Honsig-Erlenburg, W.

Distribution and threat of Grayling (Thymallus thymallus) in Carinthia (Austria)

Summary: Nearly 1/3 of investigated sections in running water courses of Carinthia (Austria) contained populations of grayling (Thymallus thymallus, in 114 sections of 28 rivers). Self reproducing stocks were recorded at 350 to 1050 m above sea level, the average water temperature in those sections ranges between 5.4 to 9.5 °C with summer temperatures exceeding 20 °C. The fish communities of that grayling rivers contain three to 12 species, the fish biomass is in the range between 10 and 200 kg/ha. In some sections of the upper River Drau a biomass of up to 700 kg/ha has been recorded. Anthropogenic impacts, especially the construction of serial impoundments in the river Drau as well as former pollutions (e.g. of the rivers Gurk, Glan, Lavant) have reduced grayling stocks considerably. The sanitation of these waste water polluted rivers led to the recovery of the grayling stocks; in many sections grayling is now the dominant fish species again. Graylings are threatened nowadays by stocking of rainbow trout (competition) and predation by hibernating cormorants. Improvements of the structural heterogeneity of the aquatic habitat can be achieved by restructuration of river sections (for example upper river Drau).