Two Pints of Water Can Contain the DNA of Thousands of Fish

Two pints of water from a 1.2 million gallon tank were all that was needed to identify 13,000 fish

Photo: Zac Wolf

Getting a precise count on a large population of any creature is a tough task—just look at the effort to U.S. government has to put into the decennial census. Counting fish is even more cumbersome: up until now, scientists' best strategies involved nets and keen eyes. But new research from the University of Washington could make taking a fish census simple.

The researchers sampled just two pint-sized glasses from the Monterey Bay Aquarium's behemoth 1.2 million gallon tank, which houses around 13,000 fish live in. Like humans, fish constantly shed cells from their skin, and researchers realized those cells could be put to use for identifying their former owners.

They used DNA probes specific to vertebrate animals to analyze the genetic components of those water samples. And their analysis revealed not just which fish lived in the tank (and had recently been thrown in as food) but also which species were most plentiful. 

The researchers were able to correctly identify bony species such as tuna and sardines, but could not pick up on cartilage-bearing species, like sharks and rays. The probes—which are still being developed and refined—also missed the tank's sea turtles. Still, lead author Ryan Kelly thinks that, "Clearly this is an effective tool in the wild when you know what you’re looking for.” He and his colleagues think that the method could eventually be used in the open ocean to take stock of wild fish in a particular area or to determine whether rare or endangered species still live there. 


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