Tide impacts the dispersion of eDNA from nearshore net pens in a dynamic high-latitude marine environment

Environmental DNA (eDNA) is increasingly used to detect animals in aquatic habitats, but uncertainty remains about the relationship between the present location of an animal relative to eDNA detections. In marine environments, physical characteristics-such as tides and currents-can influence the distribution of eDNA. In this study, we make use of hatchery net pens containing >46 million juvenile chum salmon (Oncorhynchus keta) in nearshore Southeast Alaska to test for dispersion of eDNA and the effects of tide. Initially, we collected and filtered surface water every 80 m along a 2 km transect to test eDNA attenuation over surface distance during incoming and outgoing tides on a single day. The following year, we sampled at three depths (0 m, 5 m, and 10 m) every 500 m along the same transect as well as along a perpendicular transect, to understand dispersion by depth and in additional directions. Chum salmon eDNA was quantified using species-specific qPCR. We found that surface samples showed a consistent signal of decreasing chum salmon eDNA across the 2 km transect (R-2 = 0.665), with the majority of eDNA detections within 1.5 km of the net pens. Tide had a significant effect, resulting in higher concentrations of chum DNA throughout the transect during incoming tide and a steeper decline in eDNA over distance during outgoing tide (R-2 = 0.759). Depth affected chum salmon DNA concentration, with the majority of eDNA at the surface and a decreasing amount of DNA with increasing depth. This study addresses one of the critical knowledge gaps in applying eDNA to marine fisheries management by providing empirical evidence of eDNA dispersion and demonstrating that most eDNA detections are likely from nearby individuals that are either currently or recently present. Yet even at close proximity, eDNA signal strength fluctuates and depends on the physical environmental variables during a given sampling event.