Problems affecting nitrification in commercial RAS with fixed-bed biofilters for salmonids in Chile

The present case study focused on the problems that affect the nitrification process at three commercial recirculating aquaculture systems (RAS) for salmonids with fixed-bed biofilters operating in Chile, where the main factors were found to be management problems: (1) large variations in daily feeding, which results in unstable nitrogenous compounds (TAN, NO₂⁻, NO₃⁻) concentration; (2) variable daily water exchange, producing unstable culture conditions (variations in pH and temperature); (3) high densities of culture, which results in overall bad culture conditions (high CO₂ concentration, high amount of fine solids, high oxygen consumption). When properly managed, the RAS have proven to tolerate up to 15% of daily variation in feeding, as low as 10% of daily “new” water inlet, and densities as high as 60 kg fish/m³ without showing any nitrification problems. The results from this study demonstrates that maintaining good water quality is essential to secure an efficient growth of both the target species and the nitrifying bacteria, therefore, the production strategies should consider both the target species and the nitrification process requirements.     

Growth and diet of anadromous Arctic charr after their return to freshwater

Abstract – The post smolt of anadromous Arctic charr ( Salvelinus alpinus ) from Lake Storvatn, northern Norway (70°39'48"), continued to feed and grow after their return to freshwater in late summer, but in late autumn (October) both their growth rate and feeding intensity ceased along with the drop in water temperature. In contrast, adult, sexually mature anadromous charr did not appear to feed during their freshwater residency. The stomach fullness of post smolts were similar to that of resident Arctic charr during early autumn, and their diets were similar, being dominated by pelagic prey such as surface insects, Holopedium sp., Bosmina sp. and chironomid pupae. Later in the autumn, the resident charr also included some more benthic prey, in particular gastropods, whereas the post smolts continued to feed predominantly on zooplankton and surface insects, but with a lower stomach fullness than the resident fish. This suggests that after arrival in freshwater, the post smolts continue their pelagic feeding behaviour displayed during the sea-migration.     

Comparative growth study of wild- and hatchery-produced Arctic charr (Salvelinus alpinus L.) in a coldwater recirculation system

The growth performance of Arctic charr of wild (W) and hatchery (H) origin was compared in a commercial coldwater recirculation system (Villmarksfisk, Bardu, 68°N, 19°E, Norway). The initial individual body mass was 115 g and similar between groups. The rearing temperature was 9.2 °C and the fish were held under continuous light (24:0 L:D). At the end of the experiment (day 240), the average body mass of the H fish was 451 g compared to 231 g in the W fish. The accumulated mortality of wild Arctic charr was about 40%; 10 times higher than the mortality of hatchery-produced Arctic charr (4%). The difference in growth performance and survival rate impose a great disadvantage of using this wild caught fish as compared to commercially available hatchery-produced Arctic charr in coldwater recirculation system. However, further improvements in the production chain (catching, live transport, quarantine, size grading, etc.) may still make production of wild caught Arctic charr profitable, especially as it demands a higher price in niche markets.     

Important influent-water quality parameters at freshwater production sites in two salmon producing countries

Further growth in Atlantic salmon (Salmo salar) aquaculture production is expected, and as a response to limited freshwater resources, recirculating aquaculture systems (RAS) are increasingly applied in smolt production. Knowledge of the general composition and quality of inlet-water is important for designing water-treatment to obtain optimal water quality in both flow-through and RAS systems. Based on water quality surveys in Norway (96 water sources, 1999–2006) and Chile (120 water sources, 2006–2008) inlet-water quality was evaluated. Norwegian smolt production is characterized by almost exclusively utilizing surface waters as inlet-water sources, with lake inlets constituting 88% and river inlets 12%. This results in large seasonal variations in both temperature, and inlet-water quality. In Chile, production is based on inlet-water from groundwater wells (32%), natural springs (40%) and rivers (28%). Norwegian inlet-water quality shows significantly lower pH and buffering capacity. The content of total organic carbon and total nitrogen is generally higher in Norway, while the levels the main metals of concern, aluminium (Al) and iron (Fe), show large between-site variability in both countries. In low pH waters in Norway, the concentration of inorganic (labile) aluminium exceeds recommended level (10 μg/L) in 15% of the samples. The Norwegian database documents highly variable production intensity in smolt production. The measured levels of carbon dioxide (CO₂, 11.6 ± 6.2 mg/L) and total ammonia nitrogen (TAN, 499 ± 485 μg N/L) (mean ± SD), exceed current legislative recommendations in 30% and 10.5% of the cases, respectively. RAS technology has the potential to improve a variable water quality if it proves reliable for the time intervals and production volumes needed. Thus, if necessary adjustments in water treatment to the local water quality are implemented, RAS production may well constitute a substantial part of smolt production in the future.     

Evaluation of brook trout production in a coldwater recycle aquaculture system

Brook trout (Salvelinus fontinalis) are a commercially important coldwater species reared in Wisconsin and the Midwestern United States. Brook trout are raised by private, tribal, state, and federal fish hatchery facilities in Wisconsin. Approximately 10% of private coldwater aquaculture operations are presently raising brook trout of various strains for stocking uses and a limited amount for food markets. Growing brook trout to a larger size, if they can be reared in a shorter time span, may present a potential new sector for the aquaculture market in the Midwestern US. The present study reports hatchery production attributes, i.e., growth, survival, fin condition, feed efficiency, water chemistry requirements and general husbandry of Lake Superior strain (Nipigon) brook trout reared in a recirculating aquaculture system (RAS), operated at an average temperature of 13 °C. The recycle system at NADF reared 1379 kg of brook trout over a 10-month period from fingerling (9 g) to market size (340–454 g). The trout grew faster (0.84 g/day and 0.64 mm/day) in the RAS than fish cultured in traditional flow-through tank culture utilizing ground water at 7.6 °C (0.14 g/day and 0.35 mm/day). Final average weight of RAS fish was 260 g, while the flow-through fish averaged 65 g. Final tank densities for the RAS averaged 40.4 kg/m³ while flow-through tanks averaged 31.2 kg/m³. Throughout the project, feed conversions in the RAS ranged from 0.9 to 1.3. Water quality variables such as TAN, nitrite, DO, temperature, TSS, CO₂, ph, etc. were within safe limits for brook trout and will be discussed. It does appear from this initial research project that market size brook trout can be raised successfully in a recycle system within a similar time frame as a rainbow trout produced in a Wisconsin typical flow-through facility.     

Assessment of “discreet” vertebral abnormalities, bone mineralization and bone compactness in farmed rainbow trout

The present study aimed to evaluate the importance of “discreet” vertebral abnormalities in normally-shaped rainbow trout in relation to vertebral bone condition in French fish farms. A total of 373 trout (262 ± 2 mm in total length) from 20 fish farms sampled were studied. The fish were radiographed and the axial skeleton examined for vertebral abnormalities. Vertebrae from the middle axial region (V32-38) were selected to evaluate vertebral bone condition. Bone mineralization (BM, %) was estimated by the ratio of ash and dry weight. Bone compactness (BC,%) was measured using Bone Profiler 3.23 images software on digitized radiographs of transverse sections (125 ± 10 μm). Statistical analyses were performed to test the relationships between the occurrence of vertebral abnormalities, and BM and BC. The occurrence of affected trout ranged from 0 to 55% depending on the farm. Trout displayed vertebrae with low BM (< 54.6%) and low BC (< 28.1%) in 40% and 55% of the farm, respectively. No relationships were observed between bone condition parameters (BM, BC) and the occurrence of vertebral abnormalities. These results could be explained by a wide and variable plastic response of bone characters (i.e., vertebral abnormalities, BM and BC) to the various rearing conditions in the fish farms sampled.     

Feed intake and growth of Arctic charr, Salvelinus alpinus (L.), fed diets supplemented with oxytetracycline and squid extract

This study examined the long-term effects of a potential feeding deterrent, oxytetracycline (OTC), and a putative feeding stimulant, squid extract, on feed intake, growth and dry matter (DM) digestibility in Arctic charr, Salvelinus alpinus (L.). Fish were fed one of four diets for 9 weeks: 1. commercial feed formulation (basic); 2. BM (basic plus 20 g kg⁻¹ OTC); 3. BMS (BM plus 10 g kg⁻¹ squid extract); 4. BS (basic plus 10 g kg⁻¹ squid extract). The feed intake of Arctic charr was not reduced by the addition of OTC to the basic feed, and addition of squid extract to the medicated feed did not enhance feed consumption. DM digestibility and growth tended to be reduced in groups of fish fed diets supplemented with the antibiotic (BM and BMS). This suggests that OTC may have interfered with nutrient digestion and absorption. OTC does not seem to be a feeding deterrent for Arctic charr, possibly indicating that the charr is a rather unselective feeder. Because the medicated feeds were readily accepted, there may be little to be gained by addition of feeding stimulants to these types of feeds for charr.     

Impact of a new artificial shelter on Arctic charr (Salvelinus alpinus, L.) behaviour and culture performance during the endogenous feeding period

Shelter is of major importance in many species of fish both in the wild and in aquaculture. Sheltering behaviour of Arctic charr has been poorly studied in aquaculture. A new type of shelter made of PVC agricultural drain cut in half was tested on culture performance and behaviour during the endogenous feeding period. This device offered grooves where alevins could position themselves and lie in a vertical position. A first experiment compared fish with and without shelter in incubator compartments. All fish were measured at 122, 158, and 190 days post fertilization (dpf) and observed at 126, 139 and 157 dpf. Fish provided with shelter showed better growth performances, lower mortality and started first exogenous feeding about six days later compared to fish without shelter. These effects from shelter provision were also associated with much less mobility in fish provided with shelter. At 126 dpf, all fish provided with shelter were immobile while other fish displayed horizontal stationary body movement (86%) or swam against the water current (7%). At 157 dpf, 85% of fish without shelter were immobile compared to 95% in fish provided with shelter. Both categories displayed stationary body movements. In most cases, there was one single fish per groove and a stable average value of 2.4 grooves was maintained between two successive fish throughout the study. The second experiment was a preference test where the fish were placed in similar compartments but occupied by a half shelter. We found that 61% of the fish were located under the shelter at 122 dpf while at 157 dpf, 58% were on the shelter and 42% out of the shelter. The present results revealed Arctic charr behavioural characteristics in the presence of a new type of artificial shelter at very early stages of development. This shelter enables the fish to stabilize in a vertical position without producing any movement and to choose by itself its favorite location throughout development. This device could be used to improve Arctic charr growth, performance and fish welfare during the endogenous feeding period.     

Growth in Arctic charr and rainbow trout fed temporally concentrated or spaced daily meals

The effect on growth of distributing feed over a few hours compared tomore frequent meals was tested on 1+ Arctic charr (Salvelinus alpinus L.) and 1+ rainbow trout (Oncorhynchus mykiss Walbaum). Triplicate hatchery groups for each treatment were fed at a ration level of 1%/dayeither with few meals (8 times per day divided into morning and evening)or with frequent meals (32 meals equally distributed during the day). Wefound an opposite effect of meal frequency on growth in the two species.Low feeding intensity (8 meals per day) had a significantly positive effecton growth in rainbow trout but a significantly negative effect on growth inArctic charr when compared to feeding the fish frequent meals. Theopposite response to meal frequency is likely to be an effect of thedifferences in activity during feeding. Rainbow trout feed much moreaggressively than charr which can result in feeding being a more stressfulevent. In this experiment, the specific growth rate was lower and the feedconversion ratio higher for Arctic charr compared to rainbow trout.     

The effect of low pH on physiology,stress status and growth performance of turbot (Psetta maxima L.) cultured in recirculating aquaculture systems

We evaluated the effect of low pH and low and high total ammonia nitrogen (TAN) concentrations on the physiology, stress status and the growth performance of turbot in RAS. Two experiments were conducted. In Experiment 1, turbot (466 g) were grown at control (pH 7.5; TAN ~0.5 mg/L) or low pH and high TAN (pH 5.7; TAN ~50 mg/L) for 55 days. In Experiment 2, turbot (376 g) were grown at control (pH 7.5; TAN ~0.5 mg/L), low pH and low TAN (pH 5.7; TAN ~5 mg/L) or low pH and high TAN (pH 5.7; TAN ~50 mg/L) for 59 days. In Experiment 1, final body weight, feed intake and growth were significantly lower and FCR significantly higher in turbot exposed to low pH and high TAN. In Experiment 2, only growth was significantly lower in turbot exposed to treatment low pH and high TAN as compared to fish in the control treatment and low pH and low TAN. Osmoregulation and stress indicators measured were within normal levels. In conclusion, turbot grew equally well in a water pH of 7.5 or 5.7 provided a low TAN. In contrast, low pH combined with a high TAN impaired turbot performance.     

Feed intake,growth and osmoregulation in Arctic charr,Salvelinus alpinus (L.), transferred from freshwater to saltwater at 8°C during summer and winter

The purpose of the current study was to examine seasonal changes in seawater tolerance and growth performance of anadromous Arctic charr (Salvelinus alpinus L.) held at the same temperature (8°C) during winter and summer. Charr (20–27 cm), previously reared in freshwater under natural photoperiod, were transferred either directly (DT) from freshwater to seawater (35 ppt), from freshwater to brackish water (20 ppt), or were gradually adapted (GT) to seawater over a period of 10 days. Control fish were held in freshwater. Feed intake and osmoregulatory ability were then monitored on three occasions during the following 59 days. Two experiments were carried out, one during winter (December–January) and the other during summer (June–July). In both experiments fish mortality was low. Plasma osmolalities recorded in fish transferred to seawater were within normal ranges, but osmolalities on day 10, were significantly lower in summer (313 mOsm/kg (DT), 328 mOsm/kg (GT)) than in winter (323 mOsm/kg (DT), 352 mOsm/kg (GT)). In winter, feed intake and growth rates were high in fish kept in fresh and brackish water, but charr transferred directly to seawater ate little and lost weight. Fish that were gradually adapted to seawater occupied an intermediate position. During summer the observed differences in feed intake were small and all fish had relatively high growth rates. These results suggest that Arctic charr display seasonal changes in feed intake and growth performance that parallel seasonal changes in hypoosmoregulatory capacity. The ability to survive and hypoosmoregulate in full strength seawater does not, however, seem to be a particularly good indicator of successful seawater adaptation with respect to the ability to display high rates of feed intake and growth. During winter, a gradual transfer to seawater appeared to lead to improved feeding and growth compared to direct transfer.     

Modelling the life-history variation of Arctic charr

Abstract –  A model based on proximate considerations of life histories of Atlantic salmon, Salmo salar , was examined for its applicability to fit the variation in life-history of wild Arctic charr, Salvelinus alpinus , based on a qualitative assessment of information related to growth and lipid dynamics of Arctic charr. The original salmon model is discussed in context of modifications required to account for added complexities in the life history of Arctic charr in relation to anadromy versus residency. A study from North Norway shows that individual charr that emigrate from the lakes to the sea, maintain a high growth rate in the lake in late summer and early autumn compared with resident fish. Their relatively low lipid level in autumn combined with a high rate of change of lipid during winter was associated with postponement of maturation in the anadromous individuals. Individuals that remain resident in the lake arrested growth in autumn. Their high lipid level in autumn combined with a low rate of change of lipid during winter was associated with maturation the following summer, without emigration from freshwater. Results from this and other related studies show similarities with the model derived from lipid and growth dynamics of Atlantic salmon. The adjusted charr model illustrates possible proximate explanations for the high variation in life-history strategies of Arctic charr. However, the model does not account for the characteristic return migration of immature charr into freshwater several weeks after their entry to the sea. The proximate physiological stimulus for this movement of immature fish is not entirely clear.     

Water quality and rainbow trout performance in a Danish Model Farm recirculating system: Comparison with a flow through system

The objective was to compare water quality and fish growth and mortality in a pilot scale recirculating system (RS) and a control tank in flow through system (FTS). The RS was designed after the Danish Model Trout Farm and operated with a make up water renewal rate of 9 m³ kg^-1 of fish produced. RS water quality did not decrease significantly with water flow rate decrease in the RS. During the experiment, the RS water treatment system presented solids removal efficiency of 59.6 ± 27.7% d⁻¹, ammonia oxidation of 45 ± 32 g m⁻³ d⁻¹, oxygenation yield of 392 ± 132 g of O₂ kWh⁻¹ and CO₂ degassing of 23.3 ± 11.9% pass⁻¹. In the RS, nitrite concentration was 0.15 ± 0.07 mg l⁻¹, close to the toxicity threshold; a N₂ supersaturation phenomenon was measured, probably due to the air injection depth. The biofilter and sedimentation area management has to be improved to avoid organic matter decomposition and release of dissolved elements. Even if no N₂ over-saturation apparent effect on fish performance and aspect were detected, the airlift depth has to be modified in the case of industrial development of the RS. Some improvements of the water treatment system, especially on the airlift and sedimentation area, are suggested.Concerning fish growth, no significant differences were observed between the RS and the FTS. No pathologies were detected and cumulative mortality rates (0.1%) were similar to the farm's usual data. There were no significant effects of water flow rate decrease in the RS on fish performance and energy savings were recorded to be 0.7 kWh kg⁻¹ of fish produced between RS₁ and RS₂. The global energy cost of the RS was 3.56 kWh kg⁻¹ of fish produced (0.107 € kg⁻¹ of fish produced). Even if the energy consumption of the water treatment system can be improved, the results confirm that recirculating system can be used for industrial trout on growing, without fish performance deterioration.     

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.     

Growth effects in Arctic charr reared in cold water: Feed frequency, access to bottom feeding and stocking density

The effects of time restricted feeding, possibility of bottomfeeding and stocking density on the growth of Arctic charr(Salvelinus alpinus L.) were examined in fish held at lowtemperature (<2 °C). Fish fed for a restricted time (1 h) hadsignificantly (p < 0.05) lower specific growth rate (0.15 vs0.32% per day) than those fed the same ration over an extendedtime period (12 h). Increasing stocking densities had a positive andsignificant effect (p < 0.05) on growth with SGR increasing from 0.27to 0.52% per day at 2–30 kg m^-3. Fish withaccess to feed on the tank floor had a significantly higher (p <0.05) growth rate (0.3 vs 0.13% per day) than those without thepossibility to feed from the bottom. When fish were held underconditions without access to the bottom a doubling of the feed rationdid not result in a significant (p > 0.05) increase in growth rate(0.13 vs 0.12%percnt; per day).     

Heavy metal and waste metabolite accumulation and their potential effect on rainbow trout performance in a replicated water reuse system operated at low or high system flushing rates

A six-month trial was conducted to compare the effects of high and low make-up water flushing rates on rainbow trout performance and water quality in replicated water reuse aquaculture systems (WRAS). Six identical 9.5 m³ WRAS, containing a single 5.3 m³ tank and operated at a total recirculating flow of 380 L/min were stocked with 1000 rainbow trout each (133 ± 1 g). Three WRAS were operated at high flushing rates (2.6% of total flow) and three were operated at low flushing rates (0.26% of total flow), providing system hydraulic retention times of 0.67 and 6.7 days, respectively. During a one-week period when fish were at maximum feeding (i.e., mean feed loadings of 0.53 and 5.3 kg/m³ make-up water flow high and low make-up conditions, respectively) and maximum densities (80 kg/m³), water samples were collected across all unit processes. All typical water quality parameters measured at the culture tank outlet during this week were significantly different between treatments, except for dissolved oxygen, carbon dioxide, and temperature, which were controlled. Within the low exchange WRAS, total suspended solids (TSS), carbonaceous biochemical oxygen demand, total ammonia nitrogen, un-ionized ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, dissolved organic carbon, particle counts, true color, and total heterotrophic plate counts were significantly greater, whereas UV transmittance (%) and alkalinity were significantly reduced. Of these parameters, TSS, fine particles, and heterotrophic bacteria counts were the only parameters of concern within the low exchange WRAS. The potential impacts of each water quality constituent are discussed. Element analysis indicated that concentrations of nine metals were significantly greater within the low exchange WRAS. The highest metal concentrations measured at low exchange were within safe recommended limits, with the exception of copper (0.037–0.056 mg/L), which could have reached chronically toxic levels. Although cumulative mortality was relatively low for all WRAS, a linear trend between copper concentration and mortality was evident. The highest mortality, which occurred within a low exchange WRAS, coincided with the highest copper (0.056 mg/L); and the lowest mortality, which occurred within a high exchange WRAS, coincided with non-detectable copper levels. A comparison of survival between treatments bordered significance, 99.5 ± 0.1 and 98.9 ± 0.4% for the high and low exchange WRAS, respectively. There was no significant difference in rainbow trout weight at the conclusion of the study, i.e., approximately one year post-hatch: 1401 ± 23 and 1366 ± 33 g for the high and low exchange WRAS, respectively. There were no differences in thermal growth coefficients or feed conversion ratios between the high and low exchange treatments. Rainbow trout condition factor was significantly greater within the low exchange WRAS.     

Feed-growth relationships of Arctic charr transferred from freshwater to saltwater at different seasons

Groups of Arctic charr, Salvelinus alpinus (L.), originating from an anadromous stock, were transferred directly from freshwater to seawater (35 S), from freshwater to brackish water (20 S), or were gradually adapted to seawater over a period of 10 days. A control group was kept in freshwater. Feed intake and growth were then monitored during the following 59 days. Two experiments were carried out, one during winter (December–January) and the other during summer (June–July). Water temperature was maintained at 8 C. All fish had been held under natural photoperiod and temperature conditions from the end of the first feeding period (mid-April) until the experiments were carried out. Rates of growth did not differ between the groups of charr held in fresh- and brackish water, but growth of these groups was better than that of fish reared in seawater. Relationships between individual feed intake (FI) and individual specific growth rates (SGR), within groups were expressed as: ln (SGR+1) = a + ln (FI+1), and regression lines were compared using analysis of covariance. In winter, the regression line for fish transferred directly to seawater had a significantly steeper slope, and a lower elevation, than the lines for fish transferred to brackish water or held in freshwater. Fish transferred gradually to seawater had an intermediate position. In summer, the regressions for all groups were parallel with intercepts in the seawater groups being significantly lower than in the freshwater group. This indicates that the gross feed conversion efficiency (FCE) in Arctic charr was lower in seawater than in freshwater. The results are discussed in relation to digestion, gut function and osmoregulation. yc]Keywords xc]Arctic charr (Salvelinus alpinus (L.)) xc]Feed utilization xc]Salinity xc]Season     

Winter movement activity patterns of anadromous Arctic charr in two Labrador lakes

Anadromous Arctic charr, Salvelinus alpinus, feed in the marine environment for several months during the summer and migrate back to fresh water in late summer to spawn and/or overwinter. While overwintering, anadromous Arctic charr are generally believed to reduce or cease feeding, and they are poorly described in their winter movement activity. This study used telemetry data collected from two locations to describe overwintering movement activity, including interindividual variation. Movement activity declined markedly during the ice‐covered period, suggesting opportunistic maintenance feeding was used as an energy conservation strategy. Fall and spring movement was correlated with daylight hours, and ice break‐up had a significant effect on the timing of outmigration. Movement activity was negatively correlated with body length, with smaller individuals being more active than larger fish. Although general activity patterns were evident, there were significant differences among individuals, particularly during spring immediately prior to lake departure. Lake size and individual differences in metabolic rate may account for some of this variation.     

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.     

The effect of nearly closed RAS on the feed intake and growth of Nile tilapia (Oreochromis niloticus), African catfish (Clarias gariepinus) and European eel (Anguilla anguilla)

One of the challenges that Recirculating Aquaculture Systems (RAS) are still facing is the risk that in RAS fish grow less than in flow-through systems due to the accumulation of substances originating from feed, fish or bacteria associated with the water re-use. The present study investigated whether RAS with high and low accumulation levels of these substances affect feed intake and growth of Nile tilapia Oreochromis niloticus, African catfish Clarias gariepinus, and European eel Anguilla Anguilla. One-hundred and twenty individuals of each species were used (start body weights: Nile tilapia 264.8 ± 8.3 g; African catfish 253.2 ± 2.1 g and European eel 66.6 ± 1.3 g). For a period of 39 days, growth and feed intake were compared between high and low accumulation RAS. HIGH accumulation RAS was designed for maximal accumulation of substances in the water by operating the system at nearly-closed conditions (30 L/kg feed/d), using mature biofilters and high feed loads; and (2) LOW accumulation RAS was designed to be a proxy for flow-through systems by operating at high water exchange rates (1500 L/kg feed/d), new biofilters and low feed load. HIGH accumulation RAS induced a reduction in feed intake (42%) and growth (83%) of Nile tilapia, as compared to systems that are a proxy for flow-through conditions. This effect was not observed in European eel and African catfish. The cause of this reduced feed intake and growth rate of Nile tilapia is still unclear and should be addressed in further studies.