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In this study, water quality and fish performance of traditional RAS with nitrifying biofilters were compared with systems operated under heterotrophic N assimilation (HET-N), a process where bacteria consume ammonium directly for growth and thereby remove dissolved N excreted from the fish, using three different modes of carbon addition. Using twelve identical pilot scale RAS, four treatment groups were established in triplicate: RAS with autotrophic biofilters (Control), RAS with autotrophic biofilters and acetate addition (BF RAS +Acetate), RAS without biofilter with acetate addition (BF RAS -Acetate) and RAS without biofilter and a biopellet reactor (Biopellet RAS). The nine RAS with carbon addition all had lower levels of nitrate and orthophosphate at the end of the trial compared to the three control RAS (approx. 70% less NO3- in the Biopellet RAS and 72% less PO43- in BF RAS -Acetate). Without biofilters installed, both BF RAS -Acetate and Biopellet RAS maintained acceptable water quality parameters during their respective start-up phases and were fully developed in under 3 weeks. The addition of acetate to the water caused an expected formation of bioflocs in the systems, and a significant increase in bacterial activity and turbidity. Substantial feed spill was observed in RAS with acetate addition. The absence of bacterial accumulation and no increase in turbidity in the water in Biopellet RAS suggest that the processes primarily occurred within the reactor. The overall fish mortality was <1%, however, both types of RAS with acetate addition led to reduced fish growth (7.4–20%) compared to the control RAS and the RAS with biopellets. Biopellets were found to reduce dissolved N and P, and had a fast start up time without deteriorating water quality, thereby showing promising traits for use in RAS.

Physical enrichment can improve the welfare of captive fish. Previous research has shown that fish often show preference for enriched environments, which can also result in improvements in growth performance. However, effects of enrichment are not always positive and the design and extent of the enrichment needs to be carefully considered. In this regard, information in real aquaculture scenarios is limited. The aim of this study was to serve as a proof of concept to test the feasibility of using simple PVC immersed shelters as a tool for better welfare in an organic rainbow trout farm. Our shelters induced little extra work in farm routines and had no negative effects on fish performance, health or mortality. The behavioral assessment pointed to a preference for sheltered areas in undisturbed conditions. However, no benefits were observed in terms of stress responses during standardized stress tests, and fish showed no obvious shelter‐seeking behavior after disturbance. The results in terms of shelter‐seeking behavior were probably limited by the short duration of the experiment, which was due to the farm’s routines and needs. It is recommended that strategies for enrichment in real scenarios should be tested covering a relevant part of the life cycle of the fish in captivity, to fully account for their potential to improve welfare in aquaculture

Sea trout (anadromous brown trout, Salmo trutta L.) is a popular recreational angling target species in the Baltic Sea region and beyond. Across countries, a substantial part of angled sea trout is released after capture either due to management regulations or due to voluntary decisions by the anglers. However, information about post-release impacts of catch and release (C&R) on survival and growth in saline waters is largely missing. We used a flow-through seawater raceway (4–10 °C) to investigate impacts on survival and growth of angled and released sea trout (< 40 cm) up to 29 days post release. Bleeding was common among angled sea trout, but differed between angling treatments, i.e., lure fishing with treble hook (size 4), lure fishing with single hook (size 1/0) and fly fishing with single hook (size 12). However, no mortality and no significant differences in growth were found after a 26–29 days monitoring period among a control group and the three treatment groups, but a small subset (6%) of the angled fish caught on lure had unhealed/infected hooking wounds at the end of the experiment. The results infer that adverse effects of C&R on coastal sea trout due to high post-release mortality or reduced growth can be limited and may not pose a significant problem for sea trout stocks. Nevertheless, further studies are required to corroborate these results under more natural field conditions and at higher water temperatures. In addition, further studies on long-term sublethal impacts of C&R on sea trout are needed.

Treatment development for parasitic infestation is often limited to disease resolution as an endpoint response, and physiological and immunological consequences are not thoroughly considered. Here, we report the impact of exposing Atlantic salmon affected with amoebic gill disease (AGD) to peracetic acid (PAA), an oxidative chemotherapeutic. AGD-affected fish were treated with PAA either by exposing them to 5 ppm for 30 min or 10 ppm for 15 min. Unexposed fish from both infected and uninfected groups were also included. Samples for molecular, biochemical, and histological evaluations were collected at 24 h, 2 weeks, and 4 weeks post-treatment. Behavioral changes were observed during PAA exposure, and post-treatment mortality was higher in the infected and PAA treated groups, especially in 10 ppm for 15 min. Plasma indicators showed that liver health was affected by AGD, though PAA treatment did not exacerbate the infection-related changes. Transcriptome profiling in the gills showed significant changes, triggered by AGD and PAA treatments, and the effects of PAA were more notable 24 h after treatment. Genes related to immune pathways of B- and T- cells and protein synthesis and metabolism were downregulated, where the magnitude was more remarkable in 10 ppm for 15 min group. Even though treatment did not fully resolve the pathologies associated with AGD, 5 ppm for 30 min group showed lower parasite load at 4 weeks post-treatment. Mucous cell parameters (i.e., size and density) increased within 24 h post-treatment and were significantly higher at termination, especially in AGD-affected fish, with some treatment effects influenced by the dose of PAA. Infection and treatments resulted in oxidative stress-in the early phase in the gill mucosa, while systemic reactive oxygen species (ROS) dysregulation was evident at the later stage. Infected fish responded to elevated circulating ROS by increasing antioxidant production. Exposing the fish to a crowding stress revealed t

The present study investigated the involvement of key molecular regulators of oxidative stress in amoebic gill disease (AGD), a parasitic infestation in Atlantic salmon. In addition, the study evaluated how these molecular biomarkers responded when AGD-affected fish were exposed to a candidate chemotherapeutic peracetic acid (PAA). Atlantic salmon were experimentally infected with the parasite Neoparameoba perurans, the causative agent of AGD, by bath exposure and after 2 weeks, the fish were treated with three commercial PAA products (i.e., Perfectoxid, AquaDes and ADDIAqua) at a dose of 5 ppm. Two exposure durations were evaluated – 30 min and 60 min. Sampling was performed 24 h and 2 weeks after PAA treatment (equivalent to 2- and 4-weeks post infection). At each sampling point, the following parameters were evaluated: gross gill pathology, gill parasitic load, plasma reactive oxygen species (ROS) and total antioxidant capacity (TAC), histology and gene expression profiling of genes with key involvement in oxidative stress in the gills and olfactory organ. AGD did not result in systemic oxidative stress as ROS and TAC levels remained unchanged. There were no clear patterns of AGD-mediated regulation of the oxidative stress biomarkers in both the gills and olfactory organ; significant changes in the expression were mostly related to time rather than infection status. However, the expression profiles of the oxidative stress biomarkers in AGD-affected salmon, following treatment with PAA, revealed that gills and olfactory organ responded differently – upregulation was prominent in the gills while downregulation was more frequent in the olfactory organs. The expression of catalase, glutathione S-transferase and thioredoxin reductase 2 was significantly affected by the treatments, both in the gills and olfactory organ, and these alterations were influenced by the duration of exposure and PAA product type. Parasitic load in the gills did significan

Biofilters are key components in recirculating aquaculture systems (RAS). For ensuring cost-efficient function of biofilters, characteristics of biocarriers such as shape, material, surface area, density, longevity, and price needs to be evaluated. In this study, we compared otherwise identical biocarriers made of polyethylene (PE) against ones made of polypropylene (PP) and PP biocarriers with three different levels of pigment (carbon black) addition (without added pigment, semi-pigmented, fully-pigmented). The four types of biocarriers were tested in 12 parallel moving bed bioreactors under identical conditions receiving water from a semi-commercial freshwater RAS with rainbow trout. Measurements of total ammonium nitrogen (TAN) and nitrite-nitrogen (NO2--N) removal rates and nitrification kinetics were carried out by spiking events with NH4Cl or NaNO2 two, four, and six weeks after start-up. The results showed all biofilters removed TAN only after two weeks of operation, and that biofilters with PP biocarriers had a shorter maturation time, reaching their maximum TAN removal rate at week 4, while PE biofilters reached equal maximum at week 6. Biofilters with PE had generally lower NO2--N removal rates and first-order reaction constants than PP biofilters. Pigmentation level had a minor effect on TAN and NO2--N removal rates and kinetics, as fully pigmented biocarriers initially were found to have lower removal rates and first order constants for TAN and NO2--N than semi-pigmented or biocarriers without added pigment. Overall, the results suggest that PE is inferior biocarrier material than PP, due to the longer start-up period and nitrite accumulation, which might be related to the differences in the surface characteristics between these two materials. Furthermore, the results indicate that pigment addition does not provide any extra benefit for nitrifica

The project investigated the potential of peracetic acid (PAA) as a treatment for amoebic gill disease (AGD) in Atlantic salmon. A series of studies were performed to 1) investigate the decay of PAA and the factors that influence this inherent chemical behaviour, 2) document the health and welfare consequences of using PAA in salmon smolts, and 3) evaluate the amoebicidal activity of PAA and its efficacy to treat AGD. Temperature, salinity, light and UV affected the decay dynamics of PAA. Salmon smolts could tolerate PAA doses of 0.6 to 10 ppm, however, behavioural and physiological responses were significantly influenced by duration and frequency of exposure, as well as with PAA commercial products. Stress and disease status of fish were documented to influence the responses of salmon to PAA. The application of PAA was identified to be a mild stressor and could induce both transient and systemic oxidative systems. Different physiological adaptive responses were mounted by the fish to counteract these impacts. PAA exhibited amoebicidal activity in vitro. Exposure of AGD-affected salmon to PAA using different treatment protocols resulted in an equivocal disease resolution. Though treatment could reduce parasite load, gross and microscopic pathologies persisted after the treatments. New insights about the pathophysiology of AGD had been generated particularly on the role of oxidative stress and circulating metabolites. Additional treatment optimisation is required for PAA as a treatment for AGD. Nonetheless, the project provided significant advancements in the chemistry and physiology of PAA in fish that will be crucial in developing the evidence-driven application of this disinfectant in aquaculture.

Recirculating aquaculture systems (RAS) have been promoted as a sustainable supplement to net pen aquaculture and land-based flow-through systems, and RAS is currently a commonly used production concept. RAS have numerous environmental assets such as decreased water consumption, but there are challenges related to water quality control and use of biocides in some systems. As the retention time and degree of reuse of water increases, the nutrients and organic matter accumulate causing favorable conditions for micro-organism growth, which can result in decreasing water quality including blooms of harmful micro-organisms. Treatment of these blooms includes application of disinfectants such as formalin, hydrogen peroxide and peracetic acid. Recent new knowledge on the microbial dynamic and water quality in RAS has enabled development of alternative treatment methods, such as membrane filtration, UV, and ozonation, and preliminary studies have identified the potential of biological control of micro-organisms by their naturally-occurring zooplankton predators. However, these methods are not optimized and costly (physical control) or not tested in large scale (biological control), which hampers their application in aquaculture systems. Further, the sources, dynamics and environmental control of the development of micro-organism blooms are still not understood. For instance, it is conceivable that biofilter could function as a ‘ticking bomb’ and that imbalances in the biofilter communities could be the source of micro-organism blooms in treatment tanks. The main aim of our project ‘Towards biocide-free recirculating aquaculture systems’ (TOBIFREE) was to provide new knowledge of the causes and treatments of microalgal blooms in RAS systems that could result in reduction of the use of biocides in aquaculture. Specifically we wanted to investigate 1) the biofilter communities and whether they might act as a source for micro-organism blooms in growing tanks, 2) the potential a

Plastic is commonly used as biofilter media in recirculating aquaculture systems. Because plastic is relatively expensive and may erode and emit microplastics to the environment, efforts are being made to test and develop more sustainable materials. Five alternative locally available biofilter media were compared with commercial plastic media and evaluated in duplicate in 1 m3 two pilot scale Recirculation aquaculture system. Ammonium chloride and sodium nitrite were added to the systems for 4 weeks followed by stocking 20 kg of Nile tilapia in each system. Volumetric total ammonia nitrogen (TAN), nitrite and oxygen conversion rates were assessed for ten weeks. All biofilters with local media matured and reached full capacity after six weeks, while commercial plastic biomedia matured after seven weeks. This study found that the performance of commercial plastic biomedia was similar to performance of coconut shells in terms of volumetric TAN conversion rate (VTR), volumetric nitrite conversion rate (VNR) and volumetric oxygen conversion rate (VOCR). The highest VTR recorded in this study was 599 ± 15.8 g TAN/m3/d from coconut shells while the lowest was 343 ± 8.9 g TAN/m3/d from cattle horns. Biofilters with commercial plastic media had the highest VNR (704 ± 50.3 g NO2–N/m3/d) while media made of cattle horns was the lowest (457 ± 46.1 g NO2–N/m3/d). Biofilters containing coconut shells demonstrated the highest oxygen consumption around 3.0 g/m3/d and biofilters containing charcoal consumed less than 1.0 g/m3/d of oxygen. This study suggests that coconut shells can be used in place of plastic materials in simple recirculation aquaculture system biofiltration. This study also recommends further studies on comparing coconut shells with other biomedia and assessing its effects on water quality parameters and durability.

Mucous cells, the microscopic structural hallmark of mucosal surfaces, are highly responsive to environmental changes. Here we report how the gills of Atlantic salmon (Salmo salar) smolts responded to peracetic acid (PAA), a potent oxidative disinfectant and a candidate chemotherapeutant in aquaculture, through the Mucosal Mapping showing mucous cell size, volumetric density and defence activity, coupled with two-way histopathological scoring strategies. Two hundred and forty smolts were exposed to therapeutic doses of PAA on two occasions. The initial exposure included a 5-min bathing at concentrations of 0, 0.6, 1.2, and 2.4 ppm PAA. After a two-week recovery, the treatment groups were re-exposed to the same PAA concentrations for 30 min. Gill samples were collected at 2 h, 2 days, and 2 weeks after each exposure. The dynamic changes (i.e., size, volumetric density and defence activity) of the mucous cells were analysed on the distinct mucous cell populations in the gill filament and the lamella, as well as the lamellar thickness. Lamellar mucous cells were always significantly smaller (<70 μm2) and less dense (<2% volume) than those in the filament (70–100 μm2 and 8–11% volume) giving defence activities (combination of mean mucous cell area and volumetric density in a given tissue) of 0.1–0.4 for lamellae and about 0.6–1.4 for filaments, consistent with the functions of these branchial areas. A transient sub-acute mucous cell hypertrophy was a striking response in the gill lamella to PAA during the initial and re-exposure, particularly demonstrated by the groups exposed to 0.6 and 1.2 ppm where this size change was significant. Nonetheless, the recovery was quick, suggesting more an effect of general stress than dose of PAA. Similarly, a transient reduction in hyperplasia was noted as mucous cell density decreased on the filament, but generally significant hyperplasia was not detected, and volumetric density remained unaltered regardless o

Formalin is applied in certain aquaculture systems to control parasites infestations as well as bacterial and fungal diseases. This study investigated the capacity of end-of-pipe denitrifying woodchip bioreactors to remove potentially harmful amounts of residual formaldehyde (FA) from aquaculture effluents. Formaldehyde was readily removed by experimental- and field-scale denitrifying woodchip bioreactors and the removal of FA was found to be a combination of an initial adsorption of FA to woodchip surfaces (52 ± 2.8 g FA/m3 woodchips) and microbial degradation. Volumetric FA removal rates reaching 261 ± 27 g FA/m3/d were found at FA inlet concentrations of 90 mg FA/L and hydraulic retention times (HRT) of 5 h. High FA removal efficiencies ranged from 88.3 ± 4.6–99.8 ± 0.2% found for FA inlet concentrations –up to 105 mg FA/L and HRTs between 3.4 and 15 h. Microbial FA degradation rates in woodchip bioreactors were positively correlated to temperature with a Q10 value of 2.27 and a corresponding Arrhenius temperature coefficient of 1.086 for the investigated temperature range of 7–23 ◦C. At a commercial, outdoor recirculating aquaculture system (RAS) three full-scale woodchip compartments, achieved an average volumetric FA removal rate of 29.4 ± 0.2 g FA/m3/d and a removal efficiency of 82.5 ± 0.8% during the first 24 h following addition of FA. The results demonstrated that woodchip bioreactors are efficient in removing residual FA from RAS effluents and that nitrate removal was transiently enhanced during FA removal.

The effect of water velocity on nitrification rates in fixed bed biofilters was investigated in three freshwater pilot scale RAS with rainbow trout. Removal of total ammonia nitrogen (TAN) and nitrite-nitrogen were assessed by NH4Cl spikes and tested at four different water velocities in the biofilters (1.4, 5.4, 10.8 and 16.2 m h-1) under identical conditions. Water velocities below 10.8 m h-1 significantly reduced TAN- and nitrite removal rates. The surface specific TAN removal rates correlated with the TAN concentrations at the water velocities 10.8 and 16.2 m h-1, and the first order surface removal rate constant was estimated at 0.45 m h-1. However, no correlations between TAN removal and TAN concentrations were found at the lowest velocities. Up to five-fold elevated nitrite levels were found in the RAS when biofilters were operated at 1.4 m h-1 compared to the trials at other water velocities, substantiating the significant effect of water velocity on both nitrification processes. The importance of biofilter hydraulics documented in this pilot scale RAS probably have implications for design and operation in larger scale RAS.

Foam fractionation is often considered an ineffective way of removing organic matter from freshwater due to the low surface tension of the water. There is, however, a lack of studies testing foam fractionation efficiency in replicated freshwater recirculating aquaculture systems (RAS). Foam fractionation can be applied with or without ozone. Ozone is a strong oxidizer previously shown to improve water quality and protein skimmer efficiency. To test the efficiency of foam fractionation and ozonation (20 g O3 kg-1 feed) separately and in combination in freshwater RAS, a two-by-two factorial trial was conducted with each main factor at two levels (applied or not applied). Each treatment combination was carried out in triplicates using 12 replicated pilot scale RAS stocked with juvenile rainbow trout (Oncorhynchus mykiss) and operated at a feed loading of 1.66 kg feed m-3 make-up water. The trial lasted 8 weeks and samples were obtained once a week. Ozone applied by itself significantly reduced the number of particles (83%), bacterial activity (48%) and particulate BOD5 (5-days biochemical oxygen demand; 54%), and increased ultra violet transmittance (UVT; 43%) compared to the untreated control group. Foam fractionation by itself lead to significant reductions in particle numbers and volume (58 and 62%, respectively), turbidity (62%), bacterial activity (54%) and total BOD5 (51%). A combination of both treatments resulted in a significant additional improvement of important water quality variables, including a 75% reduction in total BOD5, 79% reduction in turbidity, 89% reduction in particle numbers and 90% reduction in bacterial activity compared to the control. The removal efficiencies were within the same range as those observed in previous studies conducted with foam fractionators in saltwater systems (with or without ozone), corroborating that foam fractionation may become a useful tool