The Danube River is the most important refugium for sturgeon populations in the EU inhabited by four species (Huso huso [beluga] , Acipenser gueldenstaedtii [Russian sturgeon] , Acipenser stellatus [stellate sturgeon] , and Acipenser ruthenus [sterlet]) . Whereas the first th ree species are on the brink of extinction , the fourth species (sterlet ) shows a declining population trend since decades. In particular, t he illegal hunt for caviar is the major risk for populations . Probably, caviar is the most expensive animal products in international trade with prices exceeding 5000 € per kilo of beluga caviar even from aquaculture. Today, sturgeon fishing is no longer permitted in the Lower Danube and the Black Sea . A study was conducted to quantify the amount of illegal sturgeon caviar and meat in trade. Therefore, 149 samples of caviar and meat from the the four Lower Danube countries (Serbia, Romania, Bulgaria, and Ukraine) were investigated in a combined genetic-isotope approach .
Materials & methods
For sturgeon meat and caviar, species or hybrid identification is rarely possible by visual inspection alone. Only DNA analysis can determine the species or hybrid in question, while isotope patterns are essential for detecting wild (poached) fish and identifying the geographic origin. Sampling was managed by authorities . Caviar and tissue samples were stored frozen or in ethanol. DNA was extracted using standard procedure . A combined approach using several genetic markers (mitochondrial DNA; Single Nucleotide Polymorphisms; Microsatellites) was used for secure species/hybrid identification.
The stable isotope method is the leading standard analytical tool to verify the authenticity of biological materials. The method relies on the principle that stable non-radioactive isotopes occur in nature in different relative proportions, because biological processes (such as the water cycle) influence their abundance variations. Stable isotope patterns deliver various information on the geographical origin and the source (e.g. wild versus farmed). T he stable isotopes of the elements carbon, nitrogen and sulphur measured in sturgeon tissue reflect the available feed and therefore indirectly the source of an animal or in that study if it was wild-caught or captive-bred.
Results and discussion
Isotope analysis uncovered that wild sturgeon products were on sale in all four countries. Alarmingly high numbers were observed of wild-caught sturgeon (31 samples/21% ); additional 17 samples (11.4%) were in violation with CITES and EU regulations, and 25 cases (17%) were found of consumer deception. Of the 31 samples originating from wild-caught sturgeons, 28 yielded results with confidence intervals ≥95% . Within these samples, DNA analysis identified all Danube sturgeon species in varying proportions, but with the sterlet being the dominating species. In addition, 25 samples were sold as wild products but originated from aquaculture, thus indicating an existing consumer demand for wild sturgeon products, which fuels a niche market of wild products that guarantees greater profit . Three were fake caviar and three samples were from non-sturgeon fishes . Our results indicate the urgency of establishing an effective monitoring and enforcement network against poaching and illegal trade, which must be coordinated between all Lower Danube countries including producers (e.g. aquaculture), transit and consumer countries.