Alien Siluriformes (Pisces) recently reported in the freshwaters of the Danube Delta Biosphere Reserve

In the period 2018–2024, new species of fish belonging to the order Siluriformes (silurids) were reported in the freshwater habitats of the Romanian part of the Danube Delta Biosphere Reserve (DDBR). These species were: Ameiurus melas (black bullhead) and Ictalurus punctatus (channel catfish), which are both American catfish species belonging to the Ictaluridae family, as well as Clarias gariepinus and its hybrid, Heteroclarias (North African catfish, a cross between Clarias gariepinus and Heterobranchus longifilis , belonging to the Clariidae family). These two American catfish species have been present in the DDBR for several years, following the first recorded appearance of the species in Romanian freshwater upstream of the Danube Delta. However, the North African catfish ( Clarias gariepinus and the C. gariepinus × H. longifilis hybrid) should not be included in the list of alien species in the DDBR because they appeared accidentally in the DDBR and cannot withstand the low water temperatures in temperate zones during winter. The DDBR’s list of alien freshwater fish already includes seven species, such as Hypophthalmichthys molitrix , H. nobilis , Ctenopharyngodon idella , Pseudorasbora parva , Perccottus glenii (all five fish species are native to Asia) and Lepomis gibbosus (native to North America), as well as an extension of the range of Percarina demidoffi . Records of alien silurids confirm that two other alien fish species on the DDBR list have spread by natural dispersal from Romanian waters to the Danube Delta. Monitoring activities in the area should continue in future studies to observe possible numerical expansion and assess the impact of alien silurids on native species. For now, only Ameiurus melas raises concerns about its negative impact on native ichthyofauna.