title: Auswirkungen hydraulischer Variablen und der Ontogenie auf die Dynamik der Jungfischdrift an einem Schotterufer der österreichischen Donau
Creators: name: Lechner, Aaron Johann
Publisher: name: :none
Year: 2009

description: Die Auswirkungen hydraulischer Variablen (Durchfluss, Fließgeschwindigkeit, Sohlschubspannung, Wassertiefe) und der Ontogenie auf die Jungfischdrift wurden von April 2008 bis Juni 2008 an einem naturnahen Schotterufer der österreichischen Donau untersucht. Ich fand ein deutliches saisonales Muster, wobei die höchsten Driftdichten Mitte Juni auftraten. Insgesamt konnten sieben verschiedene Familien in der Drift nachgewiesen werden. Die dominanten Familien waren Gobiidae (55,3%), Cyprinidae (39,9%), Percidae (3,3%) und Cottidae (1,3%). Ein stadiumsabhängiger Drift war bei den Cypriniden (L1: 32,7%, L3: 23,8%) und den Perciden (J1: 49,6%) vorhanden. Der Einfluss der Hydraulik auf die Driftdynamik war gering. Generell war die Driftdichte aber negativ mit der Strömung korreliert, was auf einen aktiven Drifteintritt der Individuen hindeutet. Die Driftdichten der benthischen Cottiden nahmen bei zunehmender Sohlschubspannung ab. Eine erhöhte Mortalität, oder eine aktive Flucht der Individuen in das Interstitial könnten dafür verantwortlich sein. Ein deutlicher lateraler Driftgradient war an der Donau nicht ausgebildet. Ich vermute , dass die Strömungsstrukturen an der Schotterbank und Habitatprefärenzen der einzelnen Stadien die räumlichen Verteilung der driftenden Fische bedingen.
description: The downstream drift of early developmental stages was investigated in a large European lowland river (Danube, Austria) to ascertain seasonal and spatial dynamics as well as the effects of environmental conditions on this mode of transport. Drift sampling began mid-April and continued at seven day intervals until end-June 2008. Samples were taken exclusively after dusk. The drift nets were conical shaped with an opening of 0.78 m². The nets were set at different distances from the shore along the river bank using a stratified random block design. I found a clear seasonal pattern with peak densities in mid-June what was strongly correlated with the increasing water temperatures. A total of 7 drifting families were recorded. The dominant families were Gobiidae (55.3%), Cyprinidae (39.9%), Percidae (3.3%) and Cottidae (1.3%). A stage dependent drifting was found for cyprinids (L1: 32.7%, L3: 23.8%) and percids (J1: 49.6%), what may be attributed to major changes in fish morphology and behaviour. Environmental variables influenced the drift dynamic to a small extent. Generally densities of drifting fishes were negatively correlated with current velocity and there was no significant relation between body size and current. Because the ability to resist the water current develops with size, these findings support the assumption, that drift is rather an active decision than a passive displacement. High Shear stress values coincided with low densities of drifting cottids. As individuals of this family perform a benthic lifestyle, this probably indicates that the fish get spilled in the sediment or hide in the interstitial when current and shear stress increase. Increasing drift densities during elevated river discharge as found for cottids and early cyprinid stages may due to a loss of visual orientation in turbid water. The high abundance of older cyprinids in shallow areas may be the result of diurnal habitat shifts triggered by food demands and predation risk. I did not find clear density- or size-related patterns in the lateral distribution of drifting fishes and assume that cross currents at the upper end of the gravel bank might be the reason for these results.