Bythotrephes Research Articles

Local differentiation in the defensive morphology of an invasive zooplankton species is not genetically based

Evolutionary changes in functional traits represent one possible reason why exotic species spread to become invasive, but empirical studies of the mechanisms driving phenotypic differentiation between populations of invasive species are rare. This study tested whether differences in distal spine length among populations of the invasive cladoceran, Bythotrephes longimanus, could be explained by local adaptation or phenotypic plasticity. We collected Bythotrephes from six lakes and found that distal spine lengths and natural selection on distal spine length differed among populations, but were unrelated to the gape-limitation of the dominant fish predator in the lake from which they were collected. A common garden experiment revealed significant genetic and maternal variation for distal spine length, but phenotypic differences among populations were not genetically based. Phenotypic differences among lakes in this ecologically important trait are, therefore, the result of plasticity and not local adaptation, despite spatially variable selection on this heritable trait. The ability of Bythotrephes to plastically adjust distal spine length may explain the success of this species at invading lake ecosystems with diverse biotic environments.

Collapse of Lake WhitefishCoregonus clupeaformis(Mitchill, 1818) species pair in Como Lake, Ontario

Lake Whitefish (Coregonus clupeaformis) species pairs exist in a small number of northern temperate lakes in North America. Sympatric populations of dwarf and normal-sized whitefish represent a continuum of morphological and genetic divergence among these lakes. In this study, we used morphological and age data to assess the status of the Lake Whitefish species pair in Como Lake, Ontario (Canada). Whitefish were collected with gill-nets from spawning shoals during the fall of 2012 and 2014 and compared with archived (circa 1989) dwarf and normal-sized specimens. Our study indicates that the two previously collected forms have been replaced by a single form with a different morphology and age structure. In comparison to archived specimens, contemporary Lake Whitefish were larger, deeper-bodied and older; with more gill rakers, lateral line scales, smaller eyes and a wider interorbital width. We hypothesize that the recent invasion of Como Lake by Spiny Waterflea (Bythotrephes longimanus) has affected whitefish growth rates and changed the ecosystem conditions that previously maintained the species pair.

Differential invasion success in aquatic invasive species: the role of within- and among-population genetic diversity

Despite a well-developed theoretical basis for the role of genetic diversity in the colonization process, contemporary investigations of genetic diversity in biological invasions have downplayed its importance. Observed reductions in genetic diversity have been argued to have a limited effect on the success of establishment and impact based on empirical studies; however, those studies rarely include assessment of failed or comparatively less-successful biological invasions. We address this gap by comparing genetic diversity at microsatellite loci for taxonomically and geographically paired aquatic invasive species. Our four species pairs contain one highly successful and one less-successful invasive species (Gobies: Neogobius melanostomus, Proterorhinus semilunaris; waterfleas: Bythotrephes longimanus, Cercopagis pengoi; oysters: Crassostrea gigas, Crassostrea virginica; tunicates: Bortylloides violaceous, Botryllus schlosseri). We genotyped 2717 individuals across all species from multiple locations in multiple years and explicitly test whether genetic diversity is lower for less-successful biological invaders within each species pair. We demonstrate that, for gobies and tunicates, reduced allelic diversity is associated with lower success of invasion. We also found that less-successful invasive species tend to have greater divergence among populations. This suggests that intraspecific hybridization may be acting to convert among-population variation to within-population variation for highly successful invasive species and buffering any loss of diversity. While our findings highlight the species-specific nature of the effects of genetic diversity on invasion success, they do support the use of genetic diversity information in the management of current species invasions and in the risk assessment of potential future invaders.

The North American invasive zooplanktivoreBythotrephes longimanusis less hypoxia-tolerant than the nativeLeptodora kindtii

The predatory cladoceran Bythotrephes longimanus (spiny water flea) has been invading lakes and damaging food webs across the central part of North America since the early 1980s. To understand its niche and that of the taxonomically related and native predatory cladoceran Leptodora kindtii, we investigated species survival after 12 h exposures to low dissolved oxygen (DO) concentrations in the laboratory. Bythotrephes longimanus (n = 690) exhibited a hypoxia tolerance limit (LC50) of 1.65 mg·L−1DO (95% confidence interval: 1.59, 1.72 mg·L−1) and was significantly less tolerant of hypoxia than L. kindtii (n = 380), which exhibited an LC50of 0.58 mg·L−1DO (0.51, 0.65 mg·L−1). These lab-based physiological results are consistent with landscape-scale observations that B. longimanus successfully invades primarily mesotrophic and oligotrophic lakes, while L. kindtii inhabits a wider range of lakes that includes eutrophic ones. Climate change throughout the 21st century may increase the occurrence and severity of hypoxia in the hypolimnia of lakes and may provide a growing barrier to B. longimanus invasion.

Dietary overlap between two zooplanktivores in Lake Winnipeg, Manitoba: Rainbow smelt (Osmerus mordax) and cisco (Coregonus artedi)

Abstract Dietary niche overlap between invasive rainbow smelt (Osmerus mordax) and the native cisco (Coregonus artedi) of similar size in Lake Winnipeg was examined using stomach content analysis throughout the open water period in 2010 and 2011. Although smelt and cisco were caught together in the North Basin (n = 7 stations), rainbow smelt were exclusively caught in the North Basin (n = 39 stations), whereas cisco were predominantly caught in the South Basin (n = 31 stations). Substantial dietary overlap was evident when found together and apart, illustrating little dietary niche differentiation between the two species in sympatry and allopatry in the lake. Both smelt and cisco appear to shift from copepods in spring to large-bodied cladocerans in summer and fall. Although high dietary overlap suggests that these species have the potential to compete for prey, differential abundance in each basin of Lake Winnipeg suggests possible environmental niche divergence between these two species. Overall, this study provides important baseline information to examine the impact of another recent invasive planktivorous species (e.g. Bythotrephes longimanus, spiny water flea) on the smelt-cisco competition as well as the commercially important piscivorous walleye (Sander vitreus) population in Lake Winnipeg.

Invasive invertebrate predator, Bythotrephes longimanus, reverses trophic cascade in a north‐temperate lake

AbstractTrophic cascades can determine the structure of aquatic food webs, a role often used to manage water quality in lakes. However, trophic cascades are subject to multiple interacting drivers of environmental change. For example, invasive species can dramatically alter food webs, though we know less about invasive species effects on managed trophic cascades. Here, we investigate the changing food web dynamics of Lake Mendota (Wisconsin, U.S.A.) over a 40‐yr time period. Piscivore stocking (biomanipulation) beginning in 1988 coupled with a 1987 die‐off of cisco (the dominant zooplanktivore) led to decreased zooplanktivory that cascaded to increases in the large‐bodied herbivorous zooplankton Daphnia pulicaria and greater water clarity. In 2009, the invasive spiny water flea Bythotrephes longimanus was first detected in Lake Mendota and soon reached record densities. As a result of this invasive invertebrate predator, zooplanktivory increased to 47% of the earlier cisco period. Concomitant with the spiny water flea population explosion were declines in D. pulicaria and water clarity as both exhibited a strong negative relationship to zooplanktivory across the 40‐yr time series. We argue that the trophic cascade of 1987–1988 created an environment of high food resources and low fish planktivory that promoted the proliferation of Bythotrephes as it functionally replaced cisco in Lake Mendota's food web. Our results highlight how a manipulated, eutrophic lake was vulnerable to adverse impacts by invasive species, calling attention to the importance of understanding interactions among drivers of environmental change.

Decrease in young-of-the-year yellow perch growth rates following Bythotrephes longimanus invasion

Bythotrephes longimanus, an invasive zooplankter from Eurasia, has caused severe declines in native zooplankton communities in Rainy and Kabetogama lakes in northern Minnesota. Both lakes have experienced a 40–60% decrease in peak summer zooplankton biomass following B. longimanus establishment around 2006–2007. In these lakes, yellow perch (Perca flavescens) are a key fishery species, and young-of-the-year (YOY) yellow perch are mainly planktivorous during their first summer. This led to concern that their growth could be detrimentally affected by the depletion of zooplankton forage. We used seining data to compare growth rates of YOY yellow perch before (2001–2005) and after (2008–2012) B. longimanus establishment in Rainy and Kabetogama lakes. Nearby Lake Vermilion, assumed to have been unaffected by B. longimanus during this time period, was used as a reference for natural variation in YOY growth in the region. YOY yellow perch length was modeled as a linear function of cumulative growing degree days (GDD) throughout the summer, and the slope of the relationship was compared between pre- and post-B. longimanus time periods for the three study lakes. The two lakes with B. longimanus showed similar decreases in YOY yellow perch growth rate relative to GDD, whereas Lake Vermilion showed no evidence of a decline in growth rates during this period. The reduction in growth rates resulted in an approximate 10% decrease in mean length of YOY yellow perch at the end of the summer after B. longimanus establishment, which could lead to further effects of this invasive zooplankter at higher trophic levels.

Predicting aquatic invasion in Adirondack lakes: a spatial analysis of lake and landscape characteristics

AbstractInvasive species continue to pose major challenges for managing coupled human–environmental systems. Predictive tools are essential to maximize invasion monitoring and conservation efforts in regions reliant on abundant freshwater resources to sustain economic welfare, social equity, and ecological services. Past studies have revealed biotic and abiotic heterogeneity, along with human activity, can account for much of the spatial variability of aquatic invaders; however, improvements remain. This study was created to (1) examine the distribution of aquatic invasive species richness (AISR) across 126 lakes in the Adirondack Region of New York; (2) develop and compare global and local models between lake and landscape characteristics and AISR; and (3) use geographically weighted regression (GWR) to evaluate non‐stationarity of local relationships, and assess its use for prioritizing lakes at risk to invasion. The evaluation index, AISR, was calculated by summing the following potential aquatic invaders for each lake: Asian Clam (Corbicula fluminea), Brittle Naiad (Najas minor), Curly‐leaf Pondweed (Potamogeton crispus), Eurasian Watermilfoil (Myriophyllum spicatum), European Frog‐bit (Hydrocharis morsus‐ranae), Fanwort (Cabomba caroliniana), Spiny Waterflea (Bythotrephes longimanus), Variable‐leaf Milfoil (Myriophyllum heterophyllum), Water Chestnut (Trapa natans), Yellow Floating Heart (Nymphoides peltata), and Zebra Mussel (Dreissena polymorpha). The Getis‐Ord Gi* statistic displayed significant spatial hot and cold spots of AISR across Adirondack lakes. Spearman's rank (ρ) correlation coefficient test (rs) revealed urban land cover composition, lake elevation, relative patch richness, and abundance of game fish were the strongest predictors of aquatic invasion. Five multiple regression global Poisson and GWR models were made, with GWR fitting AISR very well (R2 = 76–83%). Local pseudo‐t‐statistics of key explanatory variables were mapped and related to AISR, confirming the importance of GWR for understanding spatial relationships of invasion. The top 20 lakes at risk to future invasion were identified and ranked by summing the five GWR predictive estimates. The results inform that inexpensive and publicly accessible lake and landscape data, typically available from digital repositories within local environmental agencies, can be used to develop predictions of aquatic invasion with remarkable agreement. Ultimately, this transferable modeling approach can improve monitoring and management strategies for slowing the spread of invading species.

Colonization dynamics of the invasive predatory cladoceran, Bythotrephes longimanus, inferred from sediment records

AbstractDespite the central role of colonization to invasion biology, little is known about the early dynamics of founder populations because this time period is often unobserved and short‐lived. This study documents with high resolution the early population ecology of an invading freshwater zooplankton, Bythotrephes longimanus (Cladocera : Cercopagidae), by measuring accumulation rates of its caudal spine in sediments. Using dated (210Pb and 137Cs) sediment cores from four spatially distinct sites in Island Lake Reservoir (Minnesota, USA), we describe its first presence, early distribution, growth trajectory, and early impacts on prey. The sediment record shows that B. longimanus first appeared and was widely distributed in the lake in 1982 (± 2 yr, standard deviation), 8 yr before its first detection in the water, making it one of the earliest documented invasions in North America and suggesting that ecosystems may serve as dispersal hubs for years prior to detection. Logistic growth models describing spine accumulation rates show that B. longimanus required about two decades to achieve an annual equilibrium (K). Prolonged buildup to K may owe to several factors, including accumulation of a sufficiently large bank of resting eggs, the obligate overwintering life stage. Early exponential growth was incongruent with the presence of a lag phase. Post invasion, Daphnia mendotae became proportionally the most abundant daphniid in the lake, but the timing of the switch in prey species composition coincided more with the proliferation of B. longimanus density and its attainment of K than with its arrival to the lake.

Biotic resistance of impact: a native predator (Chaoborus) influences the impact of an invasive predator (Bythotrephes) in temperate lakes

Introduced predators have caused some of the largest documented impacts of non-native species. Interactions among predators can have complex effects, leading to both synergistic and antagonistic outcomes. Complex interactions with native predators could play an important role in mediating the impact of non-native predators. We explore the role of the native predator context on the effect of the introduced predatory cladoceran Bythotrephes longimanus. While post-invasion impacts have been well described, studies have largely ignored the role of native predators. We used a field mesocosm experiment to determine whether Bythotrephes’ impact on prey communities is influenced by the presence of the ubiquitous native predatory insect larvae Chaoborus. The two predators exhibited niche complementarity as no change in total zooplankton prey abundance was detected across predator treatments. Rather, copepod abundances increased with decreasing abundances of Chaoborus, while cladocerans decreased with increasing abundances of Bythotrephes. Thus, the replacement of Chaoborus with Bythotrephes led to changes in the overall community structure of the zooplankton prey, but had little effect on prey total abundance. More interestingly, we found evidence of biotic resistance of impact, that is, the impact of Bythotrephes on the cladoceran community was altered when the two predators co-occurred. Specifically, the predation effect of Bythotrephes was more restricted to the shallower regions of the water column in the presence of Chaoborus, leading to a reduced impact on deeper dwelling prey taxa. Overall, our results demonstrate that the native predator context is important when trying to understand the effect of non-native predators and that variation in native predator abundances and assemblages could explain variation in impact across invaded habitats.

Zooplankton communities and Bythotrephes longimanus in lakes of the montane region of the northern Alps

Lakes in the Alps represent a considerable fraction of nutrient-poor lakes in Central Europe, with unique biodiversity and ecosystem properties. Although some individual lakes are well studied, less knowledge is available on large-scale patterns essential to general understanding of their functioning. Here, we aimed to describe crustacean zooplankton communities (Cladocera, Copepoda) and identify their environmental drivers in the pelagic zone of 54 oligotrophic lakes in the montane region of the Alps (400–1200 m) in Austria, Germany, and Switzerland, covering a spatial scale of 650 km. Moreover, we aimed to provide data on the distribution and ecological requirements of the North American invader Bythotrephes longimanus in its Central European native range. Communities were mainly dominated by widespread species typical of lowland habitats, and only a few true specialists of oligotrophic alpine lakes were present. The most frequent taxa were the Daphnia longispina complex and Eudiaptomus gracilis, with 48 and 45 occurrences, respectively. Species richness decreased with altitude and increased with lake area. The main structuring factors of community composition were chlorophyll a concentration and depth, which drove an apparent separation of mesotrophic and oligotrophic communities. Bythotrephes had 13 occurrences, showing a preference for deep oligotrophic lakes. Its presence was not coupled with lower crustacean species richness, as was repeatedly observed in North America. Additionally, it frequently co-occurred with the other large predatory cladoceran, Leptodora kindtii. B. longimanus might be considered a truly montane species in Central Europe, given its absence in lowland and alpine lakes.

Outbreak of an undetected invasive species triggered by a climate anomaly

AbstractWhen an invasive species appears at a new location, we typically have no knowledge of the population dynamics leading up to that moment. Is the establishment of invasive propagules closely followed by the appearance of the population? Or alternatively, was there an established low‐density population that was released from a constraint and crossed the detection threshold? The early stages of the invasion process are a critical gap in our knowledge, yet vitally important for the detection and management of invasions. Here, we present multiple lines of evidence supporting the lag scenario for an invasive species outbreak. The invasive predatory zooplankton, spiny water flea (Bythotrephes longimanus), was detected in Lake Mendota, Wisconsin (USA), in summer of 2009 and rapidly reached and sustained exceptionally high densities. To evaluate whether Bythotrephes' outbreak immediately followed introduction or erupted from an established low‐density population, we constructed a population model of Bythotrephes in Lake Mendota. In the model, Bythotrephes persisted indefinitely at low levels until favorable thermal conditions in 2009, the coolest July since at least 1895, allowed it to erupt to high densities and establish a large egg bank in the lake sediments. The egg bank stabilized the population in the high‐density state despite a return to nonfavorable thermal conditions, which is further supported by demographic data suggesting a constant contribution from the egg bank during the year. The prolonged lag scenario is corroborated by the detection of two individual Bythotrephes in pre‐2009 archived samples, and the detection of Bythotrephes spines in lake sediment core layers dating back to 1994 (±5 yr). Together, our results suggest that Bythotrephes persisted for at least a decade below the detection limit, until optimal thermal conditions triggered a population outbreak. This work highlights the potential for environmental conditions to trigger invasive species outbreaks from low‐density populations.

The effects of Bythotrephes longimanus and calcium decline on crustacean zooplankton communities in Canadian Shield lakes

Declining calcium concentrations and invasion by Bythotrephes longimanus are two important, often co-occurring, stressors affecting Canadian Shield lakes. However, there has been no experimental examination of how they might jointly influence zooplankton communities. We conducted a 6-week field mesocosm experiment in Havelock Lake, Haliburton, Ontario, Canada to examine the individual and joint effects of Bythotrephes and calcium along a gradient ranging from 1.2 to 2.6 mg/l on zooplankton communities. Although densities of Bythotrephes in our study are unknown, it significantly reduced total zooplankton abundance in invaded compared to uninvaded treatments by 46%, with the greatest impacts on small cladocerans and daphniids. Low calcium reduced total zooplankton and cladoceran abundances. Although Havelock Lake has the lowest calcium concentration among invaded lakes in the Muskoka–Haliburton region (1.2 mg Ca/l), an effect of calcium on individual species abundances was not detected. Additionally, we did not detect an interactive effect of both stressors. Our results suggest that lake calcium concentration may not yet be low enough to effect a strong response. However, as Bythotrephes continue to invade low calcium lakes, and as calcium concentrations further decline, we may see larger impacts on cladocerans as calcium thresholds for reproduction and growth are reached.

Leptodora kindtii Population Dynamics in the Island Region of Western Lake Erie before and after the Invasion of the Predacious Cladoceran Bythotrephes longimanus

Competition among native and non-native species can cause decreases in population size and production of both species. The native predaceous crustacean zooplankter Leptodora kindtii shares a similar niche with the invasive Bythotrephes longimanus in Lake Erie. This niche overlap may contribute to the decline in abundance and production of Leptodora in the western basin of Lake Erie. Historical (1946) and recent (2006) data were used to determine if the decline in Leptodora abundance and production was associated with the effects of Bythotrephes, which invaded Lake Erie in the mid-1980’s. Pre-invasion abundances and lengths of L. kindtii were compared with current data (2006). A change in prey community abundance, composition and dynamics were observed, relative to pre-invasion, with a marked decline in.abundance and size of L. kindtii after the invasion of Bythotrephes. Competition for food and direct predation are two explanations, among others, for the declines observed in L. kindtii size, abundance and production that have occurred since B. longimanus invasion.

Predator presence dramatically reduces copepod abundance through condition‐mediated non‐consumptive effects

Summary Predators can induce changes in prey phenotype that reduce predation risk but also incur costs (so‐called ‘non‐consumptive effects’ or NCEs). Few studies, however, have examined NCEs over time scales sufficient to link individual‐ and demographic‐level processes to changes in prey abundance. Using a laboratory experiment, we investigated how NCEs of the invasive planktivorous cladoceran Bythotrephes longimanus on the condition of native zooplankton from Lake Michigan affect population abundance. Presence of chemical cues (i.e. kairomones) from Bythotrephes led to a greater than 13‐fold reduction in abundance of diaptomid copepods in the absence of any direct predation. This dramatic reduction was a result of reduced fecundity and likely reduced survival. The costly anti‐predator response underlying the demographic effects was reduced food intake and/or assimilation as evidenced by reduced gut fullness, which led to poorer condition, as evidenced by lower lipid accumulation. These predator effects may remain undetected in studies of animal populations that fail to include sufficient efforts to search for them, and illustrate the far‐reaching potential of predator non‐consumptive effects (‘fear effects’) to influence prey populations.

Invasive species triggers a massive loss of ecosystem services through a trophic cascade

Despite growing recognition of the importance of ecosystem services and the economic and ecological harm caused by invasive species, linkages between invasions, changes in ecosystem functioning, and in turn, provisioning of ecosystem services remain poorly documented and poorly understood. We evaluate the economic impacts of an invasion that cascaded through a food web to cause substantial declines in water clarity, a valued ecosystem service. The predatory zooplankton, the spiny water flea (Bythotrephes longimanus), invaded the Laurentian Great Lakes in the 1980s and has subsequently undergone secondary spread to inland lakes, including Lake Mendota (Wisconsin), in 2009. In Lake Mendota, Bythotrephes has reached unparalleled densities compared with in other lakes, decreasing biomass of the grazer Daphnia pulicaria and causing a decline in water clarity of nearly 1 m. Time series modeling revealed that the loss in water clarity, valued at US$140 million (US$640 per household), could be reversed by a 71% reduction in phosphorus loading. A phosphorus reduction of this magnitude is estimated to cost between US$86.5 million and US$163 million (US$430-US$810 per household). Estimates of the economic effects of Great Lakes invasive species may increase considerably if cases of secondary invasions into inland lakes, such as Lake Mendota, are included. Furthermore, such extreme cases of economic damages call for increased investment in the prevention and control of invasive species to better maximize the economic benefits of such programs. Our results highlight the need to more fully incorporate ecosystem services into our analysis of invasive species impacts, management, and public policy.

A plague of waterfleas (Bythotrephes): impacts on microcrustacean community structure, seasonal biomass, and secondary production in a large inland-lake complex

The spiny cladoceran (Bythotrephes longimanus) is an invasive, predaceous zooplankter that is expanding from Great Lakes coastal waters into inland lakes within a northern latitudinal band. In a large, Boundary Water lake complex (largely within Voyageurs National Park), we use two comparisons, a 2-year spatial and a 12-year temporal, to quantify seasonal impacts on food webs and biomass, plus a preliminary calculation of secondary production decline. Bythotrephes alters the seasonal biomass pattern by severely depressing microcrustaceans during summer and early fall, when the predator is most abundant. Cladoceran and cyclopoid copepods suffer the most serious population declines, although the resistant cladoceran Holopedium is favored in spatial comparisons. Microcrustacean biomass is reduced 40–60 % and secondary production declines by about 67 %. The microcrustacean community shifts towards calanoid copepods. The decline in secondary production is due both to summer biomass loss and to the longer generation times of calanoid copepods (slower turnover). The Bythotrephes “top-down” perturbation appears to hold across small, intermediate, and large-sized lakes (i.e. appears scale-independent), and is pronounced when Bythotrephes densities reach 20–40 individuals L−1. Induction tests with small cladocerans (Bosmina) suggest that certain native prey populations do not sense the exotic predator and are “blind-sided”. Failure of prey to deploy defenses could explain the disproportionate community impacts in New World versus Old World lakes.

A review of the effects ofBythotrephes longimanusand calcium decline on zooplankton communities — can interactive effects be predicted?

Anthropogenic stressors including acid deposition, invasive species, and calcium (Ca) decline have produced widespread damage to Canadian Shield lakes, especially to their zooplankton communities. Here, we review current knowledge on the individual effects on zooplankton by the non-indigenous predator Bythotrephes longimanus and Ca decline; we identify knowledge gaps in this literature and examine the likely interactive impacts of Bythotrephes invasions and Ca decline on zooplankton. The negative impacts of Bythotrephes longimanus on zooplankton communities are well known, whereas current understanding of the effects of declining Ca on zooplankton is restricted to Daphnia spp.; hence, there is a large knowledge gap on how declining Ca may affect zooplankton communities in general. The co-occurring impacts of Bythotrephes and declining Ca have rarely been studied at the species level, and we expect daphniids, particularly Daphnia retrocurva and Daphnia pulicaria, to be the most sensitive to both stressors. We also expect a synergistic negative interaction on cladocerans in lakes with both stressors, leaving a community dominated by Holopedium glacialis and (or) copepods. Our predictions form testable hypotheses but since species and ecosystem response to multiple stressors are difficult to predict, we may actually see ecological surprises in Canadian Shield lakes as Bythotrephes continues to spread and Ca levels continue to fall.

Diet of walleye and sauger and morphological characteristics of their prey in Lake Winnipeg

Abstract Lake Winnipeg has the second largest commercial walleye ( Sander vitreus ) fishery in North America. The lake has experienced and will continue to undergo comprehensive changes such as eutrophication, invasion of non-indigenous aquatic species, and climate change. The North and South Basins of the lake differ in many ways, e.g. mean water depth and temperature, turbidity, and fish community. The study objectives were to establish a baseline for diet of walleye and sauger ( Sander canadensis ) in Lake Winnipeg, to determine whether the diets currently differ between basins and among seasons, and to investigate whether the non-indigenous rainbow smelt ( Osmerus mordax ) contributes substantially to their diet. Walleye and sauger diets in the South Basin had more diverse species composition, whereas in the North Basin, the diet consisted almost entirely of the introduced rainbow smelt (as well as troutperch ( Percopsis omiscomaycus ) for sauger). Some seasonal variation was also documented and found to be attributable to the emergence of young-of-year fish and mayfly larvae. The predominant incorporation of the abundant and large rainbow smelt in the diets of walleye and sauger in the North Basin may lead to increased growth and condition of fish in this basin. These data provide a baseline for comparison in future investigations of walleye and sauger diets as changes occur in the food web of the lake with invasion of zebra mussels ( Dreissena polymorpha ) and spiny water fleas ( Bythotrephes longimanus ).

Evolutionary stasis despite selection on a heritable trait in an invasive zooplankton.

Invasive species are one of the greatest threats to ecosystems, and there is evidence that evolution plays an important role in the success or failure of invasions. Yet, few studies have measured natural selection and evolutionary responses to selection in invasive species, particularly invasive animals. We quantified the strength of natural selection on the defensive morphology (distal spine) of an invasive zooplankton, Bythotrephes longimanus, in Lake Michigan across multiple months during three growing seasons. We used multiple lines of evidence, including historic and contemporary wild-captured individuals and palaeoecology of retrieved spines, to assess phenotypic change in distal spine length since invasion. We found evidence of temporally variable selection, with selection for decreased distal spine length early in the growing season and selection for increased distal spine length later in the season. This trend in natural selection is consistent with seasonal changes in the relative strength of non-gape-limited and gape-limited fish predation. Yet, despite net selection for increased distal spine length and a known genetic basis for distal spine length, we observed little evidence of an evolutionary response to selection. Multiple factors likely limit an evolutionary response to selection, including genetic correlations, trade-offs between components of fitness, and phenotypic plasticity.