Relationship between sea lice levels on sea trout and fish farm activity in western Scotland

The relationship between aquaculture and infestations of sea lice on sea trout, Salmo trutta L., is controversial. Here, the association between sea lice infestations on wild sea trout and characteristics of local Atlantic salmon, Salmo salar L., farms were investigated using data collected on the Scottish west coast. The proportion of sea trout with louse burdens above a critical level was positively related to the fork length of the sea trout and the mean weight of salmon on the nearest fish farm, and negatively related to the distance to that farm. The distance to the nearest fish farm did not influence the probability of infestations above the critical level beyond 31 km although there was considerable uncertainty around this cut‐off distance (95% limits: 13–149 km). The results support a link between Atlantic salmon farms and sea lice burdens on sea trout in the west of Scotland and provide the type of information required for marine spatial planning.     

Diagnosis of bacterial kidney disease by detection of Renibacterium salmoninarum by real-time PCR

Bacterial kidney disease (BKD), caused by Renibacterium salmoninarum (Rs), is a serious threat to salmon in aquaculture as well as to wild populations. We have developed a real-time polymerase chain reaction (PCR) for detection of Rs in kidney samples. The PCR is based on detection of unique parts of the 16S rRNA gene of Rs and DNA equivalent to 1-10 Rs genomes was detected per reaction. No cross-reactivity with other fish pathogenic or related bacteria could be demonstrated. Analysis of individual kidney samples collected from BKD classified populations identified 39.9% of the fish as positive by real-time PCR compared with 28.0% by polyclonal enzyme-linked immunosorbent assay (ELISA). The real-time PCR assay was found to be well suited for complementary use with ELISA for diagnosis of BKD, with the ability to detect clinical as well as covert Rs infections. The infection level determined by the polyclonal ELISA and by real-time PCR was significantly correlated.     

Infectious agent detections in archived Sockeye salmon (Oncorhynchus nerka) samples from British Columbia,Canada (1985–94)

In response to concerns that novel infectious agents were introduced through the movement of eggs as Atlantic salmon aquaculture developed in British Columbia (BC), Canada, we estimated the prevalence of infectious agents in archived return‐migrating Sockeye salmon, from before and during aquaculture expansion in BC (1985–94). Of 45 infectious agents assessed through molecular assays in 652 samples, 23 (7 bacterial, 2 viral and 14 parasitic) were detected in liver tissue from six regions in BC. Prevalence ranged from 0.005 to 0.83 and varied significantly by region and year. Agent diversity ranged from 0 to 12 per fish (median 4), with the lowest diversity observed in fish from the Trans‐Boundary and Central Coast regions. Agents known to be endemic in Sockeye salmon in BC, including Flavobacterium psychrophilum, Infectious haematopoietic necrosis virus, Ceratonova shasta and Parvicapsula minibicornis, were commonly observed. Others, such as Kudoa thyrsites and Piscirikettsia salmonis, were also detected. Surprisingly, infectious agents described only recently in BC salmon, Ca. Branchiomonas cysticola, Parvicapsula pseudobranchicola and Paranucleospora theridion, were also detected, indicating their potential presence prior to the expansion of the aquaculture industry. In general, our data suggest that agent distributions may not have substantially changed because of the salmon aquaculture industry.     

Aeromonas salmonicida infection levels in pre‐ and post‐stocked cleaner fish assessed by culture and an amended qPCR assay

Due to increasing resistance to chemical therapeutants, the use of ‘cleaner fish’ (primarily wrasse, Labridae, species) has become popular in European salmon farming for biocontrol of the salmon louse, Lepeophtheirus salmonis (Krøyer). While being efficient de‐licers, cleaner fish mortality levels in salmon cages are commonly high, and systemic bacterial infections constitute a major problem. Atypical furunculosis, caused by Aeromonas salmonicida A‐layer types V and VI, is among the most common diagnoses reached in clinical investigations. A previously described real‐time PCR (qPCR), targeting the A. salmonicida A‐layer gene (vapA), was modified and validated for specific and sensitive detection of all presently recognized A‐layer types of this bacterium. Before stocking and during episodes of increased mortality in salmon cages, cleaner fish (primarily wild‐caught wrasse) were sampled and screened for A. salmonicida by qPCR and culture. Culture indicated that systemic bacterial infections are mainly contracted after salmon farm stocking, and qPCR revealed A. salmonicida prevalences of approximately 4% and 68% in pre‐ and post‐stocked cleaner fish, respectively. This underpins A. salmonicida's relevance as a contributing factor to cleaner fish mortality and emphasizes the need for implementation of preventive measures (e.g. vaccination) if current levels of cleaner fish use are to be continued or expanded.     

Randomized clinical field trial of a bacterial kidney disease vaccine in Atlantic salmon,Salmo salar L.

A randomized, blinded clinical trial was performed to assess the relative effectiveness of five commercial and one experimental vaccine in a population of farmed fish experiencing a bacterial kidney disease (BKD) outbreak that occurred in one study cage that was part of a larger clinical field trial. A total of 6000 uniquely identified Atlantic salmon S1 presmolts were randomly assigned to vaccine groups in the hatchery and transferred to a commercial marine aquaculture site. Repeated sampling events to evaluate growth, inherent physical conditions and health status were carried out over the entire production cycle. During the second summer at sea, the study cage developed an outbreak of BKD that lasted approximately 240 days. The effectiveness of the selected vaccines was evaluated using survival analysis methods. The sole vaccine group offering protection for BKD was found to significantly decrease the hazard of dying (hazard ratio, HR = 0.68, P = 0.018) during the outbreak, compared to the industry standard, vaccine group. Additionally, during the outbreak, fish with a shortened operculum had a significantly decreased hazard (HR = 0.38, P = 0.033) compared to those fish with a normal operculum, while fish with jaw deformities had a significantly increased hazard (HR = 2.55, P = 0.001) compared to fish with normal jaw status.     

First identification and characterization of Streptococcus iniae obtained from tilapia (Oreochromis aureus) farmed in Mexico

This is the first study to isolate, identify and characterize Streptococcus iniae as the causative disease agent in two tilapia (Oreochromis aureus) populations. The populations were geographically isolated, of distinct origins, and did not share water sources. Affected fish showed various external (e.g., exophthalmia and cachexia, among others) and internal (e.g., granulomatous septicaemia and interstitial nephritis, among others) signs. All internal organ samples produced pure cultures, two of which (one from each farm, termed S‐1 and S‐2) were subjected to biochemical, PCR and 16S rRNA sequencing (99.5% similarity) analyses, confirming S. iniae identification. The two isolates presented genetic homogeneity regardless of technique (i.e., RAPD, REP‐PCR and ERIC‐PCR analyses). Pathogenic potentials were assessed through intraperitoneal injection challenges in rainbow trout (Oncorhynchus mykiss) and zebrafish (Danio rerio). Rainbow trout mortalities were respectively 40% and 70% at 10⁴ and 10⁶ CFU per fish with the S‐1 isolate, while 100% mortality rates were recorded in zebrafish at 10² and 10⁴ CFU per fish with the S‐2 isolate. The obtained data clearly indicate a relationship between intensified aquaculture activities in Mexico and new disease appearances. Future studies should establish clinical significances for the tilapia industry.     

Variations in mucous cell numbers in gills of Atlantic salmon (Salmo salar) presmolt in commercial freshwater farms in Norway

Fish gills are heavily exposed to the external milieu and may react against irritants with different cellular responses. We describe variations in mucous cell counts in gills from healthy Atlantic salmon (Salmo salar) presmolts in five recirculating aquaculture system (RAS) farms and one flow‐through farm. Based on certain criteria, mucous cells were histologically quantified in a defined lamellar region of the gills and the counts were analysed. Immunohistochemistry (IHC) was used to investigate epithelial responses. The median number of total mucous cells in the defined region was 59 per fish. Between the farms, the medians varied from 31 to 101 with the lowest in the flow‐through farm. A regression model was fitted with “total mucous cells” as the dependent variable and with “fish length” and “fish farm” as independent variables. The proportion of variation in mucous cell counts explained by the model was twice as high when “fish farm” was included compared to only “fish length.” IHC revealed proliferative responses in coherence with high mucous cell numbers. Conclusively, the variation in mucous cell counts depends on combined farm‐related factors. Establishing a baseline for mucous cell counts is fundamental in the development of high‐throughput monitoring programmes of gill health in farmed fish.     

A historical review of the key bacterial and viral pathogens of Scottish wild fish

Thousands of Scottish wild fish were screened for pathogens by Marine Scotland Science. A systematic review of published and unpublished data on six key pathogens (Renibacterium salmoninarum, Aeromonas salmonicida, IPNV, ISAV, SAV and VHSV) found in Scottish wild and farmed fish was undertaken. Despite many reported cases in farmed fish, there was a limited number of positive samples from Scottish wild fish, however, there was evidence for interactions between wild and farmed fish. A slightly elevated IPNV prevalence was reported in wild marine fish caught close to Atlantic salmon, Salmo salar L., farms that had undergone clinical IPN. Salmonid alphavirus was isolated from wild marine fish caught near Atlantic salmon farms with a SAV infection history. Isolations of VHSV were made from cleaner wrasse (Labridae) used on Scottish Atlantic salmon farms and VHSV was detected in local wild marine fish. However, these pathogens have been detected in wild marine fish caught remotely from aquaculture sites. These data suggest that despite the large number of samples taken, there is limited evidence for clinical disease in wild fish due to these pathogens (although BKD and furunculosis historically occurred) and they are likely to have had a minimal impact on Scottish wild fish.     

Control of bacterial kidney disease in Atlantic salmon, Salmo salar L., by dietary modification

Abstract. The effects of dietary treatments were investigated as a prophylactic measure to minimize the occurrence and severity of bacterial kidney disease (BKD) infections in Atlantic salmon, Salmo salar L., at Margaree Fish Culture Station, Nova Scotia, Canada. Six diets containing various levels of calcium, magnesium, zinc, iron, copper, manganese, cobalt and iodine in addition to one commercial diet were fed to post–yearling Atlantic salmon in two consecutive experiments. Natural infection was utilized to examine the effects of each diet on the prevalence of BKD. Diet containing high levels of iodine (4.5 mg/kg feed) and fluorine (4.5 mg/kg) reduced BKD prevalence rate to 3% and 5% respectively compared to 95% and 38% with commercial feed. Some other experimental diets also reduced prevalence of BKD, but to a lesser degree than the diet with additional iodine and fluorine.     

PCR survey for Paramoeba perurans in fauna,environmental samples and fish associated with marine farming sites for Atlantic salmon (Salmo salar L.)

Amoebic gill disease (AGD) caused by the amoeba Paramoeba perurans is an increasing problem in Atlantic salmon aquaculture. In the present PCR survey, the focus was to identify reservoir species or environmental samples where P. perurans could be present throughout the year, regardless of the infection status in farmed Atlantic salmon. A total of 1200 samples were collected at or in the proximity to farming sites with AGD, or with history of AGD, and analysed for the presence of P. perurans. No results supported biofouling organisms, salmon lice, biofilm or sediment to maintain P. perurans. However, during clinical AGD in Atlantic salmon, the amoeba were detected in several samples, including water, biofilm, plankton, several filter feeders and wild fish. It is likely that some of these samples were positive as a result of the continuous exposure through water. Positive wild fish may contribute to the spread of P. perurans. Cleaner fish tested positive for P. perurans when salmon tested negative, indicating that they may withhold the amoeba longer than salmon. The results demonstrate the high infection pressure produced from an AGD‐afflicted Atlantic salmon population and thus the importance of early intervention to reduce infection pressure and horizontal spread of P. perurans within farms.     

Early marine survival and movements of escaped Atlantic salmon Salmo salar L. juveniles from a land‐based smolt farm during autumn

The aim of this study was to examine early marine survival and movements of simulated escaped Atlantic salmon Salmo salar L. pre‐smolt and smolt from a commercial smolt farm during autumn. One‐third of the pre‐smolt most likely died in the immediate vicinity of the release location, whereas the corresponding mortality for smolts was lower (8.5%) during the 5‐week study period. The surviving pre‐smolt left the farm area after 2–3 days, predominantly along the shore. In contrast, most of the surviving smolts left the farm area during the first day and 54% seemed to move away from the shore and adopt a more pelagic movement pattern than pre‐smolt. The number of surviving fish recorded in the fjord decreased throughout the study period, possibly due to a combination of fish migrating out of the fjord or undetected mortality. Compared with existing knowledge on migration of released farmed smolts during spring, our results indicate less directional and slower movement rates during autumn. Only two of the tagged fish were detected upstream in the rivers following release. A rapid dispersion of escapees indicates that the potential for recapturing escapees is limited unless recapture efforts are initiated immediately after escape. Hence, there is a need for development of technology that detects and prevents escapees to enter the sea.     

The immune response of Atlantic salmon, Salmo salar L., to the causative agent of bacterial kidney disease, Renibacterium salmoninarum

Abstract. Both under-yearling and post-yearling Atlantic salmon parr produced high agglutinating antibody titres in response to a single intraperitoneal injection of killed bacterial kidney disease (BKD) cells emulsified in. Freund's complete adjuvant (FCA), Low or no response was observed in animals injected with BKD cells in saline or in animals vaccinated by hyperosmotic immersion. Immunological duration was insufficient in fish vaccinated as under-yearling parr to provide protective immunity 2 years later when the fish had become smolts. Atlantic salmon post-yearling parr injected with BKD cells in FCA demonstrated a reduced prevalence of BKD lesions compared to control animals when both were observed as smolts 1 year after vaccination.     

Genetic diversity in a Tasmanian hatchery population of Atlantic salmon (Salmo salar L.) compared with its Canadian progenitor population

The level of genetic diversity in a cultured Atlantic salmon (Salmo salar) population from Tasmania, Australia was examined at 11 microsatellite loci and compared with that in its progenitor population from the River Philip in Nova Scotia, Canada. The reference progenitor population consisted of archived scales collected from wild River Philip salmon in 1971 and 1972, not long after salmon from this river were imported into Australia in the mid‐1960s. The Tasmanian hatchery stock had a significant reduction in the mean number of alleles (31–43%) and mean allelic richness (28–39%) across all microsatellite loci compared with the wild Canadian population. Mean heterozygosity levels remained unchanged. Estimates of per‐generation effective population sizes for the Tasmanian population, based on allele frequency temporal variance with the wild progenitor population, ranged from 102–207 individuals and reflected hatchery records.     

Potential disease interaction reinforced: double-virus-infected escaped farmed Atlantic salmon,Salmo salar L., recaptured in a nearby river

The role of escaped farmed salmon in spreading infectious agents from aquaculture to wild salmonid populations is largely unknown. This is a case study of potential disease interaction between escaped farmed and wild fish populations. In summer 2012, significant numbers of farmed Atlantic salmon were captured in the Hardangerfjord and in a local river. Genetic analyses of 59 of the escaped salmon and samples collected from six local salmon farms pointed out the most likely source farm, but two other farms had an overlapping genetic profile. The escapees were also analysed for three viruses that are prevalent in fish farming in Norway. Almost all the escaped salmon were infected with salmon alphavirus (SAV) and piscine reovirus (PRV). To use the infection profile to assist genetic methods in identifying the likely farm of origin, samples from the farms were also tested for these viruses. However, in the current case, all the three farms had an infection profile that was similar to that of the escapees. We have shown that double-virus-infected escaped salmon ascend a river close to the likely source farms, reinforcing the potential for spread of viruses to wild salmonids.     

Evaluation of zebrafish (Danio rerio) as an animal model for the viral infections of fish

Zebrafish (Danio rerio) is a laboratory model organism used in different areas of biological research including studies of immune response and host–pathogen interactions. Thanks to many biological tools available, zebrafish becomes also an important model in aquaculture research since several fish viral infection models have been developed for zebrafish. Here, we have evaluated the possible use of zebrafish to study infections with fish viruses that have not yet been tested on this model organism. In vitro studies demonstrated that chum salmon reovirus (CSV; aquareovirus A) and two alloherpesviruses cyprinid herpesvirus 1 (CyHV‐1) and cyprinid herpesvirus 3 (CyHV‐3) are able to replicate in zebrafish cell lines ZF4 and SJD.1. Moreover, CSV induced a clear cytopathic effect and up‐regulated the expression of antiviral genes vig‐1 and mxa in both cell lines. In vivo studies demonstrated that both CSV and CyHV‐3 induce up‐regulation of vig‐1 and mxa expression in kidney and spleen of adult zebrafish after infection by i.p. injection but not in larvae after infection by immersion. CyHV‐3 is eliminated quickly from fish; therefore, virus clearing process could be evaluated, and in CSV‐infected fish, a prolonged confrontation of the host with the pathogen could be studied.     

Vibrogen‐2 vaccine trial in lumpfish (Cyclopterus lumpus) against Vibrio anguillarum

Lumpfish (Cyclopterus lumpus), a native fish of the North Atlantic Ocean, is utilized as cleaner fish to biocontrol sea lice infestations in Atlantic salmon aquaculture. However, bacterial infections are affecting cleaner fish performance. Vibrio anguillarum, the aetiological agent of vibriosis, is one of the most frequent bacterial infections in lumpfish, and effective vaccine programmes against this pathogen have been identified as a high priority for lumpfish. Vibrogen‐2 is a commercial polyvalent bath vaccine that contains formalin‐inactivated cultures of V. anguillarum serotypes O1 and O2, and Vibrio ordalii. In this study, we evaluated Vibrogen‐2 efficacy in lumpfish against a local isolated V. anguillarum strain. Two groups of 125 lumpfish were bath‐immunized, bath‐boost‐immunized at four weeks post‐primary immunization, and intraperitoneally (i.p.) boost‐immunized at eight weeks post‐primary immunization. The control groups were i.p. mock‐immunized with PBS. Twenty‐seven weeks post‐primary immunization, the fish were i.p. challenged with 10 or 100 times the V. anguillarum J360 LD₅₀ dose. After the challenge, survival was monitored daily, and samples of tissues were collected at ten days post‐challenge. Commercial vaccine Vibrogen‐2 reduced V. anguillarum tissue colonization and delayed mortality but did not confer immune protection to C. lumpus against the V. anguillarum i.p. challenge.     

The Three‐spined Stickleback,Gasterosteus aculeatus Linnaeus 1758, plays a minor role as a host of Lepeophtheirus salmonis (Krøyer 1837) in the Gulf of Maine

The sea louse, Lepeophtheirus salmonis (Krøyer 1837), is a significant parasite of farmed salmon throughout the Northern Hemisphere. Management of on‐farm louse populations can be improved by understanding the role that wild fish play in sustaining and providing refuge for the local population of sea lice. In this study, 1,064 sticklebacks were captured. Of these animals, 176 individuals were carrying a total of 238 sea lice, yielding a prevalence and intensity of 16.5% and 1.4 lice per fish, respectively. Detailed examination of the sea lice on the three‐spined sticklebacks captured in Cobscook Bay found two L. salmonis individuals using three‐spined sticklebacks as hosts. A 2012 survey of wild fish in Cobscook Bay, Maine, found multiple wild hosts for Caligus elongatus (von Nordmann 1832), including three‐spined sticklebacks (Gasterosteus aculeatus L.), but no L. salmonis were found in this earlier study.     

Production of market-size European strain Atlantic salmon (Salmo salar) in land-based freshwater closed containment aquaculture systems

Interest in land-based farms using recirculating aquaculture systems (RAS) for market-size Atlantic salmon (Salmo salar) continues to grow, and several commercial facilities are already rearing fish. Performance data for commercially available mixed-sex, all-female, and triploid all-female Atlantic salmon reared to market-size in freshwater land-based facilities, however, are limited, particularly for European strain fish. Accordingly, eight groups of European-sourced Atlantic salmon (five groups of diploid mixed-sex, two groups of diploid all-female, and one group of triploid all-female fish) were reared from eyed egg to market-size in a semi-commercial scale land-based aquaculture systems over five separate production cycles to quantify performance metrics. Fish reached market-size (4−5 kg) in 24.7–26.3 months post-hatch. Fish were reared at a mean water temperature of 12.3–13.7 °C from first feeding to a mean size of 466–1265 g, then 13.3–15.1 °C during growout. On average, all-female groups grew faster than mixed-sex groups; however, environmental conditions and performance of individual cohorts varied. In a comingled production cycle, diploid all-female salmon grew faster than triploid counterparts. Early maturation rates ranged from 0 % to 67 %, with a mean maturation rate of 34 % for diploid mixed-sex fish and 67 % and 13 % for two diploid all-female groups, respectively. Triploid all-female Atlantic salmon did not mature. This research confirms biological and technological feasibility of growing Atlantic salmon to market-size in land-based systems but controlling early maturation of diploid salmon remained a challenge under the conditions utilized in these trials. This research provides important data inputs to optimize operational and financial projections for existing and potential land-based Atlantic salmon farms.