Journal of Experimental Marine Biology and Ecology 183 (1994) 133-145

JOURNAL OF EXPERIMENTAL MARINE BIOLOGY AND ECOLOGY

The effects of temperature and salinity on growth and survival of juvenile tiger prawns Penaeus esculentus (Haswell)

C. 3. O’Brien*

Centre for Marine Science, University of New South Wales, Australia

Received 15 February 1994; revision received 13 June 1994; accepted 21 June 1994

Abstract

Juvenile brown tiger prawns Penaeus esculentus iive in a wide range of inshore environments but how temperature and salinity affect their growth and survival is not known. Juvenile P. esculentus

(lo-12 mm carapace length, CL) were examined at combinations of five temperatures (1.5, 20, 25, 30 or 35 “C) and six salinities (5, 15, 25, 35, 45 or 5579 over 50 days. Individual prawns were identifi~ by uropod clips and their carapace moult increment (MI) and inte~oult period (IP) histories were obtained from exuviae. After 50 days, the prawns had survived a wide range of temperature-salinity combinations. Survival was greater than 60% in waters 15 to 30 “C and 15 to 45x,; and at 35 “C between 25 and 45x,. Combinations of extreme temperatures and salinities were lethal. Moult increment varied Iittle between 20 and 35 “C or with prawn size. By contrast, IP increased markedly with decreasing temperature and to a lesser extent with prawn size and thus controlled growth rates. Fastest growth is estimated to be at 30 “C and 30%,. The condition of prawns, in terms of energy (21.0 to 22.8 kJ*g-’ ash free dry weight) and wet weight (WW) and dry weight (DW) to CL relatjonships varied little between treatments. The P. escu-

lentus juveniles in this study, obtained from near the southern limit of the species distribution, coped in temperatures and salinities that were well outside those which they would normally encounter in the wild. Cool water is the major factor which restricts their distribution to tropi- cal and subtropical Australia. Because P. esculentus is a potential aquaculture species, the im- plications of the above results for the culture of the species are discussed.

Keywords: Growth; Penaeid; Prawn; Salinity; Survival; Temperature

* Correspondence and current address: CSIRO Division of Fisheries, Box 120 Cleveland, QLD 4163, Australia.

0022-0981/94/$7.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0022-0981(94)00098-O

1. Introduction

Adult and larval penaeid prawns f = shrimp) usually live in deep water where physical conditions are relatively stable but juveniles occupy shallow, inshore areas prone to cstuarine fluctuations of salinity and temperature. Physiological studies show that juvenile penaeids arc typically euryhaline (Dal1 ct al.. 1990) and can adapt to rapid salinity changes (Bursey & Lane. 1971; Parado-Estepa et al., 1987). They are hotvevcr. sensitive to water temperature because it can inffuence ionic and osmotic regulation (Williams, 1960), oxygen consumption (Kutty et al., 1971; Bishop et al., 1980; Dali, I98 I ), growth (Zein-Eldin & Griffith. 1968; Staples & Heales. 1991; O’Brien. 1994) and behaviour (Hill, 1985).

Most of the above studies have examined the effects of either s&nit> or tcmpera- ture. Because salinity and temperature interact. however, there is a need to study these factors in combination. Such studies on penacid prawns are scarce (e.g. Williams. 1960: &in-Eldin & Griffith. 1968; Preston. 1985; Staples & Heales, 1991) and we know littlc about how these, apparently successful, estuarine dwellers respond to the temperatures and salinities that they expcriencc. Furthermore, because aquaculture of penaeids is increasing there is a need for information on how temperature and salinity affect the growth and survival of juvenile prawns and, importantly. a definition of conditions for

optimal production. f’etltreus esculenrus is endemic to Australia and adults are commercially important

(Grcy ct al., 1983; Kailola et al., 1993). Penueus esculentus has a tropical to subtropi- cal distribution so juveniles are unlikely to encounter extreme low temperatures. In summer. however, the temperatures of their shallow water habitats may reach 3.5 ‘C (pers. obs.). Moreover. juveniles are found in a wide range of salinities (Young, 197X: Penn, 1981; Watson & Mellors. 1990). In this study, the survival. growth components (moult increment. Ml. and intermoult period, IP) and condition (as weight and energy) of juvenile P. esculet~fus over a wide range of temperature-salinity combinations were examined. This will further develop our understanding of the ecology of the species as well ;ts define the optimal conditions for aquaculture.

2. Methods

Petrous esculrnrus juveniles were reared in 30 combinations of temperature ( 1% 30. 25. 71) or 35 “C) and salinity (5. 15. 35, 35. 45 or 55”;,,,) over 51) dabs. Each treatment (teinperaturc-salinity coIilbinati~~n) consisted of three tanks: two test tanks each con- taining eight identifiable prawns. and a third tank containing 11 to 14 reserve prawns to replace an! dead prawns in the test tanks and maintain the stocking density. Due to aquaria reslrictions, three separate experiments were undertaken. Each experiment consisted of two salinity and five temperature combinations.

C.J. O’Brien 1 J. Exp. Mar. Bid. Ed. I83 (19943 133-145 135

2.2. The aquarium system

Two identical closed seawater systems were maintained at different test salinities (shown in Staples & Heales, 1991). Each system was divided into five bays; each bay contained three 22-l polyethylene tanks fitted with lids and was maintained at one of the five test temperatures. The flow rate of seawater into each tank was 9 l*h-‘. Overflow water from each tank was piped through a shell filter and sump before re- circulating.

The six test salinities (maintained to k 0.5%,) were obtained by the addition of INSTANT OCEAN@ (artificial seawater salt) or deionised freshwater to Moreton Bay seawater (5 and 1 pm filtered). The five test temperatures (ma~t~ned to i 0.5 “C) were obtained by adjusting the flow of seawater to each bay through hot and cold water heat exchangers. Air was bubbled into each tank and pH was maintained between 7.9 and 8.2 with NaH,CO, (Spotte, 1970). Light (fluorescent light from the laboratory ceiling lights) was provided from 0600 to 1800 through a 130 mm diameter clear Perspex window in each tank lid.

2.3. Collection, acclimation and marking of prawns

Penaeus escuientus juveniles were caught in Moreton Bay, south Queensland (27” 31’ S, 153” 17’ E) in March 1989, December 1989 and February 1990 with a 1 x 0.5 m beam trawl fitted with a 12-mm mesh net. The water temperature and salinity during collecting ranged from 25 to 27 “C and 33 to 35x,.

In the laboratory, a double acclimation procedure was used to prepare prawns for test conditions. First, groups of about 70 prawns, 10 to 12 mm carapace length (CL), were held in each of five 160-l tanks. The water temperature in each of these tanks was raised or lowered to one of the five test temperatures by changing the water tempera- ture through 3 “C each 24 h. Prawns in the 35 “C treatments were held at 32 “C until salinity acclimation was complete. After the test temperature had been obtained in each 160-l tank, up to 30 prawns (eight prawns for each of two test tanks and between 11 and 14 prawns for a third reserve tank) were selected (r~domly) for each of the two salinity treatments and were placed into their comparable temperature bays in each of the racks. The test salinities were obtained by changing the salinity of seawater in the sump in each of the two racks through 5g0 each 24 h. After test salinities were reached, the temperature for the 35 “C treatments was adjusted from 32 to 35 “C.

Uropod clips were used to identify individual test prawns. Clipping was done with a red hot scalpel and repeated as required.

2.4. Daily procedure

Each morning, the tanks were inspected for exuviae, the prawns were counted, and the temperature, salinity and pH of the water were measured. The amount of food remaining was assessed before the tanks were siphoned clean. Prawns were fed twice daily (around 1030 and 1800) with commercial prawn pellets (Higashimaru Food Inc., Japan). The amount of food fed to the prawns in each tank was weighed and main-

tained to a slight excess. Rations were increased if all food was consumed or reduced only when 2 to 3 days of excess was noted; this avoided possible underfeeding when one or more prawns were temporarily fasting due to ccdysis.

The length of the exuvial carapaces were measured using vernier callipers (to 0.1 mm). Because exuvial carapaces were usually separated from the rest of the exu- viae. matching two or more carapaces with their respective uropods was sometimes difficult, especially if the uropods were chewed and the clipping patterns became un- clear. However, carapaces could usually be matched to their uropods by using CL to outer uropod or CL to 6th abdominal segment relationships (O’Brien. 1992). Further- more, the previous CL and moult history enabled anticipation of which prawns were going to moult and the probable size of their carapace.

2 .S. Pruwn condition

At the end of each experiment, the weight to CL relationships. moisture and energy contents were used to assess the condition of the prawns. To standardise the prawns at moult Stage C (Smith & Dali, 1985) for weight and energy analysts. prawns were removed from the tanks 24 to 48 h after they moulted; larger prawns and those reared at lower temperatures usually took the longest to reach moult Stage C. In treatments with high mortality, prawns that were exhibiting distress behaviour, e.g. swimming erratically, were removed prior to their anticipated death so enough material could be obtained for analysis.

Prawns were starved for several hours prior to their removal (so their food did not influence their weight or energy content) then killed in an ice slurry and rinsed in dis- tilled water. Moult stage was determined using the inner left uropod; if the inner left uropod was not intact, the inner right or the outer left or the outer right uropod was

used (in that order).

_‘.5. I. Weight

Prawns were blotted dry with a tissue and weighed (wet weight), labelled and stored at -50 “C prior to measuring dry weight (DW) and energy. For DW, prawns wcrc oven dried at 60 “C to constant weight (usually ~48 h) and held in a desiccator prior to weighing.

The dried prawns from each tank were pooled then ground into a tint powder using a mortar and pestle (this gave two powder samples per treatment). The powder was compressed into one to three pellets (depending on the quantity of powder available) each weighing about I g. The pellets were dried at 60 “C overnight and held in a des- iccator prior to energy determination using a LECO AC 200 ballistic bomb calori- meter. Corrections to energy values were made for ash and the heat input of the fuse wire (9.6 J.cm- ‘) but not for sulphur or acid by-products. Ashing (of two to three small samples of powder) was done in a muffle furnace at 520 ‘C for about 18 h.

C.J. O’Brien 1 J. Exp. Mar. Biol. Ecol. 183 (1994) 133-145 137

2.6. Data analysis

2.6.1. Definition of moult increment, intermoult period and survival Moult increment (MI, mm change in CL) was the difference between the size of (1)

two successive exuvial CLs or (2) the CL at death and the last exuvial CL, if death occurred at least 1 day after ecdysis or (3) the CL at removal from the experiment and the last exuvial CL. Intermoult period was the time (days) between two successive ecdyses.

Survival was based on the original 16 prawns in each treatment over 50 days. In treatments where mortality was high, prawns exhibiting distress behaviour were re- moved and killed to provide material for weight and energy analyses. These prawns were counted as dead 1 day after the day they were removed.

2.6.2. Response-surface analysis The effects of temperature (T), salinity (S) on MI, IP and survival were tested

separately using linear (7’, S), quadratic [(T:, Si where Tf = (T- 25)2 and S: = (S - 30)2) and interaction (T x S) terms in multiple regression analyses (Box and Youle, 1955). Prawn size (CL) was included as a factor in the MI and IP analyses. The re- gression coefficients of these six independent variables were used to generate response- surfaces using the equation:

Y = b0 + b,(T) + b2(S) + b,,(T2) + b,,(S*) + b&T x S)

where Y is the response (MI, IP or survival); b, is the intercept; b, to b22 are regres- sion coefficients of independent variables (Alderdice, 1972). Levels of response on the response-surface graphs are indicated by isopleths (lines).

5

15 20 26 90 35

Temperature ("C)

Fig. 1. Response-surface estimates of the percent survival of juvenile 30 combinations of temperature and salinity.

P. esculentus after 50 days reared at

138 C.J. O’Brien / J. Exp. Mar. Biol. Ecol. 183 (1994) 133-145

3. Results

3.1. Survival

After 50 days, P. esculentus juveniles had survived a wide range of temperature and salinity combinations (Fig. 1). The highest and most rapid mortalities occurred at

combinations of extreme temperatures and salinities. The quadratic effects of salinity accounted for 45 y0 of the model and indicated that the highest mortalities were at the

extremes of salinity (Table 1). In fact, most mortality occurred at low salinities e.g. all prawns reared at 5% (at all temperatures) were dead by day 50; at 55x,, high mortality only occurred at 15 and 35 “C. Temperatures (between 15 and 35 “C) had little effect on survival except at salinity extremes; the quadratic and linear effects totalled only 22% (Table 1). The response-surface equation for survival is:

% Survival = 121.609 - 1.292(T) - 0.411(T:) + 0.9(S) - O.ll(S:).

3.2. Moult increment

The size of the MI was influenced mainly by temperature (Table 2, Fig. 2). The quadratic and linear temperature effects (Tf and T) accounted for 23 and 7% of the

variance, respectively. The largest MI were at temperatures between 25 and 30 “C. However, between 20 to 35 “C at 15 to 45x0 the range of MI only differed by about 0.4 mm. Although the other four independent variables (S:, CL, T x S and S) also contributed significantly to the model their combined effects accounted for a total of 9% of the variance (Table 2). There was liktle difference between the MI of the small and large juveniles e.g. the maximum MI of 12 and 18 mm CL juveniles was 1.2 and

1.4 mm, respectively (Fig. 2). A feature of the response-surfaces (Fig. 2) is the euryhaline response (indicated by

the elongated isopleths) where, at any one temperature, there was very little change in MI over the range of salinities tested. The quadratic salinity term (Sf) was the third

most significant factor in the MI model (Table 2); thus, the major effect of salinity was at the salinity extremes and this is reflected in the more pronounced curvature of the MI isopleths at salinities >45%, and < 15x,. The response-surface equation

Table 1

Stepwise multiple regression analysis of survival of juveniles of P. esculentus reared at 30” temperature

(r, Tf) and salinity (S, Sf) combinations after 50 days

Variable Coefficient F P Model R2

s: -0.11 60.5 0.0001 0.45 T: 0.411 23.8 0.0001 0.62 s 0.9 19 0.0002 0.77 T - 1.292 6.7 0.0157 0.81

Intercept 121.609

Tf = (T- 25)‘, Sf = (S - 30)*. Coefficients were used in response-surface equations.

Table 2

C.J. O’Brien /J. Exp. Mar. Biol. Ecol. 183 (1994) 133-14.5 139

Stepwise multiple regression analysis of carapace moult increment of P. esculentus juveniles reared at 30”

temperature (T, TF) and salinity (S, ST) combinations for 50 days

Variable Coefficient F P Model RZ

T: - 0.008 631.4 0.000 1 0.23

T 0.034 241.8 0.000 1 0.30 s: - 0.0005 212.2 0.000 1 0.37 CL 0.018 34.5 0.0001 0.38 TxS - 0.0003 9.0 0.0001 0.39 s 0.010 6.8 0.0027 0.39

Intercept 0.077

Tf = (T - 25)‘, Sf = (S - 30)‘, CL = exuvial carapace length; T x S is the temperature-salinity interaction.

Coefficients were used in response-surface equations.

for MI is:

MI = 0.077 + 0.034(T) - O.OOS(r:) + 0.01(S) - O.OOOS(S:) - O.O003(T x S) + O.OlS(CL).

3.3. Intermoult period

Intermoult period was strongly influenced by temperature (Table 3, Fig. 3). The linear and quadratic (T and T:) temperature effects accounted for 44 and 20% of the var- iance, respectively. At moderate salinities, the minimum IP occurred over the tempera- ture range of about 28 to 33 “C. Below 28 “C, IP increased markedly as temperature decreased, e.g. for 12 mm CL prawns at 30X,, IP increased from 5 days at 28 “C to 10 days at 24 “C. IP also increased with prawn size; the IP of 18-mm CL prawns was about 5 days longer than that of 12-mm CL prawns. Prawn size (CL) explained the

Temperature (“C)

Fig. 2. Response-surface estimates of the carapace MI (mm) of (a) 12 mm CL P. esculenrus reared at 30 combinations of temperature and salinity.

and (b) 18 mm CL juvenile

Tahlc :

SIepwse multiple regresson analysts of mtennoult pcrtod ofJuveniles reared al 30 tcmperalurc ( 7: 7‘:) and

balinit\ (S. Sf) combinations for 50 day ----_---.. _ -._. .__ ..___._

Vanablc (‘ocff icicnt k P Model R’ -..-- ___.. ---._-.__ _ ._ _.-_._ ~.

I 1.17 17n.i 4 0.0ocI I 0.44

1.1 0.109 X0X 7 0.000 I 0.64

(‘I 11.684 325. I 0.000 I 0.70

.f ; 0.005 361 -I 0.000 i 0 ?S

.s 0. I 15 23. I 0.0001 0.75

I’r .s 0.003 I 3 9 O.O(H)2 ll.70

Intcrccp1 27.62h

7‘: = (7 .. 25)‘. .Sf = (S - 30)‘. CL = exuvial carapace length: 7.x S is Ihe temperature-salinity imeraction.

Coefficicnls were used in response-surface equations.

third highest percentage of the variation (6’i,) in the IP model. The effect of salinity on IP was similar to that in the MI model in that the quadratic salinity effect (.St) accounted for 5O., of the variability and is reflected in the curvature of the isopleths at salinity extremes. Although other sources of variation (S and T x S) also contributed significantly to the model their combined effects accounted for only 1 ‘l,, of the variance (Table 3). The response-surface equation for IP is:

IP = 27.636- 1.17(T) + O.l09(T;) + 0.115(S) t O.OOj(Sf) - O.O003(TxS) + 0.684(CL).

3.4. Growth in carapace leugfh

The response-surface equations were used to generate MI and IP for combinations of temperature and salinity in 1 “C and l& intervals. The conditions for fastest growth (in terms of CL, estimated from the greatest ratio of MI to IP) were 30 “C and 30%.

w,-

45.

L5 .f? ‘= 25. (I)

15.

5-

-

f ‘Of 9

1 -

2s 30 s5 15 20 25 Jo 35

Temperature (“C)

Fig. 3. Response-surface estimates ofthe IP (days) of(a) 12 mm CL and (b) 18 mm CL juvenile P. e.wu(enrus reared at 30 combinations of temperature and salinity.

C.J. O’Brien / J. Exp. Mar. Biol. Ecol. 183 (1994) 133-145 141

Growth decreased with prawn size from about 1.7 to I.1 mm CL*week-’ for 12- and l&mm CL juveniles, respectively. This was mainly because IP increased with size. At the optimal salinity, 30x,, the growth rates decrease as temperature decreases

(Fig, 4a). Growth was fastest at 30 “C and although prawns at 25 ‘C had a similar MI their slightly longer IP reduced their growth to about 73 % of the fastest rate. At 35 ‘C the IP was shorter but the MI was reduced so growth rates were slower. Growth at 20 “C was reduced to about 27 y0 of the fastest rate and there was virtually no growth at 15 “C.

At the optimal temperature, 30 “C, prawns grew fastest at 25 and 35%,. Growth at 15 and 45x, was about 82% of the fastest rate. Prawns at 55x, grew slower (about 56% of the fastest rate) than those at other salinities due to a smaller MI and longer IP (Fig. 4b).

3.5. Prawn condition

There were few differences in the length to weight relationships, mean moisture contents and energy values of prawns among temperature-salinity treatments. Carapace length, wet weight (WW) and DW were measured for the prawns removed from each treatment. Because the size and weight range of prawns from each treatment was

narrow, length and weight data from all treatments were pooled and the slopes of DW to CL and WW to CL relationships were compared with those of wild P. esculentus.

The slopes of both DW to CL and WW to CL relationships of prawns in this study were not significantly different (p> 0.05) from the slopes of the same relationships estimated for wild prawns (WW = 0.00056 CL 3 14, DW = 0.00012 CL3.2 (N = 202, both r2 = 0.99, O’Brien, 1992).

Temperature (“C)

0 10 20 30 40 50

Salinity Qo)

;! 45 15 5 55

0 10 20 30 40 50

Day

Fig. 4. Growth in CL ofjuvenile P. esculentus: (a) reared at five temperatures at the salinity optimum of 30%0 and (b) reared at six salinities at the temperature optimum of 30 “C. The length of each vertical step indi- cates the size of the MI. The length of each horizontal step indicates the duration of the IP.

The mean moisture contents of the prawns t-cared at each of the 30 combinations of temperature and salinity ranged from 7 1.2 to 79.2”” and tended to decrease with increasing salinity. Because P. esculenrus juveniles osmoregulate less efficiently at salinity extremes (Dali, 1981), changes in body moisture contents with salinity due to osmo- sis arc to be expected. Even so. the range of moisture contents in this study is simi- lar that found for wild P. esculenrlrs (7 1 to 79”,,, salinity z 35”,,,. O’Brien, 1992) and other Penneus species (cited in Barclay ct al.. 1983). Thus, analyses to extricate differ- ences in moisture content of prawns due to treatlnent effects were not attempted. The mean energy content of the prawns ranged from only 21.01 to 22.76 kJ.g ’ (ash free dry weight) and there were no trends in energy values between treatments. Because of the narrow range of energy values, these data were not analysed further.

4. Discussion

The growth of prawns is a function of both the size increment at each moult and the duration of the IP (Botsford, 1985). The size range of the MI of P. rsculentus juveniles was small over a wide range of temperatures and salinities. For example, in waters between 24 and 32 “C the range of MI was about 0.2 mm. By contrast, IP increased markedly with both decreasing temperature and prawn size. Thus, as MI is relatively constant, growth, as measured by the ratio of MI to IP. is mainly influenced by the duration of the IP. Consequently, the effect of a relatively small decrease in tempera- ture can be striking. For example, at 30?&,, the growth rate of 12 mm CL prawns will halve if water temperature decreases from 28 to 24 ‘C because the IP will double from 5 to 10 days. This explains the response of wild P. esculentus juveniles in subtropical, south Queensland where growth rates decrease markedly as waters cool during autumn (O’Brien, 1994).

Growth rates, in terms of CL, also decreased with increasing prawn size, mainly due

to an increase in IP. While the moult increment remained relatively similar for diffcr- ent sized prawns, the minimum IP increased from 5 days for 11 mm CL prawns to 9 days for 1X mm CL prawns. Correspondingly, the fastest growth rates were 1.7 and 1.1 mm CL,week I, respectively. This is consistent with the von Bertalanffy growth relationship which states that the instantaneous growth rate in length over time dc- creases with size (Gulland, 1975).

The optimal conditions for the growth of P. e~s~~f~~r~M.~ juveniles was estimated to be at 30 ‘C and 3O’&. These conditions arc similar to those estimated for slightly smaller P. tmvyhzsi.s juveniles under laboratory conditions (3 1 ‘C. 30‘& Staples & Healcs. 1991). Both P. exulen~us and P. merguien.sis have a similar geographical distribution in Australia. The above conditions are also similar to those reported for the fastest growth of laboratory reared P. uax~.~ and P. se@vus juveniles from Galveston Bay, in the Gulf of Mexico (30 to 32.5 “C and 25 to 35?&,, Zein-Eldin & Griffith, 1968). It is likely that most juvenile penaeids would grow quickly at temperatures around 30 “C and sahnities between 25 and 3X,:,,.

The high survival and consistent condition of the prawns in terms of wet and dry weight and energy content indicate that P. escufenrus juveniles from south Queensland

C.J. O’Brien/J. Exp. Mar. Bid. Ecol. 183 (1994) 133-145 143

cope well in a wide range of salinities and temperatures. A salinity of 5x0 was lethal but survival was high at 55x, (at temperatures between 20 and 30 ‘C). This is consistent with Dal1 (198 1) who predicted a lower lethal salinity of about 3x,, and Dal1 (1981) and Dal1 & Smith (1981) who showed that P. esculentus juveniles were more efficient osmotic and ionic regulators in high salinities. Euryhalinity is probably inherent to P. esculentus juveniles as populations are found in most warm water seagrass habitats around tropical and subtropical Australia, including Shark Bay, Western Australia (26” S), where the waters are hypersaline (from 38x0 to > 45x,,, Penn, 1981) due to restricted river flow and ocean influence, and high rates of evaporation; Weipa, north Queensland (12” 45’ S) and Moreton Bay in south Queensland (27”s) where rains flood the rivers and can lower the salinity below IO%, (Young, 1978; Haywood & Staples, 1993); and oceanic reef-tops of Torres Strait (9” 40’ S, Watson & Mellors, 1990). Furthermore, the response of each population to temperature-salinity conditions are likely to be maximised due to localised inherited abilities which are dependent on salinity-temperature history of the parent stock (Preston, 1985).

Dal1 et al. (1990) noted that only small amounts of energy are required for the regulatory activities of juvenile prawns. Thus, over a wide range of salinities the maxi- mum energy available can probably be directed into growth. By contrast, the range of water temperatures over which a prawn will function optimally is relatively narrow (around 25 to 30 “C) and at temperatures outside this range the efficiency of regula- tory activities will decrease and growth rates will be slower. However, natural popu- lations of prawns are unlikely to be continuously exposed to extreme conditions because their habitats are typically moderated by the tide and/or die1 changes. Furthermore, prawns are mobile and may avoid undesirable conditions. Thus, the use of shallow, inshore habitats, which expedite development of juvenile prawns would appear to be a useful life-history strategy, despite the potentially unstable physical nature of these habitats. It is likely that the distribution of P. esculentus juveniles around tropical and subtropical Australia is influenced by factors other than temperature and salinity e.g. the strong preference for seagrass (Staples et al., 1985; Coles et al., 1987) and its associated abundant food and shelter.

Cool water is probably the major physical factor limiting the production and distri- bution of P. esculentus juveniles. In south Queensland, high production of prawns is limited to a period of about 4 months per year when the mean water temperature is above 25 “C and growth rates exceed 1 mm CL.week-’ (O’Brien, 1994). Because of the slower growth and decreased activity of P. esculentus in cool water (Hill, 1985; O’Brien, 1994) this species may not compete as well with better adapted cool-water species and it is not surprising that south Queensland is near the southern limit of the species distribution. By contrast, in northern Australia, water temperatures exceed 25 “C for about 9 months per year (Coles & Lee Long; 1985; Haywood & Staples, 1993; Loneragan et al., 1994) and P. esculentus juveniles are abundant for longer. The greater production ofjuveniles in northern Australia is reflected in commercial catches of adults being sustained for longer.

Penaeus monodon is the preferred prawn species grown by Australian prawn farmers because of fast growth rates and marketability. However, fast growth rates of wild P. esculentus (fastest about 2.1 mm CL-week-‘, O’Brien, 1994) indicates that this

species has considerable aquaculture potential. Furthermore, the optimal temperature and salinity for growth, 30 “C and 307&,. should not be difficult to obtain in tropical .4ustralian prawn ponds and the euryhalinity demonstrated should be useful for coping with possible fluctuations in pond salinity due to the seasonal wet and dry periods that arc typical in tropical Australia.

It is likely that most of the early attempts at farming P. rsculenzus were unsuccess- ful because farmers used unsuitable diets (e.g. P. mmxfon diets) which were too low in protein. The high quality. 60”” protein diet used in this study appeared to be extremely satisfactory but it was expensive. The development of a economically viable diet will be an important prerequisite for the successful farming of P. mculentus.

Acknowledgements

This work formed part of a Ph.D. at the University of New South Wales. Support was received from an Australian Postgraduate Research Award. Thanks to CSIRO Division of Fisheries, Cleveland for the use of equipment and facilities. Drs J. MacIntyrc (University of NSW), D. Staples (Bureau of Rural Sciences, Canberra), B. Hill and P. Rothlisberg (CSIRO) gave constructive criticism of drafts of this manuscript.

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