Replication data for eDNA in deep lakes

Environmental DNA (eDNA) is an increasingly used non-invasive and cost-effective method for species detections in surveillance studies. The myxozoan endoparasite <i>Tetracapsuloides bryosalmonae</i> is the causative agent of proliferative kidney disease (PKD) in salmonid fish. PKD is a potential lethal disease of freshwater salmonids when water temperatures exceed 12-14^oC for prolonged periods, and periodically high mortality and decline in farmed and wild salmonid populations in Europe and North America have been reported in the last decades. The aim of this study was to use eDNA as a method to detect <i>Tetracapsuloides bryosalmonae</i> from large, deep, dimictic Norwegian lakes. Such habitats are expected to become increasingly important for cold-water salmonids as climate warms. The samples were collected from five lakes in southeastern Norway, and parasite DNA was detected by qPCR. The limit of detection (LOD), the target DNA concentration needed to give a positive result in 9 of 10 reactions, was determined. eDNA from T<i>. bryosalmonae </i>was detected above the LOD-level in four of the five lakes surveyed. When including sub-LOD positive samples, results indicated presence of <i>T. bryosalmonae</i> in all five lakes. These findings corresponded with the detection of <i>T. bryosalmonae</i> DNA in salmonid-kidneys in four of the lakes in a previous survey. The detection of parasites from eDNA varied between years and stations within the same lake, revealing a changing and apparently stochastic spatial distribution of parasite DNA from year to year. Even with positive detections of <i>T. bryosalmonae</i> by eDNA, this method will not be able to reveal dynamics between the different parasite hosts or detect different susceptibility between salmonid species, as could be indicated from analyses of fish tissue. Strategies for eDNA sampling in deep, dimictic lakes are discussed.