L. Forró szerk.: Miscellanea Zoologica Hungarica 10. 1995 (Budapest, 1995)

Kappus, B., Jansen, W. , Fok, P.; Rahmann, H.: Threatened lamprey (Lampetra planeri) populations of the Danube Basin within Baden-Württemberg, Germany

Discussion The general situation of the brook lamprey within the upper Danube Basin is well illustrated by a comparison of the historic and recent existence of populations. At least 20% of the populations have disappeared and the size of remaining populations has generally been reduced. These developments were paralleled over the last 200 years by structural changes of rivers and creeks, and biotic disturbances. Among the latter disturbances, stocking practices are of major importance in affecting lamprey populations. The present study clearly demonstrates that stocked salmonids kill or injure adult lampreys during spawning time. Similar results were obtained by Bohl (1993) and Kirchhofer (in press). Our findings confirm those of Bohl (1993) who reported that the conspicuous spawning behaviour makes them vulnerable to prédation by salmonids and also by burbot {Lota lota), pike (Esox lucius), and eel (A. anguilla), particularly if predator densities have been artificially increased by stocking. Since our electro-fishing surveys indicate that eels frequently occur within accumulations of detritus, sand or gravel, these fish may not only prey on adult lampreys, but also potentially eat eggs and larvae. The stocking of eels, which until recently, has been financially supported by state authorities, is also important from a different aspect. The apparently consistent presence of lampreys among the eel-batches used for stocking had, and still have, a number of noteworthy consequences. There is the chance that species of lampreys have been introduced into water bodies, which they could not have reached otherwise (i.e. ponds and lakes without inlets or outlets), or into lotie systems in which they did not occur naturally (also see Kappus & Rahmann 1994b). Although the first record of river lampreys (L fluviatilis) from the Danube River dates before the first (known) stocking of eels in 1881 (Hack 1881), this species has most likely been introduced into the Danube system via this route. A total of 2.4 million eels have been introduced into water bodies within the area of what is now Baden-Württemberg between 1893 and 1906 (Sieglin 1892). Assuming that 30% of these fish have gone into the Danube system, and that the proportion of lampreys mixed in with the eels was 0.5 %, then approximately 4000 lampreys have been accidentally introduced into this river system within 13 years. A further 1000 lampreys can be estimated to have entered the Danube in the vicinity of the city of Ulm, where eels were stocked at the beginning of the 20th century (Fischereiverein Ulm 1905). According to Holcik (1986), L. fluviatilis, L planeri and also P. marinus were among those accidentally stocked. Little is known about the fate of these fish, but it is likely that some exchange of genetic material information occurred between the L. planeri of unknown origin and the autochthonous stocks already present. This gene flow must be considered detrimental, because the evolutionary history of the introduced lampreys took place outside their "new" habitat and these animals were not adapted to the specific local conditions. Among the structural changes detrimental to lampreys, the removal of stream meanders and large scale channelization has notably resulted in a considerable loss of suitable lamprey habitat. Although these measures, which affect the whole river profile, now have to be officially authorized, regular river "maintenance" does not. Therefore, it is still common practice and indirectly encouraged by the regulations of the Ministry for Rural Affairs (MLR 1980) to invest considerable financial resources into measures, which do not consider species and habitat protection requirements (see also Berg et al. 1989). In the case of lampreys, the armoring of river banks often completely destroys actual or potential larval habitats within the lenitic, sediment covered nearshore areas (Schadt 1994). A further "maintenance" measure, which negatively affects lamprey populations, is the frequent cutting of shoreline vegetation.