Reducing phosphorus emissions from net cage fish farming by diet manipulation.

To alleviate the environmental impact of net cage fish farming in terms of phosphorous (P) emissions to the Baltic Sea, this study aimed at developing and documenting a diet concept for large rainbow trout (Oncorhynchus mykiss) in brackish water (∼15 ppt) that minimizes the excretion of dissolved P and reduces the excretion of particulate P without compromising fish performance and gonadal development. This was to be achieved by reducing the total dietary P content and matching dietary bioavailable P concentrations to fish requirements using whole-body P concentration and expected individual raw material digestibility as criteria. The diet concept was firstly tested in a laboratory mass-balance study with all female rainbow trout (∼1100 g fish-1) fed three commercial-like low-P diets with 0.74% total P, 0.67% total P, or 0.62% total P plus phytase. Comparing the highest and lowest P diets showed that it was possible to reduce the excretion of dissolved P by 87% to 0.08 g dissolved P kg-1 biomass gain without compromising P requirements and fish performance. To verify the concept on commercial scale, an 8 mm P-reduced test diet with 0.63% total P and targeted a bioavailable P concentration of 0.41% by adding phytase was tested against a commercial control diet with 0.81% total P, feeding each diet to four commercial net cages for 5½ months. Harvest data along with ovary and whole-body P analysis confirmed that there were no performance differences between treatment groups, further sustaining that the specific P discharge may be reduced from an estimated 5.1 to 3.2 kg P t-1 fish produced by minimizing the total dietary P content while tailoring the bioavailable P concentration to match fish requirements. Applying the diet concept to the current (2020) Baltic salmonid production could theoretically reduce P emissions by 147 t yr-1 including 79 t dissolved P and 68 t particulate P.

Biowaste and by-products as rearing substrates for black soldier fly (Hermetia illucens) larvae: Effects on larval body composition and performance.

Black soldier fly (Hermetia illucens) larvae can convert biowaste and by-products into body mass high in protein (~40% dry matter, DM) and lipid (~30% DM). However, the type of rearing substrate also affects the larval body composition and thus its nutritional value. Hitherto, it remains unclear how and to what extent the larval body composition can be altered by the substrate. This study was therefore performed to examine the possibilities of modifying larval body composition using different rearing substrates. To investigate this, 5-days old larvae were reared for seven days on different locally available waste and by-products: brewer's spent grain, mitigation mussels (Mytilus edulis), rapeseed cake, and shrimp waste meal (Pandalus borealis). Larval composition and performance were compared to larvae reared on a commercial chicken feed as well as a mixed feed (mixture of chicken feed and by-products, with a similar macronutrient composition to chicken feed). Larval body weight was recorded daily to determine growth over time whereas larvae and substrates were sampled at the start and end of the trial and analysed for their nutritional composition. The type of rearing substrate affected both larval body composition and growth performance. There was a clear relation between the nutritional composition of the substrate and larvae for certain fatty acids. Larvae reared on marine-based waste substrates contained a higher share of omega-3 fatty acids than larvae reared on the other substrates, indicating an accumulation of omega-3 fatty acids from the substrate. There was a strong positive linear correlation between the ash content in the substrate and larvae whereas larval lipid, protein, amino acid, and chitin content seemed more affected by larval development. Overall, this study showed that the rearing substrate affects larval composition and development, and that larval composition of certain nutrients can be tailored depending on further food and feed applications.

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.

Saline fish wastewater in biogas plants - Biomethanation toxicity and safe use.

Increasing marine land-based recirculating aquaculture systems (RAS) and stricter environmental regulations, pose new challenges to the aquaculture industry on how to treat and dispose saline fish wastewater. The fish wastewater could be incorporated into biogas reactors, but currently, the effects of salinity on the biomethanation process are poorly known. This study aimed to assess the toxicity of fish wastewater with different salinities on the biomethanation process and to propose optimum co-digestion scenarios for maximal methane potential and safe use in biogas plants. Results showed that, depending on salinity and organic content, it is possible to efficiently co-digest from 3.22 to 61.85% fish wastewater (v/v, wastewater/manure) and improve the maximum methane production rate from 2.72 to 61.85%, respectively compared to cow manure mono-digestion. Additionally, salinity was identified as the main inhibitor of biomethanation process with a half-maximal inhibitory concentration (IC50) of 4.37 g L-1, while sulphate reduction was identified as a secondary inhibitor.

Nitrate removal microbiology in woodchip bioreactors: A case-study with full-scale bioreactors treating aquaculture effluents.

Woodchip bioreactors are viable low-cost nitrate (NO3-) removal applications for treating agricultural and aquaculture discharges. The active microbial biofilms growing on woodchips are conducting nitrogen (N) removal, reducing NO3- while oxidizing the carbon (C) from woodchips. However, bioreactor age, and changes in the operating conditions or in the microbial community might affect the NO3- removal as well as potentially promote nitrous oxide (N2O) production through either incomplete denitrification or dissimilatory NO3- reduction to ammonium (DNRA). Here, we combined stable isotope approach, amplicon sequencing, and captured metagenomics for studying the potential NO3- removal rates, and the abundance and community composition of microbes involved in N transformation processes in the three different full-scale woodchip bioreactors treating recirculating aquaculture system (RAS) effluents. We confirmed denitrification producing di­nitrogen gas (N2) to be the primary NO3- removal pathway, but found that 6% of NO3- could be released as N2O under high NO3- concentrations and low amounts of bioavailable C, whereas DNRA rates tend to increase with the C amount. The abundance of denitrifiers was equally high between the studied bioreactors, yet the potential NO3- removal rates were linked to the denitrifying community diversity. The same core proteobacterial groups were driving the denitrification, while Bacteroidetes dominated the DNRA carrying microbes in all the three bioreactors studied. Altogether, our results suggest that woodchip bioreactors have a high genetic potential for NO3- removal through a highly abundant and diverse denitrifying community, but that the rates and dynamics between the NO3- removal pathways depend on the other factors (e.g., bioreactor design, operating conditions, and the amount of bioavailable C in relation to the incoming NO3- concentrations).

Nitrate removal from aquaculture effluents using woodchip bioreactors improved by adding sulfur granules and crushed seashells.

This study examined the effects on nitrate removal when adding sulfur granules and crushed seashells to a woodchip bioreactor treating aquaculture effluents. Using a central composite design, the two components were added at three levels (0.000, 0.125 and 0.250 m3/m3 bioreactor volume) to 13 laboratory-scale woodchip bioreactors, and a response surface method was applied to find and model the optimal mixture ratios with respect to reactor performance. Adding 0.125 m3/m3 sulfur granules improved the total N removal rate from 3.27 ± 0.38 to 8.12 ± 0.49 g N/m3/d compared to pure woodchips. Furthermore, the inclusion of crushed seashells together with sulfur granules helped to maintain the pH above 7.4 and prevent a production (i.e., release) of nitrite. According to the modeled response surfaces, a sulfur granule:crushed seashell:woodchip mixture ratio containing about 0.2 m3 sulfur granules and 0.1 m3 crushed seashells per m3 reactor volume would give the best results with respect to high N removal and minimal nitrite release. In conclusion, the study showed that N removal in woodchip bioreactors may be improved by adding sulfur granules and seashells, contributing to the optimization of woodchip performance in treating aquaculture effluents.

Rhythmicity and plasticity of digestive physiology in a euryhaline teleost fish, permit (Trachinotus falcatus).

Digestive physiology is considered to be under circadian control, but there is little evidence in teleost fish. The present study explored the rhythmicity and plasticity to feeding schedules of enzymatic digestion in a candidate aquaculture fish, the permit (Trachinotus falcatus). The first experiment identified the rhythms of digestive factors throughout the light-dark (LD) cycle. Gastric luminal pH and pepsin activity showed significant daily variation albeit not rhythmic. These dynamic changes were likewise observed in several digestive enzymes, in which the activities of intestinal protease, chymotrypsin and lipase exhibited significant daily rhythms. In the second experiment, the existence of feed anticipatory activity in the digestive factors was investigated by subjecting the fish to either periodic or random feeding. Anticipatory gastric acidification prior to feeding was identified in periodically fed fish. However, pepsin activity did not exhibit such anticipation but a substantial postprandial increase was observed. Intestinal protease, leucine aminopeptidase and lipase anticipated periodic mealtime with elevated enzymatic activities. Plasma melatonin and cortisol demonstrated robust daily rhythms but feeding time manipulations revealed no significant impact. Plasma ghrelin level remained constant during the LD cycle and appeared to be unaffected by differing feeding regimes as well. Taken together, the digestive factors of permit were highly dynamic during the LD cycle. Periodic feeding entrained digestive physiology and mediated anticipatory gastric acidification and intestinal enzymatic activities. This knowledge will be essential in developing feeding protocols and husbandry-related welfare strategies that will further advance this candidate finfish as an aquaculture species.

Innate immune defenses exhibit circadian rhythmicity and differential temporal sensitivity to a bacterial endotoxin in Nile tilapia (Oreochromis niloticus).

The present study investigated the daily dynamics of humoral immune defenses and the temporal influence in the sensitivity of these responses to a bacterial endotoxin in Nile tilapia (Oreochromis niloticus). The first experiment subjected the fish to two photoperiod conditions, 12L:12D (LD) and 0L:24D (DD), for 20 days to characterize the rhythms of humoral immunity. Serum alkaline phosphatase (ALP), lysozyme (LYZ), peroxidase (PER) and protease (PRO) exhibited significant rhythmicity under LD but not in DD. No significant rhythms were observed in esterase (ESA) and anti-protease (ANTI) in both photoperiod conditions. Fish reared under LD were subsequently subjected to DD while the group previously under DD was exposed to LD, and this carried on for 3 days before another set of samples was collected. Results revealed that the rhythms of LYZ, PER and PRO but not ALP persisted when photoperiod was changed from LD to DD. Nonetheless, immune parameters remained arrhythmic in the group subjected from DD to LD. Cluster analysis of the humoral immune responses under various light conditions revealed that each photic environment had distinct daily immunological profile. In the second experiment, fish were injected with bacterial endotoxin lipopolysaccharide (LPS) either at ZT3 (day) or at ZT15 (night) to evaluate the temporal sensitivity of humoral immunity to a pathogen-associated molecular pattern. The results demonstrated that responses to LPS were gated by the time of day. LPS significantly modulated serum ALP and ANTI activities but only when the endotoxin was administered at ZT3. Serum LYZ and PER were stimulated at both injection times but with differing response profiles. Modulated LYZ activity was persistent when injected at ZT3 but transient when LPS was applied at ZT15. The magnitude of LPS-induced PER activity was higher when the endotoxin was delivered at ZT3 versus ZT15. It was further shown that plasma cortisol was significantly elevated but only when LPS was administered at ZT3. On the other hand, plasma melatonin was significantly affected by LPS injection but only when exposed at ZT15. Taken together, this study shows that several key components of humoral immunity in tilapia exhibit circadian rhythms and adapt to photoperiodic changes. Further, results of the bacterial endotoxin challenge suggest that responsiveness of serum humoral factors to a biological insult is likely mediated by the time of day, highlighting the importance of circadian rhythm in the immunological functions of fish.

Humoral and mucosal defense molecules rhythmically oscillate during a light-dark cycle in permit, Trachinotus falcatus.

Circadian rhythm provides organisms with an internal system to maintain temporal order in a dynamic environment. This is typified by a 24-h cycle for a number of physiological processes, including immunity. The present study characterized the humoral and mucosal defense molecules and their dynamics during a light-dark (LD) cycle in juvenile permit, Trachinotus falcatus. All studied defense molecules were constitutively identified in serum and skin mucus. Serum generally exhibited higher levels of these defenses than skin mucus, with the exception of anti-protease (ANTIPRO). The difference in ANTIPRO, lysozyme (LYZ), esterase (ESA) and catalase (CAT) levels between serum and skin mucus was not affected by the phase of the daily cycle. However, a clear phase-dependent difference was observed in protease (PRO), globulin (GLOB), myeloperoxidase (MPO), alkaline phosphatase (ALP) and glutathione peroxidase (GPX) levels. Activities of ALP and GPX displayed significant daily rhythmicity in both serum and skin mucus. Circadian profile of ALP was identical in both biofluids, but an antiphasic feature was exhibited by GPX. GLOB and MPO levels also exhibited significant daily oscillation but only in serum with acrophases registered at ZT 14.5 and 6.15, respectively. Mucus PRO and serum ANTIPRO demonstrated significant temporal variations during a daily cycle albeit not rhythmic. Cluster analysis of the defense molecules in serum and skin mucus revealed two different daily profiles suggesting a possibility of distinct circadian control between humoral and mucosal immunity. These observations indicate that LD cycle had a remarkable impact in the defense molecules characterizing the humoral and mucosal immunity in permit. Daily rhythmic patterns of these defense molecules contribute to our understanding of the barely explored interplay of immunity and circadian rhythm in teleost fish. Lastly, the results could be useful in developing aquaculture practices aiming at modifying the immune functions of permit for improved health.