Evidence of a compensatory response in invasive Rusty Crayfish (Faxonius rusticus) following intensive harvest removal from northern Lake Michigan fish spawning reefs

Abstract

Abstract The goal of a harvest removal program to control invasive species is usually to achieve sustained reductions in population abundance. Yet removal efforts may be counteracted by density-dependent processes by increasing available resources and reducing stage-specific mortality. We tested a harvest removal strategy for invasive Rusty Crayfish (Faxonius rusticus) focused on nearshore cobble reefs, used by several native fishes as spawning habitats, in Lake Michigan with a before-after control-impact (BACI) study design. We removed 3182 Rusty Crayfish, primarily adults (> 20 mm carapace length), at two sites over two harvest seasons, expending 17,825 trap days in effort. While the BACI analysis results suggested that a statistically significant reduction in Rusty Crayfish densities assessed from diver quadrat surveys was achieved on one reef (Little Traverse Bay Crib), suppression effects were highly variable across sites. In addition, reduced densities were sustained over the egg maturation period for native fish and into the following year after removal ceased. However, by late summer/early fall between consecutive suppression efforts in 2018 and 2019 we observed a threefold increase in pre-removal densities. Size-frequency histograms from diver quadrat surveys showed higher abundance of juvenile size classes over the following spring and summer at Little Traverse Bay Crib compared to its paired reference site, suggesting a compensatory response. Evidence of compensation was corroborated by stock-recruitment analysis and a coincident increase in ovarian egg counts at Little Traverse Bay Crib between 2018 and 2019. With a proviso that we only conducted two seasons of consecutive suppression, our results emphasizes the effects of density-dependent processes in invasive species management and raises questions about efficacy of adult-only removal strategies.