Graded environmental hypercapnia in juvenile spotted wolffish (Anarhichas minor Olafsen): effects on growth, food conversion efficiency and nephrocalcinosis

Growth performance and food conversion efficiency (FCE) were investigated in juvenile spotted wolffish (Anarhichas minor Olafsen), mean (S.D.) initial weight 15.7 (4.8) g, reared at four levels of carbon dioxide (CO_2(aq)) for 10 weeks at 6 °C and 33‰. CO₂ levels averaged 1.1 (control), 18.1 (low), 33.5 (medium) and 59.4 (high) mg l⁻¹, with corresponding pH values of 8.10, 6.98, 6.71 and 6.45, respectively. In addition, kidneys from sampled fish were examined macroscopically for gross signs of calcareous deposits, i.e. nephrocalcinosis, at the start and end of the experiment. Growth was significantly reduced at the highest concentration (P<0.0001), as compared to all other groups, while no overall differences in growth rate or mean weight were seen in the range of 1.1–33.5 mg CO₂ l⁻¹ at the end of the experiment. Daily feeding rates and total food consumption were reduced at the highest concentration (P<0.001), whereas food conversion efficiency did not vary significantly between groups. Plasma chloride levels displayed a significant decrease with increasing CO₂ levels, from 151.3 mmol l⁻¹ (1.1 mg CO₂ l⁻¹) to 128.3 mmol l⁻¹ (59.4 mg CO₂ l⁻¹) at the end of the experiment, whereas plasma osmolality in the high CO₂ group was significantly higher compared to the control group at the end of the experiment (371.4 and 350.8 mOsmol kg⁻¹, respectively). Nephrocalcinosis was observed in all groups at the end of the experiment, but was most pronounced in the medium and high CO₂ group.     

Effect of dietary vitamin C supplementation on the oxygen consumption, ammonia-N excretion and Na/K ATPase of Macrobrachium nipponense exposed to ambient ammonia

The 96-h LC₅₀ of ammonia-N and the effects of dietary vitamin C on oxygen consumption, ammonia-N excretion and Na⁺/K⁺ ATPase activity of Macrobrachium nipponense exposed to ambient ammonia were investigated. The results showed that the 96-h LC₅₀ of ammonia-N was 36.6 mg l⁻¹ for the freshwater prawn, M. nipponense, at pH 8.0. When prawns were exposed to high ambient ammonia-N concentrations, the oxygen consumption rate increased and ammonia excretion decreased. Dietary vitamin C supplementation led to higher oxygen consumption and lower ammonia excretion. Na⁺/K⁺ ATPase activity increased with increased ambient ammonia-N exposure in the range of 0–18.3 mg l⁻¹, and then was reduced at ambient ammonia-N 36.6 mg l⁻¹. Na⁺/K⁺ ATPase activities of prawns fed a vitamin C-supplemented diet were significantly lower than those of prawns fed a diet which was not supplemented with vitamin C.     

Filtration of king scallops (Pecten maximus)

Filtration rates of hatchery-reared king scallop (Pecten maximus L.) juveniles, fed a single species alga diet (Pavlova lutheri (Droop) Green), were measured at a range of temperatures (6–21 °C). Weight specific filtration rate (ml min⁻¹ g⁻¹ (live weight)) of juveniles of a selected size range of 17–19 mm shell height (0.26–0.36 g live weight) increased with temperature above 16 °C and decreased below 11 °C, but was not significantly different between these two temperatures. Measurements at 16 °C using juveniles with a wider size range of 10–25 mm shell height (0.05–0.8 g live weight) gave the allometric equation: filtration rate (ml min⁻¹)=12.19×weight (g)^0.887. Filtration rate decreased significantly when the cell concentration was greater than 200 cells μl⁻¹ (4.25 mg (organic weight) l⁻¹). With six other algae food species, filtration rates similar to those with P. lutheri were only achieved with Chaetoceros calcitrans (Paulsen) Takano. All other algae species tested were cleared from suspension at significantly lower rates. Experiments with diet mixtures of P. lutheri and these other algae suggested that this was usually a reflection of lowered filtration activity, rather than pre-ingestive rejection of cells. In experimental outdoor nursery rearing systems, the filtration rate was inversely proportional to the concentration of cells in the inflow, in the range 5–210 cells μl⁻¹. It was not affected by flow rate (2–130 l h⁻¹, equivalent to 0.12–28.38 l h⁻¹ g⁻¹ (live weight)) with scallop juveniles stocked from 2 to 62 g l⁻¹. The results are discussed in relation to on-growing scallops at field sites.     

Effects of dissolved oxygen on hemolymph parameters of freshwater giant prawn, Macrobrachium rosenbergii (de Man)

Profiles of changes in physiological parameters of freshwater giant prawns, Macrobrachium rosenbergii, exposed to various dissolved oxygen (DO) levels of 7.75, 4.75, 2.75, and 1.75 mg l⁻¹ are reported. The parameters involved in osmoregulation and oxygen transport were monitored for a 6-day period. Notable depressions in hemolymph osmolality, Na⁺, K⁺, and Cl⁻ contents were observed within 24 h after exposure to hypoxia at 2.75 and 1.75 mg O₂ l⁻¹, and thereafter remained at rather steady levels, which were significantly lower than those under normoxic conditions (4.75–7.75 mg O₂ l⁻¹). The extent of depression of osmotic-related constituents, hemolymph osmolality and Cl⁻ in particular, increased with decreased DO.

Oxyhemocyanin constituted 65.46–65.84% of total hemolymph proteins under the various DO levels examined; both hemolymph oxyhemocyanin and proteins showed notable elevations 24 h after exposure to hypoxic conditions, and reached the highest and constant level by 48 h after exposure. The compensatory responses of prawns to reduced O₂ were manifested by increased O₂ uptake through augmentation of hemocyanin, which results in enhancement of oxygen binding capacity of the hemolymph. In the same period, a significant surge of the respiratory products, PCO₂ and HCO₃⁻, was also demonstrated 6 h after exposure to hypoxic conditions which resulted in hemolymph alkalosis. These processes likely resulted in an increase in water influx and consequent declines in hemolymph osmolality and ion composition. Furthermore, hyperventilation and respiratory alkalosis, indicated by increased oxyhemocyanin and pH, respectively, were found to be predominant responses of M. rosenbergii to hypoxic stress.     

Comparison of nutrients release among some maricultured animals

Integrated mariculture is a feasible method to maintain sustainable and high productivity of aquaculture. The choice of cultured animals and biofilters in the integrated system has to be made on the basis of their nutrient release rates and the clearance rate of each component of the system. We are examining the nutrient release rates among fish (mangrove snapper, Lutjanus russeli, and sea perch, Abudefduf septemfasciatus), abalone (Haliotis diversicolor), scallops (Chlamys noblis), and green mussels (Perna viridis) in the laboratory. Fish feed is the major sources of inorganic nutrient input in fish farms. The orthophosphate and ammonia release rates of minced trash fish (1593 μg P g⁻¹ day⁻¹ and 150 μg N g⁻¹ day⁻¹) were respectively 6–12 times and 4–88 times higher than those of cultivated fish. Mangrove snapper had the overall highest nutrient release rate, followed by sea perch, abalone, scallops, and mussels for nitrite and nitrate; and followed by abalone, sea perch, mussels, and scallops for orthophosphate and ammonium. Among mollusks, abalone had the highest orthophosphate (162 μg P g⁻¹ day⁻¹), nitrate (1.4 μg N g⁻¹ day⁻¹), nitrite (1.6 μg N g⁻¹ day⁻¹) and ammonium (25.0 μg N g⁻¹ day⁻¹) release rates per gram wet weight per day. Abalone released large amounts of orthophosphate, nitrite and nitrate in the experiment. Scallops and green mussels had low nutrient release rates.     

A simple test to estimate the salinity resistance of fish with specific application to O. niloticus and S. melanotheron

In order to develop a simple and accurate index of the salinity resistance of tilapia, batches of 10 juveniles (5 to 20 g) of two different species Oreochromis niloticus and Sarotherodon melanotheron reared in freshwater were subjected to gradual increases in salinity until 100% mortality. Seven daily increments of salinity were tested with 4 replicates: 2, 4, 6, 8, 10, 12 and 14 g l⁻¹ day⁻¹, while control batches were kept in fresh water. The temperature was maintained at 27 °C. The concentration of oxygen, ammonia and the pH were not limiting factors. The mortality, monitored on a daily basis, appeared after 2–51 days and was spread out over 1–20 days, depending on the increment of salinity. The higher the daily rate in salinity increase, then the shorter the time lapse before total mortality occurred. The cumulative mortality as a function of salinity fit well with simple linear regressions. The criterion of the resistance to salinity was the index MLS (median lethal salinity) defined at each daily rate as the salinity at which 50% of fish died. For S. melanotheron, the mean MLS was 123.7±3.5 g l⁻¹ whatever the daily rate in salinity. For O. niloticus, the MLS was 46.3±3.4 g l⁻¹ for daily increases in salinity ranging from 2 to 8 g l⁻¹ day⁻¹ and decreased significantly (P<0.05) above this level. The MLS-8 g l⁻¹ day⁻¹ ,which takes into account the full capacity of the fish to adapt to the increasing salinity, appeared to be a simple, optimized and efficient criterion for assessing the resistance to salinity for O. niloticus and S. melanotheron. This criterion can be a useful tool for ranking the different parental strains and hybrids of different genus and species of tilapia used in programmes of genetic selection for growth and salinity tolerance.     

Effects of elevated atmospheric CO2 on growth, photosynthesis and nitrogen metabolism in the economic brown seaweed, Hizikia fusiforme (Sargassaceae, Phaeophyta)

Hizikia fusiforme (Harv.) Okamura (brown seaweed) was cultured using aeration with two CO₂ conditions: outdoor air (actual atmospheric CO₂ concentration, averaging 360 μl l^− 1) and CO₂-enriched air (averaging 700 μl l^− 1), to investigate the possible adjustments of elevated atmospheric CO₂ to the growth, photosynthesis and nitrogen metabolism in this mariculture species. Aeration with CO₂-enriched air reduced the pH in the culture medium in comparison with aeration with air. The mean relative growth rate was enhanced when H. fusiforme was grown at high CO₂ with respect to normal CO₂. There was little change in the rate of light-saturated photosynthesis, dark respiratory rate and apparent photosynthetic efficiency, measured in natural seawater, between thalli grown in high and normal CO₂ contents. However, both the mean nitrate uptake rate and the activity of nitrate reductase at light period were increased following culture at high CO₂, indicating an enhanced nitrogen assimilation of H. fusiforme thalli with the CO₂ enrichment in culture. It was proposed that the intensive cultivation of H. fusiforme would remove nutrients more efficiently with the future elevation of CO₂ levels in seawater, which could be a possible solution to the problem of ongoing coastal eutrophication.     

Metabolic scope, swimming performance and the effects of hypoxia in the mulloway, Argyrosomus japonicus (Pisces: Sciaenidae)

The culture of the mulloway (Argyrosomus japonicus), like many other Sciaenidae fishes, is rapidly growing. However there is no information on their metabolic physiology. In this study, the effects of various hypoxia levels on the swimming performance and metabolic scope of juvenile mulloway (0.34 ± 0.01 kg, mean ± SE, n = 30) was investigated (water temperature = 22 °C). In normoxic conditions (dissolved oxygen = 6.85 mg l^− 1), mulloway oxygen consumption rate (M·o₂) increased exponentially with swimming speed to a maximum velocity (U_crit) of 1.7 ± < 0.1 body lengths s^− 1 (BL s^− 1) (n = 6). Mulloway standard metabolic rate (SMR) was typical for non-tuna fishes (73 ± 8 mg kg^− 1 h^− 1) and they had a moderate scope for aerobic metabolism (5 times the SMR). Mulloway minimum gross cost of transport (GCOT_min, 0.14 ± 0.01 mg kg^− 1 m^− 1) and optimum swimming velocity (U_opt, 1.3 ± 0.2 BL s^− 1) were comparable to many other body and caudal fin swimming fish species. Energy expenditure was minimum when swimming between 0.3 and 0.5 BL s^− 1. The critical dissolved oxygen level was 1.80 mg l^− 1 for mulloway swimming at 0.9 BL s^− 1. This reveals that mulloway are well adapted to hypoxia, which is probably adaptive from their natural early life history within estuaries. In all levels of hypoxia (75% saturation = 5.23, 50% = 3.64, and 25% = 1 .86 mg l^− 1), M·o₂ increased linearly with swimming speed and active metabolic rate (AMR) was reduced (218 ± 17, 202 ± 14 and 175 ± 10 mg kg^− 1 h^− 1 for 75%, 50% and 25% saturation respectively). However, U_crit was only reduced at 50% and 25% saturation (1.4 ± < 0.1 and 1.4 ± < 0.1 BL s^− 1 respectively). This demonstrates that although the metabolic capacity of mulloway is reduced in mild hypoxia (75% saturation) they are able to compensate to maintain swimming performance. GCOT_min (0.09 ± 0.01 mg kg^− 1 m^− 1) and U_opt (0.8 ± 0.1 BL s^− 1) were significantly reduced at 25% dissolved oxygen saturation. As mulloway metabolic scope was significantly reduced at all hypoxia levels, it suggests that even mild hypoxia may reduce growth productivity.     

Water quality control using Spirulina platensis in shrimp culture tanks

A cyanobacterium (Spirulina platensis) was co-cultured with black tiger shrimp (Penaeus monodon) for water quality control. We evaluated the effects of: (1) three S. platensis trial conditions on inorganic nitrogen concentrations at one shrimp density (S. platensis trial conditions included: absent, nonharvested and semicontinuous harvesting) and (2) two shrimp densities on inorganic nitrogen concentrations, with and without S. platensis. Semicontinuous harvesting of S. platensis at one shrimp density resulted in significantly reduced (P<0.05) inorganic nitrogen concentrations (NH₄, NO₂ and NO₃). With S. platensis absent, ammonium and nitrite concentrations ranged from 0.5 to 0.6 mg l⁻¹, while nitrate concentrations ranged from 16 to 18 mg l⁻¹ by day 44. With nonharvested S. platensis, considerable variability occurred with nitrogen concentrations. Semicontinuous harvest of S. platensis reduced nitrate to 4 mg l⁻¹, while ammonium and nitrite ranged from 0.0 to 0.15 mg l⁻¹, respectively. The factorial evaluation of shrimp density versus presence and absence of S. platensis resulted in greatly reduced nitrogenous compounds with S. platensis present regardless of shrimp density, and only moderately increased nitrogen with greater shrimp density. Without S. platensis, all nitrogen compounds were substantially elevated and shrimp survived was significantly reduced at high shrimp density.     

The hydraulically integrated serial turbidostat algal reactor (HISTAR) for microalgal production

A hydraulically integrated serial turbidostat algal reactor (HISTAR) for the mass production of microalgae was designed, constructed and preliminarily evaluated. The 9266-l experimental system consists of two enclosed turbidostats hydraulically linked to a series of six open continuous-flow, stirred-tank reactors (CFSTRs). The system was monitored and controlled using GENESIS process control software. A production study was preformed using Isochrysis sp. (C-iso) to assess system stability and production potential under commercial-like conditions. The study was performed at the following target system parameters: system dilution rate of 0.49 per day, pH 7.6, NITROGEN=10 mg l⁻¹, PHOSPHORUS=2 mg l⁻¹, and artificial illumination (photosynthetic photon flux density) from 1000 W metal halide LAMPS=800 μmol s⁻¹ m⁻². At steady state conditions, daily harvested algal paste was 1454 g (wet), mean areal system PRODUCTIVITY=47.8±3.04 g m⁻² per day (17.1±1.09 g C m⁻² per day) and mean CFSTR6 DENSITY=105.5±6.71 mg l⁻¹.     

Modeling industry-wide sediment oxygen demand and estimation of the contribution of sediment to total respiration in commercial channel catfish ponds

Data collected from 45 commercial channel catfish, Ictalurus punctatus, ponds were used to develop empirical models predicting sediment oxygen demand (SOD). Seven acceptable models were combined with a Monte-Carlo sampling distribution to predict industry-wide sediment oxygen demand (SOD_i). The SOD_i values obtained from the best equation were used in simulations to assess the effect of diurnally varying water column dissolved oxygen (DO) concentrations on SOD and the effect of pond water depth on the contribution of SOD to overall pond respiration. Estimated SOD_i ranged from 62 to 962 mg m⁻² h⁻¹, with a mean of 478 mg m⁻² h⁻¹. There was a 95% probability of mean SOD_i being ≥700 mg m⁻² h⁻¹. The effects of diurnal variation in DO concentration in the water column on expression of SOD was modeled by combining maximum SOD_i, an empirical relationship between DO and SOD, and simulated pond DO concentrations. At DO concentrations >15 mg l⁻¹, diel SOD in catfish ponds exceeded 20 g O₂ m⁻² day⁻¹. But when average diel DO was <4 mg l⁻¹ and the range of DO concentration was 6–8 mg l⁻¹, SOD decreased to 13 g O₂ m⁻² day⁻¹ because DO availability limited the full expression of potential SOD. Respiration totals for sediment (average SOD_i), plankton, and fish respiration were calculated for pond water depths ranging from 0.25 to 4 m. Although whole-pond respiration increases as pond depth increases, the proportion of total respiration represented by sediment decreased from 48 to 10% by increasing water depth over this range. The results of these studies show that SOD is a major component of total pond respiration and that certain management practices can affect the impact of SOD on pond oxygen budgets. Mixing ponds during daylight hours, either mechanically or by orienting ponds for maximum wind fetch, will increase oxygen supply to sediments, thereby allowing maximum expression of SOD and maximum mineralization of sediment organic matter. Given a mixed condition caused by wind or other artificial means, the construction of deeper ponds increases the total mass of DO available for all respiration, causing nighttime DO concentrations to decline at a slower rate, reducing the need for supplemental aeration. Because a pond’s water volume decreases over time from sediment accumulation, annual aeration costs will increase with pond age. Constructing ponds with greater initial depth will therefore reduce long-term cost of aeration, allow more flexible management of pond water budget, and reduce the long-term expense associated with pond reconstruction.     

Effect of increased water recirculation rate on algal supply and post-larval performance of scallop (Pecten maximus) reared in a partial open and continuous feeding system

In a commercial scallop hatchery spat production depends on a culture system which ensures high survival and good growth. Reuse of water with algae may increase the food exploitation and hence reduce the costs. Post-larvae of great scallop (Pecten maximus) were studied in a commercial hatchery using a partial open and continuous feeding tank system. Three different water recirculation rates (67, 83 and 92%) were tried out in two experiments with post-larvae originating from three spawning groups of ages between 43 and 57 days post-spawn, 316–886 μm shell-height and 1.1–9.6 μg ash-free dry weight. The post-larvae were held in sieves in tanks of 2500 l where a downwelling flow was maintained by airlifts. New water with a mix of monocultured algae was continuously added to the tanks at algal concentrations of 10 and 15 cells μl⁻¹ in experiment 1 (groups 1 and 2) and 2 (group 3), respectively. The algal supply to each sieve was reduced along with increased recirculation rate, but was kept between 6 and 13 cells μl⁻¹. Generally no significant differences in survival, growth or chemical content were found between the three recirculation rates, while few differences were found between and within groups. Large variation in survival was found between and within groups (1–81%). Highest survival was found in experiment 1, and where post-larvae from two settlements were used, the first settlement survived better than the second. The daily growth ranged from 15 to 62 μm shell-height and from 0.3 to 2.6 μg ash-free dry weight. The scallop post-larvae could well be reared at all three recirculation rates studied as an increase from 67 to 92% did not seem to affect the post-larval performance seriously. The algal supply, however, had to be compensated by an increasing number of cells (>10 cells μl⁻¹) when increasing the recirculation rate.     

Substitution of Chaetoceros muelleri by Spirulina platensis meal in diets for Litopenaeus schmitti larvae

The nutritional response of Litopenaeus schmitti larvae to substitution of Chaetoceros muelleri by Spirulina platensis meal (SPM) was evaluated. The substitution levels (S) were 0%, 25%, 50%, 75% and 100%, dry weight basis. Final larval length (FL) ranged from 1.98 to 3.16 mm for the different substitution levels. There was a significant relationship between S and FL, described by the following quadratic equation: FL = 2.853 + 0.01598S − 0.000233S². The substitution level (S) yielding maximum FL was 34.2%. Development index (DI) values ranged from 2.84 to 3.93 and were dependent on substitution level. The corresponding equation was DI = 3.799 + 0.00945S − 0.000189S² (P < 0.01). Maximum DI was obtained at 25.0% substitution. Survival was high (82–87%) and no significant differences were found between treatments. Protein digestibility of either microalgae was high, with 92% for SPM and 94% for C. muelleri, with no significant differences between them. The results in this study indicate that an adequate balance of nutrients in relation to the requirements of the species is critical. To simultaneously improve FL and DI, a 30% substitution of C. muelleri by SPM is suggested. This is equivalent to feeding 0.15 mg larvae^− 1 day^− 1 dry weight basis of a 70% C. muelleri/30% SPM diet, representing 0.078 mg protein larvae^− 1 day^− 1, 0.026 mg lipids larvae^− 1 day^− 1 and 2.732 J larvae^− 1 day^− 1.     

Effect of starvation on the growth and survival of post-larval abalone (Haliotis iris)

The starvation tolerance of post-larval abalone (Haliotis iris) was determined by examining post-larval growth and survival after various periods of starvation. Competent larvae (10 days old at 16°C) were induced to attach and metamorphose with 2 μM GABA. Post-larvae were either fed diatoms (Nitzschia longissima) or starved. In Experiment 1, post-larvae were starved immediately after metamorphosis for periods of 1, 2, 4, 8, 15, 20, 25 and 30 days. Starved post-larvae grew relatively well for several days after metamorphosis despite the absence of food (averages of 10.4 and 17.8 μm shell length (SL) per day after 8 days for two batches). Subsequent growth was minimal, averaging 1.7 and 0.7 μm day⁻¹ over 6–7 days for the two batches. There was no clear relationship between period of starvation and growth rate when fed. Mean daily growth rate over 3 weeks when fed ranged from 15–22 μm day⁻¹. However, the duration of starvation did have a significant effect on survival. Survival of post-larvae fed after 1–2 days of starvation was 90–100% after 3 weeks of feeding. Longer starvation periods gave progressively lower survival and post-larvae starved for 30 days all died within a week of being fed. In Experiment 2, post larvae were fed for 3 weeks after metamorphosis, then starved for 0, 3, 7, 14 or 21 days. Growth rates of starved post-larvae averaged only 5–6 μm day⁻¹ in the first week (vs. 30 μm day⁻¹ in controls), and later declined to zero. Growth resumed within a week following return to food, but the 14- and 21-day starvation treatments took 2 weeks to reach growth rates comparable to controls. The no-starvation controls and the 3- and 7-day starvation treatments all had >70% survival over 4 weeks after return to food. Survival in the 14- and 21-day starvation treatments was 15–20%, with almost all mortalities occurring in the first week after return to food. These data suggest that Haliotis iris post-larvae are relatively tolerant of starvation, so abalone farmers have a week or so to remedy food shortages before major post-larval mortality begins.     

Dopamine modulates the physiological response of the tiger shrimp Penaeus monodon

Levels of glucose, lactate, pO₂, pCO₂, HCO₃⁻, TCO₂, Na⁺, K⁺, Cl⁻, protein, and oxyhemocyanin in the hemolymph and its osmolality and pH were measured when tiger shrimp, Penaeus monodon (13.5 ± 1.5 g body weight), were individually injected with saline or dopamine at 10^− 8, 10^− 7, or 10^− 6 mol shrimp^− 1. Results showed that hemolymph glucose, lactate, pCO₂, HCO₃⁻, and TCO₂ values increased from 2 to 4 h; hemolymph osmolality, Na⁺, and total protein had increased at 2 h; and hemolymph K⁺ decreased from 2 to 8 h after the dopamine injection. All physiological parameters returned to the control values 4–16 h after receiving dopamine. The dopamine injection also significantly decreased the oxyhemocyanin/protein ratio of P. monodon which occurred at 2 h, resulting from an elevation of hemolymph protein and a slight decrease of oxyhemocyanin. These results suggest that stress-inducing dopamine caused a transient period of modulation of energy metabolism, osmoregulation, respiration, and the acid–base balance in P. monodon in adapting to this environmental stress.     

The effects of density, light and shelter on the growth and survival of African catfish (Clarias gariepinus Burchell, 1822) fingerlings

The effects of stocking density, light and shelter on the growth and survival of Clarias gariepinus fingerings was evaluated. In this experiment African catfish with initial individual mean weight 0.79±0.1 g were reared at two different stocking densities—5 fish l⁻¹ and 10 fish l⁻¹ in either sheltered or unsheltered tanks with reduced and normal light condition. In all conditions growth rate was significantly affected by stocking density. The growth rate was significantly higher at low densities and in reduced light conditions where shelter was provided. Survival rate was in excess of 79% in all treatments and was not affected by treatment.     

A laboratory scale recycling water unit for tilapia breeding

The technical features of a laboratory scale water recycling unit for experimental small scale tilapia breeding are described. Two units (1 and 2) were operated during a 6 month period, carrying a similar fish load (7·5 kg) and feeding rate (2% fish body weight/day). Unit 1 received natural illumination, while unit 2 was artificially illuminated (14/10 - light/dark cycle). Both units were equipped with a biological filter bed (substrate surface area, 3500 cm²). In unit 1, total ammonium and nitrite concentrations ranged from 0·05 to 0·5 mg liter⁻¹, while nitrate varied between 10–40 mg liter⁻¹. In unit 2 corresponding values were 0·15-3 mg liter⁻¹, 0·05–0·8 mg liter⁻¹ and 10–40 mg liter⁻¹. Temperatures ranged between 20–29°C and pH values between 7·5–6·9 in both units. Dissolved oxygen concentrations decreased gradually from 5·6 to 3·4 mg liter⁻¹ in unit 1 and from 5·6 to 2·6 mg liter⁻¹ in unit 2. Twenty-six spawnings occurred in unit 1 in March and April, while only eight spawnings occurred in unit 2, possibly because of the absence of sunlight. The significance of these results are discussed.     

Oxygen consumption rates of white sturgeon under commercial culture conditions

Oxygen consumption rates of white sturgeon (Acipenser transmontanus) were measured under commercial culture conditions. Mean fish size ranged from 0.09 to 3.8 kg (0.2–8.4 lbs). Mean daily values of oxygen consumption rates ranged from 70–330 mg kg fish⁻¹ h⁻¹. Peak oxygen consumption rates were measured to be as much as twice the mean daily values and were seen to occur in response to feeding for fish fed a ration of less than 2.6% body mass per day. With higher feed rations, peak oxygen consumption rates were a smaller percentage of the mean daily value. Multiple regression analysis showed that 93% of the variations in measured oxygen consumption rate values could be attributed to variations in feed ration.     

Age, growth and mortality of greater amberjack from the southeastern United States

Rings on 323 sectioned otoliths (sagittae) were used to determine the ages of greater amberjack, Seriola dumerili, sampled from headboats and commercial handline vessels operating out of ports from North Carolina through the Florida Keys. Age and growth determinations were difficult. Rings were identified and counted on 71% of the samples, but measurements could be made on only 48%. The oldest fish examined was 17 years old and was 1552 mm total length (TL). Back-calculated mean lengths at annulus formation were 366, 873, 1207 and 1471 mm TL for ages 1, 5, 10 and 15, respectively. The von Bertalanffy equation describing theoretical growth for TL was L_t = 1648(1 − e^{−0.119(t + 1.230)}), where T = years. The equation using fork lengths (FL) was L_t = 1514(1 − e^{−0.115(t + 1.178)}). The weight-length relationship for greater amberjack was W = 7.2 × 10⁻⁸L^2.700, where W is the whole fish weight (in kg), and L is the total length (in mm). Conversion of total length to fork length may be obtained using the equation: FL = −4.807 + 0.887(TL), and fork length to total length: TL = 20.110 + 1.111(FL). Greater amberjack are fully recruited to the fishery by age 8. Estimates of total instantaneous mortality (Z) ranged from 0.60 to 0.65 depending on the year.     

Efficacy of formalin, hydrogen peroxide, and sodium chloride on fungal-infected rainbow trout eggs

Antifungal agents are essential for the maintenance of healthy stocks of fish and their eggs in intensive aquaculture operations. In the USA, formalin is the only fungicide approved for use in fish culture. However, hydrogen peroxide and sodium chloride have been granted low regulatory priority drug status by the United States Food and Drug Administration (FDA) and their use is allowed. We evaluated the efficacy of these fungicides for controlling fungal infections on rainbow trout eggs. A pilot study was conducted to determine the minimum water flow rate required to administer test chemicals accurately in Heath incubators. A minimum water flow rate of 7.6 1 min⁻¹ was necessary to maintain treatment concentrations during flow-through chemical exposures. The antifungal activity of formalin, hydrogen peroxide, and sodium chloride was evaluated by treating uninfected and 10% fungal-infected (Saprolegnia parasitica) rainbow trout eggs (Oncorhynchus mykiss) for 15 min every other day until hatch. There were no significant differences among treatments in percent hatch or final infection for uninfected eggs receiving prophylactic chemical treatments. Eggs of the negative control group (uninfected and untreated) had a mean hatch exceeding 86%. All chemical treatments conducted on the infected egg groups controlled the spread of fungus and improved hatching success compared with the positive control groups (infected and untreated). Formalin treatments of 1000 and 1500 μl l⁻¹ and hydrogen peroxide treatments of 500 and 1000 μl l⁻¹ were the most effective. Sodium chloride treatments of 30 000 mg l⁻¹ improved fry hatch, but the compound was less effective at inhibiting fungal growths compared with hydrogen peroxide and formalin treatments.