The Mura River, which joins the Drava on the Croatian-Hungarian border and later flows together with the Drava into the Danube, constitutes the starting point of what many like to call the “Amazon of Europe”. An area stretching on almost 1 million ha of exceptional value which is home to many protected and endangered species, such as otter and beaver, white-tailed eagles, Danube salmon and, of course, sturgeons.
With a specific task in mind, to perform a visual census, as described in Janauary’s news, but also with a secret wish – hoping to meet face to face with a sterlet (Acipenser ruthenus), we dove underwater to find the secrets of the river. In January, a professional diver and the most experienced Slovenian underwater filmmaker - Ciril Mlinar - investigated the potential sterlet habitats in the Mura River in Slovenia for REVIVO, while simultaneously filming the underwater world. Four different pools with maximum depths between 4-8 m were surveyed to determine if this method could be used in (future) scientific surveys in the Danube tributaries for the confirmation and analysis of potential wintering habitats of sterlet and other migratory fish species.
Unfortunately, sterlet, like other sturgeon species, proved hard to film, as we had expected – this is a species that prefers to swim in the deepest parts of the riverbed. The abundance of other species was also very low. We managed to film only one fish species, the alien Prussian carp (Carassius gibelio). This confirms that there is a lack of suitable habitats in a degraded river ecosystem where more adaptable invasive species take over the degraded habitats abandoned by native species. But the problems don’t end here.
Plastic pollution – bottles and other containers, as well abandoned fishing hooks - is also evident from the video. The lack of sediment as seen on the video is another huge concern for the fish and other organisms of the Mura River, resulting from the sediment retention behind the hydropower dams upstream. Consequently, river scoring occurs, incising the bedrock ever deeper, impacting the groundwater table and initiating the chain of ecosystem degradations linked to this.
The comparison of the visual census and other methods, tested to identify the migratory fish habitats in the Mura and Sava Rivers, will be presented after the last fish sampling will be performed in June 2020.
For now, we are publishing this video, where winter secretes and scars of the Mura River are (finally) revealed.
Photo © Matevž Lenarčič