Fish weir
In general, fish weirs are used to catch migrating fish . In the past mainly for the procurement of food and for commercial fishing , today they are mainly used for researching fish migration .
In the case of fish weirs in rivers , a river profile is usually blocked off with guide fences in the form of rakes . Migrating fish of a certain size cannot pass the rake and are either fished at the weir (for example with nets) or fed into a trap , from which they can easily be removed.
Dynamic fish weir
The dynamic fish weir is a further development of conventional techniques. It arose from the desire for a fish weir that is easy to maintain, transportable, inexpensive and - which is particularly important in rivers - is not destroyed by floods . It was developed at the Institute for Hydrobiology and Aquatic Ecology (IHG) at the University of Natural Resources and Life Sciences, Vienna (Mühlbauer et al., 2003). The so-called "resistance board weir" (Tobin, 1994), which is used in North America to record the migration of salmon , served as a model. In the spring of 2004, just one year after the publication of the Dynamic Fish Weir, this method was already in use on four rivers in Austria and Germany.
Construction, assembly, maintenance
The dynamic fish weir consists of lattice-like panels composed of plastic pipes. One end is anchored to the sole and the other is kept above water with a float. Due to the rake structure of the fish weir, only coarse floating debris usually remains on the construction, which greatly reduces maintenance work.
Flood security
The most important property of the dynamic fish weir is its flood safety. In order to achieve this, the trap box should be made of solid steel or something similar. On the other hand, the dynamic fish weir must be as flexible as possible in order to be able to yield to the water masses in the event of flooding. As the discharge rises, the load on the weir increases until the float is no longer able to lift the float and the fish weir is pushed down and overflowed. If the outflow continues to rise, the fish weir is gradually overflowed. At the same time, more and more floating debris occurs, which can endanger the system. As a rule, the fish trap box and fish weir are already deep enough under water so that they are not damaged. In order to be able to withstand the hydraulic load on the water bed, the fish weir must be carefully processed and adequately anchored. In March 2002, the dynamic fish weir on the Pielach was flooded 4 to 5 meters high with a discharge of around 150 cubic meters per second. However, the approximately 10-year flood event did not damage the fish weir.
Dynamic fish weirs can be used primarily for the following scientific questions.
Fisheries issues
Inventories
Fishings on individual dates usually only represent snapshots. Fish weirs can also record the moving fish populations.
Quantitative uptake of fish stocks is usually not possible in large rivers. In situations where large parts of the population move into backwaters, for example to spawn there, they can be recorded and marked using a fish weir. After migrating back into the main body of water, the total population of a fish species can be determined by relatively few recaptures. By using a fish weir and marking the fish, the population of a fish species was determined for an entire section of the Danube for the first time - this is what happened in the LIFE-Huchen project for the noses immigrating from the Wachau section of the Danube that were marked at the fish weir at the Pielach estuary (Zitek et al., 2004 in preparation).
Catching spawning fish
For the rearing of autochthonous stocking material, spawning fish can be caught gently using the fish weir. Another advantage is that the spawning fish are only caught when they actually migrate to the spawning area.
Ecological issues
Monitoring of fish migration
Increased ecological awareness leads to attempts to treat rivers today in terms of sustainability and to take account of their ecological function. The EU takes this idea into account in the EU Water Framework Directive. The impairment of the passage for aquatic organisms is one of the strongest interventions in the ecology of our rivers. It is caused by power plants and river regulation. In water network projects (fish migration aids, reconnection of backwaters, ...) attempts are made to compensate for the negative effects of water fragmentation. Fish weirs can be used to check whether water networks have already been implemented and whether they are actually accepted by the fish, or the potential of fish willing to migrate can be ascertained to determine the priority of measures.
Auto-ecological studies
With a fish weir it is possible to record fish migrations in quantitative terms and their natural temporal occurrence. In combination with other methods for monitoring fish migrations (marking tests, telemetry, ...) a complete picture of the migration behavior of individual species is created.
Hydraulic engineering applications
Fish weirs can be used to keep fish that have already been fished out of the area of river construction sites.