Water Temperatures Under Ice, The Ghost Factor

Every ice angler experiences it, sooner or later: A "guaranteed spot" that crawls with fish every winter suddenly fails to produce. Such events seem especially prevalent during extreme winters, like the one we experienced last year.

The winter of 2013-14 produced thicker ice, heavier snowfall, and colder average temperatures than normal. Cleveland, Ohio, experienced more sub-zero days than during any winter for over 30 years. That was the bad news. The good news was ice fishing on Lake Erie was safer and lasted longer than it has in decades. "But we found walleyes in the same spots where we typically find them on Erie," said Craig Lewis, owner of Erie Outfitters. "The depths they used were pretty typical."

Temperature layers in the Great Lakes and other large lakes, it seems, are less likely to be affected by changing conditions above the ice. But how did the toughest winter weather in a third of a century affect fish location in other areas? In Minnesota and Wisconsin, where fish had the option, we found them deeper than what we consider "normal" on lake after lake. Where we'd been catching them at 17 to 24 feet during January for many years, we were finding walleyes 30 to 35 feet down. Some of the best anglers in the region concurred.

"Walleyes were a lot deeper last year," says Minnesota guide Brian Brosdahl. "I thought there must have been a temperature gradient acting like a barrier keeping them off their normal spots. Extended periods of frigid weather can have a dramatic effect on location. Last year we had lots of slush early, due to heavy snow. When that froze, it seemed to provide a lot less insulation against the Arctic air that reigned most of the season." 

Temperature isn't the only factor. Higher snowfall and thicker ice cover can affect fish location. Where baitfish find the most plankton or insects—and where they find the most stability and comfort—might be the most important factors when trying to locate active predators. But the ghost factor might be water temperature. It probably influences those other critical components of a working pattern.

How many ice anglers bother to observe water temperature at depth? Most perceive conditions under the ice to be pretty homogeneous, not only month-to-month, but year-to-year. But it isn't so. Temperature bands move up and down through the water column all winter, expanding and contracting, sometimes to startling extents.

The first to point it out to me was Pat Smith, an intuitive and versatile angler working with Thorne Brothers at the time. Smith is fond of finding completely overlooked walleyes in water shallower than 10 feet under the ice. "When I say shallow, I mean 3 to 8 feet, where weeds stay greenest," he said. "I find these spots in the summertime. Night spots in summer become awesome shallow spots in winter. The key is finding areas where deep water bends in close to shore. But winter weather can make or break the fishing there. Heavy snow cover can improve the shallow bite, but extremely cold weather can push frigid 33°F water down to the bottom in 8 to 10 feet of water, driving walleyes out of there. When I find 36°F or warmer water on bottom in 10 feet of water, I know walleyes will push even shallower on those key spots."

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Measuring Chances

Intrigued ever since by the concept of monitoring temperatures under the ice, last winter I decided to monitor this ghost factor haunting our efforts—to find fish. Electronics companies reflect a sudden interest in downtemp on the part of the angling public. Several underwater cameras designed for ice fishingthe Aqua-Vu 760CZ , Vexilar FSDVO DT Double Vision, and Marcum VS6255D and VS8255D—include temperature monitors in the camera. They send temperature readings back to the camera monitor for constant updates while viewing.

For the "less is more" crowd—those who want to carry less equipment—several handheld units serve nicely. The Vexilar Deptherm is a basic and dependable tool—a simple mercury thermometer inside a glass tube that allows water to enter by increments as pressure increases to measure depth. Or you can attach it to a measured line. Drop it down 5 feet, hold it there for 30 seconds, and retrieve. It maintains the temperature reading for about two minutes. Make a note of the data, drop it down 10 feet, and repeat the process until you have readings for every 5 feet down to bottom.

The Fish Hawk TD is a digital unit that saves a little time. It's about half the size of a cell phone, so it fits in a pocket (an inner pocket next to your body is the best place to carry it). Attach it to some heavy line (I used 36-pound Cortland Catfish Line), hit the "Start" button, wait for the "Ready" signal, and drop it straight to bottom. It automatically records the temperature every 5 feet all the way down to 300 feet. In-Fisherman Senior Editor Steve Quinn has used the ClineFinder from Catalina Technologies. "It's waterproof and durable," he says, "with a 50-foot cable to the digital thermometer."

Using these units last winter, we commonly found temperatures of 34°F to 35°F down as far as 25 feet after severe cold snaps. But it wasn't uncommon to read temperatures of 37°F to 38°F on the same spots after two or three days of unseasonably warm weather. I had no idea temperature regimes could change so quickly with the surface immobilized and covered by a static amount of snow and ice. 

Every species reacted differently. As a general rule, crappies moved deeper, from "normal" haunts in 12- to 17-foot depths to areas 21 to 26 feet— down. They either stopped biting or moved deeper (where possible) when faced with water 35°F or colder. Rainbow trout didn't seem to be affected by temperature—they went where the food was. That was the most dramatic contrast we foundcrappies being warmwater fish while trout are coldwater species.

Walleyes in one lake kept biting at about 22 feet for over a month, even when temperatures varied by about 4°F. But the bite wasn't the same at 34°F as it was at 38°F. Not even close. In warmer water (always accompanied by warmer air temperatures and stable weather), walleyes tended— to move higher off bottom and strike more aggressively. In colder water, they held closer to bottom and exhibited smaller strike zones. Size of the lure didn't seem to matter as much as the aggressiveness of the presentation, but subtle seemed betterespecially after several nights in the -10°F to -20F range.

Bluegills concentrate around remaining green weeds all winter, especially cabbage. For about a decade, I've been fortunate enough to find bulls grazin' in the grass all winter in depths of 8 to 12 feet on a number of local lakes. Last year, even where the cabbage was still green and healthy at those depths (which became rare by late winter), bluegills were nowhere to be found. We had to hunt 5 to 15 feet deeper in all cases. Temperature may not have been the reason for the change, but we noted that temperatures typically ranged from 33°F to 35°F in the "usual" spots and we rarely found bluegills biting in water colder than 35°F.

As hard as we worked at it, this is a small pool of data to draw conclusions from. Next winter we might find bluegills biting like crazy in 33°F water. But I doubt it. One laboratory analysis I've often quoted observed that crappies begin to lose motor function at 35°F. At 33°F, they lose control altogether and begin suspending upside down, sideways, and at other unnatural angles. Makes it difficult for crappies to capture prey at that point. Bluegills did seem more resilient than crappies, however, when frigid air pushed colder bands of water toward bottom.

After one winter of monitoring temperature, the most important aspects gradually come into focus. As with open-water fishing, temperature may or may not determine location or behavior under the ice. But it can be a good indicator.

Visualizing Change

When air temperatures drop to near or below 0°F, the coldest temperature bands near the ice expand downward. The 33°F band, normally restricted to within a few feet of the ice, extends down to 5 feet or more, and the 34°F band may extend to 10 feet or deeper, depending on the duration of the cold snap. If air temperatures rise and become unusually warm, those cold bands of temperature recede back toward the ice. Anyone can measure these phenomena, relate it to fish location, and log it in a journal for future reference.

Many factors can make responses to water temperature in winter seem trivial or dramatic. Fish have more choices in deep, structurally diverse lakes than in shallow dish-shaped lakes. Genetic makeup of local baitfish and gamefish populations could make them more tolerant or more sensitive to variations in temperature. Springs and groundwater might negate the influence of frigid temperatures. The needs of baitfish and invertebrates ultimately determine location of their predators, based on availability of certain substrates, cover, or structural types. If those options are widely distributed, fish are more likely to move for reasons involving comfort.

With that in mind, correlating temperature changes with location in winter might ultimately allow us to plan on certain responses. We can look at weather history and weather predictions and begin to visualize responses fish might be making before venturing out. And when we can't find fish in the usual spots, temperature might indicate where to hunt next so it's another tool in the location box.

When you return to spots often enough to establish a history of temperatures under the ice, you begin to realize that some spots are more stable than others. Areas exposed to more current (most lakes have sub-surface currents) tend to be less stable, and drastic temperature changes can result. Current can be good or bad, and temperature can be a deciding factor.

Finding a key spot is easier than understanding why it suddenly becomes an empty stage. Temperature might be forming unseen gates and barriers—invisible structure. It can be a ghost factor haunting your ice-fishing patterns.