Zooplankton selectivity by larval yellow perch in Green Bay, Lake Michigan

Recruitment of yellow perch (Perca flavescens) in southern Lake Michigan has been poor since 1989, whereas in Green Bay, Lake Michigan, larval yellow perch have been more abundant. Therefore, investigation of factors influencing yellow perch early life stages in Green Bay, and comparison with southern Lake Michigan conditions, can lend insight into factors limiting recruitment of this species in southern waters of the lake. We investigated diets and selectivity of larval yellow perch in Green Bay, to identify conditions of zooplankton availability that limit larval yellow perch. Larval yellow perch and zooplankton were collected in Green Bay during May and June 1998 and 1999. We identified, counted, and measured available zooplankton and larval yellow perch stomach contents, estimated larval gape size, and compared larval diets to zooplankton availability. Similarities between 1998 and 1999 in larval yellow perch diet composition and selectivity indicate that zooplankton are most likely to limit small (< 8 mm TL), gape-limited larvae, whose diets are comprised almost exclusively of copepod nauplii and small (< 0.5 mm) copepods. Differences between 1998 and 1999 further implicate the importance of small copepods. In 1999, when small copepods were less abundant in Green Bay than in 1998, a higher proportion of larvae had empty guts, and growth of larvae was slower. In southern Lake Michigan, densities of small copepods have declined precipitously since the late 1980s. Taken together, our findings indicate that availability of suitable prey for larval yellow perch in Lake Michigan warrants further investigation as a potential factor limiting recruitment during the last decade.