Recent Minnesota DNR surveys show booming Lake Superior prey fish numbers

Duluth, Minn. — Did you struggle to catch fish on Lake Superior last summer? If so, it might not be because you’re an inept angler. It could have been because the fish you sought were feasting at an all-you-can-eat buffet.

Preliminary Minnesota DNR surveys indicate that 2022 may have been an exceptional spawning year for Superior’s native prey species.

“I’ve been doing those (fall) surveys since 2012, and I’ve never seen a
fraction of those numbers,” said Cory Goldsworthy, the DNR’s Lake
Superior area fisheries supervisor.

Let’s talk about the cisco population in Lake Superior. These oily prey
species also are known as lake herring or tullibees and can grow up to
18 inches long. They travel in immense schools and provide great forage
for lake trout. They are incredibly longlived for a prey species,
sometimes surviving for 40 years or more. But they produce a strong
year-class only sporadically.

“It’s all relative,” Goldsworthy said. “1984 was epic – the biggest (cisco
year-class) we’ve ever seen. Ciscoes also produced good year-classes in
1988, 1989, and 1990. Some fish from these year-classes are still out
there.”

And then came several years of poor recruitment until 2003.

“Twenty years ago was the last good year-class event,” Goldsworthy noted,
although there were some fair years in 2005, 2009, 2014, and 2015.

“But they were kind of just blips on the radar,” he said.

Ciscoes seem to lead a roller coaster existence. When sea lampreys became
established in Lake Superior, cisco numbers declined. When rainbow smelt flourished in the lake, cisco numbers suffered.

The rehabilitation of Lake Superior’s lake trout population has been a success story, and the state DNR hasn’t stocked lakers for several years now. But all those mouths took a toll on the prey base.

A cisco’s strategy for survival is to grow quickly and find safety in numbers by forming large schools.

“A cisco will grow really fast those first two years of life and try to
outgrow getting eaten by all but the biggest predators,” Goldsworthy said.

With a rehabilitated lake trout population and 20 years without a good cisco year-class, no one was pushing the panic button just yet, but there
definitely were concerns about the population of this important prey species.

There’s a lot to learn about ciscoes and how they best reproduce. In truth, Lake Superior has been down this road with ciscoes before, and maybe
boom-and-bust cycles are the natural order of things with the species.

“Some of the lowest adult abundance that we’ve seen has produced some of the greatest year-classes,” Goldsworthy said. “It was a similar situation in the 1970s. But you need to have a minimum number of fish out there so
that there are enough adults that when conditions are right, they can
pull off a spawning and recruitment event.

“This could be the natural cycle for ciscoes,” he said.

“Maybe they do put on these epic recruitment events every 20 to 30 years
because they do live so long. But we can’t assume that what’s worked in
the past is going to work now or in the future, especially when we have
exotic species like spiny water fleas and rainbow smelt that are having
an unknown impact on the lower trophic food web. Or the next invasive
that enters the system causing additional unknown impacts that we don’t
know about until it’s too late. Everything is all right until it isn’t.”

If preliminary surveys are accurate, the stars may have aligned for
ciscoes in 2022. Ciscoes spawn in fall, but their eggs don’t hatch until
April. They tend to pull off the best year-classes when there is
substantial ice cover, although major ice cover didn’t happen last
winter.

“Ice cover events are definitely one factor that leads to successful recruitment,” Goldsworthy said. “Cold spring temperatures are another factor, but then those water temps need to warm up in the summer for those fish to grow. If it’s a warmer spring and zooplankton blooms occur early but those cisco haven’t hatched yet, once that yolk sac goes away, they pretty much starve.”

Last year’s late spring meant ciscoes likely hatched while adequate food was available. Zooplankton blooms happened when the young ciscoes were large enough to prey on them.

And then there’s the rainbow smelt factor. These tiny nonnative fish prey
on newly hatched ciscoes, but smelt numbers have been down the past
couple of years, perhaps giving ciscoes a window of opportunity.

“It’s hardly ever just one thing that you can put your finger on and say,
‘This is why we had a recruitment event,’” Goldsworthy said.

But these factors likely contributed to the high number of juvenile ciscoes
the U.S. Geological Survey found while conducting surveys last summer.
These findings were confirmed last fall when the Minnesota DNR found a
similar juvenile prey base while conducting hydro-acoustic and midwater
trawl surveys onboard the Research Vessel Blue Heron, operated by the
University of Minnesota-Duluth Large Lakes Observatory.

And it’s not just ciscoes.

Bloaters and kiyi also appear to have pulled off good year classes.

“Bloaters are more of a near-shore, bottom-oriented fish in the whitefish
family,” Goldsworthy said. “Kiyi are deep water-oriented fish. They hang
out in the deepest part of the lake.”

Bloaters and kiyi, which are often collectively known as chubs, generally
measure 6 to 9 inches, meaning they never grow large enough to avoid
predatory lake trout.

Fisheries managers will have a better understanding of the 2022 year-class when the USGS does its bottom trawl survey again this spring. Perhaps the
substantial predator base is devouring those young fish, negating the
big hatch. Goldsworthy would like to better understand how predators
select their prey.

“I’d really like to get good diet data for a 10-year period, but resources
are limited, especially for a long-term, lake-wide project,” he said.
“In 2021, when we were conducting an EPA-funded lake-wide diet study
with partner agencies around the lake, there were no juvenile ciscoes,
bloaters, or kiyi out there.

“Our perception is predators aren’t eating these juvenile fish because we
didn’t see them (in predator fish stomachs), but we didn’t see them
because they weren’t there. Now we can see who is eating who.”

If ciscoes, bloaters, and kiyi really did produce a great hatch last year, it could have profound effects.

“It could be tough fishing for anglers going into the next few years,”
Goldsworthy said. “Currently, our perspective of what Lake Superior is,
is based on a declining prey abundance. We think we have a lot of lake
trout out there and they’re starting to grow slower, meaning they are
competing for limited food resources, but we haven’t hit a really big
(prey) recruitment event in our careers. If they come through, does that
mean that predator abundance will continue to naturally expand? I don’t
know.

“We should see nice, big, fat lake trout out there. We should also see faster-growing nonnative salmon,” he said. “We should see bigger fish if they’re eating kiyi, ciscoes, and bloaters.

“If you have this huge population of native prey fish, how is that
affecting the lower trophic food web, because these fish are out there
eating, spawning and pooping, creating more energy for the part of the
food web we typically can’t see. We don’t have a good idea of what’s
going to happen because it’s something we’ve never seen before.”

Time will tell if 2022 was a big year for native prey species or if
predators quickly will knock them down. In 20 years, will anglers be
talking about the epic spawn of 2022?

“Everything is leading in the right direction of, yeah, there’s a big year-class. We’re just sitting here with our fingers crossed,” Goldsworthy said.