Comparing denitrification rates and carbon sources in commercial scale upflow denitrification biological filters in aquaculture

Aerobic biological filtration systems employing nitrifying bacteria to remediate excess ammonia and nitrite concentrations are common components of recirculating aquaculture systems (RAS). However, significant water exchange may still be necessary to reduce nitrate concentrations to acceptable levels unless denitrification systems are included in the RAS design. This study evaluated the design of a full scale denitrification reactor in a commercial culture RAS application. Four carbon sources were evaluated including methanol, acetic acid, molasses and Cerelose™, a hydrolyzed starch, to determine their applicability under commercial culture conditions and to determine if any of these carbon sources encouraged the production of two common “off-flavor” compounds, 2-methyisoborneol (MIB) or geosmin. The denitrification design consisted of a 1.89 m³ covered conical bottom polyethylene tank containing 1.0 m³ media through which water up-flowed at a rate of 10 lpm. A commercial aquaculture system housing 6 metric tonnes of Siberian sturgeon was used to generate nitrate through nitrification in a moving bed biological filter. All four carbon sources were able to effectively reduce nitrate to near zero concentrations from influent concentrations ranging from 11 to 57 mg/l NO₃–N, and the maximum daily denitrification rate was 670–680 g nitrogen removed/m³ media/day, regardless of the carbon source. Although nitrite production was not a problem once the reactors achieved a constant effluent nitrate, ammonia production was a significant problem for units fed molasses and to a less extent Cerelose™. Maximum measured ammonia concentrations in the reactor effluents for methanol, vinegar, Cerelose™ and molasses were 1.62 ± 0.10, 2.83 ± 0.17, 4.55 ± 0.45 and 5.25 ± 1.26 mg/l NH₃–N, respectively. Turbidity production was significantly increased in reactors fed molasses and to a less extent Cerelose™. Concentrations of geosmin and MIB were not significantly increased in any of the denitrification reactors, regardless of carbon source. Because of its very low cost compared to the other sources tested, molasses may be an attractive carbon source for denitrification if issues of ammonia production, turbidity and foaming can be resolved.     

Studies of limiting factors governing the waterflow requirement for Atlantic salmon (Salmo salar L.) in landbased seawater systems

In the first part of the investigation two different models are derived to obtain the direct ratio between (a) total ammonia and oxygen and (b) unionized ammonia and oxygen as limiting factors for the waterflow requirement for salmonids in landbased seawater systems. In both models the mass related ammonia quotient, expressed as the ratio between total ammonia excreted (mg NH₄---N per kilogramme fish per minute) and oxygen consumed (mg O₂ per kilogramme fish per minute) is an important factor.

The second part of the study reports measurements of the mass related ammonia quotient (diurnal cycles) of Atlantic salmon kept in a landbased tank farm and calculations of waterflow requirements. The total variation find of the mass related ammonia quotient values were between 0·03 and 0·08. Based on two of the most extreme diurnal cycles with respect to temperature and fish size, the specific waterflow requirement applying ammonia as a limiting factor was in the range 0·04–0·17 litres/kg min.

The third part of the study applies the measurements obtained for the ammonia quotient to the models. The results show that un-ionized ammonia may become the first limiting factor by combinations of a high difference in the dissolved oxygen concentration between the inlet water and the discharge, a high pH and a low threshold level for un-ionized ammonia. However, oxygen is usually found to be the first limiting factor when compared to un-ionized ammonia.

If the value 0·5 mg NH₄---N/litre is used as the limiting criterion for total ammonia, the difference in the dissolved oxygen concentration between the inlet water and the discharge should not exceed about 7 mg/litre. However, in seawater oxygenation is necessary to provide and maintain this difference. Therefore both models result in the same conclusion for moderate levels of oxygen addition.     

Marine shrimp aquaculture: a novel waste treatment system

Ecuadorian Penaeus vannamei were cultured in dirt ponds (each of approximately 163 m²) at four different stocking densities, i.e. 5 shrimp m⁻², 10 shrimp m⁻², 15 shrimp m⁻² and 20 shrimp m⁻². Experiments were carried out over three different periods during the year. Each experiment lasted for 11–14 weeks. No commercial feed was given to the shrimp. The only input to the ponds was about 30 kg of cattle manure per pond per week. Chemical composition of the cattle manure was analyzed. Water quality parameters such as temperature, pH, DO and turbidity were recorded twice daily for each experiment; nutrients (nitrite, nitrate, ammonium and phosphate), water ATP, sediment ATP, H₂S and chlorophyll were measured twice weekly for each experiment. Shrimp were sampled either weekly or bi-weekly for body weight measurements.

The results showed a negative correlation between stocking density and growth. Weekly growth ranged from 0·44 to 1·58 g week⁻¹. Survival was over 50% in all treatments and averaged at 70·8%. Under these stocking densities, shrimp production ranged from 4·4 to 18·8 kg ha⁻¹ day⁻¹. The stocking density of 15 shrimps m⁻² provides better production than the other stocking densities.

Water quality data did not relate to any shrimp growth. Water nutrient levels in pond discharge water were less than or equal to the nutrients in the incoming water in spite of the weekly addition of cattle manure and did not increase with the addition of cattle manure. No coliform bacteria were detected in any pond water samples through the study period. This indicates digestion of cattle manure in marine shrimp ponds would not pollute the environment with high concentrations of dissolved nutrients.

Thus, a marine shrimp pond can be considered a dissolved nutrient marine treatment plant converting unwanted cattle manure (1841 kg cattle manure ha⁻¹ week⁻¹ in this study) into a valuable commodity — shrimp.     

Characterization of Ca-ATPase activity in gill microsomes of freshwater mussel, Lamellidens marginalis (Lamarck) and heavy metal modulations

Freshwater mussel Lamellidens marginalis showed a Ca²⁺-stimulated ATPase activity in the microsomal fraction of gill. Mg²⁺ was also found to be an activating cation for this ATPase enzyme. Ca²⁺ ATPase showed maximal activity at 40 °C and between pH 7.5–8.0. Substrate specificity of Ca²⁺ ATPase was highest with ATP, followed by GTP and other trinucleotides such as UTP, CTP and ADP also showed some amount of hydrolysis. Ca²⁺ ATPase showed slight inhibition with ruthenium red and sodium vanadate and is insensitive to sodium azide, ouabain, oligomycin B and phenylalanine. Heavy metals like Hg²⁺, Cu²⁺ and Zn²⁺ showed 50% inhibition of Ca²⁺ ATPase activity at concentrations of 0.25 mM, 0.5 mM and 1 mM, respectively whereas Cd²⁺ and Ni²⁺ showed 39% and 28% inhibition of enzyme activity at 1 mM concentrations.     

Shipping studies with juvenile and adult Malaysian prawns Macrobrachium rosenbergii (de Man)

Four studies examined shipping factors of packing technique, density, duration, type of water and use of habitat material for shipping juvenile and adult prawns Macrobrachium rosenbergii. Prawns were shipped in double polyethylene lined 38 × 38 × 20 cm deep styrofoam boxes containing oxygenated water. At temperatures of 19–20°C, 17 g prawns could be shipped safely for 42 h at a density of 10–12 prawns per box (12–15 g liter⁻¹ shipping water). Juveniles, mean size about 6 g, could be shipped at a density of 40 per box (18 g liter⁻¹) for 24 h or 20–25 per box (9–11 g liter⁻¹) for 48 h. Use of mesh material to increase surface area in the box did not appear beneficial nor did shipping in brackish water (salinity8‰). Adults packed unrestricted resulted in survival rates substantially higher than those obtained from immobilized prawns wrapped in mesh.

During the shipment, pH and dissolved oxygen concentrations decreased whereas ammonia concentrations increased. The decreased pH levels may have reduced the ammonia toxicity by decreasing the amount of toxic unionized ammonia (NH₃) in solution. In general, dissolved oxygen concentrations appeared more closely related to survival rates than did other water quality parameters which were measured.     

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.     

Effect of pH and Decapsulation on the Toxicity of Ammonia to the Brine Shrimp Artemia franciscana

Artemia franciscana were hatched and tested for tolerance to ammonia at pH 6.5 and 8.5 in artificial seawater with a salinity of 17 ‰. Nauplii seemed to be less tolerant of ammonia at pH 6.5 despite the fact that more of the ammonia is in the ionized form (NH₄⁺). It is suggested that this greater sensitivity is a result of the NH₄⁺ competing with the Na⁺ during gut transfer. Nauplii hatched from decapulated cysts were more tolerant of ammonia than those hatched from whole cysts, probably because of a greater energy reserve.     

Cyclical fluctuations in ammonia and nitrite-nitrogen resulting from the feeding of turbot, scophthalmus maximus (L.), in recirculating systems

Daily and weekly cyclical fluctuations in ammonia (NH₄---N μg litre⁻¹) and nitrite---nitrogen (NO₂---N μg litre⁻¹) were shown to occur in marine recirculating systems containing juvenile turbot, Scophthalmus maximus (L.). A stepwise multivariant regression analysis, computed for the prefeeding morning base levels, showed that ammonia---nitrogen concentration in the fish tanks was related to inflow ammonia---nitrogen concentration, water flow rate, water temperature, mean fish weight and fish biomass. Influent ammonia concentration and water flow rate were by far the most important variables accounting for 68·0% and 16·7% of the variation in the regression respectively.

The fluctuations, which occurred at concentrations thought to be tolerated by the fish, were attributed to the amount and timing of feeding. The significance of the existence of such fluctuations was discussed together with the importance of their amplitude and frequency.     

Influence of temperature on protein utilization in juvenile European seabass (Dicentrarchus labrax)

The aim of this trial was to study the utilization of dietary protein by seabass juveniles with 5.5 g mean body weight, at two water temperatures: 18°C and 25°C. For that purpose, the fish were fed for 12 weeks, four isoenergetic diets with different protein levels (36, 42, 48, and 56%). At the end of the trial, growth rate and feed utilization were significantly better at the higher water temperature. Within each temperature, specific growth rate and feed efficiency were significantly higher with the 48 and 56% protein diets than with the other diets. At 25°C, feed efficiency was also significantly better with the 56% than with the 48% protein diet. N retention (g kg average body weight⁻¹ day⁻¹) was higher at 25°C than at 18°C but, as a % N intake the inverse was true. Although at 25°C N retention (% N intake) was not different among groups, retention in g kg ABW⁻¹ day⁻¹ was significantly higher with the 56% protein diet than with 36 and 42% protein diets. On the contrary, at 18°C N retention (g kg ABW⁻¹ day⁻¹) was similar among groups while as a percentage of N intake it was inversely related to the dietary protein level. Regarding energy utilization, at each temperature, there were no differences among dietary treatments on energy retention (g kg ABW⁻¹ day⁻¹). As a % of energy intake, energy retention significantly increased with the increase of dietary protein level at 25°C, while at 18°C, there were no significant differences among groups. Within each temperature, at the end of the trial, there were no differences among groups in proximate composition of whole fish. Apparent digestibility coefficients of dry matter, protein and energy significantly improved with the increase of water temperature but, within each temperature, there were no significant differences among groups. The results of this study indicate that, regardless of water temperature, the dietary protein requirement for growth seems to be satisfied with a diet containing 48% protein. Growth and feed efficiency were significantly higher at the higher temperature, however, protein utilization was more efficient at the lower temperature.     

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.     

Larval Development of the Giant River Prawn Macrobrachium rosenbergii at Different Ammonia Concentrations and pH Values

The effect of ammonia and pH levels on giant river prawn Macrobrachium rosenbergii larvae were evaluated to provide science-based information on safe levels of ammonia and pH for larviculture. Survival rate, developmental stage, and larval weight gain were determined for larvae kept in water with total ammonia (NH ₄-N)concentrations of 0, 1, 2, 4, and 8 mg\L and pH 7, 8, and 9. The trials were conducted in two phases: phase 1, larvae from stages I through VIII and phase 2, larvae from stage VIII until metamorphose. Oxygen consumption was determined for larvae in stages I and VIII at total ammonia concentrations of 0, 4, and 8 mg/L and pH 8. Survival rate up to stage VIII varied from 86 to 98% and did not differ for total ammonia concentrations in pH 7 and 8 and for 0 mg/L NH₄ N in pH 9. Survival rate was significantly lower (0–20%) for total ammonia concentrations from 1 to 8 mg/L (0.43–3.41 mg/L of unionized ammonia) in pH 9. Larval stage indexes (7.9–8.0 range) and weight gain (1.572–2.931 mg range) of larvae at the end of phase 1 of the experiment did not differ for the different ammonia concentration solutions, but were significantly lower in pH 9. In phase 2, no parameter differed among treatments for pH 7 and 8; however there was total mortality at pH 9 until 96h. Respiration rates diminished when larvae were exposed to total ammonia concentrations of 4 and 8 mg/L (0.28 and 0.55 mg/L of unionized ammonia), but development remained unaltered. Therefore, M. rosenbergii larvae tolerate high levels of total ammonia, while toxicity depends primarily on unionized ammonia concentrations. In addition, alkaline pH (9) acted directly on the larvae, curbing development and causing severe mortality. Larval tolerance to high ammonia and pH levels decreases for the last zoeal stages.     

Development of a specific pathogen free population of the Chinese fleshy prawn, Fenneropenaeus chinensis: Part II. Secondary quarantine

The Oceanic Institute (OI), in collaboration with researchers from the University of Arizona (UAZ), established a population of specific pathogen free (SPF) Fenneropenaeus chinensis through Secondary Quarantine (SQ) in Kona, Hawaii. Shrimp were 0.38 g (S.D. ± 0.13 g) when they entered the nursery/growout module of the SQ facility. Juveniles from five maternal families were stocked in separate 230-L nursery tanks, as was a sixth family comprised of shrimp from two of the seven original spawns. After tagging, representative shrimp from all six families were stocked in a 50-m² concrete raceway that was enclosed in a biosecure greenhouse. After 155 days in the growout module, mean shrimp weight was 23.4 g (S.D. ± 7.25 g) and females (29.1 ± 4.61 g) were 74% larger than males (16.7 ± 2.44 g). A Sprung Structure™ was erected to enclose the biosecure maturation and hatchery module of the SQ facility. Broodstock were subjected to an acclimation process where they were kept in seawater at 22 °C, followed by a reduction in water temperature of 1 °C/day until the temperature reached 17 °C. At this time, unilateral eyestalk ablation was performed on female broodstock and water temperature was further reduced at the same rate until it reached 12 °C. After 2 weeks, water temperature was raised to 17 °C at a rate of 1 °C/day to induce ovarian development. Production of an F₁ generation of Fe. chinensis was successfully accomplished using artificial insemination, and representative shrimp from this generation have tested negative for all specifically listed pathogens using histology and PCR/RT-PCR. Of particular interest with Fe. chinensis is their cold tolerance. This species may represent a valuable resource for shrimp farmers in temperate and sub-tropical climates to grow a second “winter” crop, thereby increasing production and profitability for the shrimp farmer.     

Extenders and cryoprotectants for cooling and freezing of piracanjuba (Brycon orbignyanus) semen, an endangered Brazilian teleost fish

The aim of this study was to develop a protocol for semen storage of piracanjuba (Brycon orbignyanus) by both cool storage at 4 °C and cryopreservation at − 196 °C. Semen was diluted in some fish semen extenders (Exp. 1) or in extenders combined with the antibiotic gentamycin sulfate (Exp. 2) and stored at 4 °C. Sperm motility was estimated every 24 h. Then, the effects of egg yolk (0 and 5%), cryoprotectants (dimethyl sulphoxide — DMSO, methanol, and methylglycol) and extenders (NaCl 154 mM, BTS™ Minitub and M III™ Minitub) on semen cryopreservation were evaluated (Exp. 3). Semen was added to each of eighteen cryosolutions (2 yolk concentrations × 3 cryoprotectants × 3 extenders), aspirated into 0.5-mL straws, frozen in nitrogen vapor (Taylor-Wharton, CP 300, “dry shipper”) and stored at − 196 °C. Sperm motility was evaluated after thawing at 60 °C-water bath for 8 s. The three cryosolutions that produced the highest post-thaw sperm motility were used again to freeze semen. Post-thaw semen quality was then evaluated under three tests: sperm motility, the percentage of live spermatozoa and hatching rate (Exp. 4). Piracanjuba semen diluted (1:10 total volume) in NaCl 200 mM or in Saad solution (NaCl 200 mM, Tris 30 mM) maintained motility above 35% for as long as 7 days, at 4 °C. Motility of only 7% was observed on undiluted semen after 3 days at 4 °C. There was neither beneficial nor detrimental effect of gentamycin on sperm motility at 250 μg/mL. Egg yolk addition to the cryosolution was beneficial in samples cryopreserved in NaCl 154 mM and in M III™, but detrimental for samples cryopreserved in BTS™. Methylglycol was the most effective cryoprotector compared to DMSO and methanol. Motility and percentage of live spermatozoa were similar among semen cryopreserved in NaCl–yolk, M III™–yolk and BTS™, all containing 10% methylglycol, but lower than fresh control. Hatching rates of eggs fertilized with sperm cryopreserved in NaCl–yolk or BTS™ were higher than for eggs fertilized with sperm cryopreserved in M III™–yolk, but lower than control fertilizations. The semen cryopreservation protocols developed here will be used to set up a gene bank for endangered piracanjuba populations.     

Treatment of hatchery wastewater by a sequencing batch reactor process

After settling pretreatment, the primary sludge wastewaters from a fish hatchery were treated in three five-litre bench-scale sequencing batch reactors. The sequencing batch reactors were operated concurrently under the same operating conditions. A cycle of 24 hours was adopted during the course of this study. Two different temperature regimes (22°C and 10°C) were employed, and a range of initial BOD₅ concentrations was treated. The results showed the occurrence of very high reductions in BOD₅ and COD concentrations, and very high removal of ammonia---nitrogen and suspended solids. The average five-day BOD₅ reduction was 98% for high influent concentrations, while the reduction for lower influent concentrations was 95%. The average five-day BOD₅ concentration for the effluents was less than 100 mg litre⁻¹. Nitrification was pronounced after 40 days of reactor operation. Denitrification occurred when the reactors were fed wastewaters with high BOD₅ concentrations, although it was not observed with low influent BOD₅ concentrations or at low temperatures (10°C). The overall treatment efficiency remained very high when the reactors were operated under a low temperature regime (10°C).     

Performance and operation of a rotating biological contactor in a tilapia recirculating aquaculture system

This paper describes the performance characteristics of an industrial-scale air-driven rotating biological contactor (RBC) installed in a recirculating aquaculture system (RAS) rearing tilapia at 28 °C. This three-staged RBC system was configured with stages 1 and 2 possessing approximately the same total surface area and stage 3 having approximately 25% smaller. The total surface area provided by the RBC equaled 13,380 m². Ammonia removal efficiency averaged 31.5% per pass for all systems examined, which equated to an average (± standard deviation) total ammonia nitrogen (TAN) areal removal rate of 0.43 ± 0.16 g/m²/day. First-order ammonia removal rate (K₁) constants for stages 1–3 were 2.4, 1.5, and 3.0 h⁻¹, respectively. The nitrite first-order rate constants (K₂) were higher, averaging 16.2 h⁻¹ for stage 1, 7.7 h⁻¹ for stage 2, and 9.0 h⁻¹ stage 3. Dissolved organic carbon (DOC) levels decreased an averaged 6.6% per pass across the RBC. Concurrently, increasing influent DOC concentrations decreased ammonia removal efficiency. With respect to dissolved gas conditioning, the RBC system reduced carbon dioxide concentrations approximately 39% as the water flowed through the vessel. The cumulative feed burden – describes the mass of food delivered to the system per unit volume of freshwater added to the system daily – ranged between 5.5 and 7.3 kg feed/m³ of freshwater; however, there was no detectable relationship between the feed loading rate and ammonia oxidation performance.     

SLODS™: slow dissolving standards for water flow measurements

Slow dissolving plaster standards (SLODS™) were developed to obtain integrated water flow measurements over a time scale of 20–30 days, and tested in giant clam farms in the Solomon Islands under steady flow environments. SLODS™ are composed of a gypsum plaster formulation known as Specially Prepared Gypsum (SPG). This plaster has a considerably higher compressive strength; 10 000–13 000 psi, compared with approximately 1000 psi for more common formulations of Plaster-of-Paris. Experimental calibrations in a water motion simulator over 12 days demonstrated a highly significant relationship between fractional weight loss (FWL) of SLODS™ per day, and water velocity (r²=0.96). The SLODS™ simultaneously measured water flow (over a 21 day period in August 1997) at 12 giant clam grow-out sites spread across 400 km of island archipelagoes. Mean water velocity ranged from 5 to 11 cm s⁻¹ and significantly influenced growth of giant clams. SLODS™ compared well with the established methodology of clod cards (r=0.69; P<0.01), with a clear advantage in their ability to integrate water motion over an extended time period into a single measurement. A practical application of SLODS™ to aquatic research will be the development of standardized SLODS™ (produced in a variety of sizes and shapes) calibrated to different factors affecting dissolution. This will require a more detailed analysis of the performance of SLODS™ under different flow, salinity, dissolved solute's, and temperature environments.     

Bacterial diversity of tilapia (Oreochromis niloticus) cultured in brackish water in Saudi Arabia

The bacterial flora occurring in brackish pond water, sediment, gills and intestine of healthy tilapia cultured in Saudi Arabia were estimated both quantitatively and qualitatively, and the isolates were identified to genus or species level. Total viable count of bacteria ranged from 1.4±1.5×10³ to 8.6±2.7×10³ cfu ml⁻¹; 1.2±3.1×10⁶ to 7.3±1.1×10⁷ cfu g⁻¹; 8.7±1.9×10⁵ to 2.1±0.9×10⁶ cfu g⁻¹; and 2.8±2.4×10⁷ to 1.0±1.6×10⁸ cfu g⁻¹ in the pond water, sediment, gills and intestine of brackish water tilapia, respectively. In total, 19 bacterial species were identified. The bacteria were predominantly Gram-negative rods (87%). Pond water and sediment bacteria influenced the bacterial composition of gills and intestine of tilapia. In contrast to gill bacteria, more diversification was observed in intestinal bacteria. The predominant (prevalence >10%) bacterial species were Vibrio parahaemolyticus, Vibrio carchariae, Vibrio alginolyticus, Chryseomonas sp., Vibrio vulnificus, and Streptococcus sp. in all the populations with the exception of the sediment population where Streptococcus sp. was replaced by Shewanella putrefaciens. Vibrio spp. (58% of the total isolates) dominated the total bacterial population.     

Effects of temperature change rate on nitrogen effluxes of Macrobrachium rosenbergii (DeMan)

Ammonia-N (NH4⁺ + NH³) and total nitrogen (TN) efflux rates of Macrobrachium rosenbergii were measured during stepwise temperature changes (26 °C → 22 °C → 18 °C → 22 °C → 26 °C) at different rates. Sudden temperature reductions did not affect efflux rates, but sudden temperature rises evoked rapid and high increases in the efflux rates. A direct relationship between ammonia-N excretion and water temperature was observed when gradual temperature changes were effected over a 5 h period. When the animals were acclimated to a new temperature for 72 h, no clear effects of temperature on the efflux rates were found. Sudden temperature changes resulted in an increased proportion of ammonia-N in the total nitrogen excreted. It is suggested that temperature changes during the shipment of M. rosenbergii should be avoided or made gradually.     

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.     

Measurements of oxygen consumption and ammonia excretion of Atlantic salmon (Salmo salar L.) in commercial-scale, single-pass freshwater and seawater landbased culture systems

The oxygen consumption of Atlantic salmon was measured in large culture tanks for a period of 20 months from the parr to the adult stage. In addition, diurnal sampling was conducted for estimation of both oxygen consumption and ammonia excretion. The oxygen consumption was affected especially by temperature, season and smoltification. For parr the oxygen consumption rate was 1–6 mg O₂/kg min and the ammonia excretion rate was 0·037–0·13 mg N/kg min from autumn to spring. The corresponding rates for adult salmon during the period October to July were 1·5–4·5 mg O₂/kg min and 0·075–0·13 mg N/kg min.