What hinders the journey of migratory fish?
When eels, salmon, sturgeon and other migratory fish swim through the rivers on their way to the sea and back, they also have to face a number of natural dangers. Predators with differing hunting strategies lie in wait in the different waterbodies through which they travel. Additionally, the fish have to deal with varying environmental conditions during their travels: the oxygen content and the temperature of the water is constantly subject to change. Migratory fish have to continually adapt to these natural fluctuations. Migratory fish are faced with a particular challenge when they reach the estuary – where the river meets the sea – as it is here that the salt content of the water changes.
Alongside these natural challenges, it is humans that adversely affect the journey of these fish the most. This is because many of the changes in the water-course are due to its use by humans for multiple purposes. Riverbanks are preferred areas for settlement, as the soil is richer in nutrients and more high-yielding for agriculture. Furthermore, the rivers serve as a supply of food thanks to fishing; as a source of drinking water; for the transport of goods; and for the production of energy. To this end, humans have straightened rivers and built weirs, dykes and breakwaters. This has led to the current situation of there being hardly any natural rivers left in Germany. These changes have a pronounced effect upon the habitats in and at the rivers and resultingly on migratory fish, who are dependent the river being intact, connected and sufficiently clean to be able to carry out their journey from the sea, their early life phases and back to the sea unhindered.
What consequences does human use have on migration?
Migratory fish are dependent on being able to change between different habitats throughout their lives. This change between habitats usually follows the course of the rivers with spawning taking place most upstream and subsequent life cycle stages follow on the way downstream. This route is often blocked through weirs and dams which are used in rivers for the production of energy or the stowage of water. These constructions hinder or slow down migration upstream, meaning that migratory fish such as sturgeon or salmon do not manage to reach their spawning grounds and the eels no longer reach their feeding grounds. Fish that made it upstream and travel downstream towards the weir face their own risks: they can be drawn into the turbines of power stations, thus injuring or killing them. In the critically endangered eel, this mortality can reach up to 90 % of the migrating eels at a single power station. Even if there is a bypass, fish must be able to locate and use it. Otherwise they are trapped upstream. The obstruction of migration in both directions results in the ever-lower number of migratory fish.
Weirs and dams not only interrupt the migration of the water dwellers – they also disrupt the flow of the river and, in doing so, change the environmental conditions. The current above weirs and dams slows down considerably and creates standing water which through the large surface warms up considerably in the sun. In addition, suspended solids, which are carried with the river, sink to the bottom and create thick deposits of sludge and sand. In deep reservoirs, this can lead to oxygen levels becoming scarce near the ground becoming deadly for cold seeking organisms. These changes result is a change of the fauna that lives in this area of the river with specialists being replaced and generalist taking over. As many river dwellers lay their eggs in fast-flowing, cooler areas of the river, many spawning grounds are also lost due to slow-moving reservoirs.
In addition to physical barriers, the migration of many species of fish is also impeded by water pollution. Migratory fish have to swim through sections of river on their travels which serve as deposits for sewage treatment plants or factories. The presence of many components of nutrients and harmful substances in waste water increases the oxygen consumption, reducing oxygen availability for the organisms. In addition, toxins and pollutants cause stress reactions in the fish, further increasing the energy consumption during their extended travels and weakening the animal in combating pathogens and parasites.
How can migratory fish be protected?
To ensure that human interference does not impede the migration of fish in the future, and that species populations have a chance to recover, various approaches have been developed:
One of these solutions involves enabling fish migration in spite of construction works on the river. Special fish passes can help all water dwellers to overcome obstacles such as weirs and dams without. In this scenario, migratory fish would be able to make use of special migration facilities, built in close vicinity of the weirs, on their travels downstream. These should also prevent fish reaching the turbines. On their way upstream, so-called fish ladders can help them make their way back upriver. However, these facilities have to be suitable for the different species and numbers of fish using them. First of all, the facility has to be found by the fish, which depends on the position in relation to the obstacle, the current and the detectability. The size of the facility is critical! The bigger the better. Especially large fish such as sturgeons are dependent on larger fish ladders than smaller species and fish migrating in large numbers must be provided to migrate in groups to ensure swift and safe movement. Furthermore, the passes have to be well-provided with sufficient amounts of river water so that the fish – who orientate themselves based on the current when swimming – can find the entrance to these facilities.
When the migratory fish manage to overcome these obstacles, they begin the search for suitable spawning grounds or feeding grounds in the river or sea which need to be fully functional and large enough to support large fish populations. As such, critical habitats need to be protected or restored.