Production of marketable-size tilapia,Oreochromis spilurus (Günther), in seawater cages using different production schedules*

Abstract. In Kuwait, the culture of tilapia, Oreochromis spilurus (Günther), in sea cages is limited by water temperature for mid-April to mid-November only. Using fry spawned in April, one production cycle is passible, but marketing time is limited to less than 20 days. With a market capacity of 1 tonne/day, the production of 100 tonnes/year planned for a commercial farm requires 100 marketing days. It is therefore necessary to shorten production time by stocking larger fingerlings to extend the marketing period. Four fingerling production schedules were tested, using fry spawned (1) in May, grown normally; (2) in May, stunted; (3) in August; and (4) in December. The fish were initially stocked and overwintered in fibreglass tanks. On 19 April, the fish were transferred to 1-m³ net cages at 150 fish/cage and grown to marketable size (<300g). Results showed that fish spawned in May (normal and stunted), August and December were harvested in July, August and October and can be marketed for 125, 100 and 60 days, respectively. It is apparent that the production of 100 tonnes of tilapia to be marketed over a period of 100 days is possible using fry produced in August; the profit, however, is marginal. Fry spawned in December gave the highest profit.     

The effect of high salt diet on the direct transfer of Oreochromis mossambicus,O. spilurus and O. aureus/O. niloticus hybrids to sea water

The effect of feeding a high salt diet on the survival rates of some fresh water Oreochromis (formerly Sarotherodon) species in marine culture was experimentally investigated. High salt diet (10% NaCl) was fed to O. mossambicus, O. spilurus and O. aureus/O. niloticus hybrids for a period of 4 weeks and the survival rates were estimated after each week by directly transferring the species to various salinity levels. Feeding of dietary sodium chloride considerably enhanced the survival rates of O. mossambicus up to 84%, and the O. aureus/O. niloticus hybrids and O. spilurus up to 62% and 50%, respectively, in sea water. Best survival rates were recorded after 2 weeks of feeding the salt diet for O. mossambicus and O. aureus/O. niloticus hybrids, whereas in O. spilurus best survival was not achieved until 3 weeks. Contrary to the sudden increase in plasma osmotic concentration recorded in the fish transferred directly from fresh water to 60% sea water, high salt diet feeding prior to the transfer resulted in only a slight increase in the plasma osmotic concentration in sea water.     

Osmoregulatory ability of anadromous Arctic char, Salvelinus alpinus (L.), migrating towards the sea

Abstract. Hypoosmoregulatory capacity was examined in anadromous Arctic char, Salvelinus alpinus (L.), caught whilst migrating towards the sea in the river Å-elva, northern Norway (69°04′N and 17°00′E). Metacercariae of the marine digenean Cryptocotyle lingua (Creplin) were found on 47% of the fish investigated, demonstrating that they had been in brackish or sea water on at least one previous occasion. The fish were randomly divided into three groups, and exposed to sea water (35 ppt salinity at 5–6°C) for 7 days- one group immediately after capture, and the other two groups after 16 days of acclimation in fresh water and brackish water (15–17ppt), respectively. Blood plasma osmolality, Na⁺ and Mg²⁺ levels showed only a small and transient rise during seawater exposure, and there were only minor changes in muscle water content, irrespective of prior treatment. A slight, but insignificant, reduction in hypoosmoregulatory capacity appeared to occur in the groups retained in fresh water or brackish water for 16 days prior to seawater exposure. The results did not indicate that the fish show any improvement of hypoosmoregulatory capacity following a period of acclimation in brackish water. The hypoosmoregulatory capacity of seaward-migrating Arctic char is well developed before they enter the sea, and may be comparable to that seen in salmonids which undergo a parr-smolt transformation. There was a negative correlation between blood plasma electrolyte levels and both body length and weight, indicating that hypoosmoregulatory capacity was affected by the size of the fish. The effects of body size upon the development of hypoosmoregulatory capacity are discussed with respect to estuarine residence and the constraints placed upon the survival of small fish in full-strength sea water.     

Sex reversal and growth of Oreochromis spilurus (Günther) in brackish and sea water by feeding 17α-methyltestosterone

Abstract. An investigation was undertaken to study the effect of 17α-methyiteslosterone (17-MT) on the sex reversal and growth of tilapia, Oreochromis spilurus (Günther), in brackish water (3–5 ppt) and sea water (40 ppt). Seven experiments were performed; four in brackish water and three in sea water. Yolk-sac fry (1 or 3 day age) were immersed (7 or 24 h/day) in water containing hormone at a concentration of 0–10mg/l for 4–25 days. The swim-up fry were subsequently fed either normal or hormone-impregnated diets (30–70 mg/kg) for another 35 days starting from first feeding. The growth and SGR of fry were better in groups which received hormone both in water and food as compared with water or food alone. Immersion In 5–10 ppm (24 h/day) brackish water solution for 5 days did not bring any change in growth or sex ratio. However, immersion in sea water solution for 25 days (7 h/day) at a concentration of 1 ppm induced positive growth but no change in sex ratio. Higher doses either did not affect or reduced growth and SGR but induced higher percentages (77–83%) of males. Complete masculinization (100%) was observed only in brackish water when fish were immersed in 2·5 ppm solution for 4 days (24 h/day) and later fed 50 mg/kg in diets for 35 days. These results were further confirmed when the immersion schedule was modified to 10 days (7 h/day) and feeding 50–70 mg/kg hormone. It can be concluded from these studies that in brackish water complete masculinization of O. spilurus is possible through immersion in 5·0 ppm for a minimum of 75 h followed by feeding with 50 mg/kg food for a minimum of 40 days starting from first feeding.     

Preliminary studies on feminization and growth of Oreochromis spilurus (Günther) by oral administration of 17α-ethynyloestradiol in sea water

A study was conducted on Oreochromis spilurus (Günther) fry (9.0 mm) in sea water (38-40%o) to evaluate the effects of three doses (40, 70 and 100 ppm) of a synthetic oestrogen, 17α-ethynyloestradiol (17-EE) on the feminization, specific growth rate (SGR), food conversion efficiency (FCE), condition factor (CF) and survival rate. The fry were fed a hormone-impregnated diet for 42 days, followed by a hormone-free diet for another 77 days. At the end of the experiment (119 days), successful feminization was obtained in all groups treated with 17-EE. The dose of 100 ppm was the most potent one and induced 92.2% sex reversal. At the end of the hormone-treatment phase (42 days), all groups treated with 17-EE and the untreated controls showed no significant differences in growth, SGR, FCE, CF, and survival rate. After 119 days, a significant difference was detected only in the mean body weight and total length where the EE-100 group had the highest values. However, this was due to the low survival rate in this group (37.5%) and not to any possible anabolic effect of 17-EE. The findings of this study indicate that seawater sex reversal of O. spilurus is possible and 100% sex reversal could be achieved by slightly increasing the dose and/or duration of the treatment.     

Survival rates of tilapia, Oreochromis spilurus (Günther), fingerlings reared at high densities during winter using warm underground sea water

The study was conducted to determine the optimum stocking density for rearing tilapia. Oreochromis spilurus (Günther), fingerlings in tanks during winter using warm (21-26°C) underground sea water (37%o). Seawater-acclimated fingerlings with mean weight of 2 g were stocked in eighteen 400-1 fibreglass tanks at 750 and 1000 fish m^?3. Fish were fed at the rates of 2.5, 3.0 and 3.5% day^?1 of the fish biomass. After 83 days, the mean individual daily weight gain was significantly higher (P < 0.028) at stocking of 750 fish m^?3 than at 1000 fish m^?3. Feed conversion ratio was significantly higher at stocking of 1000 fish m^?3 than at 750 fish m^?3 and at feeding rate of 3.5% day¹ than at 2.5% day^?1. However, because no significant differences were observed on survival rates between the two stocking densities and among feeding rates, it is recommended that the stocking density of 1000 fish m^?3 and a feeding rate of 2.5% day^?1 be used for optimum production of tilapia fingerlings in tanks during winter using warm underground sea water.     

Growth performance,osmoregulatory and metabolic modifications in red porgy fry,Pagrus pagrus,under different environmental salinities and stocking densities

The effects of two different environmental salinities [brackish water (BW), 12‰; sea water (SW), 39‰] and initial stock densities [low (LD), 1.0 g L^?1; high (HD), 2.0 g L^?1] on growth, osmoregulation, stress and energy metabolism of the fry Pagrus pagrus were investigated over a period of 45 days. Pagrus pagrus (n=80, 5.51 ± 0.25 g mean initial body weight) were randomly divided in eight groups. Growth, weight gain and specific growth rate increased in BW‐acclimated fish compared with SW‐acclimated fish. No differences were observed between the two stock densities tested at either environmental salinity. Plasma osmolality was lowest in BW‐acclimated specimens, but the stock density had no effect on this parameter. Branchial Na⁺,K⁺‐ATPase activity was positively correlated with environmental salinity, but unaltered at the renal level. Plasmatic parameters were enhanced by salinity and stocking conditions. At the hepatic level, triglyceride values were enhanced in BW‐acclimated fish maintained at LD. Muscle metabolites (glycogen, glucose and lactate) increased in BW‐ compared with SW‐acclimated fish; stock density had no influence. Our data suggest that changes in metabolic parameters could be correlated with the higher growth rates observed in P. pagrus acclimated to BW, while no significant effects due to the stocking density used were observed.     

Spawning performance and seed production from hybridization between Oreochromis spilurus and the GIFT strain of the Nile tilapia Oreochromis niloticus

This study compared the spawning performance of four crosses between the salt‐tolerant tilapia Oreochromis spilurus and the fast‐growing genetically improved farmed tilapia (GIFT) strain of the Nile tilapia Oreochromis niloticus: (1) O. spilurus♀×O. spilurus♂ (SS); (2) Nile ♀× Nile ♂ (NN); (3) Nile ♀×O. spilurus♂ (NS) and (4) O. spilurus♀× Nile ♂ (SN). In each cross, males and females having mean weights of 157 and 115 g, respectively were stocked in 0.43 m³ tanks at a male to female ratio of 1:3. Seeds (unhatched eggs, yolk‐sac fry and free swimming fry) were harvested biweekly for 127 days. Viable spawns resulted from all crosses. The NN cross produced the highest total seeds tank^?1 (2890), seeds (kg female)^?1 day^?1 (48.3), seeds m^?2 day^?1 (23.3) and seeds female^?1 day^?1 (7.9), followed by the SS and SN crosses. The NS cross had the lowest fecundity. Lower spawning parameters in the NS and SN crosses may be attributed to the lower social compatibility between the males and females compared with that of the pure parents. The survival and growth of the offspring from the four crosses will be compared in seawater (37–40 g L^?1) to assess whether hybrid vigor was produced.     

The development of growth and feeding equations for sea bream, Sparus aurata L., culture

The development and testing of two fish growth models is described. The equations are based on the effects of fish weight and water temperature on the specific growth rate and ration of sea bream Sparus aurata L. The models show a good fit and are proposed for prediction of harvesting time and food consumption. In particular, provided a fish-stocking technique similar to that described in this study is followed, prediction can be made on the estimation of the optimum amount of food given to avoid food wastage and save storage space, the overall rearing period from the 2-g fry stage up to the marketable size of 300-350 g, and the optimal stocking period in order to produce marketable fish at the right time and at a satisfactory selling price.     

Acclimation of rainbow trout to sea water

In some fish farms it is advantageous to transfer rainbow trout from fresh water to sea water at as small a size as possible. It has been shown that a relatively small reduction in salinity at the marine site can give appreciable improvements in survival rates during acclimation by smaller fish, thus rainbow trout of 15 g have routinely been put directly into sea cages at a salinity of 22% with mortality levels of from 1 to 8%.     

Preliminary observations on growth and survival of Oreochromis spilurus x GIFT Oreochromis niloticus F1 reciprocal hybrids in fresh and seawater

Tilapia has a significant potential for culture in saline environments. There is an increasing demand among tilapia producers to develop a tilapia hybrid that can survive well in marine water conditions. This study compared mean weight (MWT), daily growth rate (DGR), specific growth rate (SGR), survival, feed conversion ratio (FCR), condition factor (K), production rate (PR) and rate of skin lesions, and tail and fin rot as well as eye cataract in the Genetically Improved Farmed Tilapia strain (GIFT) of the Nile tilapia, Oreochromis niloticus (NN) and the salt‐tolerant Oreochromis spilurus (SS), and their F₁ reciprocal hybrids; O. niloticus ♀ x O. spilurus ♂ (NS) and O. spilurus ♀ x GIFT O. niloticus ♂ (SN) in freshwater (0 g L^?1) and seawater (40 g L^?1). Fish (3.5 g) were stocked at 150‐fish m^?3 and fed with sea bream pellets (47% protein) for 180 days. Results showed that in seawater, the SN hybrid had the highest values for MWT (165.9 g), DGR (0.9 g fish^?1 day^?1), SGR (2.14% day^?1), survival (96.3%), PR (23.9 kg m^?3) and best FCR (1.53) followed by the NS hybrid and the NN parent. Both hybrids had significantly lower rates of skin lesions and fin and tail rot than the NN genotype. In freshwater, the NN had the highest values for MWT (255.1 g), DGR (1.40 g fish^?1 day^?1), SGR (2.38% day^?1), K (2.13%) and PR (34.9 kg m^?3) followed by the NS and SN hybrids. High estimate of heterosis for MWT (41.3%), DGR (42.5%), SGR (10.7%) and survival (22.1%) was obtained in the hybrids reared in seawater, indicating that a hybrid vigor was produced and the fast growth trait from the GIFT parent was successfully combined with the salinity tolerance trait from the O. spilurus parent. The better growth performance and survival of the SN hybrid in seawater indicate that this hybrid is more suitable for culture in seawater than its reciprocal hybrid.     

Observations on the reproductive performance of three mouth-brooding tilapia species in low-salinity underground water

The reproductive performance and seed production of three mouth‐brooding tilapia species, Oreochromis spilurus (Günther), Oreochromis aureus (Steindachner) and Taiwanese red tilapia hybrid, were compared in low‐salinity underground water (3–5 g L^?1) to select species with the highest reproductive rate. Two‐year‐old males and females having average body weights of 454 and 259 g, respectively, were stocked in 2‐m³ fibreglass breeding tanks (two tanks per species) at 1:3 male to female sex ratio and at average density of three fish m^?2. Seeds (unhatched eggs; yolk‐sac fry and the swim‐up fry) were collected weekly for 112 days. The results showed O. spilurus to have significantly higher (P<0.05) mean spawning parameters expressed as total seed tank^?1 (84 154), seed kg female^?1 day^?1 (284), seed m^?2 day^?1 (190.3) and seed female^?1 day^?1 (76.3), than O. aureus and red tilapia. Monthly seed production in O. spilurus and red tilapia gradually increased from May, peaked in July and declined in August, while in O. aureus, it continued to decrease from May to August. In this study, the observed production levels of seed in O. spilurus were among the highest levels reported for tilapia; therefore, O. spilurus could be selected as a potential candidate for mass seed production in commercial tilapia hatcheries.     

Salinity and Temperature Effects on the Lipid Composition of Mullet Sea Fry (Mugil cephalus, Linne, 1758)

A large part of the extensive aquaculture in Tunisia was based on the transfer of mullet fry from the marine coastal areas to the continental fresh water lakes, the aim of this work is the monitoring of the changes made in the lipid composition of Mugil cephalus fry according to salinity and temperature acclimation. Fish acclimated to 14 °C and 0.5 psu salinity, contained 55% less total fatty acid than those acclimated to the seawater at the same temperature. Low salinity induces an increase in the percentage of some polyunsaturated fatty acid such as the 22:5 (n-3), the 22:6 (n-3) and the 20:4 (n-6), as a result of change in the level of triacylglycerol content or the activation in the synthesis of those fatty acids. The transition of the acclimation temperature from 26 to 14 °C in seawater is followed by a 215% increase in the amount of total fatty acids in the fourth week of acclimation. The total fatty acid levels of all fish at 1 week were higher than most values at 4 weeks. There was an increase of polyunsaturated fatty acids percentages due to the decrease of the temperature at the low salinity. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fatty acid composition,osmolality, Na+, K+‐ATPase activity,cortisol content and antioxidant status of rainbow trout (Oncorhynchus mykiss) in response to various dietary levels of eicosapentaenoic acid and docosahexaenoic acid

This study was conducted to evaluate the effect of dietary eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) levels on the fatty acid composition, salinity tolerance and antioxidant status of rainbow trout (Oncorhynchus mykiss). Four diets were formulated with total EPA and DHA contents of 5.41, 9.55, 13.97 and 17.88 g/kg (abbreviated as ED‐5.41, ED‐9.55, ED‐13.97 and ED‐17.88 respectively). Rainbow trout (initial weight of 90.61 ± 9.25 g) were fed the experimental diets for 8 weeks to accumulate significant differences in fatty acid composition and subsequently underwent salinity acclimation. Our results indicated that high dietary EPA and DHA significantly improved the EPA and DHA content in fish tissues. The serum osmolality of fish returned to their freshwater values in the ED‐9.55, ED‐13.97 and ED‐17.88 groups. The Na⁺, K⁺‐ATPase (NKA) activity of fish in the ED‐13.97 group changed dramatically to adapt the fish to the hypertonic environment. Moreover, there was no significant difference in the serum cortisol concentration and liver catalase (CAT) activity of fish in the ED‐13.97 group during salinity acclimation. The liver superoxide dismutase (SOD) activity in the ED‐13.97 group was significantly higher than that in ED‐5.41 and ED‐9.55 groups at the end of salinity acclimation. The muscle malondialdehyde (MDA) content in the ED‐13.97 group was significantly lower than that in the ED‐17.88 group before salinity acclimation and significantly lower than the ED‐5.41 and ED‐17.88 groups on day 7 of acclimation. The results of this study indicate that the rainbow trout in the ED‐13.97 group exhibited optimal salinity acclimation performance.     

Growth,osmoregulation and energy budget of rainbow and steelhead trout under different salinity acclimation methods and the best transition size of steelhead trout

In order to meet the demand of salmon market, Chinese scientists and entrepreneurs are working on salmon mariculture far offshore in the Yellow Sea, China. Rainbow, steelhead trout and Atlantic salmon were selected as the main culture species. The aims of the present study were as follows: (a) investigate the effect of the salinity acclimation method on the growth, osmoregulation and energy budget in two forms of Oncorhynchus mykiss, rainbow and steelhead trout and (b) explore the optimal size of steelhead trout for the seawater entry. In trial I, rainbow (mean = 99.44 g) and steelhead trout (mean = 99.01 g) were reared for 40 days after undergoing salinity acclimation at three rates: an abrupt increase in salinity from 0 to 30 g/L (T30); an abrupt increase in salinity to 14 g/L, followed by a daily increase of 2 g/L (T2) or 6 g/L (T6) until reaching 30 g/L; and no salinity exposure (control treatment) (T0). In trial II, steelhead trout with body weights of approximately 100 and 400 g were cultured for 60 days with two treatments, T0 and T2, and the specific growth rate (SGR) was calculated every 10 days. In trial I, in both kinds of fish, the optimal growth performance, survival rate, osmoregulation and energy budget were observed in the T0 treatment, followed by the T2 treatment. These results indicate that O. mykiss with a body weight of approximately 100 g can adapt to sea water with a gradual transition (T2), but they are still not suitable for the seawater entry because of low growth. Based on the recorded SGRs in trial II, our formulated regression formula revealed that approximately 200 g is the optimal size of steelhead trout for the transition to sea water.     

Growth, food conversion, maintenance and long-term survival of gilthead sea bream, Sparus auratus L., juveniles after abrupt transfer to low salinity

Juveniles of gilthead sea bream, Sparus auratus L., with individual weight 1.92 g, were reared under salinities of 8, 18, 28 and 38%o. The transfer to lower salinities (8-28%o) was abrupt. The results showed that better daily growth rates and food conversion efficiencies (FCE) when growing at 28%o gave the best performance. Statistically, salinity values above 28%o do not significantly affect daily growth rates and daily voluntary food intake. Maintenance requirement decreases linearly with increasing salinity. In particular, a l%o decrease of salinity is associated with a 0.014 (% body weight day^-1) increase of daily maintenance requirement. Mortality rates are high at salinities of 8 and 38%o (65% and 20% respectively) and minimal (1-3%) for fish reared under intermediate salinities (18-28%o). Finally, scope for growth, expressed as the minimum difference between food amount eaten and food attributed to growth, is better when the fish are kept at a salinity of 28%o. This means that most of the food amount eaten is attributed to growth and food wastage is minimal. These results confirm that gilthead sea bream fry show best performance when reared at a salinity of 28%o.     

Comparison of freshwater and marine performances of all‐female diploid and triploid Atlantic salmon (Salmo salar L.)

The performance of all‐female diploid (AF2N) and triploid (AF3N) Atlantic salmon were compared in fresh water, under commercial production conditions in 1995 and 1996 year classes. The performance of the 1996 year class was also assessed for 14 months in a commercial sea farm. Freshwater mortality was higher in the triploid groups. The majority of losses occurred in the early stages of egg development and during the first feeding period, when the incidence of non‐feeding fry was consistently higher. In growth studies, although diploid fry were significantly heavier during first feeding there were no significant differences in weight between groups some 8 months after fertilization or in presmolt growth periods from February to April in 1996 and 1997. Smolting rates were high (range 93.5–95.3%) and the incidence of deformities was low (< 1%) in both groups. Marine survival was lower in the triploid group, largely as a consequence of higher losses sustained during a period of chronic stress, when triploid losses were 9% higher. Growth patterns were similar for the first 11 months in sea water. Although graded triploid salmon were heavier in January 1998 (AF3N 1.62 ± 0.033 kg, AF2N 1.46 ± 0.36 kg, P < 0.05), when the fish were harvested in May 1998 diploid salmon were significantly heavier than triploid salmon although there was no significant difference in weights after evisceration (AF3N 2.40 kg ± 0.04 AF2N 2.49 kg ±0.03). The increase in weight of the diploids between winter and harvest reflects the growth spurt that occurs in maturing fish in the spring. Overall yields of triploid salmon in salt water were lower as a result of inferior survival.     

长江口中华鲟幼鱼血液水分、渗透压及离子浓度的变化规律

为了研究长江口中华鲟幼鱼盐度适应过程及其调节规律,将7月龄幼鱼直接转入0(淡水对照),5,10,15等4个盐度组中养殖32天,分别在0.5,1,2,4,8,16,和32 d 检测幼鱼血液水分、血清渗透压和血清Na 、Cl-、K 的浓度,结果显示：试验过程中,淡水和盐度5组中华鲟幼鱼血液各项指标始终保持一致,未呈现显著性差异。盐度10和15组,中华鲟幼鱼血液水分含量呈先下降后上升趋势,下降程度与盐度呈正相关,16 d时各组幼鱼血液水分无显著性差异。中华鲟幼鱼转入盐度10和15条件下,其血清渗透压与Na 和Cl-浓度的变化趋势一致,表现为先上升后下降,最后达到新的平衡;12 h是其上升和下降的拐点。而血清K 浓度的变化趋势与血清渗透压和Na 、Cl-离子不同,呈现先下降后趋于平稳的趋势。从结果可以看出,中华鲟幼鱼与其他广盐性鱼类一样,其盐度适应过程可分为2个阶段,即临界期和调整期。     

Screening tilapia for culture in sea water in Kuwait

Abstract. Three experiments were conducted to evaluate the potential for culturing tilapia in tanks and raceways with flowing sea water (39-41 ppt salinity) in Kuwait. In the first and third experiments, fingerling tilapia were grown for 7 to 8 months under ambient conditions. In the second experiment, larger (50-100g initial weight) male tilapia were grown for 6 months. Growth and mortality rates were determined.
Oreochromis aureus (Steindachner) grew the slowest and survived poorly. O. spilurus (Günther) and O. aureus × O. spilurus hybrids had the best survival and moderate growth rates. A red tilapia from Taiwan grew the fastest but had high mortality rates during winter. O. spilurus and the hybrid were best suited for seawater culture under ambient conditions in Kuwait.     

Size dependent early salinity tolerance in two sizes of juvenile white sturgeon,Acipenser transmontanus

The objective of this study was to investigate the influence of size on salinity tolerance in 1 year old juvenile white sturgeon. Two sizes of sturgeon (10 and 30 g) from the same spawning event (thus reducing confounding effects of genetic make-up and size) and reared in the same environment were exposed to a salinity of 0, 8, 16, 24, or 32 ppt for up to 120 h. Both 10 and 30 g fish exhibited > 93% mortality within 24 h after transfer to 24 or 32 ppt, regardless of whether they were transferred directly from freshwater (FW) or following a 48 h pre-treatment period at 16 ppt. Direct transfer from FW to 16 ppt was associated with 25 to 30% mortality, indicating that these fish have some ability to tolerate large changes in salinity for up to 5 days at this stage. Following exposure to 8 and 16 ppt, an elevation in plasma osmolarity, [Na⁺], and [Cl^?] was observed between 24 and 72 h in both 10 and 30 g sturgeon, but plasma ions and osmolarity in surviving fish at 120 h were not significantly different between groups held at 0, 8, and 16 ppt. Despite being unprepared for either direct or stepwise transfer to salinities of 24 ppt or greater, size confers some ionoregulatory advantage, as mortality occurred more slowly and the degree of ionoregulatory perturbation was less in 30 g than 10 g fish over the course of the exposures. It is not known whether the apparent advantage of size is related to a size-dependent development of ionoregulatory capacity or due to social status which can also influence ionoregulatory capacity, but age and genetic differences did not likely contribute to this size effect.