Baltic salmon, Salmo salar, from Swedish river Lule älv is more resistant to furunculosis compared to rainbow trout

Background

Furunculosis, caused by Aeromonas salmonicida, continues to be a major health problem for the growing salmonid aquaculture. Despite effective vaccination programs regular outbreaks occur at the fish farms calling for repeated antibiotic treatment. We hypothesized that a difference in natural susceptibility to this disease might exist between Baltic salmon and the widely used rainbow trout.

Study Design

A cohabitation challenge model was applied to investigate the relative susceptibility to infection with A. salmonicida in rainbow trout and Baltic salmon. The course of infection was monitored daily over a 30-day period post challenge and the results were summarized in mortality curves.

Results

A. salmonicida was recovered from mortalities during the entire test period. At day 30 the survival was 6.2% and 34.0% for rainbow trout and Baltic salmon, respectively. Significant differences in susceptibility to A. salmonicida were demonstrated between the two salmonids and hazard ratio estimation between rainbow trout and Baltic salmon showed a 3.36 higher risk of dying from the infection in the former.

Conclusion

The finding that Baltic salmon carries a high level of natural resistance to furunculosis might raise new possibilities for salmonid aquaculture in terms of minimizing disease outbreaks and the use of antibiotics.     

Evidence for an autumn downstream migration of Atlantic salmon Salmo salar (Linnaeus) and brown trout Salmo trutta (Linnaeus) parr to the Baltic Sea

In the eastern Baltic rivers, anadromous salmonid parr are known to smoltify and migrate to the sea from March until June, depending on latitude, climate and hydrological conditions. In this study, we present the first records of autumn descent of brown trout Salmo trutta and Atlantic salmon Salmo salar from the Baltic Sea Basin. Otolith microchemistry analyses revealed that these individuals hatched in freshwater and had migrated to the brackish water shortly prior to capture. The fish were collected in 2006, 2008, 2009 and 2013 from Eru Bay (surface salinity 4.5–6.5 ‰), Gulf of Finland. This relatively wide temporal range of observations indicates that the autumn descent of anadromous salmonids is not a random event. These results imply that autumn descent needs more consideration in the context of the effective stock management, assessment and restoration of Baltic salmonid populations and their habitats.     

Discrimination between farmed and free-living invasive salmonids in Chilean Patagonia using stable isotope analysis

In Chilean Patagonia relatively pristine aquatic environments are being modified by the introduction of exotic salmonids, initially through their deliberate release for sport fishing since the early twentieth century, and more recently via the accidental escape from fish farms. There is therefore a need to reliably distinguish between naturally reproducing and fugitive salmonids associated with the Chilean salmonid farming industry, the second largest in the world. We tested the ability of stable isotope analysis (SIA) and analysis of scale growth profiles to discriminate between farmed and free-living salmonids sampled around the Island of Chiloé. Juvenile Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) from aquaculture facilities were significantly more enriched in δ¹⁵N and lipid-corrected δ¹³C than river-caught individuals. Scale growth slopes during the first year in freshwater were significantly higher in farmed than in wild-caught rainbow trout, indicating faster somatic growth under hatchery conditions. Stable isotopes analysis classified 94% of juvenile Atlantic salmon and rainbow trout to their correct farm or free-living groups. Our results, therefore, can help to elucidate the origin and spread of exotic invasive salmonids in Chile, and address one of the biggest threats to native freshwater fishes in Patagonia and other temperate zones of the Southern Hemisphere.     

The Representative Research Animal: Why Rainbow Trout (Salmo gairdneri Rich.)?

In laboratory research, the rainbow trout has become a counterpart to the white rat, because that fish is an adaptable species available in much of the developed world and stocks from egg through adult are available throughout the year. Moreover, many strains are recognized, and their propagation and laboratory maintenance are not particularly demanding. Also, knowledge of rainbow trout nutrition, husbandry, diseases, immune responses, toxicology, and carcinogenesis exceeds that of any other salmonid or coldwater teleost. The rainbow trout is the logical surrogate species in many studies of other salmonids.     

Viral haemorrhagic septicaemia (VHS) outbreaks in Finnish rainbow trout farms

In Finland, viral haemorrhagic septicaemia virus (VHSV) was diagnosed for the first time in 2000 from 4 rainbow trout farms in brackish water. Since then the infection has spread and, by the end of 2004, VHSV had been isolated from 24 farms in 3 separate locations: 2 in the Baltic Sea and 1 in the Gulf of Finland. The pathogenicity of 3 of these isolates from 2 separate locations was analysed in infection experiments with rainbow trout fry. The cumulative mortalities induced by waterborne and intraperitoneal challenge were approximately 40 and 90 %, respectively. Pair-wise comparisons of the G and NV gene regions of Finnish VHSV isolates collected between 2000 and 2004 revealed that all isolates were closely related, with 99.3 to 100% nucleotide identity, which suggests the same origin of infection. Phylogenetic analysis revealed that they were closely related to the old freshwater isolates from rainbow trout in Denmark and to one old marine isolate from cod in the Baltic Sea, and that they were located close to the presumed ancestral source. As the Finnish isolates induce lower mortality than freshwater VHSV isolates in infection experiments, they could represent an intermediate stage of marine isolates evolving towards pathogenicity in rainbow trout.     

The effects of fisheries management on harvest rates of native and non-native salmonid fish species

In central Europe, both brown trout Salmo trutta and European grayling Thymallus thymallus are threatened native salmonid species with high value in recreational angling and nature conservation. On the other hand, rainbow trout Oncorhynchus mykiss and brook trout Salvelinus fontinalis are intensively stocked non-native species of high angling value but no value for nature conservation. This study tested if harvest rates of native salmonids are negatively correlated to intensive stocking and harvest rates of non-native salmonids in inland freshwater recreational fisheries. Data were collected from 250 fishing sites (river and stream stretches) over 13 years using mandatory angling logbooks. Logbooks were collected from individual anglers by the Czech Fishing Union in the regions of Prague and Central Bohemia, Czechia (central Europe) and processed by the author of this study. In result, anglers harvested 200,000 salmonids with total weight of 80 tons over 13 years. Intensive stocking of multiple salmonid species lead to slightly lower harvests of native salmonids. Inversely, intensive harvests of multiple salmonid species lead to slightly higher harvest of native salmonids. Recapture rates of stocked salmonids were relatively low (0.6%–3.7%), proving fish stocking moderately ineffective. Since the effects of non-native salmonid stocking and harvest rates on native salmonid harvest were significant but not strong, it is suggested that rivers and streams that support fishing for non-native salmonids still support fishing for native salmonids. However, this idea does not apply for fishing sites with really high intensity of non-native salmonid stocking – harvest rates of natives were very low on these fishing sites.     

Whirling disease: host specificity and interaction between the actinosporean stage of Myxobolus cerebralis and rainbow trout Oncorhynchus mykiss

Scanning electron microscopic studies were conducted on rainbow trout Oncorhynchus mykiss in the first 60 min after their exposure to the triactinomyxon spores of Myxobolus cerebralis. The results demonstrated that as early as 1 min post exposure the whole process, from the attachment of the triactinomyxon spores to the complete penetration of their sporoplasm germs, had occurred. The triactinomyxon spores sought out the secretory openings of mucous cells of the epidermis, the respiratory epithelium and the buccal cavity of trout and used them as portals of entry. Exposure experiments of the triactinomyxon spores of M. cerebralis to non-salmonid fish, such as goldfish Carassius auratus, carp Cyprinus carpio, nose Chondrostoma nasus, medaka Oryzias latipes, guppy Poecilia reticulata and also the amphibian tadpole Rana pipiens as well as to rainbow trout fry indicated a specificity for salmonids. Attempts to activate the triactinomyxon spores by exposure to mucus prepared from cyprinid and salmonid fish showed no significant differences from those conducted in tap water. The results suggest that the simultaneous presence of both mechano- and chemotactic stimuli was required for finding the salmonid fish host.     

Fish infected with Sphaerospora spp. Thélohan (Myxosporea) from waters enzootic for proliferative kidney disease of salmonids

Sphaerospores were found among three species of fish examined from waters known to be enzootic for proliferative kidney disease (PKD) of salmonids. They were detected in the renal tubules of both hatchery-reared rainbow trout (Salmo gairdneri) exposed to the infectious stage of PKD and in chubs (Gila bicolor) in the headwaters of a hatchery where PKD is enzootic. Sticklebacks (Gasterosteus aculeatus) collected near net pens where Pacific salmon had experienced a PKD epizootic were also found to harbor sphaerospores in the lumen of the kidney tubules. The latter two host species contained developmental stages of a myxosporidan in the blood and in the lumen of the kidney tubules which are similar to those of PKX, the causative agent of PKD in salmonid fish. The sphaerospores observed in the rainbow trout are the first to be observed in this species. The similarity to previously observed developmental stages, rarity, and presence of these sphaerospores in salmonid fish from a hatchery where PKD is enzootic suggest that they are the most mature stage of the PKX myxosporidan yet observed.     

A comparison of Visual Prey Detection Among Species of Piscivorous Salmonids: Effects of Light and Low Turbidities

Differences in reaction distance to prey fish by piscivorous salmonids can alter predator–prey interactions under different visual conditions. We compared reaction distances of three piscivorous salmonids commonly found in western lakes: cutthroat trout, Oncorhynchus clarki utah, rainbow trout, O. mykiss, and the nonnative lake char, Salvelinus namaycush. Reaction distances to salmonid prey were measured as functions of light and turbidity in a controlled laboratory setting. In addition, predation rates and swimming speeds of lake char preying on juvenile cutthroat trout were measured experimentally under a range of light levels. Reaction distances for cutthroat trout and rainbow trout increased rapidly as light levels increased, reaching relatively constant reaction distances at higher light levels. Reaction distances for lake char were similar to cutthroat trout and rainbow trout at the lower light levels; however, lake char reaction distances continued to increase with increasing light intensity to asymptote at distances 65% higher than those for both cutthroat and rainbow trout. Predation rates by lake char were low for the darkest light levels, increased rapidly under low light levels (0.50–0.75lx), and then declined to an intermediate rate at all higher light levels. Swimming speeds by lake char also increased rapidly from extremely low light conditions to a peak and declined to an intermediate level at light levels above 1.00lx. These results suggest that, above the saturation intensity threshold, piscivorous lake char react to fish prey at greater distances than do cutthroat trout and rainbow trout. These differences may help explain the decline of native trout following the introductions of nonnative lake char in lakes and reservoirs of western North America.     

Aquaculture, non-native salmonid invasions and associated declines of native fishes in Northern Patagonian lakes

1. Even though intensive aquaculture production of salmonids in lakes occurs in many locations around the world published studies on the survival and reproductive success of escaped cultured salmonids in freshwater ecosystems are not common. A recent expansion of aquaculture in Chile has led it to become the world's second largest producer of cultured salmonids.
2. We document the recent history of escaped and self-sustaining salmonid populations over a wide spatial scale and a long temporal scale in Chilean Patagonian lakes. Our hypotheses are that salmonid density in lakes will be higher where there is intensive aquaculture, due to greater numbers of potential escapees. Secondly, if non-native salmonids have adverse impacts on native fishes, increases in the abundance of non-native species should be associated with decreases in relative abundance of native species. Finally, if the first two hypotheses are correct we anticipate that diets of salmonids may show evidence of predation on native fishes, diet overlap with native species, and evidence of the influence of feed from aquaculture operations in the diets of salmonids and native fishes.
3. We sampled six lakes with gill nets from 1992 to 2001. Our results show that the relative abundance of free-living salmonids is closely related to the level of fish farming production. Salmonids are the top predators and in lakes with fish farming the main prey item is native fishes. The relative abundance of native fishes has decreased, most likely due to predation by salmonids.
4. Our study contributes to the understanding of the effects of non-native salmonids in oligotrophic lakes, and it provides a starting point to judge the establishment of new fish farming sites in lakes around the world.     

Gene Transfer to Fish Cells by Attenuated Invasive Escherichia coli

Genetic immunization has proved effective in a number of applications including vaccination of rainbow trout (Oncorhynchus mykiss) against the fish pathogen infectious hematopoietic necrosis virus. However, injection vaccines, especially in aquaculture, are not as desirable as oral or immersion dosing schemes. In this report we present evidence that attenuated invasive Escherichia coli can infect and deliver plasmid DNA to salmonid fish cells.     

The environmental impact of salmonid cage culture on inland fisheries: present status and future trends

This paper briefly reviews the environmental impact of the cage culture of salmonids in fresh waters, and evaluates the effects of this fish farming practice on the growth, behaviour and diseases of inland fish populations, concentrating on relationships between rainbow trout cage culture and indigenous brown trout populations. Possible future developments in salmonid cage culture are also discussed and the implications of this development for inland fisheries evaluated. The paper argues that the development of models should enable improved prediction of eutrophication from cage culture, to the benefit of the fish farmer, inland fisheries and other freshwater users, but also identifies a clear need for research in other areas of possible conflict.     

Developmental trends and aquaculture perspectives in Greece

A total of 109 Greek aquaculture projects were approved by the EC Commission between 1987 and 1991 under Regulation 4028/86, of which 65 were for sea-bass/sea-bream and 24 for mussels. The total production capacity of these projects was 13,483 tonnes, 13,025,000 sea-bass/sea-bream fingerlings and 600,000 trout juveniles. The total expenditure for grants was 12.1 billions drachmas. The geographic distribution of the projects approved by both the EC Commission and the National Developmental Law 1262/1982 (incentives for investments in Greece) are discussed in the text. A cumulative employment figure for a total of 330 aquaculture approved/established projects in Greece is about 5,500 jobs, created m remote areas where there are few other sources of employment. Total aquaculture production (excluding lagoons) increased from 2,200 tonnes in 1980 to 7,550 tonnes in 1990. A trend model, fitted to the total 1980–1990 production, predicted 10,507 tonnes for 1992. The prospects of aquaculture in Greece as well as conflict with tourism and fishing are also discussed.     

Fish Infected with Sphaerospora spp. Thélohan (Myxosporea) from Waters Enzootic for Proliferative Kidney Disease of Salmonids1

ABSTRACT. Sphaerospores were found among three species of fish examined from waters known to be enzootic for proliferative kidney disease (PKD) of salmonids. They were detected in the renal tubules of both hatchery-reared rainbow trout (Salmo gairdneri) exposed to the infectious stage of PKD and in chubs (Gila bicolor) in the headwaters of a hatchery where PKD is enzootic. Sticklebacks (Gasterosteus aculeatus) collected near net pens where Pacific salmon had experienced a PKD epizootic were also found to harbor sphaerospores in the lumen of the kidney tubules. The latter two host species contained developmental stages of a myxosporidan in the blood and in the lumen of the kidney tubules which are similar to those of PKX, the causative agent of PKD in salmonid fish. The sphaerospores observed in the rainbow trout are the first to be observed in this species. The similarity to previously observed developmental stages, rarity, and presence of these sphaerospores in salmonid fish from a hatchery where PKD is enzootic suggest that they are the most mature stage of the PKX myxosporidan yet observed.     

Modulation of fish phagocytic cells by N-terminal peptides of proopiomelanocortin (NPP)

N-terminal peptide of proopiomelanocortin (NPP, or pro-gamma-MSH) has shown to exhibit biological activity such as stimulation of adrenal mitogenesis and prolactin release-inhibiting factor activity. Structurally, studies reveal a significant difference between fish NPP from that of tetrapods, as NPPs from carp and salmonid lack gamma-MSH. Thus, fish NPP may exhibit functions different from that of mammals. The activation of phagocytic cells by NPP was analysed using rainbow trout Oncorhynchus mykiss and carp Cyprinus carpio. Rainbow trout and carp macrophages incubated with chum salmon NPP significantly enhanced the production of superoxide anion in comparison with control macrophages (without hormones). Both rainbow trout and carp macrophages had shown increased phagocytosis when stimulated administered with NPP. The above results were complemented by in vivo studies where NPP was administered to rainbow trout and carp. NPP significantly increased superoxide anion production as well as phagocytosis in macrophages. These results show that NPP in lower vertebrates activates the function of the phagocytic cells.     

Qualitative analysis of the risk of introducing Gyrodactylus salaris into the United Kingdom

Gyrodactylus salaris is a freshwater, monogenean ectoparasite of Baltic strains of Atlantic salmon Salmo salar on which it generally causes no clinical disease. Infection of other strains of Atlantic salmon in Norway has resulted in high levels of juvenile salmon mortality and highly significant reductions in the population. The parasite is a major exotic disease threat to wild Atlantic salmon in the UK. This paper qualitatively assesses the risk of introduction and establishment of G. salaris into the UK. The current UK fish health regime prevents the importation of live salmonids from freshwater in territories that have not substantiated freedom from G. salaris. The importation of other species, e.g. eels Anguilla anguilla and non-salmonid fish, represents a low risk because the likelihood of infection is very low and the parasite can only survive on these hosts for less than 50 d. Importation of salmon carcasses presents a negligible risk because harvested fish originate from seawater sites and the parasite cannot survive full strength salinity. The importation of rainbow trout Oncorhynchus mykiss carcasses from G. salaris infected freshwater sites might introduce the parasite, but establishment is only likely if carcasses are processed on a salmonid farm in the UK. A number of mechanical transmission routes were considered (e.g. angling equipment, canoes, ballast water) and the most important was judged to be the movement of live fish transporters from farms on mainland Europe direct to UK fish farms. In the future, territories may have to substantiate freedom from G. salaris and economic drivers for live salmonid imports may strengthen. Under these circumstances, legal or illegal live salmonid imports would become the most significant risk of introduction.     

Skin morphology and humoral non-specific defence parameters of mucus and plasma in rainbow trout,coho and Atlantic salmon

Susceptibility to different diseases among related species, such as coho salmon (Oncorhynchus kisutch), rainbow trout (Oncorhyncus mykiss) and Atlantic salmon (Salmo salar), is variable. The prominence of these species in aquaculture warrants investigation into sources of this variability to assist future disease management. To develop a better understanding of the basis for species variability, several important non-specific humoral parameters were examined in juvenile fish of these three economically important species. Mucous protease, alkaline phosphatase and lysozyme, as well as plasma lysozyme activities and histological parameters (epidermal thickness and mucous cell density, and size) were characterized and compared for three salmonids: rainbow trout, Atlantic salmon and coho salmon. Rainbow trout had a thicker epidermis and significantly more mucous cells per cross-sectional area than the other two species. Rainbow trout also had significantly higher mucous protease activity than Atlantic salmon and significantly higher lysozyme (plasma and mucus) activities than coho and Atlantic salmon, in seawater. Atlantic salmon, on the other hand, had the lowest activities of mucous lysozyme and proteases, the thinnest epidermal layer and the sparsest distribution of mucous cells, compared with the two other salmonids in seawater. Only coho salmon had sacciform cells. Atlantic and coho salmon had higher mucous lysozyme activities in freshwater as compared to seawater. There was no significant difference between mucous lysozyme activities in any of the three species reared in freshwater; however, rainbow trout still had a significantly higher plasma lysozyme activity compared with the other two species. All three species exhibited significantly lower mucous alkaline phosphatase and protease activities in freshwater than in seawater. Our results demonstrate that there are significant histological and biochemical differences between the skin and mucus of these three salmonid species, which may change as a result of differing environments. Variation in these innate immune factors is likely to have differing influences on each species response to disease processes.     

Multiple lines of evidence reveal complex rainbow trout responses to stream habitat enhancements

1. Habitat enhancements seek to ameliorate the detrimental effects of environmental degradation and take many forms, but usually entail structural (e.g. logs, cribs, reefs) or biogenic (e.g. carrion additions, vegetation plantings, fish stocking) augmentations with the intent of increasing fish annual production (i.e. accrual of new fish biomass through time). Whether efforts increase fish production or simply attract fish has long been subject to debate.
2. Streams of the Pacific Northwest are commonly targeted for habitat enhancements to mitigate for the detrimental effects of dams and other forms of habitat degradation on Pacific salmon. Nutrient mitigation (i.e. the practice of artificially fertilising freshwaters) is a form of biogenic habitat enhancement that attempts to mimic the enrichment effects of a natural Pacific salmon spawning event. This approach assumes nutrient augmentations alleviate nutrient limitation of primary producers and/or food limitation of primary and secondary consumers, culminating in increased fish production.
3. We conducted a multi-year manipulative experiment and tracked responses of interior rainbow trout (Oncorhynchus mykiss) to annual additions of Pacific salmon carcasses as part of an effort to enhance the productivity of salmonid populations in streams where salmon runs have been lost. We employed an integrated approach to partition the mechanisms driving numerical responses of trout populations across timescales, to assess population turnover, and to track responses to habitat enhancements across individual to population level metrics.
4. Short-term numerical increases by trout were shaped by immigration and subsequently via retention of individuals within treatment reaches. As trout moved into treated stream reaches, individuals foraged, grew, and subsequently moved to other locations such that short-term increases in fish numbers did not persist from year to year. All told, additions of salmon carcasses alleviated apparent food limitation and thereby increased secondary production of rainbow trout. However, at an annual time scale, increased production manifested as larger individual fish, not more fish within treated reaches. Fish movements and high population turnover within treated stream reaches apparently led to the subsequent dispersal of increased fish production.
5. We found multiple lines of evidence that indicated that annual additions of salmon carcasses aggregated rainbow trout and enhanced their annual production. Through this replicated management experiment, we documented dynamic individual and population level responses to a form of stream habitat manipulation across weekly and annual timescales.

     

Effects of aquaculture related stressors and nutritional restriction on circulating growth factors (GH, IGF-I and IGF-II) in Atlantic salmon and rainbow trout

The effects of aquaculture related stressors on circulating levels of GH, IGF-I and for the first time, IGF-II in Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) were investigated. Specifically, circulating growth factor levels were measured in four different experiments. Two 24 h confinement stressor procedures, (one with Atlantic salmon, the other with rainbow trout); following a hypo-osmotic stressor (freshwater bath) in salt water acclimated, adult, Atlantic salmon; and during a 22 day starvation and re-feeding protocol with juvenile Atlantic salmon. Handling and confinement resulted in significant decreases in circulating levels of all three growth factors in Atlantic salmon, and IGF-I and IGF-II (but not GH) in rainbow trout. A 2-3 h freshwater bath to remove gill parasites on a commercial Atlantic salmon aquaculture operation caused a significant decrease in circulating GH and IGF-I concentrations, but no significant change in IGF-II concentration, 2 days post bathing. Starvation for a period of 15 days in Atlantic salmon resulted in a significant increase in circulating GH levels and a significant decrease in circulating IGF-I and IGF-II. Re-feeding of starved fish for 7 days resulted in a significant decrease in GH to the concentration measured in continually fed fish, however re-feeding did not change plasma levels of IGF-I and IGF-II relative to continually starved fish. The results presented here confirm previously observed handling and confinement stressor induced effects on GH and IGF-I and, for the first time, on IGF-II in salmonids. Furthermore this study confirms the nutritional regulation of GH, IGF-I and IGF-II in juvenile Atlantic salmon.