Bits & Pieces: Factors for Strong Walleye Year-Classes

Research reveals how spring warmth, winter severity, and invasive species shape recruitment in Minnesota's top walleye lakes. Plus black catfish blotches and bass reaction strikes.

September 17, 2025 By Dr. Rob Neumann, Steve Quinn, Dr. Hal Schramm & Ralph Manns

Environmental Ecology: Factors for Strong Walleye Year-Classes

Walleyes are known as a species that often varies greatly in the success of year-classes—fish spawned in one calendar year. This factor is important as it can lead to fast or poor fishing several years later, when that group of fish reaches harvestable size. Lakes that produce strong year-classes every three or four years tend to have strong populations, supported by adults of various ages. Dr. Andrew Honsey and two other researchers from the University of Minnesota recently examined 17 variables and their potential effects on recruitment and year-class strength of Minnesota’s nine largest walleye lakes (including Minnesota’s portion of Lake of the Woods but not Lake Superior).*

These popular fisheries in the central and northern part of the state account for about 40 percent of the annual walleye harvest statewide. The team used data from the DNR’s annual standardized sampling from 1983 through 2018 and examined factors that were biological, such as population size and presence of zebra mussels and spiny water fleas; weather-related—winter severity, ice-out dates, and spring and summer temperature regimes; and anthropogenic, primarily stocking.

After considerable crunching of data to create models, results indicated that warm springs were most important in survival of a year-class. Faster warming rates after ice-out were highly beneficial, and years with late ice-outs tended to warm faster. The second-most important variable was the severity of the fish’s first winter; tough

winters took a toll on the little walleyes. The presence of invasive species was associated with reduced year-class strength, with zebra mussels more detrimental. But there was considerable variation among lakes in that factor. Indeed, the models fit some lakes better than others, with Kabetogama and Mille Lacs being the largest outliers, for unknown reasons. Fry stocking is not routinely done on these lakes but where it was, low-density stocking had positive effects on year-class strength, but boosting stocking levels had no further effect.

Overall, thermal conditions were primary factors in recruitment of year-classes. Warmer first springs and summers boosted survival, likely due to faster growth and increased availability of tiny prey. Severe winters preceding the walleye hatch were positively correlated to subsequent recruitment as well. A cold winter prior to hatching reduces abundance of the preceding year-class of walleyes, which compete and prey on hatchlings. Severe first winters reduced year-class success. Ideal conditions for a walleye year-class consisted of a rather long and cold winter preceding the hatch; late ice-out; fast spring warming; a warm first summer; and a mild first winter.

While we have no control over weather, the model provides tools for managers to examine the need for potential changes in regulations in response to walleye recruitment levels related to those conditions.

–Steve Quinn

*Honsey, A. E., Z. S. Feiner, and G. J. A. Hansen. 2020. Drivers of walleye recruitment in Minnesota’s large lakes. Can. J. Fish. Aquat. Sci. 77: 1921-1933.

Fish Health: Black Blotch Revisited

A catfish with black blotches laying on the floor of a boat. — (PHOTO / Chad Ferguson)

In-Fisherman contributor and Texas catfish guide Chad Ferguson sent us this photo of a channel catfish with what appears to be black blotch syndrome, which is most commonly seen on largemouth and smallmouth bass. Various explanations have been offered as to why fish exhibit these blotches, from genetic anomalies to UV exposure and more, but the actual cause of these “malanistic skin lesions” wasn’t known until recently.

The United States Geological Survey partnered with the Pennsylvania Fish and Boat Commission and Pennsylvania Sea Grant College Program to conduct fish population and fish health surveys, tissue collections, and to assess the prevalence of the melanistic hyperpigmentation in smallmouth bass in Pennsylvania waters.* After conducting microscopic and genetic analyses of normal skin and melanistic lesions on a sample of bass, it was determined that the black blotches were associated with a virus of the family Adomaviridae, which has been associated with black blotch in other species such as crappies, sunfish, and catfish.

More biosurveillance research is underway to determine the geographic extent of blotchy bass syndrome, track its spread, and assess possible effects on fish health.** This is the first photo we’ve seen of what appears to black blotch melanistic lesions on a catfish.

–Rob Neumann

The so-called reaction strike is just one of the many ways bass appear to humans to attack prey and lures. Before committing to a strike, bass often inspect objects that don’t seem able to move away rapidly. If prey can move out of range too fast for a successful attack, bass ignore them. Faster-moving targets require instantaneous reaction, lest the item quickly move out of range; but is this a feeding response?

Everything a black bass does, other than spawning activities, is related to feeding. Much of the misunderstanding about bass feeding stems from the false belief that bass are consummate predators, able to eat wherever and whenever they want. This is not the case. Bass and their prey are well balanced, and prey are hard to catch. Bass get enough to eat and grow, and prey avoid being eaten sufficiently to spawn and reproduce. Much of the time, bass are hungry, and many studies have shown bass stomachs are empty much of the time.

In most waters, bass can’t afford to bypass a feeding opportunity. When they refuse lures, it’s usually because they’re out of range or the bass have been made wary by a noisy approach or crude tackle. Most bass are hungry, but in a non-feeding status conserving energy, waiting for opportunities to feed when success is likely.

At times, bass truly are inactive. Once digestion begins, fish send most of their blood to the stomach, routing only minimal amounts to the nervous system for defensive purposes. In this state, they are, for all practical purposes, unable to feed. But the sudden appearance of vulnerable prey—possibly simulated by repeated casting of a lure over its nose—may attract enough attention to arouse a bass and cause it to send blood to brain and muscles. The next pass of a lure may be met with a fierce strike, which an angler may call a “reaction strike,” although it’s motivated by a feeding instinct. Also, cold water greatly limits their feeding activity.

When bass are actively feeding in schools, they all react competitively to any potential food, so we have many strikes that anglers call “reaction strikes,” but they’re simply routine feeding behavior.

–Ralph Manns