Possible reasons for late summer brown trout (Salmo trutta Linnaeus 1758) mortality in Austrian prealpine river systems

Abstract

For years, severe mortality of brown trout (Salmo trutta) in late summer has been reported in the prealpine river systems of Austria. For an initial understanding of the potential reasons for this mortality, brown trout were exposed to water under differing conditions from an affected river system. Blood parameters, histology of various organs, hepatic ethoxyresorufin-O-deethylase (EROD) activity, and occurrence of ectoparasites and fish diseases were investigated. In brown trout exposed to water from the affected river, concentrations of peripheral blood lymphocytes, granulocytes, plasma immunoglobulin and plasma lyszoyme activity decreased significantly, indicating a reduction in unspecific, innate and specific, adaptive immune response. Erythropoietic and thrombopoietic functions were also disturbed, with fish mortalities up to 100%. From the results of the present study it seems unlikely that ectoparasites, diseases, or effluent water from sewage plants were causative for the observed phenomenon. However, brown trout maintained in river water and in the dark survived longer than those under natural light conditions, and their immune systems were less drastically affected. As periods of high solar UV-radiation occurred during the experiment and the Traun River water temperature was quite variable, these factors might have had an effect on immune systems. Laboratory studies were therefore conducted to ascertain whether water temperature variations and ⁄ or UV could indeed affect the immune system. The tested temperature variations (10°C ⁄ d) suppressed the brown trout immune system, whereas UV radiation (0.045 mW ⁄ m 2 for 6.5 h per day) had no effect. However, UV-radiation did enhance the immune-suppression effect of variable temperatures.

Health Surveillance of Wild Brown Trout (Salmo trutta fario) in the Czech Republic Revealed a Coexistence of Proliferative Kidney Disease and Piscine Orthoreovirus-3 Infection

“…Kuehn et al [ 21 ] argued that the virus is the main causative agent of the proliferative darkening syndrome (PDS), a disease affecting wild brown trout populations in pre-alpine rivers in Switzerland, Germany and Austria. PDS manifests itself with pathoanatomical signs such as skin darkening, hemorrhages and necrosis of liver, spleen and kidney resulting in massive species-specific die-offs of brown trout during the late summer months [ 22 ]. On the contrary, the presence of PRV-3 was described in clinically healthy brown trout [ 23 ], and brown trout showing PDS signs also tested PRV-3-negative in other cases [ 24 ].…”

Identification of a piscine reovirus-related pathogen in proliferative darkening syndrome (PDS) infected brown trout (Salmo trutta fario) using a next-generation technology detection pipeline

“…For years, a suspicious species-specific die-off of brown trout ( Salmo trutta fario ) has been reported from pre-alpine river systems in Austria, Southern Germany, and Switzerland resulting in drastically decreased population densities in the impacted regions [ 1 , 2 ]. In the most severely affected areas, no viable populations of brown trout remain and all attempts to restock brown trout in these places have failed due to the persistence of the annual die-off [ 1 ].…”

“…Since affected brown trout develop a black pigmentation on the skin before their death, the disease was named “Schwarze Bachforelle Phänomen” in German [ 1 ], which translates into “Black Trout Phenomenon”, equivalent to “Proliferative Darkening Syndrome” (PDS) [ 3 ]. Recently it was suggested that this disease could be primarily the result of immune suppression caused by a combination of temperature variation and UV-radiation but clear evidence on the causes could not be found [ 2 ]. It was also hypothesized that there is a strong link between the Proliferative Kidney Disease (PKD), caused by the parasite Tetracapsuloides bryosalmonae , and PDS [ 4 ].…”

Exploring the link between ultraviolet B radiation and immune function in amphibians: implications for emerging infectious diseases

“…Environmental conditions not only affect pathogen growth, transmissibility and pathogenicity but also influence host behaviour, immune function and pathogen exposure regimes ( Fisman, 2007 ). Host immune function is sensitive to a range of biotic and abiotic environmental variables like temperature ( Engelsma et al , 2003 ; Raffel et al , 2006 ; Ndong et al , 2007 ; Barber et al , 2016 ), habitat quality ( Cary et al , 2014 ; Katzenback et al , 2014 ; Krynak et al , 2015 ; Makrinos and Bowden, 2016 ), nutritional status ( Venesky et al , 2012 ), competition ( Groner et al , 2014 ) and importantly, solar UVB radiation ( Kripke et al , 1992 ; Lahnsteiner et al , 2011 ; Debecker et al , 2015 ; Abu Bakar et al , 2016 ).…”