A lack of snow cover on area lakes is a primary reason for improved dissolved oxygen content this winter. That's the opinion of fisheries biologists who annually conduct dissolved oxygen tests and compile data from lakes and rivers throughout the state.
There's something else this year that has caught the attention of biologists and anglers alike, the thickness of the ice on many lakes. Fishermen on Lake Audubon are reporting 3 feet or more of ice on many areas of a lake known for its variable ice pack. While ice is often expected to reach up to 2 feet thick during a typical North Dakota winter, this year's ice pack has gone well beyond the 2-foot mark at many locations during one of the coldest winters on record.
"We checked oxygen over two weeks ago and the guys said the ice ranged from 31 to 35 inches on Audubon," said Jason Lee, North Dakota Game and Fish Department fisheries biologist, Riverdale.
Ice on some North Dakota lakes this winter is among the thickest ever experienced by many ice fishermen. These ice houses were located at the Van Hook arm of Lake Sakakawea this past week.
Randy Hiltner, NDGFD fisheries biologist at Devils Lake, said crews conducting dissolved oxygen sampling in his district encountered up to 40 inches of ice on some lakes in the northern part of the state. Ice thickness can vary from one area of a lake to another, and certainly from lake to lake, but this winter's cold has let to an unusually thick ice pack on many lakes.
"Maybe thicker than last year because last year we had snow that insulated the ice," said Hiltner. "I know I need an extension on my ice auger and I think I've only needed that one other time in the last 20 years."
A similar situation exists at Lake Darling where the ice thickness has reached 35 to 40 inches this winter. Longtime Upper Souris National Wildlife Refuge employee Duane Anderson says the last time he can recall the ice being so thick at Lake Darling was in the mid-1980s.
"I don't really remember anything like that since that time. We probably had 40 inches of ice then," said Anderson. "There's a lot of ice now. People on bare ice might make it through with a regular auger of 36 inches or so. Those in ice houses need an extension. I'm saying there is 35 to 40 inches out there."
Many ice fishermen have had to add extensions to their augers, but even then some augers are being pushed all the way down to the gear box to drill through thick ice. Cold temperatures, such as those in the minus 20 range reached last weekend, furthered the ice making process. Still, even with a very thick ice pack, dissolved oxygen content in area lakes has been relatively high.
"A possible explanation is less snow pack, that's probably the primary variable," said Hiltner.
A covering of snow limits the amount of sunlight than can penetrate ice and be utilized by plants under the ice. Too much snow leads to too little sunlight, then often to a die-off of plants. When plants die they consume oxygen, sometimes causing fish kills.
"The difference this year is that we just don't have much snow," explained Lee. "There are scattered clear spots on lakes so the sun is able to penetrate through the ice."
Lee says he can't recall seeing ice as thick as it is this winter for several years. Yes, despite the thick ice, lakes in his North-Central District generally have good to very good dissolved oxygen content.
"They are pretty good for the most part," said Lee. "Rice Lake was good. Crooked Lake looked good. Brush Lake was good. Audubon always looks good."
Rice Lake, Crooked Lake and Brush Lake have all suffered from winter kills in years past. An exception to the favorable oxygen readings this winter, said Lee, was Carbury Dam which has a history of winter kills. The dissolved oxygen reading from that impoundment was a scant 0.2 parts per million, far too little to support fish.
"We have a lot of trouble with that one," noted Lee.
Dissolved oxygen numbers in Northeast District lakes were also better than in experienced in many previous years. Other than lakes that have a history of dissolved oxygen problems, most were in very good shape.
"No surprises," said Hiltner. "Typically the readings have been much better than our readings last spring. If we don't get a huge dump of snow and a winter that goes into June, I'm not expecting too many fish kills in the northeast. Even at Lake Irvin, where it is not very deep, there's no oxygen problems. So far, so good."
Hiltner emphasizes that an unknown of amount of winter remains and that lake oxygen conditions can change before breakup. One of the state's most visited fishing lakes, Devils Lake, is in Hiltner's region. Devils Lake experienced a fish kill in some shallow backwater last year and it could happen again but, historically, the lake proper experiences little trouble with oxygen levels.
"It has actually been a very good year for perch fishing," said Hiltner. "The perch have been fairly deep, 40 to 50 feet, but overall it has been a much better perch bite than in recent years."
Devils Lake experienced a very late ice-out in 2013 with the lake not being declared ice free until mid-May. Ice-out on Lake Sakakawea in 2013 was officially recorded May 13, the third latest ice-free date in the history of the reservoir.