As Colorado Parks and Wildlife wraps up the annual effort to spawn and stock more than 130 million fertilized walleye eggs into Colorado waters, aquatic biologists have a new tool to increase egg survival and make the operation more efficient. The tool, a mobile egg battery, made its debut at the Cherry Creek State Park walleye spawn in mid-March.

Mandi Brandt is the egg battery’s architect and CPW Aquatic Biologist for Area 3 - Brush. She had one thought when she saw Nebraska using a similar contraption during their spawn: “Whoa! We need to do that.”

Even though the spark came from a neighboring state, the idea has been in Brandt’s mind for a while, “Each year, we’re trying to look at ways to make walleye spawn more efficient. For quite a few years I’ve been wanting to find a better way to rinse the eggs.”
A walleye is pulled from a net during the spawning operation at Cherry Creek State Park in March 2025. The nets are pulled and reset each day of the operation.
A walleye is pulled from a net during the spawning operation at Cherry Creek State Park in March 2025. The nets are pulled and reset each day of the operation.

The spawn process is repetitive: catching walleye in gill nets, gently squeezing the eggs from ripe females and milt from males, adding water, mixing the eggs and milt together with a feather in a precisely-timed method, adding a bentonite mud slurry to the fertilized eggs to prevent adhesion and clumping, stirring some more with a feather, rinsing the mud, fecal material, and blood clots from the eggs using filtered water from spigots, letting the eggs absorb water for an hour, then transporting them in Gatorade coolers to the fish hatchery. It’s a dance CPW has done for decades. The repetition allows biologists to look for opportunities to improve efficiency and eliminate human error.

A large female walleye delicately hangs from the net by its front teeth while being pulled in during the spawn operations at Cherry Creek State Park. Large females like this one are highly valuable for spawning operations, and getting one into the boat when it’s caught by its front teeth requires finesse and a soft touch.

A large female walleye delicately hangs from the net by its front teeth while being pulled in during the spawn operations at Cherry Creek State Park. Large females like this one are highly valuable for spawning operations, and getting one into the boat when it’s caught by its front teeth requires finesse and a soft touch.
Northeast Region Senior Aquatic Biologist Kyle Battige shows off a large female walleye before placing it in a holding tank. Females like this hold tens of thousands of eggs, and are highly valuable to the state’s spawning operations.

Northeast Region Senior Aquatic Biologist Kyle Battige shows off a large female walleye before placing it in a holding tank. Females like this hold tens of thousands of eggs, and are highly valuable to the state’s spawning operations.

Brandt hopes the mobile egg battery will do just that, “We can cut down the work of 6-12 people needed to rinse eggs to just 1-2 people. It really cuts down on the time and staff time spent cleaning eggs.”

Once she had the idea for the battery in her head, Brandt spent a few weeks bringing it to life. Her background of growing up on a farm made it easy, “Construction stuff comes naturally to me. As a farm kid, you’re always building your own things.” Add an engineering start to her college days, and it’s no surprise Brandt had the machine realized on her third drawing attempt, with help from Fishery Technician Andrew Johnson.

Brandt, Area 5 Aquatic Biologist Aubrey Pelletier, Fishery Technician Cole Trumble and Wildlife Property Technician Brody Stack spent a few days building the battery. Due to its size, the team had to construct the contraption on the barge where it will be used during the spawn at Cherry Creek.

The mobile egg battery is made to mimic the existing egg batteries in fish hatcheries, which resembles a well-stock brewery with dozens of taps. A hose pumps water from Cherry Creek Reservoir through a filter into a water tank. Filtered water is then pumped up into a head tank that feeds several large, plastic jars on the egg battery. These cylindrical jars are filled with the muddied eggs. The shape of the jars allows water flow to encourage impurities and the mud mixture to rise to the top, where it can be rinsed or siphoned off. The fertilized eggs continue to gently roll end-over-end in the lower portion of the jar to keep from sticking to each other and allow oxygen to penetrate into the eggs. After 15 minutes the golden eggs sit in clear water. Within an hour, they are ready to be loaded into coolers for transport to a hatchery.

The bentonite mud mixture is added to a pan full of eggs and milt, and then mixed gently. The mud mixture helps limit egg adhesive –– when eggs clump together –– and increases survival and viability of the eggs.

The bentonite mud mixture is added to a pan full of eggs and milt, and then mixed gently. The mud mixture helps limit egg adhesive –– when eggs clump together –– and increases survival and viability of the eggs.
Fertilized eggs sit in large jars, known as McDonald type jars, while being “rolled” –– the process of lightly mixing and stirring the eggs so they can get cleaned, absorb oxygen, and are prepared for transport to the hatchery.
Fertilized eggs sit in large jars, known as McDonald type jars, while being “rolled” –– the process of lightly mixing and stirring the eggs so they can get cleaned, absorb oxygen, and are prepared for transport to the hatchery.
As the eggs roll in their jars on the battery, the bentonite mud mixture and any other debris in the containers separate from the eggs, which settle to the bottom of the jar and continue swirling very lightly.

As the eggs roll in their jars on the battery, the bentonite mud mixture and any other debris in the containers separate from the eggs, which settle to the bottom of the jar and continue swirling very lightly.

With no way to test it before the spawn started, Brandt relied on faith in her team and design that the battery would succeed, “I was excited, not nervous, because I really felt like it was going to work after what I saw in Nebraska.”

The aspect CPW biologists are most excited about, though, is the possibility to increase egg survival rate. The old system caused eggs to lay on top of each other, reducing the amount of oxygen to the eggs and increasing clumping issues. Human error can also cause egg loss in a variety of ways, especially in the delicate rinsing steps.
Aquatic Biologist Mark Sandersen, kneeling, and Lead Fishery Technician Adam Koser, standing, check on the fertilized eggs in the battery jars at the 2025 Cherry Creek State Park spawn operations. This was the first year an egg battery has been used at the spawning site.
Aquatic Biologist Mark Sandersen, kneeling, and Lead Fishery Technician Adam Koser, standing, check on the fertilized eggs in the battery jars at the 2025 Cherry Creek State Park spawn operations. This was the first year an egg battery has been used at the spawning site.

“If you’re not super experienced, you can lose a lot of eggs if you’re not careful. We were losing quite a few on the floor of the barge through the former rinsing process. You could also inadvertently damage the eggs with the feather or with water coming from the spigots when using the old process,” said Brandt.

Where before it was possible for entire pans of fertilized eggs to be rendered unusable, Brandt now estimates only 1-2 teaspoons of eggs were lost per day at Cherry Creek this year once the system was honed in. The development of this more efficient cleaning process could also lead to higher egg output per female.

CPW aquatic biologists continually work to improve upon their product and the fisheries they manage by innovating different techniques used in fish spawning operations, fish population sampling and aquatic habitat improvements.

“Mandi (Brandt) is no exception, and I have always been impressed by how she continues to push the envelope with her unique and innovative ideas,” said Matt Nicholl, CPW Aquatic Branch Assistant Director.

The real winners in CPW’s push to continue improving and fine-tuning walleye spawn are Colorado’s anglers. Higher quality eggs lead to lower mortality rates, creating larger numbers of fry to stock into several of Colorado’s lakes and reservoirs. The mobile egg battery is a small, vital tool in the mission of CPW to manage fish populations and live up to the expectations of the multi-billion dollar angling industry.
Fertilized eggs are loaded into transport containers, large five-gallon coolers, after rolling in the jars on the egg battery. Eggs will roll for an hour to ensure they’re clean, healthy, and ready to move onto the next phase of the operation, moving to a hatchery.
Fertilized eggs are loaded into transport containers, large five-gallon coolers, after rolling in the jars on the egg battery. Eggs will roll for an hour to ensure they’re clean, healthy, and ready to move onto the next phase of the operation, moving to a hatchery.

Written by Kara Van Hoose. Kara is the Northeast Region Public Information Officer for Colorado Parks and Wildlife.