Environmental DNA captured on the fish skin mucus – a potential bias to molecular diet analyses

Molecular diet analyses from faeces appear to be an ideal alternative to traditional feeding ecology studies. Nevertheless, this method can carry a risk of contamination from the environment or from body surface of the fish itself. To tackle the contamination problem, an experiment was performed with the main aim to identify whether foreign DNA is present or absent on the fish skin mucus, and if so, the second aim was to find out if this environmental DNA (eDNA) can be removed by repeated wiping of the skin mucus. Specimens of fish were exposed to eDNA and then their fish skin mucus was wiped with two consecutive smears (using a forensic swab) that were subjected to molecular analysis. The results demonstrate that eDNA from other organisms can be captured and persist on the fish skin mucus, posing a potential risk of contamination of faeces samples. Repeated wiping of mucus reduces or eliminates foreign DNA. This study provides new insights that can contribute to the development of the molecular methods, reducing the bias and increasing the accuracy of the diet spectrum analyses.