Restoring Danube sturgeon populations through ex-situ conservation

Sturgeon are remarkable fish, sometimes called ‘living fossils’ with a slow evolutionary heritage spanning millions of years. Many sturgeon species are large and long-lived, with a so-called ‘anadromous’ lifespan traced by migrations between fresh- and saltwater environments.

Sturgeons are thus sensitive to a range of environmental pressures, and as a result, are often termed ‘flagship’ species for the wider health and connectivity of freshwater ecosystems. However, many sturgeon species are significantly threatened by human impacts, particularly the harvesting of their roe for caviar. This is often coupled with wider threats around habitat loss and the fragmentation of migration routes in river systems, for example through the construction of hydropower dams.

The Danube River has six native species of sturgeon, all of which are threatened by human activities. The European sturgeon (Acipenser sturio) and Ship sturgeon (A. nudiventris) are locally extinct, Beluga sturgeon (Huso huso), Russian sturgeon (A. gueldenstaedtii) and Stellate sturgeon (A. stellatus) are on the very brink of extinction, with the sterlet (A. ruthenus) being endangered.

To address widespread declines in Danube sturgeon populations, the MEASURES project has implemented a series of conservation and restoration measures across the basin. A key element of this work is the ex-situ conservation of Danube sturgeon. Ex-situ (or ‘off-site’) conservation is the process of protecting an endangered species by maintaining and breeding captive populations in an artificial environment, reflecting the genetic diversity in the wild.

The MEASURES project used ex-situ conservation techniques to strengthen the populations of two Danube sturgeon species: the sterlet and the Russian sturgeon. The project developed facilities where eggs harvested from autochthonous broodstock could be incubated in Danube river water where natural conditions such as temperature fluctuations and food sources could be replicated and imprinting of the fish on their natal waters can take place.

This initiative was based on the latest recommendations for ex-situ conservation by experts from the Danube Sturgeon Task Force and World Sturgeon Conservation Society affiliated with the MEASURES project. The use of natural river conditions in ex-situ sturgeon conservation has already been demonstrated on a small scale in the Austrian-Slovakian LIFE-Sterlet project.

Key to this process was the conservation of the native genotypes in the captive sturgeon populations. MEASURES researchers undertook generic analysis of the captive sturgeon broodstock in order to ensure that their offspring maintained the genetic diversity of native populations.

This analysis also allowed MEASURES researchers to better understand genetic connectivity of sturgeon populations across the Danube basin. This mapping of the ‘living gene banks’ of Danube sturgeon is a key element of the Pan European Action Plan for Sturgeons, released in 2018.

“Sturgeons are important natural heritage in the Danube River Basin and it is our responsibility to preserve them for future generations,” says Dr Thomas Friedrich, sturgeon expert at BOKU, Vienna. “While ex situ actions are at this point of utmost priority to safe the remaining gene pool, the future of our sturgeons lies in the Danube.

“Therefore, we also have to target the restoration of the ecological corridor in order for the releases to be sustainable and enabling natural reproduction and recovery in the wild,” continues Dr Friedrich. “The cooperation between the MEASURES and LIFE-Sterlet projects, as well as the WePass project, the DTF, WSCS and other stakeholders ultimately led to the implementation of necessary steps on the path to sturgeon recovery in the Danube River Basin.”

The young sturgeons reared in hatcheries are tagged and released back into the river. Their tags will give researchers new information on their migratory journeys to sea and back, which will in turn help shape future conservation and restoration planning in the Danube basin.

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Funded by the EU as part of the Danube Transnational Programme, MEASURES aimed to improve habitat quality and connectivity along the Danube in order to support populations of threatened sturgeon species, as well as other migratory fish and wider aquatic biodiversity in the basin.