Understanding why things happen the way they do under an iced-over body of water can help you catch more fish.

   For almost 40 years now, Dave Genz has been a prime mover in the advancement of ice fishing  To anglers throughout the ice belt, he is known for his ability to find and catch fish consistently, and accurately predict fish movements at different periods within the ice fishing season.

   But there is a difference between knowing that something happens and understanding why it happens.  To better understand the dynamics of iced-over bodies of water, Dave enlisted the help of Mac Strand, Ph.D., professor of entomology and aquatic ecology at Northern Michigan University, while writing his book," Bluegills!" The partnership between Dr. Strand and Dave Genz will no doubt lead to a better understanding of the 'whys' of ice fishing, that will benefit all of us who love to chase fish all winter.

   For now, a few selected topics that should make interesting reading, and help you enjoy your ice fishing more this winter.

By Dave Genz

   In the last few years, I've spent a lot of time using as Aqua-Vu underwater camera, lowering the camera down a hole in the ice and just observing things.  Through the use of the camera, I have a much better understanding of what's down there under the ice, and what it looks like.

   But I'd still like to know more about why things happen, so I can better understand why fish hold in certain areas at certain times of the year, why they move, what they're feeding on, and so forth.

   By now, a lot of serious ice anglers know that fish go through environmental changes between the time ice first forms over their heads until it melts in the spring.  Just what the changes are, and how severe the differences are between early and late winter, depends largely on how long and severe the winter is where you live.  In some regions of the so-called 'ice belt,' you may only have fishable ice for a few days or weeks.  In other locations, like my home state of Minnesota, winter hangs on for four or five months.

   Let's look at the various stages of the iced-over-period, and the changes that fish face where winter lingers.

   First Ice-At first ice, oxygen depletion is not a factor, so fish in many ways have the run of the lake.  Now is probably the best time to fish smaller waters, like small natural lakes and ponds.  Fish are often found in the same locations they were in during late fall.

   Depending on the species of fish, and the specific makeup of each body of water, you might find fish in shallow weeds, other shallow flats, inside corners, points, midlake structures or the very deepest basin.  Again, it all depends on the species of fish, its fall locational tendencies, and the food supply for that species.

Decreased Light Penetration-I call this 'the cloudy period.  It's a transition time between first ice and midwinter.  You can recognize this period by an increase in dying and dead weeds and cloudy water.

   The water might have been nice and clear earlier in the winter, but now it's cloudier.  Some people say the water got 'all riled up.' Well what riled it up? Ice is covering the surface, so there is no wind.

   This period begins about the time we get 'driving ice' in Minnesota, and we notice a definite change in the fishing.  The ice is thicker, there's less light penetration, and usually snow cover.  In your area, it might be just thicker ice and snow cover.  Even in the fringe areas of the ice belt where it can get warm enough to melt and thaw without losing all the ice the ice is not clear after a while.  Sometimes, it floods and refreezes and that makes it milky, and there's less light penetration.

   But regardless of how clear the ice is, and how much sunlight penetrates, many underwater plants have annual cycles governed by day length.  So it's actually the shortening daylight hours (photoperiod) that cause ,amu weeds to into something of a survival mode, and sort of 'mark time' until spring.  Still, this noticeable decrease in light penetration through the ice is probably the 'kicker' that causes certain weeds to brown up and start to decompose.  When plants die off or when phytoplankton population growth declines, a feeding frenzy by decomposing bacteria follows.  It becomes a cycle that causes dropping oxygen levels, as the population levels, and activity, among these 'primary decomposers' grows.

   It's interesting that there are huge, and roughly equal, populations of decomposer bacteria in all lakes, regardless of how clear, dirty, fertile, or infertile the water.  The differences become noticeable only based on how many are active at any give time in any give body of water.

   The scientists who study the workings of a lake under the ice speculate on various reasons for this clouding of the water.  One likely cause: dead and dying algae and weed particles suspended in the water, being attacked and consumed by bacteria and fungi.

   But there are other known causes.

Sometimes, phytoplankton deplete dissolved carbon dioxide levels to a point that a calcium carbonate precipitate is produced.  This phenomenon is called "lake whitening" because the water takes on a milky appearance.  Fine sediments suspended in the water can also produce a 'cloudiness' and so can large number of microscopic organisms.

   So it is the increase in activity level of decomposer bacteria (spurred by an increase in available dead and dying plants) that depletes oxygen levels.  Oxygen levels can get too low for most fishes, and can be the reason many fish vacate shallow, weedy area places that may have housed good fishing in previous weeks.

   Bottom line: It's common to see many fish species move into deeper, cleaner, clearer water once the cloudy period sets in.

   Midwinter Fishing gets tougher.  Many species are often in deeper, more oxygenated water.  I think these fish, in a lot of waters, quit feeding for a time.  I have a theory that it gets to a point where there just isn't much food in some lakes during the 'dead' of the midwinter period, and the fish go into a semi-hibernation state.  They don't move, and they don't exert energy.

   Where good fishing existed earlier in the winter, it can seem like no fish exist at all.  Being cold-blooded, some fish may not have to take in any food for a time as long as they remain dormant, a condition biologists refer to as a torpor.  When their body temperature goes down, fish don't die, bit perhaps they sense there is very little food available.

   (It's probably true, though, that for some of the cold-water fish; lake trout, big northern pike the winter, in general, may be their most active time.)

   This midwinter period is the time when larger lakes generally produce better than smaller ones.  It's not the best time of year to fish a large lake, necessarily, but it can be the time to choose a large lake over a small one.  But don't hit the large lake and go straight to where the crowd has been since first ice.  Those fish have been tapped: they've gone home in buckets.  Search out your own fish, at this time more than any other period.

   One alternative strategy that I've been using in recent years is to chase the early-ice period by traveling to other states.  Simply going south from my home base in Minnesota.  I can get to early-ice conditions after the doldrums hit.  Similarly, you can follow late-ice conditions north as they occur, as ice melts on the southern fringes of ice county.

   Late Ice Shiny wetness appears around a branch or leaf lying on the ice surface.  Snow starts melting, creeks start running, water collects on top of the ice and runs back down your hole when you drill it.

   Runoff has begun, and many fish (especially bluegills) often return to the shallows that they vacated during the cloudy period or midwinter.

   In shallow water, plants start growing again, in response to increasing day- length.  They kick out oxygen during the daylight hours, which makes the shallows hospitable again.  Insects, also in response to the lengthening photoperiod, show an impressive increase in activity.

At this time, it's common to see weeds growing under the ice.  An Aqua-Vu camera can be a great tool, because the water can still be cloudy, but you can identify green weeds growing right among decaying weeds.  The weed growing shallows can be like a flower bed in spring, where all these dead plants are surrounded by the new sprouts pushing up through and competing for the sunlight.  If you find this situation, it's a good bet some fish have found it too.

   Generally speaking, late ice is a time when many species become oriented toward shorelines and feeder creeks, or any other spots that have current, staging for the spring spawn.  Some species, in fact, spawn before the ice even melts.

  I'm going to be doing a lot more studying of lake dynamics in the coming years, as I strive to learn more of the "whys' when it comes to ice fishing.  I have some theories on underwater currents, moon cycles, oxygen levels, stuff like that.  I'll keep you posted as new information becomes available.

First printed in 2000