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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

February 2000

Perch were measured and
weighed to determine when in their life cycle the fish are most vulnerable to disease, predation or fishing. © DNR photo
Perch were measured and weighed to determine when in their life cycle the fish are most vulnerable to disease, predation or fishing.

© DNR photo

In search of perch

Why have yellow perch populations nose-dived in Lake Michigan and when will they pull out of the decline?

John Karl and David L. Sperling

Perch decline or cyclic dip? | When are perch vulnerable?
Many reasons why young perch can't survive | Lake sampling gave varied results
Narrowing reasons for the population dip | A long, slow recovery

People all around Lake Michigan are asking questions about the sustained ten-year decline in yellow perch populations. What caused the decline? How long will it go on? What can be done, if anything, to reverse it? Unfortunately, the questions are much more abundant than the answers. Like all wild species, yellow perch are part of an intricate ecosystem that is difficult to understand in every detail.

"Lake Michigan is a huge system that constantly changes," notes Bill Horns, Great Lakes fisheries coordinator for the Department of Natural Resources. "There are so many factors to consider in unraveling

why fish populations fluctuate," Horns says. "The size of the lake, nutrient loading, fish stocking programs, changing food sources, the influx of exotic organisms, changing land uses and fishing pressure all play a part. The changes are so dramatic that past history is not a good predictor of the future on Lake Michigan," he says. Moreover, the data on past perch populations in the Great Lakes is pretty spotty, Horns says.

Fish population trends prior to 1970 were based on records from commercial fish harvests. Those catches varied as the demand for fish changed, fishing technology became more efficient, and harvest regulations were tailored in each state. Also each state collected harvest figures in different ways, Horns says. "This absence of consistent lakewide data over time makes it difficult to interpret the decline we are seeing now," Horns says. "We really don't know if this is a crisis or a part of the natural variation in Lake Michigan."

Perch decline or cyclic dip?

Whether or not the perch decline is cyclic, it is certainly dramatic. From 1989 until commercial netting was closed in 1995, typical yellow perch catches in commercial nets dropped 90-95 percent per outing. And approximately 85 percent of the sport angler's catch from Lake Michigan had been yellow perch prior to the population crash.

An unpopular, but necessary step to to give the Lake Michigan perch fisher a chance to recover was curtailing its harvest by people.

In 1995, commercial harvests were cut and sport harvests were reduced from 50 to 25 fish per day. In June 1996, sport harvests were reduced to five fish per day and commercial harvests of yellow perch in Lake Michigan were closed altogether. Limited commercial catches still continue in Green Bay, but many a Friday Night Fish Fry has switched from serving local perch to serving fish more readily available from other waters.

To better understand this precipitous population crash and better manage the fishery, agencies and universities from the four states bordering Lake Michigan are collaborating. In 1994, the Lake Michigan Technical Committee formed the Yellow Perch Task Group to encourage and coordinate multistate research that is investigating the dynamics of the perch population. The Sea Grant programs of Wisconsin, Michigan, Illinois and Indiana are supporting a significant portion of this effort. Funds from the Great Lakes Fishery Trust, personnel from state management agencies, and support from commercial and sport fishing groups around the lake provide the remainder.

When are perch vulnerable?

In addition to documenting the population drop lakewide, the Yellow Perch Task Group researchers wanted to determine at what stages of their life cycle perch were most vulnerable. If fertilized eggs were not viable, then something was harming parent fish. If the eggs were fine, but the number of fish fry was low, then investigators would have to determine if fish eggs might be poisoned or preyed upon heavily after the eggs hatched. Or perhaps fry were not finding sufficient food or cover.

Scientists at the Center for Great Lakes Studies in Milwaukee compared eggs and fry collected from yellow perch from Lake Michigan, Lake Ontario, Green Bay and an inland water, Lake Mendota in Dane County. The work was a real team effort – day-to-day monitoring by the University of Wisconsin and the Sea Grant program, fish collections by DNR fisheries staff and commercial fishers, and financial support from sport anglers. This research found no problems with the earliest life stages of Lake Michigan perch. Their fertility, hatching, and survival rates were roughly comparable with perch from the other locations. However, Lake Michigan perch took roughly twice as long to develop as other perch. Wisconsin Sea Grant researcher Fred Binkowski said this is probably related to the gape of the mouth, which determines when fish can progress to the next larger food size. It could also be related to feeding behavior in the Great Lakes.

Fisheries Health Specialist Susan Marcquenski examines fish and collects organ and tissue samples on board a Lake Michigan research vessel.

© Candy Schrank
Fisheries Health Specialist Susan Marcquenski examines fish and collects organ and tissue samples on board a Lake Michigan research vessel. © Candy Schrank

Binkowski's group has also preserved more than 10,000 perch specimens from this study. These will be examined for swim bladder inflation, fish size at first feeding, growth rates, abnormal development and other early life information. The research team is also maintaining live samples of the Lake Michigan perch in the laboratory to investigate their sex ratios, growth rates and survival rates.

The percentage of female perch in the population has declined swiftly during the 1990s. In 1998, females in Wisconsin waters of southern Lake Michigan constituted a small percentage of the population. This imbalance is due in part to the faster growth rate of females compared with males. Since females grow faster than males, they are harvested sooner than males.

When young fish mature normally, the faster growth rate of females does not drastically upset the male/female balance of the population. However, the poor survival rates of young perch during the last decade mean that new females are not replacing the old ones lost to fishing and natural mortality. With few females available to spawn, the yellow perch population could collapse.

Many reasons why young perch can't survive

Though further analyses must be conducted before the results are conclusive, researchers now know several important things about the population crash: few young perch are surviving to adulthood. The causes of this "recruitment failure" are not known, but the effects are clear: with few perch surviving their early years, the average age of the population is increasing quickly. Natural mortality is now removing about 25 percent of the population each year.

The task group identified 16 factors in the early life of yellow perch that may be contributing to the population decline. These include predation by alewives, unusual weather, starvation, competition from other organisms that feed on plankton, and interference from exotic organisms like zebra mussels in the Great Lakes. Any or all of these factors may be responsible for the perch failure. Thoroughly investigating each one would require resources far beyond those currently available. Consequently, the task group is focusing on the most probable factors.

The group is also standardizing the methods used to assess perch from state to state. Previously, states used different kinds of sampling gear and measured populations differently. Standardized methods will allow states to compare their year-to-year census figures in more meaningful ways.

Lake sampling gave varied results

During the summer of 1998, perch were extensively sampled around Lake Michigan. Each captured fish was measured, marked and released. Subsequent resampling and recounts of the numbers of marked fish give researchers a statistical means of estimating the population size. Yellow perch were found to be reproducing throughout the lake, but the numbers were highly variable. According to Bill Horns, perch reproduction in 1998 was good in Indiana waters and moderate in Green Bay, but it continued to be poor in other areas of the lake.

In another study from summer of 1998, Binkowski and Brian Belonger of Wisconsin Department of Natural Resources looked at the yellow perch of Green Bay, where more perch appear to survive to adulthood than in the rest of Lake Michigan, but that's not unusual. Historically, strong year classes in Green Bay have not coincided with strong classes in southern Lake Michigan. Yet, Binkowski and Belonger found reasons for cautious optimism: The summer produced the sixth largest class of young perch in the last 20 years and the highest since 1991. This appears to be the result of an unusually warm spring, in which perch grew faster and were vulnerable to predators for a shorter period of time. Furthermore, two of their common predators, alewives and white perch, were less abundant in 1998 than in recent years.

This year's higher survival rate will probably result in greater numbers of spawning fish in the spring of 2000, but many unpredictable factors could affect the young perch between now and then. To fully restore the perch population of Green Bay, strong year classes would be needed for several years in a row.

Narrowing reasons for the population dip.

Scientists from several institutions continue to piece together other aspects of perch research. The field teams include specialists from Ball State University in Indiana; Central Michigan University; Michigan State University; Purdue University, DNR agencies from Michigan, Indiana, and Illinois; and the Illinois Natural History Survey.

Their efforts are helping the Yellow Perch Task Group better understand the population dynamics of this favorite fish. Sea Grant's Binkowski likens the work to solving a murder mystery.

"Researchers can't really explain why perch populations are declining, but they keep crossing more and more suspects off the list. This 'crime' was committed in 1990 and the trail is pretty cold," Binkowski told anglers at a summer update on perch research in Racine.

DNR fisheries biologists had documented and predicted the perch decline in presentations to the Natural Resources Board as early as 1994. Though the decline was predicted at a time anglers were still enjoying excellent fishing, surveys of young fish showed the populations of catchable-sized fish would drop dramatically.

Fisheries crews prepare to set nets and collect fish at set research plots. © DNR Fisheries
Fisheries crews prepare to set nets and collect fish at set research plots.

© DNR Fisheries

DNR netting crews in southern Lake Michigan last summer found the most young perch and three-year-old perch in the last seven years of sampling. And the sex ratio was 58 percent male to 42 percent female – much nearer the 50:50 split that can help populations rebound. Jim Francis, who led the southern Lake Michigan WorkUnit for the Wisconsin Department of Natural Resources, cautioned that we'd need at least three solid years of similarly good year classes to form a trend and consider the population as recovering or rebounding.

As Binkowski told a Wisconsin Outdoor News correspondent, it doesn't appear that perch populations dropped from fertility or genetic weaknesses, so researchers are now examining how weather, food shortage, predation and competition from exotic organisms could be harming perch.

It's possible that the invasive zebra mussels are part of the problem. The mussels are exceptionally efficient at filtering fine algae from water. Phytoplankton and small zooplankton are important food sources for larval perch once their yolk sacs are depleted after their first week of life. Removing the mass of fine plants and animals also clears the water and may make larval perch more vulnerable to predators who are sight feeders.

Larval perch have been found in the stomachs of 7 of 16 species studied by Belonger in Green Bay. Two Great Lakes invaders, white perch and alewives, consume higher percentage of larval perch in their diet as well.

"I wouldn't say predation is the whole reason for poor recruitment," Belonger said, "but it's significant in some years here in Green Bay."

Spring weather may also be an important factor in Green Bay. Belonger theorizes that pulling off a strong year class in the Bay requires steady warming trends in spring for eggs to hatch and reach the larval stage. Whether storms, steady warm-up or other factors prove more important isn't known.

In southern Lake Michigan, abundant alewives remain a strong suspect. They feed voraciously on larval perch between one-third to one-half inch in size.

A long, slow recovery

Even if scientists and managers do come to understand the causes of the decline, they may not be able to reverse it. For example, little could be done to rid Lake Michigan of zebra mussels, even if they are a significant factor in yellow perch decline.

It also could be that the yellow perch population has been fluctuating for many years. Although the current crash is the most severe, it may be part of normal ups and downs of the population: the nature of perch nature.

Binkowski sensed how difficult it is for perch anglers, commercial fishers, restaurateurs and Friday Night Fish Fry hounds to stay at bay as the research slowly unravels the combination of factors that may explain why perch declined and how they might recover.

"I sense the frustration," Binkowski said. "We're making strides, but Lake Michigan is a complicated ecosystem, and it's not likely there will be one definitive cause or answer that can explain why the population dropped or how soon it may recover."

John Karl is a science writer with the University of Wisconsin-Madison Sea Grant Program. David L. Sperling edits this publication.