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

Although inland wetlands are among the most important and impacted biodiversity hotspots worldwide, they remain poorly assessed. This is particularly concerning in regions harboring endangered endemic species and facing strong anthropogenic pressures, such as the Lower Tagus Basin in Portugal. Here we aimed to assess the use of inland wetlands in the Lower Tagus basin by native and non-native fishes and examine the environmental factors influencing community structure. Using Sentinel-2 multispectral imagery and the Normalized Difference Water Index (NDWI), we identified 409 water surfaces, 30 of which with potential to sustain fishes over time. We surveyed 11 wetlands for fishes using a standardized, multi-gear sampling approach. We found that local fish communities were dominated by non-native species, with deep, paleopotamic wetlands (e.g., Golegã and Gouxa) tending to function as reservoirs for Largemouth Bass ( Micropterus nigricans ) and Pumpkinseed Sunfish ( Lepomis gibbosus ). Conversely, parapotamic wetlands directly connected to the Tagus River (e.g., Setil and Sto. Estevão) appeared to provide migration corridors for Thin Lip Grey Mullet ( Chelon ramada ) and shallow wetlands (e.g., Caniceiras and Granho) serve as refuges for native and threatened Lisbon Arched-mouth Nase ( Iberochondrostoma olisiponense ), European Eel ( Anguilla anguilla ), and Southern Iberian Spined Loach ( Cobitis paludica ). Our findings highlight that wetlands in the Lower Tagus Basin play different roles in supporting both native and non-native fishes. Shallow and parapotamic wetlands should be prioritized in restoration actions to hamper native fish loss whereas paleopotamic wetlands require non-native eradication and control programs to hinder the spread of non-native fishes.