By Emma Bryce, December 11, 2020
HOW MANY FISH IN THE SEA? Researchers are getting closer to an answer—and improved management—by identifying the DNA traces that fish leave behind in seawater.
A liter of seawater might not look like much to the naked eye. But examine it at the microscopic level, and it explodes with information, revealing remarkable clues about the ocean it was extracted from: it can tell us how many fish are there, the diversity of those species, and even provide insights about how climate change and overfishing might be impacting their health.
All these clues arise from minuscule traces of DNA that are sloughed off in seawater from the bodies of fish as they swim by. And now, a new study shows that examining seawater samples for these genetic clues works just as well in assessing fish population sizes and diversity, as traditional catch surveys – a much more time-consuming process whereby boats capture fish and haul them up to the surface to be counted by hand.
The new sampling method could, by comparison, be a much more efficient and less costly way to assess the sustainability of fish stocks.
The researchers on the new study—published in the ICES Journal of Marine Science—launched their seawater sleuthing off the coast of New Jersey in the United States, over a period of 11 months (to account for seasonal fish migration). They took one liter water samples at specific locations just before traditional boat-based surveys commenced, so that they could effectively compare results from the two methods. Then they analysed their samples for environmental DNA – known as ‘eDNA’, the name for those telltale traces left behind by the animals that inhabit an ecosystem. Once they had these fragments, they matched them up against existing databases that revealed which species they belonged to, a process called DNA ‘barcoding’.
When they tallied up the species identified in those seawater samples, the researchers found that they showed similar levels of diversity to trawl surveys. In fact, they captured between 70 and 87% of all species, and up to 100% of common species, that were reflected in the results from trawls—meaning that effectively, the simpler seawater method works just as well.
By relying on the high density of invisible genetic traces in seawater, this method is also incredibly efficient: the study showed that DNA contained within just one litre of seawater can reveal as much information about species richness and biomass as a trawler that samples the equivalent of 66 million liters of water with its fish-catch surveys.
This isn’t the first time eDNA has been used to identify fish species in the ocean. But it’s the largest study to compare eDNA to traditional trawl surveys, and so it’s the most convincing evidence so far of how robust this method can be…