Decline of yellow perch in southwestern Lake Michigan, 1987-1997

The abundance of yellow perch Perca flavescens in southwestern Lake Michigan declined severely in the 1990s, after state and federal agencies noted a lakewide paucity of age-0 yellow perch in 1989. Regulations were changed in 1995 to reduce recreational and commercial fishing harvests, and commercial fisheries were closed lakewide in 1997. To document changes in the yellow perch population and to evaluate possible causes of the decline, we examined data collected annually in the Illinois waters of Lake Michigan during 1987-1997. Larval yellow perch were collected with plankton nets, age-0 yellow perch were collected with bottom trawls, and adult yellow perch were collected with tyke nets and gill nets. Age-0 yellow perch were rare or absent after 1988; however, larval abundance did not decline severely until after 1993. The presence of larvae and the almost complete absence of age-0 yellow perch in 1989-1993 suggest that overfishing was not initially responsible for the decline. The length at age and mean age of adult yellow perch increased steadily during the study period. The percentage of males in our samples increased to 99%. The reduction in larval catches since 1994 and the highly skewed sex ratio indicate that the number of reproducing female yellow perch was reduced below a critical level. The causes for the decline are unclear, but the demographics of the decline suggest that neither commercial fishing nor predation by alewives Alosa pseudoharengus on larvae were responsible, whereas alterations in the Lake Michigan ecosystem subsequent to the invasion by zebra mussels Dreissena polymorpha may have influenced the decline.