First record of the Danube sturgeon Acipenser gueldenstaedtii Brandt & Ratzeburg, 1833 in Al-Furat Lake, Syria: likely aquaculture escapees

Sturgeons (family Acipenseridae) are ancient aquatic vertebrates characterized by their elongated bodies, cartilaginous skeletons, and rows of bony scutes (Kuhajda, 2014). Primarily inhabiting freshwater and brackish environments, some species migrate to marine waters for spawning (Lohe, 2021). As benthic predators and prey, sturgeons play a vital ecological role (Chen et al., 2022). However, overfishing, habitat loss, and pollution have severely threatened many sturgeon populations (IUCN, 2021), necessitating urgent conservation efforts for these evolutionarily significant fishes (White et al., 2023).

The Danube sturgeon Acipenser gueldenstaedtii Brandt & Ratzeburg, 1833 is a marine species that also inhabits freshwater and brackish environments, typically at depths of 2–100 m (Riede, 2004). An anadromous species, adults migrate from the sea to rivers for spawning, while juveniles return to marine habitats to mature (Demonte et al., 2017). Native to Eurasia, its range spans the Black Sea, the Sea of Azov, and the Caspian Sea basins. This species can reach lengths of up to 2360 mm (Kottelat & Freyhof, 2007) and live for 46 years (Chugunova, 1959).

The record of the Danube sturgeon (A. gueldenstaedtii) in Al-Furat Lake (a reservoir on the Euphrates River in Syria) is significant for several ecological and biogeographical reasons, e.g., Potential Ecological Impact, Native versus Non-Native Status: If the sturgeon was introduced (intentionally or accidentally), it could compete with native fish species for resources, disrupt food webs, or introduce new parasites/diseases; Predation or Competition: Sturgeons are bottom-feeders and may alter benthic ecosystems by preying on invertebrates or outcompeting native bottom-dwelling fish; Threatened Species Implications: Since A. gueldenstaedtii is critically endangered in its native range (Black/Caspian Sea basins), its presence in Al-Furat Lake could either represent a rare refuge (if naturally occurring) or an unnatural expansion (if introduced), new pathway for invasive species, biogeographical and hydrological significance, and conservation and management concerns.

The key reasons for the cultivation of A. gueldenstaedtii are (1) caviar production—this species is highly valued for its high-quality roe, which is processed into premium black caviar, a luxury food product with significant economic value; (2) conservation and biodiversity protection—due to overfishing, habitat loss, and illegal poaching, wild populations are critically endangered. Farming helps reduce pressure on wild stocks and supports species preservation; and (3) sustainable meat production—sturgeon meat is a nutritious and marketable product. Aquaculture provides a controlled environment for efficient growth, ensuring a steady supply without depleting natural populations (Memiş et al., 2011; Vecsei, 2001).

Saygı et al. (2018) conducted a comprehensive study on the future of aquaculture in the Middle East, covering the period from 1950 to 2023. Their findings emphasized the need for more detailed analyses of aquaculture practices in the examined countries. Additionally, they recommended that governments adopt stricter regulatory measures in fisheries policies to ensure sustainable growth.

In contrast, Memis et al. (2002) highlighted the remarkable expansion of freshwater aquaculture in Turkey. Between the early 1970s and 1999, licensed fish farms surged from just 2 to 1444. Cage farming also experienced significant growth, with 57 farms established by 1999, producing 4100 tons of fish. Overall, aquaculture production skyrocketed from 3075 tons in 1986 to 63 000 tons in 1999. Inland production, primarily trout (Oncorhynchus mykiss), dominated the sector, accounting for 60% of total output. Meanwhile, marine production saw a sharp rise from 8494 tons in 1995 to 25 230 tons in 1999, driven largely by sea bream (Sparus aurata) and sea bass (Dicentrarchus labrax).

The study projected that Turkey’s aquaculture sector would continue expanding due to rising domestic demand, population growth, and increasing exports. Efforts are underway to diversify production by introducing new marine fish and shellfish species. However, bureaucratic hurdles in licensing— particularly for marine aquaculture—remain a major obstacle. The process is often cumbersome, plagued by technical knowledge gaps, and hindered by insufficient collaboration and knowledge-sharing on emerging industry developments.

Escapes of aquaculture fish into natural water bodies are common globally (Morris et al., 2008), including in Turkey (Innal & Erkakan, 2006). Such incidents often result from facility damage or predation. Sturgeon escapes have been documented previously, such as in the River Evros estuary (Koutrakis & Economidis, 2006). Here, we report an escape of A. gueldenstaedtii into the Euphrates River system, representing the first record of this species in Syrian waters.

Three juvenile A. gueldenstaedtii specimens were recorded in Tabqa City, Al-Furat Lake, Syria, on 8 October 2020, in November 2020, and on 13 November 2022. Al-Furat Lake is an artificial reservoir formed by damming the Euphrates River, measuring 80 km long, 8 km wide, and up to 60 m deep (Fig. 1) (Ministry of Environment, 2000). The specimens were collected using gillnets by local fishermen near Tabqa City.

Figure 1

Map showing the sampling locations where three specimens of A. gueldenstaedtii were collected from Syria.

Since genetic analysis was not available to be performed, identification of the specimens was solely morphological and followed established taxonomic keys (Artûhin, 1979; Bănărescu, 1964; Peseridi, 1986).

The collected specimens (three juveniles in 2020, one subadult in 2022) measured approximately 300 mm total length (TL) for the specimen from October 2020, 400 mm TL for the specimens from November 2020, 550 mm TL for the specimen recorded in 2022, and 1000 mm TL for the specimen caught in the waters around Elazığ City, Turkey and used for comparison (Fig. 2). Specimens when photographed were not alive. A. gueldenstaedtii can be identified by the following characteristics: the highest point of the body is not at the first dorsal scute (in contrast to A. schrenckii and A. nudiventris, where it is); postdorsal and/or post-anal plates are present (absent in A. baerii, A. ruthenus, A. transmontanus, and A. stellatus); plates are absent on the sides of the anal-fin base (whereas they are present in A. sturio and A. oxyrinchus); the first dorsal scute is separate from the head plates (in contrast to A. medirostris, A. mikadoi, A. brevirostrum, A. naccarii, and A. fulvescens, where it is connected); and the dorsal outlines of the head and body create an obtuse angle, with a short, blunt, and slightly rounded snout (unlike A. persicus and A. colchicus, which have uninterrupted dorsal outlines and an elongated, downward-curved snout). Additionally, the barbels are positioned closer to the tip of the snout.

Figure 2

(A) October 2020 (anonymous Fisherman; via Ahmad Aidek); (B) November 2020 (anonymous Fisherman; via Ahmad Aidek); (C) November 2022 (photo by Yosef Alabdo), (A–C) at-Tabqa, Syria; (D) July 2024 (photo by Aadel Hindawi), Elazığ, Türkiye.

The data given in this study were compared with those reported by other authors for wild populations (Artûhin, 1979; Bănărescu, 1964; Peseridi, 1986) and were found to fall within the range of these populations.

The presence of A. gueldenstaedtii in the Euphrates River system is most plausibly explained by escapes from Turkish aquaculture facilities, assisted by the absence of physical dispersal barriers. Similar expansions have been noted in South America’s Rio de la Plata Basin (Demonte et al., 2017). Hydrological modifications (e.g., dams and canals) in Turkey (Altinbilek, 2004; Yenigün et al., 2008) may further enable such dispersal.

Turkey hosts seven sturgeon species (Huso huso, A. sturio, A. stellatus, A. gueldenstaedtii, A. nudiventris, A. ruthenus, and A. persicus), many of which are threatened (Öztürk et al., 2020). While aquaculture has expanded (e.g., caviar production), conservation efforts remain critical (Wei et al., 2004).

Turkey is a primary source of non-native aquatic species in Syria and Iraq, whether through natural dispersal or accidental introduction (Tarkan et al., 2017). Analogous cases include non-native fish spreading from Ukraine to Belarus (Karatayev et al., 2008; Rizevsky et al., 2007). The recent record of A. gueldenstaedtii in Iraq (Jawad et al., 2021) supports this pattern.

The introduction of non-native A. gueldenstaedtii (Russian sturgeon) into Syrian freshwater ecosystems could have several ecological impacts, including: (1) competition with native species, Russian sturgeon may compete with native fish (e.g., Barbus spp., Capoeta spp.) for food (benthic invertebrates, small fish) and habitat, potentially reducing native biodiversity. If artificially stocked in high numbers, it could outcompete native sturgeon species (if any remain) or other bottom-dwelling fish; (2) predation pressure, as a large, omnivorous/carnivorous fish, it may prey on native fish eggs, juveniles, and invertebrates, disrupting food webs. This could negatively affect endemic or endangered species already under stress from habitat degradation; (3) habitat alteration, Sturgeons are benthic feeders; their foraging could disturb riverbed sediments, affecting water clarity and benthic communities. Dams and water extraction in Syria already stress aquatic ecosystems; adding a non-native sturgeon could exacerbate these pressures; (4) disease and parasite transmission, introduced sturgeon might bring novel pathogens or parasites that native species have no immunity against, potentially causing outbreaks; (5) hybridization risk, if native sturgeon (e.g., A. nudiventris, now rare in Syria) still exist, hybridization could further threaten their genetic integrity; and (6) socioeconomic and aquaculture conflicts, if escaped from aquaculture, Russian sturgeon could disrupt local fisheries by altering catch compositions or damaging gear. However, if properly managed, sturgeon farming could provide economic benefits (caviar production) without ecological harm.

The Syrian freshwater ecosystems (e.g., Euphrates River) are already degraded due to pollution, dams, and invasive species (e.g., Carassius auratus, Cyprinus carpio). There is no confirmed evidence of established wild populations of A. gueldenstaedtii in Syria, but accidental releases from aquaculture are possible. The potential impacts of non-native A. gueldenstaedtii (Russian sturgeon) in specific Syrian river systems, along with regulatory and ecological considerations.