Water Recirculating Aquaculture Systems Research Articles

Enhancing energy recovery from aquaculture residual materials: a focus on anaerobic digestion of African catfish (Clarias gariepinus) sediment sludge

In Germany, warm water Recirculating Aquaculture Systems (RAS) are generally integrated with biogas plants. The process sludge produced in aquaculture could be utilized to generate energy. This study investigates the potential of process sludge from commercial African catfish (Clarias gariepinus) warm water RAS, alongside associated plant production residues (whole plants and pruning residues) for energy generation through anaerobic digestion. Biogas tests, including batch, semi-Continuous Stirred Tank Reactor (CSTR), and Upflow Anaerobic Sludge Blanket Reactor (UASB) were conducted. In batch, methane yields were 229 L(N) / kg VS from the sludge and 173–184 L(N) / kg VS from the various plant substrates (cucumber, paprika, and tomato plants). During CSTR operation, mono-fermentation of sludge produced a methane yield of 265 L(N) / kg VS. Co-fermentation with 25% cucumber residues, based on VS, increased this value to 381 L(N) / kg VS. Mono-fermentation of sludge in the UASB reactor yielded a maximum of 329 L(N) /kg VS. The relatively low TS content and unfavorable C/N ratio in C. gariepinus sludge, along with the low energy density and occasional high sulfur content in the investigated plant substrates, present challenges for CSTR biogas production. These challenges can be partially mitigated through substrate combination. For mono-fermentation of African catfish RAS sludge, the UASB reactor is recommended. Improved solids separation, extraction, and concentration techniques at aquaculture operations are essential for the efficient utilization of aquaculture sludge, especially from African catfish, in biogas plants.

Dietary protein requirements of whiteleg shrimp (Penaeus vannamei) post-larvae during nursery phase in clear-water recirculating aquaculture systems

Whiteleg shrimp Penaeus vannamei farming in clear water recirculating aquaculture systems (RAS) is relatively recent, and consequently, knowledge on the shrimp dietary demands is still insufficient, particularly in the initial developmental stages. This study aimed at assessing the dietary protein requirement of whiteleg shrimp post-larvae (PL) in a clear-water recirculating aquaculture system (RAS). Six microdiets were formulated to contain 34%, 44%, 49%, 54%, 58%, and 63% crude protein (P34, P44, P49, P54, P58 and P63, respectively) and were evaluated in triplicates. Whiteleg shrimp PL (3.2 mg wet weight) were reared for 21 days in a clear-water RAS at Riasearch Lda. At the end of the feeding period, the optimal protein requirement was estimated at 47.1%, 46.4%, 47.2%, and 44.0% for weight gain, relative growth rate (RGR), feed conversion ratio (FCR), and survival, respectively. PL fed the P54, P58, and P63 diets achieved significantly higher final body weights than those fed P34. PL fed P34 showed significantly lower RGR and survival and significantly higher FCR values than those fed the remaining diets, suggesting that low protein diets may not be adequate to be used in this stage of shrimp development and/or for the clear-water RAS husbandry conditions. Moreover, diet P34 seemingly reduced the overall antioxidant status of the PL when compared to P44, P49, and P54. However, the P34 diet seems to have stimulated the PL immune mechanisms when compared to P44, P49, and P54, possibly due to increased levels of fish and algae oil. Similarly, despite the good growth performances, a diet containing 63% of protein also seemed to have compromised the overall shrimp PL antioxidant status and stimulate their immune system. Shrimp fed diet P54 showed an apparent overall superior antioxidant status when compared to the remaining diets, evidencing that using protein inclusion levels up to 54% in aquafeeds not only potentiates growth performances and survival but also can potentially be beneficial to the health status of P. vannamei PL grown in a clear-water RAS. Hence, results from this study suggest that a minimum of approximately 47% of protein should be considered when tailoring microdiets for whiteleg shrimp PL grown in a clear-water RAS, but inclusion levels up to 54% can be used with benefits to the PL antioxidant status.

Proportional up scaling of African catfish (Clarias gariepinus Burchell, 1822) commercial recirculating aquaculture systems disproportionally affects nutrient dynamics

This study reports nutrient dynamics during the “Run-in-”, “Batch-” and “Staggered 1 - 3” production phases of African catfish (Clarias gariepinus Burchell, 1822) in commercial warm water recirculation aquaculture systems (RAS) under three different stocking densities, extensive (max 50 kg m−3; 35 fish tank−1), semi-intensive (max 100 kg m−3; 70 fish tank−1) and intensive (max 200 kg m−3; 140 fish tank−1) at similar system volumes of 13.9, 15.1 and 16.9 m3. The concentrations of the dissolved macronutrients Total Ammonia Nitrogen (TAN, as NH4+-N), nitrite (NO2−-N), nitrate (NO3−-N), Total Dissolved Nitrogen (TDN), Total Oxidized Nitrogen (TON), phosphorus (measured as ortho-phosphate) (P), potassium (K), calcium (Ca) and magnesium (Mg) were monitored with colorimetric analyses and budgeted for potential use for aquaponics. We found, that stocking density and RAS maintenance affected oxygen thresholds, subsequently affecting nutrient concentrations of NO3−-N due to denitrification. K proportionally increased with feed input, while Ca, Mg and P reacted disproportionally, because of precipitation. Highest deviation was observed for TON, especially when oxygen levels dropped <6 mg L−1. Maintenance intervals, feed ratios and resulting oxygen levels changed dissolved nutrient concentrations, with highest concentrations under intensive stocking density, medium feed input and oxygen > 6 mg L−1, and lowest under semi-intensive stocking density, high feed input and oxygen > 6 mg L−1. Calculated nutrient budgets under consideration of stocking density, feed input and water exchange rates demonstrated that the production conditions affected nutrient availability and output. The macronutrient ratios TDN, P, K, Ca and Mg as well as the physical and chemical parameters changed constantly under production conditions, due to daily adjusted feeding and RAS maintenance. This must be considered during process management, in stand-alone commercial catfish aquaculture as well as in integrated systems such as aquaponics.

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.

Investigating the influence of nitrate nitrogen on post-smolt Atlantic salmon Salmo salar reproductive physiology in freshwater recirculation aquaculture systems

A major issue affecting land-based, closed containment Atlantic salmon Salmo salar growout production in water recirculation aquaculture systems (RAS) is precocious male maturation, which can negatively impact factors such as feed conversion, fillet yield, and product quality. Along with other water quality parameters, elevated nitrate nitrogen (NO3-N) has been shown to influence the reproductive development and endogenous sex steroid production in a number of aquatic animal species, including Atlantic salmon. We sought to determine whether elevated NO3-N in RAS can influence early maturation in post-smolt Atlantic salmon in an 8-month trial in replicated freshwater RAS. Post-smolt Atlantic salmon (102±1g) were stocked into six RAS, with three RAS randomly selected for dosing with high NO3-N (99±1mg/L) and three RAS set for low NO3-N (10±0mg/L). At 2-, 4-, 6-, and 8-months post-stocking, 5 fish were randomly sampled from each RAS, gonadosomatic index (GSI) data were collected, and plasma was sampled for 11-ketotestosterone (11-KT) quantification. At 4- and 8-months post-stocking, samples of culture tank and spring water (used as “makeup” or replacement water) were collected and tested for a suite of 42 hormonally active compounds using liquid chromatography/mass spectrometry, as well as for estrogenicity using the bioluminescent yeast estrogen screen (BLYES) reporter system. Finally, at 8-months post-stocking 8–9 salmon were sampled from each RAS for blood gas and chemistry analyses, and multiple organ tissues were sampled for histopathology evaluation. Overall, sexually mature males were highly prevalent in both NO3-N treatment groups by study’s end, and there did not appear to be an effect of NO3-N on male maturation prevalence based on grilse identification, GSI, and 11-KT results, indicating that other culture parameters likely instigated early maturation. No important differences were noted between treatment groups for whole blood gas and chemistry parameters, and no significant tissue changes were noted on histopathology. No hormones, hormone conjugates, or mycotoxins were detected in any water samples; phytoestrogens were generally detected at low levels but were unrelated to NO3-N treatment. Finally, low-level estrogenicity was detected in RAS water, but a NO3-N treatment effect could not be determined. The major findings of this study are i) the NO3-N treatments did not appear to be related to the observed male maturation, and ii) the majority of hormonally active compounds were not detectable in RAS water.

A Review of Factors Influencing Maturation of Atlantic Salmon, Salmo salar, with Focus on Water Recirculation Aquaculture System Environments

AbstractSexual maturation of Atlantic salmon, Salmo salar, is a complex process, with many variables having the capacity to influence the timing and prevalence of maturation and acting as promoters and/or inhibitors of sexual development. Precocious maturation has the capacity to seriously impact production in commercial aquaculture settings, and in response there has been a significant amount of research devoted to understanding this issue in order to develop remedial strategies. Very little research has been conducted specifically examining salmon maturation in land‐based, closed containment water recirculation aquaculture systems, which have recently received attention as an alternative technology for the sustainable production of market‐size Atlantic salmon. Unfortunately, the nascent closed containment salmon industry has thus far experienced high levels of precocious maturation, for reasons that are presently unclear. Given the economic challenges facing the closed containment industry's expansion, it is imperative that best management practices be developed to reduce economic losses from early maturation, in order to assist the sustainable growth of farmed Atlantic salmon production. This review provides a brief summary of published research on factors associated with early salmonid maturation, as well as information from research examining maturation and growout performance of Atlantic salmon in closed containment aquaculture systems.

Comparative economic performance and carbon footprint of two farming models for producing Atlantic salmon (Salmo salar): Land-based closed containment system in freshwater and open net pen in seawater

Ocean net pen production of Atlantic salmon is approaching 2 million metric tons (MT) annually and has proven to be cost- and energy-efficient. Recently, with technology improvements, freshwater aquaculture of Atlantic salmon from eggs to harvestable size of 4–5kg in land-based closed containment (LBCC) water recirculating aquaculture systems (RAS) has been demonstrated as a viable production technology. Land-based, closed containment water recirculating aquaculture systems technology offers the ability to fully control the rearing environment and provides flexibility in locating a production facility close to the market and on sites where cost of land and power are competitive. This flexibility offers distinct advantages over Atlantic salmon produced in open net pen systems, which is dependent on access to suitable coastal waters and a relatively long transport distance to supply the US market. Consequently, in this paper we present an analysis of the investment needed, the production cost, the profitability and the carbon footprint of producing 3300 MT of head-on gutted (HOG) Atlantic salmon from eggs to US market (wholesale) using two different production systems—LBCC-RAS technology and open net pen (ONP) technology using enterprise budget analysis and carbon footprint with the LCA method. In our analysis we compare the traditional open net pen production system in Norway and a model freshwater LBCC-RAS facility in the US. The model ONP is small compared to the most ONP systems in Norway, but the LBCC-RAS is large compared to any existing LBCC-RAS for Atlantic salmon. The results need to be interpreted with this in mind. Results of the financial analysis indicate that the total production costs for two systems are relatively similar, with LBCC-RAS only 10% higher than the ONP system on a head-on gutted basis (5.60 US$/kg versus 5.08 US$/kg, respectively). Without interest and depreciation, the two production systems have an almost equal operating cost (4.30 US$/kg for ONP versus 4.37 US$/kg for LBCC-RAS). Capital costs of the two systems are not similar for the same 3300 MT of head-on gutted salmon. The capital cost of the LBCC-RAS model system is approximately 54,000,000 US$ and the capital cost of the ONP system is approximately 30,000,000 US$, a difference of 80%. However, the LBCC-RAS model system selling salmon at a 30% price premium is comparatively as profitable as the ONP model system (profit margin of 18% versus 24%, respectively), even though its 15-year net present value is negative and its return on investment is lower than ONP system (9% versus 18%, respectively). The results of the carbon footprint analysis confirmed that production of feed is the dominating climate aspect for both production methods, but also showed that energy source and transport methods are important. It was shown that fresh salmon produced in LBCC-RAS systems close to a US market that use an average US electricity mix have a much lower carbon footprint than fresh salmon produced in Norway in ONP systems shipped to the same market by airfreight, 7.41 versus 15.22kg CO2eq/kg salmon HOG, respectively. When comparing the carbon footprint of production-only, the LBCC-RAS-produced salmon has a carbon footprint that is double that of the ONP-produced salmon, 7.01 versus 3.39kg CO2eq/kg salmon live-weight, respectively.

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

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

Observations on side-swimming rainbow trout in water recirculation aquaculture systems.

During a controlled 6-month study using six replicated water recirculation aquaculture systems (WRASs), it was observed that Rainbow Trout Oncorhynchus mykiss in all WRASs exhibited a higher-than-normal prevalence of side swimming (i.e., controlled, forward swimming but with misaligned orientation such that the fish's sagittal axis is approximately parallel to the horizontal plane). To further our understanding of this abnormality, a substudy was conducted wherein side swimmers and normally swimming fish were selectively sampled from each WRAS and growth performance (length, weight), processing attributes (fillet yield, visceral index, ventrum [i.e., thickness of the ventral “belly flap”] index), blood gas and chemistry parameters, and swim bladder morphology and positioning were compared. Side swimmers were found to be significantly smaller in length and weight and had less fillet yield but higher ventrum indices. Whole-blood analyses demonstrated that, among other things, side swimmers had significantly lower whole-blood pH and higher Pco 2. Side swimmers typically exhibited swim bladder malformations, although the positive predictive value of this subjective assessment was only 73%. Overall, this study found several anatomical and physiological differences between side-swimming and normally swimming Rainbow Trout. Given the reduced weight and fillet yield of market-age side swimmers, producers would benefit from additional research to reduce side-swimming prevalence in their fish stocks.Received March 20, 2014; accepted May 20, 2014

Evaluation of depuration procedures to mitigate the off-flavor compounds geosmin and 2-methylisoborneol from Atlantic salmon Salmo salar raised to market-size in recirculating aquaculture systems

Fish cultured within water recirculating aquaculture systems (RAS) can acquire “earthy” or “musty” off-flavors due to bioaccumulation of the compounds geosmin and 2-methylisoborneol (MIB), respectively, which are produced by certain bacterial species present in RAS biosolids and microbial biofilms. Fish cultured in RAS are generally transferred to separate depuration systems that are flushed with water in a single pass or operated with limited water recirculation (with no biofilter), in order to purge these unpalatable flavors. Technologies and standard operating practices that optimize purging kinetics for Atlantic salmon Salmo salar and other species cultured in RAS are needed to improve the consistency and efficacy of depuration. A 2×2 factorial trial was conducted to evaluate techniques to mitigate off-flavor from Atlantic salmon cultured to 3–5kg in a semi-commercial scale freshwater RAS. Twelve replicated depuration systems (0.5m3) were used to evaluate four combinations (n=3) of the following standard operating procedure and system design parameters: (1) disinfection of depuration systems as a 1h static treatment using 250mg/L hydrogen peroxide (H2O2) prior to stocking fish, (2) no disinfection prior to stocking fish, (3) presence of water aeration media within gas transfer columns of depuration systems, and (4) absence of water aeration media within gas transfer columns of depuration systems. Food-size Atlantic salmon were stocked within the depuration systems and kept off feed for 10 days. Six salmon were harvested from the original RAS on Day 0 and fileted for baseline assessment of off-flavor concentrations. Thereafter, filet samples (n=3–4) were taken on Days 3, 6, and 10 to evaluate off-flavor kinetics. Hydrogen peroxide disinfection of depuration systems resulted in significantly reduced off-flavor in salmon filets during the depuration period. Results also indicated that the presence of high-surface-area water aeration media shielded biofilms from complete disinfection, resulting in less and slower off-flavor removal from Atlantic salmon filets; while depuration systems void of media resulted in greater and more rapid off-flavor reduction. Thus, water aeration media should not be used in depuration systems because of the challenges posed for effective cleaning, disinfection, and inactivation of off-flavor producing bacteria that may be present, and unit processes and locations that are difficult-to-clean should be excluded. In addition, a wide range of off-flavor concentrations were measured within individual salmon, indicating that one salmon is not a representative sample size to determine market suitability.

Flavobacterium branchiophilum and F. succinicans associated with bacterial gill disease in rainbow trout Oncorhynchus mykiss (Walbaum) in water recirculation aquaculture systems

Bacterial gill disease (BGD) is a common and occasionally devastating disease that affects numerous cultured fish species throughout the world (Starliper & Schill 2012). Outbreaks of BGD tend to occur when environmental conditions deteriorate, and opportunistic pathogens can more readily cause overt disease (Bullock 1972; Schachte 1983). The putative causative agent, Flavobacterium branchiophilum, has been shown to induce BGD under laboratory conditions (e.g. Lumsden et al. 1995; Ostland et al. 1995); however, diagnosis of BGD in the field is generally carried out through light microscopy and/or observation of clinical signs. Therefore, the identity of the bacterial specie(s) present on the gills of BGD-affected fish in culture settings is most often based on visual assessment alone. The observed bacteria using microscopy are presumed to be F. branchiophilum based on organism morphology (numerous long, thin rods; Fig. ​Fig.1.),1.), with this presumption supported by previous BGD research (e.g. Byrne et al. 1995; Ostland et al. 1997; Derksen, Ostland & Ferguson 1998) and various F. branchiophilum-specific diagnostic approaches that have been developed. The latter include ELISA (MacPhee et al. 1995), IFAT (Heo, Kasai & Wakabayashi 1990), and PCR (Toyama, Kita & Wakabayashi 1996). A comprehensive examination of bacterial species present on the gills of naturally infected fish, however, has not been carried out and is desirable to enhance our understanding of BGD and to inform further research (e.g. potential vaccine development). We therefore sought to induce BGD through environmental manipulation (as opposed to pathogen challenge) and to identify the bacterial species involved in typical BGD outbreaks. Our findings are presented in this short communication.

Comparing the effects of high vs. low nitrate on the health, performance, and welfare of juvenile rainbow trout Oncorhynchus mykiss within water recirculating aquaculture systems

Previous research indicates that rainbow trout Oncorhynchus mykiss begin to exhibit health and welfare problems when cultured within water recirculating aquaculture systems (WRAS) operated at low exchange (6.7 days hydraulic retention time) and a mean feed loading rate of 4.1kgfeed/m3 daily makeup flow. These studies could not conclusively determine the causative agent of the health and welfare issues, but accumulation of mean nitrate nitrogen (NO3-N) to approximately 100mg/L was determined to be a potential cause of abnormal swimming behaviors such as “side swimming” and rapid swimming velocity. A subsequent controlled, 3-month study was conducted to determine if NO3-N concentrations of 80–100mg/L resulted in chronic health issues for rainbow trout. Equal numbers of rainbow trout (16.4±0.3g) were stocked within six replicated 9.5m3 WRAS. Three WRAS were maintained with a mean NO3-N concentration of 30mg/L (“low”) resulting from nitrification, and three WRAS were maintained with a mean concentration of 91mg/L (“high”) via continuous dosing of a sodium nitrate stock solution in addition to nitrification. All six WRAS were operated with equal water exchange (1.3 days mean hydraulic retention time) and mean feed loading rates (0.72kgfeed/m3 daily makeup flow), which provided enough flushing to limit the accumulation of other water quality concentrations. Rainbow trout growth was not significantly impacted by the high NO3-N treatment. Cumulative survival for fish cultured within the high NO3-N WRAS was lower and bordered statistical significance, which resulted in total rainbow trout biomass that was significantly lower for this group at study's end. In addition, a significantly greater prevalence of side swimming rainbow trout occurred in the high NO3-N treatment, as was observed during previous research. Swimming speeds were generally greater for rainbow trout cultured in the high NO3-N treatment, but were not always significantly different. Although most water quality variables were controlled, significant differences between treatments for the concentrations of other water quality parameters inhibited definitive conclusions regarding the effect of NO3-N. However, due to the unlikely toxicity of confounding water quality parameters, study results provided strong evidence that relatively low NO3-N levels, 80–100mg/L, were related to chronic health and welfare impacts to juvenile rainbow trout under the described conditions.

The Impact of Water Exchange Rate and Treatment Processes on Water-Borne Hormones in Recirculation Aquaculture Systems Containing Sexually Maturing Atlantic Salmon Salmo salar

A controlled seven-month study was conducted in six replicated water recirculation aquaculture systems (WRAS) to assess post-smolt Atlantic salmon (Salmo salar) performance in relation to WRAS water exchange rate. Unexpectedly high numbers of precocious sexually mature fish were observed in all WRAS toward the end of the study period; therefore, a separate investigation was conducted to quantify the levels of water-borne hormones (cortisol (C), testosterone (T), 11-ketotestosterone (11-KT), progesterone (P), and estradiol (E2)) to determine the impact of WRAS exchange rate, as well as transit through the unit processes, on soluble hormone concentrations.Triplicate water samples were collected at three separate sites in each of the six WRAS: pre-unit processes, post-unit processes, and at the makeup water influent. Water samples were concentrated and separate quantifications were carried out for each target hormone using enzyme immunoassay kits. Results indicated that among the hormones examined, only T was associated with higher concentrations in low exchange WRAS compared to high exchange WRAS. Water passage through the unit processes was associated with a significant reduction in concentration of 11-KT, in both high and low exchange WRAS. Water-borne concentrations of T, 11-KT, and E2 were significantly higher than influent makeup water; the majority of C and P concentrations were not significantly different between WRAS and makeup water samples. No significant differences were noted in the prevalence of apparently sexually mature fish or gonadosomatic indices in either sex between treatments, except a significantly higher prevalence of apparently mature female fish in low exchange WRAS. Overall, these findings suggest that, under the conditions of this study, C, P, E2, and 11-KT do not accumulate in lower exchange WRAS, and that, aside from 11-KT, the WRAS unit processes do not impact hormone concentration. Furthermore, the observed precocious sexual maturation was mostly unrelated to WRAS exchange rate.

Comparing the effects of feeding a grain- or a fish meal-based diet on water quality, waste production, and rainbow trout Oncorhynchus mykiss performance within low exchange water recirculating aquaculture systems

Feeding a fish meal-free grain-based diet (GB) was compared to feeding a fish meal-based diet (FM) relative to water quality criteria, waste production, water treatment process performance, and rainbow trout Oncorhynchus mykiss performance within six replicated water recirculating aquaculture systems (WRAS) operated at low exchange (0.26% of the total recycle flow; system hydraulic retention time=6.7 days). Rainbow trout (214±3g to begin) were fed the GB diet within three WRAS and the FM diet within the other three WRAS for 3 months. Feeding the GB diet resulted in significantly greater total ammonia nitrogen (TAN) throughout the study, as well as significantly greater total suspended solids (TSS) and carbonaceous biochemical oxygen demand (BOD) over the greater part of the study. Greater counts of fine solids (2–30μm) were associated with the GB diet. Water clarity was improved for the GB diet as reflected by significantly reduced true color and increased ultraviolet transmittance. Total and dissolved phosphorous, as well as the daily mass captured per kg feed of total phosphorous were significantly lower within the effluent associated with the GB diet. The daily mass of total nitrogen and TSS captured per kg feed and discharged from each WRAS was equal between diets, but waste load among three discharge flows varied. Waste removal efficiency across unit processes was similar between diets, with the exception of solids removal efficiency across the microscreen drum filter and the radial flow settler, which was generally lower for the GB diet. Rainbow trout growth, feed conversion, condition factor, and survival were similar between diets. Survival was 99.5±0.2% for both diet treatments. Skin-on fillet yield and whole-body protein levels were significantly greater at the end of the study for trout fed the GB diet. Overall, the GB diet evaluated during the present study proved to be a viable feed option for use within a low exchange WRAS. However, further refinements to grain-based diet formulations and/or use of ozone to improve water quality when feeding GB diets within low exchange WRAS could be beneficial.

Abnormal swimming behavior and increased deformities in rainbow trout Oncorhynchus mykiss cultured in low exchange water recirculating aquaculture systems

Two studies were conducted to evaluate rainbow trout Oncorhynchus mykiss health and welfare within replicated water recirculating aquaculture systems (WRAS) that were operated at low and near-zero water exchange, with and without ozonation, and with relatively high feed loading rates. During the first study, rainbow trout cultured within WRAS operated with low water exchange (system hydraulic retention time (HRT)=6.7 days; feed loading rate=4.1kg feed/m3 daily makeup flow) exhibited increased swimming speeds as well as a greater incidence of “side swimming” behavior as compared to trout cultured in high exchange WRAS (HRT=0.67 days; feed loading rate=0.41kg feed/m3 daily makeup flow). During the second study, when the WRAS were operated at near-zero water exchange, an increased percentage of rainbow trout deformities, as well as increased mortality and a variety of unusual swimming behaviors were observed within WRAS with the highest feed loading rates and least water exchange (HRT≥103 days; feed loading rate≥71kg feed/m3 daily makeup flow). A wide range of water quality variables were measured. Although the causative agent could not be conclusively identified, several water quality parameters, including nitrate nitrogen and dissolved potassium, were identified as being potentially associated with the observed fish health problems.

The effects of ozonation on performance, health and welfare of rainbow trout Oncorhynchus mykiss in low-exchange water recirculation aquaculture systems

A controlled four-month trial was conducted to compare the effects of ozonation (oxidation–reduction potential setpoint=250mV) versus no ozonation on rainbow trout Oncorhynchus mykiss performance, health, and welfare in replicated WRAS operated at low exchange rates (0.26% of the total recirculating flow) and high feed loadings (3.9kg/day per m3/day makeup flow). Rainbow trout at 74±2g (mean±standard error) in size were randomly stocked into six replicated 9.5m3 WRAS (1000 fish per system). Ozonation of the three treatment WRAS began two months post-stocking following biofilter acclimation when fish were 295±1g. Fish were maintained at densities between 40kg/m3 and 80kg/m3 throughout the study at water temperatures of approximately 13–17°C. By study's end, percentage survival for both groups was high (>98%). Fish in ozonated systems weighed significantly (p<0.05) more at study's end than fish from the non-ozonated systems (1161±6g vs. 993±12g, respectively). Histopathological evaluation revealed increased levels of gill epithelial hyperplasia and hypertrophy, as well as hepatic lipidosis, in fish from ozonated systems; however, all lesions were subclinical. Fin condition was good overall in both groups, although fish from non-ozonated WRAS had better (p<0.05) dorsal fin indices than fish in the ozonated cohort. The major blood chemistry finding was higher (p<0.05) urea nitrogen in fish from non-ozonated systems (15.33±0.90mg/dL, vs. 6.800±0.470mg/dL in fish from ozonated WRAS). Overall, the results of this study indicate that raising rainbow trout to market size in ozonated WRAS improves fish performance without significantly impacting their health and welfare.

The effects of ozone and water exchange rates on water quality and rainbow trout Oncorhynchus mykiss performance in replicated water recirculating systems

Rainbow trout Oncorhynchus mykiss performance and water quality were evaluated and compared within six replicated 9.5m3 water recirculating aquaculture systems (WRAS) operated with and without ozone at various water exchange rates. Three separate studies were conducted: (1) low water exchange (0.26% of the total recycle flow) with and without ozone; (2) low water exchange with ozone versus high water exchange (2.6% of the total recycle flow) without ozone; and (3) near-zero water exchange (only backwash replacement) with and without ozone. Mean feed loading rates for WRAS operated at high, low, and near-zero exchange were 0.40, 3.98, and 55.9kgfeed/m3 makeup water, respectively. Ozone significantly reduced total suspended solids, color, and biochemical oxygen demand and resulted in a significant increase in ultraviolet transmittance (%) (P<0.10). Ozone also created ambient water quality within low exchange WRAS that was comparable to that of WRAS operated at high water exchange (P>0.10). Additionally, dissolved copper and iron were significantly lower within WRAS operated with ozone (P<0.10). Dissolved zinc was also consistently lower in WRAS operated with ozone, but not significantly (P>0.10). In Studies 1 and 3, total ammonia nitrogen and nitrite nitrogen were slightly lower within the ozonated systems, but were not always significantly lower. In all studies, ozone did not prevent nitrate nitrogen accumulation. At the conclusion of Study 1, rainbow trout growth was significantly greater within low exchange WRAS operated with ozone (P=0.001). At the conclusion of Study 2, rainbow trout growth was similar between treatments (P=0.581), indicating that fish grew equally as well within ozonated WRAS operated at 1/10th the flushing rate as the non-ozonated and high flushing control systems. Overall, ozone created an improved water quality environment within low and near-zero exchange WRAS that generally resulted in enhanced rainbow trout growth rates, survival, feed conversion, and condition factor.

The effects of carbon dioxide on performance and histopathology of rainbow trout Oncorhynchus mykiss in water recirculation aquaculture systems

Chronic exposure to elevated levels of dissolved carbon dioxide (CO2) has been linked to reduced growth, physiological disturbances and negative health outcomes in intensively reared fish. Although pumping to a degassing tower can lower concentrations of dissolved CO2 in water recirculation aquaculture systems (WRAS), pumping can be a significant cost for operators. A 6-month trial was conducted to compare the effects of high (24±1mg/L; partial pressure=8.79mmHg) and low (8±1mg/L; partial pressure=2.91mmHg) dissolved CO2 concentrations on rainbow trout Oncorhynchus mykiss performance and health in replicated WRAS operated at low-exchange rates (0.26% of the total recirculating flow). Rainbow trout (62±1g) were randomly stocked into six replicated WRAS and into three small tanks within a flow-through system to provide a physiological comparison. All study fish were maintained at densities between 25 and 80kg/m3, at water temperatures of approximately 13–14°C, and at dissolved oxygen concentrations of approximately saturation. A 24-h photoperiod was provided and all fish groups were fed equal portions every 2h during the study period. Fish health and performance were assessed with daily mortality and monthly length and weight data collection, as well as multiple tissue samplings for histopathological assessment. At the study's end, percentage survival for both groups was high (>97%). No significant (p<0.05) differences in growth or survival were observed between CO2 treatments. No nephrocalcinosis or related pathologies were noted. Skin and gill pathologies were common in both treatment groups; however, there were few statistically significant differences between groups for any of the tissue types evaluated: high CO2 fish were more likely to exhibit lymphocytic portal hepatitis, while the low CO2 treatment group exhibited greater gill epithelial hyperplasia. None of the pathologies observed were substantive or likely to cause mortality. The results of this study indicate that raising rainbow trout to market size in WRAS with CO2 concentrations of 24mg/L does not significantly affect their overall health and performance.

Degradation and effect of hydrogen peroxide in small-scale recirculation aquaculture system biofilters

From an environmental point of view, hydrogen peroxide (HP) has beneficial attributes compared with other disinfectants in terms of its ready degradation and neutral by-products. The rapid degradation of HP can, however, cause difficulties with regard to safe and efficient water treatment when applied in different systems. In this study, we investigated the degradation kinetics of HP in biofilters from water recirculating aquaculture systems (RAS). The potential effect of HP on the nitrification process in the biofilters was also examined. Biofilter elements from two different pilot-scale RAS were exposed to various HP treatments in batch experiments, and the HP concentration was found to follow an exponential decay. The biofilter ammonia and nitrite oxidation processes showed quick recuperation after exposure to a single dose of HP up to 30 mg L−1. An average HP concentration of 10–13 mg L−1 maintained over 3 h had a moderate inhibitory effect on the biofilter elements from one of the RAS with relatively high organic loading, while the nitrification was severely inhibited in the pilot-scale biofilters from the other RAS with a relatively low organic loading. A pilot-scale RAS, equipped with two biofilter units, both a moving-bed (Biomedia) and a fixed-bed (BIO-BLOK®) biofilter, was subjected to an average HP concentration of ∼12 mg L−1 for 3 h. The ammonium- and nitrite-degrading efficiencies of both the Biomedia and the BIO-BLOK® filters were drastically reduced. The filters had not reverted to pre-HP exposure efficiency after 24 h, suggesting a possible long-term impact on the biofilters.

The impact of water exchange rate on the health and performance of rainbow trout Oncorhynchus mykiss in water recirculation aquaculture systems

A controlled six-month study was conducted in six replicated water recirculation aquaculture systems (WRAS) to assess fish health indicators in relation to high feeding/low flushing conditions. Juvenile rainbow trout (Oncorhynchus mykiss) were stocked in six identical WRAS (1000 fish per system), and were maintained over the study period between 25 kg/m3 (minimum) and 80 kg/m3 (maximum) densities. Three WRAS received a relatively high makeup water exchange rate (2.6%), while three systems received only 0.26% exchange; mean feed loadings of 0.39 and 4.1 kg/day per m3/day of makeup water flow were maintained for the high and low exchange treatments, respectively. At the end of the study period, there were surprisingly no significant (p<0.05) differences in average fish weight between treatment groups. Percentage survival was excellent in both high and low makeup system populations (99.5%±0.1 and 98.9%±0.4, respectively). Histopathological evaluation revealed a significant increase in splenic and skin lesions in low makeup system fish relative to those sampled from the high makeup systems, while lesions in the posterior kidney were more prevalent in the high makeup system fish. Plasma chloride and blood urea nitrogen were also significantly different between fish from the two treatment groups, and caudal fin quality assessment showed significantly greater fin erosion in low makeup system fish. Despite these clinical and subclinical differences, there appeared to be no major treatment effect on overall fish performance.