Phenological extremes could jeopardize walleyes in north-temperate lakes

A 2024 study by Martha E. Barta et al. looked at phenological extremes and how those might affect walleye in 194 Midwestern lakes. Growing variability in things such as ice off and on could be causing trophic mismatches, or mismatches in the timing of peak predator and prey abundance. This could mean negative effects for walleye spawning and recruitment. Local Department of Natural Resources (DNR) biologist Zach Feiner was also a part of this study.

“Ice off phenology shifted earlier, about three times faster than walleye spawning phenology over time,” the abstract of the study said. Over time, these spawning deviations caused decreases in offspring survival. The study predicted these mismatches could continue to worsen, increasing these negative effects to walleye populations.

Phenological shifts have been documented by many studies in recent years. One of the most well-documented of those shifts, as stated in this paper of this recent study, is a shift to earlier and earlier shift in spring biological events that are associated with rising temperatures. Other studies have linked warming temperatures and earlier spawning of spring spawning fish species as well. 

“The mechanisms associated with these temporal shifts are elusive, given that the strength and direction of phenological change may vary within and among species,” the paper said, citing Thackeray et al. 2010. Several studies have looked at a directional change in temporal trends, but there is also evidence that environmental variability will also continue to increase. Understanding species’ responses to this increased variability, then, will likely become more important as well. 

Trophic mismatches that occur due to changes in phenology over time can break down food webs as the prey abundance for a predator species may not align at the time of the life cycle where the predator would be feeding on that prey. The paper stated that these mismatches, when prolonged, have the ability to substantially decrease offspring survival and recruitment in predators such as walleye. This, in turn, could threaten sustainability of the entire ecosystem. 

Ice off brings about increases in primary productivity in lakes, bringing about production of food resources for larval fish. If larval fish are not present during the time that this appropriately-sized prey is abundant, this causes a mismatch that can threaten that year’s offspring growth and possibly survival. The study looked to answer three questions. Researchers wondered if walleye spawning or ice-off phenology shifting over time. They also wanted to find out what relationship there might be between walleye spawning and ice-off. Third, they wanted to look to see whether trends in walleye spawning phenology varied with lake characteristics or with walleye stocking.

The team used an extensive dataset in their study. The paper said they did find a “slight trend” toward earlier peak spawning dates in walleye throughout the Upper Midwest by about .4 days per decade. This result was somewhat small, but other studies, the paper mentioned, did show a more marked change in spawning of other Midwest percids. For instance, a 2016 study found a change of per decade in Lake Michigan yellow perch. 

“Trends in walleye spawning phenology may be less directly responsive to ice phenology than originally predicted,” the paper said. “The nearly static nature of spawning dates may also suggest a larger role for photoperiod in moderating spawning dates than previously thought.”

This study found that ice-off dates shifted earlier at a rate of 1.1 days per decade, which was approximately three times faster than changes in walleye spawning. The temporal coupling of walleye and the peak abundance period of their prey could demonstrate a threat to walleye populations. 

While ice-off is directly related to climate conditions each year, this study shows that walleye may not be able to shift their spawning phenology to fit those conditions on a year-after-year basis. This could have the effect of reducing walleye populations that are currently doing well, but in cases of walleye populations that are already declining, that picture if even gloomier. 

This study did find a possible difference in trends on stocked lakes. 

“Lakes with walleye populations supported entirely by natural recruitment trended toward earlier spawning, while lake supported partially or entirely by stocking showed no change,” the paper said. A 1999 study showed that brood stock collection and aquaculture practices could have selective effects on spawning behavior as well. Researchers the effects on stocked fish warranted additional research. Additional research, the paper said, should also be conducted on the influence of other climatic and non-climatic variables on phenology as well. 

Deviations in spawning dates based over the last four decades have become much larger. The corresponding lack of change in walleye spawning phenology is predicted to bring about a 23% decrease in walleye natural recruitment across the study range. 

The study did have some limitations that were acknowledged in the paper. Mark-recapture surveys were limited to after ice-off. This could mean that walleye spawning had taken place earlier than researchers might have been able to account for occasionally. Additionally, the correlation between ice off and surveying dates could artificially elevate the relationship between spawning phenology and ice-off phenology. 

The potential for these trophic mismatches to continue to occur and to become more extreme, the paper said, researchers, with the help of local and Tribal communities, should continue to look for ways to mitigate the negative of climate change effects that are predicted to continue over time. 

“Our results further demonstrate the need for careful mechanistic examination of population and community phenology to evaluate susceptibilities to phenological change and better inform management strategies that can increase ecosystem resilience and prepare for potential shifts to new ecosystem states,” the study concluded. 

Beckie Gaskill may be reached via email at [email protected].