Best of the Blog, 2016: What Do Fish Do Under the Ice?

We’re taking a look back at some of the CFL’s most popular blog posts from 2016. Here’s one originally published on back in 2015 about what life is like under the ice – considering Lake Mendota still hasn’t frozen, maybe this one will be better in 2017? Happy New Year!
These guys can't wait for ice-off so they can start sneakin' Photo: Fargo-Moorhead Dive News
These guys can’t wait for ice-off. Photo: Fargo-Moorhead Dive News
[Originally posted Jan. 30, 2015] – While we’ve been spared (so far) by any sort of climate shenanigans like a polar vortex this winter, our lakes have had a nice thick cover of ice on for a month or more. And lots of people ask variations of the question – what’s life like for fishes under the ice?
Well, the short answer is “Not much different than the rest of the year.” They eat and breathe and try to avoid becoming lunch.But there are some interesting elements to winter life, so we asked CFL grad student, Alex Latzka to help explain. (NOTE: Alex is now a post doctoral research at McGill University)
Up on land, while everything from bears to bats to turtles to raccoons is all curled up, cozy and hibernating, fishes have no such luxury. They have to keep moving enough to pass water over their gills and continue to breathe. But, still, Latzka says, everything slows way down.
“Most forage fishes (like bluegill, or other species that serve as prey for bigger fish) have to stay really still and not expend a lot of energy,” Latzka says. “So they are hanging out in structure, or weed beds, where there’s cover and some food.”

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Alex Latzka holds up an impressive pike pulled form the ice of an “undisclosed” Madison lake. Dan Oele assists.

Predators, like northern pike, are then hanging out near the weed beds waiting for something to venture out. Still, Latzka says, “They’re not eating a lot.” Cool water fish like pike and bluegill lose weight in winter, since they can’t find enough food to make up for the “metabolic cost,” or calories they’re burning just staying alive.
In fact, according to the book, “Ecology of Teleost Fishes,” by Robert J. Wootton, in some instances, fish like bluegill won’t eat all winter long, relying on fat stores they accumulated in the fall to get by.

Burbot. Image: New York DNR
Burbot. Image: New York DNR

On the opposite end of that spectrum, cold-water species of fishes thrive in these conditions. While a pike looks a little worse for wear in spring, species like burbot come out of the experience fat and happy. We don’t have burbot in Lake Mendota, but we once had lake trout and cisco, other cold water species that loved long winters, since they weren’t confined to their summer hangout in the coldest, deepest parts of the lake.

And that brings us to another winter issue for fish under the ice – oxygen. With the “lid” on the lake, precious little oxygen enters the water column and  fish are left to survive on what got mixed into the lake in the fall. That’s one reason they slow down so much – being all active and burning through their oxygen reserve is a bad idea.
Oxygen is particularly limited at the bottom of the lake, where microbes are decomposing all of the organic matter that settled out during more productive months. Thanks to the overabundance of nutrients driving all of that growth (and algal blooms) during the summer and fall, Lake Mendota has a LOT of organic matter resting on the bottom and those microbes use up a ton of oxygen breaking it down. The end result can be either hypoxic “low-oxygen” or anoxic “no-oxygen” conditions at the bottom of the lake.
These so-called “dead zones” can also form in summer and are a big reason why you don’t see lake trout or cisco in Lake Mendota anymore – those fishes couldn’t survive when the only remaining parts of the lake with cold-water habitat kept running out of  oxygen every summer.

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This is as much excitement as this pike (and Alex Latzka and Jereme Gaeta) will have all winter! Photo: A. Hinterthuer

One final thing about life under the ice that Latzka finds fascinating, is that, in winter, a lake’s habitats get a lot less diverse. “In warmer months, the temperature can vary all over the lake,” he says. Water is nice and warm at the surface, still quite cold down deep, and all sorts of temperature in between those two extremes.
Every species can spread out in search of their optimal temperature comfort zone.
But in winter the entire lake is, more or less, the same temperature. Usually 3 or 4 degrees Celsius. And that leads to behavior you won’t see quite so dramatically in summer.
“If you find warm water inlets,” Latzka says, “and let me qualify by saying ‘Be careful walking on ice near warm water inlets!’ But, where you have pulses of warm water, a lot of species of fish are going to want to be in it, because those levels of temperature change, just a couple of degrees, effects how active they’re going to be, how much they can eat and they’ll grow better in those conditions.”
So there you have it – the not-quite-exciting story of life under ice for our fishes. They mostly stay still, hang out in the same spot and, like a lot of us, wait for spring and open water to get back in action.

2 thoughts on “Best of the Blog, 2016: What Do Fish Do Under the Ice?”

  1. Nice post, thanks. I think another interesting effect that wasn’t mentioned is the dietary changes that occur when a lake is iced over. During those cold months insect and crustacean activity virtually comes to a standstill. So species that might predate heavily on these sources must switch to a more fish-based diet. This requires more energy to be expended in chasing down food, at a time when low oxygen levels create a sluggishness in fish.
    And another thought on oxygen…. the “lid” definitely blocks exposure to atmospheric oxygen. But it also greatly reduces light penetration. This slows photosynthesis in both plants and algae and reduces the internal oxygen production.
    All things considered it’s a wonder that any fish survive at all beneath the ice.
    Thanks again, love when fishing gets a little scientific!

  2. “And discoveries like this can help Gabonese officials make more informed decisions, as they seek to develop their natural resources responsibly.” You think so? You think ‘discovery’ of this heretofore scientifically unnamed fish will have any significance in the endless quest for resources from an endless proliferation of a single species’ need for finite resources? Not realistic

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