A newly frozen lake shimmers like a mirror in the morning sun. Up close, that mirror becomes a window to the world below. I enjoy sliding across the new surface above stands of cabbage and watching fish dart from my shadow or hoofprint on their ceiling.
This thin layer of ice seals the watery world below from the onset of tempestuous winter. Below the ice, however, the environment is dynamic. New chemical and biological processes proceed, affecting fish in many ways.
Temperature gradients, currents, and oxygen levels position baitfish, predators, and the far smaller organisms that base the entire food web. As snow packs the surface and the ice layer grows, the world below becomes more difficult to decipher. Sonar and underwater cameras provide insights while fishing results verify or refute the countless theories we propose.
In the North Country, the ice fishing season can be divided into several periods that extend from first ice to ice-out. These ice stages begin after fall turnover, when cold, low-oxygen water from the bottom is mixed with surface water. The lake soon becomes uniform in temperature, oxygen content, and water quality. As lakes continue to chill, water temperature falls to 39°F.
If windy weather prevails, lakes cool below 39°F without freezing. But at that temperature, a cold, calm night can cause ice-over from bank to bank if no currents interfere. The stable conditions that existed after turnover are sealed. Without wind, particles settle and lakes often reach maximum clarity in the few weeks after ice-up.
During this early period of stability, fish are free to roam shallow and deep. Structure, cover, current, light, and prey availability typically determine the location of each species and size class of fish. In our experiences and according to ice fishing lore, first ice brings a good bite. The first anglers daring enough to reach key spots often make great catches. The reasons for a good bite aren't entirely clear. Several factors seem to contribute.
First, early ice often sustains what had been a good fall bite, while fishing becomes easier without boat control hassles from wind and waves. Before ice cover forms, walleyes, panfish, and pike typically have established feeding regimes that continue after ice-up. Walleyes hold on major structural elements like humps and offshore bars, feeding most actively at dusk in clear lakes. Panfish patterns vary, some along weedlines in more fertile waters, otherwise dropping into deep basins in other waters.
Key for pike, good green vegetation remains on productive flats well into the ice season. Later, weeds wither as light levels are reduced. Favorite late-fall waypoints often produce once ice forms.
Second, ice is thin, so drilling lots of holes to cover water is easy. Limited snow cover facilitates walking. Soon enough, anglers can make use of 4-wheelers. Temperatures also usually are milder prior to the deep freeze of January and February, making fishing more pleasant and efficient as well. Finally, abundant light and oxygen keep fish relatively frisky, although warmwater species move and feed in slow motion.
As soon as a solid ice layer forms, thermal stratification again affects a lake. Water is densest (heaviest) at 39°F. Ice initially forms at 32°F and is at its lightest in this state. Meanwhile, heavy water sinks. Cold air temperatures grow the ice layer, which in turn cools water below it. At the ice-water interface, water is barely above 32°. In moderate depths, it remains in the mid-30°F range. In a layer on the bottom lies a zone of 39° water, the warmest available. Density differences maintain this stratification unless currents causing mixing.
As winter progresses, ice thickens and snow shrouds the underwater world. Plants decay and go into senescence, no longer producing oxygen. This decay, along with respiration of zooplankton, invertebrates, and fish gradually reduce dissolved oxygen levels. Healthy vegetation and phytoplankton produce oxygen but their activity is light-dependant, and it's reduced as ice and snow build. In oligotrophic lakes, changes in oxygen levels aren't substantial enough to shift fish location.
Fishing action for walleyes, pike, crappies, and sunfish typically declines as the midwinter period progresses. Fishing pressure and harvest take a toll on popular lakes and popular spots within those lakes, evidenced by the appearance of "Swiss cheese." Fish also tend to roam more as prey numbers fall and habitat quality declines. They're harder to locate and often less inclined to bite.
Heavy snow presses down the ice sheet, causing water to seep from ice holes and heaves. Snowy, slushy conditions that bog trucks, clog snowmobile treads, and make walking a strain bring a drop in fishing pressure at this time.
For fish, habitat is reduced as well. The upper portions of the lake are frozen and water directly below frigid. The bottom layer may also be inhospitable. We call this situation "the big squeeze" as fish are constricted into the middle zone of a lake.
Limit catches of walleyes become tough to attain, and average size of panfish often shrinks. Big fish are available but odds of catching them drop. In regions with only a couple month of ice cover, the midwinter slump is less obvious. At the southern range of the ice belt, this may offer the only ice fishing of the year.
If thick ice and snow linger in late winter, water quality in many lakes worsens. As the layer of oxygen-depleted water increases along the bottom of fertile lakes, fish are pushed upward. In fertile lakes, fish stop feeding when environmental conditions become severe.
In extreme cases, fish are forced to nuzzle up against the bottom of the ice pack, actually forming domes and grooves as they move or fin in place. If there isn't relief, fish kills result. Some productive, shallow fisheries in the North Country teeter on the edge of winterkill. Every decade or so, loss of most fish is followed by fast growth for the remaining populations.
In deeper, less fertile lakes, late ice brings something of a rejuvenation of ice fishing. Walleyes and pike often feed more actively toward the end of winter. Indeed, this period signals the onset of prespawn feeding for these early spawning species.
As daytime temperatures rise above freezing, snow and ice melt from the top of the ice pack. As it melts, the reverse of ice-up occurs. Water seeps into the ice, pockmarking it. And as the land warms, it thaws the ice pack from the shore outward, making travel tricky. Banks exposed the sun thaw even faster. As the top of the ice melts, the entire floe rises since it's floating on warmer water below.
At the end of the season, die-hard anglers put planks across rotting shoreline ice to reach the remaining hard ice. To do this, you must aware of ice conditions that can change by the hour and avoid unsafe situations.
Underwater, greater light penetration and inflowing water reoxygenate bays and seem to stimulate a new boom in plankton. This activity attracts minnows and then panfish, followed by bass and pike that return to shallower spots they'd vacated in late January.
Anglers make great catches on shallow weedy flats, at times fishing just below the ice. Action can be fast as you watch crappies and bluegills check out a presentation or dart into view as they strike. Panfish often scatter as a big pike cruises by, perhaps en route to a spawning marsh. This short period of fast fishing is followed shortly by ice-outanother ice season is finished.