Non-Invasive Monitoring of Tetracapsuloides bryosalmonae by eDNA: Influencing Environmental Parameters

Tetracapsuloides bryosalmonae, the causative agent of Proliferative Kidney Disease (PKD), affects salmonid populations in the northern hemisphere. In Switzerland, PKD monitoring to date has been conducted irregularly using invasive sampling. However, non-invasive detection protocols based on environmental DNA (eDNA) have been established, and there is still a lack of knowledge about environmental parameters that may influence detection. In this study, we evaluated which sampling period would be best for eDNA-based monitoring of T. bryosalmonae and which environmental parameters might influence the outcome by regularly sampling water from six Swiss rivers over an entire year and testing for presence of T. bryosalmonae DNA by droplet digital PCR (ddPCR). The T. bryosalmonae infection status of bryozoans from all investigated rivers was also assessed. Water temperature, precipitation, and air temperature were examined as environmental parameters. We found T. bryosalmonae DNA in all rivers by ddPCR. Positive results were almost exclusively retrieved between late spring and early autumn (mid-April until end of October). 98.3% of T. bryosalmonae positive water samples were collected when average daily water temperatures were above 8°C, and 71.9% of the positive samples when average daily water temperatures were above 14°C. Occupancy modeling corroborated the influence of water and air temperature on detecting T. bryosalmonae eDNA. Precipitation caused early clogging of filters in some cases and reduced detection by reducing sample volumes, suggesting when sampling for eDNA should be avoided. Finally, recommendations for a future eDNA-based monitoring of T. bryosalmonae are provided.