Trout, sturgeon, shad, Danube salmon, barbel - are all migratory fish, which take care of the health of rivers and wetlands, by maintaining a complex food chain. In addition, many of them provide a livelihood and food security for millions of people around the world. But the populations of migratory freshwater fish species have decreased, on average, by 93% in Europe since 1970, and those of sturgeon, by 91% between 1970 and 2016. When it comes to the number of species, in the Danube we now only find four: the sterlet, the Russian sturgeon, the beluga, and the starry sturgeon. Not surprisingly, given the growth of the hydropower sector, overfishing, climate change and pollution, all human induced causes for population decline.
Wetlands play a key role in maintaining water quality and are essential types of ecosystems for migratory fish species, but globally they are disappearing three times faster than forests. Therefore, the proposed MEASURES Strategy emphasizes the need for green infrastructure to manage the risk of floods and nature-based solutions, such as ecological restoration of wetlands and of riparian forests. Thus, both people and migratory fish thrive. Communities have cleaner water, lower air temperatures and green spaces for recreation, while protecting livelihoods such as fishing and sustainable aquaculture.
Breeding migration is an integral part of the natural life cycle of all migratory fish and, implicitly, of sturgeons in the Danube, which makes them particularly sensitive to the impact of physical barriers, such as dams. Perhaps the most famous dam is the Iron Gates I, where, immediately after the inauguration, the catches of beluga and Russian sturgeon reached a maximum level, because many migrating sturgeon remained captive downstream of the dam. Between 1972 and 1976, 115.7 tons of beluga and Russian sturgeon were fished here, 25% more than the total amount fished during the 5 years prior to the construction of the dam. After 1976, however, catches dropped dramatically, reaching only 37.3 tons between 1980 and 1984 when the Iron Gates II dam was built, and since then, populations have continued to decline.
Assessing and mitigating or removing the effects of barriers to migration from the ecological corridor is one of the measures proposed in the MEASURES Strategy for the Danube Corridor.
Sturgeons reproduce during the Spring, and for this they need temperatures between 8-16 ºC, as well as a rocky substrate (preferred by the beluga), sandy or sticky (preferred by the starry sturgeon). Therefore, sediment extraction has had a negative impact, changing the habitat of migratory fish, especially sturgeon. Moreover, the regularization of the river changed the speed of the water flow, which ended up transporting even more sediments. The strategy proposed in MEASURES emphasizes the need for nature-based solutions for navigation. Better waterway solutions could be a combination of hydrostructural works, with wise sediment management, artificial islands and natural shoreline reinforcement.
Under the MEASURES project, almost 9000 sturgeon-tagged yearlings were released into the Danube until the end of 2020. Sturgeon repopulation actions take place in Spring and Autumn, when temperatures are neither very low nor very high (during the Summer transport conditions are more difficult to achieve, the water being very hot and the need for oxygen higher), and sturgeons can find food in the sea. The MEASURES strategy for the Danube corridor includes measures designed to secure and protect populations of migratory fish species by ensuring the existence of genetically viable juveniles, by repopulating endemic species (Russian sturgeon in Romania and sterlet in Hungary) and monitoring these activities.
From the Black Forest Mountains to Sulina, the Danube has shaped over time 10 countries and countless communities and cultures. Today, 83 million people live in the Danube river basin, of which over 20 million depend directly on the Danube for drinking water.
Rivers and streams play a vital role for millions of people - from fishermen, locals, to brewers, hotel owners, to people who have clean tap water in the city. Real solutions to protect these resources, as well as migratory fish, must be shaped involving all stakeholders. One of the measures proposed in the MEASURES Strategy for the Danube Corridor refers to improving the participation of the general public, all stakeholders and supporting national networks for migratory fish.
Fishermen were, in the past, the only ones who knew best how fish stocks were doing as well as where the best fish breeding places were situated. Today, thanks to technology and research & monitoring actions in conservation projects, we can collect scientific data about the corridors and favorite places for feeding and reproduction, from the fish themselves. For this purpose, fish must be tagged before being released, and in MEASURES we used several types of tags. Coded wire tags - as thick as a hair, which must be extracted to be read, PIT - similar to those used for pets, but also external plastic marks, with the contact details of the organization that did the restocking action. Thus, through scientific catches we can find out important details such as in the case of the juvenile released at Isaccea on October 16, 2020 which arrived, five days later, on the Chilia branch, 17 km from the where the river reaches the Black Sea, near Vâlcov ( Ukraine). Such research and monitoring actions are also included in the MEASURES Strategy for the Danube Corridor.
The full moon in August is also called the Sturgeon Moon, and some say that the name was given by the American Indians, who fished lake sturgeon (Acipenser fulvescens), this being the first source of fresh meat, in Spring. But the main purpose for which the sturgeons were fished (legally, but also illegally) is caviar. This black gold, preferred by the Tsar of Russia, but also by the kings of England (Edward II raised the sturgeon to the rank of a royal fish) is still a symbol of luxury for consumers. Sturgeon fishing is prohibited in the Danube segment after the Iron Gates, as well as in countries with access to the Black Sea, in order to give fish stocks time to recover. However, a ban on fishing for these species has led to overexploitation of other migratory fish species, shad.
Danube sturgeons are an important indicator of ecosystem health. They live mainly in the Black Sea and migrate upstream on the Danube and other large rivers to breed. They grow to a length of 7.2 meters and can live up to 100 years. Due to their long life cycle and delayed adulthood, sturgeons are particularly vulnerable to exploitation and other threats, including pollution, and stocks need several years to recover. Danube water becomes polluted by ships that discharge wastewater into the water before docking, by boarding houses that do not have sewage treatment plants, as a result of agricultural and industrial processes (pesticides and other chemicals applied in agriculture), as well as by spills from mining operations and accidentally, or with microplastic.
The Black Sea is an important habitat for sturgeons and other migratory fish. Here, they feed, and some species also winter here - the beluga lives at great depths, 130-180 m of the sea. That is why it is important that all countries with access to the Black Sea work together to protect them. In addition to the fact that all sturgeon species are protected by the CITES Convention since 1998, they are also protected by the legislation of each country. Sturgeon fishing has been banned in Romania since 2006 and Bulgaria since 2011, in Turkey since 1958 (for those under 10 kg, and for 1996 in total), Ukraine since 1996, Georgia since 1967 and Russia since 2005. In the MEASURES Strategy for the Danube corridor, the importance of national and international collaboration is highlighted by measures such as: creating national plans and connecting National Migratory Fish Networks.
Specialized sturgeon farms and those for other migratory fish play a key role in protecting these wild populations. Using fish caught in the wild as well as a scientific methodology - which includes a breeding plan, analysis of each specimen and calculations of crosses between specimens that ensure a high degree of genetic variety, sturgeon juveniles obtained this way have a better chance of survival. Through the repopulation actions with such juveniles, the viability of the breeding populations of migratory fish increases. In order to keep track of existing farms and yearlings resources, it is necessary to create, implement, update and ensure the maintenance of a common database on sturgeon operations and ex-situ activities, a recommendation included in the MEASURES Strategy for the Danube Corridor.
Climate change will also affect migratory fish, especially those considered megafauna (such as the beluga sturgeon). Projections for the next 30 years show a decrease in water flow in the Danube basin, for the river and its tributaries, which may lead to the extinction of some species or the replacement of native species with more tolerant invasive species. Migratory fish are particularly exposed to the impact of climate change because they have a very low tolerance for habitat degradation and low oxygen levels, which will limit migration. Such projections should be taken into account for the development of National Plans for Migratory Fish Species. These are essential for the well-being of fish species, but also for the other elements of the habitat, as well as for the people who need them - from fishermen, to producers, to consumers.