CPW launches walleye study to assess population dynamics at Lake Pueblo, Chatfield and Cherry Creek
Colorado Parks and Wildlife
Colorado Parks and Wildlife aquatic research section and aquatic biologists are conducting a walleye demographic study at the three reservoirs the state utilizes in its spring spawning operation.
CPW stocked 47 million walleye fry across the state in 2019, the most of any species, and those fry come from spawning operations at just three Colorado reservoirs - Cherry Creek, Chatfield and Lake Pueblo. CPW collected 126 million walleye eggs last spring from its spawning effort to meet the demand of its hatcheries.
The study being conducted is called Fall Walleye Index Netting (FWIN). It was developed by the Ontario Ministry of Natural Resources and Forestry for assessing walleye populations in a standardized way. CPW has adapted that protocol to work here in Colorado. It involves the netting of fish from across a reservoir at varying depths and measuring what is caught to get an idea of what the population density is and the structure of the fish in the lake.
“The Canadians related the catch rate of walleye in their FWIN nets to independent estimates of density and developed a relationship between the two," said Adam Hansen, Lake and Reservoir Researcher with CPW. "This relationship indicates that every one walleye captured in a FWIN net equates to about one walleye per hectare in a lake or reservoir. By following the FWIN protocol, we can get a rapid estimate of how many walleyes are in a reservoir and compare this number directly among all three spawning reservoirs.”
This study actually launched in the fall of 2018 at Lake Pueblo after anglers expressed interest in changing harvest regulations there, which are fairly restrictive to protect its walleye broodstock. Broodstock is a group of mature individuals used in aquaculture for breeding purposes. Lake Pueblo is vital in CPW’s walleye spawning operation, but researchers wanted to assess the population there to see if regulations could be relaxed to let anglers harvest more fish.
“One of the reasons we did a FWIN survey is to look at the potential density of walleye, the make-up of their population, whether they grow fast or slow, and how well they survive to adulthood. With all that information we can start developing population models to figure out how much harvest the population could actually withstand without compromising the broodstock,” Hansen said. “Do we have some room to relax the regulations and let people take more fish home, particularly those less than 18 inches long, or is the fishing pressure so high on Lake Pueblo that we should maintain current regulations because they work and it is a vital broodstock?”
So that is how it all started, from the question about Lake Pueblo and its harvest regulations. Then with the spawning operation this past spring, another question arose.
Lake Pueblo and Cherry Creek were through the roof with record spawning numbers, but Chatfield’s performance was poor. So the FWIN method was started this October to look at Cherry Creek and Chatfield and assess what factors could be affecting performance between the two reservoirs.
“Should we put all of our resources into spawning fish at Cherry Creek and not return to Chatfield until it is more productive, or should we continue to run a spawn operation at both Chatfield and at Cherry Creek,” Hansen said is one question that has been asked. “We do not know what is best until we sample and compare each population in a standardized way, and the fall period is the best time to do that. We are in the early stages of understanding poor performance at Chatfield, but the first step is to go out, catch fish, and see what the overall population looks like in relation to our other broodstocks.”
In other words, CPW is comparing the health of the walleye population in all three reservoirs to figure out what is going on and what may need to change at Chatfield.
“You can look at relative patterns in the size structure of walleyes, how fast they grow and how well they survive, and from there, start to diagnose potential factors contributing to the low abundance of spawning adults," Hansen said. "For example, factors such as high fishing pressure and post-release mortality of intermediate sized fish or poor growth and survival of our stocked young-of-year during spring should leave different clues in the data,” Hansen said.
The fall is a good period to assess the walleye population - even if that means doing so in five-degree temperatures like they did on a brisk Oct. 30 morning at Chatfield, the last day of netting this year.
“They are not spawning, so all the mature fish, immature fish, all sizes are intermixed during the fall, making it easier to get an in-depth look at the entire population,” Hansen said. “With mature fish, females are starting to develop their eggs and males are starting to develop their testes for spawning next spring, so it is a good time to assess their maturity status. We can then look at how old they are when they reach mature stages and analyze the differences among reservoirs.”
All the fish collected were measured and dissected so the otoliths (ear bones) could be extracted. Biologists cut a section through the otoliths to age the fish. For every year of life they put a ring down, just like a tree does.
“In a population where female fish grow quickly, get big and mature fast, they produce more eggs, start spawning sooner, and are generally available to collect eggs from for longer,” Hansen said. “If they are growing slow and it takes them a while to reach mature size, that means they are spending more time in the reservoir not spawning. The longer they are in the reservoir, the more opportunities they have to be caught by anglers or to die from natural causes before we can collect eggs from them during our spawn operation.”
That is why the fish caught are dissected, so they have the age data to match up with their measurements to get a picture of the growth and maturity dynamics.
All fish harvested in the study were filleted and donated, much of that went to the Denver Rescue Mission and the remainder was donated by wildlife officers to families in need.