Yellow Perch Update

December 1998

By John Karl

People all around Lake Michigan are asking questions about the nine-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. The numbers and health of the fish are determined by a host of interacting factors.

Some Basic Facts

Researchers know several important things about the population crash. The most important one is that 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.

A related fact is that the percentage of females in the population has declined swiftly during the 1990s. In 1998, females in Wisconsin waters of southern Lake Michigan constituted only 20 percent of the perch population. This imbalance is due in part to the faster growth rate of females compared with males. Because females grow faster than males, they are harvested sooner than males.

When recruitment occurs at normal levels, the faster growth rate of females does not drastically upset the male/female balance of the population. However, the recruitment failure during the last decade means 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.

Learning More

In response to this alarming situation, agencies and universities from the four states bordering Lake Michigan have launched a collaborative research and management effort. In 1994, the Lake Michigan Technical Committee of the Great Lakes Fishery Commission formed the Yellow Perch Task Group and charged it with developing a multi-state research initiative on the dynamics of the perch population. A significant portion of this effort is being supported by the Sea Grant programs of Wisconsin, Michigan, and Illinois-Indiana. The remainder is backed by funds and personnel from state and tribal fishery management agencies and commercial and sport fishing groups around the lake.

Drawing on the expertise of university scientists and agency biologists from the region, the task group identified 16 factors that may be contributing to the population decline, all related to the early life of yellow perch. These include predation by alewives, unusual weather, and starvation of larval perch. Any or all of these factors—and possibly others—may be responsible for the recruitment 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 task group is also standardizing assessment methods 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 assessments in more meaningful ways.

Good News and Bad News

The taskgroup found mixed results in sampling and tagging assessments during the summer of 1998. Yellow perch were found to be reproducing throughout the lake, but the numbers were highly variable. According to Bill Horns, Great Lakes Fisheries Coordinator for the Wisconsin Department of Natural Resources and member of the task group, reproduction last spring was good in Indiana waters and moderate in Green Bay, but it continued to be poor in other areas of the lake. Horns also said fishing will not improve until the lake sees several years in a row of strong reproduction.

Wisconsin Sea Grant Research

During the summer of 1997, Fred Binkowski and his colleagues at the Wisconsin Aquatic Technology and Environmental Research (WATER) Institute in Milwaukee examined the early life history of perch from Green Bay, greater Lake Michigan, Lake Ontario, and Lake Mendota in Madison. This research, supported by UW Sea Grant, found no problems with the earliest life stages of Lake Michigan perch—fertility, hatching, and survival rates were roughly comparable with perch from the other locations. As they matured, however, Lake Michigan perch took twice as long as the other perch to advance from one food size to the next. Binkowski said this is probably related to the gape of the mouth, which determines when fish can progress to the next larger food size. However, it could be also be related to feeding behavior. Further analyses must be conducted before the results are conclusive.

Binkowski’s group has preserved more than 10,000 perch specimens from this study. These will be examined for swim bladder inflation, size at first feeding, growth rates, abnormal development, and other early life information.

Binkowski has also maintained live samples of the Lake Michigan perch in his laboratory for the past year and a half. His group is investigating their sex ratios, growth rates, and survival rates. A sex ratio close to 50:50, as he expects to find, would mean that the low percentage of females in Lake Michigan is not due to biological factors but to females being harvested faster than males.

More UW Sea Grant-sponsored research into the problem was launched last summer. In March, Binkowski and his colleagues began a four-year project to examine the genetic condition, growth rates, population size, and mortality rates of young-of-the-year perch in southern Lake Michigan. The team spent much of the summer on the lake, gathering data on the population size, growth rates, and movements of larval and post-larval yellow perch. They also studied the interactions of perch with potential predators. These data are being analyzed this winter.

The researchers also accomplished the critical first steps of several laboratory experiments, collecting and preserving thousands of samples of larval and post-larval perch. These samples will be analyzed for potential maternal effects on the condition of young perch. This genetic analysis is a measure of the fishes’ health and its effect on their survival.

In another study this past summer supported by UW Sea Grant and the Wisconsin DNR, Binkowski and Brian Belonger of WDNR looked at the yellow perch of Green Bay, where recruitment problems have not been as severe as in the rest of Lake Michigan.

Binkowski and Belonger found reasons for cautious optimism in Green Bay: This summer’s young-of-year class was the sixth largest in the last 20 years, and the highest since 1991. This appears to be the result of an unusually warm spring, which fostered higher growth rates than usual. The faster perch grow, the shorter time they are vulnerable to predators. Furthermore, two of their common predators, alewives and white perch, were less abundant this year than in recent years.

This year’s higher survival rate will probably result in greater numbers of spawning fish in the spring of 2000, although many unpredictable factors could affect the class 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.

Other Sea Grant Research

Other teams of researchers supported by the Michigan and Illinois-Indiana Sea Grant programs are examining the population size, growth rates, and survival rates of yellow perch in southern Lake Michigan. They are also measuring fat content to determine the fishes’ health and examining possible alewife predation on perch larvae and starvation of young yellow perch due to competition for zooplankton from exotic species such as zebra mussels and the spiny water flea. UW Sea Grant will relay highlights of these investigations as results develop.

Other Efforts

This winter, the WDNR will conduct its annual graded mesh gill net assessment, which will provide information on the status of all year classes (see graph at right showing data from 1986 - 98 in two age brackets).

Other aspects of the perch problem are being addressed by researchers from the following institutions: 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.

The Nature of Nature

Fisheries managers around Lake Michigan have responded to the alarming yellow perch situation by closing commercial fishing in their waters. Daily bag limits for sport fishing have been reduced throughout the lake.

The efforts of the Yellow Perch Task Group promise a better understanding of the population dynamics of this favorite fish. However, 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 were primarily responsible for the decline of yellow perch.

The size of the yellow perch population has fluctuated for many years. Although the current crash is the most severe, it may be part of normal ups and downs of the population. Until much more can be learned about this complicated system, such dramatic changes can be considered simply "the nature of nature."

UW Sea Grant will keep you informed as Sea Grant researchers and the Yellow Perch Task Group assemble more pieces of the perch puzzle.

For More Information:
Fred Binkowski, Senior Scientist, UW-Milwaukee Center for Great Lakes Studies, (414)382-1705
John Karl, Science Writer, (608) 263-8621

See also:
    Major Perch Research Effort Launched (03/28/98)
    Perch Research: Phase One Complete (10/6/97)
    UW Sea Grant Researchers Tackle Perch Problem (8/20/97)
    Where Have All the Yellow Perch Gone? (4/16/97)

Created in 1966, Sea Grant is a national network of 29 university-based programs of research, education and outreach dedicated to the protection and sustainable use of the United States' coastal, ocean and Great Lakes resources. The National Sea Grant Network is a partnership of coastal states, private industry and the National Sea Grant College Program , National Oceanic & Atmospheric Administration, U.S. Department of Commerce . The University of Wisconsin Sea Grant College Program is administered by the Sea Grant Institute on the UW-Madison campus in Madison, Wisconsin.

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