The growth of disk abalone, Haliotis discus hannai at different culture densities in a pilot-scale recirculating aquaculture system with a baffled culture tank

The growth rate of disk abalone, Haliotis discus hannai, energy consumption and changes in water quality were monitored in a pilot-scale recirculating aquaculture system (RAS) for 155 days. Baffles were installed in the RAS culture tanks to enlarge the attachment area and clean out solid waste materials automatically by hydraulic force only. The experimental disk abalones, of shell length 24.5 ± 0.5 mm, were cultured at three stocking densities, 700, 1300 and 1910 individuals/m² bottom area, in triplicate. The abalones were fed with sea mustard, Undaria pinnatifida, once a week. The abalone feed conversion rates and daily growth rates ranged from 24.5 to 25.9 and 0.32 to 0.36%, respectively. Their daily shell increments and survival rates ranged from 67.7 to 78.6 μm/day and 87.6–92.2%, respectively. The growth in weight tended to decrease at a culture density of 1300 individuals/m² bottom area, while shell increments and survival rates were acceptable at this density. The total power consumption for heating was 1185.4 kW, comprising 30.2% of the total power consumption, while the average water exchange rate was only 2.9% per day. The total ammonia nitrogen stabilized below 0.07 mg/L, after conditioning of the biofilter. The NO₂⁻–N, NO₃⁻–N and total suspended solid concentrations were also maintained within acceptable ranges for the normal growth of disk abalone. The use of the RAS with these newly designed culture tanks for disk abalone culture produced 1300 individuals/m² bottom area with a water exchange rate of only 2.9% per day and used about one-tenth of the heat energy of a conventional flow-through system.     

Peracetic acid degradation and effects on nitrification in recirculating aquaculture systems

Peracetic acid (PAA) is a powerful disinfectant with a wide spectrum of antimicrobial activity. PAA and hydrogen peroxide (HP) degrade easily to oxygen and water and have potential to replace formalin in aquaculture applications to control fish pathogens, for example the ectoparasite, Ichthyophthirius multifiliis.We studied water phase PAA and HP decay in three aquaculture situations, i) batch experiments with two types of system waters, ii) PAA decay at different fish densities, and iii) degradation of PAA in submerged biofilters of recirculating aquaculture systems (RAS). Furthermore, effect of PAA on the nitrification activity and the composition of the nitrifying population were investigated.PAA and HP decay showed first order kinetics. High dosage PAA/HP in water with low COD inhibited HP removal, which was not observed in water having a higher COD content. PAA decay was significantly related to fish stocking density, with half life constants for PAA of 4.6 and 1.7 h at 12 and 63 kg m^− 3, respectively.PAA application to RAS biofilter showed rapid exponentially decay with half life constants of less than 1 h, three to five times faster than the water phase decay rates.Biofilter surface specific PAA removal rates ranged from 4.6 to 13.9 mg PAA m^− 2 h^− 1 and was positively correlated to the nominal dosage. Low PAA additions (1.0 mg L^− 1) caused only minor impaired nitrification, in contrast to PAA application of 2.0 and 3.0 mg L^− 1, where nitrite levels were significantly increased over a prolonged period, albeit without fish mortality. The dominant ammonium oxidizer was Nitrosomonas oligotropha and the dominant nitrite oxidizer was Nitrospira. Based on the present findings and other recent results from field and in vitro studies, application perspectives of PAA are discussed.     

Evaluation of the impact of nitrate-nitrogen levels in recirculating aquaculture systems on concentrations of the off-flavor compounds geosmin and 2-methylisoborneol in water and rainbow trout (Oncorhynchus mykiss)

Aquatic animals raised in recirculating aquaculture systems (RAS) can develop preharvest “off-flavors” such as “earthy” or “musty” which are caused by the bioaccumulation of the odorous compounds geosmin or 2-methylisoborneol (MIB), respectively, in their flesh. Tainted aquatic products cause large economic losses to producers due to the inability to market them. Certain species of actinomycetes, a group of filamentous bacteria, have been attributed as the main sources of geosmin and MIB in RAS. Previous studies have demonstrated that certain nutritional factors can stimulate or inhibit bacterial biomass and geosmin production by certain actinomycetes. In the current study, the effects of two nitrate-nitrogen (NO₃^--N) levels (20–40 mg/L and 80–100 mg/L) on geosmin and MIB levels in culture water and the flesh of rainbow trout (Oncorhynchus mykiss) raised in RAS were monitored. Water and fish tissue samples were collected over an approximately nine-week period from six RAS, three replicates each of low and high NO₃^--N, and analyzed for geosmin concentrations using solid phase microextraction–gas chromatography–mass spectrometry. Results indicated no significant difference in geosmin concentrations in water or fish flesh between the low and high NO₃^--N RAS. Therefore, higher NO₃^--N levels that may occur in RAS will not adversely or beneficially impact geosmin-related off-flavor problems.     

Evaluating the chronic effects of nitrate on the health and performance of post-smolt Atlantic salmon Salmo salar in freshwater recirculation aquaculture systems

Commercial production of Atlantic salmon smolts, post-smolts, and market-size fish using land-based recirculation aquaculture systems (RAS) is expanding. RAS generally provide a nutrient-rich environment in which nitrate accumulates as an end-product of nitrification. An 8-month study was conducted to compare the long-term effects of “high” (99 ± 1 mg/L NO₃-N) versus “low” nitrate-nitrogen (10.0 ± 0.3 mg/L NO₃-N) on the health and performance of post-smolt Atlantic salmon cultured in replicate freshwater RAS. Equal numbers of salmon with an initial mean weight of 102 ± 1 g were stocked into six 9.5 m³ RAS. Three RAS were maintained with high NO₃-N via continuous dosing of sodium nitrate and three RAS were maintained with low NO₃-N resulting solely from nitrification. An average daily water exchange rate equivalent to 60% of the system volume limited the accumulation of water quality parameters other than nitrate. Atlantic salmon performance metrics (e.g. weight, length, condition factor, thermal growth coefficient, and feed conversion ratio) were not affected by 100 mg/L NO₃-N and cumulative survival was >99% for both treatments. No important differences were noted between treatments for whole blood gas, plasma chemistry, tissue histopathology, or fin quality parameters suggesting that fish health was unaffected by nitrate concentration. Abnormal swimming behaviors indicative of stress or reduced welfare were not observed. This research suggests that nitrate-nitrogen concentrations ≤ 100 mg/L do not affect post-smolt Atlantic salmon health or performance under the described conditions.     

Effects of live rock on the reef-building coral Acropora digitifera cultured with high levels of nitrogenous compounds

Reef-building corals are sensitive to excessive nitrogenous (N) compounds. To maintain levels of inorganic nitrogenous compounds low in coral aquaria, various technologies, mechanical, chemical and biological, have been applied. As one of the biological techniques, “live rock,” which can be defined as a dead coral skeleton covered with crustose coralline algae (CCA), has long been applied for coral aquaria. Until recently, however, there has been little evidence for the effectiveness of live rock in removal of N compounds from coral aquaria. Demonstrating comparative experiments with live rocks, here we report that the live rock is capable of removing N compounds and reduces the mortality of reef-building coral. We cultured the reef-building coral Acropora digitifera with the sea cucumber Holothuria atra as a natural nitrogen producer. H. atra increased the concentration of the inorganic N compounds (NH₄⁺, NO₃⁻ and NO₂⁻) that resulted in high coral mortality. The presence of the live rock remarkably reduced the concentrations and sustained a high coral photosynthetic activity. We detected the functional genes amoA and nirS within the live rock, suggesting the occurrence of both nitrifying and denitrifying bacteria. These results support the idea that “live rock” is an effective biofilter that can maintain water quality suitable for reef-building corals.     

Effects of feeding frequency on metabolism of juvenile walleye

The effects of feeding frequency (1, 2, 3, or 15 times daily) on oxygen consumption (OC, mg O₂ kg⁻¹ h⁻¹) and ammonia excretion, (AE, mg TAN kg⁻¹ h⁻¹) of walleye (Stizostedion vitreum) are described. Walleye were reared at a practical culture density of 35·4 kg m⁻³ in a single-pass system at 23·2°C. Diurnal variation in metabolic rates were related to feeding, not to photoperiod. Minimum OC rates occurred 30 min before the first feeding of the day, which was the longest average time since the last feeding. Metabolic rates increased immediately after feeding. The maximum rates for OC were 36–49% higher than the minimum rates, and 14–22% higher than the 24-h mean rate. Maximum rates for AE were 137–409% higher than the minimum rates, and 39–87% higher than the mean rates. There was a highly significant difference in the mean metabolic rates related to feeding frequency. The mean OC rate for 1 feeding day⁻¹ (222·0 mg O₂ kg⁻¹ h⁻¹) was greater than the mean rates for 2, 3 and 15 feedings day⁻¹. The OC rate for 1 feeding day⁻¹ was 50·6% greater than the rate for 15 feedings day⁻¹ (147·4 mg O₂ kg⁻¹ h⁻¹), the lowest mean rate. Mean and maximum oxygen/feed ratio (OFR, kg O₂ kg⁻¹ feed fed day⁻¹), varied inversely with feeding frequency. The mean ammonia/feed ratio (AFR) was similar for all but the 3 feedings day⁻¹ treatment, but the maximum AFRs for 2 and 15 feedings day⁻¹ were lower than the AFRs for 1 and 3 feedings day⁻¹. AE was directly proportional to OC; the regression equations were highly significant, but specific for feeding frequency.     

Limiting factors associated with nitrification in closed blue crab shedding systems

A study of factors limiting crab densities in closed blue crab Callinectes sapidus shedding systems was conducted using scaled down experimental units. Nitrification beds, activated carbon, dolomite and plants were used to maintain water quality. Nitrite (NO₂N) was found to be the most critical toxic element accumulating in the system as a result of the nitrification process. Concentrations of approximately 20 mg liter^?1 NO₂N and above caused increased mortality in intermolt crabs. Mortality in molting crabs was observed at concentrations as little as 2 mg liter^?1 NO₂N. Dissolved oxygen (DO) was identified as the factor limiting the efficiency of the nitrification beds. As DO concentrations decreased, the rate of nitrification slowed, apparently causing nitrification to be inhibited at the nitrite to nitrate conversion step. As nitrite concentrations increased, high mortalities resulted, further increasing the loading in the systems and depressing DO concentrations, due to the high BOD exerted by the dead crabs. Elevated crab populations were maintained in the systems when aeration and flow increases were supplied to the nitrification beds.     

Effects of abrupt salinity increase on nitrification processes in a freshwater moving bed biofilter

The nitrification process is a widely used biological approach responsible for ammonia and nitrite removal in recirculating aquaculture system (RAS) biofilters. Given this pivotal role, the influence of different water quality parameter on nitrification efficiency is important information for RAS operations. One influencing parameter is salinity, and salinity fluctuations in freshwater RAS biofilters are reported to affect the nitrifying bacteria. This study investigated the effects of abrupt increase in salinity in freshwater RAS on substrate-dependent (1’-order) as well as substrate independent (0’-order) nitrification rates. A 100% inhibition was found for surface specific removal (STR) of total ammonia nitrogen (TAN) and surface specific nitrite removal (SNR) when salinity was abruptly increased to 25‰ and above. A fast turnover (i.e. steep decline in [NH₄-N⁺] and [NO₂-N⁻]) were observed at lower salinities (≤10‰), while limited/no degradation of either ammonia or nitrite was seen at salinities above 25‰. At low substrate loading (1’-order process), removal rate constants (k_1a) of 0.22 and 0.23 m d^-1 were observed for ammonia and nitrite degradation, respectively, declining to 0.01 m d^-1when adding marine RAS water increasing the salinity to 15‰. Similar observations followed at high nutrient loadings (0’-order process) with STR and SNR of 0.10 and 0.12 g N m^-2 d^-1, respectively, declining to 0.01 g N m^-2 d^-1 at 15‰. When salinities of 25‰ and 35‰ were applied, neither TAN nor nitrite degradation was seen. The results thus demonstrate a pronounced effect of salinity changes when freshwater RAS biofilters are subjected to fast/abrupt changes in salinity. RAS facility operators should be aware of such potential effects and take relevant precautions.     

Larviculture of the painted river prawn Macrobrachium carcinus in different culture systems

The objective of this study was to evaluate different hatchery systems used for the larviculture of the Macrobrachium carcinus based on survival, larval development and production of post-larvae. The experimental culture was carried out in three phases designated as Phase I (Zoea VI to VIII – Z_{VI – VIII}), Phase II (Zoea VIII to X – Z_{VIII – X}), and Phase III (Zoea X to PL – Z_{X – PL}), with densities of 30, 27.5 and 25 larvae / L, respectively. The M. carcinus larvae (Z_VI) were reared in four culture systems, two being open (Greenwater – GW and Clearwater – CW) and two being closed (Biofloc – BFT and Bio-filter – RAS), distributed in twelve 10 L plastic containers, filled with 20 ppt brackish water, equipped with constant aeration, and water circulated by air lift and heated with thermostat (∼30 °C). The GW treatment was maintained with Chlorophyceae algae in the density of 3–5 × 10⁵ cells/mL. In the CW, the water was previously filtered through a 5 μm mesh screen, sterilized with 10 ppm active chlorine and, dechlorinated with vitamin C and subjected to aeration for 24 h. The BFT received water rich in bioflocs that was matured prior to the experiment and used molasses as a source of organic carbon. In the RAS, the culture water circulated through an external “Dry-Wet” biological filter. The feeding was carried out ad libitum four times daily, alternating a wet diet formula with a commercial diet, which was supplemented with newly hatched Artemia nauplii at a rate of 40–50 per larvae/day. Temperature, dissolved oxygen and pH were monitored daily and the salinity two times per week. Total ammonia, nitrite, nitrate, orthophosphate, alkalinity, total suspended solids, chlorophyll-a, COD and BOD were also analyzed. The best water quality (P < 0.05) was obtained in the RAS, with 0.49 (±0.38), 0.23 (±0.22), and 9.0 (±1.5) mg/L of TAN, NO₂-N and NO₃-N, respectively. In the GW, the nitrogen species showed high fluctuations and higher concentrations at 2.32 (±1.68), 3.53 (±3.53) and 18.2 (±12.9) mg / L of TAN, NO₂-N and NO₃-N, respectively. Considering the three phases (Z_{VI – PL}), the overall survival was 0.03, 1.97, 2.23 and 17.32 % for the BFT, CW, GW and RAS, respectively. When considering the phases separately, the survival in Phase I (Z_{VI – VIII}) was highest in the GW system at 58.7 % while the RAS was the highest in Phases II (Z_{VIII – X}) and III (Z_{X – PL}) at 70.6 % and 60.3 %, respectively. The BFT showed 8.4 (±3.5) PL/L, which was higher (P < 0.05) than that obtained in the RAS (2.8 ± 1.2 PL/L) and the GW (1.3 ± 1.1 PL/L) and similar to that obtained in the CW (5.6 ± 2.0 PL/L). Thus, the larviculture for the M. carcinus may be optimized by adopting a multiphase management strategy, which the intermediate larval stages (Z_{VI – IX}) are reared in the GW system and the final stages (Z_{X – PL}) are reared in the BFT system.     

The effects of crowding on mackerel (Scomber scombrus L.) — Physical condition and mortality

Series of trials in which mackerel (Scomber scombrus L.) were confined in keepnets at different stocking densities are described. From simple confinement trials it was found that 50% of the fish died after 48 h at a stocking density of 30 fish m⁻³, equivalent to 6.5 kg m⁻³. Trials in which fish were held at stocking densities, and for a duration, comparable to those experienced in a “dried up” purse seine prior to “slipping”, showed that up to 90% of “slipped” fish died within 48 h of release. The primary cause of death was probably physical damage, particularly skin loss, caused by abrasion, although there is some evidence that mackerel have a healing process which can accommodate minor skin abrasions. A tagging trial showed a small but significant increase in mortality due to the tagging procedure.     

Waste excretion of marble goby (Oxyeleotris marmorata Bleeker) fed with different diets

Marble goby (Oxyeleotris marmorata Bleeker), with its high demand and price, has a great potential as a profitable commercial aquaculture candidate in Malaysia and Southeast Asia region. Efforts are being made to produce this species in a better controlled culture environment like recirculating aquaculture system (RAS) due to poor growth performance and disease problems shown by conventional cage and outdoor pond culture systems. Quantification of waste excreted by fish is critical to RAS design. This study was conducted to characterize the waste excretion rates of marble goby fed with different diets (live food and minced fish). Ammonia-N (TAN), urea-N, nitrite-N (NO₂-N), nitrate-N (NO₃-N), total-N (TN), organic-N (ON), feces-N, 5-day biochemical oxygen demand (BOD₅) and total suspended solid (TSS) produced from marble goby were determined over a 72-h excretion period. Under given experimental conditions, the results showed that feed type had significant influence on the waste excretion rates, with marble goby fed live tilapia (Oreochromis niloticus) exhibiting significantly (P < 0.05) the lowest amount of waste excretion comparable to that of fish fed live common carp (Cyprinus carpio) and minced scads (Decapterus russellii). This indicates that feeding marble goby with tilapia poses less adverse effects on water quality and is thus a suitable diet for this species. The waste excreted by the fish is composed of nitrogenous excretion (TAN, Urea-N, ON, Feces-N), and productions of dissolved biodegradable organic substances (BOD₅) and TSS (TSS_feces + TSS_water). About 58-71% of the nitrogen consumed in food was excreted and its rate depended mainly on the feed type. TAN was the chief end-product of protein metabolism; about 74-84% of the daily total nitrogenous excretion was TAN. Urea-N accounted for 13-21% of the daily total nitrogenous excretion indicating that urea-N is an important nitrogenous excretory end-product in marble goby. The waste excretion data presented in this study can be served as a pre-requisite for designing a RAS for this species. The overall BOD₅ and TSS production found in this study also point to the need for including bio-filtration unit and suspended solids removal mechanism in the RAS design.     

Effects of alkalinity on ammonia removal,carbon dioxide stripping,and system pH in semi-commercial scale water recirculating aquaculture systems operated with moving bed bioreactors

When operating water recirculating systems (RAS) with high make-up water flushing rates in locations that have low alkalinity in the raw water, such as Norway, knowledge about the required RAS alkalinity concentration is important. Flushing RAS with make-up water containing low alkalinity washes out valuable base added to the RAS (as bicarbonate, hydroxide, or carbonate), which increases farm operating costs when high alkalinity concentrations are maintained; however, alkalinity must not be so low that it interferes with nitrification or pH stability. For these reasons, a study was designed to evaluate the effects of alkalinity on biofilter performance, and CO₂ stripping during cascade aeration, within two replicate semi-commercial scale Atlantic salmon smolt RAS operated with moving bed biological filters. Alkalinity treatments of nominal 10, 70, and 200 mg/L as CaCO₃ were maintained using a pH controller and chemical dosing pumps supplying sodium bicarbonate (NaHCO₃). Each of the three treatments was replicated three times in each RAS. Both RAS were operated at each treatment level for 2 weeks; water quality sampling was conducted at the end of the second week. A constant feeding of 23 kg/day/RAS was provided every 1–2 h, and continuous lighting, which minimized diurnal fluctuations in water quality. RAS hydraulic retention time and water temperature were 4.3 days and 12.5 ± 0.5 °C, respectively, typical of smolt production RAS in Norway.It was found that a low nominal alkalinity (10 mg/L as CaCO₃) led to a significantly higher steady-state TAN concentration, compared to when 70 or 200 mg/L alkalinity was used. The mean areal nitrification rate was higher at the lowest alkalinity; however, the mean TAN removal efficiency across the MBBR was not significantly affected by alkalinity treatment. The CO₂ stripping efficiency showed only a tendency towards higher efficiency at the lowest alkalinity. In contrast, the relative fraction of total inorganic carbon that was removed from the RAS during CO₂ stripping was much higher at a low alkalinity (10 mg/L) compared to the higher alkalinities (70 and 200 mg/L as CaCO₃). Despite this, when calculating the total loss of inorganic carbon from RAS, it was found that the daily loss was about equal at 10, and 70 mg/L, whereas it was highest at 200 mg/L alkalinity. pH recordings demonstrated that the 10 mg/L alkalinity treatment resulted in the lowest system pH, the largest increase in [H⁺] across the fish culture tanks, as well as giving little response time in case of alkalinity dosing malfunction. Rapid pH changes under the relatively acidic conditions at 10 mg/L alkalinity may ultimately create fish health issues due to e.g. CO₂ or if aluminium or other metals are present. In conclusion, Atlantic salmon smolt producers using soft water make-up sources should aim for 70 mg/L alkalinity considering the relatively low loss of inorganic carbon compared to 200 mg/L alkalinity, and the increased pH stability as well as reduced TAN concentration, compared to lower alkalinity concentrations.     

Discharge management in fresh and brackish water RAS: Combined phosphorus removal by organic flocculants and nitrogen removal in woodchip reactors

The current study combined P and N removal using organic flocculant chemicals and woodchip bioreactors in both freshwater and brackish water (7 ppm) recirculating aquaculture systems (RAS). The use of carbon (C) containing flocculant chemicals in the process was hypothesized to further stimulate C-demanding N removal (denitrification) in bioreactors. The trial of combined P and N removal consisted of four treatments: freshwater and brackish water RAS with and without the addition of supernatant from flocculation process to the woodchip reactor. Duplicate woodchip reactors were used per treatment and the trial was run for six weeks. 56% and 49% of P was removed from fresh and brackish sludge water, respectively. The nitrate-N (NO₃-N) removal rate was improved in the treatment when supernatant from flocculation process was used together with RAS discharge water when compared against the control. In brackish water RAS, the improvement was more pronounced (from 6.6–16.5 g NO₃-N m⁻³ d^-1) than in freshwater RAS (from 5.1–6.5 NO₃-N m⁻³ d^-1). In the freshwater bioreactors using supernatant, N was largely discharged as a nitrite-N (NO₂-N). High NO₂-N concentrations in freshwater reactors allude to incomplete denitrification reactions taking place. The results suggest that the organic flocculants did provide an additional C source for denitrification, which improved the N-removal process. However, in freshwater RAS this might have been partly due to untargeted processes such as DNRA (dissimilatory nitrate reduction to ammonium), and/or insufficient denitrification reactions taking place (excessive NO₂-N production).     

Effect of water recirculation on seawater quality and production of scallop (Pecten maximus) larvae

Scallop larval production systems in Norway have changed from the use of batch to continuous flow through systems (FTS) during the last decade. Energy use to heat water in both larval and spat nurseries is considerable. Two experiments (June 2010 and February 2011) using water recirculation technology (RAS) were performed in large scale systems (3500 L larval tanks) supplied with continuous addition of algal feed, and 20% renewal of seawater.In the RAS a gradual increase in CO₂, decrease in pH and dissolved oxygen was observed over time. This was most obvious during experiment two, when the total organic carbon content increased in both FTS and RAS. The total bacterial number was lower and more stable in FTS than in the RAS. The variations in seawater quality parameters were smaller during the first experiment compared to the second, when values of oxygen saturation were reduced to <70%, pH was 7.8 and NO₃⁻ reached 5 mg L⁻¹. Even though these changes would seem less beneficial for survival and growth of scallop larvae, results showed that the survival at the end of the larval stage was higher in the FTS, but the yield of competent larvae ready for settlement was not significant different (p > 0.05) due to large variations between tanks. The CV% was 28.9% in FTS, while it was 49.9% in RAS. In FTS the mean yield was 40.2%, while it was 26.5% of initial number of larvae in RAS. Large variations in survival and yield were found between the larval tanks as well as gradual reduction in pH and oxygen in RAS tanks. The results indicate that there is a large potential for 80% reduction in water use by utilizing recirculation technology.     

Evaluation of Stocking Density during Second‐Year Growth of Largemouth Bass,Micropterus salmoides,Raised Indoors in a Recirculating Aquaculture System

Largemouth bass (LMB), Micropterus salmoides, are a highly desirable food fish especially among Asian populations in large cities throughout North America. The primary production method for food‐size LMB (>500 g) has been outdoor ponds that require two growing seasons (18 mo). Indoor, controlled‐environment production using recirculating aquaculture system (RAS) technologies could potentially reduce the growout period by maintaining ideal temperatures year‐round. Researchers conducted a 26‐wk study to evaluate optimal stocking densities for growout of second‐year LMB to food‐fish size in an indoor RAS. LMB fingerlings (112.0 ± 38.0 g) were randomly stocked into nine 900‐L tanks to achieve densities of 30, 60, or 120 fish/m³ with three replicate tanks per density. The RAS consisted of a 3000‐L sump, ¼ hp pump, bead filter for solids removal, mixed‐moving‐bed biofilter for nitrification, and a 400‐watt ultraviolet light for sterilization. Fish were fed a commercially available floating diet (45% protein and 16% lipid) once daily to apparent satiation. At harvest, all fish were counted, individually weighed, and measured. Total biomass densities significantly increased (P ≤ 0.05) with stocking rate achieving 6.2, 13.2, and 22.9 kg/m³ for fish stocked at 20, 60, and 120 fish/m³, respectively. The stocking densities evaluated had no significant impact (P > 0.05) on survival, average harvest weight, or feed conversion ratio which averaged 92.9 ± 5.8%, 294.5 ± 21.1 g, and 1.8 ± 0.3, respectively. After approximately 6 mo of culture, LMB did not attain target weights of >500 g. Observed competition among fish likely resulted in large size variability and overall poor growth compared to second‐year growth in ponds. Additional research is needed to better assess the suitability of LMB for culture in RAS.     

End-of-pipe single-sludge denitrification in pilot-scale recirculating aquaculture systems

A step toward environmental sustainability of recirculat aquaculture systems (RAS) is implementation of single-sludge denitrification, a process eliminating nitrate from the aqueous environment while reducing the organic matter discharge simultaneously. Two 1700 L pilot-scale RAS systems each with a 85 L denitrification (DN) reactor treating discharged water and hydrolyzed solid waste were setup to test the kinetics of nitrate and COD removal. Nitrate removal and COD reduction efficiency was measured at two different DN-reactor sludge ages (high θ_X: 33–42 days and low θ_X: 17–23 days). Nitrate and total N (NO₃⁻ + NO₂⁻ + NH₄⁺) removal of the treated effluent water ranged from 73–99% and 60–95% during the periods, respectively, corresponding to an overall maximum RAS nitrate removal of approximately 75%. The specific nitrate removal rate increased from 17 to 23 mg NO₃⁻-N (g TVS d)⁻¹ and the maximal potential DN rate (measured at laboratory ideal conditions) increased correspondingly from 64–68 mg NO₃⁻-N (g TVS d)⁻¹ to 247–294 mg NO₃⁻-N (g TVS d)⁻¹ at high and low θ_X, respectively. Quantification of denitrifiers in the DN-reactors by qPCR showed only minor differences upon the altered sludge removal practice. The hydrolysis unit improved the biodegradability of the solid waste by increasing volatile fatty acid COD content 74–76%. COD reductions in the DN-reactors were 64–70%. In conclusion, this study showed that single-sludge denitrification was a feasible way to reduce nitrate discharge from RAS, and higher DN rates were induced at lower sludge age/increased sludge removal regime. Improved control and optimization of reactor DN-activity may be achieved by further modifying reactor design and management scheme as indicated by the variation in and between the two DN-reactors.     

Stressing fish in Recirculating Aquaculture Systems (RAS): does stress induced in one group of fish affect the feeding motivation of other fish sharing the same RAS?

As a consequence of water re‐use and high stocking densities, Recirculating Aquaculture Systems (RAS) may lead to an accumulation of substances released by the fish into the water, e.g. cortisol and alarm pheromones. This study investigated the effect of stressing fish on the feeding motivation of other fish not subjected to stress but sharing the same water of stressed fish. Two identical RAS were used (operated at 30 L kg feed^?1 day^?1) and contained grouped (stressed fish) and individually (receiving water from stressed fish) housed Nile tilapia. A stress test was applied in grouped housed fish on days 17 and 55. Feeding behaviour (intake and latency) was recorded in the individually housed fish 3 days before, during and 3 days after the stress test. The results showed no effect on feeding behaviour in fish receiving the water from stressed fish. These results could be a consequence of insufficient cortisol/alarm cues' release by the stressed fish into the water or inactivity of such substances, either due to a trapping effect of humic acids or due to degradation in the nitrification and denitrification processes. Future research is needed to clarify how these processes may affect the water concentration of cortisol and alarm pheromones and should be extended by measuring other behavioural and physiological traits.     

Applying biofloc technology in the culture of juvenile of Piaractus brachypomus (Cuvier, 1818): Effects on zootechnical performance and water quality

The aim of this study was to assess the zootechnical performance and water quality of a cachama blanca (Piaractus brachypomus) culture using biofloc technology (BFT) versus a system with daily water exchange (DWE). To do this, 180 juveniles (mean initial weight: 5.40 ± 0.19 g) were distributed in 12 circular plastic tanks with stocking densities of 20 or 40 individuals m³; then, they were cultured for 91 days. BFT treatments kept a C:N ratio approximately of 15:1. Temperature, pH and oxygen were monitored daily, while the other variables were measured weekly. Most productive variables were significantly influenced by both culture system and stocking density with significantly higher values of daily weight gain, total weight gain and total length for fish kept in DWE 20. However, only minor differences were observed within the BFT system. With the exception of the toxic nitrogen compounds (NH₄⁺ and NO₂^?), all the other water quality parameters were within the acceptable ranges for the cultivation of tropical fish. Microorganisms started to settle from the first week. A total of 23 genera were present, the most outstanding of which being seven genera of ciliates and three rotifers, rhizopods and chlorophytes. In conclusion, both systems BFT and DWE are useful for increasing the production of P. brachypomus in captivity. Additionally, the BFT system can potentially be applied for growing juveniles of this specie in regions with scarce water resources.     

Effect of water velocity on ammonium and nitrite removal in pilot scale fixed bed biofilters

The effect of water velocity on nitrification rates in fixed bed biofilters was investigated in three freshwater pilot scale RAS with rainbow trout. Removal of total ammonia nitrogen (TAN) and nitrite-nitrogen were assessed by NH₄Cl spikes and tested at four different water velocities in the biofilters (1.4, 5.4, 10.8 and 16.2 m h⁻¹) under identical conditions. Water velocities below 10.8 m h⁻¹ significantly reduced TAN- and nitrite removal rates. The surface specific TAN removal rates correlated with the TAN concentrations at the water velocities 10.8 and 16.2 m h⁻¹, and the first order surface removal rate constant was estimated at 0.45 m h⁻¹. However, no correlations between TAN removal and TAN concentrations were found at the lowest velocities. Up to five-fold elevated nitrite levels were found in the RAS when biofilters were operated at 1.4 m h⁻¹ compared to the trials at other water velocities, substantiating the significant effect of water velocity on both nitrification processes. The importance of biofilter hydraulics documented in this pilot scale RAS probably have implications for design and operation in larger scale RAS.     

Quality of brackish aquaculture sludge and its suitability for anaerobic digestion and methane production in an upflow anaerobic sludge blanket (UASB) reactor

Intensive recirculating aquaculture systems (RAS) produce high volumes of biosolid waste. The high salinity of brackish/marine sludge limits its use in landfill sites and waste outflows and it is a source of pollution. A reduction in sludge mass would therefore minimize the potential environmental hazard and economic burden stemming from its disposal. The aims of the current study were: 1) to characterize brackish aquaculture sludge (BAS) from three RAS in order to test for potentially suitable treatments, and 2) to test the BAS's suitability for anaerobic digestion in an upflow anaerobic sludge blanket reactor (UASB). Brackish sludge from three intensive RAS was collected periodically and analyzed for a variety of physical and chemical parameters. The mean sludge electrical conductivity and pH values ranged from 4.0 to 8.6 mS cm^− 1 and 7.0 to 7.7, respectively. A low sludge redox potential averaging − 80 mV and dissolved oxygen concentrations of less than 1 mg l^− 1 indicated the existence of anaerobic conditions. Volatile solids comprised 56 to 76% of the dry weight and the sludge volume index ranged from 44 to 69 ml g ^− 1. High concentrations of total nitrogen and total carbon were also observed, resulting in a C:N ratio ranging between 8.1 and 10.3. Toxic and/or inhibitory compounds for methanogenesis such as nitrites, nitrates and sulfides were almost absent. Sludge BOD₅ ranged from 10 to 30% dry weight. These data suggest that BAS may be used in anaerobic digestion and methanogenesis without pretreatment. This concept was tested by digesting aquaculture sludge in UASB reactors. Despite the high sulfate and phosphate concentrations in the BAS, these were found not to be inhibitory to methanogenesis. Up to 70% sludge-mass reduction and an average of 40% methane production were demonstrated.