“There are plenty of fish in the sea.” But which kinds of fish are where, and when? It’s a big question that scientists are now one step closer to answering.

Scientists at the Center for Oceans Solutions at Stanford University and the University of Washington are helping to usher in a new era of ocean monitoring using bold new tools based on cutting-edge DNA sequencing technology. Their research has received funding from a the Stanford Woods Institute's Environmental Venture Projects seed grant program (read more).

Tracking the presence and abundance of fishes and other ocean wildlife is a critical aspect of understanding marine ecosystems and applying science-based policy. Traditional methods of ocean monitoring can be time-consuming, expensive and error-prone because they often involve boat-based scuba divers counting individual animals. In some cases, these methods can be invasive, as trawling or other fishing techniques are used to collect animals.

The new technology is based on environmental DNA, or “eDNA,” found in seawater. As marine animals go about their lives, they slough off tissue and dead skin cells, just as pets shed hair on the couch. These cells break down and leak their naked, free-floating DNA into the surrounding seawater. Scientists can now collect this seawater, extract eDNA from it and sequence it to reveal the identity of the animals present.

With the advent of faster and cheaper DNA sequencing, researchers have increasingly explored these methods to study microbes or, more recently fish, in freshwater lakes.  

Now, as detailed in a new article just published in the journal PLOS ONE, Center for Ocean Solutions scientists have successfully tested this technology in the ocean for marine fish. The study’s authors are Ryan Kelly, formerly an Early Career Fellow at the Center for Ocean Solutions and now a faculty member at University of Washington, Center Early Career Fellows Jesse Port and Kevan Yamahara, and Center Science Director Larry Crowder, a Woods senior fellow.

The group sought a closed environment so that they could compare the results of the eDNA sequencing with the identities of the animals known to be present. What better place to test than the 1.2 million gallon Open Sea tank exhibit at the Center’s partner, the Monterey Bay Aquarium?

Kelly and his colleagues collected water from the Open Sea tank itself, from the food added to it and from the seawater pipes that supply it with water. They detected the majority of the bony fish residents – tuna and sardines – from their eDNA. The relative abundances of eDNA roughly matched the abundances of animals in the tank, suggesting that in the future these tools will provide a window into not just which animals are there, but also how many.

The technology is so sensitive that Kelly and his colleagues were even able to detect eDNA from non-tank residents, including other fishes and even humans. These eDNA sources appear to have come from the fish food and the tank’s seawater supply pipe, drawn from the Monterey Bay. The bay itself receives terrestrial input from agricultural runoff, human use and other sources.

Some of the tank residents, such as a sea turtle and giant sunfish, were not detected in the test. perhaps because of technological bias in the current tools or sample size or because these organisms shed DNA differently than their neighbors.

“To really make this approach relevant to monitoring, further enhancement and refinement of the technology will be needed, including more molecular work, a better understanding of eDNA fate and transport in the ocean, and broader species detection," Port explained.

Interestingly, the test detected sea otter eDNA from the tank’s seawater supply pipe, originating either from nearby exhibits housing otters or from wild otters living in Monterey Bay near the pipe’s intake. This finding demonstrates that these methods may be applicable to marine mammal monitoring in Monterey Bay, a key priority for the management of this important ecosystem.

The project also illustrated the ability of eDNA sequencing technology to allow scientists to monitor fish populations from the comfort of dry land, based on a sample of seawater. The test in the aquarium’s Open Sea tank exhibit showed that, while a few refinements are needed, this technology is poised to complement and strengthen ocean monitoring in the wild.