Salmon Farming Research Articles

The effect of subsidies and reform options on profit efficiency of trout farms: Focus on energy subsidies

Energy and imported trout egg subsidies hold a pivotal role for Iranian trout farms. They aim to decrease production costs, yet their impact on the profit efficiency of farms remains dubious. The first objective of this paper is to assess how the current subsidy scheme affects the profit efficiency of hatchery and propagation trout farms in Mazandaran, Iran. We examine several subsidy reform scenarios to enhance profit efficiency as a second objective. Standard profit efficiency (SPE) is used to acquire the efficiency of the trout farms using data envelopment analysis. Results show that the majority of farms are inefficient and highly dependent on subsidies. Comparing farms with high efficiency against those with low efficiency as a third aim of research highlights the importance of farms holding a propagation license, engaging in vertical integration, achieving adult trout production surpassing 81 t, and maintaining a fish total feed consumption to trout production ratio below 0.74. Findings from reformation scenarios reveal 1) redirecting the current subsidy towards purchasing the extruder machines for fish feed production, 2) transferring funds to subsidize electricity based on the farm's electricity consumption, and 3) opting for the production of genetically modified eggs gives higher SPE. Reformation can also reduce the burden on public funds and dependency on importation.

Comparison of transcriptome responses in blood cells of Atlantic salmon infected by three genotypes of Piscine orthoreovirus.

Piscine orthoreovirus (PRV) infection is common in aquaculture of salmonids. The three known PRV genotypes (PRV-1-3) have host species specificity and cause different diseases, but all infect and replicate in red blood cells (RBCs) in early infection phase. PRV-1 is the causative agent of heart and skeletal muscle inflammation (HSMI) in farmed Atlantic salmon (Salmo salar), PRV-2 causes erythrocytic inclusion body syndrome (EIBS) in coho salmon (Oncorhynchus kisutch), while PRV-3 induces HSMI-like disease in farmed rainbow trout (Oncorhynchus mykiss). PRV-3 can also infect A. salmon without causing clinical disease and has been shown to cross-protect against PRV-1 infection and HSMI, while PRV-2 or inactivated adjuvanted PRV-1 vaccine only partially reduced HSMI pathologic changes. In the present work, we studied the transcriptional responses in blood cells of A. salmon two- and five-weeks post infection with PRV-1, PRV-2, PRV-3, or post injection with inactivated PRV-1 vaccine. PRV-1 and PRV-3 replicated well in A. salmon blood cells, and both induced the typical innate antiviral responses triggered by dsRNA viruses. Two weeks post infection, PRV-3 triggered stronger antiviral responses than PRV-1, despite their similar viral RNA replication levels, but after five weeks the induced responses were close to equal. PRV-2 and the InPRV-1 vaccine did not trigger the same typical antiviral responses as the replicating PRV-1 and PRV-3 genotypes, but induced genes involved in membrane trafficking and signaling pathways that may regulate physiological functions. These findings propose that the protection mediated by PRV-3 against a secondary infection by PRV-1 occur due to a potent and early activation of the same type of innate immune responses. The difference in the timing of antiviral responses may give PRV-1 an evolutionary edge, facilitating its dissemination to A. salmon heart, a critical step for HSMI development.

The Atlantic salmon gill transcriptional response to natural infection with HPR0-ISAV (Isavirus salaris) in three Norwegian smolt farms.

Infectious Salmon Anaemia virus (ISAV) is an orthomyxovirus that causes large economic losses in Atlantic salmon (Salmo salar L.) aquaculture. All virulent ISAV variants originally emerged from a non-virulent subtype, ISAV-HPR0. Transient ISAV-HPR0 infections are common in both freshwater and marine environments. ISAV-HPR0 infects juveniles, marine salmon at on-growing sites, and broodstock salmon. The shift in virulence from ISAV-HPR0 to the virulent HPRΔ is suggested to be a stochastic event that depends on the virus's replication frequency. Therefore, reducing the capacity to maintain ISAV-HPR0 infection within individual farms may limit the risk of emerging pathogenic ISAV variants and ISA disease. The absence of infection-related clinical signs and the lack of experimental models limit our understanding of ISAV-HPR0-host interactions. We characterise the host transcriptional response to natural ISAV-HPR0 infection, using Atlantic salmon gill tissues collected on three Norwegian smolt farms. The comparison of all infected (qPCR-positive) and non-infected (qPCR-negative) individuals revealed a classic antiviral response in the gills of ISAV-HPR0 infected fish in a site-independent transcriptomic analysis. Complementary analyses showed that the response to infection varied considerably between sites. Site-specific differences could be associated with a range of factors that are challenging to control in field studies, such as fish size, the stage of infection, and the presence of additional microorganisms. Our findings enhance our understanding of how Atlantic salmon respond to ISAV-HPR0 infection, pinpointing common HPR0-induced antiviral response genes. Future studies should investigate whether these candidate genes limit virus replication in the gill for risk of novel transitions to virulence.

Spatial density, externalities, and productivity of salmon aquaculture farms

This paper estimates the effect of spatial proximity to other salmon farms on aquaculture firms’ costs. Open cage salmon aquaculture farms cause and face externalities, such as various diseases and salmon lice. Hydrodynamic forces and the spatial density of farms influence the transmission of these externalities. We analyze the effects of salmon farm proximity on production cost using a translog cost function estimated from a panel data set of Norwegian aquaculture farms. We employ two indicators of the spatial density of aquaculture production. The first indicator is the mean distance to the nearest farm, and the second indicator is the number of farms within 20 km distance for all the farms. We find that increased proximity to other farms is associated with significant cost increase. The results imply that there are economic welfare effects due to the farm location structure and the spatial biological externalities between farms.

IPNV Inactive Vaccine Supplemented with GEL 02 PR Adjuvant: Protective Efficacy, Cross-protection, and Stability.

Recently, outbreaks of infectious pancreatic necrosis (IPN) in worldwide rainbow trout farms caused by IPN virus (IPNV) strains belonging to genogroup 1 and genogroup 5 are reported. In this study, formaldehyde-inactivated vaccines supplemented with or without GEL 02 PR adjuvant were developed by using both genogroups IPNV strains and were intraperitoneally injected into rainbow trout. At 30 days post-vaccination, the viral loads of IPNV challenged rainbow trout (Oncorhynchus mykiss) in the vaccine groups were significantly decreased compared with those in the PBS group (P < 0.05), about 2.4 log and 2.5 log in the genogroup 1 and 5 IPNV inactivated vaccine with or without GEL 02 PR adjuvant groups, and the protective effect was not weakened after storage of the adjuvant vaccines at 4 °C for 12 months. Each vaccine could stimulate the expression of CD4, CD8, and IgM, and the adjuvant vaccines induced higher neutralizing antibody titers. In the long-term protection test, both the adjuvant vaccines could still effectively reduce IPNV viral loads in fish at 120 days post-vaccination, and the genogroup 5 IPNV inactivated vaccine showed cross-protection against the genogroup 1 IPNV strain. In the study of cell lines and virus seeds, CHSE-214 was successively passaged to 30 generations, and its growth characteristics and sensitivity to IPNV remained stable. When IPNV was passaged to 10 generations, the viral titers were not affected, but mutations were found in the VP2 protein of both genogroup 1 and 5 IPNV strains. The study is conducive to the improvement of IPNV vaccine development for rainbow trout.

Generation of IgM+ B cell-deficient Atlantic salmon (Salmo salar) by CRISPR/Cas9-mediated IgM knockout

Infectious diseases pose significant challenges to Norwegian Atlantic salmon aquaculture. Vaccines are critical for disease prevention; however, a deeper understanding of the immune system is essential to improve vaccine efficacy. Immunoglobulin M (IgM) is the main antibody involved in the systemic immune response of teleosts, including Atlantic salmon. In this study, we used CRISPR/Cas9 technology to knock out the two IgM genes in Atlantic salmon. High-throughput sequencing revealed an average mutagenesis efficiency of 97% across both loci, with a predominance of frameshift mutations (78%). Gene expression analyses demonstrated significantly reduced membrane-bound IgM mRNA levels in head kidney and spleen tissues. Flow cytometry revealed a 78% reduction in IgM+ B cells in peripheral blood, and Western blot analyses showed decreased IgM protein levels in serum. Notably, an upregulation of IgT mRNA was observed, suggesting a potential compensatory mechanism. This work presents the first application of CRISPR/Cas9 to disrupt an immune-related gene in the F0 generation of Atlantic salmon, and lays the foundation for generating a model completely lacking IgM+ B cells which can be used to study the role of B cells and antibodies. This study has implications for advancing immune research in teleosts and for developing strategies to improve salmon health and welfare in aquaculture.

Salmon Consumption Behavior Prediction Based on Bayesian Optimization and Explainable Artificial Intelligence

Predicting seafood consumption behavior is essential for fishing companies to adjust their production plans and marketing strategies. To achieve accurate predictions, this paper introduces a model for forecasting seafood consumption behavior based on an interpretable machine learning algorithm. Additionally, the Shapley Additive exPlanation (SHAP) model and the Accumulated Local Effects (ALE) plot were integrated to provide a detailed analysis of the factors influencing Shanghai residents’ intentions to purchase salmon. In this study, we constructed nine regression prediction models, including ANN, Decision Tree, GBDT, Random Forest, AdaBoost, XGBoost, LightGBM, CatBoost, and NGBoost, to predict the consumers’ intentions to purchase salmon and to compare their predictive performance. In addition, Bayesian optimization algorithm is used to optimize the hyperparameters of the optimal regression prediction model to improve the model prediction accuracy. Finally, the SHAP model was used to analyze the key factors and interactions affecting the consumers’ willingness to purchase salmon, and the Accumulated Local Effects plot was used to show the specific prediction patterns of different influences on salmon consumption. The results of the study show that salmon farming safety and ease of cooking have significant nonlinear effects on salmon consumption; the BO-CatBoost nonlinear regression prediction model demonstrates superior performance compared to the benchmark model, with the test set exhibiting RMSE, MSE, MAE, R2 and TIC values of 0.155, 0.024, 0.097, 0.902, and 0.313, respectively. This study can provide technical support for suppliers in the salmon value chain and help their decision-making to adjust their corporate production plan and marketing activities

Assessment of a semiquantitative scoring system for mild-to-moderate gill lesions in Atlantic salmon reared in recirculating aquaculture systems in Norway.

Compromised gill health is a critical cause of forfeited welfare in Atlantic salmon farming. Detecting and quantifying the early onset of gill disease is important to reveal initial inflicting stimuli. We collected gill samples of 45 Atlantic salmon from 2 commercial recirculating aquaculture systems (RASs) spanning fry-to-market-size fish with no clinical signs of gill disease. Gill samples were assessed histologically by 3 independent raters with different levels of experience. Semiquantitative scoring for 7 types of gill changes was carried out for 10 filaments per gill (450 filaments total) over 3 rounds on anonymized samples. Scores were summarized for each type of gill change. The assumed clinical relevance for each change was transformed into a category score, followed by an assessment of agreement within (intra) and between (inter) raters. A generalized linear model estimated the difference in score levels between raters. For each rater, intra-rater agreement was high for 6 gill changes and moderate for 1 gill change. Inter-rater agreement was moderate to almost-perfect, except for 2 gill changes; generalized linear model regression revealed systematic differences in score usage between the raters. Our scoring protocol worked satisfactorily for mucous cell amount, lamellar clubbing, lamellar hypertrophy and/or hyperplasia, and aneurysms, despite different levels of expertise in histologic evaluation. Intra-rater agreement was consistent, but differences existed for interlamellar hypercellularity, lamellar inflammation, and degeneration. Scoring subclinical gill changes is a challenge, and our scoring system for mild-to-moderate lesions may enable early intervention to limit the detrimental effects of poor gill health in RAS farming.

Associations Between Delousing Practices and Pasteurellosis in Farmed Atlantic Salmon.

Infections with bacteria of the genus Pasteurella have increased in occurrence in Atlantic salmon (Salmo salar) farms in Norway since 2018. This increase coincides with increased use of non-medicinal treatments against the parasitic salmon louse, Lepeophtheirus salmonis, in the farms. Here, we analysed the statistical association between the use of non-medicinal delousing methods and pasteurellosis in salmon farming in western Norway, from 2018 to 2023. The analysed data covered 1161 production cycles from 356 farming localities, in which Pasteurella was detected in 166 production cycles from 115 localities. Results showed that in months with one or more thermal delousing using heated water to remove the lice, the odds for detection of Pasteurella were 2.5 times as high as in months with no delousing (95% credible interval, c.i.: 1.8, 3.6). In months with one or more mechanical delousing involving brushing or flushing, the odds were 1.8 times as high as in months with no delousing (c.i.: 1.04, 2.9). Delousing with freshwater was not associated with increased odds for Pasteurella. The odds for Pasteurella increased with increasing salmon weight and following a Pasteurella diagnosis in the previous production cycle. These results indicate that the use of thermal and mechanical delousing may play a role in maintaining the current pasteurellosis situation in western Norway, and that freshwater delousing may be a favourable alternative in this respect.

Hindcasting Farmed Salmon Mortality to Improve Future Health and Production Outcomes.

Intracellular, free-floating and biofilm-forming bacterial pathogens have been implicated in summer mortality of farmed Chinook salmon, Oncorhynchus tshawytscha, in New Zealand. A mortality event in 2022 in the Pelorus Sound, Marlborough, was linked to high water temperatures (> 18°C), and bacterial skin disease associated with Piscirickettsia spp. (=Rickettsia-like organisms) and Tenacibaculum species. To understand the progression of infection and potential drivers of the mortality, simulation of the event was conducted using a networked susceptible, infectious and recovered (SIR) model. Parameter exploration shows that reconstruction of observed mortality rates across three affected farm sites was possible. The best SIR simulations identified plausible values for key drivers of disease, which are consistent with previously estimated disease parameter ranges for Piscirickettsia salmonis. Our modelling shows the 2022 Pelorus Sound event likely experienced spread of bacterial pathogens within healthy fish populations at salmon farm sites over a 10-week long incubation period, before elevated mortality was observed. We show evidence that vaccine use at one site likely prevented 10% higher mortality and that an alternative site for the vaccination could have further reduced mortalities. This result highlights the importance of future vaccine developments in aquaculture and the potential to improve vaccine efficacy through considered site selection.

From Metagenomes to Functional Expression of Resistance: floR Gene Diversity in Bacteria from Salmon Farms

Background. The increase in antibiotic resistance in human-impacted environments, such as coastal waters with aquaculture activity, is related to the widespread use of antibiotics, even at sub-lethal concentrations. In Chile, the world’s second largest producer of salmon, aquaculture is considered the main source of antibiotics in coastal waters. In this work, we aimed to characterize the genetic and phenotypic profiles of antibiotic resistance in bacterial communities from salmon farms. Methods. Bacterial metagenomes from an intensive aquaculture zone in southern Chile were sequenced, and the composition, abundance and sequence of antibiotic resistance genes (ARGs) were analyzed using assembled and raw read data. Total DNA from bacterial communities was used as a template to recover floR gene variants, which were tested by heterologous expression and functional characterization of phenicol resistance. Results. Prediction of ARGs in salmon farm metagenomes using more permissive parameters yielded significantly more results than the default Resistance Gene Identifier (RGI) software. ARGs grouped into drug classes showed similar abundance profiles to global ocean bacteria. The floR gene was the most abundant phenicol-resistance gene with the lowest gene counts, showing a conserved sequence although with variations from the reference floR. These differences were recovered by RGI prediction and, in greater depth, by mapping reads to the floR sequence using SNP base-calling. These variants were analyzed by heterologous expression, revealing the co-existence of high- and low-resistance sequences in the environmental bacteria. Conclusions. This study highlights the importance of combining metagenomic and phenotypic approaches to study the genetic variability in and evolution of antibiotic-resistant bacteria associated with salmon farms.

Investigation of influential parameters in rainbow trout aquaculture based on robust mass production planning

Investigation of influential parameters in rainbow trout aquaculture based on robust mass production planning

Oral immunization with attenuated Salmonella Typhimurium as a carrier of DNA vaccine against infectious hematopoietic necrosis virus (IHNV) in rainbow trout (Oncorhynchus mykiss).

Infectious hematopoietic necrosis virus (IHNV) is a serious pathogen in the salmonid aquaculture industry and leads to economic losses in the world. This study aimed to develop a new oral DNA vaccine designed to protect rainbow trout against infection by IHNV. Fish were administered via the oral route by the attenuated Salmonella enterica serovar Typhimurium as a carrier of pcDNA3.1-IHNG (glycoprotein (G)) plasmid for 7 days and finally, fish were challenged by 105pfuml-1 IHNV. The results revealed that the antigen gene was identified in different tissues of rainbow trout at 15-, 30-, and 45 days post-vaccination (dpv). Also, the recombinant vaccine elicited both an innate and specific immune response, resulting in a significant upregulation of the expression levels of ifn-1, mx-1, vig-1, igm, and igt. In addition, serum levels of neutralizing antibodies were observed to be elevated in the vaccinated fish, in contrast to the unvaccinated fish, following 30 dpv. Compared to trout that received empty S. Typhimurium, notable differences in cumulative percentage mortality were evident among the vaccinated fish. The relative percent survival (RPS) was recorded at 58.2% for the group that received oral vaccine, while the group that received empty S. Typhimurium exhibited an RPS of 18.2%. Therefore, our results showed that this bacterial vector can be a candidate carrier for pcDNA3.1-IHNG plasmid that may be employed to confer protection to rainbow trout against IHNV.

Risk Assessment of Harmful Algal Blooms in Salmon Farming: Scotland as a Case Study.

This study explored harmful algal bloom (HAB) risk as a function of exposure, hazard and vulnerability, using Scotland as a case study. Exposure was defined as the fish biomass estimated to be lost from a bloom event, based on the total recorded annual production. Hazard was estimated from literature-reported bloom events. Vulnerability was calculated from records of the number of employees (2020), as an estimate of aquaculture-based employment. The dinoflagellate Karenia mikimotoi was identified as the HAB species with the highest frequency of reported bloom events in Scotland, with variable spatial and temporal reports, but environmental and climatological variables regulating these events are currently unknown. The Shetland Islands region exhibited the highest combined HAB risk, with the highest scores in all three components. Vulnerability was particularly important to overall risk within an island setting, where a larger proportion of the population was dependent on aquaculture. The analysis demonstrated the potential to evaluate the economic and social consequences of HAB events on the aquaculture industry. As fish-killing HABs and fish health impacts are likely under-reported, more transparent reporting of events and related fish health and physiological consequences is recommended for a more quantitative application of this approach.

Methylation of foreign DNA overcomes the restriction barrier of Flavobacterium psychrophilum and allows efficient genetic manipulation.

Bacterial cold-water disease (BCWD) caused by Flavobacterium psychrophilum is a problem for salmonid aquaculture worldwide, and current control measures are inadequate. An obstacle in understanding and controlling BCWD is that most F. psychrophilum strains resist DNA transfer, thus limiting genetic studies of their virulence mechanisms. F. psychrophilum restriction enzymes that destroy foreign DNA were suspected to contribute to this problem. Here, we used F. psychrophilum DNA methyltransferases to modify and protect foreign DNA from digestion. This allowed efficient conjugative DNA transfer into nine diverse F. psychrophilum strains that had previously resisted DNA transfer. Using this approach, we constructed a gene deletion mutant that failed to cause disease in rainbow trout. Further genetic studies could help determine the molecular factors involved in pathogenesis and may aid development of innovative BCWD control strategies.

Profitability Analysis of Cold-Water Fish Farming in Northern Pakistan: A Case Study of Aziz Trout Farm District Chitral

Analysis of economic profitability is essential for establishing the financial viability of any business enterprise. In the context of trout farming in the Pakistani district of Chitral, this study evaluates the financial implications of this profitable and sustainable technique. In July of 2020, a structured questionnaire was used to collect data on profitability indicators for the study. The investigation reveals that the overall cost of producing 398 kilograms of fish on the farm was 596,013 rupees. This cost analysis offers great insight into the economic ramifications of trout farming in the region. In addition, the study indicates a good Benefit-cost ratio of 1.67, demonstrating that the benefits of trout farming outweigh the associated expenses. Additionally, the Rate of Return on Investment (RRI) is an amazing 0.78, further demonstrating the enterprise's profitability. The Variable Cost (RVC), which has a large value of 0.60, is one of the main aspects that contribute to the profitability of trout farming. This illustrates the potential profitability and cost-effectiveness of trout farming in District Chitral. The study demonstrates that fish farming, specifically trout farming, offers individuals a lucrative option to support themselves. With its favorable financial indicators and potential for long-term profitability, trout farming is a viable source of income for residents in the region.

Comparative analysis of the stress and immune responses in Atlantic salmon (Salmo salar) inoculated with live and inactivated Piscirickettsia salmonis.

Piscirickettsiosis causes the highest mortality in Atlantic salmon (Salmo salar) farming, and prophylactic treatment has not provided complete protection to date. In this study, we analyzed the immune and metabolic responses of Atlantic salmon inoculated with live and inactivated Piscirickettsia salmonis, monitoring plasma markers related to immune and stress responses. The fish were inoculated with inactivated P. salmonis, live P. salmonis, and culture medium (as control group). Blood and head-kidney samples were collected on days 3, 7, and 14 post-inoculations (dpi). Glucose and lactate levels did not show statistical differences, while cortisol levels increased from day 3 to day 14 in fish inoculated with live P. salmonis and only at 7 dpi in those inoculated with inactivated P. salmonis. Furthermore, anti-P. salmonis IgM-type immunoglobulins increased up to 14 dpi in fish inoculated with live P. salmonis but showed no change in those inoculated with inactivated P. salmonis. Meanwhile, immune markers involved in type I responses (tnfα-1, ifnγ, and cd8β) and regulatory responses (il10, tgfβ-1, and cd4-1) displayed differences between fish inoculated with live and inactivated P. salmonis. In fish inoculated with live P. salmonis, there was a clear pattern of increase at both 3 and 14 dpi, while those inoculated with inactivated P. salmonis showed a greater increase at 3 dpi. Our findings suggest that the nature of antigen may influence humoral immunity (anti-P. salmonis IgM) and the gene expression of markers involved in type I and regulatory immune responses in Atlantic salmon.

MicroRNA-203-3p participates in antiviral immune response by negatively regulating TRAF3 in the rainbow trout (Oncorhynchus mykiss).

MicroRNAs (miRNAs) are highly conserved endogenous non-coding RNAs that play a crucial role in fish immune response by regulating gene expression at the post-transcriptional level. In recent years, the viral diseases caused by infectious hematopoietic necrosis virus (IHNV) have caused significant economic losses in rainbow trout (Oncorhynchus mykiss) aquaculture, whereas the immune regulatory mechanisms of miRNAs involved in rainbow trout resistance to IHNV infection remains largely undefined. In this study, we analyzed the structural characteristics of Oncorhynchus mykiss tumor necrosis factor receptor-associated factor 3 (OmTRAF3) by bioinformatics software and explored the molecular mechanism of miR-203-3p in rainbow trout resistance to IHNV by regulating OmTRAF3 in vivo and in vitro. The open reading frame (ORF) of OmTRAF3 gene was 1 731 bp and encoded 576 amino acids including an N-terminal RING finger domain, two zinc finger domains, a coiled-coil domain, and a C-terminal MATH domain. The expression pattern analysis showed that the expression of miR-203-3p and OmTRAF3 in immune-related tissues (head kidney, spleen, and liver) and liver cells of rainbow trout infected with IHNV varied with some regularity and had opposite trends at key time points, and a targeting relationship between miR-203-3p and OmTRAF3 was confirmed using a dual luciferase reporter gene assay. Further, we found that in vivo and in vitro overexpression of miR-203-3p significantly reduced the expression of OmTRAF3, downstream immune-related genes (OmTANK, OmIKKε, OmIFN1, and OmISG15) and promoted IHNV copy number replication, while silencing of miR-203-3p yielded opposite results. More importantly, OmTRAF3 and downstream genes as well as IHNV copy number changed accordingly with the silencing of OmTRAF3. The above results revealed that miR-203-3p participates in the immune response against IHNV by targeting OmTRAF3, and provide a theoretical basis for the screening of antiviral drugs in rainbow trout.

Fish Sludge as Feed in Circular Bioproduction: Overview of Biological and Chemical Hazards in Fish Sludge and Their Potential Fate via Ingestion by Invertebrates

ABSTRACTAquaculture farming discharges large volumes of fish sludge, consisting of particulate organic matter from feed spill and feces. Fish sludge from land‐based salmonid aquaculture systems can potentially be used to feed low‐trophic species such as marine polychaetes and insects. Further use of these invertebrates as feed may provide a more sustainable feed chain. However, food security must not compromise feed or food safety. The use of fish sludge as feed is currently not permitted in the European Union (EU). A regulatory assessment that considers the integration of fish sludge in the circular bioeconomy requires knowledge of the potential chemical and biological risks involved. This review has compiled data from existing literature on the occurrence of chemical and biological hazards, as well as the physical properties of fish sludge. The potential risks along this feed chain, from fish sludge to novel feed ingredient production, are discussed in light of the EU regulatory framework. Within the available, but scarce, literature on biological hazards in fish sludge, there is no clear evidence that fish sludge is unsafe for use as a feed material for invertebrates if treatment criteria established in the EU regulations are followed. Scarcity implies a level of uncertainty, but not necessary of risk. For the chemical hazards, some elements are of concern and may exceed regulatory limits, and there are limited data on the occurrence of organic pollutants. This review highlights considerable knowledge gaps concerning the use of fish sludge in a circular feed and food system.

In vivo host-pathogen dynamics and immune responses to Piscirickettsia salmonis using the zebrafish model.

Research on Piscirickettsia salmonis, a critical pathogen affecting salmon aquaculture, has been hampered by inadequate host models to study the infection process in vivo. Here, we highlight zebrafish as a surrogate host model for in-depth analysis of P. salmonis infection by using fluorescent dyes and protein markers to visualize host-pathogen interactions. Using transgenic zebrafish lines with fluorescent immune cells, we observed a strong increase in neutrophil numbers in the infected larvae, resembling an emergency granulopoiesis response. We observed the presence of P. salmonis in the caudal hematopoietic zone accompanied by an increased expression of immune marker genes, such as the neutrophil marker mpx, interleukins and interferons, both in zebrafish larvae and adult zebrafish kidneys, features of emergency granulopoiesis in response to uncontrolled bacterial burden. Our results underscore the importance of zebrafish as a valuable model for studying P. salmonis infections.