Whole Ecosystem Effects of Removing Invasive Rusty Crayfish from a North Temperate Lake

The rusty crayfish (Orconectes rusticus) is an invasive species that impacts all trophic levels of aquatic ecosystems.  However, in some systems they do not achieve high densities and do not negatively affect native biota.  We conducted a whole-lake experiment to remove rusty crayfish from a northern Wisconsin lake to induce a shift from a high-density to a low-density crayfish lake.  Crayfish were trapped from 2001-2008, and fishing regulations were changed in 2001 to increase fish predation on crayfish.  Our objectives were to 1) reduce rusty crayfish density; and 2) evaluate the response of native fishes, non-crayfish invertebrates, and macrophytes to the crayfish removal.  The rusty crayfish population was monitored using catch rates and independent mark-recapture estimates.  Populations of native fishes were estimated using mark-recapture, and diet compositions were analyzed for four years in which rusty crayfish density was high (2000-2003) and three years in which rusty crayfish density was low (2004, 2009-2010).  To evaluate the food web response of the crayfish removal, macroinvertebrates were sampled for two years when rusty crayfish densities were high (2002-2003) and three years when crayfish densities were low (2008-2010).  Bioenergetics modeling was used to estimate the consumption of invertebrates by fishes. Macrophyte cover and species composition was sampled annually throughout the crayfish removal.  Rusty crayfish catch rates declined by two orders of magnitude, and have remained low following the cessation of trapping.  Stock recruitment analysis of rusty crayfish revealed no compensatory mechanisms that could lead to increased reproductive success at low adult densities. Populations of Lepomis spp. increased as a result of the crayfish removal, while other littoral fish populations remained approximately constant.  The response of invertebrates varied among taxa and habitat types.  Total invertebrate density declined by nearly 80% in years following the crayfish removal in non-cobble habitats, but remained relatively constant in cobble (the preferred habitat of rusty crayfish).  Snail density increased 200 fold in cobble habitat, while all other taxonomic groups either declined or exhibited no response in all habitat types.  Macrophyte cover and species richness increased as rusty crayfish declined, with minimal effects on invertebrate densities. Total consumption of invertebrates by fishes increased as crayfish density declined.  Contrary to expectations, most invertebrate taxa appear to be more strongly affected by indirect effects (i.e., fish consumption) than by direct effects of rusty crayfish. This experiment has implications for restoration of other invaded lakes, as well as for invasive species prevention efforts.