Attraction Flow and Fish Migration Observations in Nature Like Bypass-Channels

摘要

In two nature like bypass channels at the Marchfeldkanal (MFK), fish passage has beenmonitored over a period of 5 years. The functioning has been verified while observingmore than 180.000 individuals out of 39 species.Examinations of the attraction flow (AF) at the bypass entrance under various dischargeconditions demonstrate, that it depends on the proportion between the discharge in thebypass QB and in the Marchfeldkanal channel QMFK. The affect of the bypass-channeloutflow has been measured up to 10 m downstream in the Marchfeldkanal profile.Proportions QB : QMFK < 1 : 10 yield significantly higher flow velocities at the entrance ofthe bypass, than in the other part of the channel. By the proportions QB : QMFK > 1 : 30physical effects of the incoming bypass-discharge could not be measured.Further investigations have been made to find out the functional relations betweenattraction flow and the amount of fish-individuals and/or the observed species.Therefore various correlation analyses have been applied. Although the dependentvariable fish-individuals have been separated into subgroups (stagnophilous,indifferent, rheophilous species), no significant correlation between attraction flow andthese variables could be detected.The null – hypothesis that a quite noticeable AF would increase the number ofmigrating fish and would help the fish to find the entrance of the bypass system, dad tobe rejected. Increasing the flow within the bypass in order to achieve a higher attractionflow, may cause supercritical flow conditions at flumes between pools. Therefore, theparameter AF at the bypass entrance is not a proper decision tool for determining theoptimal flow in the bypass channels at the MFK.