Professor Einar Eg Nielsen, DTU Aqua

“The ones that thrive are the ones that grow up in the wild”

Gener og genomer Fiskeri og fiskebestande Fisk og skaldyr Økosystemer

What is biodiversity? We talk to Professor and Population Geneticist Einar Eg about this.

When Professor Einar Eg was a boy, he spent many hours at Skjern River in Jutland; he was a keen angler from early childhood. There, he also heard many proud angling stories, but the salmon were conspicuous by their absence. The salmon in the river had become almost mythical in the stories—everyone talked about them, but no one, or perhaps a single old man a year, got a salmon on the hook. Why? 

That question kickstarted Einar Eg’s professional path. His fascination with fish and their secrets led him to study biology with a focus on fish. At the end of his studies, a blessing in disguise happened: 

“I’d actually started writing my master’s thesis on aquaculture and hake, but that didn’t work out, so I asked a fellow student at Aarhus University for help. And suddenly I was sitting there doing genetic analyses on a bag of salmon from Skjern River!” says Einar Eg.

From there, it was clear sailing to DTU Aqua’s freshwater site in Silkeborg, where he is now a professor and coordinator of the population genetics group. Conservation and biodiversity have always been the obvious issues to pursue for him—but what is biodiversity?

“As a geneticist, I’m tempted to say that biodiversity basically means genetic diversity at different levels,” says Einar Eg, and elaborates: 

“Biodiversity is usually defined top down or bottom up: there are different ecosystems, and within these ecosystems there are different species, and within the species there’s genetic diversity. And genetic diversity can be both at the population level and directly between individuals like you and me.”

A salmon isn’t just a salmon 

“Biodiversity, in my research and consultancy on genetic diversity within species, is about preserving a sufficiently large, effective population size. To put it simply, it’s all the individuals in a species that pass their genes on to the next generations and thus ensure genetic variation,” he says.

There’s a rule of thumb called the 50/500 ratio. That is, a population must consist of at least 50 individuals to avoid inbreeding and 500 to avoid loss of genetic diversity:

“For example, it’s no use having 100,000 salmon in Skjern River if only 20 of them spawn and contribute to the next generation,” says Einar Eg.

When he came to Silkeborg in the early 1990s, it was still assumed that ‘a salmon is a salmon’. But pretty quickly throughout the decade, that view of species evolved into a widespread understanding that a salmon isn’t just a salmon:

“A salmon is an individual that belongs to a population with a gene pool of its own. For those of us who work with genetics, it’s always been abundantly clear that there are different levels of biodiversity.” 

From release to recovery 

Previously, fisheries managers did not pay much attention to managing the genetically separate stocks. But according to Einar Eg, the new DNA technology now allows statisticians to make excellent stock assessment models that include genetic analyses. DTU Aqua uses these models to provide advice to the Danish Ministry of Food, Agriculture and Fisheries on fishing quotas, sustainable management of fish stocks, and more. 

“When we started working with trout management, there was an idea to build a central farming facility and then pump trout into all streams in Denmark from the central place. Most of the trout that were released were from actual fish farming strains, which had been bred for certain characteristics for generations.”

“But then, in the early 90s, when we started to detect genetic differences between wild trout populations, it became clear that farmed fish adapt to the fish farming environment in a way we don’t want in the natural environment.”

“The ones that do best are the ones that have the right genes for their given environment and have grown up in the wild. This is what the genetic research shows,” says Einar Eg. 

Therefore, the management of fish stocks in rivers and lakes has changed from focusing on releasing juvenile fish to focusing on restoring fish habitats. 

“However, in certain cases, such as with the salmon in Skjern River, where the population was critically low, you have to help out. But in those cases you release the juveniles from the genetically and ecologically closest healthy population—you no longer take juveniles from random farmed populations,” says Einar Eg.

Incidentally, he says, many locals and fishing associations have known for a long time that genetic biodiversity is important: 

“In West Jutland, they’re very proud of their local salmon. They know that if we lose it, we can’t just release some others. These salmon have adapted genetically to the conditions in West Jutland over thousands of years.” 

Biodiversity isn’t static 

When we talk about biodiversity and the survival of species, Darwin comes into the picture—how do you see your work on biodiversity in light of Darwin’s theory of evolution? 

“Well, there’s this saying that ‘Nothing in biology makes sense except in the light of evolution,’” replies Einar Eg. 

“So if you don’t understand evolution, you can’t work with biology. And my colleagues have come up with a variation: ‘Nothing in evolution makes sense except in the light of population genetics.’ This means you have to understand that evolution, i.e. the selection and adaptation that’s going on, is happening within populations.”

“It’s no use having a static picture of biodiversity and saying that now it's fixed, like in a zoo—we can’t just safeguard a single population of a species, because it can disappear over time. We need to safeguard many populations of the same species to ensure genetic diversity—genetic diversity is the fuel that will ensure the survival and long-term adaptation of species in a changing world.”

Where do we humans fit in?

“Well, it might be tempting to say that humans are also a species in evolution and a larger biodiversity system, and then just leave it at that when it comes to all the species that are going extinct. But since we now affect nature more than ever, that view doesn’t hold up in the system.” 

“Today we know that we can’t see species in isolation—for example, the salmon in West Jutland are actually the last original lowland salmon in Europe. So the global perspective is important for the survival of species.”

“And our view of nature is changing from a utilitarian view to a natural view—i.e. nature for nature’s sake. Because exploiting nature is also an experience of nature—we want to be in nature."

"Then, of course, you can always discuss how many resources we should spend on biodiversity and which species we should make a special effort to save,” says Einar Eg. 

The fate of the houting

Speaking of species in need of help, after this interview in the spring of 2022, Einar Eg will visit Ribe in Jutland. On the agenda is the endangered North Sea houting, a fish so rare that its name doesn’t ring a bell with any of the laypeople I ask in my social circle.

According to, the houting is a salmonid fish that is closely related to the common whitefish, which we know from Jutland’s fjords and lakes.

“Where we are today with our work, we have a good overview of the genetic biodiversity of salmon, trout and houting. There used to be houting in all the rivers and streams in the Wadden Sea area, but today there’s only one robust and viable population of about 3,000 in Vidåen,” says Einar Eg.

Something more sensible to do

In the early 90s, there was scepticism about research into and protection of genetic diversity within species.

“At the first meeting I attended with the Danish Fisheries Agency and the Danish Fishermen PO about unsustainable releases of foreign salmon directly into the sea near Bornholm, someone asked me if I couldn’t find something more sensible to do than be a geneticist!” Einar Eg says with a smile.

"But we fairly quickly got some victories for population genetics in the management of those West Jutland salmon in the 90s. The streams in West Jutland were designated as EU habitats, the salmon were protected for five years, and the genetic advice we provided was used in the future management.”

“There were also other successes that led to a more holistic and biodiversity-focused management of fish stocks. For example, our fish management people now go out and advise and help municipalities remove barriers, re-establish habitats, lay out spawning gravel, etc. Measures that also benefit species other than salmon and trout.”

The population genetic research group in Silkeborg is one of the largest and most renowned in the world when it comes to fish and fisheries. Among other things, they look at environmental DNA in water samples, which makes it possible to map species in a specific area, search for DNA from a particular species, or examine an area for invasive species.

Currently, the group is undertaking a task for the Danish Environmental Protection Agency, which involves examining which ray species we have in Danish waters, where they’re caught, and whether fishermen and researchers are able to visually tell the difference between them.

What do you hope your research will have contributed to in 10 years’ time?

“What I hope—and this is what we’ve been working on for a long time—is that diversity within species comes into focus even more than it has been. And I see signs that this is happening,” says Einar Eg, and points to the new generation of researchers and nature conservationists:

“The young people don’t question the fact that, for example, there are different stocks of cod. And it’s my hope and belief that population genetics will become a common approach to how we manage fish species.” 

“And I'm not actually worried. I see clear signs that it’s going that way. So I have more faith in the future of working with biodiversity today than I did ten years ago. So when I retire one day, I think it’ll be under control,” Einar Eg concludes.

LÆS artiklen på dansk.


Einar Eg Nielsen gained a PhD in population genetics from Aarhus University in 1998. The same year, he was appointed as a researcher at Danish Institute for Fisheries Research (now DTU Aqua). 

Now, he is a Professor of Fisheries Genetics and Research Coordinator for the research area ‘Population Genetics’ (since 2010), which has 15 employees. He is also an Honorary Professor at the University of Queensland, Australia.

His research group uses molecular genetic/genomic methods to study the spatial and temporal distribution of genetic/genomic variation within species of fish and shellfish as well as the associated evolutionary processes. The group also conducts research into how this information can be used in biodiversity conservation, resource management, and practical traceability of fish and fish products.

The research also includes analyses of environmental DNA (eDNA) and DNA barcoding to map species-level biodiversity in both saltwater and freshwater, as well as in commercial catches.

Read more about the population genetics research at DTU Aqua