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

This may start a cycle of radiation mediated increase in macrophyte and algal growth, which is additionally enhanced by nutrient input. While the higher plant growth leads to increased sedimentation (and the perceived necessity for its removal), decaying algal material often results in anoxic sediment layers preventing ammocoete settlement. All these physical disturbances change flow dynamics, prevent the formation of pool and riffle sequences, and disrupt the frequent, but locally distributed accumulation of finer sediments. These altered rivers also frequently lack fast flowing, shallow reaches with gravel-sized sediments (Waterstraat 1989). Apart from the "regulation" of rivers, many of the very small water bodies have also been heavily degraded by their partial transformation into completely lined channels or culverts. Thus, both larval habitats and adult spawning sites have largely disappeared. If these habitats are present, they are often spatially separated and do not provide functional units in terms of lamprey reproduction. Prime examples for these negative developments are found at the Rot River in the vicinity of Bussmannshausen, the lower reaches of the Lauchert, the Stehbach at Rotten acker and over most of the course of the Danube. Hand in hand with the loss of suitable lamprey habitat, migration barriers have separated the remaining habitats into isolated patches, with limited biotic exchange. Ammoecetes carried downstream during periods of flooding are potentially able to bypass barriers and become newly established on sediments downstream. However, after metamorphosis, the adults are unable to return to their native spawning grounds and spawning must take place under less favourable conditions or may completely fail. As a consequence, local extinction of populations occurs (Kirchhofer in press, Bohl 1993, Schadt 1991). Such a scenario provides a reasonable explanation for the present disjunct distribution of the lamprey populations of the upper Danube Basin. The once interconnected populations of the corresponding creeks and rivers have often been reduced to isolated, small units which dynamics largely follow the metapopulation concept (Hanski & Gilpin 1991; Fig. 5). Thus, locally high population densities at some of the sites cannot hide the fact that the brook lamprey of the upper Danube Basin must be considered strongly endangered. Despite dealing with quite different types of waters, Dehus (1990) arrives at similar conclusions analyzing the status of lamprey populations in Northern Germany (Schleswig-Holstein). Obviously, efficient conservation measures have to be defined and implemented to guarantee the long-term survival of the brook lamprey (L. planeri) throughout its Central European range. One such concept has been proposed by Kappus & Rahmann (1994b), in which all interventions into known or potential lamprey habitats should be assessed with regard to their compatibility with the existence of a self-sustaining, reproducing lamprey population, prior to their actual implementation. Acknowledgements Wc thank the Government of Baden-Württemberg, Ministerium für Ländlichen Raum, Ernährung, Landwirtschaft und Forsten, for the financial support of this study (Staatshaushaltsplan 1990, Kap. 0802 Tit.Gr. 74, Projekt Nr. 27-90.35).