Beluga with a wound of a hook (Lenhardt, 2003)
Key words: threats, extinction, poaching, dams, pollution, habitat fragmentation
Sturgeons represent a unique and commercially valuable group of ancient fishes, especially because of their meat and caviar, and for these reasons they are highly vulnerable and exploited near to extinction. The recovery of sturgeons is slow and measures to protect these species are essential to be established in practice. There are many associations that are dealing with their protection and conservation but negative anthropogenic actions are still present, especially in the Danube River Basin. And not only sturgeons are endangered by these effects, other fish species are endangered too. But protection of existing but dwindling sturgeon stocks is still possible.
Treats to the survival of the sturgeon species
All sturgeon species according to the IUCN categorization have status EN (endangered) except sterlet, which is the only resident of our rivers and whose status is VU (vulnerable).
The main and biggest threat to sturgeons is fishing or more precisely poaching, since legal fishing has been banned since 2006. Because of their size, their meat quality and caviar, the bladder, skin and bones were utilized for centuries, and sturgeon fishing might have been an important factor for colonization of the Danube River basin (Friedrich, 2012).
Figure 1. Sturgeon butchering in Hamburg XIX century (Friedrich, 2012)
Figure 2. Poaching (left photo) and Leskovar animation (right photo)
Figure 3.Captured beluga (Петровић,1998).
Another threat to their survival is the construction of dams that prevented their migration upstream along the river in order to seek suitable places for spawning (habitat fragmentation).
Figure 4. Iron Gate Dam on the Danube between Romania and Serbia (Lenhardt,2014)
Figure 5. Dams on the rivers that belong to the Danube River basin (https://www.iucn.org/)
The third threat is water regulation, not only the construction of dams, but adaptation in order to use the river for other purposes, such as navigation or urbanization. Therefore often riverbeds are changed and adapted for human utilization, but thus threatening sturgeon and other species habitats. Since sturgeon species inhabiting the river bottom and feed on bottom fauna, invertebrates, changes in the regulation of waterways affect loss of habitat, food, places suitable for spawning.
Figure 6. Natural habitat of sturgeon (Lenhardt, 2005)
Figure 7. Changed riverbeds due to hydrotechnical works (https://hidrotehnicki+radovi+slike)
Also, pollution poses a threat to the survival of these species, especially pollution that is deposited or settled down at the bottom of the river where sturgeon species are most often moving and feeding. The pollution of the river affects living organisms in it. Such waste is often carried by the river and is deposited on the bottom of the river, and instead of tasty invertebrates sturgeon can find can. Beside solid waste, there are dissolved pollutants in the form of heavy metals, pesticides, chemicals, medical waste and wastewater from households of industrial wastewater.
Figure 8. The pollution of the river, Leskovar illustration (left); industrial waste water (right)(http://millenniumwaterstory.org/Problemsofwstewater)
Alien sturgeon species pose a serious threat to autochthonous sturgeon stocks through possible hybridization, competition or diseases. The hybrid species were created most by human by crossing two different species for better yield. So crossed species are breeding in ponds, but if they manage to escape into nature they can make certain changes. Negative effects of sturgeon hybridization include loss of genetic variability, or the disappearance of whole forms.
Figure 9. Hybrid species formed by crossing the starlet and beluga
Also the introduction of certain non-native species, such as the occurrence of other species of fish, that are not representatives of the fish fauna characteristic for a given climate, may adversely affect state, because they occupy existing habitat and food to native species. Potential impacts documented up to now are: competition with native fish species for food and already critical habitat, and spawning sites, introduction of parasites and diseases, and indirect environmental impact through habitat degradation, by effects on benthos and water turbidity through feeding (Jarić et al. 2018).
Figure 8. First record of the Poliodon spathula 2006 in the Danube below the Iron Gate dam II, one of about a dozen copies that have escaped from aquaculture in Romania (Poliodon is a resident of Mississippi) (Lenhardt et al. 2006).
Sturgeons are on the edge of extinction due to human impact of overfishing and changing the living conditions in rivers. They play an important role in river ecology. The loss of a certain ecotype of sturgeon within a certain river system represents an irreplaceable loss of biodiversity (Bloesch, 2005). Migration is a part of the life-cycle of all sturgeon species and the availability of – and access to – sturgeon habitats in the riverine/marine system is a basic prerequisite for the conservation and restoration of sturgeon biodiversity in the Danube River Basin and the Black Sea. Beside protection from poaching, priority is to make dams in the Lower Danube passable for sturgeons, which will make access to upper Danube River sections and to major tributaries such as the Tisza, Sava and Drava Rivers (Bloesch, 2005). There are many different organizations that are working on protection and conservation of sturgeon species but this issue should have ethical and cultural significance to all of people worldwide. It is important to do all the necessary steps to protect them.
Nowdays there are five sturgeon species present or expected to be present in Danube River basin, while one is extinct. It is our ethical mission to try to educate people how to save these precious creatures on the planet to future generation. More about them you can find in the article DINOSAUR FISH: Sturgeon fish species of the Danube River (link to other article).
- Bloesch, J., Jones, T., Reinartz, R., & B. Striebel (Editors), 2005: Action Plan for the Conservation of the Sturgeons (Acipenseridae) in the Danube River Basin. Convention on the Conservation of European Wildlife and Natural Habitats, Strasbourg.
- Friedrich, T. (2012). Master Thesis: Historical distribution, current situation and future potential of sturgeons in Austrian rivers. Vienna.
- Jarić, I. (2009). Master’s thesis: Population viability analysis of the Danube sturgeon. Uppsala University.
- Jaric, I., Smederevac-Lalic, M., Cvijanovic, G., Bronzi, P., Lenhardt, M. and Gessner, J. (2018). Status of paddlefish in Europe as an object of aquaculture and a potential invader. 8th International Symposium on Sturgeon, September 10-16, Vienna, Austria, Conference proceedings, 15-5.
- Lenhardt, M., Hegediš, A. & Jarić, I. (2005). Action plan for sturgeon species management in fishery waters of Republic Serbia. Institute for Biological Research “Siniša Stanković”, Belgrade; Developed for Ministry of Science and Environmental Protection of Republic Serbia, 1-21.
- Lenhardt M., Hegediš A., Mićković B., VišnjićJeftić Ž., Smederevac M., Jarić I., Cvijanović G., Gačić Z. (2006). First record of the North American paddlefish (Polyodon spathula Walbaum, 1792) in the Serbian part of the Danube River. Archives of Biological Sciences, Belgrade 58(3): 27-28.
- Петровић, М. (1998). Сабрана дела Књига 14 Рибарство, Завод за уџбенике и наставна средства Београд.
Autochthonous – native,indigenous, originating in the place where found
Hybridization – the process of combining different varieties of organisms to create a hybrid
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