Associations between stream habitat characteristics and native and alien crayfish occurrence

Human mediated introductions of non-indigenous crayfish species (NICS) are responsible for their rapid colonisation of European freshwaters. The introduction of North American crayfish is furthermore linked to the spread of crayfish plague and the decline of indigenous crayfish species (ICS). As the management of ICS and NICS have become necessary, a detailed knowledge on their distribution and ecological requirements is needed. We studied the current range of native noble crayfish Astacus astacus and stone crayfish Austropotamobius torrentium, as well as alien signal crayfish Pacifastacus leniusculus in Carinthia (Austria) and evaluated environmental and physical habitat features in streams with and without crayfish. Meanwhile, the loss of many ICS populations was recorded and alien P. leniusculus was found to be widespread in this region. Most of the habitat features of streams having crayfish differed significantly from sites lacking crayfish for at least one investigated native or alien species. Furthermore, multivariate and regression analyses showed specific differences in the habitat use of the investigated crayfish. Our results showed that the presence of alien P. leniusculus was associated with larger and smoother sloped lowland rivers, while the occurrence of the two native species was confined to smaller streams either at higher altitudes and with distinct physical habitat conditions (A. torrentium) or with moderate water temperatures (A. astacus). This study helps to identify potential refuge areas for the endangered native species and to predict the further spread of the most common non-native crayfish species in European streams.     

A laboratory study of the effects of shelter availability and invasive crayfish on the growth of native stream fish

1. The alteration of stream habitats by urbanisation reduces the availability of shelter. Reduced shelter availability may increase both predation risks and metabolic costs, negatively affecting the growth performance of stream fish. Although urbanised streams often allow the establishment of invasive species, the additive or interactive effects of shelter availability and invasive species are rarely explored. The invasive red swamp crayfish (Procambarus clarkii) is a strong shelter competitor and predator in streams. 2. We conducted a stream‐channel experiment to investigate how shelter availability and this invasive crayfish species affect the growth performance (measured as the loss in body mass under starved conditions) of two native benthic species, Japanese crucian carp (Carassius auratus complex) and an endangered cobitid fish (Lefua echigonia). 3. We found that both the absence of shelter and the presence of crayfish increased body mass loss in the crucian carp. However, the growth performance of the crucian carp was improved by the presence of shelter except in the presence of crayfish, in which case the shelter available did not ameliorate the negative effect of the crayfish on growth performance (i.e. an interactive effect). This result suggests that crayfish may affect the growth performance of the crucian carp through chemical cues as well as through shelter occupancy. 4. In contrast, shelter availability is the primary factor affecting the body mass loss of the cobitid fish. However, the growth performance of these fish was unaffected by the presence of crayfish. The change in the growth performance of the cobitid fish in response to the absence of shelter was greater than that of the crucian carp. This finding suggests that the cobitid fish may be more vulnerable than the crucian carp to a reduction in shelter availability. 5. Our study demonstrates that reduced shelter availability and/or crayfish invasion can have significant, indirect negative effects on the growth performance of native fish, but whether those effects are interactive might vary depending on the species. Our findings have clear implications for stream restoration and habitat assessment.     

Habitat-specific effects of particle size, current velocity, water depth, and predation risk on size-dependent crayfish distribution

This study assessed effects of abiotic (current velocity, water depth, particle size) and biotic (predation risk for crayfish, size distribution and densities of predatory fish) variables on habitat- and size-specific distribution patterns of lotic crayfish (Orconectes obscurus) using field surveys and tethering experiments. Additionally, particle size manipulations were used with predation assays to assess habitat-specific interactions since the average particle size increased from deep pools to shallow pools to riffles. Large crayfish had the highest densities in deep pools and were associated with increased water depth, whereas small and medium crayfish had the highest densities in shallow pools and were strongly associated with increased particle size and decreased water depth. Regardless of size, crayfish in deep pools had significantly lower survival than in shallow pools and riffles. However, only small crayfish showed consistent differences in predation risk by habitat type and were significantly more vulnerable to predation than larger crayfish. Additionally, large rocky refugia resulted in significantly higher survival of small crayfish in the combined particle manipulation/tethering experiment. Overall, predation appears to be a key mechanism structuring habitat-specific distribution patterns for only small O. obscurus. Large substrates may be particularly important in habitats where both small crayfish density and predation risk are high.     

Chemical cues modify species interactions: the ecological consequences of predator avoidance by freshwater snails

Chemical signals released by predators or injured prey often induce shifts in the traits of prey species, which may in turn affect species interactions. Here we investigate the role that chemical cues play in mediating species interactions in the littoral food web of lakes. Previous studies have shown that predators induce shifts in the morphology, life history, and behavior of the freshwater snail Physella, but the ecological consequences of developing these inducible defenses are not well documented. We observed habitat use of the freshwater snail Physella gyrina along a depth gradient in a natural lake, and found they increased their use of covered habitats with increasing depth. We hypothesized that this habitat shift was due to changes in the level and type of predation risk, and that the habitat shift would affect periphyton standing crops. These hypotheses were tested in a mesocosm experiment in which we manipulated the presence of molluscivorous fish and crayfish. Predators were confined to cages and snail density was identical in all treatments, so any effects of predators were mediated through trait shifts induced by chemical cues. In the presence of fish, Physella moved under cover, but in the presence of crayfish, Physella avoided cover and moved to the water surface. These non‐lethal effects of predators on snail habitat use influenced the interaction between snails and their periphyton resources. In the presence of fish, periphyton standing crop in covered habitats was reduced to just 8% of periphyton in the absence of fish. Crayfish had no significant effect on periphyton in covered habitats, but they reduced periphyton in near‐surface habitats to 39% of the standing crop in the absence of crayfish. The combined effects of fish and crayfish were generally intermediate to their individual effects. We conclude that because chemical cues often have strong effects on individual traits and trophic interactions are sensitive to trait values, chemical cues may play an important role in shaping the structure and dynamics of food webs.     

Habitat complexity reduces aggression and enhances consumption in crayfish

We tested the hypothesis that increased habitat complexity would reduce intraspecific interactions among crayfish (Orconectes propinquus), and result in an increase in the consumption rate of prey at different crayfish densities. The effect of crayfish density, food level (trout eggs), and habitat complexity on prey consumption by crayfish was quantified in the laboratory. There was a significant difference in the consumption rate between different food levels. When food was scarce, almost all trout eggs were consumed regardless of crayfish density or habitat complexity. When food was unlimited, there was a significant positive linear relationship between eggs consumed per crayfish and habitat complexity. However, the relationship was not significant when trials without habitat were deleted from the analysis. We found that habitat complexity significantly reduced intraspecific aggression. Our findings suggest that a minimal amount of habitat complexity can reduce interactions among predators, ultimately resulting in increased prey consumption.     

Species-specific pattern of crayfish distribution within a river network relates to habitat degradation: implications for conservation

The degradation of aquatic habitats has increasingly become one of the most important factors influencing the distribution of freshwater species worldwide. We analysed the occurrence of three crayfish species, Astacus astacus, Austropotamobius torrentium and Orconectes limosus, in relation to habitat degradation status (based on Directive 2000/60/ES), stream morphology, geographical characteristics and land cover. In total, we analysed 6,768 sites within the Czech Republic (Central Europe), of which 6,187 sites lacked crayfish; among the remainder, A. astacus was present in 507 sites, O. limosus occurred in 44 sites and A. torrentium was present in only 30 sites. The analysis revealed that A. astacus preferred streams of better water quality that were not surrounded by agricultural land or settlements. This species also preferentially occurred in smaller streams with stony bottom substrata that were located at higher altitudes. Austropotamobius torrentium occurrence was associated with the natural character of the water body and the presence of protected areas at higher altitudes. Conversely, the non-native crayfish species O. limosus was typically recorded at lower altitudes in downstream reaches surrounded by agricultural land and with deteriorated water quality. The degree of environmental (human) pressure besides differences in habitat characteristics at sites with ICS and NICS may be of general importance for conservation strategies aimed at central European native crayfish species, primarily because the habitat-driven co-occurrence pattern (and its possible changes in the future) may strongly influence interspecific relationships, such as direct competition and the spread of infectious diseases between crayfish species.     

Influences of native and non-native benthivorous fishes on aquatic ecosystem degradation

Invasive common carp Cyprinus carpio has long been identified as a contributor to water quality deterioration, disrupted ecosystem processes, and shifts in biological assemblage structure. In contrast, little information is available regarding the effects of native benthivorous species on aquatic systems despite their functional similarity to common carp. Effects of common carp and the native black bullhead Ameiurus melas on sediment resuspension, nutrient concentrations, macrophytes, and assemblage structure of zooplankton and macroinvertebrates, were experimentally evaluated. We observed decreased water clarity, increased nutrient concentrations, decreased macrophyte biomass, and decreased benthic macroinvertebrate abundance and biomass associated with common carp, regardless of the presence of black bullhead. In contrast, black bullhead increased total phosphorus, and chlorophyll a concentrations and copepod abundance, but had little or no effect on other measured water quality and biological variables when compared to control conditions. Overall, experimental results suggest that although black bullhead tend to be tolerant of degraded ecosystems, they are not a source of physical changes to the environment known to be responsible for perpetuating degraded water quality (e.g., stable state shifts). Therefore, increased abundance of native species, such as black bullhead, following the invasion of common carp, may serve as an indicator of ecological conditions and should not to be assumed as causative.     

Contrasting impacts of invasive engineers on freshwater ecosystems: an experiment and meta-analysis

Invasion by common carp (Cyprinus carpio) and red swamp crayfish (Procambarus clarkii) in shallow lakes have been followed by stable-state changes from a macrophyte-dominated clear water state to a phytoplankton-dominated turbid water state. Both invasive carp and crayfish are, therefore, possible drivers for catastrophic regime shifts. Despite these two species having been introduced into ecosystems world-wide, their relative significance on regime shifts remains largely unexplored. We compared the ecological impacts of carp and crayfish on submerged macrophytes, water quality, phytoplankton, nutrient dynamics, zooplankton and benthic macroinvertebrates by combining an enclosure experiment and a meta-analysis. The experiment was designed to examine how water quality and biological variables responded to increasing carp or crayfish biomass. We found that even at a low biomass, carp had large and positive impacts on suspended solids, phytoplankton and nutrients and negative impacts on benthic macroinvertebrates. In contrast, crayfish had a strong negative impact on submerged macrophytes. The impacts of crayfish on macrophytes were significantly greater than those of carp. The meta-analysis showed that both carp and crayfish have significant effects on submerged macrophytes, phytoplankton, nutrient dynamics and benthic macroinvertebrates, while zooplankton are affected by carp but not crayfish. It also indicated that crayfish have significantly greater impacts on macrophytes relative to carp. Overall, the meta-analysis largely supported the results of the experiment. Taken as a whole, our results show that both carp and crayfish have profound effects on community composition and ecosystem processes through combined consequences of bioturbation, excretion, consumption and non-consumptive destruction. However, key variables (e.g. macrophytes) relating to stable-state changes responded differently to increasing carp or crayfish biomass, indicating that they have differential ecosystem impacts.     

Differences Found in the Macroinvertebrate Community Composition in the Presence or Absence of the Invasive Alien Crayfish,Orconectes hylas

Introductions of alien species into aquatic ecosystems have been well documented, including invasions of crayfish species; however, little is known about the effects of these introductions on macroinvertebrate communities. The woodland crayfish (Orconectes hylas (Faxon)) has been introduced into the St. Francis River watershed in southeast Missouri and has displaced populations of native crayfish. The effects of O. hylas on macroinvertebrate community composition were investigated in a fourth-order Ozark stream at two locations, one with the presence of O. hylas and one without. Significant differences between sites and across four sampling periods and two habitats were found in five categories of benthic macroinvertebrate metrics: species richness, percent/composition, dominance/diversity, functional feeding groups, and biotic indices. In most seasons and habitat combinations, the invaded site had significantly higher relative abundance of riffle beetles (Coleoptera: Elmidae), and significantly lower Missouri biotic index values, total taxa richness, and both richness and relative abundance of midges (Diptera: Chironomidae). Overall study results indicate that some macroinvertebrate community differences due to the O. hylas invasion were not consistent between seasons and habitats, suggesting that further research on spatial and temporal habitat use and feeding ecology of Ozark crayfish species is needed to improve our understanding of the effects of these invasions on aquatic communities.     

Population-level metrics of trophic structure based on stable isotopes and their application to invasion ecology

Biological invasions are a significant driver of human-induced global change and many ecosystems sustain sympatric invaders. Interactions occurring among these invaders have important implications for ecosystem structure and functioning, yet they are poorly understood. Here we apply newly developed metrics derived from stable isotope data to provide quantitative measures of trophic diversity within populations or species. We then use these to test the hypothesis that sympatric invaders belonging to the same functional feeding group occupy a smaller isotopic niche than their allopatric counterparts. Two introduced, globally important, benthic omnivores, Louisiana swamp crayfish (Procambarus clarkii) and carp (Cyprinus carpio), are sympatric in Lake Naivasha, Kenya. We applied our metrics to an 8-year data set encompassing the establishment of carp in the lake. We found a strong asymmetric interaction between the two invasive populations, as indicated by inverse correlations between carp abundance and measures of crayfish trophic diversity. Lack of isotopic niche overlap between carp and crayfish in the majority of years indicated a predominantly indirect interaction. We suggest that carp-induced habitat alteration reduced the diversity of crayfish prey, resulting in a reduction in the dietary niche of crayfish. Stable isotopes provide an integrated signal of diet over space and time, offering an appropriate scale for the study of population niches, but few isotope studies have retained the often insightful information revealed by variability among individuals in isotope values. Our population metrics incorporate such variation, are robust to the vagaries of sample size and are a useful additional tool to reveal subtle dietary interactions among species. Although we have demonstrated their applicability specifically using a detailed temporal dataset of species invasion in a lake, they have a wide array of potential ecological applications.     

Prevalence of the Crayfish Plague Pathogen Aphanomyces astaci in Populations of the Signal Crayfish Pacifastacus leniusculus in France: Evaluating the Threat to Native Crayfish

Aphanomyces astaci, the crayfish plague pathogen, first appeared in Europe in the mid-19^th century and is still responsible for mass mortalities of native European crayfish. The spread of this parasite across the continent is especially facilitated by invasive North American crayfish species that serve as its reservoir. In France, multiple cases of native crayfish mortalities have been suggested to be connected with the presence of the signal crayfish Pacifastacus leniusculus, which is highly abundant in the country. It shares similar habitats as the native white-clawed crayfish Austropotamobius pallipes and, when infected, the signal crayfish might therefore easily transmit the pathogen to the native species. We investigated the prevalence of A. astaci in French signal crayfish populations to evaluate the danger they represent to local populations of native crayfish. Over 500 individuals of Pacifastacus leniusculus from 45 French populations were analysed, plus several additional individuals of other non-indigenous crayfish species Orconectes limosus, O. immunis and Procambarus clarkii. Altogether, 20% of analysed signal crayfish tested positive for Aphanomyces astaci, and the pathogen was detected in more than half of the studied populations. Local prevalence varied significantly, ranging from 0% up to 80%, but wide confidence intervals suggest that the number of populations infected by A. astaci may be even higher than our results show. Analysis of several individuals of other introduced species revealed infections among two of these, O. immunis and P. clarkii. Our results confirm that the widespread signal crayfish serves as a key reservoir of Aphanomyces astaci in France and therefore represents a serious danger to native crayfish species, especially the white-clawed crayfish. The prevalence in other non-indigenous crayfish should also be investigated as they likely contribute to pathogen transmission in the country.     

Preemptive legislation inhibits the anthropogenic spread of an aquatic invasive species, the rusty crayfish (Orconectes rusticus)

The enactment of legal policies is often recommended to prevent anthropogenic introductions of invasive species. In this paper, we evaluated the effectiveness of proactive state legislative policies in deterring colonization by rusty crayfish (Orconectes rusticus) and the expected spread into previously uninvaded states using network-based spatial analysis. We found that the presence of regulations was positively associated with the presence of rusty crayfish (p = 0.027), but often regulations were put into effect subsequent to the invasion. Regulations that did not explicitly prohibit transport, applied to specific drainages, or prohibited only rusty crayfish were not effective. However, preemptive legislation was effective in reducing the likelihood of invasion, if regulations prohibited the transport of all live crayfish species between water bodies, as only 1 state which passed such legislation prior to invasion by rusty crayfish was subsequently colonized (S = 12, p = 0.031). Five states are likely to be invaded by rusty crayfish via range expansion across interstate drainages, and dispersal rates suggest that all of these states could be colonized within 10 years. While it is unlikely that regulations will prevent dispersal-based invasion across state lines through shared drainages, preemptive legislation can significantly reduce anthropogenic spread of aquatic invasive species between drainages and effectively retard the expansion of rusty crayfish. Our results suggest that the most effective form of legislation is one that does not require individuals to identify species, thus we recommend states enact policies that explicitly prohibit the transport of all live crayfish between water bodies.     

Population structure and genetic analysis of narrow-clawed crayfish (Astacus leptodactylus) populations in Turkey

The genetic differentiation among Turkish populations of the narrow-clawed crayfish was investigated using a partial sequence of cytochrome oxidase subunit I gene (585 bp) of 183 specimens from 17 different crayfish populations. Median joining network and all phylogenetic analyses disclosed a strong haplotype structure with three prominent clades diverged by a range between 20 and 50 mutations and substantial inter-group pairwise sequence divergence (5.19–6.95 %), suggesting the presence of three distinct clades within the Anatolian populations of Astacus leptodactylus. The divergence times among the three clades of Turkish A. leptodactylus are estimated to be 4.96–3.70 Mya using a molecular clock of 1.4 % sequence divergence per million years, pointing to a lower Pliocene separation. The high level of genetic variability (H _d  = 95.8 %, π = 4.17 %) and numerous private haplotypes suggest the presence of refugial populations in Anatolia unaffected by Pleistocene habitat restrictions. The pattern of genetic variation among Turkish A. leptodactylus populations, therefore, suggests that the unrevealed intraspecific genetic structure is independent of geographic tendency and congruent with the previously reported geographic distribution and number of subspecies (A. l. leptodactylus and A. l. salinus) of A. leptodactylus.     

The bad and the super-bad: prioritising the threat of six invasive alien to three imperilled native crayfishes

Multiple species invasions and limited resources for management require prioritisation of deleterious effects of invaders on imperilled native species. This study prioritises the threat of six non-indigenous crayfish species (NICS) to three indigenous crayfish species (ICS) in southwestern Germany, a European region with high diversity of crayfish species and freshwater habitats. Using multivariate statistical analyses and niche-based species distribution models, the (1) contemporary and potential range overlap, (2) habitat overlap, and (3) rate of spread of the nine species were assessed. Predicted and contemporary range overlap with ICS was consistently the highest for the alien signal crayfish. Environmental niches of ICS tended to be associated with cooler temperatures (except for white-clawed crayfish), lower Human Influence Index, and higher terrain slope than that of alien Orconectes and Procambarus species, but were mostly similar to that of signal crayfish. Habitat overlap was found to be the highest between signal crayfish and ICS. In contrast to Orconectes and Procambarus species, signal crayfish also invade headwaters, where the most ICS populations occur. Range expansion during the past 15 years was the highest for signal crayfish, followed by Orconectes species. Because of the great potential to invade as-yet isolated refuge areas and spread at a high rate, signal crayfish is of the highest concern for conservation of ICS and should be primarily targeted by prevention and control measures. However, it merely represents the ‘worst of the worst’, since all NICS of North American origin are natural reservoirs of crayfish plague, a fatal disease of ICS.     

Effects of Predation Risk on Habitat Selection by Water Column Fish, Benthic Fish and Crayfish in Stream Pools

Predation risk can affect habitat selection by water column stream fish and crayfish, but little is known regarding effects of predation risk on habitat selection by benthic fish or assemblages of fish and crayfish. I used comparative studies and manipulative field experiments to determine whether, (1) habitat selection by stream fish and crayfish is affected by predation risk, and (2) benthic fish, water column fish, and crayfish differ in their habitat selection and response to predation risk. Snorkeling was used to observe fish and crayfish in, (1) unmanipulated stream pools with and without large smallmouth bass predators (Micropterus dolomieui >200 mm total length, TL) and (2) manipulated stream pools before and after addition of a single large smallmouth bass, to determine if prey size and presence of large fish predators affected habitat selection. Observations of microhabitat use were compared with microhabitat availability to determine microhabitat selection. Small fish (60–100 mm TL, except darters that were 30–100 mm TL) and crayfish (40–100 mm rostrum to telson length; TL) had significantly reduced densities in pools with large bass, whereas densities of large fish and crayfish (> 100 mm TL) did not differ significantly between pools with and without large bass. Small orangethroat darters (Etheostoma spectabile), northern crayfish (Orconectes virilis), and creek chubs (Semotilus atromaculatus) showed significantly greater densities in pools without large bass. The presence of large smallmouth bass did not significantly affect depths selected by fish and crayfish, except minnows, which were found significantly more often at medium depths when bass were present. Small minnows and large and small crayfish showed the greatest response to additions of bass to stream pools by moving away from bass locations and into shallow water. Small darters and sunfish showed an intermediate response, whereas large minnows showed no significant response to bass additions. Response to predation risk was dependent on prey size and species, with preferred prey, crayfish and small minnows, showing the greatest response. Small benthic fish, such as darters, are intermediate between small water column fish and crayfish and large water column fish in their risk of predation from large smallmouth bass.     

Invasive Crayfish Threaten the Development of Submerged Macrophytes in Lake Restoration

Submerged macrophytes enhance water transparency and aquatic biodiversity in shallow water ecosystems. Therefore, the return of submerged macrophytes is the target of many lake restoration projects. However, at present, north-western European aquatic ecosystems are increasingly invaded by omnivorous exotic crayfish. We hypothesize that invasive crayfish pose a novel constraint on the regeneration of submerged macrophytes in restored lakes and may jeopardize restoration efforts. We experimentally investigated whether the invasive crayfish (Procambarus clarkii Girard) affects submerged macrophyte development in a Dutch peat lake where these crayfish are expanding rapidly. Seemingly favourable abiotic conditions for macrophyte growth existed in two 0.5 ha lake enclosures, which provided shelter and reduced turbidity, and in one lake enclosure iron was added to reduce internal nutrient loading, but macrophytes did not emerge. We transplanted three submerged macrophyte species in a full factorial exclosure experiment, where we separated the effect of crayfish from large vertebrates using different mesh sizes combined with a caging treatment stocked with crayfish only. The three transplanted macrophytes grew rapidly when protected from grazing in both lake enclosures, demonstrating that abiotic conditions for growth were suitable. Crayfish strongly reduced biomass and survival of all three macrophyte species while waterfowl and fish had no additive effects. Gut contents showed that crayfish were mostly carnivorous, but also consumed macrophytes. We show that P. clarkii strongly inhibit macrophyte development once favourable abiotic conditions for macrophyte growth are restored. Therefore, expansion of invasive crayfish poses a novel threat to the restoration of shallow water bodies in north-western Europe. Prevention of introduction and spread of crayfish is urgent, as management of invasive crayfish populations is very difficult.     

Invasive crayfish reduce food limitation of alien American mink and increase their resilience to control

Trophic relationships between invasive species in multiply invaded ecosystems may reduce food limitation relative to more pristine ecosystems and increase resilience to control. Here, we consider whether invasive predatory American mink Neovison vison are trophically subsidized by invasive crayfish. We collated data from the literature on density and home range size of mink populations in relation to the prevalence of crayfish in the diet of mink. We then tested the hypothesis that populations of an invasive predator reach higher densities and are more resilient to lethal control when they have access to super-abundant non-native prey, even in the absence of changes in density dependence, hence compensatory capacity. We found a strong positive relationship between the proportion of crayfish in mink diet and mink population density, and a negative relationship between the proportion of crayfish in mink diet and mink home range size, with crayfish contribution to mink diet reflecting their abundance in the ecosystem. We then explored the consequence of elevated mink density by simulating a hypothetical eradication program with a constant harvest in a Ricker model. We found that mink populations were more resilient to harvest in the presence of crayfish. As a result, the simulated number of mink harvested to achieve eradication increased by 500 % in the presence of abundant crayfish if carrying capacity increased by 630 %. This led to a threefold increase in time to eradication under a constant harvest and an approximately 20-fold increase in the cumulative management cost. Our results add to evidence of inter-specific positive interactions involving invasive species, and our simple model illustrates how this increases management cost.     

Predation Threat Affects Behaviour and Habitat Use by Hatchery Brown Trout (Salmo Trutta L.) Juveniles

The effect of predation threat on behaviour and habitat use by brown trout (Salmo trutta L.) juveniles were studied in four indoor and six outdoor experimental channels. Northern pike (Esox lucius L.) was used as a predator. Brown trout were more active during the night compared to day, but in the presence of pike the increase in activity was less than in treatments without predator. Aggressive interactions between trout were higher during the day when pike was absent. In the presence of pike trout tended to use more coarse substrate classes as subdominant habitat suggesting increased use of cover and also higher or lower water depths. Our results increase knowledge of the effects of predation threat on habitat selection and behaviour by brown trout.     

Habitat, predation, and coexistence between invasive and native crayfishes: prioritizing lakes for invasion prevention

Conditions fostering coexistence of native species with invasive species have received little attention in invasion biology, especially for closely related invasive and native species. We used long-term datasets on multiple replicate invasions to define conditions under which native virile crayfish (Orconectes virilis) can coexist with invasive rusty crayfish (O. rusticus). We examined multiple drivers of coexistence involving habitat use and predation at between-lake and within-lake scales to derive predictions that could guide prioritization efforts to prevent future introductions of rusty crayfish and mitigate impacts of existing invasions. Lakes in which native species persisted for many years had significantly less cobble and sand habitats, and significantly more vegetated habitats compared to lakes from which native crayfish have been displaced. In the presence of rusty crayfish, virile crayfish alter their habitat use to vegetated habitats relative to habitat use in the absence of rusty crayfish. Such vegetated habitats had greater plant standing crop, plant species richness, and sediment percent organic matter compared to vegetated sites occupied by rusty crayfish. Our results suggest that low abundance of cobble habitat and altered habitat use allows native crayfish to coexist with the rusty crayfish invader. At the within-lake scale, virile crayfish persist by escaping predation in the vegetated habitats, despite suboptimal abiotic conditions. By understanding these abiotic and biotic conditions that promote coexistence, managers could enhance native crayfish persistence by targeting high cobble lakes for efforts to prevent the introduction of invasive crayfish, and targeting vegetated habitats for protection in already invaded lakes.