Population, ecology and habitat associations of the endangered mudminnow (Umbra krameri) in Croatia

The European mudminnow ( Umbra krameri Walbaum, 1792) is one of the most threatened freshwater fish in Europe, listed under Annex II of the EU Habitats Directive and classified as Endangered in Croatia. Despite its conservation importance, ecological knowledge on this species in the southern Danube basin remains limited. We investigated the distribution and habitat preferences of U. krameri in Croatia (Fig. 1,) combining historical records with intensive field surveys conducted between 2016 and 2021. A total of 212 localities were surveyed using electrofishing, of which 26 localities confirmed the presence of the species across the Drava, Mura, and Sava River systems. Populations were most abundant in the Drava drainage, particularly in the Virovitica–Podravina County, where some canals supported densities of 2–3 individuals per m² and U. krameri comprised up to 90% of the fish community. Habitat analyses revealed strong associations with shallow canals and ponds characterized by dense submerged and floating vegetation, slow or stagnant water, and organic load (Fig. 2). Interestingly, stable populations were also recorded in some faster-flowing streams of the Mura system, suggesting a wider ecological amplitude than previously assumed. In the Sava system, populations were more fragmented, restricted to small forest ponds and side-channels (e.g., Žutica forest), where they co-occurred with native loaches and invasive taxa ( Carassius gibelio , Ameiurus melas ). Principal component analysis indicated that vegetation cover, water depth, and presence of other fish species were key factors discriminating occupied from unoccupied sites. Our findings confirm that U. krameri tolerates habitats with low oxygen and high organic input, but remains vulnerable to habitat fragmentation and competition with invasive species. The Croatian populations of U. krameri are of exceptional importance for the long-term survival of the species in the Danube basin. The Drava and Mura populations represent some of the most abundant and genetically interconnected groups, while the Sava system holds small, isolated, and genetically unique remnants. These results underline the urgency of conserving suitable habitats, maintaining connectivity, and controlling invasive species to safeguard this critically threatened fish.