The sample location is not the whole story
Fish shed genetic material into water through sources such as mucus, cells and waste. Researchers can filter that water and test for DNA. A positive result is valuable evidence, but it does not attach a tiny location tag to the fish that produced it. Water moves, and the genetic signal moves with it.
Sources: s2
A test beside salmon pens
NOAA researchers studied chum-salmon DNA around net pens in Amalga Harbor, Southeast Alaska. Sampling in 2021 and 2022 examined distance, depth and tides around a known source. Most detections occurred within 1.5 kilometres of the pens, and concentrations changed with depth and tide. Those details make the result more useful than a generic claim that DNA travels a fixed distance.
The experiment provides evidence about those conditions. It does not establish a universal 1.5-kilometre radius for every species, current, season or assay.
Sources: s1
What makes the clue stronger
NOAA's later roadmap for fisheries applications identifies false detections, missed detections, dispersal and the link between DNA quantity and abundance as practical challenges. It also explains why genetic observations can complement nets, acoustics and video rather than supply every biological measurement themselves.
A proposed island survey could retain sampling coordinates, depth, time, assay identity and control results, then compare repeat samples with other observations. One positive tube would not settle whether a population lives at the sampling point; transport still needs investigation. One negative tube would not establish absence; the test may have missed material that was present.
Sources: s2