Abstract
For many years, brown trout (Salmo trutta fario) mortalities within the pre-alpine Isar River in Germany were reported by the Bavarian Fisheries Association (Landesfischereiverband Bayern e.V.) and local recreational anglers during August and September. Moribund fish seemed to be affected by proliferative darkening syndrome (PDS). In addition, proliferative kidney disease (PKD) caused by Tetracapsuloides bryosalmonae was discussed. To investigate this phenomenon, the present field study monitored brown trout mortalities by daily river inspection in 2017 and 2018. Moribund brown trout (n = 31) were collected and examined using histology, immunohistochemistry, qPCR, and quantitative stereology. Our investigations identified 29 (93.5%) brown trout affected by PKD. Four brown trout (12.9%) displayed combined hepatic and splenic lesions fitting the pathology of PDS. The piscine orthoreovirus 3, suspected as causative agent of PDS, was not detectable in any of the samples. Quantitative stereological analysis of the kidneys revealed a significant increase of the renal tissue volumes with interstitial inflammation and hematopoietic hyperplasia in PKD-affected fish as compared to healthy brown trout. The identified T. bryosalmonae strain was classified as part of the North American clade by phylogenetical analysis. This study highlights PKD and PDS as contributing factors to recurrent autumnal brown trout mortalities.
Highlights
In the last decade, high mortalities and a dramatic decline of the brown trout populations were reported by the Bavarian Fisheries Association (Landesfischereiverband Bayern e.V.) and local recreational anglers in the pre-alpine Isar River in Southern Bavaria (Germany)
Our results demonstrate that proliferative kidney disease (PKD) and proliferative darkening syndrome (PDS) contribute to the recurrent mortalities of Isar contribute to the recurrent mortalities of Isar brown trout
Due to introduction of rainbow trout into German rivers in the early 19th century, autochthonous brown trout populations had to face a strong habitat and food competitor which led to a brown trout decline or even disappearance in many Southern German rivers [30]
Summary
High mortalities and a dramatic decline of the brown trout populations were reported by the Bavarian Fisheries Association (Landesfischereiverband Bayern e.V.) and local recreational anglers in the pre-alpine Isar River in Southern Bavaria (Germany). These mortalities were predominantly observed in the warmer summer months. Facing the advent of global climate change, aquatic ecosystems are endangered because increasing water temperature is an important. Anglers described apathetic behavior and black discoloration of moribund brown trout These rather unspecific symptoms have been described for various different fish diseases [4].
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