First record of black bullhead (Ameiurus melas Lesueur, 1819) and the occurrence of hybridization with brown bullhead (A. nebulosus Rafinesque, 1820) in Belgium

Ictalurid bullheads are freshwater fish species native to North America that were introduced to Europe in the late 19th century, where they negatively impact native aquatic species. In Belgium, brown bullhead (Ameiurus nebulosus) was introduced decades ago, and is already established and widespread. Here, we report the first occurrence of a congeneric non-native invasive bullhead species in Belgium, the black bullhead (Ameiurus melas). In early May 2023, electric fishing in the Zonderikbeek, a stream situated in the northeast of Flanders (Northern region of Belgium), led the capture of a suspicious bullhead specimen. Diagnostic PCR analysis identified this specimen as a hybrid between black and brown bullheads. The following spring, in 2024, 21 Ameiurus specimens were additionally captured in a nearby lake that connected to the Zonderikbeek. Diagnostic PCR revealed that 17 of the captured individuals were pure black bullheads, three were pure brown bullheads, and one specimen appeared to be a hybrid. Morphological examination of the main distinguishing key features of these bullhead species, such as the pigmentation the anal and caudal fin membrane, gill-raker count, and the serration of the pectoral spine, were in line with these molecular analyses. Notably, hybrid individuals were morphologically characterized with features of both A. melas and A. nebulosus. The discovery of multiple individuals of A. melas, along with hybrids with the already established A. nebulosus is concerning especially given the extensive network lakes, creeks, and small rivers in vicinity of these observations. This interconnected system of water bodies could facilitate the further dispersal of both species and their hybrids. Given the significant potential ecological and socio-economic impacts of both species, urgent management efforts are needed to remove or contain not only the brown bullhead, but also the black bullhead as emerging non-native invasive species in Belgium.