PARENT SESSION
Oral Session #61: Aquatic Ecology: Fish - Communities and interactions. Presiding: J. Chick.
Thursday, August 9, 2001. 8:00 AM to 11:45 AM. Hall of Ideas F.
Multiple trophic level responses to a planktivore die-off and sustained piscivore stocking in eutrophic Lake Mendota.
LATHROP, RICHARD1,2, JOHNSON, BRETT3, JOHNSON, TIMOTHY4, VOGELSANG, MICHAEL1, CARPENTER, STEPHEN2, HRABIK, THOMAS2, KITCHELL, JAMES2, MAGNUSON, JOHN2, RUDSTAM, L5, STEWART, R1, 1 2 3 4 5
ABSTRACT- Walleye (Stizostedion vitreum) and northern pike (Esox lucius) fingerlings were heavily stocked during 1987-1992 to increase piscivory with a goal of reducing planktivory and increasing herbivory in eutrophic Lake Mendota. Fish have been stocked at lesser rates in subsequent years to maintain populations. In response to the increased piscivore densities, fishing pressure and catch rates all increased rapidly for the two piscivores even though regulations on bag limits and minimum size limits were imposed to protect the fishery. By 1998, population densities remained high for walleye. Northern pike densities and size structure also improved coupled with a dramatic decline in exploitation rates, especially after more restrictive bag/size limits were imposed in 1995. Following a major die-off of a key planktivore in 1987, planktivory rates were much lower from 1988 through 1998 allowing the large-bodied Daphnia pulicaria to flourish compared to earlier years. Increased herbivory rates have reduced algal densities and increased water clarity since 1988, although an increase in phosphorus (P) loadings during the 1990s stimulated summer blue-green algal blooms and likely offset much of the increased herbivory effect. We discuss biotic community responses at multiple trophic levels to the planktivore die-off and piscivore stocking, with reference to natural variations in P loading rates and dominant planktivore species including responses to an upsurge in planktivore biomass in 1999.
KEY WORDS: Lake Mendota, Piscivore stocking, Trophic cascade