GRAND FORKS -- Brad Olson of Grand Forks went to Ryan Pond on Saturday, Feb, 23, in hopes of getting some underwater footage of rainbow trout smacking the soft plastic fishing baits he markets as a partner in Northern Lights Plastics.

What he saw when he lowered the lens on his underwater camera was yet another unfortunate reminder of just how severe this winter has been. The bottom of the pond was covered with dead bluegills, which most likely had succumbed to winterkill.

The ice was 40 inches thick, and another foot of snow covered the ice, Olson said, leaving only four feet of water under the ice.

The North Dakota Game and Fish Department also stocks rainbow trout in the pond at King’s Walk Golf Course in Grand Forks, and it has become a popular urban fishery in recent years.

If the dead bluegills Olson encountered are any indication, at least a portion of the rainbow trout population in Ryan Pond likely succumbed to winterkill, as well.

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What causes winterkill?

An unfortunate phenomenon during winters with thick ice and heavy snow, winterkill occurs when lack of sunlight hampers photosynthesis, the process by which plants produce oxygen. As the plants decay, they actually use up the dissolved oxygen that remains in the water, and fish die-offs such as the one occurring at Ryan Pond result.

Todd Caspers, district fisheries biologist for the North Dakota Game and Fish Department in Devils Lake, said dissolved oxygen levels in early February already were low at Matejcek Dam in Walsh County, Battle Lake in Eddy County and Niagara Dam in Grand Forks County, and all were likely to suffer winterkill to some extent.

Small, shallow lakes generally are most susceptible.

Fisheries crews are trying to hit lakes across the state to sample dissolved oxygen levels as part of annual wintertime assessments, but the abundance of snow on the landscape is making the job difficult, said Greg Power, fisheries chief for the North Dakota Game and Fish Department in Bismarck.

Crews either are getting stuck or equipment is breaking down, Power said.

So far, the extent of winterkill is about as expected, Power said, but the lengthening days and the resulting increase in daylight improves prospects for lakes where winterkill hasn’t yet occurred but that might be on the brink.

In the meantime, the amount of snow and slush on the lakes has made access difficult for ice fishing enthusiasts, as well, Power said.

“In terms of fishing -- not much at all given the snowpack,” he said in an email. “In some cases, the only traffic our staff are seeing are tractors pulling off stranded ice houses.”

It’s been that kind of a winter.