Passive Water Filtration Enhances the Detection of Endangered Fish Species in Rivers: Anguilla anguilla and Salmo...

The assessment of vulnerable species is crucial for population management. Environmental DNA (eDNA) provides multiple advantages in the monitoring of endangered populations as it is a non-invasive and highly sensitive technique that causes minimum disturbances and allows the detection even in cases of low population numbers. Anguilla anguilla and Salmo salar are two diadromous species exposed to several threats both in the sea and rivers. They are cataloged as Critically Endangered and Near Threatened respectively (Interntional Union for Conservation of Nature) so it is especially important to use indirect sampling methodologies to monitor their populations. Here, quantitative PCR in environmental samples was performed using two different sampling techniques, one active in situ filtration system and one passive system with metaprobes deployment. Moreover, we have compared the DNA quantity and number of individuals obtained with electrofishing in the case of Salmo salar. We found that metaprobes are more efficient in DNA capture and suggest their incorporation in future monitoring programs aiming scarce species detection. Results also showed a trend where higher salmon abundance is associated with higher eDNA concentration when using filtered water. It is demonstrated that both methods are useful for salmon and eel monitoring, and we suggest prioritising metaprobes in cases of low abundance species detection and water filtration as a quantitative snapshot of DNA concentration.