Effects of swimming speed and dissolved oxygen on geosmin depuration from market-size Atlantic salmon Salmo salar

Common off-flavor compounds, including geosmin (GSM) and 2-methylisoborneol (MIB), bioaccumulate in Atlantic salmon Salmo salar cultured in recirculating aquaculture systems (RAS) resulting in earthy and musty taints that are unacceptable to consumers. To remediate off-flavor from market-ready salmon, RAS facilities generally relocate fish to separate finishing systems where feed is withheld and makeup water with very low to nondetectable GSM and MIB levels is rapidly exchanged, a process known as depuration. Several procedural aspects that affect salmon metabolism and the associated rate of off-flavor elimination, however, have not been fully evaluated. To this end, a study was carried out to assess the effects of swimming speed and dissolved oxygen (DO) concentration on GSM levels in water and fish flesh during a 10-day depuration period. Atlantic salmon (5–8 kg) originally cultured in a semi-commercial-scale RAS (150 m³ tank) were exposed to a concentrated GSM bath before being transferred to 12 replicated partial reuse depuration systems (5.4 m³ total volume). Two swimming speeds (0.3 and 0.6 body lengths/sec) and two DO levels (90% and 100% O₂ saturation) were applied using a 2 × 2 factorial design (N = 3), and each system was operated with a 5-h hydraulic retention time, creating a water flushing to biomass ratio of 151 L/kg fish biomass/day. Geosmin was assessed at Days 0, 3, 6, and 10 in system water and salmon flesh. A borderline effect (P = 0.064; 0.068) of swimming speed was measured for water and fish, respectively, at Day 3, where slightly lower GSM was associated with low swimming speed (0.3 body lengths/sec); however, differences were not detected at Days 6 or 10 when salmon are commonly removed for slaughter. Overall, this research indicates that significant improvements in GSM depuration from RAS-produced Atlantic salmon are not expected when purging with swimming speeds and DO concentrations similar to those tested during this trial.     

Depuration system flushing rate affects geosmin removal from market-size Atlantic salmon Salmo salar

Common off-flavor compounds including geosmin (GSM) bioaccumulate in fish cultured in recirculating aquaculture systems (RAS) resulting in unpalatable fillets that are objectional to consumers. Most RAS facilities relocate fish from grow-out tanks to separate depuration systems with increased water flushing to remediate pre-harvest off-flavors, but certain aspects of this procedure have not been optimized including characterization of water exchange rates that effectively diminish off-flavor. To this end, a study was carried out to evaluate the effects of flushing rate and associated depuration system hydraulic retention time (HRT) on GSM removal from Atlantic salmon Salmo salar originally produced in a semi-commercial scale freshwater RAS. Twenty-six fish (5−7 kg each) were stocked into twelve replicate depuration systems operated with system HRTs of 2.4, 4.6, and 11.3-h, respectively (N = 4). Geosmin was assessed at intervals in system water and fish flesh over a 10-day feed withholding period. Waterborne GSM concentration was affected by flushing rate and associated system HRT (P < 0.05). Depuration systems operated with an 11.3-h HRT had greater waterborne GSM levels at 3, 6, and 10 days post-stocking compared to 2.4 and 4.6-h HRT. A similar trend was generally reflected in salmon flesh. Residual GSM levels were successively higher in fillets on Day 6 from depuration systems with increasingly longer HRT. Geosmin levels were greatest in salmon flesh from the 11.3-h HRT treatment on Day 10, but fillet GSM between the 2.4 and 4.6-h HRT was similar. This research indicates that lowest residual GSM is achieved in water and Atlantic salmon flesh in depuration systems with increased flushing and shorter HRT, i.e., 2.4–4.6-h under conditions of this study. Selection of optimal flushing rate to remediate off-flavor from RAS-produced Atlantic salmon may also be dictated by water and energy use metrics and site-specific water availability among other factors.     

Culture environment and the odorous volatile compounds present in pond-raised channel catfish (<Emphasis Type="Italic">Ictalurus punctatus</Emphasis>)

A headspace solid-phase micro-extraction (SPME) coupled with GS-MS method was used to measure volatile compounds in fillets from musty off-flavor, muddy off-flavor, and on-flavor channel catfish (Ictalurus punctatus), along with water and soil samples from the farm ponds in which the fish had been raised. Two ponds of each type of flavor were selected, and five fish, water, and soil samples were collected from each pond. Linear and multiple linear regression analyses were carried out between/among off-flavor strength and volatile compound contents to investigate their possible correlations. The combination of two strong off-flavor compounds, 2-methylisoborneol (MIB) and geosmin (GSM), was probably mainly responsible for the musty off-flavor in the catfish fillets, and an odorous alcohol, 1-hexanol, was correlated with muddy off-flavor (p =?0.015). There was a strong correlation between beta-cyclocitral and MIB in a pond that gave musty off-flavor catfish contents (p =?0.006), suggesting that these compounds might be generated by similar cyanobacteria. The contents of GSM, MIB, and beta-cyclocitral were high in the water of ponds that yielded off-flavor fish, indicating that catfish might acquire these compounds from pond water.     

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.     

Development of phytoplankton communities and common off-flavors in a biofloc technology system used for the culture of channel catfish (Ictalurus punctatus)

The use of biofloc technology production systems continues to increase in the aquaculture industry worldwide. Recent research demonstrated that outdoor biofloc systems can be used to produce high yields of channel catfish (Ictalurus punctatus). However, studies have not yet been performed to determine the development and composition of phytoplankton communities and related off-flavor problems in these biofloc production systems. In this study, water samples were collected biweekly from May to November and channel catfish samples were collected during harvest in November from nine 18.6 m² biofloc culture tanks. Water and fillet samples were analyzed for levels of the common off-flavor compounds geosmin and 2-methylisoborneol (MIB). The development and composition of phytoplankton communities in each culture tank was also monitored. In addition, water and biofloc samples were evaluated to assess the microbial sources of geosmin and MIB within the culture tanks. Phytoplankton (including algae and cyanobacteria attached to bioflocs) biomass, as determined by concentrations of chlorophyll a in the water, gradually increased in all tanks over time. Phytoplankton communities that developed in the culture tanks were dominated by fast-growing, unicellular and small colonial types of green algae (chlorophytes) and diatoms (bacillariophytes) and slower growing, small colonial types of cyanobacteria (cyanophytes). A positive correlation (p < 0.05) between cumulative feed addition and chlorophyll a concentration was found. Although geosmin and MIB were present in the culture water of each tank during most of the study, levels were typically low and only one tank yielded catfish with geosmin and MIB in their flesh at levels high enough to be designated as off-flavor. A positive correlation (p < 0.05) between cumulative feed addition and MIB concentrations in the water of culture tanks indicates a greater potential for MIB-related off-flavor problems at high feed application rates. The microbial sources responsible for production of geosmin and MIB in the culture tanks remain unknown.     

Controlling of geosmin and 2‐methylisoborneol induced off‐flavours in recirculating aquaculture system farmed fish—A review

Recirculating aquaculture system (RAS) is an increasingly popular alternative to open aquaculture production systems. However, off‐flavours and odours can accumulate in the fish flesh from the circulating water and decrease the fish meat quality. Off‐flavours are typically caused by geosmin (GSM) and 2‐methylisoborneol (MIB) that are lipophilic compounds formed as secondary by‐products of bacterial metabolism. Even though GSM and MIB are not toxic, they often are disliked by consumers, and both have very low human sensory detection limits. Multiple methods have been suggested to remove or decrease GSM and MIB in fish, including ozonation, advanced oxidation processes (AOP)s and adsorption removal from water using activated carbon and/or zeolites. So far, purging with fresh water is the only efficient method available to remove the off‐flavours. There are multiple analytical methods available for the extraction and separation of GSM and MIB from fish flesh and water. This review discusses the current knowledge of GSM and MIB formation, the challenges faced by RAS farms due to these compounds and process solutions available for their removal.     

A novel approach for ammonia removal from fresh-water recirculated aquaculture systems,comprising ion exchange and electrochemical regeneration

A new physico-chemical process for ammonia removal from fresh-water recirculated aquaculture systems (RASs) is introduced. The method is based on separating NH₄⁺ from RAS water through an ion-exchange resin, which is subsequently regenerated by simultaneous chemical desorption and indirect electrochemical ammonia oxidation. Approach advantages include (1) only slight temperature dependence and no dependence on bacterial predators and chemical toxins; (2) no startup period is required and the system can be switched on and off at will; and (3) the fish are grown in much lower bacterial concentration, making the potential for both disease and off-flavor, lower. A small pilot scale RAS was operated for 51 d for proving the concept. The system was stocked by 105 tilapia fish (initial weight 35.8 g). The fish, which were maintained at high TAN (total ammonia nitrogen) concentrations (10–23 mgN L⁻¹) and fish density of up to 20 kg m⁻³, grew at a rate identical to their established growth potential. NH_3(aq) concentrations in the fish tank were maintained lower than the assumed toxicity threshold (0.1 mgN L⁻¹) by operating the pond water at low pH (6.5–6.7). The low pH resulted in efficient CO₂ air stripping, and low resultant CO_2(aq) concentrations (<7 mg L⁻¹). Due to efficient solids removal, no nitrification was observed in the fish tank and measured nitrite and nitrate concentrations were very low. The system was operated successfully, first at 10% and then at 5% daily makeup water exchange rate. The normalized operational costs, calculated based on data derived from the pilot operation, amounted to 28.7 $ cent per kg fish feed. The volume of the proposed process was calculated to be ∼13 times smaller than that of a typical RAS biofilter. The results show the process to be highly feasible from both the operational and economical standpoints.     

Effects of Temperature on the Uptake and Depuration of 2-Methylisoborneol (MIB) in Channel Catfish Ictalurus punctatus

The microbial metabolite 2-methylisoborneol (MIB) imparts a muddy off-flavor to channel catfish Ictalurus punctatus . Uptake and depuration of MIB from fish are important considerations in the design and implementation of systems to remove off-flavors from fish prior to processing. The kinetics of MIB uptake by channel catfish were determined by placing fish in 6.5, 14.0, 25.0, and 34.0 C water containing approximately 1.0 μg/L chemically-synthesized MIB. Fish were sacrificed following 0, 2, 4, 8, and 24 h exposure to MIB. Fillet tissue samples were subjected to gas chromatographic and fat content analysis. The model for MIB uptake was:
MIB in fillet tissue (μg/kg) =−0.61 ± 4.2 [log( h ± 1)] ± 0.0076( T ) ( h ) ± 0.089( T ),
where h is the duration of exposure to MIB in hours and T is the water temperature in degrees C. The model accounted for 74% of the total variation observed in the tissue MIB concentrations and indicated that the fillet fat content was not strongly correlated with MIB uptake. To investigate the depuration of off-flavors, exposure to MIB was halted. Tissue samples were obtained 4, 8, 24, 48, and 72 h from fish held at 6.5, 14.0, 25.0, and 34.0 C. These data yielded the model:
MIB in fillet tissue (μg/kg) = 3.6 ± 0.176( T ) – 2.06 [log( h ± 1)] – 0.00296( T ) ( h ) ± 0.197 (% fat),
where h is the duration of exposure to MIB in hours, T is the water temperature in degrees C, and % fat is the % fat in the fillet tissue. The model accounted for 67% of the total observed variation in tissue MIB concentrations. Reducing the fat content of fish and optimizing water temperatures may augment MIB removal from fish tissues prior to processing.     

Pulse versus continuous peracetic acid applications: Effects on rainbow trout performance,biofilm formation and water quality

Peracetic acid (PAA) products are being introduced to aquaculture as sustainable disinfectants. Two strategies are used to apply PAA: high dose pulse applications, or low dose continuous application. In the present study, their impacts on fish health and water quality were investigated in triplicate flow-through tanks stocked with rainbow trout. The gentler and shorter water cortisol increase measured along twice-per-week pulse applications of 1 mg L⁻¹ PAA indicated a progressive adaptation of fish. In contrast, the continuous application of 0.2 mg L⁻¹ PAA caused no stress to fish. Meanwhile, no mortality and no impact on growth or innate cellular immunity were observed. The pulse applications restricted biofilm formation, and partially inhibited nitrification. Additionally, the highest oxygen concentration and stable pH were observed. In contrast, the continuous application promoted biofilm formation, and caused a pH increase and intermediate oxygen concentration. The contrast was probably due to different susceptibility of microbes to PAA-induced oxidative stress. To summarize, pulse PAA applications cause minor stress in fish, but have advantages over continuous application by ensuring better water quality.     

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.     

Water delivery capacity of a vacuum airlift – Application to water recycling in aquaculture systems

A study was undertaken to measure the water flow (Q_w) delivered by a vacuum airlift designed for recirculating aquaculture systems (RAS) in fresh (<1‰ of salinity) and sea water (35‰ of salinity). The vacuum airlift consists of two concentric tubes connected at their top to a depression chamber. The water rises in the inner tube as a result of air being injected in its lower section and flows back through the external downcomer tube. The vacuum airlift was adjusted at three different lengths: 2, 4 or 6 m and water discharge could be lifted from 0 to 30 cm. Air flow rate (Q_g) varied from 0 to 80 L min⁻¹. Different types of air injectors were tested, delivering different bubble sizes (0.1–5 mm) depending on porosity and functioning at low or high injection pressure. Results show an increase in water flow when pipe length and air flow were increased and lift height reduced. Water flow also depended on the type of water and ranged from 0 to 35 m³ h⁻¹ (0–580 L min⁻¹) for fresh water and only from 0 to 20 m³ h⁻¹ (0–330 L min⁻¹) for sea water (for a 6 m high vacuum airlift). This difference was attributed to the smaller bubble diameter and higher gas holdup (ɛ_g) observed in sea water (0–20%) compared to fresh water (0–10%). When bubbles were present in the downcomer tube, they created a resistance to flow (counter-current airlift) that slowed down liquid velocity and thus water flow. Increasing the vacuum made it possible to use low air injection pressures and high injection depths. Vacuum also increased bubble size and airflow (20 L min⁻¹ at atmospheric pressure to 60 L min⁻¹ at 0.3 barA) and thus water flow rates. With RAS, the presence of fish feed in water rapidly increased water flow delivered by the airlift because of changes of water quality and gas holdup. When working with low head RAS (under 0.3 m), vacuum airlift could save up to 50% of the energy required for centrifugal pumps. An empirical predictive model was developed and calibrated. Simulation shows a good correlation between predicted values and measurements (R² = 0.96).     

Seasonal Trends in Flavor Quality of Channel Catfish,Ictalurus punctatus,from Commercial Ponds in Mississippi

Seasonal trends in types and intensities of fish flavors were determined for channel catfish, Ictalurus punctatus, from 10 ponds located on a commercial farm in west-central Mississippi. Fish were sampled bimonthly from July-September, 1990 and monthly from October, 1990-July, 1991. Sensory evaluation indicated that, on a yearly basis, 19% of ponds could be considered on- flavor. Off-flavors were strongest from July through September when 76% of ponds had fish tainted by 2-methylisoborneol (MIB). In nine ponds, the onset of MIB off-flavor episodes could be correlated with the presence of an MIB-producing cyanobacterium Oscillatoria chalybea. In seven ponds, fish lost the MIB off-flavor within 3 to 8 weeks after the fist date in September that 0. chalybea was noticed to be absent from the water. In June, 0. chalybea reappeared and caused MIB off-flavors in eight ponds. During the winter and spring, ponds were affected by woody (22%) and decay (25%) offflavors. The most prevalent algal species-Raphidiopsis brookii, 0. aaardhii. and Microcystis aeruginosa-were not associated with off flavor problems. On average, MIB off-flavor episodes lasted for 115 days (range: 27-344), and seven ponds were considered free of off-flavors for an average of 74 days (range: 14-140) in October, November, and December.     

Identification of conditions underlying production of geosmin and 2-methylisoborneol in a recirculating system

Geosmin and 2-methylisoborneol (MIB) are semi-volatile terpenoid compounds produced as secondary metabolites by benthic and planktonic cyanobacteria, several genera of fungi, and various actinomycetes. These off-flavor compounds pose a heavy economic burden in the aquaculture industry rendering fish unmarketable unless purified by purging with large quantities of clean water. In the present study, the presence of off-flavor compounds was examined in a recirculating aquaculture system (RAS) for tilapia culture. In this zero-discharge system, where water from the fish basins is recirculated through parallel aerobic (drum filter and a trickling filter) and anaerobic treatment loops (sedimentation/digestion basin), concentrations of geosmin and, in particular, MIB were highest in the aerobic treatment loop. Lowest concentrations were detected in the anaerobic treatment loop. This latter finding pointed toward a possible reduction of these compounds in this basin. Two bacterial strains of the streptomycetes family were isolated from the aerobic, organic-rich, drum filter and the nitrifying trickling filter. In vitro tests with these isolates, closely related to Streptomyces roseoflavus and Streptomyces thermocarboxydus, revealed that MIB production exceeded geosmin production under all conditions tested and was significantly higher under aerobic than under anoxic conditions. Under the latter conditions, with nitrate as an electron donor, the S. roseoflavus-like isolate was capable of denitrification. Based on the results obtained in this study, it was concluded that aerobic, organic-rich conditions stimulate the growth of actinomycetes and subsequent production of geosmin and MIB in the system. The observed reduction of these compounds in the anaerobic water treatment component may serve in designing treatment steps aimed at alleviating the problem of geosmin and MIB accumulation in recirculating systems.     

Physiology Approaches to the Management of Off-Flavors in Farm-Raised Channel Catfish,Ictalurus punctatus

To obtain optimal yields of channel catfish, Ictalurus punctatus, large quantities of feed are added to ponds. Nutrients released from feed support dense algal and bacterial populations. Although some microbes produce oxygen and remove wastes, certain taxa produce the muddy/earthy off-flavor metabolites, 2-methylisoborneol (1-R-exo-1, 2, 7, 7-tetramethyl-bicyclo-[2, 2, 1]-heptan-2-ol) (MIB) and geosmin (1α, 10β-dimethyl-9α-decalol). Currently, off-flavors are one of the biggest problems affecting the channel catfish industry. Fish exposed to water containing either geosmin or MIB rapidly concentrations of these compounds in their tissues. Conversely, fish placed in water free of off-flavor metabolites exhibited markedly reduced concentrations of MIB after 8 hours and continued to improve in flavor quality throughout 24 hours, indicating a progressive purging or clearing of off-flavor compounds from their tissues. Relatively lean (< 2.5% fat) fish lost MIB more rapidly than fish with greater fat contents (> 2.5% fat). This paper proposes that aquaculture production systems should be managed for maximum production efficiency and yields, and that fish containing off-flavors then could be purged in special facilities. Purging systems that rely on a constant flow require large amounts of water and may not be widely practical. Systems that recirculate water may be more feasible. However, biological filters and other components of recirculating systems may become sources of off-flavors. The early detection of off-flavor-producing taxa and the competitive exclusion of problematic populations may be useful in preventing off-flavor production in recirculating purging systems.     

Evaluating the effects of prolonged peracetic acid dosing on water quality and rainbow trout Oncorhynchus mykiss performance in recirculation aquaculture systems

Peracetic acid (PAA) is an effective disinfectant/sanitizer for certain industrial applications. PAA has been described as a powerful oxidant capable of producing water quality benefits comparable to those expected with ozone application; however, the water oxidizing capacity of PAA in aquaculture systems and its effects on fish production require further investigation, particularly within recirculation aquaculture systems (RAS). To this end, a trial was conducted using six replicated RAS; three operated with semi-continuous PAA dosing and three without PAA addition, while culturing rainbow trout Oncorhynchus mykiss. Three target PAA doses (0.05, 0.10, and 0.30 mg/L) were evaluated at approximately monthly intervals. A water recycle rate >99% was maintained and system hydraulic retention time averaged 2.7 days. Rainbow trout performance metrics including growth, survival, and feed conversion ratio were not affected by PAA dosing. Water quality was unaffected by PAA for most tested parameters. Oxidative reduction potential increased directly with PAA dose and was greater (P < 0.05) in RAS where PAA was added, indicating the potential for ORP to monitor PAA residuals. True color was lower (P < 0.05) in RAS with target PAA concentrations of 0.10 and 0.30 mg/L. Off-flavor (geosmin and 2-methylisoborneol) levels in culture water, biofilm, and trout fillets were not affected by PAA dosing under the conditions of this study. Overall, semi-continuous PAA dosing from 0.05-0.30 mg/L was compatible with rainbow trout performance and RAS operation, but did not create water quality improvements like those expected when applying low-dose ozone.     

Hydrogen peroxide oxygenation and disinfection capacity in recirculating aquaculture systems

Hydrogen peroxide (H₂O₂) treatment is an alternative for disinfection in aquaculture, which may be advantageous as it dissociates and disinfects while increasing water oxygen concentration. Yet, accurate dosing remains undeveloped in Recirculating Aquaculture Systems (RAS). Dosage requirements can depend on organic burden, stocking density, feeding frequency, salinity, temperature and biofilter performance. The present case study investigated the dual effect of H₂O₂ application for oxygen enrichment and disinfection when continuously applied to a RAS rearing European seabass. H₂O₂ addition equivalent to 2.4 and 15.8 H₂O₂ mg L⁻¹ were applied for 4 h per day in three 5-days experiments. H₂O₂ was injected at the inlet of protein skimmer and/or the rearing tanks in or without combination with traditional disinfection methods. Water microbial load and oxygen saturation were determined, along with stress markers glucose and cortisol in blood plasma of fish. Doses of 15.8 mg L⁻¹ H₂O₂ steadily increased oxygen levels in holding tank water from ∼50 % to over 100 % saturation while reducing microbial load (from 604.4 CFU ml⁻¹ in the rearing tanks before dosing to 159.8 CFU ml⁻¹ after application), achieving suitable conditions for commercial fish densities in RAS. The doses used had negligible impact on biofilter performance and did not affect the fish in terms of stress. Overall results indicate H₂O₂ is effective for disinfection and oxygenation of RAS systems when applied at appropriate dosage and we recommend the protein skimmer as the safest position in order to protect the bacterial community of the biofilters and the reared fish.     

Relevance of dietary oil source for contraction and quality of pre-rigor filleted Atlantic cod, Gadus morhua

Rigor contraction and quality development were studied during storage of pre-rigor fillets of farmed Atlantic cod (body weight 2 kg). For eight weeks prior to slaughtering, the fish were fed diets containing either 100% fish oil (M-group) or 60% fish oil and 40% soybean oil (S-group). Filleting was performed 0.5 h after slaughtering and the fillets were stored at 6 °C for 48 h post-mortem. The contraction rate was faster for the S-group, but after 48 h storage the contraction was 21% of the initial fillet length for both fish groups. The ATP content analysed 1 h post-mortem was lower of the S-group (4.5 μm g^− 1 ww) compared with the M-group (6.1 μm g^− 1 ww). After 16 h, the ATP content was similar for both fish groups (1.7-2.2 μm g^− 1 ww). The muscle pH analysed 1 h post-mortem was 0.2 unit lower of the S-group (pH 6.9) than of the M-group (pH 7.1). From 8 to 12 h, the pH fell significantly to 6.3 and 6.6 for the S-group and M-group, respectively. From 24 h and onwards, the pH was similar of both fish groups (pH 6.2). Texture development differed significantly due to dietary treatment. The S-group had highest initial breaking force, demonstrating that the texture of this group was firmer immediately after slaughtering. The breaking force of the S-group dropped to a stable level after 4 h, whereas the breaking force of the M-group decreased continuously during the storage period. The breaking force was similar of both groups after 16 h storage. The L*-value (lightness) increased during the first 4-6 h storage. From 8 to 16 h, the L*-value was stable or slightly decreasing. The L*-value was consistently higher of the M-group. In conclusion, dietary inclusion of soybean oil resulted in faster energy depletion post-mortem, faster rigor contraction, faster reduction of the breaking strength and lower L*-values.     

Effect of high-level fish meal replacement by plant proteins in gilthead sea bream (Sparus aurata) on growth and body/fillet quality traits

Juvenile gilthead sea bream (initial body weight ca. 100 g) were reared in an indoor flow through marine water system for 1 year. Fish were fed two isoenergetic [19.2 kJ g⁻¹ dry matter (DM)] and isoproteic (426 g kg⁻¹ DM) diets either based on fish meal (diet FM) or on a mixture of plant protein sources (diet PP), replacing 75% of fish meal protein. The growth trial was conducted in duplicate, two tanks for each dietary treatment. Growth performance and feed utilization were registered. Fillet quality parameters were evaluated and sensory analyses on cooked fillet were performed. Both groups had similar weight gain and specific growth rates. Feed intake was higher in sea bream fed diet FM (0.48 versus 0.44), while feed efficiency and protein efficiency ratio were significantly higher in sea bream fed PP (0.83 versus 0.77 and 2.0 versus 1.76, respectively). Sea bream fed diet FM had a lower hepatosomatic index (0.80 versus 0.87%), and a higher fillet yield (45.9 versus 44.9%). The fillet from sea bream fed diet FM had higher moisture (696 versus 682 g kg⁻¹), lower lipid levels (91 versus 100 g kg⁻¹) with higher levels of n‐3 polyunsaturated fatty acids (PUFA) and monounsaturated fatty acids (MUFA), while the PP fed sea bream presented a higher level of PUFA n‐6. There were minor differences in muscle free amino acid levels between the two diet groups. As regards sensory evaluation of cooked fillet, the judges were unable to discriminate the two dietary groups of fish. Summarizing, the results demonstrate the possibility to use diets containing high levels (750 g kg⁻¹) of plant ingredients in gilthead sea bream without affecting growth performance and with minor effects on quality traits of commercial size sea bream.     

Microbial valorization of solid wastes from a recirculating aquaculture system and the relevant microbial functions

This study was performed to establish valorization technology for solid wastes from a seawater recirculating aquaculture system (RAS) by using beneficial microorganisms. An efficient microbial agent (KBM-1) was selected based on the degradation activity of the RAS solid wastes (20% slurry) in a lab-scale reactor system considering the removal rates of chemical oxygen demand, solid material, total nitrogen, ammonium-N, and nitrate-N and the production of organic acids as electron donors for denitrification. The microbial consortium KBM-1 was particularly efficient in the removal of ammonium-N and nitrate-N with removal efficiencies of 42% and 50%, respectively, in eight days and in the rapid production of organic acids (230 mg L⁻¹, 3.5 mM, 0.018 kg m⁻³ d⁻¹) after two days. There was a concomitant removal of NO₃--N (41%, 0.005 kg N m⁻³ d⁻¹) after two days when a significant production of organic acids occurred. Comamonas sp. was a dominant genus after eight days in all treatments. The level of nitrate-N in the treatments with KBM-1 decreased by 50.4% after eight days, as opposed to that of the control sample (27.7%), indicating the potential denitrification activity of Citrobacter freundii and Comamonas sp. The bioaugmented species (Sporolactobacillus inulinus, Lactobacillus mali, Lactobacillus casei, and Clostridium tyrobutyricum), constituting 41% of the total communities, appeared to facilitate the growth of indigenous microbial communities that were involved in the degradation (hence valorization) of solid wastes (mostly remaining fish feed and fish feces) into simple metabolites (organic acids and inorganic materials such as ammonium, nitrite, nitrate, and CO₂). The simultaneous generation of organic acids through the valorization of solid wastes and their subsequent reuse in the denitrification of an RAS biofilter system can provide a significant contribution to the eco-friendly management of RASs and provide meaningful economic merit to the solid wastes of RASs.     

Application of biofloc technology for Indian major carp culture (polyculture) along with water quality management

In this study, the efficacy of biofloc system was assessed for culturing Indian major carps, e.g., rohu (Labeo rohita), catla (Catla catla), and mrigal (Cirrihinus mrigala) at varied stocking densities (STD) from 4.28, 8.57, and 12.85 fish m⁻³ of tank volume. Biofloc system could efficiently remove inorganic nitrogen from culture water and substantially improved the growth parameters of fishes as compared to the control system without the application of biofloc. Among different STDs in biofloc system, water quality and floc formation were noted to be higher with a STD of 4.28 fish m⁻³ and the average concentration of NH₄ – N, NO₂ – N and NO₃ – N of 0.61 mg L⁻¹, 0.35 mg L⁻¹ and 1.8 mg L⁻¹, respectively was recorded lowest as compared to the other STD. As a result, catla and rohu could achieve a specific growth rate of 1.1 % day⁻¹ and 0.98 % day⁻¹, respectively. However, in the other two STDs, the respective concentrations were noted in the range of acceptable water quality, and the growth rates were satisfactory. It can be concluded that the biofloc system is efficient to culture IMC in polyculture mode amd therefore, can be directly applied in scaled-up culture modules.