(* = Kurzbeitrag)
Zunahme des Brutbestands der Wasseramsel Cinclus cinclus am Küsnachter Bach von 1987 bis 2002.
(von 1994 bis 2006 vergeben)
population increase, breeding density, river structure, freshwater biodiversity, acidity, food supply, Switzerland
Bestandesentwicklung, Bestandesdichte, Fliessgewässer, Nahrungsangebot, Sauerstoffgehalt, Temperatur, Wasserqualität, Diversität
Küsnachter Bach, Zürich
Increasing breeding pair numbers of Dippers Cinclus cinclus from 1987 to 2002 at the Küsnacht brook near Zurich, Switzerland. The Küsnacht brook is a small river with a total length of 8.6 km. Its catchment of 12 km 2 consists of agricultural (50 %), forest (35 %) and urban (15 %) areas and lies to the East of Lake Zurich on an alluvial moraine with a maximum altitude of 853 m a.s.l. Dippers establish their territories between 690 m a.s.l and the mouth of the river at Lake Zurich at 406 m a.s.l. Including all subsidiary inlets a total of 8.05 km are potentially settled by Dippers. The survey started in 1987 with a breeding population of 7 pairs. After continuous growth the population consisted of 16 colour-ringed pairs in 2002, leading to a (conservatively counted) population density of 2.0 breeding pairs/km (along an extreme stretch of 2.8 km 9 pairs were breeding; i.e. 3.3 pairs/km). In 1997 a maximum of 7.5 successful nestlings per breeding pair were produced; after that total and relative number of offspring declined slightly. Such a high number of breeding pairs in a relatively isolated population has not been reported from anywhere else and I therefore list potential reasons for this surprisingly high density: An altitude of 400 to 700 m a.s.l. is preferred by Dippers in Central Europe. The river’s fall of 40 m per km (gradient 4 %) is theoretical. Effectively this steep gradient is modified by more than a hundred steps and the flow of the water is disrupted by gravel and stones and the water level reaches 40 cm. The water of heavy and abrupt rains is partly collected and equalized by catch basins of a water treatment plant (ARA). In 1992 the treatment process was adjusted to a higher standard. This improvement and the effect of aeration at the watersteps lead to absolute oxygen saturation in the Küsnacht brook. The water’s favourable quality is indicated by several species of caddisflies (Trichoptera), mayflies (Ephemeroptera) and stone flies (Plecoptera), known as preferred saprob indices species. As a consequence of the calcareous substrate the water shows a high degree of hardness. Furthermore, calcareous deposition occurs and holes, niches and potential nesting sites are created at the front side of the steps. Acid conditions do never appear; the water is of alkaline quality about pH 8.0. After its treatment the clarified water’s temperature is elevated. During winter this elevation can reach as much as 3 °C and the Küsnacht brook never freezes over, at least in the middle part. As a consequence there is always a predictable refuge for the Dippers, even at air temperatures far below 0 °C. They are able to stay year round in the same territory; in the most unfavourable case they have to move a short distance to the unfrozen stretch. Food supply, i.e. the density of aquatic macroinvertebrates, particulary fly (Diptera) and caddisfly larvae, mayfly and stone fly nymphs is sufficient (approx. 2000 macroinvertebrates per m 2 ). In the years 1993 to 1995 a total of 9 nest boxes or shelfs were installed in the surroundings of previous nests (to allow better access for controlling and ringing). In 2002 only 4 of them were used, and we can conclude that there is a sufficient supply of natural nesting sites.
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