Understanding Recruitment of Lake Michigan Fishes: The Importance of Size-Based Interactions Between Fish and Zoopiankton

Abstract

Mechanisms controlling recruitment of fishes appear to be strongly size dependent. It is now established that size-selective predators can dramatically reduce zooplankton size, but little is known about the effects of zooplankton size on growth and recruitment of fish through the post-larval stage. As fish grow, their optimal prey size increases; if large zooplankton are uncommon, growth rates may be reduced, prolonging vulnerability to predation or other size-dependent mortality sources and thus reducing recruitment. Most Lake Michigan fishes, including offshore species such as bloater (Coregonus hoyi) and nearshore species such as yellow perch (Perca flavescens), shift from feeding on zooplankton in their early years to feeding on benthic prey or to piscivory. Predation mortality on many larval and juvenile fishes including bloater and yellow perch has been shown to be size or growth rate dependent. As alewife (Alosa pseudoharengus) increased in abundance in Lake Michigan in the 1960s, large zooplankton declined and both bloater and perch recruitment was poor. In the late 1970s and early 1980s, alewife declined, large zooplankton increased, and strong year classes of bloater and perch were formed. Based on these dynamics and recent research on resource use, foraging behavior, and recruitment dynamics of larval and juvenile fishes, we suggest two hypotheses. First, young-of-year and juvenile pelagic fishes may have the major size-structuring effects on epilimnial zooplankton in Lake Michigan. And second, if large zooplankton are uncommon, as they were in Lake Michigan in the 1960s, growth rates and recruitment of native fishes will be reduced. Size-based interactions between fish and zooplankton appear to have important implications for growth and recruitment success of fishes.