The research team set out to answer three questions: how many inland wetlands remain in the Lower Tagus Basin, which fish species actually use them, and what environmental factors shape those communities. The basin, stretching along the Tagus River between Golegã and Lisbon, was once a dynamic mosaic of oxbows, marshlands, and seasonally flooded habitats. Today it is dominated by intensive agriculture, and the wetlands that persist are often isolated, artificialized fragments cut off from the river channel by hydrological barriers.

To build an inventory, the researchers turned to remote sensing. They used Sentinel-2 multispectral imagery at 10-meter resolution, captured during periods of null rainfall between August and October 2019 to avoid counting rain-flooded surfaces, and applied the Normalized Difference Water Index (NDWI), which exploits the contrast between reflected near-infrared radiation and visible green light to distinguish water from soil and vegetation. Across the study area of roughly 3,927 square kilometers below 50 meters elevation, the method identified 409 water surfaces. After screening out surfaces that were too small or too large, heavily artificialized, or had dried out in recent years, 30 wetlands with the potential to sustain fish over time remained. That figure far exceeds the eleven wetlands previously recorded for the region, suggesting these habitats are more prevalent than earlier assessments assumed.

From those 30 candidates, 11 wetlands were selected for detailed fish and environmental assessment, ranging from 0.5 to 9.0 hectares and covering a combined 51.2 hectares. Fish were sampled between May and June 2021 using a standardized multi-gear approach that combined electrofishing, beach seine nets, and baited minnow traps, with sampling effort scaled to wetland size. Electrofishing transects lasted 10 to 30 minutes depending on wetland category, seine nets were dragged across one to three locations, and pairs of minnow traps were deployed at three to nine points for up to 510 minutes. Captured native fish were identified, counted, and returned; non-natives were euthanized in compliance with Portuguese and European Union regulations on handling wild animals.

The surveys yielded 8,272 individual fish representing five native and eight non-native species. The picture was stark: non-native species occurred in every wetland, averaging 4.1 species per site, while natives averaged just 1.2 species and were entirely absent from two wetlands, Santana and Muge. The Eastern Mosquitofish (Gambusia holbrooki) was by far the most abundant species, with 4,769 individuals captured across all wetlands, followed by the Pumpkinseed Sunfish (Lepomis gibbosus) with 652 individuals in half the sites. Total catches per wetland ranged from 16 to 3,567 fish, and native abundance was consistently dwarfed by non-native numbers.

Yet the surveys also revealed where the remaining native fish persist. The endangered Lisbon Arched-mouth Nase (Iberochondrostoma olisiponense), a species endemic to the Iberian Peninsula and restricted to the Lower Tagus, was found in only two wetlands, Granho and Caniceiras, both shallow sites. The Critically Endangered European Eel (Anguilla anguilla) appeared in many wetlands, while the Near Threatened Thin Lip Grey Mullet (Chelon ramada) and Southern Iberian Spined Loach (Cobitis paludica) were also recorded. These findings confirm that even heavily altered wetlands still function as refuges for threatened species, albeit in greatly reduced numbers compared with historical records from the region.

To understand what drives these patterns, the team analyzed environmental conditions at each wetland using nine habitat variables, including temperature, conductivity, dissolved oxygen, pH, chlorophyll, turbidity, depth, and vegetation cover, plus landscape variables such as connectivity class, elevation, and hydrological distance to the Tagus River. A Principal Component Analysis summarized the main gradients, with the first axis capturing a contrast between connectivity and elevation-area, and the second reflecting variation in conductivity and vegetation cover. Fish community structure was then related to these variables through Canonical Correspondence Analysis, a multivariate technique suited to unimodal species responses along ecological gradients.

The results were statistically robust. The overall model was significant (p = 0.002), and every environmental variable tested, including connectivity, conductivity, depth, and vegetation cover, had a significant effect (p ≤ 0.005). The first two canonical axes together explained about 69% of the variation in species-environment relationships. The analysis showed that wetlands fall into distinct functional roles. Deep, paleopotamic wetlands isolated from the river, such as Golegã and Gouxa, act as reservoirs for Largemouth Bass (Micropterus nigricans) and Pumpkinseed Sunfish, likely reflecting local introductions by anglers. Parapotamic wetlands directly connected to the Tagus, such as Setil and Sto. Estevão, serve as migration corridors for the Thin Lip Grey Mullet, whose feeding migrations can carry it several kilometers upstream. Shallow wetlands like Caniceiras and Granho provide spawning, feeding, and nursery habitat for the endemic nase.

The dominance of non-native fish is not merely a matter of numbers. Species such as mosquitofish, pumpkinseed, and Gibel Carp are known to outcompete native fish, while predatory Largemouth Bass and Zander threaten amphibians and waterbirds. Common Carp, by uprooting macrophyte beds, can degrade habitat for threatened waterbirds as well. The researchers note that this biotic homogenization mirrors patterns seen in wetlands across France and Spain and is consistent with findings throughout the Tagus Basin, making it unlikely to be an artifact of sampling. Because multiple gears were used and effort was standardized through a Multi-Gear Mean Standardization method that weights catchability and effort across gear types, the team is confident the pattern reflects real ecological conditions.

The conservation implications are clear and actionable. The authors recommend prioritizing shallow and parapotamic wetlands for restoration actions, including habitat enhancement, pollution control, water abstraction regulation, and control of invasive aquatic vegetation, to hamper further native fish loss. Paleopotamic wetlands, by contrast, require targeted eradication and control programs for non-native fish to prevent them from serving as invasion reservoirs. The recent EU Nature Restoration Law offers a policy pathway to raise water levels on a more permanent basis, safeguarding areas for endangered species. Recent designations, such as the classification of Gouxa and Paul das Caniceiras as Local Natural Reserves, recognize the value of these habitats. The researchers also call for future surveys covering multiple seasons and unsampled wetlands, and suggest environmental DNA metabarcoding and citizen science as tools to deepen understanding and engage local communities. With 89% of inland wetlands worldwide remaining unprotected, studies like this one provide the evidence base needed to decide which fragments of a vanishing landscape are worth fighting for first.

Subject of Research: Use of inland wetlands by native and non-native fish communities in the Lower Tagus Basin, Portugal

Article Title: Assessing the use of inland wetlands by fishes in the Lower Tagus Basin, Portugal

Article References: Ribeiro, D., Dias, D., Gago, J., Martelo, J., Rivaes, R., da Costa, L. M., Santos, C. D., Ribeiro, F., Catalão, J., & Magalhães, M. F. (2026). Assessing the use of inland wetlands by fishes in the Lower Tagus Basin, Portugal. Nature Conservation, 65, 65-83. https://doi.org/10.3897/natureconservation.65.190071

Image Credits: AI Generated

DOI: 10.3897/natureconservation.65.190071

Keywords: inland wetlands, Lower Tagus Basin, non-native fish, Iberochondrostoma olisiponense, remote sensing, NDWI, fish conservation, invasive species, wetland restoration, Portugal, freshwater biodiversity, Sentinel-2