We don’t see it because they’re under water, but fish need oxygen, too.
Granted, they get it differently than us humans but just as sunlight and oxygen are critical for people, it’s critical for fish.
One problem snowy states like North Dakota face is that while it melts to provide moisture and water to maintain lake levels, it can also limit how much sunlight penetrates the ice, allowing underwater plants to produce oxygen through photosynthesis.
Lack of sunlight is also detrimental to aquatic vegetation, causing it to die, decompose, and reduce the amount of oxygen in a lake, described North Dakota Game and Fish Department Fisheries Management Leader Scott Gangl.
While the air humans breathe has about 200,000 parts per million of oxygen, water concentration is typically about 4 to 8 parts per million (PPM) of oxygen, according to the Ocean Conservancy.
Different fish species require different levels of oxygen dissolved in water, measured in parts per million, for breathing. For example, rainbow trout have an optimum level of about 5 PPM while walleye and perch do better when dissolved oxygen levels remain above 2 or 3 PPM. Fish can become lethargic with low oxygen levels. Bullheads, however, can handle levels below 2 PPM.
Oxygen levels below 1 PPM are lethal for most game fish, Gangl continued.
Heavy snow blocks sunlight, causing aquatic vegetation to die and decompose. That can create lower dissolved oxygen levels and possible winter fish kills. It’s even more problematic is snow occurs early in and stays throughout the winter.
North Dakota lakes can experience winter fish kills on any given year, Gangl added. Shallow lake depth can also factor into potential mortality.
Game and Fish Department fisheries biologists have conducted winter dissolved oxygen samplings across various lakes since the 1950s, typically after mid-February “when (oxygen) conditions are apt to be at their worst,” Gangl continued. “They try to select the deepest spot on the lake and measure oxygen levels in 1 meter increments to get a profile of oxygen throughout the water column.”
Oxygen levels tend to be lowest at the bottom of a lake as decomposed matter settles. If oxygen levels are around 1 to 2 PPM biologists may re-sample to help determine the extent of oxygen depletion.
The information helps fisheries managers determine spring plans for potential fish stocking or possibly transporting catchable-sized fish into the lake, if necessary. They also use the information to help them determine how soon an impacted lake can be brought back as a fishable lake again.
In some instances of major fish losses, anglers might drill a hole while ice-fishing, only to have dead minnows come up as they pull up an auger.
That lake likely has a problem because minnows tend to survive in lower dissolved oxygen levels than other species.
Another indication is the distinct aroma of rotten eggs, which occurs when hydrogen sulfide gas is produced because oxygen levels are at zero
Sometimes it takes ice-out to determine if a lake has winter kill. Numerous dead fish along the shoreline in the spring also means a fish kill.
Lakes with partial or complete fish kill become a priority for fisheries managers when they factor their available catchable-sized fish and smaller fingerlings for spring stocking.
The Game and Fish Department manages more than 400 water bodies across the state yet it is one of the nation’s smallest fish and wildlife management agencies in the country in terms of number of total employees. With limited staff it’s not always possible to know what lakes may have some degree of winter (or summer, for that matter) fish kill.
Gangl suggested anglers contact the Game and Fish Department if they find large numbers of dead fish –or minnows – when they drill a hole on their late season ice-fishing excursion.
Let them know if a bunch of dead fish line the shoreline of one’s early season open water fishing lake.
Anglers can stay abreast of how their favorite lake – or a new one they might want to check out – fared this winter on the updated “Where to Fish” app on the Game and Fish Department website.
Biologists update the status of any lakes with known winter fish kill when data becomes available, Gangl described.