Record walleye stocked in North Dakota

BISMARCK, N.D. (AP) -- This was a good year for raising walleyes at the Garrison Dam National Fish Hatchery -- a record-setter in terms of the number of walleyes stocked in North Dakota lakes.

Thanks to new pond liners at the hatchery and some weather-related issues, more than 11 million walleyes were stocked in state lakes this year, a record for the North Dakota Game and Fish Department, The Bismarck Tribune reported.

The U.S. Fish and Wildlife Service's hatchery at the dam and the Game and Fish Department for years have worked in harmony to grow multiple species for not only North Dakota lakes, but for other states as well.

Rob Holm, hatchery manager, said the biggest change that has resulted in increased production has been the installation of new pond liners.

Holm said in 2009, the last of the old clay-lined, sand-bottomed rearing ponds at the hatchery were retro-fitted with 40-mil polyethylene liners.

He said the new liners, at a cost of about $90,000 apiece, have increased the efficiency of how the hatchery is able to raise not only walleyes, but other species of fish in a number of ways.

Holm said because the spawning time of different species overlap, growing fry quickly and efficiently enables the hatchery to turn around the ponds from one species to another.

"It makes double-cropping more of an option," he said.

Walleyes and northern pike feed on a larval form of midges called chironomids during crucial stages of their growth.

Jerry Tishmack, a fisheries biologist at the hatchery for the past 20 years, said the liners keep the chironomids from burying themselves in the sand and mud, making them more accessible to the walleye and pike fry.

He said they also seem to relate better to the smooth surface of the liners and reproduce more prolifically.

Tishmack said the midges, which are naturally occurring in this part of the country, overwinter in standing water. When they hatch in the spring, it's a smorgasbord for fish.

Tishmack said this year's 9.7 million walleyes produced at the hatchery is third on the list for all-time production. Two years ago the record of 10 million was set and last year that number was 9.9 million.

He said the presence of midges seem to be a hit and miss proposition from year to year, depending on the water situation. After the flood of 2011, there were plenty to go around, and the walleyes and the pike responded.

Since then, Tishmack said the hatchery has flooded areas surrounding the ponds in an effort to make sure there is a good supply of standing water going into the fall and winter.

Holm said another change that has made a difference is water temperature. He said the sand and clay bottoms in the ponds had eroded over time since 1962 when they were built, and they were losing water.

Water from Lake Sakakawea had to be pumped in to keep the ponds filled, he said.

"The water comes from the bottom of the lake so it was cold, about 40 degrees," Holm said.

The optimal temperature for raising species like walleye is around 65 degrees.

Walleyes are hatched indoors in what are called battery jars and are tiny at birth. They are immediately put in the ponds and five weeks later, are big enough to be stocked in lakes.

Tishmack said since the liners have been installed, the survival rate for the walleye fry has been about 60 percent. Historically, it had been about 50 percent.

And the size of the walleyes that are now going into lakes has improved as well. In the past, fry about ¾-inch long were the norm. These days they are about 1 1/4 inches long.

To put that in perspective, Tishmack said the walleyes raised in the lined ponds this year averaged about 1,100 fish per pound. The old ponds yielded about 1,800 walleyes per pound.

The bigger the fish are, the better chance they have of surviving once they are stocked.

The Game and Fish Department's fisheries production and development supervisor Jerry Weigel said records are nice, but ultimately it comes down to an issue of need.

Back in the 1990s, for instance, Weigel said if Lake Sakakawea or Devils Lake needed 6 million or 7 million walleyes stocked, that's what the hatchery would produce.

But back then, North Dakota didn't have 400 or so managed lakes to stock.

Recent production numbers are a reflection of high water in the past several years creating prairie lakes where there were none before.

"We have a lot of walleye lakes now that didn't exist before," Weigel said.

Through the recent high-water years, lakes like Alkaline, Marvin Miller, Sibley and others have become the poster children of what happens when prairie lakes spring up.

"It's kind of a new phenomenon," Weigel said. The flooded vegetation of these lakes is rich in nutrients and invertebrates, the bottom rungs on the ladder that is the aquatic food chain.

These lakes are generally shallower, fertile and full of fathead minnows and have become outstanding walleye lakes.

Weigel said it has allowed the Game and Fish Department to try some different management strategies on some lakes.

In the past, conventional strategies called for stocking perch for the forage fish, pike for the predator fish to keep the perch from overtaking the lake and then walleyes, in the hope they would take off and do well.

Now, Weigel and his crews stock just walleyes in some lakes. As for this year's record stocking, only time will tell, Weigel said.

"If it (the stocking) takes, there will be great fishing in three years," he said. "But right now there are trophies in a lot of these lakes. There is unprecedented fishing on the prairie."

Tishmack said there are so many variables when it comes to growing fish, and most of them are out of the hands of biologists.

Temperature, both water and air, are key elements, he said.

If conditions are right, pike in particular will thrive. Prior to the installation of the liners, the water temperature on the east unit ponds where most of the walleye are reared was around 35 degrees. Now it runs around 46 degrees.

Warmer water sparks the hatches of a number of zooplankton, invertebrate species on which the fish feed. Warmer water means more food for the fry.

In the case of pike, Tishmack said they will first feed on microscopic zooplankton called rotifers. As the fry grow, they turn to larger invertebrates.

The midges are the key, he said. If they are not present during the third week, Tishmack said, the pike will swim through clouds of other potential food sources before cannibalizing each other.

Tishmack said fisheries biology to a large part is trial and error -- finding out what works and what does not -- and making adjustments.

And those adjustments have been paying dividends in the past decade in terms of efficiency and production.

"We've gained 10 percent in the past several years," Tishmack said. "But it's in Mother Nature's hands. ... If you don't have chironomids, you don't make fish. Bottom line."

Copyright 2013 The Associated Press.