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.     

Water quality,waste production,and off-flavor characterization in a depuration system stocked with market-size Atlantic salmon Salmo salar

Land-based Atlantic salmon, Salmo salar, grow-out facilities utilize depuration to remediate off-flavor. Water used in this process is either discharged or repurposed as supply water in recirculating aquaculture systems (RAS). Both approaches require an understanding of water quality and waste production for water treatment decisions and compliance with pollution discharge standards; however, these data were lacking. Therefore, a study was carried out to characterize these parameters. To begin, 311 salmon (5–6 kg) originally cultured in freshwater RAS were stocked at 100 kg/m³ in an 18 m³ depuration tank. Feed was withheld 1 day before transfer and throughout the 7-day study period. Hours after stocking, total suspended solids (TSS), total phosphorus (TP), and total ammonia nitrogen (TAN) levels spiked, and concentrations declined thereafter. Delta TSS and TP were negligible by the end of the trial; however, TAN plateaued, indicating that salmon began to catabolize somatic tissue in the absence of feeding. Geosmin and 2-methylisoboreol levels in water and fish were low throughout the study. This research indicates that residual waste production occurs while depurating Atlantic salmon. Procedural refinements and recommendations were gleaned including locality for introducing depuration system water within RAS and extension of the feed withholding period before depuration.     

Effect of peracetic acid on levels of geosmin, 2-methylisoborneol,and their potential producers in a recirculating aquaculture system for rearing rainbow trout (Oncorhynchus mykiss)

In recirculating aquaculture systems (RAS)s, off-flavors and odors, mainly caused by geosmin (GSM) and 2-methylisoborneol (MIB), can accumulate in the flesh of fish from RAS water, reducing the profitability of production. In this study, peracetic acid (PAA) was applied in three application intervals to pump sumps of rainbow trout (Oncorhynchus mykiss) reared in RAS. Using a real-time polymerase chain reaction (qPCR), the potential off-flavor producers were quantified using geoA and MIB synthase genes. Streptomyces was identified as the major GSM producer, and biofilters showed the highest number of potential off-flavor producers. Concentrations of GSM and MIB were analyzed in the circulating water and in the lateral part of the fish fillet. In water, concentrations up to 51 ng L⁻¹ (GSM) and 60.3 ng L⁻¹ (MIB) were found, while in the fillet, these were up to 9.8 ng g⁻¹ (GSM) and 10.2 ng g⁻¹ (MIB), decreasing with increasing number of PAA applications. PAA applications reduced the levels of off-flavor compounds, although this was insufficient to fully prevent the accumulation of GSM and MIB.     

Geosmin causes off‐flavour in arctic charr in recirculating aquaculture systems

The ‘earthy’ and ‘muddy’ off‐flavours in pond‐reared fish are due to the presence of geosmin or 2‐methylisoborneol in the flesh of the fish. Similar off‐flavours have been reported in fish raised in recirculating aquaculture systems (RAS); however, little information is available regarding the cause of these off‐flavours. Our hypothesis was that earthy and muddy off‐flavour compounds, found previously in pond‐raised fish, are also responsible for off‐flavours in fish raised in RAS. In this preliminary study, we examined water, biofilms in RAS and fillets from cultured arctic charr known to have off‐flavours and requiring depuration using instrumental [solid‐phase microextraction procedure and gas chromatograph‐mass spectrometry (GC‐MS)] and human sensory analyses. Geosmin was present in the samples taken from the biofilter and on the side walls of the tanks. Two‐methylisoborneol was only found in low levels in the samples. The GC‐MS results indicated the presence of geosmin in the fillets (705 ng kg^?1), but lower levels were found in the water (30.5 ng L^?1). Sensory analyses also detected an earthy flavour (i.e., geosmin presence) in the fillets, and, therefore, it appears that geosmin is the main compound responsible for the off‐flavour in RAS. Further studies are being performed to identify the microorganisms responsible for geosmin production in RAS.     

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.     

The use of acoustic acceleration transmitter tags for monitoring of Atlantic salmon swimming activity in recirculating aquaculture systems (RAS)

Successful operation of recirculating aquaculture systems is dependent on frequent monitoring of the optimal function of water treatment processes in order to maintain environmental conditions for optimal growth and welfare of the fish. Real time monitoring of fish status is however usually not an integrated part of automatized systems within RAS. The aim of this study was to evaluate the use of implanted acoustic acceleration transmitters to monitor Atlantic salmon swimming activity. Twelve salmon post-smolts were individually tagged and distributed in three tanks containing salmon at start density of 50 kg m⁻³. The tagging did not cause any mortality and all individuals increased their body weight during this study. Following initial recovery, acceleration data were continuously logged for one month, including treatment periods with exposure to hyperoxic (170% O₂ saturation) and hypoxic (60% O₂ saturation) conditions, and different tank hydraulic retention times (HRT; 23 and 58 min). Changes in-tank dissolved oxygen levels to hyperoxic and hypoxic conditions reduced the total activity of Atlantic salmon in this study. On the contrary, increased and reduced tank HRT increased the total activity levels. Feeding periods induced a sharp increase in the Atlantic salmon swimming activity, while irregular feeding caused larger oscillations in activity and also lead to increased swimming activity of the tagged fish. Atlantic salmon responded with a maximum recorded total activity to stress caused by technical problems within the system and consequent changes in the RAS environment. The results of this study indicate that Atlantic salmon respond quickly with changed swimming activity to changes in the water quality and acute stress caused by normal management routines within RAS. The use of acoustic acceleration transmitters for real time monitoring of swimming activity within aquaculture production systems may allow for rapid detection of changes in species-specific behavioural welfare indicators and assist in the refinement of best management practices. In addition, acceleration tag could potentially serve as a valuable research tool for behavioural studies, studies on stress and welfare and could allow for better understanding of interaction between fish and RAS environment.     

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.     

Production of market-size European strain Atlantic salmon (Salmo salar) in land-based freshwater closed containment aquaculture systems

Interest in land-based farms using recirculating aquaculture systems (RAS) for market-size Atlantic salmon (Salmo salar) continues to grow, and several commercial facilities are already rearing fish. Performance data for commercially available mixed-sex, all-female, and triploid all-female Atlantic salmon reared to market-size in freshwater land-based facilities, however, are limited, particularly for European strain fish. Accordingly, eight groups of European-sourced Atlantic salmon (five groups of diploid mixed-sex, two groups of diploid all-female, and one group of triploid all-female fish) were reared from eyed egg to market-size in a semi-commercial scale land-based aquaculture systems over five separate production cycles to quantify performance metrics. Fish reached market-size (4−5 kg) in 24.7–26.3 months post-hatch. Fish were reared at a mean water temperature of 12.3–13.7 °C from first feeding to a mean size of 466–1265 g, then 13.3–15.1 °C during growout. On average, all-female groups grew faster than mixed-sex groups; however, environmental conditions and performance of individual cohorts varied. In a comingled production cycle, diploid all-female salmon grew faster than triploid counterparts. Early maturation rates ranged from 0 % to 67 %, with a mean maturation rate of 34 % for diploid mixed-sex fish and 67 % and 13 % for two diploid all-female groups, respectively. Triploid all-female Atlantic salmon did not mature. This research confirms biological and technological feasibility of growing Atlantic salmon to market-size in land-based systems but controlling early maturation of diploid salmon remained a challenge under the conditions utilized in these trials. This research provides important data inputs to optimize operational and financial projections for existing and potential land-based Atlantic salmon farms.     

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.     

The effects of ozonation on select waterborne steroid hormones in recirculation aquaculture systems containing sexually mature Atlantic salmon Salmo salar

Steroid hormones have been shown to accumulate in recirculation aquaculture system (RAS) water over time; however, their influence on the reproductive physiology of fish within RAS remains unknown. Whether ozonation impacts waterborne hormone levels in RAS has likewise not been fully evaluated. To this end, a controlled 3-month study was conducted in 6 replicated RAS containing a mixture of sexually mature and immature Atlantic salmon Salmo salar to determine whether ozone, as typically applied in RAS to improve water quality, is associated with a reduction in waterborne hormones. Post-smolt Atlantic salmon (1253 ± 15 g) were stocked into each RAS; 109 of 264 fish placed in each system were sexually mature males, and 5 were mature females. Water ozonation, controlled using an ORP set-point of 290–300 mV, was applied with the pure oxygen feed gas within the low-head oxygenators of 3 randomly selected RAS, while the remaining 3 RAS did not receive ozone. The RAS hydraulic retention time was 6.9 ± 0.3 days. Study fish were raised under these conditions for 12 weeks; during weeks 10 and 12, triplicate water samples were collected from the following locations in each RAS: i) culture tank, ii) makeup water, iii) pre-biofilter, iv) post-biofilter, and v) post-gas conditioning. Concentrations of 3 waterborne hormones – testosterone, 11-ketotestosterone (11-KT), and estradiol (17β-estradiol) – were quantified using enzyme immunoassays (EIA). Estradiol was significantly reduced by ozonation; testosterone and 11-KT were also reduced by ozonation, although these reductions were not observed across all sampling locations and events. Testosterone and 11-KT concentrations, however, were significantly reduced following water passage through the biofilters of both ozonated and non-ozonated RAS. The results of this study demonstrate the potential for ozone to be used in RAS as a means of preventing the accumulation of steroid hormones. Further research is required to assess whether reducing hormones in this manner impacts precocious sexual maturation in RAS-produced Atlantic salmon.     

Evaluating the microbial effects of stocking freshwater snails (Physa gyrina) in water reuse systems culturing rainbow trout (Oncorhynchus mykiss)

A study was conducted to analyze the effects of snails (Physa gyrina) on biofilm, bacterial abundance, off-flavor-producing bacteria, and off-flavor compounds in reuse aquaculture systems culturing rainbow trout (Oncorhynchus mykiss). Eight experimental-scale systems were used, including four with and without snails. Mean heterotrophic bacteria counts in water were lower (P < 0.05) in systems with snails. Submerged surfaces of sumps containing snails were nearly biofilm-free, while sumps without snails were coated with biofilm. Geosmin levels in trout fillets from snail-stocked systems were generally lower but not statistically different from the controls. Rainbow trout health and performance was not affected by snail presence.     

The effects of long-term 20 mg/L carbon dioxide exposure on the health and performance of Atlantic salmon Salmo salar post-smolts in water recirculation aquaculture systems

Previous research and experience has linked elevated dissolved carbon dioxide (CO₂) to reduced growth performance, poor feed conversion, and a variety of health issues in farm-raised fish, including Atlantic salmon Salmo salar. Supplemental control measures in water recirculation aquaculture systems (RAS) to reduce CO₂ accumulation, however, such as increased water pumping to decrease tank hydraulic retention time, can represent significant costs for operators. We exposed post-smolt S₀ Atlantic salmon (197 ± 2 g, 423 days post-hatch) to either high (20 ± 1 mg/L) or low (8 ± <1 mg/L) dissolved CO₂ in six replicated freshwater RAS for 384 days to investigate differences in performance and health as the salmon were grown to harvest size. All RAS were operated at moderate water exchange rates (1.0% of the total recirculating flow), a 24-h photoperiod was provided, fish were fed to satiation, and densities were maintained between 40 and 80 kg/m³. Over the study period, dissolved oxygen was kept at saturation, mean water temperature was 14.1 ± 0.1 °C, and alkalinity averaged 237 mg/L as CaCO₃. At study’s end, no significant differences in fish weight (high CO₂ mean weight = 2879 ± 35 g; low CO₂ mean weight = 2896 ± 12 g), feed conversion ratio (1.14 ± 0.12 vs. 1.22 ± 0.13, respectively), or thermal growth coefficient (1.45 ± 0.01 vs. 1.46 ± 0.01, respectively), were observed. No significant differences in survival (high CO₂ mean survival = 99.1 ± 0.4%; low CO₂ mean survival = 98.9 ± 0.3%) or culls due to saprolegniasis (3.5 ± 1% vs. 3.0 ± 1%, respectively) were determined, and no nephrocalcinosis was observed through histopathological evaluation. Blood gas and chemistry evaluation revealed higher pCO₂, bicarbonate, and total CO₂, and lower chloride and glucose, in the high CO₂ cohort. Molecular analyses of gill enzyme regulation showed significantly higher expression of Na⁺/K⁺ ATPase α1a in high CO₂ fish at 3-weeks post-challenge, indicating physiological adaptation to the higher CO₂ environment without any noticeable long-term impacts on health or performance. Overall, the results of this study suggest that, at 237 mg/L as CaCO₃ mean alkalinity, post-smolt Atlantic salmon can be raised in freshwater RAS to harvest size with up to 20 mg/L CO₂ without significantly impacting fish health and performance.     

Fillet Quality and Processing Attributes of Postsmolt Atlantic Salmon,Salmo salar,Fed a Fishmeal‐free Diet and a Fishmeal‐based Diet in Recirculation Aquaculture Systems

Many studies have evaluated the adequacy of alternate ingredient diets for Atlantic salmon, Salmo salar, mainly with focus on fish performance and health; however, comprehensive analysis of fillet quality is lacking, particularly for salmon fed these diets in recirculation aquaculture systems (RAS). To this end, a study was conducted comparing fillet quality and processing attributes of postsmolt Atlantic salmon fed a fishmeal‐free diet (FMF) versus a standard fishmeal‐based diet, in replicate RAS. Mean weight of Atlantic salmon fed both diets was 1.72 kg following the 6‐mo trial and survival was >99%. Diet did not affect (P > 0.05) processing and fillet yields, whole‐body proximate composition(fat, moisture, protein), fillet proximate composition, cook yield, fillet texture, color, or omega‐3 fatty acid fillet content, including eicosapentaenoic acid and docosahexaenoic acid levels. Whole‐body ash content was greater in salmon fed the FMF diet. The FMF diet resulted in a wild fish‐in to farmed fish‐out ratio of 0:1 per Monterey Bay Aquarium's Seafood Watch criteria due to its fishmeal‐free status and use of lipids from fishery byproduct. Overall, fillet quality and processing attributes were generally unaffected when feeding a diet devoid of fishmeal to postsmolt Atlantic salmon cultured in RAS. [Correction added on 7 September 2017, after first online publication: the P value in Abstract has been changed from “P < 0.05” to “P > 0.05”.].     

The use of additional lighting and artificial photoperiods to recondition early maturing Atlantic salmon (Salmo salar) in Tasmania

Salmonid culture in Tasmania experiences high rates of early maturation due to the rapid growth rates achieved under favorable environmental conditions. The present study investigates the use of artificial photoperiod to recondition previously mature Atlantic salmon to provide a larger fish with higher market value. Individual groups of previously mature female Atlantic salmon (Salmo salar) were put under 24 hour artificial light from either May, June, July or maintained under ambient conditions during the austral winter. Fish maintained under lights attained a reversion from dark skin pigmentation, typical of mature individuals, to a silver sea-going coloration more rapidly than the control group. In addition flesh pigmentation, ovarian absorption and body wall thickness was significantly greater than that of the control fish resulting in a higher market price. The study demonstrates that artificial lights can facilitate the reconditioning of previously mature Atlantic salmon under southern hemisphere conditions.     

Integrating activated sludge membrane biological reactors with freshwater RAS: Preliminary evaluation of water use,water quality,and rainbow trout Oncorhynchus mykiss performance

Onsite research indicates that activated sludge membrane biological reactors (MBRs) are an effective waste treatment technology for aquaculture effluents. MBRs produce a filtered permeate that is nearly free of dissolved nutrients, organics, and solids; therefore, this technology could be well-suited for integration within the process control loop of recirculation aquaculture systems (RAS). A four-month study was carried out to evaluate the feasibility of incorporating single-vessel MBRs within freshwater RAS while culturing rainbow trout Oncorhynchus mykiss. Triplicate RAS with and without MBRs (controls) were evaluated; mRAS and cRAS, respectively. System backwash water of mRAS was processed and retained within MBRs which allowed increased water recycling, while cRAS utilized standard dilution rates to limit nitrate accumulation. On average, mRAS required six and a half times less makeup water. Mean daily water replacement of the RAS volume for mRAS and cRAS was 1.2 ± 0.4 and 7.8 ± 0.5%, respectively (P < 0.05). A range of water quality concentrations were significantly greater in mRAS including chloride, carbon dioxide, heterotrophic bacteria count, pH, nitrate-nitrogen, total ammonia-nitrogen, total phosphorous, and true color, as well as dissolved concentrations of calcium, copper, magnesium, and sulfur. Alkalinity and ultraviolet transmittance levels were significantly lower in mRAS. These culture environment differences did not affect rainbow trout growth, feed conversion, or survival (P > 0.05). In addition, concentrations of common off-flavor compounds (geosmin and 2-methylisoborneol) in water and fish flesh were not affected by MBR presence. Improvements for future MBR integration with RAS were realized including optimization of MBR permeate rates, increased RAS water exchange through the MBRs, and infrequent supplementation of a carbon source to enhance denitrification efficiency and alkalinity recovery. Overall, incorporating MBRs within RAS resulted in substantial water savings and was biologically feasible for rainbow trout production.     

Mercury comparisons between farmed and wild Atlantic salmon (Salmo salar L.) and Atlantic cod (Gadus morhua L.)

Wild and farmed Atlantic salmon ( Salmo salar L.) and Atlantic cod ( Gadus morhua L.) were collected to assess changes in mercury with size in wild vs. farmed fish. Mercury concentrations were compared with Health Canada and United States Environmental Protection Agency consumption guidelines. Lipid dilution of mercury was examined by comparing lipid-extracted (LE) and non-lipid-extracted (NLE) flesh samples in both farmed and wild fish. Mercury concentrations in the flesh and liver of farmed salmon were significantly lower than concentrations in wild salmon of similar fork length ( P <0.001), possibly due to growth dilution in rapidly growing farmed fish. Mercury concentrations were higher in LE tissue compared with NLE ( P <0.05), suggesting lipid dilution of mercury in farmed fish with a high lipid content. Farmed cod, which do not grow more rapidly than wild cod, did not have significantly different flesh and liver concentrations compared with wild cod of similar fork length ( P >0.05). Between species of farmed fish, cod had significantly higher mercury concentrations than salmon ( P <0.05), but neither farmed nor wild salmon mercury concentrations exceeded federal consumption guidelines. These results suggest that rapid growth rates and a high lipid content may play important roles in regulating concentrations of contaminants such as mercury.     

Depuration and starvation improves flesh quality of grass carp (Ctenopharyngodon idella)

Farmed grass carp (Ctenopharyngodon idella) at commercial size were transported to a natural lake for long‐term depuration while being food deprived. The effect of depuration time on the quality of fish fillets was investigated based on proximate compositions, textural parameters and flavour characteristics. The results showed that protein and lipids, but not carbohydrates, were the major source of energy for grass carp during depuration and starvation. Textural parameters that included hardness, springiness, gumminess and cohesiveness increased significantly after depuration, as well as water‐holding capacity of fish muscle. Taste and odour characteristics of grass carp muscle were obviously changed by depuration based on tests by an electronic tongue and nose. Off‐flavour volatile compounds, such as nonanal and hexanal, were reduced after depuration. In conclusion, the quality of grass carp fillets was improved effectively by long‐term depuration and food deprivation. More than 20 days of depuration was appropriate for the enhancement of grass carp quality before marketing.     

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.     

大西洋鲑、三倍体虹鳟和金鳟的肌肉营养成分与品质特性

为了解大西洋鲑(Salmo salar)、三倍体虹鳟(Oncorhynchus mykiss)、金鳟(Oncorhynchus mykiss)3种鱼肌肉营养成分和品质特性,利用生化分析、物性分析方法分析3种鱼肌肉的营养成分、氨基酸和脂肪酸组成、肉色、系水力和质构特性。结果表明,大西洋鲑、三倍体虹鳟、金鳟肌肉的水分质量分数分别为62.91%、67.15%、73.02%,粗蛋白质量分数分别为22.39%、21.03%、22.11%,粗脂肪质量分数分别为14.64%、17.16%、5.11%。3种鱼肌肉的滴水损失、黄色值(b~*)、羟脯氨酸含量、内聚性均显著不差异(P0.05)。3种鱼肌肉的硬度和咀嚼性由低到高依次为大西洋鲑、三倍体虹鳟、金鳟,而pH值的结果则与之相反(P0.05)。大西洋鲑和三倍体虹鳟肌肉的灰分、解冻损失、蒸煮损失、回复性、弹性和红色值(a~*)差异不显著(P0.05),但灰分、蒸煮损失、回复性均小于金鳟肌肉的对应指标(P0.05),弹性和红色值(a~*)则均大于金鳟肌肉的对应指标(P0.05)。大西洋鲑、三倍体虹鳟、金鳟肌肉中必需氨基酸含量占氨基酸总量分别为42.28%、41.84%、41.63%(质量分数),必需氨基酸/非必需氨基酸(EAA/NEAA)比值分别为73.25%、71.94%、71.32%,均符合联合国粮农组织/世界卫生组织(FAO/WHO)对优质蛋白质的评价标准;3种鱼肌肉中均检测到22种脂肪酸,组成丰富,其中不饱和脂肪酸含量较高。综上所述,3种鱼的肌肉都是符合人体营养需求的优质水产品,其中大西洋鲑和三倍体虹鳟肉质接近,且都优于金鳟的肉质。     

Performance and welfare of Atlantic salmon,Salmo salar L. post-smolts in recirculating aquaculture systems: Importance of salinity and water velocity

Producing a larger post-smolt in recirculating aquaculture systems (RAS) could shorten the production time in sea cages and potentially reduce mortality. Knowledge of the biological requirements of post-smolts in closed-containment systems is however lacking. In the present study, the effects of salinity and water velocity on growth, survival, health, and welfare of Atlantic salmon reared in RAS were examined. Salmon smolts were stocked in three separate RAS with salinities of 12, 22, and 32‰ and subjected to high (1.0 body lengths per s⁻¹) or low (0.3 body lengths second⁻¹) water velocity. Growth performance, survival, welfare, and physiological stress responses were monitored until the fish reached a bodyweight of around 450 g. Growth rate was higher at lower salinity and higher water velocity generally had a positive effect on growth in all salinities. Feed conversion ratio was lower at 12‰ compared to the 22 and 32‰ when the fish were between 250 and 450 g. Higher mortality, elevated plasma cortisol levels, higher incidence of cataract, and a higher expression of stress-induced genes in the skin (iNOS, Muc5ac-like) indicated a negative effect of higher salinity on fish welfare. Male maturation was low (<1%), and not affected by salinity or water velocity.