Very few juvenile yellow perch have occurred in yearly surveys of the northern Illinois waters since 1988. The decline in the number of juvenile yellow perch reaching adulthood has caused a shift from a population dominated by young fish to an older population. With relatively few young fish entering the population, the lakewide abundance of yellow perch has declined. Additional evidence that the population has declined is the increased growth of yellow perch that we have observed, since food resources do not need to be shared by as many individuals. Three year-old yellow perch are now reaching lengths that it would have taken 6 years to reach just a decade ago.
Many factors are possible causes of the decline of yellow perch stocks in Lake Michigan.
No strong evidence exists that exploitation was a leading factor in the current decline because, prior to 1994, the number of larval yellow perch we found in our surveys did not drop appreciably. It is more likely that a combination of biological factors has affected yellow perch in the recent past and that the suite of factors may vary between years and locations within the lake. Yellow perch have naturally cyclic population trends but, more recently, exotic species as well as changes in food availability and the chemistry of the lake likely have impacted the yellow perch population. Competition with exotic species like the alewife and smelt has changed the food habits and availability of food for juvenile perch. In addition, these exotics may also prey on larval yellow perch, reducing the number that survive until the fall.