Counting the Invisible Fish

Scientists don’t need to see a fish to know it’s there.

Published December 9, 2015

They can swim, they can hide, but fish always leave a trail. Wherever animals roam, they (and we) leave behind a smattering of cells—whether from skin, fur, hair, mucus, or waste. Scientists have learned to detect these genetic calling cards, or “signatures,” to determine whether a species is present in a given ecosystem without ever having to spot an actual animal.

Jeff MacAdams, a Hakai researcher and graduate student at the University of Victoria in British Columbia, is developing a method for using environmental DNA (eDNA) to calculate salmon density in a stream. At his study site, a hatchery along Goldstream River on Vancouver Island, he correlates how many coho salmon in a specific amount of moving water it takes to detect genetic material in a given number of two-liter samples. Out in the field, the equation can be reversed—the researcher can estimate fish abundance based on the number of positive samples.

MacAdams is one of a growing number of scientists employing eDNA in their work—researchers recently used eDNA to determine the density of chinook salmon in the Upper Columbia River of Washington and British Columbia, for example, and others are mapping the same species in the Yukon—though this may be the first time eDNA has been applied to coho, a particularly elusive salmon species.

Eventually the technique could replace—or at least complement—traditional fish-counting methods. “The real strength is how sensitive it is,” MacAdams says. With eDNA, fewer researchers can monitor more streams, requiring only fragments of cells to quantify even the most evasive of fish.


Jeff MacAdams’ research is supported by a grant from the Tula Foundation, which also funds Hakai Magazine. The magazine is editorially independent of the institute and foundation.