Wanzenböck, Josef, Gassner, Hubert, Lahnsteiner, Barbara, Hassan, Yasmin, Hauseder, Gudrun, Doblander, Christine, and Köck, Günter
In order to analyse potential influences of soda industry effluents on the ecological integrity of Traunsee (TS) we investigated the fish community of the lake in comparison with a reference lake (Hallstättersee HS) and used a reconstruction from the (older) literature concerning the original species composition of Traunsee. Published `Index of Biotic Integrity (IBI)' metrics were considered to be of limited value due to the relatively low species number in the oligotrophic, Alpine lake. Therefore we included, in addition to species composition, studies on egg distribution, larval fish densities, life-history parameters (i.e. growth, maturity, fecundity, age and size composition), stress levels and heavy metal content of the dominant whitefish (Coregonus lavaretus), as well as overall fish density and biomass (using hydroacoustics) to assess the ecological status of the fish community. Two of the original 18 species have disappeared from the lake, presumably in connection with the introduction of non-native eel (Anguilla anguilla) and the construction of a power plant in the outflow. Silt from grinded limestone together with highly alkaline pore waters is emitted via industrial wastewater from Sodaworks and covers part of the lake bottom. We observed that eggs of whitefish were spawned mainly in the main inflowing river and close to the shore, thus avoiding the silty areas and making the anticipated damage to the reproductive potential of whitefish neglectable. This was corroborated by larval surveys done weekly on both lakes from January to May, which showed halve the density of whitefish larvae in TS compared to HS. Estimates of potentially spawning fish from hydroacoustic surveys resulted in a ratio of 1(TS) : 3 (HS). Analysis of whitefish revealed that they are growing faster in TS and have higher fecundity leading to some compensation of lower abundance. High levels of fishing in TS might have led to this pattern and to depressed yields as indicated by the age composition. Level of oxidative stress and heavy metal content were not discernible from the reference lake. Therefore we concluded that negative impacts on the ecological status of the fish community resulted from fisheries mismanagement and a power plant situated in the outflow of the lake, considered to have damaged spawning places for some species, but not from soda industry effluents. [ABSTRACT FROM AUTHOR]