Review of infectious agent occurrence in wild salmonids in British Columbia,Canada

Wild Pacific salmonids (WPS) are economically and culturally important to the Pacific North region. Most recently, some populations of WPS have been in decline. Of hypothesized factors contributing to the decline, infectious agents have been postulated to increase the risk of mortality in Pacific salmon. We present a literature review of both published journal and unpublished data to describe the distribution of infectious agents reported in wild Pacific salmonid populations in British Columbia (BC), Canada. We targeted 10 infectious agents, considered to potentially cause severe economic losses in Atlantic salmon or be of conservation concern for wild salmon in BC. The findings indicated a low frequency of infectious hematopoietic necrosis virus, piscine orthoreovirus, viral haemorrhagic septicaemia virus, Aeromonas salmonicida, Renibacterium salmoninarum, Piscirickettsia salmonis and other Rickettsia-like organisms, Yersinia ruckeri, Tenacibaculum maritimum and Moritella viscosa. No positive results were reported for infestations with Paramoeba perurans in peer-reviewed papers and the DFO Fish Pathology Program database. This review synthesizes existing information, as well as gaps therein, that can support the design and implementation of a long-term surveillance programme of infectious agents in wild salmonids in BC.     

Studies on the inactivation of selected viral and bacterial fish pathogens at high pH for waste disposal purposes

This study investigated the use of alkaline hydrolysis at ambient temperature for inactivation of selected fish pathogens in fish tissues under conditions approximating those that are likely to be found in the aquaculture industry. Infectious salmon anaemia virus (ISAV) and Lactococcus garvieae have been determined in a previous study to be the most resistant virus and bacteria to pH 12 from a wide range of viruses and bacteria tested. They were spiked at high titres into fish extracts that were then treated with 1 m sodium hydroxide (NaOH). Viable L. garvieae was not detected in the treated fish extract after 1 h, and ISAV was not detected after 24‐h exposure. Field mortalities of Atlantic salmon, Salmo salar L., caused by infectious pancreatic necrosis virus were treated by alkaline hydrolysis at ambient temperature. The macerated fish mortalities contained a high titre of virus (3.38 × 10⁸ TCID₅₀ g^?1) that was reduced to approximately 2.2 × 10³ TCID₅₀ g^?1 after 24‐h exposure to NaOH, and virus was not detected after exposure for 48 h. The results suggest that alkaline hydrolysis at ambient temperature has potential as a biosecure treatment method for fish by‐products containing fish pathogens.     

A mortality event in wrasse species (Labridae) associated with the presence of viral haemorrhagic septicaemia virus

Viral haemorrhagic septicaemia (VHS) is an infectious disease of farmed and wild fish and has an extensive host range in both freshwater and marine environments. In December 2012, a wrasse population consisting of ballan, Labrus bergylta (Ascanius), corkwing, Symphodus melops (L.), cuckoo, Labrus mixtus L., goldsinny, Ctenolabrus rupestris (L.), and rock cook, Centrolabrus exoletus (L.), held at a marine hatchery in the Shetland Isles, Scotland, experienced a mortality event. Approximately 10 000 wrasse were being held at the facility on behalf of an Atlantic salmon, Salmo salar L., aquaculture company prior to being deployed for the biological control of parasites on marine pen Atlantic salmon, aquaculture sites. Fish Health Inspectors from Marine Scotland Science initiated a diagnostic investigation, and subsequent diagnostic testing confirmed the site to be VHSV positive by qRT-PCR and virus isolation followed by ELISA. A VHSV genotype-specific qRT-PCR assay revealed that the isolates belonged to genotype III, the European marine strain of the virus. The virus genotype was further confirmed by nucleic acid sequencing of the partial nucleoprotein (N) and glycoprotein (G) genes followed by BLAST nucleotide searches. This study reports for the first time the detection of VHSV within multiple wrasse species and highlights the need for a comprehensive risk-based approach to the use of wrasse and other finfish species as biological controls within the aquaculture industry.     

Evaluation of sea lice abundance levels on farmed Atlantic salmon (Salmo salar L.) located in the Broughton Archipelago of British Columbia from 2003 to 2005

Salmon farming began in British Columbia (BC) in the 1970s and in 2006, aquaculture represented BC's largest agricultural export. Along with this growth in production has been a growth in controversy, including the concern that sea lice originating from Atlantic salmon farms negatively impact wild juvenile pink salmon in the Broughton Archipelago. To understand the dynamic interaction between wild and farmed fish, data for on‐farm abundance of sea lice are required. In this study, 33 000 Atlantic salmon from 20 active farms were examined over 3 years. Two species of lice were found: Lepeophtheirus salmonis and Caligus clemensi. Inter‐annual and seasonal variations in abundance levels occurred with lower levels of L. salmonis in 2003 compared with 2004 and 2005, while C. clemensi levels were highest in 2003. The abundance of L. salmonis was greater on older farmed fish. The findings are compared with European and eastern Canadian sea lice reports, and possible sources of sea lice on farmed salmon are discussed.     

A survey of salmon gill poxvirus (SGPV) in wild salmonids in Norway

In 2016, the Norwegian health monitoring programme for wild salmonids conducted a real‐time PCR‐based screening for salmon gill poxvirus (SGPV) in anadromous Arctic char (Salvelinus alpinus L.), anadromous and non‐anadromous Atlantic salmon (Salmo salar L.) and trout (Salmo trutta L.). SGPV was widely distributed in wild Atlantic salmon returning from marine migration. In addition, characteristic gill lesions, including apoptosis, were detected in this species. A low amount of SGPV DNA, as indicated by high Ct‐values, was detected in anadromous trout, but only in fish cohabiting with SGPV‐positive salmon. SGPV was not detected in trout and salmon from non‐anadromous water courses, and thus seems to be primarily linked to the marine environment. This could indicate that trout are not a natural host for the virus. SGPV was not detected in Arctic char but, due to a low sample size, these results are inconclusive. The use of freshwater from anadromous water sources may constitute a risk of introducing SGPV to aquaculture facilities. Moreover, SGPV‐infected Atlantic salmon farms will hold considerable potential for virus propagation and spillback to wild populations. This interaction should therefore be further investigated.     

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.     

Real-time PCR detection of Parvicapsula pseudobranchicola (Myxozoa: Myxosporea) in wild salmonids in Norway

The myxozoan genus Parvicapsula contains 14 species infecting fish, some of which are known to cause severe disease in farmed and wild salmonids. Parvicapsula pseudobranchicola infections were first reported from seawater-reared Atlantic salmon, Salmo salar, in Norway in 2002 and have since then been an increasing problem. The present study describes a Taqman real-time PCR assay for specific detection of P. pseudobranchicola. The Taqman assay targets the 18S rRNA gene of P. pseudobranchicola and is able to detect as few as ten copies of the target sequence. Using the described assay, P. pseudobranchicola was detected in both farmed and wild salmonids, indicating that wild Atlantic salmon, sea trout, Salmo trutta, and Arctic char, Salvelinus alpinus, may be natural hosts of the parasite. Parvicapsula pseudobranchicola was found in samples from wild salmonids in the far south and the far north of Norway, displaying a wide geographic range of the parasite. Farmed salmonids showed P. pseudobranchicola infection levels many folds higher than that observed for wild sea trout, indicating that farmed Atlantic salmon are subjected to an elevated infection pressure compared with wild salmonids.     

New Zealand rickettsia‐like organism (NZ‐RLO) and Tenacibaculum maritimum: Distribution and phylogeny in farmed Chinook salmon (Oncorhynchus tshawytscha)

A total of 777 fish from three growing regions of New Zealand Chinook salmon farms comprising of five sites were tested. Quantitative PCR was used to determine the distribution of New Zealand rickettsia‐like organism and Tenacibaculum maritimum. Genetic information from these bacteria were then compared with strains reported worldwide. Using this information, suggested associations of pathogens with clinically affected fish were made. NZ‐RLO was detected in two of the three regions, and T. maritimum was detected in all regions. Three strains of NZ‐RLO were identified during this study. Based on analysis of the ITS rRNA gene, NZ‐RLO1 appears to be part of an Australasian grouping sharing high similarity with the Tasmanian RLO, NZ‐RLO2 was shown to be the same as an Irish strain, and NZ‐RLO3 was shown be closely related to two strains from Chile. Based on multi‐locus sequence typing, the New Zealand T. maritimum was the same as Australian strains. NZ‐RLOs were detected more frequently in fish with skin ulcers than fish without skin ulcers. While additional research is required to investigate the pathogenicity of these organisms, this is the first time that NZ‐RLOs have been associated with the development of clinical infections in farmed Chinook salmon.     

Seasonal dynamics of microbial community in an aquaculture system for Nile tilapia (Oreochromis niloticus)

Studies on bacterial abundance and diversity will improve the understanding of the microbial ecology to optimize aquaculture production, water quality, disease control and environmental impact of effluents. We comparatively evaluated an aquaculture system in dry and rainy seasons by Fluorescent in situ Hybridization (FISH) and culture‐dependent methodology. Overall, a negative correlation between seasonality in bacterial and flagellates abundance was observed. Alpha‐, beta‐, gamma‐proteobacteria and Cytophaga‐Flavobacter were observed with seasonal variation. Putative pathogenic bacteria such as coagulase‐negative Staphylococcus, non‐fermenter Gram‐negative rods (Pseudomonas sp. and Burkholderia sp.), and Enterobacteriaceae were also detected in this study with significant seasonal variation. Focusing on medically important bacteria, our data show that microbial diversity in the environment associated with aquaculture, as it is practiced, may be altered in growth ponds used for fish cultivation. As an ecological consequence, potentially pathogenic bacteria might be released in high concentrations to the downstream environments posing potential threats to human and animal health.     

Isolation,molecular characterization and antimicrobial susceptibility of Aeromonas spp. obtained from Tiger Grouper (Epinephelus fuscoguttatus) and Marble Goby (Oxyeleotris marmoratus) fish in Sabah,Malaysia

Aeromonads are ubiquitous in aquatic environments and have been implicated in fish and human infections. In this study, we isolated, studied antimicrobial susceptibility patterns and screened the existence of 15 virulence genes in aeromonads from two famously consumed fish species—seven marine Tiger Grouper (Epinephelus fuscoguttatus) and eight freshwater Marble Goby (Oxyeleotris marmoratus) from the aquaculture hatchery in Sabah, Malaysia. A total of 30 aeromonads (17 A. caviae, 9 A. rivuli, 4 A. dhakensis) were identified using PCR targeting GCAT gene, rpoD‐restriction fragment length polymorphism and multi‐locus phylogenetic analysis. All 30 strains were resistant to amoxicillin and cephalothin and five strains were multidrug‐resistant. Nine virulence genes (lip, ela, eno, fla, aerA, hylA, dam, alt and ser) present in A. dhakensis, suggesting the virulence potential of this species as a fish pathogen. This study offers as a baseline for future studies in monitoring and managing these two fish in aquaculture industry.     

New salmonid hosts for Tenacibaculum species: Expansion of tenacibaculosis in Chilean aquaculture

The success and sustainability of Chilean aquaculture largely depends on the control of endemic and emerging pathogens, including several species of the genus Tenacibaculum. Tenacibaculum dicentrarchi and “Tenacibaculum finnmarkense” have been detected and confirmed in Chilean Atlantic salmon (Salmo salar). However, no outbreaks of tenacibaculosis in rainbow trout (Oncorhynchus mykiss) or coho salmon (Oncorhynchus kisutch) have been reported, either in Chile or globally. The aims of this study were to determine whether the mortalities recorded for rainbow trout and coho salmon from five marine fish farms located in the Los Lagos, Aysén, and Magallanes Regions could be caused by Tenacibaculum spp. The diseased fish exhibited cutaneous haemorrhages, tail and peduncle rots, and damage on the mouth and tongue. Microbiological analysis of infected external tissues yielded 13 bacterial isolates. The isolates were identified as members of the genus Tenacibaculum through biochemical analysis (e.g. Gram-stain negative, straight rods, filamentous cells and motile by gliding), but differences existed in biochemical results, making species-level identification through biomolecular tools essential. The 16S rRNA analysis found that the majority of isolates were more closely related to “T. finnmarkense” than T. dicentrarchi, while the phylogenetic trees resulting from multilocus sequence data recovered the four main clades (clades I to IV) identified by Olsen et al. (2017, Veterinary Microbiology, 205, 39). This is the first documented occurrence of clinical tenacibaculosis in farmed rainbow trout and coho salmon globally, and it extends the known host distribution of this pathogen in Chile. Moreover, we confirm the presence of Tenacibaculum species in the Chilean Patagonia. These findings highlight the importance of establishing preventative measures to minimize the spread of this disease within the Chilean marine aquaculture industry, as well as the need for monitoring initiatives worldwide in these farmed fish species.     

Infectious pancreatic necrosis virus in fish by‐products is inactivated with inorganic acid (pH 1) and base (pH 12)

The aquaculture industry needs a simple, inexpensive and safe method for the treatment of fish waste without heat. Microbial inactivation by inorganic acid (HCl) or base (KOH) was determined using infectious pancreatic necrosis virus (IPNV) as a model organism for fish pathogens. Salmonella and spores of Clostridium perfringens were general hygiene indicators in supplementary examinations. IPNV, which is considered to be among the most chemical‐ and heat‐resistant fish pathogens, was reduced by more than 3 log in 4 h at pH 1.0 and pH 12.0. Salmonella was rapidly inactivated by the same treatment, whereas spores of C. perfringens were hardly affected. The results indicate that low and high pH treatment could be particularly suitable for fish waste destined for biogas production. pH treatment at aquaculture production sites could reduce the spread of fish pathogens during storage and transportation without disturbing the anaerobic digestion process. The treatment could also be an alternative to the current energy‐intensive steam pressure sterilization of fish waste to be used by the bioenergy, fertilizer and soil improver industries.     

Infectious salmon anaemia – pathogenesis and tropism

Infectious salmon anaemia (ISA) is a serious disease of farmed Atlantic salmon caused by the aquatic orthomyxovirus infectious salmon anaemia virus (ISAV). ISA was first detected in Norway in 1984 and was characterized by severe anaemia and circulatory disturbances. This review elucidates factors related to the pathogenesis of ISA in Atlantic salmon, the dissemination of the virus in the host and the general distribution of the 4‐O‐acetylated sialic acids ISAV receptor. The knowledge contributes to the understanding of this disease, and why, almost 30 years after the first detection, it is still causing problems for the aquaculture industry.     

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.     

围网养殖对华阳河湖鱼类群落结构的影响

为了发展渔业生产,人们将大量自然湖泊通过围网的方式投放鱼苗进行养殖。目前,这种养殖模式对鱼类群落的影响没有得到有效的评估。本研究于2016年在华阳河湖围网养殖区和非养殖区分别设置采样点对鱼类进行季度调查,探讨围网养殖对鱼类群落结构的影响。结果显示,在围网养殖区共采集鱼类6目12科35属46种,非养殖区采集鱼类6目11科40属57种,其中以鲤形目种类最多,分别占养殖区和非养殖区鱼类总数的65.22%和63.16%。非养殖区的优势种为?和短颌鲚,而鲢、鳙是围网养殖区的优势种。与非养殖区相比,围网养殖区山溪河流性鱼类百分比从21.05%降低至15.22%。通过双因素方差分析解析养殖与季节对鱼类群落结构的影响,发现围网养殖区的鱼类密度、物种数、优势度指数都显著低于非养殖区,而均匀度指数显著高于非养殖区。通过SIMPER分析得出,造成养殖区和非养殖群落结构差异的重要贡献物种为短颌鲚、?、鲤、太湖短吻银鱼、鲫、鱵、鲢和鳙。     

Polyculture of sea scallops (Placopecten magellanicus) suspended from salmon cages

Commercial and developmental operations for the culture of the seascallop, Placopecten magellanicus, are present in AtlanticCanada and New England. In an experiment designed to examine the commercialfeasibility of polyculture of scallops with Atlantic salmon(Salmo salar), we measured growth andsurvival of sea scallops grown in suspension at two salmon aquaculture sites innortheastern Maine (Johnson Cove (JC) and Treats Island (TI)). Sea scallop spatwere grown in pearl nets and deployed on drop lines containing ten nets inAugust 1994. One drop line of ten nets was sampled about every four months andscallops were counted, measured and weighed. Scallop tissues were also analysedfor paralytic shellfish toxins (PSP). The maximum level of PSP recorded duringthe study was 1174 g STX equiv.·100 gtissue^–1 (excluding adductor muscle weight). After one year,shell heights were 53.6 and 56.4 mm, growth rates were 0.11 and0.12 mm per day and wet adductor muscle weights were 3.3 and 4.1g (TI and JC, respectively). These growth rates were comparable tosea scallops grown in suspension culture to a nearby scallop aquaculture siteand other areas in Atlantic Canada. Reduced rates of survival were found duringthe latter part of the experiment and were attributable, in part, to heavyfouling, predators and high stocking density. The potential for supplementalincome, diversification of the salmon aquaculture industry, and feasibility ofculturing scallops at adjacent sites to salmon operations does exist.     

Atlantic salmon in the North Pacific

The first catches of Atlantic salmon, Salmo salar L., in British Columbia (BC) waters occurred in 1987. The first reported escape of Atlantic salmon (2000 individuals) occurred in 1988. From 1988 to 1995, 97 799 Atlantic salmon were reported escaped from net pens in BC but the true number was higher as not all escapes are reported. Since 1987 a total of 9096 Atlantic salmon was caught in the coastal marine waters of BC, Washington and Alaska, and 188 were caught in fresh water. Most catches occurred in the Johnstone Strait area, where the abundance of salmon farms is highest. The most distant recovery occurred in 1994 when an Atlantic salmon was caught near the western end of the Alaska Peninsula. There have been no reports of successful reproduction of Atlantic salmon in the wild and no feral juveniles have been found. Atlantic salmon caught in the ocean in BC have substantial amounts of adipose tissue and they are heavier at length than fish caught in Alaska. The proportion of fish with prey items in their stomachs is generally low but higher in Alaska (13.1%) than in BC (5.8%). Most fish caught in fresh water are either maturing or mature.     

Case definition for clinical and subclinical bacterial kidney disease (BKD) in Atlantic Salmon (Salmo salar L.) in New Brunswick,Canada

Bacterial kidney disease (BKD) is considered an important cause of loss in salmon aquaculture in Atlantic Canada. Causative agent of BKD is the Gram‐positive bacteria Renibacterium salmoninarum. Infected salmon are often asymptomatic (subclinical infection), and the disease is considered chronic. One of the challenges in quantifying information from farm production and health records is the application of a standardized case definition. Case definitions for farm‐level and cage‐level clinical and subclinical BKD were developed using retrospective longitudinal data from aquaculture practices in New Brunswick, Canada, combining (i) industry records of weekly production data including mortalities, (ii) field observations for BKD using reports of veterinarians and/or fish health technicians, (iii) diagnostic submissions and test results and (iv) treatments used to control BKD. Case definitions were evaluated using veterinarians’ expert judgements as reference standard. Eighty‐nine and 66% of sites and fish groups, respectively, were associated with BKD at least once. For BKD present (subclinical or clinical), sensitivity and specificity of the case definition were 75–100% varying between event, fish group, site cycle and level (site pen). For clinical BKD, sensitivities were 29–64% and specificities 91–100%. Industry data can be used to develop sensitive case definitions.     

Epizootiology of Parvicapsulaminibicornis in Fraser River sockeye salmon, Oncorhynchusnerka (Walbaum)

Late-spawning Fraser River sockeye salmon, Oncorhynchus nerka , stocks have suffered significant prespawn mortality associated with an unusually early freshwater migration pattern and the myxosporean parasite Parvicapsula minibicornis . Surveys of migrating adult salmon from several spawning populations were conducted in 1999 and 2000 to determine the extent of infection with P. minibicornis , when and where the parasite first becomes detectable during migration, and whether early migrating stocks might be used as sentinels to assess risk of infection in late-spawning stocks. Posterior kidney, preserved in 95% ethanol, was examined for P. minibicornis in stained histological sections and using a polymerase chain reaction (PCR) test. The prevalence of this parasite in all Fraser River sockeye salmon stocks examined was high (range 47–100% infected). In contrast, P. minibicornis was not detected in the fish tested from the two sockeye salmon stocks outside the Fraser River drainage in either 1999 or 2000. The parasite was also not detected histologically or by PCR in the kidney tissue of the fish from the Fraser River that were sampled in salt water or early during their freshwater migration up the river. These findings and the progression in the prevalence and intensity of infection as the fish from three stocks (early Stuart, Weaver Creek and Cultus Lake) were monitored over time, suggest salmon acquired the parasite either in the lower Strait of Georgia or in the lower Fraser River before the confluence of the Harrison River. In both 1999 and 2000 the parasite was present in all Fraser River sockeye salmon stocks sampled, which suggests that early Stuart salmon may be valuable as a sentinel stock for the presence of the parasite in later-spawning stocks.     

The development of the Norwegian wrasse fishery and the use of wrasses as cleaner fish in the salmon aquaculture industry

Norway leads the world aquaculture production of Atlantic salmon Salmo salar and farmed Norwegian Atlantic salmon is currently consumed around the globe. However, sea lice infestation is a major problem faced by the salmon aquaculture industry in Norway and elsewhere. The use of wild-caught cleaner fish, mainly wrasses, has been recommended over the other available methods as the most economical and environmentally friendly option to control sea lice infestation in salmon farming. Here, we review the development of the Norwegian wrasse fishery and the use of wrasses as cleaner fish. In this document, we address the sea lice problem and introduce the main wrasse species employed as cleaner fish, document the cleaning behaviour of wrasses, present the development of a new wrasse fishery associated with the salmon aquaculture industry, and finally, we identify the main challenges associated with the intensive use of wild-caught cleaner wrasses and provide some insight for future directions of the wrasse fishery and further development of aquaculture techniques to supply salmon facilities with domesticated cleaner fish.