Ordinarily, when biologists want to know what kinds of fish are migrating through the local waters, they have to do fishnet trawls. Not only is this approach labor-intensive, but it's also stressful for the fish that get netted and released. According to scientists at New York's Rockefeller University, however, there may be an easier and more fish-friendly alternative: just look for the animals' DNA in water samples.

Fish are continuously passing their DNA into the water, in the form of excretions along with sloughed-off body slime. Previous research had indicated that even in small samples of both freshwater and seawater, there was enough of this "eDNA" (environmental DNA) to identify dozens of species that were in the area.

In the new project, a Rockefeller team led by Mark Stoeckle collected 1-liter water samples on a weekly basis for six months, from New York's East and Hudson Rivers. Analysis of the eDNA in those samples suggested which fish species were in the vicinity at the time – these findings were in keeping with migratory data that had been gathered between 1988 and 2015 using traditional fishnet trawls.

"For the first time, we've successfully recorded a spring fish migration simply by conducting DNA tests on water samples," said Stoeckle.

Throughout the course of the study, a total of 42 fish species were identified. A paper on the research was recently published in the journal PLOS ONE.