Minnesota DNR crews getting set for Mille Lacs walleye population estimate
Garrison, Minn. — It’s time for Minnesota DNR Fisheries crews to get back out on Lake Mille Lacs and conduct their first of two (in back-to-back years) population estimates done during a five-year cycle.
If only the weather would cooperate and quickly melt away the considerable amount of ice that still covers the lake.
Such a scenario also was present the last time the DNR launched the first of two years of marked-recapture work in 2013.
“Last year would have really been nice,” said Tom Jones, the Minnesota DNR’s regional treaty coordinator, who will be heading up data collection.
Basically, a population estimate is calculated from the fish originally caught and marked and the marked fish recaptured several weeks later. That information gives officials a ratio from which to calculate the population. The estimate is of walleyes 14 inches and larger, so it’s not a complete estimate of spawning biomass, since some of the smaller female walleyes captured wouldn’t yet be spawners.
First, the walleyes are captured roughly around the time they spawn, which is why the presence of lake ice could be an obstacle this year. In 2013, the walleyes started spawning beneath the ice.
Some of the walleyes are captured at night via electrofishing boats along the shorelines – a method that tends to capture more males than females.
“Electrofishing is more efficient,” said Eric Jensen, the DNR’s large lake specialist for Mille Lacs. “But you tend to get more males because of how they spawn. The males kind of wait near shore for the females to come in. You tend to catch a lower proportion of females.”
So, day crews set trap nets off shore a ways, a method that tends to catch more females.
All of the fish are marked with yellow tags that protrude from the fish and that are fitted on the fish with what looks like a retail tagging gun, Jensen said.
After the fish have spawned, and have mixed back in with their non-tagged counterparts, DNR crews head back out and set short-term gill nets, which soak for between 20 and 40 minutes.
“Because we process so quickly, as long as water temperatures stay low, they will be unharmed by it,” Jensen said of a method that otherwise typically kills fish that get stuck in those types of nets for extended periods of time.
DNR crews will note the ratio of tagged to untagged fish in the nets, and also keep track of the location where they are tagged, their sex, and other things about the fish.
The estimate is considered more accurate with more fish captured, which is why late ice is a factor, and part of the reason the DNR tends to do these estimates in back-to-back years.
“Generally, we do two years in a row so we can reproduce the results – just to make sure we don’t have some type of wild card,” Jensen said, noting that it is just one more set of data for fisheries managers to use. “We don’t have an annual data series with these population estimates, but we can still track trends over time with it.”
And the estimates tend to mirror the DNR’s annual fall sampling index, Jones said.
In 2013, late ice prevented the DNR from setting as many trap nets as it would have preferred, and the total number of walleyes tagged was about 7,000, only a third of the 20,000-fish goal.
“We had a lot of trouble going after them with the nets,” Jensen said. “But we were able to get our eletrofishers in between the shore and the ice.
Jensen said that all anglers are encouraged to put back any tagged fish they catch during the wintertime, when there has continued to be limited harvest, despite catch-and-release seasons for state anglers the past two and upcoming open-water seasons.
Anybody who does catch a walleye with the yellow tag should write down the number and report it to the DNR, which will then send the angler information collected about that particular fish, including where it was originally tagged and any other times it has been captured by anglers.
The most recent report was completed in 2014. It included data from that year, 2013, and 2008, and showed that the walleye population in 2013 and 2014 was about one-third of what it was in 2008 (or, an estimated 722,000 to 216,000 fish in 2013 and 232,000 walleyes in 2014).
That’s because, Jones said, a proportion of the adult fish present in 2008 were dying off every year, but there were few fish being “recruited,” or replacing them.
“Getting to the cause of that, why we didn’t gain more fish, that is more complicated,” Jones said.
The vaunted 2013 year-class of walleyes were in the process of being spawned when the 2013 population estimate was being conducted, and would not have shown up the following year, since the estimate only takes in walleyes 14 inches or longer.
But that year-class, the only substantial year-class that’s since been spawned (though the verdict is still out on the last couple of year-classes), is expected to factor big into this year’s population estimate, whose results probably won’t be available to the public until October, Jones said.
Jones said he wouldn’t be surprised if the estimate doubled this year from that most recent 2014 estimate (of 232,000 walleyes), but, even so, as has been an ongoing conundrum for those concerned about an open-water state harvest of walleyes, fisheries managers say they still want another strong year-class of fish present before harvest can be increased.
“They (the 2013 year-class of walleyes) should make up about 15 to 20 percent of the estimate, and they are probably going to be closer to half,” Jones said.