Lake water conditions: winter-kill and turnover
| Home Waters |
By Jack Simpson
As the local lakes are just now beginning to open their ice surfaces, I thought it would be timely to present a short explanation of some of the conditions that affect our fishery immediately after ice-off.
The Cariboo Region fisheries techs have completed their oxygen content testing of the ‘problem lakes’ of the area and results are mostly encouraging. With the exception of Kloacut Lake, west of Big Creek and Fletcher Lake, the lakes that have suffered either partial or total winter-kills in the recent past are in good shape. Kloacut has winter-killed, again.
‘Winter-kill’ is something that is a naturally-occurring part of stillwaters on the Interior Plateau. It is caused when lakes with no freshwater inlets suffer a serious reduction of oxygen in the water: enough of a reduction that some or all fish cannot breathe. This reduction is usually caused by two things that occur over the winter:
1. Snow cover on the ice prevents the penetration of sunlight and extensive weed-beds die off as a result of that loss of sunlight. Depending on the water chemistry and contents of the lake bottom, this weed die-off will create carbon dioxide in enough quantities that it will displace all or most of the oxygen in the water. Low or no oxygen, fish die.
2. Certain lakes have mineralization left over from ancient volcanic action (Till Lake being a prime example), if those lakes have an extensive weed bed die-off the rotting of the dying weeds can release huge quantities of Hydrogen Sulphide (H2S), a deadly gas that will obviously kill any fish in that body of water.
Both these conditions are resolved naturally after ice-off. The depleted oxygen is replenished by the wave action that aerates the water surface, then mixes with the remaining water at turnover. The H2S evaporates on the water surface and dissipates into the air. This will create a rotten-egg smell around any lake that has suffered from that condition.
Turnover:
When the ice surface on a lake melts in spring, the surface water level is near freezing, making that surface water level denser and colder than the water lower in the water column of the lake. As that surface level of water is warmed by the spring air, it gradually becomes equal in temperature and density with the lower levels and proceeds to mix with the rest of the lake water, supplying the oxygenated level with the entire water body, replenishing the entire lake with oxygenated water. This is turnover, which occurs normally within 7 to 10 days after ice-off, depending on air temperatures and wind action in that period of time.
For the fisher: prior to turnover, fish live and feed in the oxygen-rich upper level of lake water. AFTER turnover, the oxygen is diluted by the non-oxygenated water and the actual density of oxygen through out is quite sparse, leaving fish rather listless and not particularly interested in feeding. Until the water stratifies into an oxygenated upper half of the lake water (usually 10-14 days after turnover) fishing is quite poor, except in shoal areas where surface water is oxygenated by wave action.
In practical terms, after ice-off, fish like crazy, because after turnover, you may as well take a week or two off.
Preserve our waters and our incredible fishery. That fish you release will be more than 2 lbs bigger next year.
Jack Simpson, on Home Waters
Scientific Anglers, Ross Reels & Fly Rods ProStaff
Email: This e-mail address is being protected from spambots. You need JavaScript enabled to view it
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