Crayfish Abundance Research Articles

Can intensive trapping to reduce the abundance of an invasive crayfish benefit an imperiled congener?

Abstract Management aimed at reducing the population sizes of established invasive species can be beneficial for the conservation of native species. Crayfish are some of the most widespread and impactful invasive species in freshwater ecosystems. This study evaluated the efficacy of intensive baited trapping as a tool to reduce the abundance of an invasive crayfish (Procambarus spiculifer) in small streams and benefit an imperiled congener (Procambarus pictus). The invasive crayfish was captured and removed from a 1‐km stream reach through three rounds of intensive trapping conducted over a 16‐month period. The abundance of the invasive species declined significantly in the trapping reach after the initial round of trapping. Throughout the year following the onset of trapping, its abundance within the trapping reach was less than half of that at control sites. The sex ratio of invasive crayfish collected in traps was approximately 1:1, and 31% of captured individuals were likely to be immature based on size class. The abundance of the imperiled species did not change significantly over the study period, but 1 year may not be long enough to detect a response. Overall, the data suggest that periodic intensive trapping can effectively reduce the abundance of this invasive crayfish in small streams, and the extent to which this benefits the imperiled species remains an open question. Future research examining how trapping can best be applied to achieve lasting effects (including trap type, season and barriers to immigration) would be valuable for understanding the conservation potential of this management technique.

Characteristics of the Stone Crayfish Population along a Disturbance Gradient—A Case Study of the Kustošak Stream, Croatia

The stone crayfish, Austropotamobius torrentium, is a native European freshwater species sensitive to habitat alterations such as stream channelization and urban development, putting its populations at risk. This study aims to assess how habitat alteration and subsequent water quality changes affect the stone crayfish population in the Kustošak Stream (Croatia) through three selected sites under different levels of anthropogenic disturbance: (1) undisturbed; (2) recently modified, and (3) long-time modified sites. At each site, crayfish were captured, measured, and marked to estimate population size, structure, and crayfish condition. Additionally, we examined whether water quality (18 measured physicochemical parameters) affects relative crayfish abundance. We also used mitochondrial sequencing and microsatellite genotyping to assess species genetic diversity and population connectivity and to compare results among the sites. The results showed that habitat alteration caused an overall reduction in population abundance and changes in size structure; however, it had not yet resulted in detectable genetic differences. Partial least squares regression showed that crayfish abundance was affected by the physicochemical parameters of water, including, among others, oxygen, calcium ions, nitrates, pH, and water temperature. We discuss our findings in the context of the effects of anthropogenic disturbance on the viability and persistence of this EU priority species.

Long-term macrophyte and snail community responses to population declines of invasive rusty crayfish (Faxonius rusticus).

A central focus of invasive species research has been on human efforts to eradicate invaders or reduce their abundance to mitigate the worst of their impacts. In some cases, however, populations of invasive species decline without human intervention, which may inform management responses to these invaders. Such is the case of the invasive rusty crayfish (Faxonius rusticus) in northern Wisconsin, USA, where systematic population monitoring since 1975 has revealed population declines in approximately half of the lakes surveyed. Population declines of invasive species without human intervention remain understudied, but there is even less research on how communities respond following such declines. Using 10 lakes in Vilas County, Wisconsin, we investigated community recovery of habitat (macrophytes) and prey (freshwater snails) of F. rusticus following up to 33 years of declines of this invader in some lakes using a dataset with a rare, long-term span over which consistent data were collected (1987, 2002, 2011, and 2020). We compared community responses in lakes where F. rusticus populations reached a peak and subsequently declined (boom-bust lakes) and lakes where our dataset only captured the decline of F. rusticus (bust lakes) to reference lakes with consistently high or low crayfish abundance over time. We found partial recovery of macrophytes and snails in the bust and boom-bust lakes where F. rusticus has declined, with recovery of macrophyte abundance and richness in the boom-bust lakes achieving levels observed in the low-crayfish reference lakes. Snail abundance and richness increased after declines of F. rusticus, though not to the level of the low-crayfish reference lakes, suggesting that snail recovery may lag macrophyte recovery because snails are dependent on macrophytes and associated periphyton for habitat. The recovery we document potentially represents long-term ecosystem resilience of lakes to biological invasions. Our results suggest that lake communities may recover without active restoration interventions after invasive crayfish population declines, although identifying which lakes experience these natural declines remains a priority for future research and management.

Long-term trends in crayfish invasions across European rivers

Europe has experienced a substantial increase in non-indigenous crayfish species (NICS) since the mid-20th century due to their extensive use in fisheries, aquaculture and, more recently, pet trade. Despite relatively long invasion histories of some NICS and negative impacts on biodiversity and ecosystem functioning, large spatio-temporal analyses of their occurrences are lacking. Here, we used a large freshwater macroinvertebrate database to evaluate what information on NICS can be obtained from widely applied biomonitoring approaches and how usable such data is for descriptions of trends in identified NICS species. We found 160 time-series containing NICS between 1983 and 2019, to infer temporal patterns and environmental drivers of species and region-specific trends. Using a combination of meta-regression and generalized linear models, we found no significant temporal trend for the abundance of any species (Procambarus clarkii, Pacifastacus leniusculus or Faxonius limosus) at the European scale, but identified species-specific predictors of abundances. While analysis of the spatial range expansion of NICS was positive (i.e. increasing spread) in England and negative (significant retreat) in northern Spain, no trend was detected in Hungary and the Dutch-German-Luxembourg region. The average invasion velocity varied among countries, ranging from 30 km/year in England to 90 km/year in Hungary. The average invasion velocity gradually decreased over time in the long term, with declines being fastest in the Dutch-German-Luxembourg region, and much slower in England. Considering that NICS pose a substantial threat to aquatic biodiversity across Europe, our study highlights the utility and importance of collecting high resolution (i.e. annual) biomonitoring data using a sampling protocol that is able to estimate crayfish abundance, enabling a more profound understanding of NICS impacts on biodiversity.

Community assembly of prairie farm ponds: build it and they will come, stock it and they won't

Dam construction affects freshwater ecosystems worldwide. While there is much focus on large impoundments, farm ponds are overlooked despite their near-ubiquity across human-altered landscapes. Within the Great Plains of North America, there are millions of farm ponds, yet little is known about the fish communities and factors structuring them. We propose a conceptual model where fish, amphibian, and crayfish abundances differ along a pond size and water permanency gradient and are further influenced by interactions among species. In the summer of 2021, we sampled 100 farm ponds across central Kansas, primarily on private land. Pond size and permanency explained community structure with smaller and less permanent ponds being dominated by amphibians and crayfish while larger ponds were dominated by stocked sportfish. Distribution modeling revealed a negative correlation between stocked fish and other community components indicating potential interactions. If we are to conserve headwater stream species, especially those that are threatened or endangered, strategies that integrate farm ponds seem necessary given their prevalence on the landscape.

Evaluating the Efficacy of Approaches to Control Invasive Populations: A Conceptual Model Development for the Signal Crayfish

Invasive crayfish are among the major threats to freshwater ecosystems, with the signal crayfish, Pacifastacus leniusculus, being one of the most successful crayfish invaders in Europe. Approaches to invasive crayfish control range from manual and physical to biological and biocidal control methods. However, all of these approaches have their drawbacks and have limited efficacy. Among traditional approaches with minimal impacts on environment and non-target species, manual control via trapping is the most frequently applied. More innovative approaches comprise, among others, usage of sterile male release technique, whose efficacy in the field is yet to be fully tested, especially how it combines with more traditional approaches. A good alternative to costly and logistically challenging field comparisons of these approaches and their combinations is population modeling. Population models can integrate all relevant species-specific biological and ecological information and can be applied to identify management scenarios of highest impact on invasive crayfish abundances. In this study, we developed a conceptual population model of the invasive P. leniusculus following the Pop-GUIDE framework. In addition, we expanded on the framework to increase its applicability to other fields beyond environmental risk assessment. Finally, we discuss potential application of the model and its future use as a management tool.

Investigation of crayfish abundance in proximity to bridges, with emphasis on the Threatened Big Sandy crayfish (C. callainus)

The Big Sandy Crayfish, Cambarus callainus, is a large-bodied crustacean which inhabits third order streams within the Levisa, Tug, and Russel Fork basins throughout the conjunction of West Virginia, Virginia, and Kentucky. In 2016, C. callainus was federally designated as Threatened under the Endangered Species Act (ESA), which ushered in long‑needed attention and protection for the species. ESA benefits are broad in scope and highly beneficial to the species persistence and recovery. However, such broad levels of protection necessitate intense levels of scrutiny and regulation for development projects transpiring near confirmed or expected occurrences of protected species. West Virginia as a whole needs updated infrastructure, and this is especially true for the southern coalfields where C. callainus persists. Due to recent surges of funding through the Infrastructure Investment and Jobs Act, West Virginia is set to receive $3.7 billion dollars designated specifically for the repair or replacement of bridges and highways. Many bridges on the docket for maintenance/reconstruction cross streams within C. callainus distribution. Herein, I propose a C. callainus crayfish survey and habitat modeling project to determine whether this species exhibits preference, avoidance, or indifference to those bridge crossings in most dire need of repair or replacement. This project will enable private and public infrastructure entities to more efficiently generate mitigation and harm‑reduction protocols, and will reduce unexpected expenditures related to ESA protections.

Combined per‐capita and abundance effects of an invasive species on native invertebrate diversity and a key ecosystem process

Abstract Invasive alien species (IAS) are leading to the homogenisation of taxonomic and functional biodiversity, with negative consequences for key ecosystem processes in fresh water. Invasive signal crayfish (Pacifastacus leniusculus) is expected to disrupt detritus‐based food webs by affecting leaf breakdown and/or by decreasing invertebrate density and diversity through predation. The combination of per‐capita and abundance effects of P. leniusculus in invaded ecosystems is still largely unknown. A four‐week field experiment was established in Rabaçal and Tuela Rivers (NE Portugal) to assess effects of P. leniusculus on invertebrate taxonomic and functional diversity and leaf litter breakdown following a gradient of invasion. We controlled the presence and absence of crayfish by placing the animals and leaf litter inside cages at six sites (three sites per river) according to the crayfish in situ abundance (absent, low, high). Cages were covered with coarse‐ or fine‐mesh net to allow or prevent invertebrates from accessing the leaves. Results showed that higher crayfish in situ abundance led to a decrease in abundance, richness and Shannon diversity of invertebrates and to changes in the communities’ structures. Higher crayfish abundance led also to a decrease in invertebrate functional redundancy and an increase in the percentage of invertebrate taxa with resistance forms. Leaf litter breakdown increased with crayfish presence and decreased at sites with higher crayfish abundance. Overall, signal crayfish changed the community structure of invertebrates, with potential severe long‐term effects on native communities and leaf litter breakdown. Given the widespread distribution of signal crayfish (and other crayfish species), their ecological impacts should be assessed carefully, especially in pristine freshwater ecosystems such as those described here.

Factors Determining the Abundance of Red Swamp Crayfish (Procambarus clarkii) in a Large Lake Connected to the Yangzte River

Invasive species and habitat degradation are the main reasons for freshwater biodiversity loss. Reports on the habitat degradation and invasion ecology of Red Swamp crayfish (Procambarus clarkii) are few, although it is one of the most devastating invasive species in freshwater ecosystems. Based on a three-year investigation during 2017–2019 in West Dongting Lake, this study used Principal Component Analysis (PCA), Generalized Linear Model (GLM) and Bayesian model to analyze the relationships of P. clarkii stocks and environment factors in natural and modified wetlands. The results showed that the abundance of P. clarkii was positively correlated with total nitrogen, total phosphorus, water temperature, pH, water depth, and water transparency; while it was negatively correlated with dissolved oxygen and redox potential. The difference between P. clarkii stocks in natural and modified wetlands was nonsignificant. The P. clarkii stock decreased yearly, as levels in both 2018 and 2019 were significantly lower than in 2017. We concluded that inter-annual variation of the hydrological regime plays an important role in P. clarkii dynamics, and thus it is of great importance to ensure that the water level and flow velocity in summer to control the invasive P. clarkii.

Immune Response in Crayfish Is Species-Specific and Exhibits Changes along Invasion Range of a Successful Invader.

Simple SummaryA tiny fraction of all introduced species worldwide successfully establishes viable populations that rapidly spread and negatively impact native biodiversity and ecosystem functioning—i.e., become invasive. Even so, invasive species exert adverse environmental, economic, and human health impacts globally. Thus, identification of successful invaders is one of the immediate challenges in invasion biology. Recent studies highlight species’ immunity as an important component of invasion success since it enables invaders to adapt to the novel conditions as they expand their range. Here, we analyzed whether the immune response of the successful crayfish invader, the signal crayfish, changes as the invader encounters different environmental conditions and novel counterparts. We used several parameters, frequently applied to assess crayfish immunological status, and compared their cumulative pattern between sites along the signal crayfish invasion range in the Korana River, Croatia, as well as between the invasive signal crayfish and the native narrow-clawed crayfish. Immune response differed between native and invasive species and exhibited variations along the invasion range, which were mostly affected by water temperature and crayfish density. Our results indicate that changes in immunity may occur during range expansion, and imply that immunity could have a role in invasion success of invertebrate invaders.Immunity is an important component of invasion success since it enables invaders’ adaptation to conditions of the novel environment as they expand their range. Immune response of invaders may vary along the invasion range due to encountered parasites/microbial communities, conditions of the local environment, and ecological processes that arise during the range expansion. Here, we analyzed changes in the immune response along the invasion range of one of the most successful aquatic invaders, the signal crayfish, in the recently invaded Korana River, Croatia. We used several standard immune parameters (encapsulation response, hemocyte count, phenoloxidaze activity, and total prophenoloxidaze) to: i) compare immune response of the signal crayfish along its invasion range, and between species (comparison with co-occurring native narrow-clawed crayfish), and ii) analyze effects of specific predictors (water temperature, crayfish abundance, and body condition) on crayfish immune response changes. Immune response displayed species-specificity, differed significantly along the signal crayfish invasion range, and was mostly affected by water temperature and population abundance. Specific immune parameters showed density-dependent variation corresponding to increased investment in them during range expansion. Obtained results offer baseline insights for elucidating the role of immunocompetence in the invasion success of an invertebrate freshwater invader.

Spatial and temporal patterns in native and invasive crayfishes during a 19‐year whole‐lake invasive crayfish removal experiment

Abstract Understanding where, when, and how native species persist in the face of invasive species‐driven ecosystem change is critical for invasive species management and native species conservation. In some cases, ecological interactions among native and invasive species are spatially structured, and spatial segregation can be a key coexistence mechanism for ecologically similar taxa. We evaluated 19‐years of spatially explicit crayfish community data from a long‐term whole‐lake experiment, which includes 8 years of invasive rusty crayfish (Faxonius rusticus) removal followed by 11 years of post‐removal data collection. We quantified the within lake spatiotemporal patterns of virile crayfish (F. virilis) and rusty crayfish, and relate their dynamics to site‐level habitat conditions. In response to removal efforts, rusty crayfish catch rates declined by >95%, and native virile crayfish catch rates increased by more than 20‐fold. Ten years after ceasing removals, rusty crayfish have stayed at this relatively low abundance, and the virile crayfish population has remained stable. During removal, rusty crayfish abundances decreased non‐uniformly throughout the lake. Only after rusty crayfish populations were at their lowest levels did the native virile crayfish population begin to show signs of a recovery. Virile crayfish recovery was highly localized within the lake, and likely influenced by habitat and rusty crayfish abundance. Initially, virile crayfish made the most substantial resurgence in an area of the lake with rocky habitat conditions, but through time their distribution shifted into adjacent suboptimal macrophyte and muck habitats as rusty crayfish became more abundant in nearby areas. In general, when the two species overlapped in space, virile crayfish abundance stayed low, or the population shifted to adjacent areas with fewer competitively dominant rusty crayfish. Our results suggest that habitat heterogeneity allowed virile crayfish to maintain a foothold despite high rusty crayfish densities. Removal efforts led to the recovery of virile crayfish, and spatial segregation facilitated both species coexisting at comparable abundances for a decade. Our results highlight that invasive species control, even in the absence of complete eradication, can benefit native species and that spatially structured interactions can promote coexistence.

Do crayfish affect stream ecosystem response to riparian vegetation removal?

Abstract Riparian vegetation management alters stream basal resources, but stream ecosystem responses partly depend on top‐down interactions with in‐stream consumers. Large‐bodied omnivores can exert particularly strong influences on stream benthic environments through consumption of food resources and physical disturbance of the benthos. Trophic dynamics studies conducted within the context of reach‐scale riparian vegetation manipulations can provide insights into the interactions and relative importance of top‐down and bottom‐up controls that determine ecosystem response to riparian change. Here, we examine how top‐down control by native crayfish omnivores (Cambarus bartonii) interacts with abiotic conditions created by reach‐scale removal of riparian rhododendron (Rhododendron maximum) in the southern Appalachian Mountains of the U.S.A. We conducted 32‐day trophic experiments by nesting five pairs of electrified (crayfish excluded) and non‐electrified (crayfish access) plots within each of two 300‐m stream reaches (one control and one rhododendron‐removed) for 1 year pre‐removal and 2 years post‐removal. Algal growth responded positively to the reduced canopy cover (post‐rhododendron removal) only under low flow conditions combined with the absence of top‐down control by crayfish. Leaf decomposition rates were reduced by c. 40% in the absence of crayfish, but higher inputs of rhododendron leaf litter during the summer following rhododendron removal reduced the effect of crayfish presence on decomposition. Riparian rhododendron removal also significantly increased benthic sediment and fine benthic organic matter, but crayfish exclusion did not affect these stream properties. Potential long‐term reductions in crayfish abundance could reduce the top‐down effects of crayfish and ultimately lead to higher algal growth and reduced leaf decomposition rates in streams where rhododendron is managed through removal.

Distribution of crayfish in the Southern Basin of Lake Michigan and the Greater Chicago Region

Crayfish represent important links in aquatic food webs because they have diverse, omnivorous diets and are an important source of energy for fishes and birds. Crayfish have the ability to increase sediment transport through bioturbation, some are considered ecosystem engineers due to their burrowing habits, and crayfish invasions have been linked to large declines in biodiversity and changes in ecosystem structure and function. Despite their ecological importance and the threats that invasive crayfishes pose, the distribution of crayfishes in the Laurentian Great Lakes is not well studied. Here, we report on four years of intensive crayfish surveys in the southwestern portion of the Lake Michigan Basin, a region with diverse freshwater ecosystems and few previous records of crayfish distribution. From 2015 to 2018, baited minnow traps and SCUBA were used to document the distribution and abundance of crayfish across streams, rivers, inland lakes, and Lake Michigan. Six species of crayfish were captured, including two invasive species. The invaders are the widely distributed and abundant Faxonius rusticus (rusty crayfish) and Procambarus clarkii (red Swamp crayfish), a species early in the invasion phase. Native species were found in fewer habitat types and were less abundant than invasive F. rusticus. Comparing our results to previous sampling showed that native crayfish distribution and diversity have declined at the same time that F. rusticus has spread over recent decades. There is potential for new and recently introduced invaders, such as the red swamp crayfish, to further alter ecosystems.

The dynamics of the invasion of the rusty crayfish,Faxonius rusticus(Girard, 1852) (Decapoda: Astacidea), in two New York state, USA stream systems

AbstractI used 20 years of sampling data to describe the dynamics of the invasion of the rusty crayfish, Faxonius rusticus (Girard, 1852), of two stream systems in New York state (USA) and compare this invasion to invasions of this species of lakes elsewhere in North America. Rusty crayfish typically increased rapidly in abundance while displacing native crayfishes, becoming > 90% of crayfish captured in an average of 9 years. As in lakes, overall crayfish abundance increased as the invasion progressed. The F. rusticus population spread at a median rate of 3 km yr–1 upstream and 8.1 km yr–1 downstream. The hypothesis that an Allee effect would occur at the invasion front was not supported, since the rate of change of F. rusticus abundance did not increase as the invasion progressed. Qualitatively, the dynamics of F. rusticus’s invasion of streams are similar to previous findings for lakes, suggesting that the main processes affecting the invasions are similar.

Does river channelization increase the abundance of invasive crayfish? Survey of Faxonius limosus in small Central European streams

The aim of this study was to assess whether regulated river reaches show higher numbers of the spiny cheek crayfish Faxonius limosus, a common freshwater invader in Europe. Man-made alterations of riverine habitats may increase their susceptibility to biological invasions. This phenomenon is commonly studied in the context of dam reservoirs; however, the impact of river channelization and the resulting riverine habitat homogenization on the success of invasive species are mostly lacking. Surveys were conducted in natural and channelised river stretches in five rivers of south-central Poland. General linear mixed model analysis showed that crayfish abundance is usually higher at sites located within regulated reaches. The likely reason for the detected differences in abundance of crayfish may be related to decreased food-web complexity and lack of predators in a homogenised habitat. The results suggest that the channelization of small watercourses may increase the abundance of invasive species F. limosus and its pressure on native European crayfish. However, due to limited data obtained in field, our study should be considered as a pilot one.

Irruptive White Ibis breeding is associated with use of freshwater crayfish in the coastal Everglades

AbstractAs avian reproductive success is generally prey limited, identifying important prey types or sizes and understanding mechanisms governing prey availability are important objectives for avian conservation ecology. Irruptive White Ibis (Eudocimus albus) nesting at coastal colonies in the southern Everglades numbered over 100,000 nests in the 1930s. A century of drainage and altered hydrologic patterns reduced aquatic prey availability and eliminated large nesting events; nesting activity in recent decades has been typically less than 5% of historical peaks. Hydrologic restoration is expected to increase ibis nesting activity, but which prey types will support high nesting effort is less clear. In 2017 and 2018, we collected food boluses from White Ibis chicks at coastal colonies in Everglades National Park. We also monitored regional nesting activity from 1999 to 2018. In 2017, the region had 1,075 nests, typical of the past several decades; but in 2018, there were 30,420 nests, representing the highest recorded nesting activity in 87 yr. Prey composition varied between years; estuarine crabs dominated nestling boluses in 2017, while crayfish and fish were dominant prey in 2018. Crayfish, especially Procambarus alleni, were heavily exploited by ibis early in the 2018 breeding season, while fish were used more at the end. Crayfish abundances in wetlands near the colonies were higher prior to 2018, and more crayfish-producing short-hydroperiod wetlands remained available for ibis foraging in 2018. Our results support previous studies indicating that crayfish are important prey for breeding ibises and suggest that unprecedented, extensive flooding of seasonal wetlands promoted crayfish production and initiated the irruptive breeding in 2018. Our observations indicate that rehydration of the southern Everglades could restore ibis nesting activity at coastal colonies, but further investigations of hydrologic variation, crayfish production, and ibis foraging and nesting activity will be helpful to understand these dynamics and the importance of short-hydroperiod wetlands.

Pathways to impact for aquatic conservation science via multi‐modal communication and stakeholder engagement

Abstract Translating scientific evidence into practical actions is a primary goal of scientists working to inform evidence‐based policy and decision‐making, but how can authors best facilitate new conservation interventions following publication of their research? This case study used two articles in Aquatic Conservation: Marine and Freshwater Ecosystems (AQC) to examine the mechanisms and barriers to translating published aquatic science into social, ecological and policy interventions for the conservation of threatened Murray crayfish (Euastacus armatus). These articles revealed how steep declines in Murray crayfish abundance arose from the loss of in‐stream and riparian habitats preferred by crayfish, and demonstrated the social reasons for conserving Murray crayfish to protect the wellbeing of a diversity of people (e.g. Traditional Owners, landholders, ecotourism operators). Following publication of these articles in AQC the authors used multi‐modal communication (e.g. written briefs, personal discussions, seminars, magazine articles, media broadcasts) to translate their findings to stakeholders and the general public. Analysis of relevant documents (e.g. management plans, public information leaflets) and stakeholder interviews uncovered a range of interventions arising from the two AQC articles. Social interventions (e.g. new public signage, ecotourism briefings) were the quickest to occur after publication (1–2 years). Although some changes in management were apparent (e.g. revised Murray crayfish action plans, monitoring programmes), systemic barriers (e.g. policy/funding timeframes) have prevented some ecological and policy interventions from being realized. Diverse communication strategies can help rapidly translate published science into new conservation actions by giving stakeholders the right evidence in the right way to inform their particular practice. Open‐access summaries that explain findings within readily shareable media (e.g. graphical/video abstracts) can be particularly effective in raising early awareness. Some types of intervention, however, may require a long‐term perspective and assistance from boundary‐spanning knowledge brokers with an aptitude for evidence‐based policy development.

English

Abstract Anthropogenic modifications to river-floodplain systems can decouple floodplains from mainstem inputs, alter flood pulse dynamics, and disrupt population dynamics and trophic web stability of aquatic biota. The Atchafalaya River Basin (ARB) receives an annual flood pulse from the Mississippi River that contributes to high crayfish abundance. Conversely, reduced crayfish abundance in the Barataria Basin (BB) is attributed to the system no longer receiving an annual flood pulse from the Mississippi River. Therefore, the purpose of this research was to determine if the absence of an annual flood pulse and reduced crayfish abundance influenced the diets of carnivorous fishes by examining stomach contents of fishes from both basins. Stomach contents were grouped as crayfish, fish, non-crayfish invertebrate, and herpetological. Although the percent occurrence of crayfish in fish stomachs differed between floodplain inundation and low-water periods in the ARB, crayfish were still the major diet constituent of ARB fishes during both periods. Non-crayfish invertebrate was the major diet constituent in BB fishes, with crayfish ranking as the second fewest diet constituent present. Our results demonstrate how flood pulse dynamics influence crayfish, and ultimately trophic webs, in large river-floodplain systems.

Size-Specific Advantage in Shelter Competition Between the Mountain Madtom and Crayfishes

AbstractShelter is a crucial component to many species' survival, and when shelter becomes limited, resulting competition can have negative effects on growth and survival. In Little Chucky Creek, a tributary to the Nolichucky River in Tennessee, the severe population decline of Chucky Madtoms Noturus crypticus has been partially attributed to an increase in the abundance of cavity-dwelling crayfish after the establishment of two nonnative species. Although it has been suggested that the crayfish exclude the cavity-dwelling fish from shelter, we are not aware of studies that have been conducted to demonstrate that crayfish directly outcompete madtoms in this regard. Our objective was to experimentally test the hypothesis that shelter competition between crayfish and Mountain Madtoms Noturus eleutherus, a surrogate species for Chucky Madtoms, is a function of relative size. We conducted behavioral trials in which shelter was the limited resource for the two potential competitors. We recorded the madtoms' success at occupying the provided cover object for 5 d as well as health condition at the end of the competition phase. Both madtom occupancy and health condition were positively correlated with increasing relative size. As the size differential increased between madtoms and crayfish, madtoms were more successful at occupying the cover object, and overall condition was greater at the end of the trial. Conversely, when madtoms were smaller than crayfish, individuals were more commonly excluded from the cover object or even killed. Juvenile madtoms experienced 100% mortality. We concluded that crayfish exhibit a size-specific competitive advantage over Mountain Madtoms when shelter is limited, and that crayfishes may catalyze fish population declines at least partially through shelter exclusion and predation on juveniles.

Stable resource polymorphism along the benthic littoral-pelagic axis in an invasive crayfish.

Although intraspecific variability is now widely recognized as affecting evolutionary and ecological processes, our knowledge on the importance of intraspecific variability within invasive species is still limited. This is despite the fact that understanding the linkage between within‐population morphological divergences and the use of different trophic or spatial resources (i.e., resource polymorphism) can help to better predict their ecological impacts on recipient ecosystems. Here, we quantified the extent of resource polymorphism within populations of a worldwide invasive crayfish species, Procambarus clarkii, in 16 lake populations by comparing their trophic (estimated using stable isotope analyses) and morphological characteristics between individuals from the littoral and pelagic habitats. Our results first demonstrated that crayfish occured in both littoral and pelagic habitats of seven lakes and that the use of pelagic habitat was associated with increased abundance of littoral crayfish. We then found morphological (i.e., body and chelae shapes) and trophic divergence (i.e., reliance on littoral carbon) among individuals from littoral and pelagic habitats, highlighting the existence of resource polymorphism in invasive populations. There was no genetic differentiation between individuals from the two habitats, implying that this resource polymorphism was stable (i.e., high gene flow between individuals). Finally, we demonstrated that a divergent adaptive process was responsible for the morphological divergence in body and chela shapes between habitats while difference in littoral reliance neutrally evolved under genetic drift. These findings demonstrated that invasive P. clarkii can display strong within‐population phenotypic variability in recent populations, and this could lead to contrasting ecological impacts between littoral and pelagic individuals.