Using eDNA to Track Fish Populations
by Sydney Widell
UW-Madison undergrad Sydney Widell spent her summer at Trout Lake Station as the summer science communication intern as she followed researchers around and sent back a ton of entertaining and informative dispatches from the field – like this one from August 20th.
At the start of every fishing season, the Wisconsin Department of Natural Resources (WDNR) embarks on the arduous task of setting catch limits on the state’s popular game fish like walleye.
In order to estimate how many fish anglers may remove from a lake while still keeping the population healthy, the WDNR first needs to estimate how many fish are in the lake to begin with and how those populations are changing. Not an easy task, considering that there can be thousands upon thousands of fish in a single 100-acre lake.
And more than 1,000 of the state’s 15,074 lakes fit that bill.
Enter CFL graduate student Mike Spear, who may be on the brink of developing a radical new way to estimate fish populations by decoding their DNA.
Mike is based in Madison, but I got the chance to catch up with him in the field on Bearskin Lake when he came up north to sample last week.
As fish swim through a lake, Mike explained, they constantly shed genetic material in the form of skin, eggs and waste. The amount of environmental DNA — or eDNA — in the water might give researchers like Mike clues about the size of the fish populations it came from.
“These fish are literally broadcasting their presence in DNA,” Mike said. “What we want to do is see if we can use environmental DNA to get population estimate information in a much quicker, less disruptive and less costly way.”
Right now, Mike is specifically looking at walleye — a species that the WDNR is mandated by law to make population estimates of annually. The WDNR collects data on 15 different lakes each year, and Mike is surveying its 2018 sample sites. That way, he’ll be able to compare his findings to theirs.
In the spring, researchers capture and tag walleye on a lake using nets. Later on, they return to the lake, and using a method called electrofishing to stun fish, quickly net them and check for tags before safely releasing them back to the lake.
Based on sex ratios, the size of the lake and the number of tagged walleye they caught a second time, they can extrapolate the size of the entire population.
But, for the WDNR, surveying the 15 lakes takes weeks and requires a lot of personnel.
If Mike can establish a relationship between the amount of walleye eDNA in a lake and the population estimates made by the WDNR, he could potentially model entire populations based entirely on eDNA.
“The idea behind that is we have all these walleye swimming around in a lake, and it takes a lot of time and effort to pull those walleye out of the lake and count them,” Mike said. “But we can, just by taking a water sample, find and count the number of walleye DNA molecules that are floating around in the water.”
Mike will spend the fall looking for segments of genetic code unique to walleye, or a “walleye signature,” in the samples he’s taking now.
Last year’s sampling yielded results that looked a lot like the WDNR’s, he said.
“The relationship was strong enough that we’re doing it again this year to see if it holds up year to year,” Mike said. “It could be a tool going forward [fisheries managers] might want to adopt.”
You can find more of Sydney’s words and pictures on our blog.