Europe’s Wetland Fishes Face Silent Decline as Scientists Unite to Save Them

Freshwater wetlands are among the most biologically valuable ecosystems in Europe, yet the specialized fish species that inhabit their shallow, densely vegetated waters are slipping toward decline with remarkably little public attention. A new special issue of the journal Nature Conservation, published by Pensoft, now brings together eleven studies that map the distribution, genetics, ecology, and conservation of these overlooked animals, offering the most coordinated picture to date of what is happening to Europe’s wetland fishes and what can still be done to save them.

The collection grew out of an international conference called The Fish of Muddy Waters, held in Vienna, Austria, in October 2025. Over three days, researchers presented 52 studies, including 42 oral presentations and 10 posters, all focused on fish species specifically adapted to the floodplain wetlands of European rivers. The eleven papers that made it into the special issue represent only a fraction of that work, but they capture the geographic and thematic breadth of the effort, spanning from the Tagus floodplain wetlands of Portugal to the Danube lowlands of the Pannonian Basin, and from Croatia to northern Germany.

Three species anchor the issue: the European mudminnow (Umbra krameri), the European weatherfish (Misgurnus fossilis), and the crucian carp (Carassius carassius). Despite differences in their ranges and conservation status, these fishes share a defining ecological trait. They are all strongly associated with shallow, structurally complex wetlands that are frequently isolated from larger water bodies or only temporarily connected to them. That specialization makes them excellent indicators of wetland integrity, but it also makes them acutely vulnerable when those habitats disappear, degrade, or become fragmented.

The physics and chemistry of wetland waters explain much of this vulnerability. Because these systems are shallow and packed with vegetation, summer temperatures can climb steeply, and dissolved oxygen fluctuates dramatically, with hypoxic conditions frequently developing in isolated or heavily vegetated zones. Such conditions favor fish species with physiological or behavioral adaptations to low oxygen concentrations and elevated temperatures, while excluding less tolerant species typical of larger rivers and lakes. In other words, the very features that make wetlands hostile to ordinary fish are precisely what allow wetland specialists to persist, which is why hydrological alteration is such a direct threat to them.

That threat is mounting on multiple fronts. Land-use change, pollution, and climate change are degrading wetlands across the continent, and the loss, degradation, and fragmentation of suitable habitats increasingly interact with biological invasions. Remnant populations of native wetland fishes now face competition, predation, pathogen transmission, and hybridization from invasive non-native fishes. Among the experimental studies in the special issue is work examining habitat use and potential competition with invasive Asian weatherfishes, a group that exemplifies how introduced species can exploit the same niches that native specialists depend on.

The methodological range of the new papers is striking. Some analyze species distributions in relation to habitat preferences and habitat deterioration across Portugal, Croatia, northern Germany, and the Pannonian Basin. Others probe population genetics to understand how fragmentation has shaped the diversity of remaining populations. Applied conservation contributions span conservation genetics, the engagement of citizen scientists in monitoring, the development of regional action plans, and ultimately the assessment of conservation translocation success, a critical but often neglected step in determining whether reintroduction programs actually work.

For the European mudminnow, the special issue dedicates several papers covering its genetics, distribution, and translocation outcomes, and additional conference findings were folded into an updated monograph on the species. This depth of attention reflects the mudminnow’s status as one of Europe’s most iconic wetland specialists, a fish confined to small, often ephemeral water bodies whose survival depends on the fine-grained management of local hydrology. The broader lesson, the editors argue, is that conservation requires more than simply protecting the remaining localities where these species still occur.

Effective action, according to the synthesis of the special issue, must combine the maintenance or restoration of appropriate hydrological regimes and habitat structure with invasive species management, preservation of genetic integrity, and long-term population monitoring. Hydrological connectivity deserves particular nuance. Periodic reconnection can restore natural floodplain processes and benefit native specialists, whereas permanent connections may facilitate the spread of invasive competitors and predators into refugial habitats, effectively destroying the isolation that native wetland fishes rely upon. Getting this balance right is one of the central technical challenges facing wetland restoration across Europe.

The findings arrive at a politically significant moment. The authors note that the research is directly relevant to the implementation of the EU Nature Restoration Regulation, particularly for the identification, restoration, and monitoring of freshwater habitats and habitats of species of conservation concern. Translating distributional, ecological, and genetic knowledge into regional action plans, carefully designed translocations, and measurable assessments of conservation success is presented as the pathway from science to policy, ensuring that restoration targets are grounded in what these species actually need rather than in generic habitat goals.

Preventing further losses, the editors conclude, requires coordinated cross-border conservation that combines habitat restoration, invasive species management, long-term monitoring, and, where appropriate, reintroduction programs that respect the local genetic variability of selected species. By raising awareness of the plight of the European mudminnow, the European weatherfish, and the crucian carp, the researchers hope to stimulate further conservation action and help reverse the ongoing population declines of fishes that have, for too long, thrived unnoticed in Europe’s muddy waters.

Subject of Research: Biology and conservation of European wetland fishes

Article Title: Special issue in the journal “Nature Conservation” (Pensoft): biology and conservation of wetland fishes in Europe

Article References: Special issue in the journal “Nature Conservation” (Pensoft): biology and conservation of wetland fishes in Europe. (n.d.). https://doi.org/10.3897/natureconservation.65.215711

Image Credits: AI Generated

DOI: 10.3897/natureconservation.65.215711

Keywords: wetland fishes, European mudminnow, European weatherfish, crucian carp, floodplain wetlands, invasive species, conservation genetics, habitat restoration, hydrological connectivity, EU Nature Restoration Regulation, Nature Conservation journal, Vienna conference