Distribution and age structure situation of selected fish species in austrian rivers during the 2016 to 2018 GZÜV period | Verbreitung und Altersstruktursituation ausgewählter Fischarten in österreichischen Fließgewässern der GZÜV-Periode 2016 bis 2018

In the course of the implementation of the EU Water Framework Directive and the Austrian Water Act, fish in streams and rivers play a decisive role in achieving the objectives. For this purpose, it was necessary to develop a standardized methodology, which required a standardized sampling and assessment definition in addition to a water body type specific approach with common guidelines. Part of the assessment scheme for the fish ecological status is the age structure situation of the species stock of the guiding species and typical accompanying species. From a descriptive point the occurrence of a species and the age structure situation (5-level: very good to poor) give a rough overview on qualities with regard to habitat or its accessibility. The nine species considered here, with a focus on rhithral species, include bottom- and water column-oriented, short- and medium-distance migratory, as well as several current and spawning guilds. The metrics used in the Fish Index Austria (e.g. occurrence of fish species, guilds, age structure) were found to be strongly indicative of stress, with lack of connectivity and hydromorphological deficits to be cited.To date, 220 stretches from various fish regions and bioregions have been evaluated in the 2016 to 2018 GZÜV period; stretch selection by those responsible in the respective federal states took place predominantly in areas of the respective water bodies that are affected by hydromorphological stressors.Burbot could be detected in only 20% of the sampled sites where it would have been expected to form a population (i.e. with the status of a guiding species or typical accompanying species) (n = 70). Chub was found in 82% (n = 98), Grayling in 69% (n = 116), Brown Trout in 97% (n = 200), Brook Loach in 74% (n = 83), Minnow in 55% (n = 58), Gudgeon in 70% (n = 81), Huchen in 20% (n = 49) and Bullhead in 81% (n = 174) of the respective sites.Consequently, the medium-distance migratory species burbot and huchen have significantly lower occurrence numbers, followed by minnow. Brown trout, on the other hand, had the highest distribution value.The percent distribution of very good and good age structure for each species was: Burbot 3% (n = 70), Chub 40% (n = 98), Grayling 17% (n = 116), Brown trout 55% (n = 200), Brook Loach 43% (n = 83), Minnow 31% (n = 58), Gudgeon 46% (n = 81), Huchen 4% (n = 49), Bullhead 48% (n = 174).Similar to the occurrence ratios, only a small number of stretches where the age structure can be considered very good or good is evident in the medium-distance migratory species. Brown trout again had the highest percentage of stretch at 55%.