In contrast with similar removal efforts in inland lake ecosystems, our study is among the first to offer evidence of a compensatory response in fecundity and juvenile recruitment in Rusty Crayfish following repeated harvest removals (Hein et al. 2006, 2007; Tetzlaff et al. 2011; Hansen et al. 2013a). But while those studies did not find evidence of density-dependence, part of their management intervention strategy was to target juveniles through increased fish predation and thus may have masked a compensatory response. In that regard, our findings are unique but not entirely unexpected. For example, Hawaiian spiny lobster (DeMartini et al. 1993) exhibit compensatory increases in fecundity in response to commercial harvest. Nonetheless, our study raises questions about the conditions under which density dependence can manifest in a population. The most obvious difference between our study and those previously published is the environmental context and the management intervention strategy. Lake Michigan is oligotrophic, resulting from decades of filtering by non-native dreissenid mussels (Evans et al. 2011). Therefore, Lake Michigan may be more resource limited than typical inland lakes, which may lead to density dependent responses in juveniles when larger size classes are reduced and competition for limited resources is diminished. In this study, the effect of adults on juveniles was evident in the fitted curves from the stock-recruitment analysis. More comprehensive investigation of the relationship between primary production and Rusty Crayfish population dynamics is needed to help explain why our findings differed from previous studies.
Numerous examples can be found in the literature of aquatic invasive species removal programs producing similar responses in the target species populations. For example, after two years of harvest removals of European Perch (Perca fluviatilis) from New Zealand ponds, young-of-the-year quickly dominated the population structure of treatment ponds compared to control ponds (Ludgate and Closs 2003). A study on Common Carp (Cyprinus carpio) removals from interconnected natural lakes in southeastern South Dakota, USA found an inverse relationship between Carp survival and removal rate (Weber et al. 2016). An eradication program targeting European Green Crab (Carcinus maenas) in a small California estuary saw a 30-fold single year increase in population abundance compared to nearby estuaries with no eradication efforts (Grosholz et al. 2021). Finally, despite removing over 450,000 Lake Trout from Yellowstone Lake, USA over a 15 year period, population growth rates remain above replacement (Syslo et al. 2011). Our study adds to the growing body of evidence that while mechanical removal programs can be effective at producing short-term reductions in population densities, alternative management strategies and/or high sustained efforts are necessary to achieve long-term suppression and the desired ecological responses often stated as program goals.
Cannibalism is characteristic of many age or size-structured populations and can be an important mechanism for population regulation through negative density dependence (Polis 1981; Van Buskirk 1989; Post et al. 1999). Common across decapod crustaceans, cannibalism can be a successful strategy when heterospecific prey densities are low because conspecifics are energy rich and possess the correct nutrient compositions relative to the species in question (Parsons et al. 2013). Although we did not study cannibalism in Rusty Crayfish in this study, cannibalism is frequently observed in other crayfish populations and has been discussed in the context of invasive crayfish removal or suppression (Houghton et al. 2017; He et al. 2021). Previous research has indicated that size-segregation occurs among Rusty Crayfish on Lake Michigan reefs (Kvistad et al. 2021b). Further, evidence of compensatory population dynamics from this study may indicate that cannibalism is a potentially important mechanism on these habitats. Empirical studies of cannibalism on Lake Michigan reefs are needed. However, if verified, this knowledge could have important implications for crayfish management, as it would imply that adult-only harvest removal strategies alleviate intraspecific predation on the juvenile population.
Various aspects of population biology are emphasized in invasion biology (Sakai et al. 2001), but such knowledge is rarely used to predict management outcomes prior to application in the field. Yet quantifying population growth parameters and simulating perturbations can lead to valuable insights for guiding suppression efforts (Govindarajulu et al. 2005; Day et al. 2018). Mathematical models can provide scientific knowledge on the effectiveness of various management strategies which can help decision makers direct resources appropriately (N’Guyen et al. 2018). In one example, Milligan et al. (2017) used a hybrid mathematical model of Red Swamp Crayfish (Procambarus clarkii) and California Newt (Taricha torosa) population dynamics to determine optimal crayfish trapping regimes and predict time to local newt extinction under alternative trapping scenarios. Simulation studies leveraging knowledge of population biology and carefully designed models could save considerable time and effort and improve conservation outcomes by guiding management decisions. Incorporating aspects of density-dependence in population growth models through mechanisms such as competition or cannibalism could help improve Great Lakes invasive crayfish management strategies.
Programs focused only on mechanical removal usually must maintain high levels of effort over long timeframes to achieve desired ecological responses. Rapidly maturing species with high fecundity are among the most difficult to manage through mechanical removal alone, especially if the most widely available capture technology selects only for adult size classes (Stuecheli 1991). When programs do succeed, they usually combine multiple management strategies to ensure broad coverage of all life-history stages which can be sustained without continuous human intervention, such as various forms of biological control (Hein et al. 2007; Thresher et al. 2014). Thus while mechanical control strategies used in isolation often fail to achieve desired ecological outcomes, combining mechanical removals with alternative strategies (e.g., predator manipulation) can be highly successful (Hein et al. 2007; Thresher et al. 2014; Bajer 2019).
According to the enemy-release hypothesis, successful invaders often reach abundances near carrying capacity due to lack of regulation from co-evolved predators, pathogens, and competitors (Keane 2002; Mitchell and Power 2003; Torchin et al. 2003; Colautti et al. 2004). While the reefs in this study support populations of Rusty Crayfish predators, such as Smallmouth Bass (Micropterus dolomieu) and Rock Bass (Ambloplites rupestris), little is known about their population dynamics at nearshore reefs, especially during the fall when Rusty Crayfish are most active and susceptible to predation. Due to the open nature of the Great Lakes ecosystem compared to inland lakes and streams, migration of natural enemies may play an important role in predator-prey dynamics on reefs. In Lake Michigan, Smallmouth Bass have been documented migrating great distances offshore after spawning in the summer (Kaemingk et al. 2011). Therefore, Smallmouth Bass might be absent or only present in low densities in the late summer and fall when juvenile Rusty Crayfish recruitment occurs in Lake Michigan (Kvistad et al. 2021b). As such, a spatio-temporal mismatch of predator and prey in Lake Michigan may hinder the use of natural predators as a control mechanism. The use of native predators as a form of biocontrol on Rusty Crayfish could conceivably be achieved, however, through the use of enclosures to manipulate foraging locations and behaviors (Ward-Fear et al. 2009, 2010).
Recolonization after removal is another important consideration for harvest-management programs. Both the degree of isolation and the mobility potential of the targeted species can affect the outcome of harvest-management programs. Even in loosely connected ecosystems, relatively low immigration rates occurring infrequently (e.g., after heavy flooding in a system of interconnected lakes) can lead to rapid population recovery following harvest removal efforts (Weber et al. 2016). Buckley (2017) speculated that rapid recolonization occurred on Lake Michigan reefs after a period of intense trapping due to a rapid recovery observed in relative abundance following cessation of trapping. Our observations of traps further offshore catching more Rusty Crayfish later in the season support the possibility of recolonization occurring from outside the immediate treatment area. However, spatially explicit studies focused on measuring recolonization rates would be useful for improving understanding of immigration and emigration on reef habitats. Unfortunately, our mark-recapture efforts were not sufficient to characterize movement on reef habitats due to low-recapture rates. One potential reason for our low recapture rates is that we could not monitor whether any individuals left the study area. Nonetheless, we observed individuals traveling considerable distances suggesting a high degree of mobility. Movement dynamics of invasive species, including quantifying rates of immigration and emigration following removal efforts, is an area of research in need of greater attention (Green and Grosholz 2021). Such information could help conservation practitioners develop better suppression targets by considering meta-population dynamics and information about local connectivity across habitats. For Rusty Crayfish on GL reefs, our results suggest that successful control may require either removal across a larger buffer area (to mitigate recolonization of a core treatment area) or deployment of a more effective barrier to prevent recolonization.