Introduction: Luc is a volunteer at WWF. He has studied Aquaculture and Marine Resource Management at Wageningen University and Research. From this year on, Luc will pursue a PhD on rheophilic fish species in the Meuse river in the Netherlands.
Global appearance of sturgeons
The family of sturgeons consists of 25 species, of which the global distribution is limited to the Northern hemisphere (Zhang et al., 2013). This characteristic fish is exceptionally old (200 million years, while the dinosaurs went extinct 65 million years ago), and is one of the most ancient fish that is still alive (Bloesch., 2006). All species of sturgeon are threatened with at least some risk of extinction (Lenhardt., 2006), which is also the case for the 6 species of sturgeon that natively inhabit the Danube river basin (Bloesch et al., 2006).
Species of sturgeon in the Danube river basin
Considering sturgeons in the Danube river, we particularly refer to the six native inhabitants in this area (Bloesch., 2006):
- Acipenser gueldenstaedti (Danube or Russian sturgeon)
- Acipenser nudiventris (Fringebarbel or Ship sturgeon)
- Acipenser ruthenus (Sterlet)
- Acipenser stellatus (Stellate or Starred sturgeon)
- Acipenser sturio (Common or Atlantic sturgeon)
- Huso huso (Beluga or Great sturgeon)
Stock assessments are often complex to make for these species as they are only occasionally caught, and it is difficult to determine whether a species is truly extinct. However, based on the very low catch numbers, the ship sturgeon and Atlantic sturgeon have been predicted to be extinct in the Danube river (Jarić., 2009; Lenhardt., 2006).
Besides the native inhabitants, some species were introduced in the Danube river for caviar/sturgeon meat production. These species include (Bloesch., 2006):
- Polyodon spathula (North American paddlefish)
- Acipenser naccarii (Adriatic sturgeon)
- A. baeri (Siberian sturgeon)
- A. ruthenus x Huso huso (bester)
Sturgeon ecology
All species of Danube river sturgeon feed on invertebrates and/or fish (Bloesch et al., 2006). The sterlet feeds solely on invertebrates, while the beluga sturgeon is a true predator as its diet consists only of marine and freshwater fish (Reinartz., 2002). Sturgeons typically possess protractile barbels to dig up insects, larvae, ringworms or molluscs (oysters etc.)(Bloesch et al., 2006. Their eyes are characteristically small relative to their body, and they barely use their sight to locate their prey (Bloesch et al., 2006).
Besides the relatively small eyes, long snout, and ancient body protection, sturgeons possess some additional interesting traits. Some species of the remarkable sturgeon family can live to an old age, and Beluga sturgeons of > 100 years have been mentioned (Bloesch., 2006). Moreover, sturgeons can reach large body sizes. Again, the Beluga takes the cake, and stunning lengths of 8 meters have been reported for this species (Bloesch., 2006)! Besides this, sturgeons possess a genetically unique adaptive potential to river conditions, which has allowed them to survive for roughly 200 million years on this planet (Friedrich et al., 2019). Despite their remarkable adaptivity to changing conditions, they are still susceptible to the enormous changes humans have recently introduced in their ecosystems. As a result of their long lifespans, sturgeons typically reach sexual maturity at a late age. This makes them particularly susceptible to overfishing. For this reason, the presence of sturgeons is considered as one of the best indicators for riverine ecosystem health and sustainable fisheries management (Friedrich et al., 2019).
Migration
All species of sturgeon exhibit migratory behaviour to at least some extent, in which they show directed traveling behavior to reach spawning grounds (Bloesch., 2006; Lenhardt., 2006). This migratory behaviour is innate, meaning that they intuitively know in which direction they should migrate. Based on migration patterns, the native sturgeons of the Danube river can be divided into two groups: anadromous and potamodromous. The anadromous fish, a group of fish that also includes salmon, tend to spend their adult life at sea for feeding, to migrate back to the freshwater to reach spawning areas. Sturgeons belonging to this group include the Beluga, Atlantic, and stellate sturgeon, and they commonly use the Caspian (black) sea as a marine feeding ground. The other group is characterized by a migration pattern that completely takes place in freshwater, called a potamodromous migration pattern. Considering the native sturgeons in the Danube river, typical potamodromous species include the sterlet and ship sturgeon, for which both feeding and spawning sites are present in freshwater areas (Bloesch., 2006). Interestingly, different individuals of one species do not necessarily follow the same migration pattern. Particularly the Russian sturgeon has multiple populations that display either anadromous or potamodromous patterns. Furthermore, even for a true anadromous species like the Beluga, the migratory patterns can differ between individuals. This is expressed in two migration peaks during spring and fall. The ‘spring peak fish’ migrate – and reproduce in the river during spring. The ‘fall peak fish’ on the other hand migrate to the river during fall, hibernate in calm, lower riverine areas during winter, and continue their travel during spring to reproduce. A true hibernating fish! During these spawning migrations, large distances tend to be covered. For example, while both feeding and spawning areas are present in freshwater areas for the sterlet, this smallest species of sturgeon was still found to migrate 300km to find a suitable spawning habitat. The beluga sturgeon still takes the cake when talking about long-distance migrations, as their migrations have been reported to cover thousands of kilometers (Bloesch., 2006; Friedrich et al., 2019)!
Spawning
Sturgeons start reproducing at a late age. Sterlets for instance become sexually mature between 4-7 years (Sterlets), and Russian sturgeons between 12-16 (Bloesch., 2006). In order to achieve successful reproduction, a sufficient amount of good-quality, functional spawning habitat is required. Reproduction takes place in the beds of main rivers, and gravel of stony substances must be present at a depth of around 3-25 meters. Furthermore, external variables such as temperature and water current speed play an essential role in reproduction success. All sturgeon species require a low water current speed, and every species has its’ own optimum temperature to lay eggs. For example, the sterlet requires a temperature between 12 – 17 degrees for effective hatching of the eggs (Bloesch., 2006). All in all, there a quite some requirements for a good spawning ground. Once a sturgeon has found a suitable, spawning site, this individual does not simply forget this site! Homing behaviour has been suggested for sturgeons, meaning that they return to their same spawning sites for reproduction (Bloesch., 2006).
Threats
All of the sturgeon species that are native to the Danube river basin are currently classified as either vulnerable, endangered or critically endangered (Strat & Gheorghe., 2023). Sightings of some of the sturgeon species like the Atlantic sturgeon and ship sturgeons are really scarce, making it difficult to judge whether these species still occur in the Danube river. In fact, the Atlantic sturgeon has been predicted to already be extinct since 1970 (Jarić., 2009). The other four species, the Beluga, Russian, stellate sturgeons, and sterlet are still present in the Danube river basin, but are all seriously endangered as well. For example, Lenhardt et al (2006) predicted that the Russian sturgeon has a high change go extinct around 2050.
The main threats to large, migratory fish in the Danube River Basin, including sturgeons are 1) overfishing, 2) water quality & habitat degradation, and 3) the introduction of constructions (such as dams) that block migration routes (Friedrich et al., 2019; Lenhardt et al., 2006; Bloesch et al., 2006). Overfishing has historically been a common threat to sturgeons in the Danube river due to the high economic value of sturgeon eggs (caviar) and meat. The first fishing events trace back all the way to the 5th and 6th century B.C. (!) when Greek colonies exploited sturgeons. Already in the 18th century, fishing of migratory sturgeon in the Austrian part of the upper Danube river has been banned due to rapidly declining stocks (Bloesch., 2006). Besides commercial – and recreational fishing, poaching and illegal trade nowadays further challenge sturgeon stocks (Bloesch., 2006). Habitat degradation can mainly be attributed to agricultural and industrial practices, and urbanization (Lenhardt et al., 2006). Two dams that form a critical obstacle in the Danube river are the two Iron Gate hydroelectrical dams that hamper the migration of anadromous sturgeon species between the middle and lower basin of the Danube river (Friedrich, 2019). Furthermore, habitat degradation has reduced the amount of suitable, functional spawning habitat. In fact, when sufficient spawning habitat is lacking, sturgeons have the ability to reabsorb the eggs they have produced (Friedrich et al., 2019), meaning that the standing stock will not be strengthened by new offspring.
Action! Sturgeon conservation
The vast number of threats has necessitated the initiation of action plans to conserve sturgeon stocks in the Danube river. The former Soviet union realized this early on, and they initiated the Sturgeon Ranging Plan in 1950 in which they aimed to replenish sturgeon stocks in the Caspian sea. Populations of stellate and Russian sturgeon were successfully restored in this area, partly due to the release of many sturgeon stocks raised in hatcheries (Secor et al., 2000). Additionally, degraded spawning grounds in the Volga river – Caspian sea region were supplemented with artificial spawning reefs, which proved to be an effective way to stimulate the reproduction of stellate and Russian sturgeon (Secor et al., 2000). Since then, many conservation actions have been launched. One of the most important conservation efforts was the “Action plan for conservation of sturgeons in the Danube River Basin”, which was developed in 2005 (Friedrich., 2019). Although some conservational efforts have shown to be fruitful, the effectiveness of action plans is challenged by the difficulty of estimating the current state and trends of sturgeon stocks in the Danube River. The Danube river is the most international river basin in the world, and to further boost the effectiveness of conserving the beloved sturgeon, continental cooperation and careful monitoring are required (Friedrich., 2019).
References
Bloesch, J., Jones, T., Reinartz, R., & Striebel, B. (2006). An action plan for the conservation of sturgeons (acipenseridae) in the Danube River Basin. Österreichische Wasser- Und Abfallwirtschaft, 58(5–6), 81–88. https://doi.org/10.1007/bf03165708
Friedrich, T., Reinartz, R., & Gessner, J. (2019b). Sturgeon re‐introduction in the Upper and Middle Danube River Basin. Journal of Applied Ichthyology, 35(5), 1059–1068. https://doi.org/10.1111/jai.13966
Jarić, I., Lenhardt, M., Cvijanović, G., & Ebenhard, T. (2009). Acipenser sturio and Acipenser nudiventrisin the Danube - extant or extinct? Journal of Applied Ichthyology, 25(2), 137–141. https://doi.org/10.1111/j.1439-0426.2009.01227.x
Lenhardt, M., Jaric, I., Kalauzi, A., & Cvijanovic, G. (2006). Assessment of extinction risk and reasons for decline in Sturgeon. Biodiversity and Conservation, 15(6), 1967–1976. https://doi.org/10.1007/s10531-005-4317-0
Reinartz, R. (2002). Sturgeons in the Danube River: Biology, Status, Conservation.
Secor, D., Arefjev, V., Nikolaev, A., & Sharov, A. (2000). Restoration of sturgeons: lessons from the Caspian Sea Sturgeon Ranching Programme. Fish and Fisheries, 1(3), 215–230. https://doi.org/10.1111/j.1467-2979.2000.00021.x
Strat, D., & Gheorghe, I. (2023). Conservation status and effectiveness of the national and international policies for the protection and conservation of sturgeons in the Danube River and Black Sea Basin. Diversity, 15(4), 568. https://doi.org/10.3390/d15040568
Zhang, X., Wu, W., Li, L., Ma, X., & Chen, J. (2013). Genetic variation and relationships of seven sturgeon species and ten interspecific hybrids. Genetics Selection Evolution, 45(1). https://doi.org/10.1186/1297-9686-45-21
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