The weatherfish has long fascinated biologists. Its common name stems from the folk belief that the fish becomes unusually active when weather conditions change, and its biology is genuinely peculiar: like several other cobitid loaches, it can supplement its gill respiration by absorbing oxygen through the lining of its intestine, an adaptation that allows it to survive in warm, stagnant, oxygen-poor waters where most fish would suffocate. That same tolerance, the new research suggests, may now be the key to its persistence. As a nocturnal species that spends its life hidden in muddy sediments and dense vegetation, the weatherfish is notoriously difficult to study, which helps explain why many aspects of its ecology and biogeography remain poorly resolved even today.

Its conservation status reflects this uncertainty. The species is listed as Least Concern on the European Red List, yet it is classified as Vulnerable in Germany’s national Red List, and regional assessments within the North German Plain span nearly every category from Critically Endangered in North Rhine-Westphalia to Least Concern in Brandenburg. Mecklenburg-Western Pomerania, the north-easternmost federal state, sits at the north-western edge of the species’ natural range and is now considered to hold the main German stronghold of the weatherfish. Genetic and palaeontological evidence indicates that the genus Misgurnus began colonising Central Europe during the Pliocene, but that its permanent establishment in the North German Plain occurred only around the end of the Pleistocene, roughly 11,500 years ago.

To map the species’ fortunes, the researchers drew on a state-wide fish and cyclostome database containing over 67,000 records compiled by 2024, roughly half of which came from surveys conducted by the Society for Nature Conservation and Landscape Ecology, with the remainder contributed by environmental authorities and volunteer organisations. Because quantitative data such as individual counts were available for only a limited subset of records, the team used the relative detection frequency, the proportion of weatherfish records among all fish records, as a proxy for abundance. All georeferenced records were transferred onto a 10 by 10 kilometre grid, and habitat preferences were analysed using electrofishing records only, to eliminate biases introduced by methods of varying efficiency.

The results were striking. Drainage ditches accounted for by far the highest proportion of weatherfish detections, with a binomial logistic regression indicating roughly seventy-fold higher odds of detecting the species in ditches than in lakes, the reference category. Oxbows, small standing waters, rivers, brooks and canals all ranked significantly above lakes as well, but none came close to the ditches. This pattern held even after the researchers applied a standardised index that corrects for differences in species richness among waterbody types. The finding is all the more remarkable given that the state’s landscape, shaped by glacial retreat, once teemed with natural wetlands. River oxbows and small floodplain waters, widely assumed to be the weatherfish’s original habitats, have largely disappeared as the region was converted into intensive farmland, leaving the estimated 30,000 kilometres of melioration ditches as the species’ principal refuge.

The fish communities sharing these ditches tell a consistent story. Tench, pike, nine-spined sticklebacks and three-spined sticklebacks were the species most frequently recorded alongside the weatherfish in ditch sections where it occurred, and all are known for their tolerance of warm, oxygen-depleted, vegetation-choked conditions. No other species showed a preference for ditches as pronounced as the weatherfish’s, underscoring that the loach is not merely a generalist but a specialist of these extreme environments. The authors note that temporary extremes such as high temperatures, low oxygen and even water scarcity may actually enhance the ditch habitat’s suitability for the weatherfish, boosting its competitiveness relative to less hardy species.

Across the 35-year period from 1990 to 2024, the frequency of weatherfish detections remained relatively stable, typically fluctuating between 0.5 and 2.0 percent of all fish records, with a slight increase visible over the past fifteen years. The species was found across nearly the entire state, with the exception of a few Baltic Sea islands and some water-deficient areas, yet it was rarely abundant. More than 70 percent of records were based on the capture of only one or a few individuals, and many such observations could not be confirmed in subsequent sampling. The historical record itself carries a curious artefact: until the late 1970s, documented occurrences were confined to the western part of the state, mirroring the old border between the former Duchies of Mecklenburg and the Prussian Province of Pomerania, a boundary that evidently shaped where early naturalists looked rather than where the fish actually lived.

The monitoring programme, launched between 2008 and 2010 at six sites and expanded to ten by 2016, revealed a more turbulent picture beneath the apparent stability. Nine of the ten monitored sites are artificial waterbodies, mostly drainage ditches; only one, the Alte Elde, an old riverbed of the River Elde, is natural. Population densities were typically low, often below one individual per 100 square metres, but several populations occasionally exploded to densities far exceeding the average, with one site reaching 91 individuals per 100 square metres. Length-frequency distributions showed that these surges were driven almost entirely by a single size cohort, meaning a single successful age class, which points to highly irregular reproduction or recruitment. In years with low densities, samples consisted mostly of medium to large fish, consistent with long-lived adults persisting through repeated breeding failures.

This boom-and-bust dynamic has direct consequences for conservation policy. Under the EU Habitats Directive, population assessments are normally based on two sampling events within a six-year reporting period, but the authors argue that such snapshots are highly problematic for a species whose numbers can swing by orders of magnitude between years. They recommend incorporating results from previous reporting periods into assessments, increasing the number of sampling events, and ideally moving to annual surveys. Populations that reproduce only sporadically are also inherently more vulnerable to local extinction than those with continuous recruitment, which makes the connectivity between subpopulations, the metapopulation structure, critical for the species’ long-term survival.

The practical implications are sobering. The drainage ditches that now sustain the weatherfish are fragmented by thousands of bed sills and weirs, likely numbering in the tens of thousands across north-eastern Germany, and they suffer elevated fish mortality from intensive mechanical maintenance. The authors call for the least invasive maintenance regimes possible, including temporally and spatially alternating measures, manual vegetation clearing, and the creation of refuge habitats such as connected alcoves and small standing waters. Improving upstream passability within ditch systems and their connecting watercourses is identified as the single most important step toward stabilising the overall population. Intriguingly, the rapid return of beavers to the region may help: their dams flood large areas, reconnecting isolated waterbodies and creating exactly the kind of shallow, vegetated, oxygen-poor habitat in which the weatherfish, against all odds, appears to be quietly thriving.

Subject of Research: Ecology, distribution and population dynamics of the European weatherfish in Mecklenburg-Western Pomerania

Article Title: Distribution, habitat, and population dynamics of the European weatherfish ( Misgurnus fossilis L., 1758) in the north-easternmost part of Germany

Article References: Krappe, M., & Weidig, A. (2026). Distribution, habitat, and population dynamics of the European weatherfish (Misgurnus fossilis L., 1758) in the north-easternmost part of Germany. Nature Conservation, 65, 25-48. https://doi.org/10.3897/natureconservation.65.188825

Image Credits: AI Generated

DOI: 10.3897/natureconservation.65.188825

Keywords: European weatherfish, Misgurnus fossilis, drainage ditches, Mecklenburg-Western Pomerania, Habitats Directive, population dynamics, wetland conservation, electrofishing monitoring, metapopulation connectivity, fish ecology, Germany, species conservation