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

Results Distribution of lamprey populations within the Danube basin of Baden-Württemberg A total of 50 lamprey populations within the upper Danube system were detected in the field or known to have existed based on reliable historic records. Of these, 40 populations have been confirmed to exist during the last 20 years. A minimum of 10 brook lamprey populations have become extinct since 1820, while a much higher number must realistically be assumed to have disappeared without ever being recorded (Kappus et al. 1994). The 50 confirmed populations can be separated into two centres of distribution: one group of 26 populations are clustered within the "Schwäbische Alb" area in the vicinity of the cities of Ulm and Sigmaringen, the other group of 22 populations is centred in the headwaters region of the Danube in the area of the rivers Brigach and Breg (Fig. 1). The relatively low number of populations (n=4) in those tributaries entering the Danube from the south is most likely caused by glaciation effects, as this southern area was largely covered by ice during the last Riss-glaciation. Hazards to lamprey populations of the Danube basin of Baden-Württemberg In the following section the specific hazards affecting each of the presently known 40 lamprey populations found in the study area are described. The number of different threats for each population can range between 0 and 7 (Table 2). Generally, the populations in the "Schwäbische Alb" area face an average of 3.3 identifiable threats, whereas this number is only 2.5 for the populations in the Danube headwaters area. If the different hazards are ranked according to their number of occurrences, with 32 listings, in-stream migration barriers must be considered the number one threat to lamprey populations, followed by the sealing of river banks through deployment of large and flat rocks and, thirdly, the input of organic nutrients (Table 3). Other hazards, which occurred less frequently, may nevertheless pose a major threat to an individual population, were river channelization, minimal low flow conditions, removal of sediments, toxicants, and fish stocking practices. Physical disturbances of river integrity Morphometrical changes due to construction measures represent the major threat to lamprey populations of the upper Danube Basin. Eighty percent of all rivers with known lamprey populations were affected by one or more horizontal barriers (Fig. 2), forming effective obstacles to upstream (and downstream) movement of lampreys (and other fish). A total of 102 barriers have been identified in the rivers with known lamprey populations. As the mean number of 5.3 barriers indicates, the affected rivers have been dammed several times and an unbroken sequence of "impoundments" continues for a major section of the river or follows each other with only minor distances in between. Such conditions destroy the former continuum of suitable lamprey habitat and render potential survivors to small groups of animals with no hope for long-term survival. In addition to migration barriers, river channelization may be equally destructive to lamprey habitat. In this case, flow conditions are often altered in such a way, that the fine particulate sediments, necessary as ammoecete (feeding) habitat, have been washed away and cannot be re-established. A third factor negatively affecting lamprey populations by disturbing the river