Comparative fish stock surveys at Johnsbach and Enns as part of the EU LIFE project

In the course of the EU LIFE project Nature Conservation Strategies for Forest and Wild River in the Gesäuse (2005-2009), renaturation measures took place at the Johnsbach and Enns rivers. In the present work, the fish stocks collected before and after the implementation of these steps by means of electrofishing were compared in order to examine the measures with regard to their fish ecological suitability. Despite the measures, the catch figures at the Johnsbach fell, while at the Enns they remained the same. The assessment on the basis of the Fish-Index-Austria showed that good ecological status can only be achieved at the Johnsbach by adapting the mission statement in the lower sections; in the upper section, the assessment is insufficient due to the insufficient biomass. At the Enns, too, an insufficient state of class 5 was calculated in the Gesäuse section due to too low biomass values; in the Upper Gesäuse section, only class 4 is reached, even taking into account the occupied Huchen, since here, in addition to biomass, the species spectrum also ensures an unsatisfactory state. In addition, a mapping of transverse structures was carried out at the Johnsbach in order to be able to evaluate them with regard to their passability for fish. This was done by comparing the height and flow velocity values with the values of the swimming and jumping performance of the fish species from the literature. This showed that impassable obstacles, at least for coppices and juvenile salmonids, are still present in all sections of the Johnsbach where thresholds exist. Since the periods between the completion of the construction work and the fishing for post-monitoring were very short at both the Enns and the Johnsbach, a longer observation period is recommended in order to monitor the development of the measures taken and, if necessary, to be able to take further necessary steps.