Winterkill: why some BC lakes lose their trout

Every few years an angler arrives at a favourite lake in spring and finds dead trout along the shore as the ice goes off. That is winterkill: the fish suffocated under the ice. It is a natural event in shallow, productive lakes, it is more common after a long hard winter, and it can empty a lake that was fishing well the fall before.

How a lake runs out of oxygen

In open water a lake takes oxygen from the air at its surface and from the plants and algae growing in it. Ice seals off the air. Snow on top of the ice blocks the light, and without light the plants stop making oxygen. Meanwhile the dead weed and algae from summer are rotting on the bottom, and rot uses oxygen. All winter the lake spends oxygen and gets none back. If the ice stays long enough and the lake holds too little water for the amount of rot, the oxygen runs out before spring and the fish die.

Which lakes are most exposed

Shallow lakes, and above all lakes that are shallow across most of their area. A deep hole in one corner holds only a little of the lake's water.
Rich lakes: the weedy, insect-rich water that grows big trout fast also grows the most to rot in winter.
Long winters: early freeze-up, late ice-off, and heavy early snow that shuts out the light for months.
Low water: a lake drawn down or in drought goes into the winter with less water, and less oxygen, to spend.

What an aerator does

Some lakes where winterkill is a known risk are fitted with aerators that pump air into the water through the winter, keeping oxygen up and a patch of the lake open. The open water and the thin ice around an aerator are dangerous all winter. Signs mark aerated lakes. Stay well away from the open area and anything near it.

How to tell after ice-off

Dead fish along the shore as the ice leaves are the clearest sign, though a few dead fish can follow any winter. A lake that fished well last fall and gives nothing for weeks in spring, while nearby lakes fish normally, may have lost its fish. Fisheries agencies survey and report winterkill on many managed lakes, and often restock them. If you see a fish kill, report it to the province.

Checking a lake before a spring trip

The Freshwater Fisheries Society of BC, which raises and releases trout into many of the province's lakes, publishes winterkill reports for the lakes it manages, and in some years a list of lakes it rates at high risk going into winter. After a long, snowy winter those are worth reading before a long drive. The maximum depth on each lake's page is a rough first check, with one caution: winterkill turns on how much of the lake is shallow, not on its single deepest point, so a deep hole does not make a lake safe from it.

If your lake is a shallow one

Fish it hard in the fall and in the first weeks after ice-off, which is when shallow lakes fish best anyway.
After a long snowy winter, check the published winterkill reports before a long drive in spring.
Keep a deeper lake nearby as the back-up.

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