SPICER — Walleye numbers continue to decline in Green Lake.

It’s led the Minnesota Department of Natural Resources to consider reducing the walleye bag limit on the lake by half, from the current, six-fish possession limit to three.

A public meeting on the proposal will be held from 6 to 8 p.m. on September 18 at Dethlef’s Center in Spicer.

The DNR is already asking anglers what they think about the possible reduction. The DNR is currently conducting a creel survey on the lake to measure angler success rates.

Anglers being questioned about the idea of reducing the bag limit have been receptive. At this point, roughly 55 percent of those questioned say they support or strongly support the reduction. Another 25 percent are neutral and 20 percent are mildly or strongly against it, according to Dave Coahran, fisheries supervisor with the Minnesota Department of Natural Resources in Spicer.

July 21 marks the five year anniversary of the discovery of zebra mussels in Green Lake, and it’s no coincidence that walleye numbers are down. Zebra mussels are adversely affecting the walleye population, according to the fisheries supervisor.

But walleye numbers were on the decline well before the arrival of this invasive aquatic species. “You can’t blame it all on zebra mussels because it started 20 years ago,” said Coahran.

Water clarity has been improving in the lake thanks to the installation of the Glacial Lakes (formerly Green) Sanitary Sewer system and other efforts to reduce the nutrients flowing into the lake. As the water clarity improves, habitat changes are occurring that favor sight feeding fish like smallmouth and largemouth bass, bluegill, and crappies over walleye and yellow perch.

In response to the clearing water, light-sensitive and cold-water loving walleye are going deeper, and deeper still. Although it seems counter-intuitive, water temperatures are warmer in clear lakes. The sunlight penetrates to greater depths and adds more energy to the water.

A warming climate may also contribute to the challenges walleye are facing.

Coahran pointed to research on Mille Lac Lakes that showed how improving water clarity adversely affects walleye. The same is happening on Green Lake. As the walleye move to deeper locations, they often encounter lower oxygen levels and less food.

In short, walleyes are spending less time in the desirable habitat in which they thrive, the fisheries supervisor pointed out.

Zebra mussels gobble up phytoplankton, which is the base of the lake’s food chain. Less phytoplankton means there is less zooplankton, the microscopic animals that rely on it.

This is likely harming walleye numbers, said Coahran. Newly-hatched walleye and perch are among the fish that have less zooplankton to feed on and grow.

Fish can change eating patterns as zebra mussels take a bigger share of the lake’s energy by devouring the phytoplankton. Whether that is happening yet in Green Lake, we do not know, according to the fisheries supervisor.

The DNR is beginning studies to look at the yellow perch population in the lake, and how zebra mussels are affecting the waters. This study could help answer some of the questions.

But at this point, it’s impossible to say how much of the decline in walleye numbers can be attributed directly to zebra mussels, said Coahran.

With many contributing causes, the decline is evident.

Each year, the Spicer fisheries crew sets gill nets to track walleye numbers. The numbers have been declining steadily, and reached their low point this past year, said Coahran. The decline has been the most pronounced in the last five or six years.

Prior to the arrival of zebra mussels, the fisheries crew expected to find 10 or more walleye in each gill net. Today, they are lucky to find six, he said.

Anecdotal evidence from anglers indicates that walleye catch rates are down as well. And without a doubt, anglers are finding the walleye much deeper than in years past.

Coahran said the fisheries staff has been changing its stocking practices as the decline continues. They are no longer stocking small fry or even fingerling,. There just isn’t the zooplankton food supply desired for their growth.

Now, the staff is stocking 5,000 yearling walleyes of 10-inches in size each year. They are marked with an antibiotic that makes it possible to identify stocked from naturally produced fish in the lake. That allows the DNR to track the success of the stocking.

This much is known: Natural walleye reproduction is steadily declining, said the fisheries supervisor.