Proliferative kidney disease (PKD) is an emerging parasitic disease that affects wild and farmed salmonid fishes worldwide. Currently, it is widespread in Europe with records in many countries. This study assessed the status of PKD in the river Wulka, a small Austrian river, which was known to host a vital brown trout population despite having high water temperatures. Contrary to the initial hypothesis that the river was free from PKD, we observed an overall PKD prevalence of 92%. Noninfected fish were restricted to the uppermost river section. Twenty‐two out of 87 PKD‐positive fish of all age classes showed clinical signs, while five individuals exhibited signs of recovering from PKD infection. The severity of clinical signs was significantly dependent on the fish length and age, with smaller or younger individuals being more severely affected. Comparison of data from the earlier fish sampling events with those results from 2017 and 2018, together with the analyses of the pattern of water temperature since 2010, indicates that Wulka's brown trout population might become extinct in the near future.

Water temperature is one of the most important abiotic parameters in rivers having direct and indirect effects on fish. Especially cold-water species like the brown trout (Salmo trutta) are limited by high temperatures. Beside direct physiological stress, higher water temperatures also reinforce the emergence of diseases. In this study we investigate thermal regimes of rivers based on a large-scale dataset covering Austria (∼70,000 km²). The analyses aim to clarify to what extent water temperatures support the emergence of proliferative kidney disease (PKD) and assemble physiological stress for brown trout under current and future climate conditions. Data from 274 gauging stations at 184 rivers were used to calibrate a water temperature model and to investigate critical water temperature thresholds. Therefore, we developed a risk assessment scheme to identify river reaches that already have or will develop critical thermal regimes for brown trout in respect of PKD emergence and thermal physiological stress. The results revealed severe changes in the thermal regimes of the investigated rivers under climate change. Furthermore, the variable characterizing riparian vegetation played a vital role to explain cooling of the water in downstream direction. In respect of PKD, the amount of river reaches having unlikely outbreaks of PKD decreased from 72.6% under current conditions to 37.7% in the far future RCP8.5 scenario. Within small rivers that currently showed optimal thermal regimes over large extents (10,244 km), the habitat suitability will be reduced by combined effects of PKD and physiological stress to 6,554 km. In general, suitable habitats of S. trutta will shift upstream, thus to higher altitudes, and to smaller, alpine rivers in Austria. The warming leads to physiological stress that induces a diminished growth due to the non-positive transition of caloric values to growth as well as cardiac dysfunction in brown trout. These factors will further restrict the distribution of brown trout. However, the results also underline the enormous importance of the alpine region as a future refuge for brown trout in Central Europe. Thus, this study will help to inform managers to timely develop robust conservation strategies.

Background: Renibacterium salmoninarum and Mycobacterium sp. are important bacterial pathogens of fish. R. salmoninarum is the causative agent of bacterial kidney disease, a Gram-positive bacterium mostly known for causing chronic infections in salmonid fish, while multiple species belonging to the Mycobacterium genus have been associated with mycobacteriosis in fish as well as in human. The objective of this study was to determine the prevalence of these two bacterial pathogens in populations of wild brown trout (Salmo trutta fario) in four rivers (Kamp, Wulka, Traun and Ybbs) in Austria. Results: A total of 457 kidney samples were examined for both bacterial agents using nested and conventional PCR as well as bacterial cultivation on KDM-2, histological examination and immunohistochemistry. Molecular evidence showed an estimated prevalence level of 0.94% for R. salmoninarum in 2017 while the bacterium could not be detected in 2018 and histology showed signs consistent with a low-level chronic inflammation in the kidney of infected fish. Similarly, no fish were found positive for Mycobacterium in 2017 but in 2018, the prevalence was found to be 37.03% in the Kamp river (4.08% across all rivers). The sequencing data confirmed that these fish carried Mycobacterium sp. although the precise species of Mycobacterium could not be ascertained. Conclusions: This survey constitutes the first insight into the prevalence rate of R. salmoninarum and Mycobacterium sp. in wild brown trout (Salmo trutta fario) populations in Austria. Both of these pathogens were only detected in the summer months (June and July), which might suggest that the stress linked to increased water temperature could act as stressor factor and contribute to the outbreak of these diseases. The age of the fish might also play a role, especially in the case of Mycobacterium sp. as all the infected fish were in their first summer (June).

Proliferative kidney disease (PKD) is a disease found in salmonid fish that is widespread in Europe and North America. The dependency of the clinical signs on the water temperature is extensively reported in rainbow trout, but detailed information on brown trout is lacking. In this study, juvenile brown trout were exposed to the spores of Tetracapsuloides bryosalmonae and then kept at different ambient water temperatures (16°C, 19°C and 22°C) for 10 weeks along with recording of morbidity throughout the experiment. At 6, 8 and 10 weeks post‐exposure, fish from each temperature group were sampled and underwent pathoanatomical examination to survey disease progression. At 16°C, brown trout showed a significantly higher survival probability compared to those kept in 19°C and 22°C water. Additionally, the parasitic burden (MSQ) was higher and the clinical signs were more pronounced in the brown trout kept at 19°C and 22°C compared with the ones kept at 16°C. This study highlights the correlation of PKD outbreaks and water temperature increases related to global climate change, which will impact the future distribution of brown trout in natural waters.

Nontuberculous mycobacteria constitute a subgroup among the Mycobacterium genus, a genus of Gram-positive bacteria that includes numerous pathogenic bacteria. In the present study, Mycobacterium spp. were detected in natural water samples from two Austrian rivers (Kamp and Wulka) using three different primers and PCR procedures for the identification of the 16S rRNA and hsp65 genes. Water samples were collected from the Kamp (45 samples) and Wulka (25 samples) in the summer and winter of 2018 and 2019. Molecular evidence showed a high prevalence of Mycobacterium sp. in these rivers with prevalence rates estimated at approximately 94.3% across all rivers. The present study represents the first survey into the prevalence of Mycobacterium sp. in natural water in Austria. Because nontuberculous mycobacteria have known pathogenic potential, including zoonotic, these findings may have implications for health management and public health.