Walleye fry were hatching last Friday at the DNR's season hatchery in Park Rapids, just below the dam in the Fish Hook River.
According to fisheries specialist Scott Muhm, the connection between the temperature of the river water and the timing of the eggs' hatching is no coincidence.
"Whatever the temperature of the Fish Hook River is, is what's in here," Muhm said about the water circulating through the hatching jars, tiers of walleye batteries and the big fry tank that runs down the center of the shed.
Muhm pointed out how oxygenated river water flows directly into each jar, allowing it to bubble up through the eggs. Newly hatched fry swim over a spout into the batteries, descend tier by tier and finally drain into the fry tank.
The hatchery has a limited ability to heat or cool the water, speeding up or slowing the eggs' development. The equipment is mainly used to synchronize the hatching of white sucker fry to feed the hungry baby muskellunge occupying a separate corner of the hatchery.
With the walleye, Muhm said, the river's temperature ensures the eggs hatch at the right time.
This year, for instance, "Our first lot of walleyes were taken on the 19th of April," he said about the DNR netting, spawning and returning the fish during the warm days leading up to this spring's April 27 ice out.
When walleye are weaned
Based on studies of walleye populations in area lakes, the Park Rapids DNR hatchery is aiming to stock around 60 million walleye this year.
"Walleyes, when they hatch, have a yolk sac, and they can live on that three to four days," said Muhm. "After that, they have to eat something."
Fisheries staff transfer about half of the walleye fry to fishing lakes, he said, "and the other half go into what we call rearing ponds - small, shallow natural lakes with no other fish. We'll put them there in the spring and harvest them with nets in the fall, when they're four to six inches long, and then stock fishing lakes with them."
Meanwhile, the growing baby walleye move from eating plankton to a diet of minnows.
"It's a boom-bust return, from fry to fingerling in the fall," Muhm said. "When it works, it's a real cheap way to raise good fingerlings. You'll get a lot of blanks, and then you'll hit a home run in one or two ponds, and that will carry your stocking quota to the fall."
Also important to know are what makes some lakes good for stocking fry versus fingerlings - that is, four- to six-inch fish.
"Usually, your shallower, fertile lakes are pretty good candidates for walleye fry stocking," he said. "Garfield Lake in Hubbard County is a good fry stocking lake."
Meanwhile, he said, DNR puts fingerlings in lakes where fish like northern pike would eat the fry.
"So, your Island, Eagle, Potato, Fish Hook chain is all stocked with fingerlings," said Muhm.
An important tool
There are some lakes the DNR doesn't stock at all because they do all right by themselves.
"Stocking is a tool, a management tool that's very effective, but it's a tool," said Muhm. "It's not a cure-all. If mother nature is working, you'll never beat mother nature."
Some Hubbard County lakes where the DNR hasn't found a need to stock walleye in recent years include Big Sand Lake and Kabekona Lake.
How does DNR know where stocking is or isn't needed? Muhm explained, "We do lake surveys. We put out trap nets, gill nets, and do (electrofishing surveys) in the summertime, if we want to see what kind of fish are in the lakes."
In addition to walleye, the Park Rapids hatchery also has a rare, "old-school" program for feeding white sucker fry to inch-long baby muskies, then transferring them to grow-out ponds where they can forage on fathead minnows.
Muhm noted that the state's other hatcheries feed young muskies a dry diet and keep them in a more controlled environment.
"They're more successful doing it on a dry diet, because you've got more control in a hatchery environment," he admitted. "When you put them out in a pond, you're subject to the weather; and you know Minnesota weather - up and down, up and down. When you get a cold front, and those are real small fish, you don't get a lot of phytoplankton production. That's a weak link in the chain."
Nevertheless, he said, "I do like muskies on a sucker diet. It's boom-bust, but when it works, it's fantastic. I think the end product is pretty neat, because those fish have been on a natural diet from the get-go."
Muhm hopes this year's run of 270,000 muskie eggs will produce at least 30,000 mature fish for local fisheries, as well as 15-20,000 for private producers. By October, they should be 11 to 12 inches long and ready to stock fishing lakes all over the state.
"The anglers like them," he said. "It's a very popular program. The state has kind of spread muskie lakes around the state, so you don't have to drive across the state to fish muskie."
Regarding Park Rapids' position close to some natural muskie lakes, Muhm added, "We're lucky to have a lot in this area."