PARENT SESSION
Oral Session # 25: Invasive Species II: Aquatic Systems.
Presiding: G Rilov
Tuesday, August 5. 8:00 AM to 11:30 AM, SITCC Meeting Room 205.

Impacts of invasive crayfish on trophic structure in lake littoral zones.

Wilson, Karen^*,1, Willis, Theodore², Vander Zanden, M. Jake², ¹ Carleton College, Northfield, MN² Center for Limnology, Madison, WI

ABSTRACT- In this study we investigate the impact of an invasive crayfish, Orconectes rusticus, on the food web structure of lake littoral zones in northern Wisconsin. The rusty crayfish is invasive across the upper Midwest and northeastern United States, as well as into parts of Canada. In northern Wisconsin lakes, O. rusticus often dominates benthic biomass, reaching trap abundances from 2 - 20 times the abundance of native species. High densities of rusty crayfish are negatively correlated with snail, macrophyte and macroinvertebrate abundances. O. rusticus obtains a size refuge from most predatory fishes at carapace lengths > 35 mm. We hypothesized that, at high densities, larger rusty crayfish represent the top of the near-shore benthic food web, forcing littoral fishes to switch to pelagically-derived food resources. To test this hypothesis, we assessed trophic structure in 5 high-density rusty crayfish lakes and 5 low-density rusty crayfish lakes in northern Wisconsin. We used stable isotopes (delta-¹³C and delta-¹⁵N) to measure trophic position and prey source (littoral vs. pelagic) for a trophic range of littoral fishes (smallmouth bass, bluegill, johnny darter, and yellow perch), crayfish, macroinvertebrates, snails and mussels. We also assessed ambient snail and macroinvertebrate populations using a benthic suction sampler in both cobble and sand habitats. Initial results suggest significantly fewer macroinvertebrates and snails in lakes with high densities of rusty crayfish; we expect that these differences will be reflected in strong shifts in trophic structure throughout the littoral food web.

Key words: trophic structure, Orconectes rusticus, invasive species