

15

““.““”

20

SALINITY

3. Oxygen consumption at metabolic postlarvae exposed to salinity changes.

Fig.

critic

oxygen

+ I? schmitti

25

I”

30

”

35

Oloo

concentration

levels

of P. schmitti and P. setiferus

268

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7-+

:

setiferus

I?

*

P schmitti

I

*e%+__zJ

l-

0

I

I

1

1

1

5

10

15

/

/ 20

SALINITY,

30

25

37-38

%m

Fig. 4. Survival of P. schmitti and P. seriferus postlarvae

exposed to salinity changes.

For P. schmitti postlarvae, the maximum MS,, was obtained in animals kept at 38%0 salinity. The minimum MS,, was obtained in shrimp kept at 15%0 salinity (Table 1). The MS, was also affected by salinity with maximum values in 38%0 and minimum in postlarvae kept at 15%0 (Table 1). The maximum differences between MS,,, and MS, was obtained in shrimp kept at 38, 30, 20 and 15%0 salinity. Between these groups there were no significant differences (Table 1). The value obtained from the difference between MS,,, and MS, at 25%0 salinity was significantly lower than that obtained at

setiferus

0

*I?

1

1

schmitti

I

1

6

10

15

SALINITY, Fig.

5. Energy deficit (END = (MS, - MS,,,)/MS,,,) salinity changes.

T

1

I

20

26

30

37-36

%o

of P. schmitti and P. setiferus postlarvae

exposed to

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the other salinities (Table 1). In P. schmitti, the energy deficit (END) was found to be between 11.9% (38%0 salinity) and 26.3% (30%0 salinity), with intermediate values at the other salinities (Table 1). For P. setiferus postlarvae (Table 31, the maximum MS,,, was obtained in animals kept at 1%0 salinity. The minimum MS,,, was obtained in shrimp kept between 20 and 37%0 salinity. The MS, had a behaviour similar to that of the MS,,,, with the highest values in animals exposed to 1%0 salinity and the lowest values between 20 and 30%0 salinity. The maximum differences between MS,,, and MS, were obtained in shrimp kept at 30, 10 and 1%0 salinity and the lowest values in postlarvae kept at 37%0 salinity (Table 3). END as a function of salinity reached a peak for the postlarvae of both species at 30%0 (Fig. 5). At this salinity, END was 26% followed by a lower value at 25%~ salinity. In P. schmitti, an increase in the END was registered in 20 and 15%0 salinities, with values closer than that obtained in 30%0 salinity (Table 1 and Fig. 5). For P. setijkrus, an increase in the END was registered in 20%0 salinity followed by a decrease in the shrimp kept at 15%0 salinity. END increased in salinities of 10 and 15%0, followed by a reduction in shrimps kept at 1%0 salinity (Table 3 and Fig. 5). 4. Discussion In the present study, a change in the oxygen consumption was observed in relation to salinity in the postlarvae of both species. For P. setiferus, oxygen consumption increased with decreasing salinity, while for the P. schmitti the highest metabolic rate was obtained at the highest salinity. The osmoregulatory ability of P. setiferus is well known. McFarland and Lee (1963) showed that P. setiferus is able to maintain the osmotic pressure of the haemolymph practically constant at between 5 and 30%0 salinity. In P. setiferus, the increase in the oxygen consumption observed in relation to the decrease in salinity (Table 3) can be interpreted as an increase in the energy requirements necessary to satisfy the osmotic adjustments of the internal medium. These adjustments represent an investment of 0.28 and 1.39 J day-’ mg-’ wet weight after a change of salinity from 20-37%0 to 5-15%0 and 1%0, respectively. Although information is not available for the osmotic regulatory ability of P. schmitti, it is possible that P. schmitti may be a euryhaline and osmotic regulator. In contrast to P. setiferus, P. schmitti postlarvae showed a decrease in the oxygen consumption in relation to a decrease in the salinity between 38 and 30%0 (Table 1). It seems that P. schmitti requires an increase in metabolic energy due to the important osmotic adjustments in high salinities. At 38%0, P. schmitti postlarvae required 84% more energy than for shrimp maintained at the other salinities. As a consequence, P. schmitti should be classified as a euryhaline species. According to the results obtained in this study, P. setiferus and P. schmitti postlarvae proved to be oxygen conformers at all experimental salinities. Above 5 mg 1-l) the oxygen concentration was not limiting and oxygen consumption was observed as a plateau. The inflection point of the curves obtained was associated with the critical oxygen level (COL). In both species, COL (4.5-5 mg 1-l) was close to saturation level. This means that both species of postlarvae are very sensitive to lower oxygen concentra-

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tions, independent of salinity. These values are close to the COLs reported for P. japonicus (5 mg l- ‘; Egusa, 1961) and P. monodon (4-4.3 mg 1-l ; Liao and Chien, 1994). One of the indicators utilized in the present study in order to evaluate the effects of the reduction in the dissolved oxygen was the metabolic scope (MS), derived from transformation of oxygen consumption to energy (J day-’ rng-’ wet weight) units. In the present study, the MS,,, and the MS, of both species of postlarvae were modified by both oxygen concentration and salinity. The difference between these values was considered as an indicator of the loss of energy provoked by an oxygen concentration below the critical oxygen level. This index was called energy deficit (END). In both species, values of END between 9 and 26% of the MS,,, were registered, depending on the experimental salinity. The largest END was registered at 30%0 salinity for P. setiferus and at 30 and 15%0 salinity for P. schmitti. At these salinities, an MCOC of 4.5 mg 1-l produced an END of between 25 and 26%. That is, an oxygen concentration considered ‘adequate’ for the growth of other species of shrimp (P. vannamei juveniles; Seidman and Lawrence, 19851, might decrease the metabolic potential of P. setiferus and P. schmitti postlarvae (PL,,_ ,8). The significance of a reduction of this nature could be associated with lower growth and/or a lower activity level (Seidman and Lawrence, 19851, and/or a lower ability to respond to the pathogenic organisms that are usually present in culture ponds (LeBlanc and Overstreet, 1991). Therefore, with an oxygen deficit, the loss of mobility might make the organism vulnerable to predation and/or cannibalism, especially when the animals are cultivated at high densities. A reduction in activity at low oxygen concentrations has been reported for Penaeus shrimp. Seidman and Lawrence (1985) observed a reduction in muscular activity and ingestion rates for juvenile P. vannamei cultivated at oxygen concentrations lower than 2 mg 1-i. According to the results obtained in this study, salinities of between 5 and 15%0 and of 25%0 proved to be best for P. setiferus and P. schmitti, respectively. Tolerance for the decrease in the oxygen concentration was greater at these salinities than at other salinities. Here the critical oxygen level (MCOC), survival and END proved to be less than those registered at other salinities. Although these salinities do not correspond to the isosmotic point (28%0) of P. setiferus, they are frequently found in the estuary habitat (2-23%0) where these animals are fished (3.5-6.8 lo6 kg per year) (Day et al., 1982). One of the objectives of the prebreeding period is to stimulate the rapid growth of the postlarvae in the conditions that they will experience during the fattening period. In these culture conditions, the animals are kept at a high density (40-60 PL rn-‘) and within an intensive feeding program (AQUACOP, 1986). Likewise, in the intensive cultivation it is common to seed postlarvae at high densities (90-150 PL m-‘1, which are maintained for up to 2 months; the animals are then ‘split up’ in order to reduce starting densities and to improve their growth (Liao and Chien, 1994) With the aim of reducing the effects of the low oxygen concentrations on P. setifkrus and P. schmitti postlarvae, a careful reduction of the salinity should be considered (at least 5%0 per day), which optimizes the metabolic scope of shrimps from sea water to 25%0 for P. schmitti and 15%0 for P. setiferus. If there is an adequate infrastructure for careful handling of

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the salt water, a salinity of 5%0 could be recommended for the cultivation of P. setiferus postlarvae. At this salinity a decrease of up to 4.5 mg 0, 1-l would not modify the metabolic potential of the postlarvae, this being the minimum oxygen concentration at could be obtained. Survival rates greater than 80% with levels of which the MS,,, production of between 3000 and 6000 kg haa’ have been reported for the intensive culture of P. setiferus. In these culture conditions, salinity was maintained between 18 and 25%0, and the dissolved oxygen between 4.8 and 5.1 mg 0, l- ’ (Hopkins et al., 1993). The interaction between oxygen concentration and other environmental factors should be studied before establishing the minimum quantities of oxygen needed for the cultivation of P. set&us and P. schmitti. The relationship between the dissolved oxygen and other factors, such as pH, ammonium, density of the animals, and temperature, should also be researched. More analysis has already been carried out for P. monodon (Allan et al., 1990; Allan and Maguire, 1991, 1992), which is one of the most successful species regarding production for the worldwide cultivation of shrimps (Csavas, 1994).

Acknowledgements This research was carried out at the Centro Regional de Investigaciones Pesqueras de Lerma, Campeche, Mexico, and at the Centro de Producci6n de Postlarvas de Tunas de Zaza, Cuba, through a collaboration agreement between the Instituto National de la Pesca, Universidad National Autonoma de Mexico, and the Universidad de la Habana, Cuba. Financial support was given by DGAPA-UNAM (project IN200994) and INP to Dr. Luis A. Soto and Dr. Carlos Rosas.

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