The effect of salinity on growth and survival of juvenile black bream (Acanthopagrus butcheri)

Aquaculture 210 (2002) 219 – 230 www.elsevier.com/locate/aqua-online

The effect of salinity on growth and survival of juvenile black bream (Acanthopagrus butcheri) Gavin J. Partridge *, Greg I. Jenkins Aquaculture Development Unit, WA Maritime Training Centre, 1 Fleet St., Fremantle, Western Australia 6160, Australia Received 26 February 2001; received in revised form 16 September 2001; accepted 19 September 2001

Abstract Growth and survival of juvenile black bream (Acanthopagrus butcheri) were determined at salinities from 0 to 60 ppt (in 12-ppt increments) and from 0 to 12 ppt (in 4-ppt increments) in two separate trials of 6 and 4 months duration, respectively. Juvenile black bream were able to survive and grow at salinities ranging from freshwater (0 ppt) to 48 ppt. Osmotic stress was evident at 60 ppt, however, survival was not significantly affected. Fish reared at 24 ppt in trial 1 had a specific growth rate of 2.34 F 0.03%/day, a rate significantly higher only to those fish reared at 60 ppt (2.16 F 0.04%/ day). Growth was greater at 24 ppt in association with the highest food intake and most efficient FCR. Although both food intake and FCR were not significantly higher than those obtained with fish reared at 12, 36 and 48 ppt, the combination of the two factors being optimised at 24 ppt lead to the greatest growth. Analysis of data from the second trial found no significant difference in the growth rate of black bream reared at salinities ranging from freshwater to 12 ppt, with SGR ranging from 1.92 F 0.05% /day to 2.05 F 0.02% /day. Variable results in freshwater between the two trials suggested that total hardness of freshwater may influence survival and/or an ontogenetic change in salinity tolerance may occur. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Salinity tolerance; Black bream; Acanthopagrus butcheri; Growth; Survival

1. Introduction The black bream (Acanthopagrus butcheri) is an estuarine species of Sparid found throughout southern Australia and is one of the most important commercial and recreational species in this region (Kailola et al., 1993; Yearsley et al., 1999; Sarre et al., 2000a). Unlike most Sparidae, black bream complete their entire life cycle within their natal *

Corresponding author. Tel.: +61-8-9239-8032; fax: +61-8-9239-8081. E-mail address: [email protected] (G.J. Partridge).

0044-8486/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 ( 0 1 ) 0 0 8 1 7 - 1

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estuarine environment (Sarre and Potter, 1999). As such, their population cannot be supplemented from surrounding marine waters or other estuaries, leaving them highly susceptible to overfishing (Potter et al., 1996). Evidence suggests that stocks of black bream are either fully or over-exploited (Lenanton et al., 1999), particularly in the Blackwood River, Western Australia (Valesini et al., 1997; Sarre and Potter, 1999). Due to the decline in this population, research into culturing this species for stock enhancement began in 1991 (Lenanton et al., 1999). Due to their hardiness and euryhaline nature, this research has also generated interest in utilising the species for stocking salt affected inland water bodies for recreational fishing (Jenkins, 1997). As a result, over 200,000 juvenile black bream were made available for stocking into a variety of water bodies on freehold land ranging from small freshwater ponds to large saline lakes. Fish were stocked at low density and, in almost all cases, left to forage for naturally occurring food. Results of these stockings, in terms of growth and survival, were highly variable. However, those fish stocked into freshwater performed poorly (Sarre et al., 2000b). Although it is well known that black bream are capable of tolerating a wide range of salinities (Sarre and Potter, 1999), the optimal salinity for maximising growth and survival has not yet been determined, nor has growth performance been evaluated at hypo- or hypersaline extremes. A series of laboratory experiments were, therefore, undertaken to determine the influence of salinity on growth and survival rates of black bream to assist aquaculturists in deciding which water bodies would be most suitable for this species. The first of two trials quantified growth and survival over a 6-month period at salinities ranging from freshwater to 60 ppt, in 12-ppt increments. The second trial quantified these variables over a 4-month period at salinities ranging from freshwater to 12 ppt, in 4-ppt increments.

2. Materials and methods 2.1. Experimental system Each treatment was replicated four times in 140-l recirculating glass aquaria. Flow rate through each aquarium was maintained at 4 l/min and temperature was maintained with a thermostatically controlled immersion heater. Temperature variation was the same within and between treatments and ranged seasonally between 22 and 24 jC. Tanks were vacuumed daily and a 10% water exchange administered to prevent the build up of nitrate. The desired salinities were obtained by either adding artificial sea salt (‘Ocean Nature’ Aquasonic) to seawater or by diluting seawater with freshwater. The freshwater used in trial 1 was sourced from a metropolitan catchment dam, prior to the chlorine administration point and had a total hardness of 35 ppm. A chemical contamination of this water source prevented its use in the second trial. Council-supplied, potable town water, with a total hardness of 120 ppm, was therefore used in trial 2 after filtration through activated carbon. Temperature, pH, dissolved oxygen and salinity were measured daily. Total ammoniacal nitrogen and nitrite nitrogen were determined weekly via spectrophotometry according to Dal Pont et al. (1974) and Martin (1972), respectively. pH was maintained above 7.8 by the regular addition of sodium bicarbonate.

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3. Growth and survival trials Black bream used in the current trials were obtained from the Aquaculture Development Unit hatchery, where they were reared from naturally spawning broodstock held at 36 ppt. Juvenile bream from the same cohort were closely graded to ensure a uniform size distribution, then acclimated from 36 ppt to their respective salinities at the rate of 4 ppt/h. Fish in trial 1 were 4 months old at the commencement of the trial and had an initial wet weight of 0.78 F 0.02 g (standard error). Those used in trial 2 were 6 months old at the start of the trial with an initial wet weight of 3.65 g F 0.10 g. After acclimation, each aquarium was stocked with 100 fish (trial 1) or 80 fish (trial 2). Due to a shortage of aquaria, only three replicates of the 60-ppt treatment were tested in trial 1. Due to inconsistent results obtained with the freshwater treatment of trial 1, eight replicates of this treatment were incorporated into trial 2. Fish were fed daily on ‘native fish food’ (Ridley Agriproducts; 52% protein, 12% fat) with the diet particle size increasing from 1 to 3 mm as fish increased in size. Fish in each tank were fed slowly to satiety from a preweighed feed container. Daily consumption (grams) was recorded as the difference in weight of this container, plus the dry weight equivalent of fresh food offered. As this diet is not formulated specifically for black bream, a dietary supplement of fresh food (prawns, mussels or white bait) was given three times per week to ensure that growth was not compromised by the possibility of a nutritionally incomplete diet. At four weekly intervals, the water level in each aquarium was lowered to 25 l and a random subsample of 20 fish (less the number of mortalities that occurred during the previous month) was taken to assess growth and to reduce stocking density. At the end of the fifth month of trial 1, when only 20 fish remained in each tank, all fish were removed, weighed then returned to the tanks for the final month. Blood was taken twice during each trial to determine plasma osmolality. Blood was taken directly from the heart with a 22gauge, heparinised needle from a subsample of those fish removed for growth assessment. Due to the small size of the fish, blood from several fish from each replicate was pooled. Plasma was separated from haematocrit after centrifugation at 5000 rpm for 2 min (microcentrifuge, Denver Force 7) and plasma osmolality quantified using a cryoscopic mini-osmometer (Osmomat 030). Food conversion ratios (FCR) were calculated at the end of each trial by dividing the total (wet) biomass gain in each tank by the total consumption. Biomass gain was calculated by summing the weights of all monthly subsamples, mortalities and the fish remaining at the end of the trial, then subtracting from this the initial biomass stocked into the tank. Total consumption included the dry weight equivalent of all fresh food consumed in addition to the dry weight of consumed pellets. At the completion of each trial, a sample of 10 whole fish were pooled from each replicate and dried at 90 jC for 48 h to determine body water content.

4. Statistical analysis Growth [weight gain and specific growth rate, Eq. (1)], total food consumption, FCR and osmolality data were compared by analysis of variance after assumptions of

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homogeneity of variance and normality were verified using the Bartlett and Shapiro– Wilk W tests, respectively. Percentage survival and moisture content data were analysed in the same manner after arcsine transformation. Tukey – Kramer’s HSD test was used to compare differences between treatment means. All analyses were carried out on JMP (SAS Institute) and, unless otherwise stated, all statements of statistical significance refer to the 0.05 probability level.  SGRð%=dayÞ ¼

lnðWf Þ  lnðWi Þ t2  t1

  100

ð1Þ

5. Results 5.1. Trial 1: growth and survival at salinities of 0– 60 ppt in 12-ppt increments After the first month of the trial, survival in all treatments was greater than 96% with no significant differences between salinities ( P = 0.29) (Table 1). During the second month, all fish in the freshwater treatment died. After the first weighing, there was no significant difference in weight between these fish and those reared between 12 and 48 ppt (P = 0.49) and they appeared in good health. All measured parameters were considered to be well within the tolerable range for black bream (pH = 7.8, DO = 86%, TAN = 0.07 ppm, NO 2 –N = 0.13 ppm). Postmortem analysis revealed that the fish had died as a result of exposure to a toxin, possibly a contaminant in the header tank from which water exchanges were made. With the exception of the freshwater treatment, survival at all salinities was greater than 90% with no significant differences in survival between salinities during any monthly period (Table 1). Growth of black bream over the 6-month period is shown in Fig. 1. Maximum specific growth rate was achieved from fish reared in 24 ppt (2.34 F 0.03%/day). However, there were no significant differences in growth between these fish and those reared in salinities of 12, 24, 36 and 48 ppt (Table 2). Fish reared in 60 ppt had the lowest specific growth rate (2.16 F 0.04%/day), significantly lower only to those reared in 24 ppt.

Table 1 Percent survival of juvenile black bream reared under various salinities over a 6-month period (trial 1) Salinity (ppt)

Month 1

Month 2

Month 3

Month 4

Month 5

Month 6

0 12 24 36 48 60 *

97.3 F 1.9a 99.5 F 0.5a 99.8 F 0.3a 99.8 F 0.3a 96.5 F 2.2a 99.7 F 0.3a

0 99.7 F 0.3a 99.4 F 0.6a 100 F 0.0a 99.7 F 0.3a 100 F 0.0a

0 99.6 F 0.4a 100 F 0.0a 100 F 0.0a 99.6 F 0.4a 100 F 0.0a

0 100 F 0.0a 100 F 0.0a 99.4 F 0.6a 100 F 0.0a 98.3 F 1.7a

0 100 F 0.0a 100 F 0.0a 100 F 0.0a 100 F 0.0a 100 F 0.0a

0 100 F 0.0a 97.5 F 1.4a 100 F 0.0a 100 F 0.0a 90 F 7.6a

Within each month, values sharing the same letter are not significantly different. Data are means F S.E. (n = 4, * n = 3).

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Fig. 1. Change in wet weight over 6 months of juvenile black bream grown in six different salinities (trial 1).

Analysis of variance found a significant effect of salinity on FCR ( P = 0.0003). Those fish reared in 24 ppt had the lowest FCR of 1.34 F 0.03. However there were no significant differences in FCR in fish reared in salinities from 12 to 48 ppt (Table 2). Those reared in 60 ppt had a significantly higher FCR (1.74 F 0.06) than all other

Table 2 Growth, body water content, food consumption, FCR and plasma osmolality of juvenile black bream reared under various salinities over a 6-month period (trial 1) 12 ppt Weight gain (g) SGR (% /day) Body water content (%) Total consumption (g) FCR Osmolality (month 3) (mosM/kg) Osmolality (month 6) (mosM/kg)

24 ppt a,b

33.62 F 1.38 2.25 F 0.02a,b 64.0 F 0.2a 1939 F 66a 1.43 F 0.05a 330 F 4a 348 F 6a

36 ppt a

48 ppt a,b

39.54 F 2.00 2.34 F 0.03a 63.1 F 0.2a 2001 F 21a 1.34 F 0.03a 353 F 21a

34.58 F 1.98 2.27 F 0.03a,b 63.9 F 0.9a 1902 F 23a 1.41 F 0.03a 345 F 9a

365 F 8a,b

361 F 5a,b

60 ppt * a,b

32.52 F 1.28 2.23 F 0.02a,b 64.4 F 0.7a 1892 F 31a 1.46 F 0.04a 365 F 48a 375 F 3b

28.66 F 1.91b 2.16 F 0.04b 67.4 F 1.0b 1711 F 24b 1.74 F 0.06b 528 F 18b 385 F 6b

Within each row, values sharing the same letter are not significantly different. Data are means F S.E. (n = 4, * n = 3).

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treatments. In addition, those fish reared in 60 ppt consumed significantly less food (1711 F 24 g) during the experimental period than those in all other treatments (Table 2). Those reared in 24 ppt consumed the greatest amount of food (2001 F 20 g), however, there were no significant differences in consumption in fish reared from 12 to 48 ppt. At the end of the third month, there were no significant differences in plasma osmolality in fish held from 12 to 48 ppt salinity, with values ranging from 330 to 365 mosM/kg (Table 2). The plasma osmolality of fish kept at 60 ppt was, however, significantly higher than all other treatments (528 F 18 mosM/kg). After 6 months, there were no significant differences in plasma osmolality in fish held from 24 to 60 ppt salinity (Table 2). Plasma osmolality of fish held at 12 ppt (348 F 6 mosM/kg) was, however, significantly lower than those held at 48 ppt (375 F 3 mosM/kg) and 60 ppt (385 F 6 mosM/kg). At the completion of the trial, there was no significant difference in body water content in fish reared between 12 and 48 ppt with values ranging from 63.1 F 0.2% to 64.4 F 0.7%. Those fish reared in 60 ppt, however, had a significantly higher moisture content than fish in all other treatments (67.4 F 1.0%) (Table 2). 5.2. Trial 2: growth and survival at salinities of 0– 12 ppt in 4-ppt increments Survival of fish in salinities greater than 0 ppt was greater than 98% at each month (Table 3). Survival in freshwater was greater than 97% in 3 of the 4 months but was significantly lower (83.6 F 1.48%) than all other salinities in the second month. The rate of growth of the black bream is shown in Fig. 2. At the completion of the 4month trial, fish weighed between 31.5 and 36.5 g with no significant difference between treatments in terms of weight gain or specific growth rate (Table 4). One-way analysis of variance showed a significant effect of salinity on FCR ( P < 0.0001)(Table 4). No significant differences occurred between salinities of 4, 8 and 12 ppt, with values ranging from 0.96 to 1.05. The FCR value obtained for fish reared in freshwater (0.87 F 0.02) was not significantly different to that obtained in 4 ppt (0.96 F 0.04). Likewise, there were no significant differences in total consumption between salinities of 4, 8 and 12 ppt (ranging from 1223 to 1271 g). However, consumption at 0 ppt (1165 F 8 g) was significantly less than at 4 ppt (1271 F 24 g) (Table 4). There were no significant differences in plasma osmolality between the salinities tested at month 2, with values ranging from 336 F 7 mosM/kg at 0 ppt to 360 F 12 mosM/kg at Table 3 Survival of juvenile black bream reared under various salinities over a 4-month period (trial 2) Salinity

Month 1

Month 2

Month 3

Month 4

0* 4 8 12

98.0 F 0.04a 99.7 F 0.04a,b 99.7 F 0.04a,b 100 F 0.00b

83.6 F 1.48a 99.2 F 0.77b 100 F 0.00b 99.6 F 0.38b

97.5 F 0.73a 99.5 F 0.58a,b 100 F 0.00b 99.5 F 0.00a,b

97.9 F 1.18a 98.6 F 1.64a 100 F 0.00a 99.3 F 0.00a

Within each month, values sharing the same letter are not significantly different. Data are means F S.E. (n = 4, * n = 8).

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Fig. 2. Change in wet weight over 4 months of juvenile black bream grown in four different salinities (trial 2).

12 ppt (Table 4). The same pattern of plasma osmolality with respect to salinity was observed at month 4 and again there were no significant differences in plasma osmolality between treatments (Table 4). Body water content of fish at the end of the trial ranged from 67.1 F 0.1% to 69.2 F 0.1%. Those fish held at 0 ppt had significantly higher body water content at the completion of the trial compared to all other treatments (Table 4). Table 4 Growth, body water content, food consumption, FCR and plasma osmolality of juvenile black bream reared under various salinities over a 4-month period (trial 2) 0 ppt * Weight gain (g) SGR (% /day) Body water content (%) Total consumption (g) FCR Osmolality (month2) (mosM/kg) Osmolality (month 4) (mosM/kg)

4 ppt a

32.83 F 0.72 2.05 F 0.02a 69.2 F 0.1a 1165 F 8a 0.87 F 0.02a 336 F 7a 361 F 8a

8 ppt a

32.53 F 2.04 2.04 F 0.05a 67.1 F 0.1b 1271 F 24b 0.96 F 0.04a,b 351 F 12a 375 F 8a

12 ppt a

27.87 F 1.83 1.92 F 0.05a 67.7 F 0.0b,c 1223 F 39a,b 1.05 F 0.03b 347 F 5a

29.85 F 1.04a 1.98 F 0.03a 68.1 F 0.1c 1235 F 14a,b 1.04 F 0.02b 360 F 12a

367 F 7a

386 F 8a

Within each row, values sharing the same letter are not significantly different. Data are means F S.E. (n = 4, * n = 8).

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6. Discussion Although there have been many studies investigating salinity tolerance in Salmonids and freshwater fish (Brett, 1979; Kilambi, 1980; Clarke et al., 1981; McKay and Gjerde, 1985; McCormick et al., 1989; Morgan and Iwama, 1991), few have investigated the tolerance of marine or estuarine species to a variety of salinities. The data presented in this study demonstrate that black bream are extremely tolerant of a wide range of salinities when fed to satiety and maintained at temperatures of 22– 24 jC. Although it has been hypothesised that maximum growth should occur in an isosmotic environment of 10 F 2 ppt due to lower osmoregulatory energy demands (Brett, 1979), interspecific differences in salinity optima are documented. Brocksen and Cole (1972) investigated the optimal salinity for growth of three marine species, the Bairdiella (Bairdiella icistia), orangemouth corvina (Cynoscion xanthulus) and the sargo (Anisootremus davidsoni) and found the optimal range to be 33 – 37 ppt, when tested at salinities from 29 to 41 ppt. Alternatively, Lambert et al. (1994) found that when grown at 7, 14 and 28 ppt, Atlantic cod (Gadus morhua) had the best growth at 14 ppt, up to 63% better than when grown at 28 ppt. The Arctic cisco (Coregonus autumnalis) showed no significant difference in growth between 6 and 30 ppt (Fechhelm et al., 1993) and Swanson (1998) obtained the best growth of milkfish (Chanos chanos) at a salinity of 55 ppt, when grown at salinities of 15, 35 and 55 ppt. Direct comparison of the results of the current study with those of salinity optima of other sparid species is difficult due to the varying salinity ranges and increments studied. Based on the available literature, however, it appears that sparids have optimal growth in a salinity range between 12 and 28 ppt. When tested at salinities ranging from 8 to 38 ppt, in 10-ppt increments, Klaoudatos and Conides (1996) found the optimal salinity for the gilthead sea bream (Sparus auratus) to be 28 ppt. Woo and Kelly (1995) obtained the best growth of golden-line sea bream (S. sarba) in a salinity of 15 ppt, when tested at salinities of 7, 15 and 35 ppt. Further detailed investigation of salinities between 12 and 36 ppt is required to determine more accurately the optimum salinity for growth of juvenile black bream. Few marine euryhaline species have been shown to tolerate and grow well in freshwater. Red drum (Sciaenops ocellata) and mullet (Mugil cephalus) have both been shown to survive and grow in freshwater, but at a lower rate than their optima of 20 and 28 ppt, respectively (DeSilva and Perera, 1976; Crocker et al., 1981). Wu and Woo (1983) tested the tolerance of three marine fish to freshwater, including one sparid species, black sea bream (Mylio macrocephalus), and all three died. Dendrinos and Thorpe (1985) found that European sea bass (Dicentrarchus labrax) died within a few days in freshwater. The good growth and high survival rates obtained in trial 2 with fish in freshwater may have been the result of the increased hardness of the scheme water (120 ppm) compared to that from the catchment dam used in the trial 1 (35 ppm). The effect of increasing water hardness on survival of striped bass (Morone saxatilis) has been shown by Grizzle et al. (1985). Postharvest survival of this species was improved from 16% to 80– 99% when water hardness was increased from 10 – 20 up to 200 ppm. Further investigation into the tolerance of black bream to freshwater with varying degrees of hardness is required, especially considering the water bodies into which black bream have been stocked vary considerably in ionic composition. Since the fish used in the second trial were older than

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those used in trial 1, their superior performance may have also been due to a greater tolerance to freshwater at an older age. Support for this theory is provided by Weng (1971) who observed no mortalities in black bream older than 1 year after transfer to freshwater, whereas 20% of fish younger than 1 year died upon transfer. This increased tolerance to low salinity with increasing age is also supported by observations on the distribution of black bream in their natural environment. Sampling of black bream in the Swan river estuary in Western Australia demonstrated that the majority of large fish ( > 250 mm total length) were caught in salinities below 20 ppt, whereas smaller individuals were found in areas where the salinity ranged from 20 to 30 ppt (Sarre, 1999). It has been shown that holding fish in different salinities can result in differences in both consumption and FCR, however, responses differ between species. In the current study, growth of black bream was greatest in 24 ppt due to a high food intake and the most efficient FCR. Although both factors were not significantly better than those obtained at 12, 36 and 48 ppt, the combination of the two factors being optimised at 24 ppt lead to the greatest growth. Klaoudatos and Conides (1996) obtained a similar result with gilthead sea bream, with the highest food intake and best conversion efficiency resulting in optimum growth at 28 ppt. In contrast, optimal growth of Atlantic cod at 14 ppt was due only to improved conversion efficiency, and not differences in food intake (Lambert et al., 1994). Appetite depression at suboptimal salinities has, however, been observed for many species including mullet (DeSilva and Perera, 1976), red drum (Crocker et al., 1981) and gilthead sea bream (Dendrinos and Thorpe, 1985). The low FCR obtained with black bream held in freshwater during the second trial may have been the result of the significantly higher body water content of these fish. Alternatively, the decreased consumption at this salinity could have resulted in a low FCR. With intake effectively restricted below the maximum ration, FCR may have been improved due to improved digestibility at a lower food intake (Brett, 1979). The significantly higher FCR obtained in 60 ppt could have been due to changes in gut evacuation rate, caused by fish drinking excess seawater in an effort to overcome the loss of water to the hyperosmotic environment (Lambert et al., 1994). This theory is supported by the significantly higher body moisture content of fish reared at 60 ppt. Evaluation of diet digestibility at each salinity may have clarified the variation in FCR between salinities. Tort et al. (1994) reported that the plasma osmolality of gilthead sea bream in full strength seawater ranged from 350– 400 mosM/kg. This range is similar to that obtained in the current study at salinities ranging from 0 to 48 ppt, indicating that black bream are capable of efficient osmoregulation within this salinity range. The fact that no significant differences in body moisture content occurred in this range supports this observation. At the end of the third month, those fish held in 60 ppt had a plasma osmolality of 528 mosM/kg. Although above the normal range of osmolality values for most euryhaline teleosts (Bond, 1979), some species have the ability to maintain long-term plasma osmolality levels over 600 mosM/kg (Nordlie, 1987). As the collection of this blood sample coincided with a short period of disease, including lesions and white spot, it is possible that this level of osmolality may have been a temporary consequence of this additional stress. The fact that the fish recovered from these conditions and their plasma osmolality decreased to 385 mosM/kg supports this theory. Alternatively, the drop in osmolality after 6 months could have been the result of an ontogenetic change in tolerance

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to hypersaline water or an induced increase in osmoregulatory ability. The fact that large black bream have been observed in the wild in salinities as high as 114 ppt, as they become locked in small bodies of evaporating water (G.A. Sarre, personal communication), could support either of these two theories and places black bream amongst the most euryhaline species described (Nordlie, 1987). Evidence of ontogenetic changes in salinity tolerance have been observed for other species including puffer fish (Takifugu rubripes) (Han et al., 1995), red drum (Crocker et al., 1981) and various species of tilapia (Oreochromis spp.) (Watanabe et al., 1985, 1990). Further work on the ontogenetic changes in salinity tolerance of black bream will be useful in identifying the minimum age at which black bream can be stocked into hypersaline water. Results from concurrent field studies investigating the performance of black bream in inland water bodies have observed poor survival in freshwater. These fish were usually not supplied with supplemental food, but left to forage on naturally occurring food. In addition, mortalities generally occurred in winter, suggesting that under conditions of low temperature and/or low food availability or quality, black bream are less tolerant of other stressors such as freshwater. Klaoudatos and Conides (1996) support this theory by suggesting that low salinity tolerance is based on food quality and availability. In addition, unpublished data collected by the authors has shown that low temperature tolerance of black bream juveniles is significantly reduced in freshwater. Further studies investigating the interactive effects of ration size/quality, low temperature and salinity will be beneficial for future inland stockings.

Acknowledgements This study was funded by the Fisheries Research and Development, Project #97/309. The authors would like to thank Anthony Aris and Bruce Ginbey for their technical assistance, Dr. Gavin Sarre, Dr. Brett Glencross and Dr. Sagiv Kolkovski for their constructive criticism of the manuscript and Dr. Yuk Wing Cheng for his assistance during early analysis of the data.

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