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| Juvenile Walleyes in Lake Metonga (C)Engbretson Underwater Photo |
by Eric Engbretson
Something was going very wrong in Lake Metonga. The
2,000-acre lake in Wisconsin’s north woods had been a successful walleye lake
for decades, but by the late 1990s, natural reproduction and walleye
recruitment had slowed to a trickle. To make matters worse, by 2004, the adult
walleye population had sharply declined.
Spawning habitat was excellent, but young fish, either stocked or
natural, were not surviving to adulthood.
Mike Preul, fisheries biologist for the Mole Lake Sokaogon
Chippewa Community has been involved with fisheries work on Lake Metonga for
twenty years. He took a closer look at
the lake to see what changes might explain why walleyes were suddenly having
such a rough time.
The fish community in Lake Metonga had always included black
bullheads, but now fishermen were telling Mike how often they were catching the
small catfish. “We were doing some routine electro shocking one day and when
the stunned fish floated to the surface, all you could see were yellow bellies.
That’s when I knew there was a problem”. The abundance of bullheads had become
enormous. “We did some simple math and calculated what a bullhead eats in a
year. They were taking up a lot of space in the system and stressing the
available forage for walleye.” The
theory was they were outcompeting walleyes for important forage at critical
times when walleyes need an abundance of specific-sized food. “We knew stocking wasn’t working, so the idea
was that if we could create a kind of void, the walleyes might rush in to fill
it.”
Like many fisheries managers, Mike’s resources were limited,
and he privately wondered if this was really a battle he could win. His plan
was to create conditions where walleyes could thrive, and that meant doing
something about how much of the lake’s biomass was tied up in bullheads. His plan reversed the fortunes of the lake’s
walleyes almost overnight.
In the spring of 2008, Mike began the first of what
would become an annual bullhead removal on Lake Metonga. The work was
labor-intensive. “At first we set fyke nets, but the fish weren’t moving into
them in sufficient numbers, so the decision was made to electroshock for
bullheads”. Mike’s crew worked until
their arms ached, shocking and netting the whiskered fish out of the lake. By
summer’s end, they had removed 13,337
pounds of bullheads. All the fish were donated to the public, to food banks
and to nearby wildlife rehabilitation centers for the feeding of raptors.
Into
the newly created void, Mike stocked 5,000 large fingerling walleye. The next spring, another 6,216 pounds of bullheads were
removed and over two million walleye fry were stocked. But this time, something
different happened. Substantial numbers
of young walleyes survived from the two years of stocking and began to show up and be counted in fall
recruitment surveys.
At first there seemed to be no end to the steady stream of
stunned bullheads that rose to the surface every time Mike’s crew flipped on
the electrodes of his shocking boat. But gradually, fewer and fewer bullheads
appeared, and it took longer to fill up the tubs on the boat with fish. This
process became a ritual every spring on Lake Metonga and by 2012 the total
catch rate had dropped by 87%. During
the same time, the abundance of walleye fingerlings was steadily increasing.
During 2011 there was a large year class consisting of
purely naturally reproduced fish. With the resurgence of naturally reproduced
walleye, stocking was no longer needed after 2012. With bullhead numbers
reduced and in better balance with other fish, walleye production in Lake
Metonga boomed. Eventually, both natural recruitment and adult walleye density
reached historic highs. For Mike Preul, this felt like the kind of victory that
fisheries managers too rarely experience.
“I won’t lie to you, it was a lot of hard work some days, but the
results have been so amazing.”
Today Mike Preul spends only a few days each spring shocking
for bullheads. It’s become pure maintenance now, like mowing his lawn. “As long
as they don’t get out of control, I think we’re good”, smiles Mike. Walleye production has it’s up and down years
but has remained reliable overall. What
Mike learned on Lake Metonga could fill a book. His bio-manipulation method of
removing bullheads to rebuild embattled walleye populations was successfully
employed on Patten Lake in nearby Florence County and worked like pure magic.
When that lake’s over-abundant population of bullheads were severely reduced,
the walleyes quickly came back in record-setting density.
Keeping walleye lakes healthy and productive is an ongoing
issue, and all lakes face their own unique challenges. Overall, there likely
aren’t a great number of walleye lakes affected by bullhead populations, but
where such lakes exist, Mike Preul’s pioneering work is one silver bullet that
fisheries managers can now add to their arsenal.
(For
further information, questions or comments about his work, Mike Preul can be
contacted at mike.preul@scc-nsn.gov)
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