Large-Lake Responses to Declines in the Abundance of a Major Fish Planktivore—the Lake Michigan Example

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Abstract
Alewife abundances declined dramatically in southeastern Lake Michigan over 1973-77, several years before the lakewide decline occurred. The regional effects of this decline on adult copepod abundances, zooplankton biomass, and water clarity are examined. In the offshore region, the two largest copepods, Limnocalanus macrurus and Diaptomus sicilis, increased in abundance during the mid-1970''s, reflecting the decrease in alewife predation. Limnocalanus macrurus abundances declined in later years, reflecting increased predation pressures from the increasing bloater population. The small-bodied D. minutus and the medium-bodied D. ashlandi exhibited no apparent response to the decline in alewife abundance. Large-bodied D. oregonensis and small-bodied C. bicuspidatus thomasi declined in abundance. Size-selective fish predation pressures continued to remain high in the inshore region: increased abundances of yellow perch and rainbow smelt apparently compensated for the alewife decline. Zooplankton biomass, zooplankton mean dry weight, and water clarity apparently were not affected by the decline in either the inshore or offshore region. The results of this study are evaluated in terms of the lakewide decline in alewife abundance, the summer 1983 dominance of Daphnia pulicaria in offshore waters, the 1983 marked improvement in offshore water clarity, and later changes in summer offshore D. pulicaria populations.

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