Disease of rainbow trout caused by Pseudomonas luteola

Bacteria isolated from rainbow trout, Oncorhynchus mykiss, kept in a farm, in Turkey. During the outbreak, 40% of the rainbow trout (10-40 g) died. Typical clinical signs were exophthalmia, dark pigmentation, hemorrhage at the base of the pectoral, pelvic, anal fins and around the vent. Internal signs were enlarged spleen, pale liver and intestine filled with yellowish fluid. Liver, kidney and spleen of diseased fish, were aseptically streaked on Tryptic Soy Agar. After incubation, pure cultured colonies were observed and biochemically characterized with API 20 NE and other biochemical tests. Cultured bacterial 16 S rDNA gene was sequenced. Based on biochemical characteristics and sequence of 16 S rRNA, the causative bacteria were identified as Pseudomonas luteola. This study reports the first P. luteola infection in fish.     

Ceratomyxa shasta genotypes cause differential mortality in their salmonid hosts

Ceratomyxa shasta is a myxozoan parasite of salmonid fish. In natural communities, distinct genotypes of the parasite are associated with different salmonid hosts. To test the hypothesis that genotypes of C. shasta cause differential mortality, the polychaete host was experimentally infected with different parasite genotypes. Genotype I was obtained from Chinook salmon, Oncorhynchus tshawytscha, and genotype II from either coho salmon, O. kisutch, or rainbow trout, O. mykiss, We then challenged four salmonid strains: Chinook and coho salmon that occur in sympatry with the parasite and allopatric Chinook salmon and rainbow trout. Parasite genotype I caused mortality only in Chinook strains, although mortality in the allopatric strain also occurred from exposure to genotype II. A second experiment demonstrated that genotype II could be separated into two biotypes based on differential mortality in rainbow trout and coho salmon. These differential patterns of mortality as a result of infection by certain genotypes of C. shasta support field observations and suggest a co‐evolutionary relationship between these parasites and their hosts.     

Effects of fish species composition on Diphyllobothrium spp. infections in brown trout – is three‐spined stickleback a key species?

Subarctic populations of brown trout (Salmo trutta) are often heavily infected with cestodes of the genus Diphyllobothrium, assumedly because of their piscivorous behaviour. This study explores possible associations between availability of fish prey and Diphyllobothrium spp. infections in lacustrine trout populations. Trout in (i) allopatry (group T); (ii) sympatry with Arctic charr (Salvelinus alpinus) (group TC); and (iii) sympatry with charr and three‐spined stickleback (Gasterosteus aculeatus) (group TCS) were contrasted. Mean abundance and intensity of Diphyllobothrium spp. were higher in group TCS compared to groups TC and T. Prevalence, however, was similarly higher in groups TCS and TC compared to group T. Zero‐altered negative binomial modelling identified the lowest probability of infection in group T and similar probabilities of infection in groups TC and TCS, whereas the highest intensity was predicted in group TCS. The most infected trout were from the group co‐occurring with stickleback (TCS), possibly due to a higher availability of fish prey. In conclusion, our study demonstrates elevated Diphyllobothrium spp. infections in lacustrine trout populations where fish prey are available and suggests that highly available and easily caught stickleback prey may play a key role in the transmission of Diphyllobothrium spp. parasite larvae.     

Impact of origin and condition of host fish (Salmo trutta) on parasitic larvae of Margaritifera margaritifera

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Suitability of different salmonid strains as hosts for the endangered freshwater pearl mussel (Margaritifera margaritifera L.)

• 1. The complex life cycle of endangered European freshwater pearl mussel Margaritifera margaritifera L. involves an obligatory parasitic phase on a host fish. Knowledge on the host–parasite interaction and on the suitability of different host fish species and strains is required both for the management of wild fish and mussel populations as well as for improving the efficiency of captive breeding methods.
• 2. In this study, the suitability of different salmonid strains for hosting glochidia was tested, including Danube salmon (Hucho hucho L.) and three brown trout (Salmo trutta L.) strains from inside and outside the freshwater pearl mussel distribution range. All brown trout strains as well as Danube salmon were successfully infected with freshwater pearl mussel glochidia and encystment of mussel larvae was detected.
• 3. One brown trout strain originating from the natural pearl mussel distribution range was identified as the most suitable host, revealing the highest fish‐weight‐normalized infection rates and highest glochidial growth rates, whereas endemic Danube salmon was least suitable. Under natural conditions, the role of Danube salmon may be attributed to the long‐distance dispersal of glochidia in the Danube system, whereas sedentary brown trout appear to be the most important hosts at a local scale.
• 4. Successful infection of suitable hosts and the maintenance of these host–parasite systems in calcareous water were demonstrated in this study. These results indicate that neither the infection process nor the encystment phase of freshwater pearl mussels is dependent on low lime concentrations.
• 5. The results of this study suggest that careful selection and management of appropriate host fish strains is mandatory for sustainable conservation management of freshwater pearl mussel populations. Copyright © 2010 John Wiley & Sons, Ltd.

     

Pseudomonas putida infection in rainbow trout

A disease outbreak occurred during the Spring of 2006 among rainbow trout, Oncorhynchus mykiss, kept in growout tanks. During the outbreak 35% of the rainbow trout died. Typical clinical signs were ulceration on the dorsal surface of the fish. Based on biochemical characteristics and sequence of 16S rRNA, the causative bacteria isolated from diseased fish were identified as Pseudomonas putida. Histologically, skin ulcer initially was characterized by sloughed off epithelia and epithelial necrosis. In the advanced form of the disease, almost all layers of the skin down to the epidermis were lost. Outside of Japan, this study reports the first P. putida infection in fish.     

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.     

Food interrelationships between brown trout, Salmo trutta L., and rainbow trout, Oncorhynchus mykiss (Walbaum), in a small put-and-take stillwater fishery

Abstract. The food of brown trout, Salmo trutta L., and rainbow trout, Oncorhynchus mykiss (Walbaum). in a small stillwater put-and-take fishery in Surrey, England was examined by stomach content analysis of fish caught by angling between the months of April and September 1985. Overall, brown and rainbow trout tended to utilize different food sources. Mann-Whitney U-tests showed most major food items to be eaten in significantly different amounts, the most important exceptions being chironomid pupae and adult Diptera. In all months except April, rainbow trout utilized mainly midwater food, particularly Cladocera. Brown trout fed mainly on benthic food organisms and fish. Spearman rank correlations showed diets to be dissimilar during this period, with significant negative correlations in May and August. These results indicate an absence of interspecific competition for food, and imply spatial separation of brown and rainbow trout. In April both brown and rainbow trout fed extensively on the temporary bottom fauna, mainly trichopteran and megalopteran larvae, and diets were significantly positively correlated.     

Hybrids between Salmonidae species. Hatchability and growth rate in the freshwater period

In 1972 a hybridization experiment with salmonids was initiated at the Fish Breeding Experimental Station, Sunndalsøra. The purpose of this experiment was to study the viability, performance traits and the fertility of hybrids. Hybrids were produced from the following members of the salmon family: Salmon (Salmo salar), sea trout (Salmo trutta), brown trout (Salmo trutta), rainbow trout (Salmo gairdneri) and arctic char (Salvelinus alpinus). All crosses attempted, except those involving the rainbow trout, were relatively successful. Considerable differences in the hatching rates of the different hybrids were observed. All crosses with the arctic char were heavier than the better pure bred specimens at 11 months. In all other crosses the hybrids were lighter than the purebred specimens. The experiment is continuing, it is evident that more research is necessary before the potential value of these hybrids for fish farming can be properly assessed.     

Are brown trout Salmo trutta fario and rainbow trout Oncorhynchus mykiss two of a kind? A comparative study of salmonids to temperature‐influenced Tetracapsuloides bryosalmonae infection

Proliferative kidney disease (PKD) of salmonids caused by Tetracapsuloides bryosalmonae causes high mortalities of wild brown trout (Salmo trutta fario) and farmed rainbow trout (Oncorhynchus mykiss) at elevated water temperatures. Here the aim was to compare the temperature‐dependent modulation of T. bryosalmonae in the two salmonid host species, which display different temperature optima. We used a novel experimental set‐up in which we exposed brown trout and rainbow trout to an identical quantified low concentration of T. bryosalmonae for a short time period (1 hr). We followed the development of the parasite in the fish hosts for 70 days. PKD prevalence and parasite kinetics were assessed using qPCR. Exposures were performed at temperatures (12°C and 15°C) that reflect an environmental scenario that may occur in the natural habitat of salmonids. T. bryosalmonae infection was confirmed earliest in brown trout kept at 15°C (day 7 post‐exposure) while, in all other groups, T. bryosalmonae was not confirmed until day 15 post‐exposure. Moreover, significantly greater infection prevalence and a faster increase of parasite intensity were observed in brown trout kept at 15°C than in all other groups. These results indicate that PKD is differentially modulated by water temperature in related host species.     

Quantitative trait loci (QTL) associated with resistance of rainbow trout Oncorhynchus mykiss against the parasitic ciliate Ichthyophthirius multifiliis

The parasitic ciliate Ichthyophthirius multifiliis has a low host specificity eliciting white spot disease (WSD) in a wide range of freshwater fishes worldwide. The parasite multiplies rapidly whereby the infection may reach problematic levels in a host population within a few days. The parasite targets both wild and cultured fish but the huge economic impact of the protozoan is associated with mortality, morbidity and treatment in aquacultural enterprises. We have investigated the potential for genetic selection of WSD-resistant strains of rainbow trout. Applying the DNA typing system Affymetrix^® and characterizing the genome of the individual fish by use of 57,501 single nucleotide polymorphisms (SNP) and their location on the rainbow trout chromosomes, we have genetically characterized rainbow trout with different levels of natural resistance towards WSD. Quantitative trait loci (QTL) used for the selection of breeders with specific markers for resistance are reported. We found a significant association between resistance towards I. multifiliis infection and SNP markers located on the two specific rainbow trout chromosomes Omy 16 and Omy 17. Comparing the expression of immune-related genes in fish—with and without clinical signs—we recorded no significant difference. However, trout surviving the infection showed high expression levels of genes encoding IgT, T-cell receptor TCRβ, C3, cathelicidins 1 and 2 and SAA, suggesting these genes to be associated with protection.     

A simplified method to estimate Diphyllobothrium spp. infection in salmonids

Some fish parasites constitute severe management problems as they may cause mortality of their fish host or are important zoonoses of humans. Parasite assessments are therefore critical to keep track of infections. If conventional sampling techniques can be simplified, parasite assessments might be easier to obtain, less time‐consuming and more extensive. In this study, we compare the assessed number of Diphyllobothrium spp. cysts (CYST) with the counted number of Diphyllobothrium spp. plerocercoid larvae recovered using a conventional digestive technique (LARV). The aim was to determine the potential of using CYST as a simplified methodology for assessing Diphyllobothrium spp. infection in salmonids. In total, 365 brown trout and 424 Arctic charr were sampled from nine lakes in subarctic Norway. Strong correlation, significant linear relationship and large amount of explained variation were found between log₁₀CYST and log₁₀LARV in both fish species. The method had a slight, but not significant tendency to work better in charr compared to trout. In addition, absolute difference between CYST and LARV increased at parasite intensities >100 indicating that the method has reduced functionality when estimating parasite intensity in heavily infected salmonid populations. However, overall, using this simplified and less time‐consuming methodology, a good indication of Diphyllobothrium spp. intensity, abundance and prevalence was obtained. We suggest that this method provides a sound proxy of the Diphyllobothrium spp. burden and have the potential to be used in parasite assessment during fish monitoring and fisheries management surveys, particularly if the time and resources for detailed parasite studies are not available.     

The occurrence of proliferative kidney disease (PKD) in cultured and wild fish: further investigations

Abstract. In an investigation of the occurrence of proliferative kidney disease (PKD) in freshwater fish other than rainbow trout, 18 species of wild fish and seven species of fish raised in cultivation wore sampled from waters where the disease occurred annually in rainbow trout, Oncorhynchus mykiss (Richardson). Results revealed that certain wild stocks of brown trout. Salmo trutta L., grayling, Thymallus thymallus L., and pike, Esox lucius L., were infected with PKD, as were cultivated Atlantic salmon, Salmo salar L., parr, brown trout and char, Salvelinus alpinus (L.). Microscopical examination revealed the presence of the PKX cell in these species and also intraluminal protozoa possibly related to the PKX cell, which were not found in the rainbow trout. Other species of freshwater fish had myxosporidan infections but, unlike PKD infection, there was little host/parasite tissue response. The PKX cell as a myxosporidan stage is discussed and the presence of the disease in wild fish is reported.     

An alien fish threatens an endangered parasitic bivalve: the relationship between brook trout (Salvelinus fontinalis) and freshwater pearl mussel (Margaritifera margaritifera) in northern Europe

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Potential population and assemblage influences of non‐native trout on native nongame fish in Nebraska headwater streams

Non‐native trout are currently stocked to support recreational fisheries in headwater streams throughout Nebraska. The influence of non‐native trout introductions on native fish populations and their role in structuring fish assemblages in these systems is unknown. The objectives of this study were to determine (i) if the size structure or relative abundance of native fish differs in the presence and absence of non‐native trout, (ii) if native fish‐assemblage structure differs in the presence and absence of non‐native trout and (iii) if native fish‐assemblage structure differs across a gradient in abundances of non‐native trout. Longnose dace Rhinichthys cataractae were larger in the presence of brown trout Salmo trutta and smaller in the presence of rainbow trout Oncorhynchus mykiss compared to sites without trout. There was also a greater proportion of larger white suckers Catostomus commersonii in the presence of brown trout. Creek chub Semotilus atromaculatus and fathead minnow Pimephales promelas size structures were similar in the presence and absence of trout. Relative abundances of longnose dace, white sucker, creek chub and fathead minnow were similar in the presence and absence of trout, but there was greater distinction in native fish‐assemblage structure between sites with trout compared to sites without trout as trout abundances increased. These results suggest increased risk to native fish assemblages in sites with high abundances of trout. However, more research is needed to determine the role of non‐native trout in structuring native fish assemblages in streams, and the mechanisms through which introduced trout may influence native fish populations.     

Epidemiology of epizootic haematopoietic necrosis virus (EHNV) infection in farmed rainbow trout, Oncorhynchus mykiss (Walbaum): findings based on virus isolation, antigen capture ELISA and serology

Abstract. The epidemiology of epizootic haematopoietic necrosis virus (EHNV) infection was studied in farmed rainbow trout, Oncorhynchus mykiss (Walbaum). Estimates of mortality during five outbreaks on a commercial farm from 1986 to 1992 ranged from 0033 to 0.2% per day and total mortality did not exceed 3–4% in any outbreak. Affected fish were 0+ and less than 125 mm forklength. Clinical signs were non-specific, and laboratory examination was required to confirm the diagnosis. At the height of the outbreak in 1992, EHNV was demonstrated by virus isolation and antigen capture ELISA in 89% of clinically affected fish and 51% of dead fish, while the prevalence of infection in apparently healthy in-contact fish was 4%. Two and 4 months later the virus was not detected in a group of apparently healthy fish that had been affected earlier. Antibodies specific for EHNV were not found in rainbow trout from the infected farm; however, strong humoral responses were detected by ELISA in two immunized fish, indicating that the virus was immunogenic. These data suggested that EHNV was poorly infective but highly virulent in rainbow trout. Clinical EHNV infection was positively correlated with high rearing density and a low rate of water exchange, and therefore, with presumed poor water quality. Water temperature, which ranged from 11 to 17°C during outbreaks, did not appear to determine the incidence of clinical infection. EHNV infection in farmed rainbow trout was preceded by infection in free-living redfin perch, Perca fluviatilis L., in the water catchment, but it was uncertain whether this represented the source of infection for rainbow trout.     

Observations on open-water densities of sea lice larvae in Loch Shieldaig, Western Scotland

Sea trout (Salmo trutta L.) stocks on the West Coast of Scotland and Ireland have decreased due to reduced survival during the marine phase of their life cycle. Lice‐infested sea trout returning to rivers could indicate that parasite burdens are contributing to the decline. Salmon farms represent a potential source of substantial quantities of sea lice, leading to a conjectured link between parasites on salmon fish farms and infestations on wild sea trout. To investigate the potential infective pressure on sea trout in Loch Shieldaig, offshore and sub‐littoral plankton samples were collected and analysed for sea lice nauplius and copepodid stages. During the plankton survey, numbers of sea lice on a fish farm in the loch reached a maximum in November 2001. Soon after, numbers of sea lice larvae peaked in open‐water samples and then in sub‐littoral samples. Nauplii were found adjacent to the farm and occurred less elsewhere. This study reports a concentration of sea lice larvae at the head of Loch Shieldaig and indicates a possible relationship between sea lice numbers on the fish farm and lice larvae densities in the open‐water of the loch and in the sub‐littoral zone.     

Acoustic conditioning for recall/recapture of escaped Atlantic salmon and rainbow trout

Escape of salmon from sea cages is a problem that continues to plague the aquaculture industry. Data collected during the past 15 years from Norway, Scotland, Ireland, Canada and U.S. suggest significant impacts on natural runs of fish and economic losses to producers. The present report investigated the feasibility of using acoustic conditioning as a means of recalling/recapturing escaped fish. Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) were found to respond to frequencies in the range of 50 - 400 Hz equally well. Subsequently, both species were conditioned to a 250 Hz acoustic tone during feeding. Juvenile and sub-adult fish readily conditioned to the acoustic signal within four days, with the maximum number of fish responding (85% salmon, 96% trout) by day seven. To assess retention of conditioning, fish were exposed to a single tone without feed reinforcement every one, two or four weeks. Salmon and trout continued to respond for a seven month period with no significant decrease (88% salmon, 97% trout) in response. No significant differences were observed in the response of either species to tones differing in frequency by up to 200 Hz (89% salmon, 96% trout) and intensity by 20 dB (91% salmon, 96% trout). Both species were reproducibly recalled to a cage or feeding ring in a 3.7 m tank, but were reluctant to re-enter the cage. The findings indicate that salmon and trout are readily conditioned to acoustic signals and retain that conditioning for an extended period of time without reinforcement. These characteristics suggest that acoustic conditioning has potential as a means to recall escaped salmon and when coupled with recapture, can reduce interactions with wild stocks and losses to the producer.     

Distribution and prevalence of T. bryosalmonae in Austria: A first survey of trout from rivers with a shrinking population

The first evidence of proliferative kidney disease (PKD) in an Austrian river (the River Kamp) was documented in 2016, and no information on the PKD infection status of trout in other rivers was available. Since then, brown trout (Salmo trutta fario) and rainbow trout (Oncorhynchus mykiss) have been collected from rivers in Upper and Lower Austria for different diagnostic purposes. In this study, we summarize the recent findings of a first survey concerning the distribution of Tetracapsuloides bryosalmonae, the causative agent of proliferative kidney disease (PKD), from these samples. Between September 2015 and October 2017, a total of 280 brown trout and 39 rainbow trout were collected from 21 rivers in the provinces of Upper and Lower Austria. T. bryosalmonae was detected by PCR of kidney tissue in 17 of 21 sampled rivers and in 138 of 280 brown trout as well as in 11 of 39 rainbow trout. Pathological signs of PKD (e.g., hypertrophy of the kidney) were observed in 33 analysed brown trout and six rainbow trout samples. No correlations between fish infected by T. bryosalmonae and the parameters size and age class, condition factor, geological origin of the streams and distribution within the river course were found, while positively tested fish are significantly increased at sampling sites exceeding water temperatures of 15°C for median periods of 115 days. The prevalence within the affected streams or stream sections is highly variable, and in single rivers, infection rates of up to 90% are confirmed.