Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

A fish under the ice. © Terry Margenau

To increase a fish's chance of survival when released, minimize handling and limit its exposure to cold air.
© Terry Margenau

December 2009

A cold world with an icy ceiling

Ice covers the fishes' world for more than four months a year. How do they adapt?

Alisa Santiesteban

We all have our comfort zones when it comes to temperature. Especially during Wisconsin's long winters, we really have no choice: You have to figure out how to wrap up and stay comfortable. You can fight the mental game and just bear it, or embrace the weather as the season brings on the cold stuff.

We've adapted to spending time outdoors when the daylight is short, the temps drop and the winds are stiff. Skiing, sledding, ice fishing and a host of other cold-friendly activities are fun on a bright day. Yet, just as we enjoy our time outdoors, we take solace in the fact that we can always head inside and warm up somewhere. Not everything is so lucky.

Once lakes freeze over, fish have to make due with colder temperatures. Though it sounds like a horrible fate, remember that the piscean world generally stays within a 40 degree temperature range year-round. Unless they freeze-out, lakes don't drop below freezing and they are rarely warmer than the mid-seventies. Wisconsin's waters are wonderful habitats, and fish have strategies to survive even some of the harshest conditions.

So where do the fish go when the temperature plummets and ice blankets the water? Most don't go anywhere...they just adapt. Nature has guided them through this annual cycle for thousands of years, and they are prepared to spend a long winter in lower light at lower temperatures.

The frigid wonderland beneath the ice is not well explored, but biologists do understand the importance of metabolism, food and oxygen: key components in underwater winter survival.

Activity and food go hand in hand

During early winter, the lakes have turned over, the water holds plenty of oxygen, food has been stirred up and fish are active. Even though the water doesn't drop below freezing, as winter progresses food becomes more scarce, the waters are still, light penetration drops off and oxygen levels begin to drop too. Fish have learned to adapt to survive.

Fish are poikilotherms, that is, "coldblooded," and can modify their metabolism to the environment.

"To conserve their energy and lower their needs for food and oxygen, most fish decrease their activity during the winter months," says DNR Fisheries Supervisor Terry Margenau.

During winter, anglers can see some of those changes. For instance, anglers are more likely to find food in the stomachs of the fish they catch compared to other times of the year, Margenau observes.

"When water temperatures are around 75 degrees F in the summer, it may only take a fish 24 hours to digest its prey," he says. "But in winter, the same food item may take an entire week to digest."

Fish are often divided into three groups: coldwater, coolwater and warmwater species that thrive in different temperature ranges.

"When it comes to the winter, different fish choose different activity levels, just like people," laughs Margenau. "Some stay somewhat active, some get more sluggish, and others just hunker down."

Fish such as lake trout, whitefish and brown trout are coldwater fish at heart that remain fairly active throughout the winter months. The onset of winter brings few changes for them. In fact, the cooling water during winter months allows such species to expand their range, venturing into habitats that are normally off-limits during summer months when water temperatures rise and oxygen levels may drop off.

Walleye, northern pike and panfish also adapt well to cooler water and keep feeding at the onset of winter. In fact, they feed heartily during first ice as protective weed cover for their prey dies back and small prey fish head for the shallower, relatively warmer waters. Other species, like bass and muskies become more sluggish and only feed to meet base maintenance needs in winter.

Some fish need even less food energy to get by and really restrict their movements in winter. Bullheads tend to pass the winter buried in the sand. Carp become dormant and some catfish take on a state of near hibernation barely moving, slowing their respiration rate and rarely feeding.

"We've tracked seasonal flathead catfish movements with sonic tags and radio tags, says DNR fisheries biologist Al Niebur. "Both techniques show that once flatheads settle into their winter habitats, they move very little. "I would definitely characterize their behavior during the winter as a type of hibernation," Niebur says.

When DNR fish researchers first started sampling flatheads by SCUBA diving, the researchers thought they could release tagged fish at the surface and the big cats would be active enough to swim the short distance back to the safety of their winter group, but the researchers soon found the fish were so sluggish that surface releases would not work.

"We quickly found out from our tagging studies that these fish could not swim back down to the bottom against the current. They usually ended up in areas significantly downstream of their original wintering site," says Niebur. "We now have divers take all the fish we tag down to the bottom of the river pool before releasing them."

It is interesting to note that the closely related channel catfish are more active in winter, keep feeding and are often caught by anglers through the ice. Go figure.

When it comes to food in winter, most fish are fairly opportunistic according to Margenau.

"Take northern pike. Panfish are not a preferred prey food for pike during most of the year because they are tough to swallow, are a big size, and are very effective at using weed beds for cover. Yet, when you're cleaning pike that are caught in the winter, you will often find they have eaten bluegill as a mainstay of their diet," Margenau says. Consider that during summer months when aquatic plant growth is at its maximum, panfish have lots of places to hide. When the seasonal vegetation dies back in late fall, this hiding cover is greatly reduced. It's a perfect advantage for a coolwater predator like a pike. They're not picky during the cold months."

Photo of a northern underwater © Terry Margenau
A northern cruises past a fish crib looking for an easy meal.
© Terry Margenau

Most fish won't expend too much energy to find food in winter. They can't afford to spend much time or effort chasing down other fish to fill their bellies. The trade-off is that since food resources are limited, growth rates during the cold months slow way down. If food supplies are really low, this poor diet can result in stunting where the fishes' normal growth rates are retarded.

Layin' low

So what habitat do fish look for to protect them during more sedentary periods over winter? Places that can best meet their needs – as warm as practical, as much oxygen and food as possible, protected from predators and as much light as is plausible.

First, a short chemistry lesson. During summer, on my many swimming trips to our local lakes, I know that generally the deeper I dive, the colder the water gets. During winter there's a bizarre phenomenon that the frigid water actually gets relatively warmer the deeper you descend in a lake. The reason has to do with how water molecules arrange. As water cools, the molecules sink and pack tighter and tighter together (increasing density) until the temperature drops to 39 degrees F. At that point, the water molecules are heavier than the water at the surface (around 32 degrees F), causing a topsy-turvy effect where warmer water is at the bottom.

This oddity is important to our finned friends beneath the ice. Once shallow waters near shore freeze, many fish will stay together and high-tail it to deeper, warmer areas of the lake unless they are top-of-the-food chain predators. Some fish, like bullheads, even take it a little bit further and burrow in the mud or gravel for a little extra warmth.

For instance, hibernating flathead catfish congregate in close proximity to each other and to large riprap or wood structures for protection, according to Niebur. However, in waters with big open areas and little cover, fish just hunker down in the deeper areas of the lake or deeper channels in rivers.

If you know where fish cribs or areas of vegetation are located in a lake, there's a good chance you'll find fish there in the winter. If you're unsure of these areas, look for a cluster of tip-ups, bored ice holes and ice shanties. Avid ice anglers often know best and they leave clues where they are finding fish.

"Several fish species will hang around fish cribs and any remaining green vegetation for good reasons," says Margenau – to eat and not be eaten.

"Bluegills for example, use cribs for protection from predators like northern pike. The pike use these areas to find food like bluegill. And anglers," adds Margenau, "use these areas to find both!"

Fish wintering in rivers face the additional challenge of unrelenting currents. Trout will often give up the fast-flowing riffles and runs they inhabit in the summer and move downstream seeking out deeper, quieter pool habitat in the lower reaches of streams.

In the Mississippi River, bluegill will move into the backwaters where they find the quieter waters more suitable. Walleye, sauger and pike prefer to stay in the same places they occupy in fall and spring, but they move to the deeper rock areas that provide cover, escape and the chance to find congregating forage fish. Smallmouth bass may be found in the main channel or in backwaters but also take refuge behind warmer wing dams or closer to shore. Each fish has slightly different preferences but knows what's best for it.

In search of oxygen

As winter presses on, ice thickens, snow builds up, and diminishing supplies of oxygen are continually consumed below the ice. Low oxygen levels test fish and can leave them vulnerable. Shallow lakes with abundant vegetation are at especially high risk for dropping dissolved oxygen levels.

"Some fish are more sensitive to low oxygen than others, though the reasons for that aren't well known," says DNR fish ecologist Paul Cunningham. "Low oxygen is most unforgiving for fish like bass, trout and bluegill. Northern pike, walleye, crappie and yellow perch have added tolerance, and fish like fathead minnows and bullheads come out on top in their ability to handle the lowest amounts of oxygen."

One of the most successful species in low-oxygen conditions is the mud minnow because it can take in air through its gills and also breathe air directly as needed to survive. John Magnuson, director emeritus of the Center for Limnology at the University of Wisconsin-Madison, observed that mud minnows breathe oxygen trapped in bubbles just beneath the ice.

Fish under low-oxygen conditions move upward in the water column searching for more oxygen-rich water, or they migrate to oxygenated inlets to get through the harshest months, he explained. When fish become stressed from low oxygen, their activity levels really drop off and feeding basically shuts down until conditions improve.

If these low-oxygen conditions persist for a period of time, winterkill or "freeze-out" may occur, killing some or all of the fish, says Cunningham. "Partial winterkills are expected every year, but we haven't seen severe winterkills for more than 20 years," he notes. "In some waters, partial winterkill is just a natural and beneficial process that results in faster growth rates for the survivors."

Hundreds of Wisconsin's lakes that are more prone to severe winterkill are equipped with aeration systems that sustain open water during winter allowing oxygen exchange and uptake. Artificial aeration is an expensive, energy-intensive option.

Limit how long your catch is out of the water

You would want to be in the frigid waters in winter about as much as a fish wants to be exposed to the air. So it's best to try to keep the fish in the water as much as you can if you intend to return your ice-fishing catch to the water.

"At any temperature colder than freezing, a fish's gills are very susceptible to freezing when exposed to air," says Sue Marcquenski, DNR fish health specialist. "It is best to keep the fish in the water as much as possible while removing the hook."

When temperatures start running below 10 degrees F, a fish's eyes are also at risk of being damaged.

"When it's really cold, you might notice the fish's eyes become opaque when they're out of the water," says Marcquenski. "It's possible that the cornea has frozen and although the eyes clear up when the fish is put back in the water, the cornea or lens might have been damaged. Fungus or bacteria in the water can also infect the damaged cornea, perhaps leading to blindness.

Along the same lines, it's a common misperception that cold water reduces the likelihood of infections in fish, so it's important to be careful when handling fish.

"Cold temperatures don't prevent infections, they simply slow the infection down because the fish's metabolism is so much slower," says Margenau. "If a fish gets an internal or external injury, it may subsist throughout the winter, but when spring weather comes and the waters warm up, the infection gets worse and can be lethal."

Additionally, ice anglers fishing in water deeper than 25 feet should take care to pull hooked fish up slowly, to lower the risk of enlarging gas (swim) bladders. The gas bladder contributes to a fish's buoyancy control that allows it to suspend at a favorable water depth without having to swim too hard. If a fish is brought up too quickly, the bladder can rapidly expand causing the stomach to be pushed into the fish's throat. It can be difficult for these fish to survive if released, though unhooking the fish quickly and helping it upright itself in the water will increase its chances of adjusting the bladder on its own.

In summary, fish are hardy and adopt many different strategies for making it through the cold months. Those species that can't make it through the winter weather disappeared here long ago. Just like their human counterparts, the hardy survivors below the water adapted to the broad range of temperatures they are sure to experience in the upper Midwest.

Alisa Santiesteban writes for DNR's Fisheries Management and Habitat program.

Winter fish phenomena
  • Drunken Fish – After first-ice and before snow cover has had a chance to block out the sun, a maximum amount of light reaches the green plants below that photosynthesize and trap oxygen below the surface. Just below the ice, you may see fish swimming on their sides appearing to be lightheaded because of too much oxygen. Anglers often report seeing these "drunken fish."
  • Early Spawners – Most Wisconsin fish spawn in spring, summer or fall, but the burbot prefers the middle of winter, under the ice. It is Wisconsin's earliest spawner. Spawning usually takes place at night and by daytime the spawning grounds are deserted. Surface water temperatures are close to freezing, there is no nest, and the burbot provide no care to the eggs. Burbot (also called lawyer, eelpout and lingcod) are members of the freshwater cod family.
  • Anchor Ice – Occasionally when it is extremely cold, fast-flowing rivers can form ice on the bottom, called anchor ice. Sometimes when a stream is on the verge of freezing, ice starts to form on the stream bed. This can be very damaging, freezing to death trout eggs, developing trout buried in the gravel for a long winter's rest, and invertebrates that live in the stream beds and in the small spaces between gravel and rocks.