Monitoring presence and abundance of two gyrodactylid ectoparasites and their salmonid hosts using environmental DNA

Abstract

Background: Invasive species represent a major challenge for the conservation of biodiversity. The invasive ectoparasitic fluke Gyrodactylus salaris is considered one of the major threats to the Atlantic salmon (Salmo salar), and the parasite has so far been detected in 50 rivers in Norway. Aims: We investigate environmental DNA (eDNA) as a tool for detecting and assessing relative abundance of G. salaris and Atlantic salmon, upstream and downstream of a recently constructed artificial migration barrier in the River Driva in Norway. In addition, we also use eDNA to assess abundance of the less pathogenic G. derjavinoides and its main host, the brown trout (S. trutta). Material & Methods: We filtered 1 L and 10 L of water through a 0.45 μm cellulose filter and a 2.0 μm glass fiber filter, respectively, at nine different localities along the river. Concentrations of eDNA were assessed using droplet digital PCR (ddPCR) and compared to parasite abundance based on conventional methodology using electrofishing and the counting of individual parasites on juvenile salmon. Results: All four species could successfully be detected from water samples using two different protocols varying in sample volumes, filter types, and DNA‐isolation methods. However, eDNA‐occupancy modeling revealed that the probability of detecting the two gyrodactylid species was higher when filtering 10 L water through a 2.0 μm glass fiber filter (p > .99) than when filtering 1 L water through a 0.45 μm cellulose filter (p = .48–.78). The eDNA concentrations of the two fish species were markedly higher below the migration barrier, reflecting the expected higher biomass of fish. For the two gyrodactylid parasites, eDNA concentrations showed a peak upstream of the migration barrier and decreased below the migration barrier. The observed pattern was consistent with parasite abundance based on conventional methodology. Discussion: Assessing abundance in rivers using eDNA is challenging and potentially influenced by downstream accumulation and dilution from tributaries, but our results suggest that G. salaris eDNA concentrations were indicative of parasite abundance. Conclusion: We conclude that eDNA is an efficient way of monitoring gyrodactylid parasites and their salmonid hosts, and we suggest that eDNA should be incorporated into future monitoring of G. salaris.