Land-based Recirculating Aquaculture Systems Research Articles

Measure-correlate-predict methods to improve the assessment of wind and wave energy availability at a semi-exposed coastal area

To reduce the carbon footprint of near-shore and land-based Recirculating Aquaculture Systems (RAS), there is interest to harness renewable energy from local sources. When estimating power availability from wind and waves, it is imperative to ensure that the metocean resource data accurately represents the energy flux at the specific site. Here, four measure-correlate-predict (MCP) models are investigated for wind speed, significant wave height and peak wave period. The MCP relationships are obtained from higher spatial resolution wind and wave data, from a weather research and forecasting model, met-mast measurements at Stord airport (wind), and spectral downscaled DNORA/SWAN model (wave), correlated to the NORA3 database over a common time interval. These relations are used to improve the predictions of metocean parameters from the NORA3 model at the RAS location for the period 2012–2021, and hence allow more accurate estimates of renewable power availability. The prediction accuracy of the MCP models improves in all four cases when an appropriate data sorting criterion and algorithm is implemented. Between 2012 and 2021, average wind power density and wave power density are lowest in July at 182 W/m2 and 0.6 kW/m, and highest in December at 538 W/m2 and 7.5 kW/m, respectively.

Exposure to sublethal levels of H2S does not influence the short-term growth or feed utilization in post-smolt Atlantic salmon (Salmo salar) during restrictive feeding

Hydrogen sulfide (H2S) related mass mortality events in land-based recirculating aquaculture systems (RAS) are associated with the sudden release of substantial amounts of H2S, resulting in acutely toxic exposure. However, the production and release of H2S can also occur at slower constant rates, leading to continuous exposure to sublethal H2S concentrations, the effects of which are unknown. Here, we examined growth rates, feed conversion ratios, and apparent nutrient digestibility in post-smolt Atlantic salmon, following a 10-day exposure to one of two sublethal concentrations of H2S, compared against a control group. The H2S concentrations of the low exposure (LE) and moderate exposure (ME) groups throughout the trial were 1.8 ± 0.8 μg/L and 4.1 ± 1.9 μg/L, respectively. Neither exposure regime affected growth, feed utilization, or the apparent nutrient digestibility. The results suggest that Atlantic salmon tolerate exposure to sublethal H2S concentrations for 10 days, without a discernible effect on the production performance. For aquaculture systems, specifically RAS, this means that while higher concentrations of H2S should be strictly avoided, the presence of low H2S concentrations for a limited period might not be detrimental to fish welfare or production. The RAS environment is complex, and the potential combined effects of H2S and factors pertinent to RAS may influence fish performance differently in a production setting.

Impact of freshwater rearing history on Atlantic salmon gill response to viral stimulation post seawater transfer

Land-based recirculating aquaculture systems (RAS) have risen in prevalence in recent years for Atlantic salmon production, enabling intensive production which allows increased growth and environmental control, but also having the potential for reducing water use and eutrophication. The Atlantic salmon has an anadromous life history with juvenile stages in freshwater (FW) and on-growing in seawater (SW), enabled by a transformational process known as smoltification. The timing of smoltification and transfer of smolts from FW to SW is critical under commercial production with high mortalities during this period. The impact of FW rearing system on immune function following seawater transfer (SWT) is not well understood. In this study parr were raised in either RAS or a traditional open-LOCH system until smolting and then transferred to a common marine environment. Two-weeks post-SWT fish were immune stimulated with a viral mimic (poly I:C) for 24 h to assess the ability to mount an antiviral immune response, assessed by whole transcriptome analysis of gill tissue, an important immune organ in fish. We show that unstimulated smolts reared in the LOCH had higher immune gene expression than those reared in RAS as determined by functional analysis. However, following stimulation, smolts reared in the RAS mounted a greater magnitude of response with a suite of immune genes displaying higher fold induction of transcription compared to LOCH reared smolts. We suggest RAS smolts have a lower steady state immune-associated transcriptome likely due to an unvarying environment, in terms of environmental factors and lack of exposure to pathogens, which shows a compensatory mechanism following stimulation allowing immune ‘catch-up’ with those reared in the LOCH. Alternatively, the RAS fish are experiencing an excessive response to the immune stimulation.

Comparison of three unionid mussel species in removing green microalgae grown in recirculating aquaculture system effluent

Global increase in aquaculture production has created a need to reduce its environmental impacts. Nutrients could be recycled especially at land-based recirculating aquaculture systems (RAS) by cultivating green microalgae in aquaculture effluent. However, microalgae are difficult to harvest. As a multi-trophic solution, mussels could be used in harvesting microalgae. We tested three European freshwater mussels (duck mussel Anodonta anatina, swan mussel A. cygnea, and swollen river mussel Unio tumidus) for filtering two common green microalgae (Monoraphidium griffithii and Selenastrum sp.) grown in RAS effluent. Mussels decreased microalgal concentrations in the tanks 42–83% over three consecutive trials. Algal concentrations at the end of each trial were lowest for both microalgae in tanks containing Anodonta mussels. Clearance rates were higher for Anodonta mussels than for U. tumidus. Mussels biodeposited more microalgae to tank bottoms when M. griffithii was filtered. Ammonium concentration decreased or did not change in tanks with M. griffithii, but increased in tanks containing Selenastrum sp. These results suggest that of the tested species Anodonta mussels and M. griffithii show best potential for RAS effluent bioremediation application. We conclude that a co-culture of microalgae and unionid mussels could be used for recycling nutrients in aquaculture.

A Comparative Analysis of the Nutritional Quality of Salmon Species in Canada among Different Production Methods and Regions

Nutritional information of fresh seafood, including salmon, is not commonly available to the public, which can lead to misconceptions. The aim of this study was to determine the nutritional content of salmon fillets, comparing: (1) Canadian salmon, both wild (pink, chinook, and sockeye) and farmed (Atlantic salmon); (2) Canadian farmed Atlantic salmon grown in ocean net pens or land-based recirculating aquaculture systems (RAS); and (3) farmed Atlantic salmon raised in Canada compared with Scotland, Chile, and Ireland. Samples were purchased from retail stores in Canada and analyzed for moisture, crude protein, total lipid, fatty acids, amino acids, cholesterol, mercury, and color. The greatest differences in nutritional content were between species, rather than if it was wild or farmed. Compared to salmon raised in net pens, salmon raised in RAS had three times more eicosapentaenoic acid (EPA) + docosahexaenoic acid (DHA) per serving (0.7/100 g vs. 2.3/100 g, respectively), twice as much omega-3s (14% vs. 30%) and redder in color (24.7 vs. 30.1) but higher in saturated fats (18% vs. 24%). Scottish salmon had over double the amount of EPA + DHA per 100 g (1.6 g) than salmon from Canada (0.70 g), Chile (0.66 g), and Ireland (0.61 g). While nutritional content differed among salmon types, each type can provide dietary essential nutrients that can benefit consumers.

Early warning through video monitoring: Dissolved hydrogen sulphide (H2S) affects Atlantic salmon swimming behavior in recirculating aquaculture systems

Hydrogen sulphide (H2S) poses a major threat in marine land-based recirculating aquaculture systems (RAS) leading to acute mortality in sensitive fish species such as Atlantic salmon (Salmo salar). To date, little is known about the effects of H2S on the physiology and behavior of the species. The present study analyzed Atlantic salmon swimming behavior in response to H2S in a controlled trial. The setup consisted of two Recirculating Aquaculture Systems (RAS) in parallel. The control RAS comprised of one fish tank (800 L, 10 kg fish/m3 (≈ 70 fish)), while the exposure RAS included two fish tanks (800 L; 10 and 30 kg fish/ m3 (≈ 70 and 200 fish)). Swimming behavior was monitored via a submerged custom-built stereo camera system and an overhead surveillance camera. Fish (smolt, ≈ 114 g) were exposed once a day for 10 consecutive days to increasing H2S concentrations, from ≈ 1- up to ≈ 68 μg/L (2 μM). Continuous measurements of dissolved H2S, O2 and CO2 were taken using a real-time monitoring system. Three swimming parameters were extrapolated from video recordings using machine learning algorithms: i) speed, ii) pattern (representing whether the fish swim in a straight or zigzagging direction) and iii) dispersion (indicative of schooling behavior). The results showed that fish reacted rapidly to H2S, with a stress response characterized by faster swimming speed, erratic pattern, and loss of schooling behavior. The response was concentration-dependent, increasing linearly up to 30–40 μg/L, above which a clear threshold was observed. Notably, concentrations around 40–50 μg/L, induced significantly greater behavioral changes compared to lower concentrations, and further increases in H2S did not lead to additional changes in behavior. Swimming parameters quickly returned to basal levels, comparable to the one's prior exposure, once H2S was no longer present in the water. This study provides new insights on the sensitivity of Atlantic salmon to acute H2S exposure and highlights the potential behind the use of machine vision as an early warning tool for poor water quality in RAS.

Salinity as a tool for strain selection in recirculating land-based production of Ulva spp. from germlings to adults

The genus Ulva is globally distributed and has been thoroughly studied because of its functional biochemical composition, rapid growth rates and opportunistic features, and interest in Ulva cultivation is growing worldwide. In Europe, mostly near- and on-shore flow-through cultivation systems are used and land-based recirculating aquaculture systems (RAS) using fresh water or artificial seawater have not been developed for Ulva. While RAS provides quality control and can be located inland, maintenance costs are high. Using selected strains adapted to low-salinity could reduce seawater production costs and improve the economic feasibility. Therefore, our study assessed how salinity can be used as a tool for strain selection and optimization of functional traits. Growth rates and antioxidant activity of three species (four strains) of tubular and foliose Ulva from the NE-Atlantic and Mediterranean (foliose: Ulva lacinulata – two geographical strains, tubular: Ulva linza and Ulva flexuosa) were followed for three weeks at salinities ranging from 10 to 30 PSU. The tubular strains achieved optimal growth at a lower salinity than U. lacinulata. However, growth rates of both foliose strains were higher than of tubular strains, even at sub-optimal salinity. Therefore, U. lacinulata is a good candidate for RAS with artificial seawater, and the cost of salt can be reduced by up to 33.3% (20 PSU) without significantly reducing the growth rate of U. lacinulata. Higher antioxidant activity was achieved by reducing the salinity to 10 PSU for 10 days, suggesting that the functional traits of cultivated Ulva lacinulata can be optimized prior to harvest.

Commercial production of Florida pompano (Trachinotus carolinus) larvae at low salinity induces variable changes in whole-larvae microbial diversity, gene expression, and gill histopathology

IntroductionSalinity presents economic and technical challenges in land-based recirculating aquaculture systems (RAS) in the U.S. warm water marine finfish aquaculture industry. Many studies have shown euryhaline fish reared at salinities closer to their iso-osmotic salinity can yield enhanced production performance as well as potential reduced costs to farms. However, there is potential for osmotic stress in fish larvae to negatively impact larvae microbiome and innate immune system. Florida pompano (Trachinotus carolinus) is a popular sportfish has been targeted for land-based RAS due to its impressive market value and euryhaline capacity. This study investigated the impacts of rearing Florida pompano larvae at salinities closer to their iso-osmotic salinity.Materials and methodsLarvae were cultured at 10, 20, and 30 ppt in triplicates, and larvae samples were collected for histopathology, microbiome, and whole transcriptomics analysis every three days from hatching until the time of weaning (24 days post hatch [DPH]). Water samples were also taken for microbiome analysis on every other larval sampling day. DiscussionThese changes were driven more by metamorphosis, causing an increase in expression of antioxidant genes (cat, gss, gsto1, and scara3) than by the presence of potentially pathogenic genera, which failed to induce an immune response (low or unchanged expression of downstream elements of the NOD1 or TLR5 pathways). These findings provide baseline information on Florida pompano low salinity tolerance in larviculture during early developmental stages. In addition, we have shown minimal effects on the immune system at salinities as low as 10 ppt. This work has important implications for larval health management and can be used to refine and direct future research regarding improving commercial production of warm water marine species

Sense of place and perceived community change in perceived impacts of and cooperation with local aquaculture development in the US

The emotional, social, and other bonds people have with specific locations, labeled “sense of place” (SoP), have been covered by diverse academic literatures, but conceptual and methodological contradictions and omissions leave their effects unclear. For example, the role of SoP in support of or resistance to siting of potentially hazardous facilities is uncertain due to mixed findings, disparate methods, and no consideration of mechanism(s) for SoP effects. We use a survey (n = 523) of Americans living near three proposed or operating land-based recirculating aquaculture systems (RAS) facilities to explore how multiple dimensions of SoP might affect local support for these developments, with perceived community change and perceived facility impacts as serial mediators. Direct effects of SoP were absent, but indirect effects of identity with place, and especially of social bonding with local family and friends, increased support via positive evaluations of past community change and/or perceived facility impacts; bonding with nature did not affect facility support, or even perception of the facility's environmental impacts. Examining effects of different dimensions of SoP, as well as the mediating effects of seeing community change generally as positive, may provide both the basis for explanatory mechanisms of SoP effects and for divergent findings; however, causal claims would depend on longitudinal or experimental methods.

Determination of the Oxidative Stress Biomarkers of 8-Hydroxydeoxyguanosine and Dityrosine in the Gills, Skin, Dorsal Fin, and Liver Tissue of Atlantic Salmon (Salmo salar) Parr.

Oxidative stress is a condition caused by an imbalance in the occurrence of reactive oxygen species in the cells and tissues of organisms. An ultra-performance liquid chromatography–electrospray ionization tandem mass spectrometry (UPLC–ESI–MS/MS) method was developed for the simultaneous determination of two oxidative stress biomarkers, 8-hydroxydeoxyguanosine (8OHDG) and dityrosine (DIY), in the gills, skin, dorsal fin, and liver tissue of Atlantic salmon (Salmo salar) parr. The use of target analyte-specific 13C and 15N internal standards allowed quantification of each target analyte to be performed through the standard solvent calibration curve. The relative recoveries [mean ± (relative standard deviation%)] of 8OHDG and DIY were 101 ± 11 and 104 ± 13% at a fortified concentration of 10 ng/mL (8OHDG) and 500 ng/mL (DIY), respectively, ensuring the accuracy of the extraction and quantification. The chromatographic separation was carried out using a gradient elution program with a total run time of 5 min. The limits of detection (LODs) were 0.11 and 1.37 ng/g wet weight (w.w.) for 8OHDG and DIY, respectively. To demonstrate the applicability of the developed method, it was applied in 907 tissue samples that were collected from Atlantic salmon parr individuals reared in an experimental land-based recirculating aquaculture system (RAS) treated with peracetic acid. Moreover, the possibility of using the dorsal fin as an alternative matrix for the minimally invasive assessment of oxidative stress in Atlantic salmon parr was introduced. To our knowledge, 8OHDG and DIY were used for the first time as biomarkers for biomonitoring the fish health (oxidative stress) of Atlantic salmon parr in RAS.

Impact of ozone treatment on dissolved organic matter in land-based recirculating aquaculture systems studied by Fourier transform ion cyclotron resonance mass spectrometry

In land-based recirculating aquaculture systems (RAS), the accumulation of dissolved organic matter (DOM) can have detrimental effects on water quality impacting the system performance, microbial community, and consequently fish health and welfare. Ozone is used in the RAS water treatment process to improve water quality and remove DOM. However, little is known about the molecular composition of DOM in RAS and its transformation when exposed to ozone. In this study, we performed a detailed molecular characterization of DOM in RAS and explored its transformation induced by ozonation of RAS waters. Ultra-high resolution (UHR) Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) was used to characterize the DOM matrix of RAS waters (pump-sump and tanks) and to evaluate its transformation by ozonation. The analysis of DOM extracted from makeup water and feed samples allowed for the determination of DOM sources in RAS prior to ozonation. The CHO and unsaturated group of compounds were the most abundant class found in water samples. On the contrary, the DOM from feed samples was unique and consisted mainly of CHO, CHON and unsaturated group of compounds. After the ozonation of RAS waters, humic-like and unsaturated compounds [positive oxygen subtracted double bond equivalent per carbon (DBE–O)/C)] were decomposed, particularly the CHO-DOM that contained fewer -CH2- features. Fulvic-like compounds and several hundred saturated compounds [negative (DBE–O)/C)] were formed post ozonation, particularly the CHON and CHONS group of compounds that were associated with fish diets, makeup waters and transformation products from the ozonation of the RAS waters. This study showed that the high accuracy of the ultra-high resolution FTICR MS can be applied to characterize and monitor the changes of DOM at a molecular level in RAS waters. To our knowledge, this is the first study where FTICR MS was incorporated for the characterization of DOM and its sources in RAS.

Temporal changes in skin and gill microbiomes of Atlantic salmon in a recirculating aquaculture system – Why do they matter?

Mucosal surfaces are key components of teleost health, providing defence against opportunistic pathogens and other insults. Maintaining the integrity of mucosal surfaces and their associated microbial communities, especially the gill and skin that have large surface areas exposed to the environment is essential. Production of Atlantic salmon in land-based recirculating aquaculture systems (RAS) has increased significantly in recent years as it allows greater control over stability of the environment in which fish are reared, reduces water demand and minimises environmental impacts. However, little is known about the impact of the RAS environment upon the temporal dynamics of skin and gill mucosal microbiomes. In this study we examined microbial communities in gill mucus, skin mucus and rearing water throughout freshwater (FW) RAS production, and at 1-week and 4-weeks following transfer to seawater (SW) in open cage production using 16S rRNA sequencing. Microbial diversity and richness in skin and gill mucus of fish reared in a RAS system were temporally dynamic. Dynamics in richness and diversity were similar in the two mucosal tissues, and to some extent also mirrored that of the surrounding water. Dysbiosis indicated by an abrupt decline in diversity during FW production coincided with an increase in the relative abundance of two taxa belonging to the RAS-biofilter-associated nitrogen-cycling genus Hydrogenophaga in RAS tank water and this was also observed in gill and skin mucus. Extensive overlap in core taxa was observed between gill and skin mucus, but host-specific cores were non-existent during the dysbiotic event with all cores present in the rearing water. Diversity remained stable during the transition from FW to SW, but distinct community composition and core taxa were observed in the two environments. Although RAS are closely controlled, significant temporal variation could be observed in temperature as well as levels of CO2 and nitrogen compounds, reflecting the increasing biological load within the system over time. The results presented here suggest that, in terms of microbiomes, dysbiosis may occur in both the RAS environment and fish mucosal surfaces over time, but microbial communities have the capability to recover.

Fish Prisons and Bluehouses: Perceived Risks and Benefits of Land-based Aquaculture in Four US Communities

ABSTRACT The farming of aquatic species in water environments, aquaculture is presently the fastest-growing food producing sector worldwide yet is unfamiliar to many Americans. In this study, we examine perceptions of land-based recirculating aquaculture systems (RAS), a novel approach to raising fish. Through in-depth interviews (n = 71) with diverse stakeholders in four US communities, we explore how individuals make sense of the risks and benefits associated with proposed or existing RAS facilities and situate these judgments in the context of “naturalness.” As a hybrid of fishing and industrial farming, land-based RAS can both support and undermine perceived naturalness, thus posing both perceived benefits and risks to local environments and economies. As a form of restoration, some RAS projects bring economic and environmental revitalization to communities; however, this restoration is understood in the context of a site’s historical use. Findings contribute to emerging environmental scholarship on food systems communication, and offer practical applications for public communication surrounding aquaculture development.

The effects of two water temperature regimes on Atlantic salmon (Salmo salar) growth performance and maturation in freshwater recirculating aquaculture systems

There is increasing investment in and use of land-based recirculating aquaculture systems (RAS) to produce market-size Atlantic salmon (Salmo salar) despite limited understanding of economic feasibility at commercial scale. High incidence of early maturing fish in RAS growout is one obstacle to profitability for this production method. Well-defined, RAS-specific water temperature thresholds that maintain high fish growth performance while minimizing early maturation are needed to accurately forecast bioplans, operational costs, and gains from reduced maturation downgrades. Accordingly, the objective of this research was to compare growth performance and maturation status of diploid, mixed-sex Atlantic salmon grown to ~1.3 kg in land-based RAS at a standard production temperature of 14 °C or at a cooler temperature of 12 °C. Survival (97.7 ± 0.6 vs 98.1 ± 0.3%), final weight (1292 ± 25 vs 1355 ± 31 g), condition factor (1.57 ± 0.01 vs 1.58 ± 0.01), feed consumption (409.2 ± 6.2 vs 449.3 ± 15.8 kg/RAS), and feed conversion ratio (1.11 ± 0.02 vs 1.15 ± 0.01) were similar between temperature treatments while thermal growth coefficient (2.45 ± 0.02 vs 2.18 ± 0.02) was significantly higher at 12 °C. Maturation prevalence (20.4 ± 3.5 vs 32.1 ± 1.4%) was significantly higher at 14 °C and gonadosomatic index of immature fish skewed higher at 14 °C. Overall, lower water temperature reduced prevalence of maturation in RAS while maintaining similar production performance; however, more than 20% of salmon cultured at 12 °C matured indicating even lower temperature, additional manipulations to the RAS environment, or use of all-female stocks are needed to optimize Atlantic salmon growout in RAS for reduced maturation.

Assessing the risk of climate change to aquaculture: a national-scale case study for the Sultanate of Oman

Aquaculture is expanding globally and is an increasingly important component of world food security. However, climate change can impact aquaculture through a variety of mechanisms varying by location and aquaculture type with implications for future productivity. Understanding the risks that climate change poses on different culture systems in different locations is important to enable the design of targeted adaptation and resilience building actions.Here we present an aquaculture climate risk assessment framework, applied to the aquaculture sector of the Sultanate of Oman, that identifies the sensitivity and exposure of different components of the sector to climate change risk.Oman has aspirations to significantly expand aquaculture over the next decade focussing on coastal shrimp ponds, finfish sea cages, land-based recirculating aquaculture systems, and ponds and raceways. We quantify overall climate risk as the combination of four risks: (1) species’ temperature sensitivity, (2) flooding and storm surge exposure, (3) low-oxygen hazard and (4) disease vulnerability. Shrimp culture is identified as highest risk due to high exposure of shrimp ponds to flooding and storm surges, and high disease vulnerability. Seabream cage farming also faces high risk due to high thermal sensitivity and high potential of low-oxygen levels affecting sea cages. Following the risk assessment a stakeholder workshop was conducted to identify targeted adaptation measures for the different components of the sector. The framework for assessing climate risk to aquaculture demonstrated here is equally applicable at the regional, national or sub-national scale to support design of targeted resilience building actions and enhance food security.

Effect of stocking density on growth of largemouth bass (Micropterus salmoides) cultured in containers in a land‐based recirculating aquaculture system (C‐RAS)

To study the effect of different stocking densities on the growth of largemouth bass (Micropterus salmoides) cultured in containers in a land-based recirculating aquaculture system (C-RAS), 13,500 fish with initial average body mass of 137.3 ± 5.46 g were selected and stocked at 1000 fish/container (group 50 fish/m3), 1500 fish/container (group 75 fish/m3) or 2000 fish/container (group 100 fish/m3), with three replicates for each group. Thirty fish were randomly collected from each container on Days 30, 60 and 90 for measurement and analysis. The final body mass, body mass gain rate, daily body mass gain, final body length, body length growth rate and daily body length growth in group 50 fish/m3 were significantly higher than those in groups 75 fish/m3 and 100 fish/m3 (p < 0.05). The relationship between body mass and body length followed a power function, with R2 values of 0.9968, 0.9896 and 0.9902 respectively. Based on our comprehensive analysis, the largemouth bass in this study grew fastest at the density of 50 fish/m3. As the first study of the effect of stocking density on growth of largemouth bass in a C-RAS, these results provide a reference for aquaculture using the relatively new C-RAS system.

Low turbidity in recirculating aquaculture systems (RAS) reduces feeding behavior and increases stress-related physiological parameters in pikeperch (Sander lucioperca) during grow-out.

There is a tendency to farm fish in low turbidity water when production takes place in the land-based recirculating aquaculture systems (RAS). However, the effect of water turbidity on stress and performance is unknown for many species cultured in RAS. The effect of different turbidity treatments as Formazine Attenuation Units (0 FAU, 15 FAU, and 38 FAU) on feed intake performance (latency, total feeding time, and total feed intake) and physiological blood stress parameters (cortisol, lactate, and glucose) in medium-sized pikeperch ((Sander lucioperca) n = 27, undetermined sex and age) of initial body weights of 508.13 g ± 83 g (at FAU 0, 15, and 38, respectively) was investigated. The rearing system consisted of 9 rectangular tanks (200 L per tank). Fish were housed individually (n = 1, per tank, n replicates per treatment = 9). All tanks were connected to a recirculation system equipped with a moving bed biofilter. Feed intake in pikeperch kept at low turbidity (0 FAU) was 25% lower than pikeperch kept at high turbidity (38 FAU) (P < 0.01) and also significantly (10.5%) lower compared to feed intake in pikeperch kept at intermediate turbidity (15 FAU) (P < 0.01 for 0 FAU vs. 15 FAU, feed intake sign. Value as the main effect is P < 0.01). Pikeperch kept at low turbidity showed significantly slower feeding response (latency time) towards pellets entering the tank, shorter feeding times (both P < 0.05), and higher glucose blood concentration (73%) in contrast to pikeperch kept at highest turbidity. A reduction of 25% feed intake has obvious economic consequences for any fish farm and present data strongly emphasize the importance of considering the species-specific biology in future RAS farming.

Microbial communities in full-scale woodchip bioreactors treating aquaculture effluents

Woodchip bioreactors are being successfully applied to remove nitrate from commercial land-based recirculating aquaculture system (RAS) effluents. In order to understand and optimize the overall function of these bioreactors, knowledge on the microbial communities, especially on the microbes with potential for production or mitigation of harmful substances (e.g. hydrogen sulfide; H2S) is needed. In this study, we quantified and characterized bacterial and fungal communities, including potential H2S producers and consumers, using qPCR and high throughput sequencing of 16S rRNA gene. We took water samples from bioreactors and their inlet and outlet, and sampled biofilms growing on woodchips and on the outlet of the three full-scale woodchip bioreactors treating effluents of three individual RAS. We found that bioreactors hosted a high biomass of both bacteria and fungi. Although the composition of microbial communities of the inlet varied between the bioreactors, the conditions in the bioreactors selected for the same core microbial taxa. The H2S producing sulfate reducing bacteria (SRB) were mainly found in the nitrate-limited outlets of the bioreactors, the main groups being deltaproteobacterial Desulfobulbus and Desulfovibrio. The abundance of H2S consuming sulfate oxidizing bacteria (SOB) was 5–10 times higher than that of SRB, and SOB communities were dominated by Arcobacter and other genera from phylum Epsilonbacteraeota, which are also capable of autotrophic denitrification. Indeed, the relative abundance of potential autotrophic denitrifiers of all denitrifier sequences was even 54% in outlet water samples and 56% in the outlet biofilm samples. Altogether, our results show that the highly abundant bacterial and fungal communities in woodchip bioreactors are shaped through the conditions prevailing within the bioreactor, indicating that the bioreactors with similar design and operational settings should provide similar function even when conditions in the preceding RAS would differ. Furthermore, autotrophic denitrifiers can have a significant role in woodchip biofilters, consuming potentially produced H2S and removing nitrate, lengthening the operational age and thus further improving the overall environmental benefit of these bioreactors.

A Temporally Dynamic Gut Microbiome in Atlantic Salmon During Freshwater Recirculating Aquaculture System (RAS) Production and Post-seawater Transfer

Atlantic salmon aquaculture is undergoing an expansion of land-based recirculating aquaculture systems (RAS), especially for freshwater (FW) stages of production. Juvenile salmon undergo parr-smolt transformation, also known as smoltification and become pre-adapted to tolerate seawater (SW). One aspect requiring study is the development of microbial communities during this time, especially in RAS systems. Here we analyzed temporal changes in microbiome associated with the intestine in Atlantic salmon during smolt production in a commercial RAS production facility and followed the same cohort of fish post-seawater transfer (SWT), using 16S rRNA gene sequencing. Microbial diversity and richness showed an increase over time across FW production, but declined sharply and significantly 1-week post-SWT before re-establishing itself with a completely different community structure after 4 weeks. Core microbial taxa could be assigned to three distinct categories; (1) omnipresent, (2) salinity specific, or (3) transient. By including diet and water samples in the analyses, we classified true core taxa associated with the host, those associated with the diet, and transient cores associated with microbial communities in tank water. The rising trend observed in microbial richness in the water may be a consequence of a temporal increase in organic load while dominance of Vibrionaceae may be attributed to the higher temperatures maintained during RAS production and above average natural water temperatures post-SWT. Functional analysis suggests modulation of metabolic pathways post-SWT, but downstream impacts on fish growth and health in a commercial setting remain to be elucidated. A deeper understanding of the interplay between microbial composition and functionality can play a role in optimizing fish performance in tightly regulated RAS production.

Biofilters are potential hotspots for H2S production in brackish and marine water RAS

The production of hydrogen sulphide (H2S) is a major concern and challenge in marine land-based recirculating aquaculture systems (RAS). In order to gain a thorough understanding on H2S production dynamics in RAS, an anaerobic batch reactors experiment was performed, evaluating the production potential of total dissolved sulphide (TDS) and H2S and characterizing the microbial communities in different RAS environments (solid waste, biomedia+RAS-water, biomedia+seawater [SW], and RAS-water) during 33 days. The highest H2S concentrations were achieved in the reactors with biomedia+RAS-water, where biofilms growing on biomedia hosted a high abundance of sulphate reducing bacteria (SRB), which activated once anoxic conditions and carbon sources were introduced. The biofilms in the biomedia+SW reactors produced lower amounts of H2S than the biomedia+RAS-water reactors, due to the lower abundance of SRB. The solid waste produced H2S at low rate, the production being limited by the substrate competition between SRB and other microbial groups. No production of H2S was observed in the reactors with only RAS-water, due to the absence of SRB and suitable carbon sources. Based on measurements of production of H2S in this study, a moving bed biofilter submerged in brackish water (17 ppt) can potentially produce 139 g of H2S per biofilter m3, when the biomedia is not adequately mixed and oxygenated. Altogether, the results demonstrate that although SRB are found both in biofilters and solid waste, these two environments exhibit different H2S production dynamics. Biofilters produce large quantities of H2S under anoxic conditions, while solid waste deposited to the rearing tanks exhibits a lower but constant production of H2S. However, both of these H2S sources can be suppressed through fast removal of solids from rearing tanks and optimal operational conditions for biofilters.