We conducted an exhaustive search of the published literature to address our questions. This search aimed to determine the scope of published literature between 2000 and 2024 concerning the number of published articles identifying a crayfish invasion pathway in the Great Lakes Basin. This time period was chosen due to the expanding usage of the internet following the new millennium and to provide insight into modern crayfish invasion pathways in the GLB. The search was conducted principally in Web of Science using a brute force approach that sought to use multiple combinations of keywords and replications of the search with quotations around certain keywords. The full list of keywords and combinations we used (along with search results) is provided in Suppl. material 1: table S1. We focused our efforts strictly on the peer-reviewed literature. Our decision to focus on peer-reviewed literature uses a reliable and indexed source of material that facilitated the collation of information of crayfish invasive pathways in this system.

To summarize our search procedure, we performed keyword queries for research papers on crayfish invasion pathways reported A) generally for the entire GLB and B) for individual states and provinces within the GLB (Ontario, Minnesota, Wisconsin, Michigan, Illinois, Indiana, Ohio, Pennsylvania, and New York). We also included the terms “non-indigenous” and “invasive species” to pull more papers that could report relevant information about crayfish but might not be the main topic of the paper. We further included three crayfish species in our searches (common name and species names) -- red swamp crayfish (Procambarus clarkii), marbled crayfish (Procambarus virginalis), and rusty crayfish (Faxonius rusticus) as these three species are of general management interest across the basin. In total, our search included up to 9 unique search terms and 92 total search combinations of these terms, including repeat searches where we modified specific keywords using quotation marks. We then identified the relevant literature for our review based on several decision criteria that filtered for papers pertinent to our study’s questions. These criteria include whether the geographic scope of the paper fell within the GLB, if the paper identified/attributed any crayfish invasion pathways in the article, and if the paper was published during the period 2000–2024. These criteria allowed us to identify the recent history of identified crayfish invasion routes in the GLB and identify trends regarding the location of work, species of interest, history of work, and pathways used by crayfish to enter the Great Lakes Basin. We then categorized each article’s invasion pathway(s) into one of the 6 general (Hulme) pathways (Release, Escape, Contaminant, Stowaway, Corridor, and Unaided). Finally, the specific pathway, if discussed, such as aquarium release or bait release, was recorded as a subcategorization of the general invasion pathway.

From 2000 to 2024, our investigation revealed an average of ~2 articles per year that addressed crayfish invasion pathways in the GLB, indicating a modest output of papers that identified crayfish invasion pathways. Notably, during the first half of this period, we found an average of 0.91 articles per year that identified the invasion pathway used by crayfish to reach a location. In contrast, we observed an increase in the latter half of the period to 2.5 articles per year that identified the invasion pathway. This is an almost threefold rise in the rate of publications that identify the crayfish invasion pathway in their study. This increased trend in annual publications highlights the importance of research on crayfish invasion pathways. The majority of articles identified a single invasion pathway used by crayfish.

The spatial distribution of research papers on crayfish pathways encompassed several geographic foci (Fig. 1). Wisconsin, Michigan, and Illinois emerged as the primary research locales for recent literature on crayfish invasion pathways, accounting for 57% of regionally focused articles. Of the thirteen articles published in Wisconsin eight were released between 2001 and 2007 while in Michigan eight of the eleven articles have been published since 2017. The increase on crayfish invasion pathway research in Michigan occurred following the confirmation of red swamp crayfish within the state (Sard et al. 2023). The remaining 45% of papers identified invasion pathways in Canada, Indiana, New York, Ohio, and Pennsylvania. Only one state in the GLB, Minnesota, had no papers that identified crayfish invasion pathways specifically in that state. Seven papers on crayfish invasion pathways focused on the GLB region, with one additional nationwide analysis that included the GLB. This distribution highlights the geographic focus of existing research and underscores the importance of expanding the range of locations where we examine crayfish invasion pathways. We noticed that certain researchers and their lab groups (i.e. David M. Lodge – 7 publications) are well represented among the studies that identify crayfish invasion pathways, resulting in the high concentrations of papers on a subset of the states within the GLB (see Suppl. material 1: table S2).

Non-native crayfish are introduced through a variety of invasion pathways. When we assessed the specific crayfish invasion pathways found in the GLB, the most common were natural dispersal (26%) and bait release (34%) (Fig. 2). Over fifty percent of all pathways identified in the literature were from these two categories. Escape from the retail trade was the third most common specific pathway found in the GLB (11%). The remaining specific pathways identified in the literature comprised less than 30% of crayfish invasion pathways. We found several relevant trends when assessing the first and second half of the review period. We noticed that from 2000 to 2012 over 70% of pathways identified were from either natural dispersal or bait release, while these two pathways comprised only 50% of specific pathways from 2013 to 2024 (Fig. 2). This 20% decrease suggests an increase in new pathways, awareness of previously unrecognized or underestimated pathways, and/or a decreased reliance on historical literature to identify invasion pathways for crayfish in the GLB. We observed four times more publications identifying the retail trade in the latter half of the assessment period. We also identified ten new pathways in the literature from 2013–2024 that did not appear in previous years. Only one specific pathway category, general release, was not reported from 2013–2024. The loss of this invasion pathway could be due to the increased specificity with which authors identified invasion paths and explained the increase in the number of described pathways found in the invasive crayfish literature for the GLB from 2013–2024.

The majority of crayfish invasions in the GLB occurred via release or unaided dispersal when using the general invasion pathway categorizations from Hulme et al. (2015). 42% of all the invasions across the study period occurred via the release pathway, which suggests an important role of people in the establishment of invasive crayfish populations. A caveat is that many invasions that were reported occurring via release are potentially confounded by the citation of previous work. In other words, many papers attributed a release as the invasion pathway without independent verification of this pathway as the source of an invasive population (Bobeldyk and Lamberti 2008, 2010; Szela and Perry 2013; Arcella et al. 2014; Glon et al. 2017). Unaided dispersal was the second most common general pathway found in the literature and comprised 36% of the total pathways identified. The interconnected nature of waterways in the Great Lakes Basin could explain the prevalence of unaided dispersal following establishment in this system (Bouvier et al. 2009; Milt et al. 2018; Morreale et al. 2023). Escapes made up ~15% of the remaining general invasion pathways, while stowaway and corridor introductions comprise less than 7% of the reported general invasion pathways. Stowaway was the only reported general pathway to appear only prior to 2012. Every other reported general pathway category was represented throughout the review period.

We found invasion pathways explicitly linked to 13 crayfish species (Suppl. material 1: fig. S1) spanning three crayfish families (Astacidae, Cambaridae, and Parastacidae) (Crandall and Buhay 2008). The Cambaridae and Astacidae comprise Holarctic species in Europe and Western Asia (Astacidae) and the United States (Cambaridae and Astacidae), while the Parastacidae are strictly Neotropical and Australasian. The reported species included species native to the United States that are expanding their range through natural dispersal, and those introduced through accidental or intentional translocations, primarily from the Australasian and Eastern European biogeographic regions (Gherardi 2007, 2011).

Astacidae crayfish

Two Astacid crayfish were reported in the literature, one native to Eastern Europe and the other native to the Western United States (Crandall and Buhay 2008). The narrow-clawed crayfish (Pontastacus leptodactylus) (Eschscholtz, 1823) is native to Eurasia, with a broad range extending from eastern Europe through the Russian Urals with a southern range margin extending into Turkey (Davidson et al. 2021). Only a single study reported this crayfish occurring within the geographic scope covered in this review. In contrast, the other Astacid crayfish reported was the signal crayfish (Pacifastacus leniusculus) (Dana, 1852), native to the Upper Pacific coast of the United States (Larson and Olden 2011). This species is a well-documented invasive species in Europe, owing to intentional stocking to supplement fisheries harvests (Henttonen and Huner 1999; Bohman et al. 2006; Jussila et al. 2015). The species has proven to be an aggressive species to native European crayfish as well as a major carrier of crayfish plague (Aphanomyces astaci), to which European species are highly susceptible (Bubb et al. 2004; Bohman et al. 2006; Ruokonen et al. 2018). In the United States, P. leniusculus is predominantly restricted to the Pacific coastal states as the Rocky Mountains and the Great Basin are significant biogeographic barriers to their natural dispersal (Larson and Olden 2011). Our review found only a single publication identifying an invasion pathway for Astacid crayfish as part of an analysis on improving surveillance for the Great Lakes (Davidson et al. 2021). No Astacid crayfish are confirmed to be established in the GLB.

Parastacidae crayfish

Two species of crayfish from Parastacidae were reported to have a potential invasion pathway in the literature reviewed. These two species, Cherax tenuimanus (Smith, 1912) and Cherax destructor (Clark, 1936) were explicitly mentioned in a single publication examining the assessment of regulations for AIS and surveillance of AIS in the GLB (Davidson et al. 2021). Parastacid crayfish are found only in the Southern hemisphere; these species originated in Australia (Furse 2014). These species of Parastacid crayfish are of interest in the pet and aquarium trade, providing opportunities for their intentional or accidental release in new environments (Horwitz 1990; Chucholl 2013). To prevent the introduction of Parastacid crayfish into the GLB, it will be essential to assess the policies and surveillance of the retail and private trade of live crayfish while educating the public about the dangers of AIS (Lodge et al. 2016; Hoff et al. 2021). No Parastacid crayfish were reported to be established in the GLB.

Cambaridae crayfish

Thirteen species of Cambaridae crayfish were identified in the literature review. The preponderance of these species’ invasion paths was focused on Faxonius rusticus, the rusty crayfish. Rusty crayfish are native to the Ohio River Basin in the United States and have long been a species of concern in the GLB (Wilson 1994; Kuhlmann and Hazelton 2007; Bobeldyk and Lamberti 2010; Olden et al. 2011; Tobler and Morehouse 2013). This species helped give rise to the concept of the “bait-bucket introduction,” and these historical studies of crayfish invasion pathways are still commonly cited as the pathway for rusty crayfish invasion (Capelli and Magnuson 1983; Hobbs et al. 1989; Ludwig Jr. and Leitch 1996). Most research identifying rusty crayfish invasion pathways has come from a relatively small group of authors. The other Cambarid species that has received the most attention is Procambarus clarkii, the red swamp crayfish. Red swamp crayfish is a global invader that has historically been introduced as a food source in the aquaculture industry and today are often transported in the retail, aquarium, and scientific trade (Harper et al. 2002; Oficialdegui et al. 2020). Native to the Southern United States and Northern Mexico, red swamp crayfish have a variety of ecological impacts in invaded ecosystems (Gherardi 2006; Oficialdegui et al. 2020). The other remaining Cambarid species are understudied in comparison to rusty and red swamp crayfish (most being reported in at least one publication), with these species’ invasion pathways being identified in broad threat analyses or reviews in the GLB (Lieb et al. 2011; Peters et al. 2014; Davidson et al. 2021). If crayfish invasions into the GLB continue, further examination of these other Cambarid species will be vital. They may utilize different pathways to expand their ranges in the GLB from the more frequently studied rusty and red swamp crayfish.

After reviewing published research over the past 24 years, there is substantial evidence that release and natural dispersal are the principal crayfish invasion pathways currently active within the GLB. Furthermore, the literature concludes that these two pathways are coupled: where release happens, dispersal to surrounding systems inevitably follows (Byron and Wilson 2001; Aquiloni et al. 2005; Gherardi 2006; Sard et al. 2023). The coupling of these pathways creates challenges for researchers and managers who seek to reduce the subsequent adverse effects of the invasion. Focusing efforts to constrain the release pathway would require a combined approach of public engagement and legislative enforcement to encourage awareness and proactive engagement (Seekamp et al. 2016). Inspection authorities could also develop training and workshops for agents and commercial owners to increase compliance and self-reporting (Oele et al. 2015). However, efforts to constrain natural dispersal are challenging to implement. Complete eradication of established invasive crayfish populations may be an impractical goal; instead, early detection and rapid response should be prioritized to prevent the establishment of new populations. One example of a successful early detection and rapid response to a crayfish invasion occurred in Eastern Wisconsin where an invasive population was successfully eradicated. This case study highlights the importance of early detection and rapid response and the difficulty in eradicating crayfish populations.

Another novel takeaway is that there is some consistency in the rate of work being published that explores crayfish invasion pathways throughout the GLB. However, given the bulk of research output was centered on three states and only a few species, researchers across the GLB currently have a relatively narrow lens for gauging active crayfish invasion pathways within the region. On the one hand, this could be interpreted as a positive sign that introductions do not occur at a rapid rate and that current management measures intended to prevent introductions (through release prohibitions, outreach programs, or regulations) effectively limit introduction rates. On the other hand, this narrow focus highlights the need for broader investigations that account for lesser-studied pathways, species, and geographic regions to ensure management strategies remain robust to potential future introductions.

What we also noticed from author affiliations was the high frequency of partnerships being leveraged by management and non-management entities to publish these research articles (e.g., the state Department of Natural Resources collaborating with academic laboratories). Such collaborations underscore the importance of integrating management and academic expertise to study crayfish invasions in the GLB effectively. For example, we identified that a current shortcoming of crayfish invasion pathway research in the GLB is an over-reliance on historical citations to identify the invasion pathway without further scrutiny (e.g., Capelli and Magnuson 1983; Ludwig Jr. and Leitch 1996). Alternate plausible invasion pathways can be uncovered through closer examination of newly established populations (e.g., Sard et al. 2023), and this work can further benefit from combining resources through partnerships and collaborations (Capelli and Magnuson 1983; Ludwig Jr. and Leitch 1996; Sard et al. 2023). Our findings, therefore, suggest that cooperative research efforts will be critical to current and future crayfish pathways research and can enhance our ability to communicate more clearly with the public and policymakers concerning where specifically to focus outreach and legislative efforts (Folke et al. 2005; Davidson et al. 2021; Hoff et al. 2021). Future pathways research for crayfish will likely need to rely on existing or more broadly established partnerships to fully assess and quantify pathway risks.

One area that can benefit from well-informed legislative efforts concerns the retail trade (e.g., aquarium shops, biological suppliers, wholesalers, and individual breeders), which could unintentionally (or otherwise) trade species under incorrect species and common names that would easily escape the attention of inspectors (Chucholl 2013; Olden and Carvalho 2024). Strengthening retail trade practice oversight, pet/aquaria ownership responsibility awareness, oversight of restricted species in biological supply for classrooms, and public education on the ecological consequences of introducing non-native species could mitigate this risk. Additionally, more systematic efforts to monitor and audit retail pathways would help detect and address these issues before they result in further introductions (Oficialdegui et al. 2025). Improving the accessibility to pet surrender and rehoming organizations may curtail the number of releases in the increasingly global and online ecosystem where a variety of species are readily available to individuals.

Through this review, we found steady progress in the last 24 years in researching crayfish invasion pathways within the GLB. Our findings add neatly to the global body of work on AIS invasion pathways, but despite this, significant gaps that limit our understanding of their full scope remain. Keller et al. (2011) noted historical knowledge deficiencies in invasion pathways in Europe, although they identified similar (ornamental trade) and different pathways (fisheries stocking) specific to this region . In contrast for South America, Loureiro et al. (2015) and de Oliveira et al. (2023) discussed the importance of the pet retail trade and aquaculture for invasive red swamp crayfish introductions, but little information is available overviewing active crayfish invasion pathways for this extremely biodiverse region of the world. While comprehensive and large-scale studies like ours for crayfish invasion pathways remain sparse, fortunately new research interest is increasing to examine the complexity of invasion pathways across larger and global spatial scales (e.g., Oficialdegui et al. 2019, 2020; Olden and Carvalho 2024). We also note that the available research disproportionately focuses on a narrow range of species and locations, leaving other potential pathways and regions underexplored. A reliance on older studies to identify pathways underscores the need for updated and comprehensive investigations into contemporary pathways, particularly new pathways identified in the second half of the review period. White papers, governmental reports, public websites, online databases (i.e. Great Lakes Aquatic Nonindigenous Species Information System), and theses/dissertations (i.e. gray literature) would likely further inform our regional understanding of crayfish invasion pathways. Strengthening collaborations between management entities, academic researchers, and policymakers to increase attention and efforts address emerging threats like misidentifications in the retail trade, will be critical to prevent future invasions. Existing researchers who are active in this area are valuable resources for managers and policymakers. We would encourage these groups to collaborate with researchers who are active in this space to implement science-based approaches and techniques. By broadening the scope of research and improving public outreach and regulatory frameworks we can more effectively safeguard the GLB’s ecosystems against the continued threat of invasive crayfish.