A fish census in a glass of water? DNA offers clues.

DNA researchers at the Monterey Bay Aquarium have shown they can determine the types of fish inhabiting a particular ecosystem with just a glass of water.

Researchers can now carry out a fish survey using just a glass of the water in which the fish live.

A group of researchers collected about two pint glasses of water from California's Monterey Bay Aquarium's 1.2 million-gallon open sea tank. They then analyzed the DNA in the water samples to determine what fish species were present in the tank. 

Researchers compared this DNA to primers – short bits of DNA – from earlier studies. If a match was found, then the specific type of fish could be determined, said Ryan Kelly, assistant professor of marine and environmental affairs at the University of Washington.

"Imagine that you are looking at a Velcro and how one side sticks to the other," Dr. Kelly, who was involved in the study, told the Monitor. "Primers work the same way."

The sea tank at Monterey Bay Aquarium, which is among the 10 largest aquariums in the world, was proposed because the fish that inhabit the tank are already known. In that way, researchers could compare the accuracy of their new technique by comparing what species of fish their DNA study revealed to what was already present in the tank.

Scientists involved in the study successfully identified eight bony fishes in the tank. The technique also picked up DNA from long-dead Atlantic menhaden, fish that had been processed, transported, and added to the tank as food. Sardines and tuna made up the greatest amount of biomass, revealed the findings.  However, researchers could not identify the turtles and cartilaginous fish such as rays and sharks. Kelly said that these kinds of biases in detection are inevitable, highlighting the need to focus on the design of more primers.

"It might be unpleasant to think about when going for a swim in the ocean, but the water is a soup of cells shed by what lives there," said Kelly, who is the lead author of the paper. "Every one of those cells has DNA and if you have the right tools you can tell what species the cell came from," he added.

This process of using DNA is less expensive and could help us understand marine ecology better and quickly look at the presence of invasive and endangered species, Kelly says.

"Clearly this is an effective tool in the wild when you know what you're looking for," Kelly said in a University of Washington press release.

The findings of the study were published in the journal PLOS One.