The surveys on fish stocks in Austrian lakes according to the implementation of the EU WFD revealed missing fish species in some cases.
If the assessment of biological quality element fish in lakes results in “moderate” ecological status class or even worse, the EU WFD requires improvements. Often small bodied fish were missing especially the occurrence of the minnow (Phoxinus phoxinus) decreased during the last 50 years. In some lakes the minnow is deemed extinct; in 38% of the investigated lakes (charr lakes, minnow lakes) it was not possible to detect native minnow. Small bodied fish are are of little concern to commercial fisheries but play an important roles in aquatic ecosystems. Minnow is classified as sentinel species in the lake type Minnow lake (“Elritzensee”).
The minnow did not decrease due to a single event, and the circumstances of the decrease are rather unknown. At the present state of knowledge a combination of rising water temperature and stocked predators might be responsible. Moreover, negative impacts on stocks could have increasing bathing activities, massive catch of minnow as bait fish (forbidden nowadays) or the consequence of eutrophication during the decades of 1970 and 1980. The occurrence of the minnow in the lakes Traunsee, Fuschlsee and Erlaufsee is historically well documented; fisheries managers corroborate the occurrence of minnow until 1970. Even high fishing effort in Fuschlsee (2008), Traunsee (2012) and Erlaufsee (2013) did not serve to detect the minnow. Hence the minnow is presumed to be extinct in these three lakes.
The aim of the present investigation is the restoration of minnow stocks in the lakes mentioned above. First, we look for suitable spawning habitats, second, mother fish from the catchment area of the lakes are kept in our Fishery Kreuzstein and third, we breed offspring suitable for each lake. Every year the lakes are stocked with the offspring and after five years a monitoring performance will be done.
Mother fish for the lakes Fuschlsee and Traunsee stem from lakes Hallstätter See and Langbathsee. The mother fish for Erlaufsee stem from Hubertussee near Mariazell. The mother fish are well adapted to aquaculture; they are held in round tanks and are fed with special fish food. After spawning in May or June we rear the used number of offspring, and in autumn we release the offspring into the lakes (Traunsee 20.000 individuals, Fuschlsee 10.000, Erlaufsee 5.000). In addition we exposed data loggers for temperature measurement at a depth between 40 cm and 60 cm. The loggers record and store temperature hourly.