Tiger Prawn in Seagrass

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    N.R. Loneragan M.D.E. Haywood D.S. HealesR.A. Kenyon R.P. Pendrey D.J. Vance

    Estimating the inuence of prawn stocking density and seagrass

    type on the growth of juvenile tiger prawns (Penaeus semisulcatus):results from eld experiments in small enclosures

    Received: 14 September 2000 / Accepted: 8 January 2001 / Published online: 3 July 2001 Springer-Verlag 2001

    Abstract Fine mesh enclosures (0.9 m2 in basal area,1 m high, with 100 lm mesh) and a jet-net retrieval

    system were developed to test the inuence of juvenileprawn stocking density on growth rates in (1) dierentmonths (April and October/November) and (2) dierenttypes of intertidal seagrass beds in the Embley Riverestuary of tropical Australia. Small juvenile tiger prawns(36 mm in carapace length, CL) were stocked inenclosures at densities of 432 prawns per enclosure(4.435.5 prawns m2) on a high biomass seagrass bed(about 70 g m2 of mostly Enhalus acoroides) and onewith low biomass (about 10 g m2 of mostly Haloduleuninervis). After 23 weeks in the enclosures, recoveryrates, and hence possibly survival, were greater on thehigh biomass Enhalus than on the low biomass Halod-

    ule. However, not all sh and crustaceans could beexcluded from the enclosures. Growth rates were twiceas fast on the high biomass Enhalus than on the lowbiomass Halodule. It is likely that the high biomassEnhalus, with its greater surface area, supported moreepiphytic ora and fauna and reduced the potential forinterference competition between prawns, comparedwith the low biomass Halodule. Growth rates on Enh-alus were signicantly faster at a stocking density of4 prawns per enclosure (1.3 mm CL week1) than at astocking density of 16 and 32 prawns per enclosure(both 0.8 mm CL week1), and did not dier signi-cantly between April and October/November (temper-

    atures were about 30C at both times). The meangrowth rate at 8 prawns per enclosure (1.1 mm CLweek1) did not dier signicantly from those at 4, 16and 32 prawns per enclosure. These results from two

    seagrass beds suggest that the carrying capacity forjuvenile tiger prawns was greater in the high biomass

    Enhalus than the low biomass Halodule bed.

    Introduction

    The carrying capacity of a habitat is determined by bothenvironmental and biological factors, such as habitatcomplexity, availability of food, competition and pre-dation. Seagrasses are a conspicuous feature of shallow-water coastal systems, where they stabilize sediments,take up nutrients and provide food and habitat for an-imals (Bell and Pollard 1989). In general, seagrass beds

    with high biomass support more animals than those withlow biomass.

    Tiger prawns (Penaeus semisulcatus and P. esculentus)spawn in oshore waters. [Note that although the tax-onomy of the penaeids has been revised recently (Pe rezFarfantes and Kensley 1997), we have chosen to use theolder nomenclature because there is some controversyover the revised names (W. Dall, CSIRO MarineResearch, personal communication)]. Over a period of34 weeks their planktonic larvae are advected to shal-low coastal areas, where they settle as postlarvae on bedsof seagrass and algae (Dall et al. 1990; Haywood et al.1995; Liu and Loneragan 1997). In the Gulf of Car-

    pentaria, tropical Australia, postlarval tiger prawns re-cruit to shallow water nursery habitats betweenSeptember and May, which covers the prewet (Sep-temberDecember) to late-wet seasons (FebruaryMay)(Loneragan et al. 1994; Vance et al. 1996).

    The densities of postlarval and juvenile tiger prawnson beds of seagrass in northern Australia rarely exceed24 ind. m2 (Vance et al. 1996; Loneragan et al. 1998),which is low in comparison with other similar-sizedcrustaceans (Haywood et al. 1998). They are also low incomparison with brown shrimp in saltmarshes(820 ind. m2: Zimmerman et al. 1984; Minello et al.1989) and recently settled blue crab megalopae in

    Marine Biology (2001) 139: 343354DOI 10.1007/s002270100572

    Communicated by G.F. Humphrey, Sydney

    N.R. Loneragan (&) M.D.E. Haywood D.S. HealesR.A. Kenyon R.P. Pendrey D.J. VanceCSIRO Division of Marine Research,Cleveland Marine Laboratories, PO Box 120,Cleveland, Queensland 4163, Australia

    E-mail: [email protected]

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    Chesapeake Bay (2550 ind. m2: Pile et al. 1996). Whatlimits the densities of juvenile tiger prawns and what arethe greatest densities of tiger prawns that seagrass bedscould support? Answers to these questions contribute tothe stock assessment of the shery and to the selection ofthe best release sites for stock enhancement programs(see Blankenship and Leber 1995; Munro and Bell 1997;Rothlisberg et al. 1999).

    In the western Gulf of Carpentaria of northern Aus-tralia, juvenile tiger prawns are found at higher densitiesin intertidal and shallow, subtidal beds of seagrass andalgae than in beds in deeper waters (Loneragan et al.1994). Their densities also vary with the biomass ofseagrass: higher densities are found in seagrass beds withhigh biomass (>100 g m2) than in those with lowerbiomass (3 ha) in-tertidal beds of seagrass (Enhalus acoroides, Halodule uninervis andHalophila ovalis) and smaller (approximately 1 ha) subtidal beds ofalgae (Fig. 1). Tides in this region vary from about 2 to 3 m inamplitude. The growth of juvenile tiger prawns (Penaeus semi-sulcatus) was estimated at dierent densities of prawns on twoseagrass beds: a bed referred to as Enhalus dominated by thebroad-leafed E. acoroides (leaves up to 1 m long, 1020 mm wide),which grows at low shoot densities (

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    seagrass when the enclosures were deployed. Enclosures wereplaced over a central augur with a 2 m long iron post and held inplace by putting 10 kg weights on top of the enclosure.

    After preliminary trials in Moreton Bay, southeasternQueensland, an experiment using 12 enclosures and densities of 4,16 and 32 prawns per enclosure was completed in the Embley Riverestuary in November 1993. After 10 days, however, animals such as

    mud crabs (Scylla serrata) had burrowed under many of the en-closures, allowing prawns to escape. Burrowing, possibly by bluecrabs (Callinectes sapidus), was also a problem in predator-exclu-sion experiments with brown shrimp (Penaeus aztecus): data from25% of the enclosures were excluded from analysis because of``signicant burrows'' (Minello et al. 1989). In later experiments, anextension was added to the base of each enclosure to try and ex-clude burrowing animals (Fig. 2b). The extension of aluminum

    plate (0.2 m deep) welded into a square frame, was pushed into thesubstrate to a depth of 0.2 m. The enclosure was then tted tightlyinto the extension frame. Field observations suggested that thisextension reduced the burrowing activity but did not exclude itfrom all enclosures.

    The eect of the enclosures on dissolved oxygen, light andtemperature were measured in Moreton Bay by placing a loggerinside and outside the enclosures. Over 4 days, dissolved oxygeninside the enclosures diered by

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    stocked at each density in enclosures ranged from 3.5 to 6.3 mmCL (Table 2). Three-way ANOVAs were used to test for dierencesin carapace length at the start of the experiment between stockingdensities, experiments and seagrass beds. Dierences betweentreatments were then tested by the RyanEinotGabrielWelschmultiple range test. The mean carapace length of prawns dieredbetween experiments (F3,58=11.35, P

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    the mean carapace length at the end of the experiment, to eliminateprawns that were unlikely to have been stocked in the enclosures.The upper size limits were calculated by applying a fast growth rateof 2 mm CL week1 (O'Brien 1994a) to the largest prawn stocked ineach enclosure; they ranged from 8.2 to 11.0 mm CL.

    The relationships between carapace length and wet weight wereused to estimate the mean wet and dry weights of the prawns ineach enclosure at the start of each experiment. The regression

    equations for estimating wet weights were:Apr : log wet wt 3:60 3:70logCL

    r2 0:95; P < 0001; n 17

    Nov : log wet wt 3:46 3:43logCL

    r2 0:97; P < 0:0001; n 265

    The growth rates of juvenile P. semisulcatus stocked in the enclo-sures were assumed to be linear, as is the case for small individualsof both P. merguiensis (Haywood and Staples 1993) and P. escu-lentus (O'Brien 1994a,b). The weekly growth rates in each enclosurewere estimated from the dierences in mean carapace length andwet weight between the start and end of the experiment, and ad-justed for the length of the experiment: e.g. mean growth incarapace length mean CLend mean CLstart=days 7. Data

    were analyzed using both an ANOVA approach and a regressionapproach. For the ANOVAs, only those enclosures in which 40%of the stocked prawns were recovered were used in analyses. Thisensured that at least two prawns (stocking density: 4 ind. per en-closure) were used to calculate the mean size at the end of theexperiment and that enclosures where major disturbances occurredwithin the enclosure, e.g. bioturbation, predation, escape ofstocked prawns, were excluded from the analysis. On Enhalus, atotal of 51 enclosures were stocked with prawns, and 26 contained40% of the apparently original prawns at the end of the experi-ments (Table 1). On Halodule, only 6 of the 21 enclosures stockedwith prawns had 40% of the original prawns. In three cases, theenclosures at Halodule had lifted from their bases in very strongwinds in October 1994 and were not retrieved.

    Analysis of growth data

    The dierent densities used in the experiments (Table 1), and theexclusion of enclosures with recovery rates of

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    Proportion of prawns retrieved from enclosures

    The mean proportion of prawns recovered was highestat a stocking density of 4 prawns per enclosure in allexperiments (Fig. 4). The highest mean recovery ratewas 94% on Enhalus at a density of 4 prawns per en-closure, in April. The mean retrievals for all stockingdensities >4 prawns per enclosure did not exceed 50%(Fig. 4). The recovery rates on Halodule were lower, andmore variable, than those on Enhalus (Fig. 4). Thehighest mean recovery rate on Halodule was about 40%,

    at 4 prawns per enclosure.A total of 415 sh, representing 22 taxa were

    retrieved from 68 enclosures (Table 4). Some taxa, par-ticularly atherinids and Ambassis spp., tended to shelterin the lee of enclosures and may have been trapped whenthe retrieval net was placed over the enclosure. Thesetwo groups of sh accounted for 135 sh (33% of thetotal). Seven of the taxa, and about 30% of the indi-viduals, were known predators of prawns. All sh, ex-cept one, were

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    taxa of crustaceans, mainly Portunus pelagicus, and 111individuals were retrieved. Note that although largeP. pelagicus (>60 mm carapace width, CW) have beenfound with Penaeus latisulcatus in their cardiac stomachs(de Lestang et al. 2000), the P. pelagicus found in en-closures were generally small (mean size: 31 mm CW;Table 4), and P. pelagicus were not successful in cap-turing juvenile tiger prawns in the laboratory (Haywood,CSIRO Marine Research, unpublished data). For thesereasons, P. pelagicus was not classied as a prawnpredator. None of the other crustaceans were knownpredators of prawns.

    The recovery rates of prawns were negatively corre-lated with the stocking density of prawns in all experi-ments (r=0.39, P=0.001, n=68). They were notcorrelated with the total numbers of sh retrieved fromthe enclosures in any experiments (r=0.12, P=0.37,n=62), or the numbers of sh that were prawn predators(r=0.08, P=0.55, n=62) or the numbers of non-pre-dators (r=0.10, P=0.45, n=62). Only the starting

    stocking density (partial r2=0.14) and the mean cara-pace length at the start of the experiment (partialr2=0.04) were tted to the stepwise multiple regressionequation for recovery rates; both coecients had nega-tive slopes. Together these variables accounted for only18% of the variation in recovery rates.

    Growth rates of stocked prawns ANOVA

    The mean growth rates in carapace length for enclo-sures where >=40% of the prawns were recoveredwere always higher on Enhalus than Halodule at the

    same stocking density (Fig. 5). The highest meangrowth rate was 1.29 mm CL week1 at 4 prawns perenclosure on Enhalus in April, and the lowest was0.37 mm CL week1 on Halodule at 16 prawns perenclosure in November (Fig. 5). The highest growthrate in an individual enclosure was 1.54 mm CL week1

    at 4 prawns per enclosure on Enhalus (November), andonly 0.82 mm CL week1 on Halodule (November).

    The mean carapace length at stocking was not asignicant covariate in any of the ANCOVA models.Results from the three ANOVAs (i.e. without a co-variate) have therefore been presented. The ANOVAto test for dierences in growth rates in carapace

    length between months and densities on Enhalusshowed that growth rates did not dier signicantlybetween months but were signicantly faster at adensity of 4 (mean1 SE: 1.30.07 mm CL week1)than 16 prawns per enclosure (0.80.12) (Table 5).From limited data, growth rates at a density of32 prawns per enclosure were also similar in April(0.70.01, n=3) and November (0.8, n=1) (Fig. 5).

    The second ANOVA to test for dierences in growthrates between dierent stocking densities on Enhalus,ignoring month, showed that the growth rates dieredsignicantly between stocking densities (Table 5). Themean growth at a stocking density per enclosure of 4

    Fig. 5a, b Penaeus semisulcatus. Mean growth rates (1 SE) ina carapace length and b wet weight, of juvenile tiger prawns atdierent stocking densities, on seagrass beds of high (Enhalus) and

    low (Halodule) biomass, in the Embley River estuary, northernAustralia. Only enclosures where recovery rates of prawns were40% of stocked prawns were used to calculate mean growth

    Table 5 Penaeus semisulcatus. Mean squares and levels ofsignicance (superscript) for analyses of variance to test dierencesin growth rates in carapace length (CL) and wet weight (wet wt)between prawn stocking densities, months and seagrass beds (see``Materials and methods''). Bold superscript indicates signicantterms in the ANOVA. Level of treatment in parentheses

    Source of variation df Mean square

    CL Wet wt

    Density, month (Enhalus)Prawn density (4, 16) 1 0.83160.001 0.00540.01

    Month (April, November) 1 0.03490.42 0.00010.69

    DensityMonth 1 0.02330.51 0.00020.64

    Error 17 0.0504 0.00068Density (Enhalus)

    Prawn density (4, 8, 16, 32) 3 0.46500.0002 0.00580.003

    Error 22 0.0460 0.00095Density, Seagrass

    Prawn density (4, 8, 16) 2 0.30250.008 0.004550.02

    Seagrass (Enhalus, Halodule) 1 0.77920.0007 0.007170.01

    DensitySeagrass 2 0.06220.31 0.001190.31

    Error 22 0.0505 0.00096

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    (1.30.06 mm CL week1) was signicantly faster thanthose at 16 (0.80.15) and 32 (0.80.02). The meangrowth rate at 8 prawns per enclosure (1.10.15) didnot dier signicantly from those at any of the otherdensities.

    The third ANOVA to test for dierences in growthrates between seagrass beds and stocking densitiesshowed that growth was signicantly faster on Enhalus(1.10.06 mm CL week1) than Halodule (0.50.06).Growth rates were signicantly faster at stocking den-sities per enclosure of 4 (1.20.08) and 8 (1.00.13)than at 16 (0.60.10) (Table 5). The densityseagrassinteraction was not signicant (Table 5).

    The trends in growth rates in wet weight with dierentmonth, density and seagrass type followed a very similarpattern to those for carapace length (Fig. 5; Table 5).For example, the third ANOVA showed that growthrates in wet weight were signicantly faster on Enhalus(0.100.008 g week1) than Halodule (0.0370.013) andfaster at stocking densities of 4 (0.100.010) and 8

    (0.110.023) than at 16 (0.050.009) prawns perenclosure (Table 5).

    Growth rates of stocked prawns correlations

    The growth rates of prawns from all enclosures and thosewhere 40% of prawns were recovered declined with the``average'' density of prawns (Table 6; Fig. 6). Althoughthe proportion of variation in growth rate explained byaverage density was greater when enclosures with lowrecovery rates were excluded from the analysis (particu-larly on Enhalus), the slopes of the lines were similar for

    both data sets (Enhalus: 0.019 vs. 0.023; Halodule:0.043 vs. 0.036; Fig. 6). The decline in growth rateswith stocking density was about 1.52 times more rapidon Halodule than Enhalus (Fig. 6).

    The mean carapace length at stocking the enclosureswas also tted to the multiple regression equations ofgrowth rates and average density for Enhalus but notHalodule (Table 7). It was the second variable tted tothe relationship and accounted for a partial r2 of 21%

    for all the data and 10% for those enclosures with highrecovery rates. The lowest growth rate (0.4 mm CLweek1) on Enhalus was recorded at an average stockingdensity per enclosure of 15 prawns, with a mean cara-pace length of 6.1 mm at the start of the experiment.

    Table 7 Penaeus semisulcatus. Multiple regression equations forthe relationships between growth rates (G, in carapace length, mmweek1) and ``average'' stocking density of prawns (AD), and themean carapace length of prawns (CL) at the start of the experimenton the Enhalus and Halodule seagrass beds. No model is shown forHalodule for 40% prawns recovered as no signicant relationshipwas found (n=6). AD was tted rst, CL second. Bold superscriptindicates signicant terms in multiple regressions

    Seagrass bed, model Partial r2

    EnhalusAll data (n=37) 0.410.001

    G 0:0221AD 0:203CL 2:168 (AD=0.19, CL=0.21)Recovery40% (n=26) 0.550.001

    G 0:0252AD 0:139CL 1:924 (AD=0.44, CL=0.10)HaloduleAll data (n=14) 0.410.001

    G 0:041AD 0:808

    Table 6 Penaeus semisulcatus. Pearson correlation coecients andtheir signicance (superscript) for the relationships between growthrates (carapace length, mm week1) and stocking and averagedensity of prawns, and the mean carapace length of prawns at thestart of the experiment on the Enhalus and Halodule seagrass beds.Bold superscript indicates signicant terms in correlations

    Seagrass bed,variable

    Correlation

    All enclosures Recovery40%

    Enhalus (n) (37) (26)Stocking density 0.380.02 0.670.0002

    Average density 0.440.01 0.670.0002

    Carapace length 0.410.01 0.280.17

    Halodule (n) (14) (6)Stocking density 0.650.01 0.58022

    Average density 0.640.01 0.65016

    Carapace length 0.070.81 0.51030

    Fig. 6a, b Penaeus semisulcatus. Regression relationships betweengrowth rates (G) of juvenile tiger prawns and average stockingdensity (AD) in a all enclosures and b those enclosures where 40%of prawns were recovered, in the Embley River estuary, northernAustralia

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    Diet of prawns in enclosures

    The mean gut fullness of juvenile Penaeus semisulcatus inenclosures on Enhalus beds did not dier betweenstocking densities of 4, 16 and 32 prawns per enclosureduring the April; mean gut fullness ranged from 7.7 (4prawns) to 8.2 (32 prawns). These values were alsosimilar to to those for prawns caught in beam trawls andscoop-nets in April (Heales et al. 1996).

    The main items of diet were similar at the dierentprawn densities in the enclosures with unidentied mat-ter, diatoms, copepods and lamentous algae exceeding90% frequency of occurrence in all cases (Fig. 7). How-ever, six categories of diet (ostracods, foraminiferans,kinorhynchs, unidentied eggs, oligochaetes and bry-ozoans) were found more frequently in prawns at4 prawns per enclosure than either of the higher densi-ties. Five items (foraminiferans, ostracods, polychaetes,kinorhynchs and oligochaetes) were also found morefrequently in prawns at densities of 16 than 32 prawns

    per enclosure.

    Discussion

    Recovery rates and design of enclosures

    The mean recovery rates (and hence probably survival)of small, grooved tiger prawns (Penaeus semisulcatus)after 23 weeks were highest (8095%) at the loweststocking density (4 prawns m2) on the high biomassseagrass Enhalus acoroides. Recovery rates on this sea-grass bed declined to 50% or less at higher stocking

    densities (50% at 8.9 and 18 prawns m2, and 2050%at 36 prawns m2). The survival of larger Penaeusvannemei (about 14 mm CL) in cages on mudats in aMexican lagoon also declined with prawn stockingdensity: after 6 weeks, survival was 97.5% at 2.5 prawnsm2, 83.8% at 8.5 prawns m2, and 54.1% at 19 prawnsm2 (Edwards 1977). The decline in recovery rates withincreasing stocking density is possibly due to a reductionin available food within the enclosures. At higherstocking densities, the prawns would also encounterother individuals more frequently, and, if intra-speciescompetition was signicant, this could also lead to lowernumbers surviving.

    We were not able to exclude all sh or all burrowingactivity from the enclosures. The loss of prawns fromenclosures could therefore also have been due to preda-tion by the sh in the enclosures and possibly by bur-rowing animals, e.g. the mantis shrimp Squilla. However,there was no signicant correlation between the recoveryrates of prawns and the number of sh predators found

    in the enclosures. These results suggest that if predationwithin the enclosures was important, it is not a linearfunction. It is likely that at higher stocking densities, shencounter prawns more frequently and possibly developa search image for prawns, which could lead to higherpredation rates at high densities of prawns. Anotherfactor contributing to the variable recovery rates ofprawns is likely to be the burrowing activity of animals inthe intertidal seagrass, creating opportunities for prawnsto escape. Despite the modications to the enclosuredesign, we were not able to control all burrowing activity.

    The recovery rates of juvenile tiger prawns from en-closures were higher, and less variable, on the high

    Fig. 7 Penaeus semisulcatus.Frequency of occurrence ofdietary items found in the fore-guts of juvenile tiger prawns atstocking densities of 4, 16 and32 prawns per enclosure on ahigh biomass seagrass bed(Enhalus), in the Embley Riverestuary, northern Australia

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    biomass, large-leafed E. acoroides than the low biomass,small-leafed Halodule uninervis (mean recoveries ofabout 2040% on Halodule; Fig. 4). This is possiblybecause epiphytic ora and fauna are likely to be moreabundant on the high biomass bed than the low biomassbed, which could lead to lower survival. Enhalus alsoprovides much more surface area than Halodule, whichcould reduce the number of encounters between prawns,and hence the potential for competition.

    Various designs of enclosures or cages have been usedto study the survival and growth of juvenile prawns. Ourenclosures (0.90 m2 basal area) are smaller than thoseused in saltmarsh (1.8 m2: Minello et al. 1989) andmudat studies (23 m2: Edwards 1977), with a nermesh size (100 lm mesh). The ne mesh of our enclo-sures limited the planktonic food available to prawnswithin the enclosures and hence our calculations ofgrowth and survival are possibly underestimations ofnatural conditions. Because of the 2.4 m tidal range inthe Embley River system, our enclosures were closed at

    the top unlike those on the saltmarsh, where tidalvariation was small (Minello et al. 1989). The prawnsused in our study (36 mm CL) were much smaller thanthose used in other studies of prawn growth and survival(>8 mm CL: Edwards 1977; Minello et al. 1989) and instudies on the impact of prawns on the benthic fauna(1020 mm CL: Nelson 1981; 2050 mm CL: Leber1985). The use of larger prawns in enclosure experimentsmakes it feasible to mark individual prawns (e.g.Edwards 1977), thus increasing the potential for infor-mation on growth and survival.

    In the eld, predation by sh is thought to be one ofthe major factors aecting the survival of juvenile prawns

    (e.g. Minello et al. 1989; Dall et al. 1990; Haywood et al.1998). The success of sh predators in capturing juvenileprawns is inuenced by a number of variables, includingthe type of substrate and the density of the prawns. Thus,the survival of tiger prawns (P. esculentus and P. semi-sulcatus: Kenyon et al. 1995), brown shrimp (P. aztecus:Minello and Zimmerman 1985; Minello et al. 1989) andblue crabs (Callinectes sapidus: Pile et al. 1996) is higheron vegetated than on bare substratum. The type of veg-etation and the amount of structure that it provides canalso modify survival rates. In the laboratory, juveniletiger prawns were twice as successful in evading a visualsh predator in tall seagrasses with high biomass and

    structure (Cymodocea serrulata and Syringodium isoe-tifolium) than in short seagrasses with low biomass andstructure (Halodule uninervis and Halophila ovalis: Ken-yon et al. 1995). For three species of sh, predation rateson postlarval brown shrimp were 1.23 times higher atpostlarvae densities of 167 than 56 postlarvae m2

    (Minello et al. 1989). The survival of banana prawnpostlarvae (P. merguiensis) was much lower whensettlement densities were very high (>20 ind. m2) thanwhen they were low (Haywood and Staples 1993). Thesendings highlight the importance of selecting the bestrelease sites and most appropriate stocking density forany stock enhancement program.

    Inuence of prawn stocking density on growth

    Our results from two seagrass beds in northern Australiasuggest that the main factors aecting the growth ofprawns in enclosures were the stocking density ofprawns and the type of seagrass bed. Both the ANOVAand regression approaches show that growth rates inenclosures on the Enhalus bed, with high biomass andstructure, were about twice as fast as those on theHalodule bed with low biomass and structure. The typeof substrate also aects the growth of juvenile P. aztecusand P. vannamei. P. aztecus in enclosures grew faster onsaltmarsh than on bare substrate (Minello and Zimm-erman 1991); caged P. vannamei grew faster in a vege-tated habitat (Ruppia) than on bare substrate (Edwards1977). These faster growth rates in habitats with vege-tation are likely to be due to a greater abundance offood, and a reduction in interactions between prawns.

    On the high biomass Enhalus bed, ANOVAs showedthat growth rates at a density of about 4 prawns m2

    were about 1.6 times faster than those at 18 and36 prawns m2. However, growth rates at densities of 4and 9 prawns m2 did not dier, and both exceeded1 mm CL week1. Although recovery rates at higherstocking densities were variable, the growth rate fromone enclosure where 31 of the 32 prawns were recovered(0.8 mm CL week1) was similar to the mean for threeenclosures where 1820 of the 32 prawns were recovered(0.7 mm CL week1). The results from the multiple re-gression analysis of the growth rate data for the Enhalusbed show that once the average density was tted to theequation, the size of the stocked prawns was also asignicant variable in explaining variation in growth

    rates growth rates declined as the size of the prawnsincreased.

    The mean growth rates on Enhalus at densities of 4and 9 prawns m2 are comparable with those estimatedfrom length-frequency analysis for the closely relatedbrown tiger prawn P. esculentus in the Gulf of Carpen-taria (1 mm CL week1: Loneragan et al. 1994) andMoreton Bay in southeastern Queensland (12 mm CLweek1 in the warmer months: O'Brien 1994a). From theregression approach, a predicted growth rate of 1 mmCL week1 is found at an ``average'' density of about12 prawns per enclosure on Enhalus (13 prawns m2)(Fig. 6b). The densities of juvenile prawns on the sea-

    grass beds in the western Gulf of Carpentaria andMoreton Bay did not exceed 2 prawns m2, and wereusually

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    Although our results suggest that density has amarked eect on growth, the frequency of occurrence ofdietary items in the foreguts of prawns in enclosures wasrelatively unaected by the stocking density of theprawns. Thus, both the foregut fullness and the fre-quency of occurrence of the main items in the foreguts ofprawns after 2 weeks in enclosures were similar at alldensities in April, and similar to those for juvenileP. semisulcatus under natural conditions (Heales et al.1996). However, the frequency of occurrence of fooditems may not provide a good indication of the abun-dance of food items in the enclosures. It is dicult toquantify the relative abundance of food items in the in-testines of prawns because food is processed through agastric mill. Estimating the relative abundance of fooditems in the enclosures would have provided better in-formation on the impact of prawns on their availablefood. However, this would have been dicult with theretrieval method that was used during our study. In otherstudies, very high densities of prawns have depleted the

    abundance of their potential food items. For example,Penaeus duorarum at densities of 160 prawns m2 de-pleted virtually all taxa of macrobenthos in seagrass(Halodule wrighti), particularly polychaetes and amphi-pods (Nelson 1981). High densities of P. duorarum(40 ind. m2) also depleted the numbers of amphipods,decapods and mollusks in seagrass (Thalassia testudi-num), and the depletion was greater when less seagrasswas present (Leber 1985).

    In our study, growth was not aected by the month ofthe experiment (April, the latewet season, or November,the prewet season), despite a lower salinity in April.However, temperatures were high and virtually identical

    in both months (30C). At lower temperatures(

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    Liu H, Loneragan NR (1997) Size and time of day aect the re-sponse of postlarvae and early juvenile grooved tiger prawnsPenaeus semisulcatus (Decapoda: Penaeidae) to natural andarticial seagrass in the laboratory. J Exp Mar Biol Ecol211:263277

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    Gulf of Carpentaria, Australia. Mar Biol 119:133143Loneragan NR, Wang Y-G, Kenyon RA, Staples DJ, Vance DJ,Heales DS (1995) Estimating the eciency of a small beamtrawl for sampling tiger prawns Penaeus esculentus andP. semisulcatus in seagrass by removal experiments. Mar EcolProg Ser 118:139148

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