Long‐Term Declines in Body Size of the Invasive Rusty Crayfish (<i>Faxonius rusticus</i>) in Temperate Lakes

ABSTRACT Invasive species often experience phenotypic change during the invasion process. For example, many invasive species are larger in their non‐native than native ranges, but some invasive species experience body size declines with time since invasion. Mechanisms of these phenotypic changes are poorly known, likely due to a paucity of long‐term datasets and complex interactions among abiotic and biotic factors that affect body size over time. We use a long‐term dataset (1980–2020) in 17 lakes of Wisconsin, United States to investigate trends in body size for an invasive species, the rusty crayfish ( Faxonius rusticus ). We relate F. rusticus body size to crayfish relative abundance as catch‐per‐unit effort (CPUE) from baited trapping, as well as modelled lake temperatures and pelagic primary productivity, using a structural equation model (SEM). Our use of an SEM allowed us to investigate the direct effect of time on F. rusticus on body size, as well as indirect effects through factors like warming of lakes due to climate change. We found that F. rusticus body size has declined by 10% over the past four decades in our study lakes, from a mean of 36.6 mm total carapace length in 1980 to 32.8 mm in 2020. Faxonius rusticus individuals were larger when lakes were warmer, but the overall effect of time on declining F. rusticus body size was stronger than this predictor. Relative abundance as CPUE had no effect on F. rusticus body size, rejecting a role for density dependence in explaining adult body size of this invasive crayfish. Declining F. rusticus body sizes have accompanied population declines of this invasive crayfish in some Wisconsin lakes, and both trends provide potential for post‐invasion ecosystem recovery. As one example, smaller F. rusticus individuals may be less effective or active predators on fish nests. By contrast, declining F. rusticus body size may also create opportunities for serial or over‐invasion by future crayfish invaders, sustaining a need to discourage crayfish introductions to this region.