Effects of Alewife (Alosa Pseudoharengus) Predation on Zooplankton Community Structure in Honeoye and Conesus Lakes

Abstract

The objective of this study was to describe and compare the chemistry and alewife and piscivore abundance between Honeoye and Conesus Lakes to identify possible reasons of the continuing presence of large-bodied zooplankton in Honeoye Lake in the presence of alewife. Alewives were accidentally introduced into Conesus Lake in the late 1970's. By 1985, large Daphnia had disappeared and Crustacea size in Conesus Lake had decreased from 1.03 mm in 1972 to 0.5 mm in 1985 and has continued to decrease to 0.29 mm in 1988 and 1993. Alewives were confirmed in Honeoye Lake by 1991, but unconfirmed reports go back to 1988. Unlike Conesus Lake, Honeoye Lake still has a large-bodied zooplankton population. The weighted mean length of Crustacea in Honeoye Lake is 0.64 mm. The classical effects on zooplankton size distribution of alewife predation that occurred in Conesus Lake by 1985 were not happening in Honeoye Lake despite the fact that the alewife in Honeoye Lake were selecting those Daphnia that were greater than one millimeter in length. There were significant differences in chemistry between the two lakes. However, none of these differences could account for differences in the zooplankton communities. Chlorophyll a was significantly higher in the epilimnion of Conesus Lake, so it could be assumed that there would be ample forage to support a large cladoceran population. Nitrate + nitrite and total phosphorus were not significantly different between the two lakes. Soluble reactive phosphorus was significantly higher in the epilimnion of Conesus Lake. The density of alewives in Honeoye Lake (0.08 fish/minute of trawling) is much less than that of Conesus Lake (0.5 fish/minute of trawling). Possible reasons for this density difference were explored. The winter water temperature of Honeoye Lake does not drop low enough (5° C) to affect alewife survival. A substantial piscivore community could be keeping the alewife population in check. There is an extremely large population of walleye in Honeoye Lake (20,000), which is one-fifth the volume of Conesus Lake (10,000 walleye). Other possible fish that could be responsible for keeping the alewife density low are small and large mouth bass, pickerel and panfish, such as yellow perch and sunfish. All things considered, the only difference between Honeoye and Conesus Lakes that could account for the differences in planktivore density and zooplankton community structure appears to be the top down effects of their respective piscivore communities.