Fish migration
As fish migration developed by many fish species is migratory behavior called, allowing them to use different habitats. For reproduction, other factors such as currents, temperature, and oxygen content are important, such as for growth or nutrition. That is why fish - similar to migratory birds - migrate within connected water systems in order to find the optimal conditions for the respective needs.
Most river fish species native to Europe depend on such migrations; especially the species that perform spawning migrations depend on the closeness of their river and its interconnectedness with all necessary sub-habitats.
Reasons for the hikes
There are several reasons the fish migrate. One of them is the development cycle that all native fish species go through. The animals migrate in order to find the ideal resources and living conditions in their respective stages of development. The hikes can be differentiated depending on their function:
- Spawning migrations: The life cycle of a new generation of fish begins with the laying of eggs and their fertilization. For this purpose, specific habitats are sought, depending on the fish species. To achieve this, the adults migrate upriver from their winter habitat. After the so-called "yolk sac broodlings", young animals that are still fed by the yolk sac, have developed into larvae, these or the further developed small juvenile fish migrate to foraging habitats.
- Food migrations: The food habitats do not necessarily coincide with the spawning habitats and so both the adult fish after spawning and the juveniles after their metamorphosis have to migrate between these habitats. Often the foraging biotopes are calmer than the spawning habitats, which require a rapid overflow to supply the eggs with oxygen. In addition, the food itself is often distributed in the running water in a temporally and spatially variable manner, which requires the fish to adapt.
- Winter hikes: Many fish species reduce their activity at lower temperatures. In winter, for example, they often retreat to winter habitats, which are calmer.
- Propagation: Rivers are very dynamic systems that are constantly subject to change. In order to ensure a natural repopulation of impoverished river sections, the aquatic organisms have to be very mobile. A flood can, for example, “comb out” certain river sections for a short time, which have to be repopulated after the flood event.
- Countercurrent migration: Due to drift, especially of the larvae and juveniles or triggered by flood events, the fish populations experience a population shift downriver. By migrating in the opposite direction, they can compensate for these losses and thus ensure a more or less even population distribution over different river sections.
The reasons for the fish migration can therefore be summarized in four strategies of the fish:
- Optimizing food intake
- Escape from adverse conditions
- Optimization of the reproductive success
- Creation of new populations
For these strategies, one can identify three functional habitats between which migrations take place: the wintering habitat , the feeding habitat and the reproductive habitat .
Migratory types of fish
The distance covered on the hikes can vary between a few meters and thousands of kilometers. The fish migrate both for different reasons and between different bodies of water (salt and fresh water). There are three types of hikes:
- Oceanodromy: migrations within the seas (salt water)
- Potamodromy: migrations within fresh water
- Diadromia: walks between sea and fresh water
The distance covered by a hike does not play a role in the classification.
The diadromous fish migration can in turn be divided into three further classes:
- anadromous: reproduction in fresh water, growth phase in the sea, e.g. Atlantic salmon ( Salmo salar )
- catadromic: reproduction in the sea, growth phase in fresh water, e.g. European eel ( Anguilla anguilla )
- amphidrom: Regular change between sea and fresh water resp. Life in the brackish zone, e.g. flounder ( Platichthys flesus )
An example of fish migration is the migration of salmon , which migrate in large flocks from the sea to the upper reaches of the rivers to spawn. The salmon cover distances of many hundreds of kilometers. Conversely, the eel migrates from the rivers to spawn into the sea in the Sargasso Sea .
Endangerment of the fish
The reasons for the decline in species observed in some populations of European fish fauna over the past few decades are diverse. However, changes in the habitat of aquatic organisms play an important role. Structural interventions in the natural dynamics of flowing waters have been changing the river landscape in Europe enormously for more than a hundred years: the rivers have straightened land reclamation, barriers for infrastructure, buildings for flood protection and, last but not least, the use of hydropower ( river power plants ) for energy generation in individual cases broken up into isolated small parts. In addition to the general state of the water, the free passage for the fish plays a particularly important role. In addition to the linear networking along the course of the river, this also includes the lateral networking of the main waters with smaller tributaries. Depending on the type of fish, falls and steps from a height of 20 to 50 cm are an insurmountable barrier for the animals. It is therefore clear: the more obstacles, also known as transverse structures, the fish have to overcome upstream, the less they make it and the smaller their habitat has become.
Possible solutions
The ecological model of an intact river landscape therefore calls for the biological continuity of the rivers, the so-called flowing water continuum. This means that migratory fish species must have the opportunity to swim through the running water from the mouth to the source, including the backwaters. In the case of weirs that cannot be overcome by migratory fish, the blockade of the hiking trails must be removed using bypasses, so-called fish ladders .
To create such fishways, knowledge about the migratory behavior of the individual species and their behavior patterns and orientation options in the area of such obstacles is necessary. Comprehensive research into fish migration, for example through monitoring with the help of fish weirs, is therefore an essential part of the conservation of the fish species concerned.
Individual evidence
- ↑ Baier, E. (2013). Ecological principles and site evaluation for the realization of the prototype of a fish passage. Bachelor thesis ETH Zurich ( Memento of the original from May 27, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. .
- ^ Ministry for the Environment and Nature Conservation, Agriculture and Consumer Protection of the State of North Rhine-Westphalia MUNLV (2005): Handbuch Querbauwerke . Dusseldorf.
- ^ A b Northcote, TG (1978): Migratory Strategies and Production in Freshwater Fishes. In Ecology of Freshwater Production (pp. 326-359). Oxford: Blackwell Science.
- ↑ Baras, E. & Lucas, MC (2001): Migration of freshwater fishes. Oxford: Blackwell Science.
- ↑ Fischwanderung.ch ( Memento of the original from May 26, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
