Crayfish are critical components of freshwater ecosystems. In some ecosystems crayfish can comprise more biomass than all other benthic invertebrates combined. Despite their role as keystone species, crayfish are often an understudied organism. This is particularly true for Michigan, where the last comprehensive crayfish survey took place in 1975. Since this time, many non-native species, including the rusty crayfish (Orconectes rusticus), have been introduced into the Great Lakes region that could alter crayfish communities across the state. Further, documenting crayfish species composition and distributions in Michigan is becoming increasingly critical due to the imminent invasion of the red swamp crayfish (Procambarus clarkii). The Michigan Department of Natural Resources (MDNR) has implemented legislation to help discourage the introduction of invasive crayfishes, but the lack of recent surveys prevents the determination of whether any non-native crayfish, other than rusty crayfish, are already established in the state. To fill these knowledge gaps we designed and implemented a stratified random survey of streams in Michigan’s lower peninsula to assess the presence, distribution, and habitat associations of native and non-native crayfish species, as well as documented the expansion of rusty crayfish. Our results indicated that the distribution of most crayfish species are widespread throughout the state, while others have expanded their range, including the invasive rusty crayfish. Our results also suggest that rusty crayfish likely influence the habitat associations of some native crayfish species. Although no red swamp crayfish were discovered in Michigan, our assessment indicated risks associated with entry vectors red swamp crayfish could use to enter the state. Introduction would appear to be likely unless management actions are taken to prevent red swamp crayfish entry into Michigan via the pet, food, and bait trade.
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