Shift in pike, Esox lucius L., predation pressure following the introduction of ruffe, Gymnocephalus cernuus (L.) to Loch Lomond

Rapid and dramatic change in the fish community of Loch Lomond has resulted from a series of fish introductions in recent years. A comparison of the diet of pike, Esox lucius L., in 1989–1990 with data from 1955–1967, prior to recent introductions, demonstrates a shift in prey choice. In 1955–1967 powan, Coregonus lavaretus (L.), dominated in the diet (57% of prey by number) by 1989–1990 the introduced ruffe, Gymnocephalus cernuus (L.), was the commonest prey species (44% by number). This shift in pike predation to an abundant population of introduced ruffe has consequences for native species. Assuming that the pike population has not increased in response to increased food availability due to the introduction of ruffe, pike predation pressure on native species will be relieved. This is likely to have the greatest effect on powan. Comparison of the predation rate in 1955–1967 with 1989–1990 supports the hypothesis that the rate of predation on powan has declined, although the effect that this may have on the powan population is unclear, as the role of predation in the regulation of population size is unknown for this species.     

Prey vulnerability to a gape-size limited predator: behavioural and morphological impacts on northern pike piscivory

The northern pike ( Esox lucius ) is a selective and important predator in lake ecosystems. Prey size in pike is limited by pike gape size, which is a linear function of pike body length. Here we show that the absolute gape-size limit in pike is greater than previously considered, and that maximum ingestible prey size is limited by prey body depth. Further, we experimentally show that pike prefer shallow-bodied roach before deeper-bodied common bream, and small prey sizes within each prey species. Handling time in pike increases with prey body depth, and since common bream are deeper-bodied than roach, handling time is longer for bream than for roach of the same length, but equal considering body depth. Prey handling time is suggested to be a major cost to the pike, since it increases the risk of losing the prey, as well as exposure to predation, kleptoparasitism and cannibalism. Consequently, prey vulnerability is determined by risk of predation and intraspecific interactions, and behavioural preferences in the pike, and not by pike gape-size limits. The consequences for natural populations is evaluated by analysing size structures of predator and prey fish populations in a eutrophic lake.     

Ontogenetic niche shifts and resource partitioning in a subarctic piscivore fish guild

The feeding ecology of three piscivorous fish species (perch (Perca fluviatilis), pike (Esox lucius) and burbot (Lota lota)), was studied in the subarctic Pasvik watercourse (69 °N), northern Norway and Russia. All three species primarily occupied the benthic habitats in the watercourse. Perch and burbot exhibited distinct ontogenetic niche shifts in food resource use, perch changing from a dominance of zooplankton to zoobenthos to fish, and burbot from zoobenthos to fish. Fish prey dominated the diet of all the investigated size-classes of pike, but small-sized pike (<20 cm) were not represented in the sample. Fish prey size was positively related to predator size in all three species. Whitefish (Coregonus lavaretus) was the dominant prey of pike and large-sized burbot and perch. Nine-spined sticklebacks (Pungitius pungitius) was also an important prey and appeared to be a dietary stepping-stone enhancing the transition from invertebrate feeding to consumption of large-sized whitefish prey for all three predators. A cluster analysis separated the different size groups of the three predator species into five functional feeding groups, most of them containing two or all three species. Within these feeding groups, and especially among the piscivorous size-classes, there was a strong and significant interspecific overlap in prey selection, and the dietary similarities between the species were in general much larger than the intraspecific similarities between ontogenetic stages. All three piscivorous species are important top predators in the aquatic food web of the watercourse, and their ontogenetic diet shifts and resource partitioning patterns generate a substantial food web complexity in this subarctic ecosystem.     

Piscivore efficiency and refuging prey: the importance of predator search mode

In predator-prey interactions, the efficiency of the predator is dependent on characteristics of both the predator and the prey, as well as the structure of the environment. In a field enclosure experiment, we tested the effects of a prey refuge on predator search mode, predator efficiency and prey behaviour. Replicated enclosures containing young of the year (0+) and 1-year-old (1+) perch were stocked with 3 differentially sized individuals of either of 2 piscivorous species, perch (Perca fluviatilis), pike (Esox lucius) or no piscivorous predators. Each enclosure contained an open predator area with three small vegetation patches, and a vegetated absolute refuge for the prey. We quantified the behaviour of the predators and the prey simultaneously, and at the end of the experiment the growth of the predators and the mortality and habitat use of the prey were estimated. The activity mode of both predator species was stationary. Perch stayed in pairs in the vegetation patches whereas pike remained solitary and occupied the corners of the enclosure. The largest pike individuals stayed closest to the prey refuge whereas the smallest individuals stayed farthest away from the prey refuge, indicating size-dependent interference among pike. Both size classes of prey showed stronger behavioural responses to pike than to perch with respect to refuge use, distance from refuge and distance to the nearest predator. Prey mortality was higher in the presence of pike than in the presence of perch. Predators decreased in body mass in all treatments, and perch showed a relatively stronger decrease in body mass than pike during the experiment. Growth differences of perch and pike, and mortality differences of prey caused by predation, can be explained by predator morphology, predator attack efficiency and social versus interference behaviour of the predators. These considerations suggest that pike are more efficient piscivores around prey refuges such as the littoral zones of lakes, whereas perch have previously been observed to be more efficient in open areas, such as in the pelagic zones of lakes.     

Cost of prey capture, growth rate and ration size in pike, Esox lucius L., as functions of prey wheight

Prey intake, amount of time feeding, pursuit and manipulation times and growth rate were studied in the pike, Esox lucius , in relation to prey weight. Predator and prey were observed in tanks at 15°C and with a 14:10 h LD photoperiod. A total of 444 captures were recorded from five 1 g prey weight groups varying from 0 to 4.9 g. Ten pike were used weighing between 50 and 149 g.
As prey weight increased, the number and weight of prey consumed decreased as did the time spent feeding, which became concentrated into the first trial of the day. Pursuit time did not vary with prey weight, but manipulation time and the weight of prey gained per unit pursuit plus manipulation time increased with prey weight.
The growth rate of pike was a positive function of ration size and a negative function of the amount of time spent capturing each gramme of prey (capture cost). Growth rate was also correlated with the mean length and weight of prey eaten.
The most profitable prey, as estimated from optimal foraging theory, need only be available once every 84 min for pike to maximize their net energy gain. The data show that feeding behaviour can have a direct influence on the reproductive success of pike through its effect on growth rate.     

Foraging among cannibals and kleptoparasites: effects of prey size on pike behavior

The northern pike (Esox lucius) is an important and selective piscivorethat chooses smaller prey than predicted from energy / timebudgets. In a laboratory experiment, we investigated pike predatorybehavior to explain this selectivity. Northern pike feedingon different prey sizes in aquaria were observed when foragingalone, when in the presence of chemical cues from similar-sizedor larger conspecifics, and when in the presence of conspecifics thatwere allowed to interact with the focal pike. The results showthat prey handling time increases with prey size and that theduration of manipulating and handling prey inflicts a risk ofexposure to cannibals and kleptoparasites on the pike. Therefore,the risk of falling victim to cannibals or kleptoparasites increaseswith prey size. Attracting and experiencing intraspecific interactorscan be regarded as major fitness costs. Chemical cues from foragingconspecifics have only minor effects on pike foraging behavior.Furthermore, the ability to strike and swallow prey head first improvespike predatory performance because failing to do so increases handlingtime. Our findings emphasize the increasing potential costswith large prey and explain previous contradictory suggestionson the underlying mechanisms of behavior, selectivity, and trophiceffects of northern pike predation.     

Foraging behaviour and capture success in perch, pikeperch and pike and the effects of prey density

The effect of school size on capture success in three different piscivores, perch Perca fluviatilis , pikeperch Stizostedion lucioperca and pike Esox lucius , was investigated. Roach Rutilus rutilus were used as prey in a pool experiment where individual predators were presented prey at densities of one, two, four, eight and 16 prey, respectively. Treatments were replicated seven times for each predator species. Perch was at first virtually unable to capture a prey from a school and suffered a significant confusion effect with increasing prey density. The effect, however, was limited in the long run, as the perch was a very effective predator in its hunting strategy where it singled out and repeatedly attacked single prey irrespective of prey density or school size. Pikeperch and pike were able to attack and capture prey at any prey density equally successfully and thus did not suffer from a confusion effect. Neither did these predators receive any apparent advantages from increasing prey density.     

Crayfish predation on the common pond snail (Lymnaea stagnalis): the effect of habitat complexity and snail size on foraging efficiency

Optimal foraging theory was used to explain selective foraging by the introduced signal crayfish (Pacifastacus leniusculus) on the thin-shelled common pond snail (Lymnaea stagnalis). Crayfish predation efficiency was studied in relation to habitat complexity and snail size. In a pool experiment (area 1.3 m²) single adult crayfish were allowed to feed on four size classes of snails for one week. A pair-wise preference trial (aquarium experiment) tested if adult crayfish selectively predated on particular size classes of snail and if prey value (expressed as snail dry mass per handling time) could explain the size range of snails chosen. Crayfish preferred the smallest size classes of snails in both pool and aquaria experiments. In the pool experiment crayfish had a strong effect on snail survival. Habitat complexity did not affect overall snail survival, but resulted in reduced predation pressure on the smallest size classes of snails. Handling time and shell-thickness increased exponentially with increasing snail size, and the two smallest size classes had the highest prey values. The results suggest that crayfish can structure the abundance and size distribution of thin-shelled snails, through size-selective predation and reduction of macrophytes. The mechanisms behind the choice of snails may be based on prey value and reduced exposure time to predators and conspecifics. Crayfish effects on snail size distribution may be less pronounced in complex habitats such as macrophyte beds. This revised version was published online in July 2006 with corrections to the Cover Date.     

Crayfish learning abilities: how does familiarization period affect the capture rate of a new prey item?

This study explores the effect of the length of learning period on capture rate of a previously unfamiliar prey by an invasive freshwater crayfish (Procambarus clarkii). Juvenile crayfish were subjected to different periods of contact (learning period) with a larvae prey (Chaoborus sp.). The length of the learning period significantly affected the number of prey consumed by the predator. Our results indicate that the naive crayfish require less than 12 h to learn to maximize capture rate of this prey. The learning coefficient, adopted in the present study, may be useful in exploring predation capabilities of alien species in newly invaded habitats.     

Invasive pike establishment in Cook Inlet Basin lakes,Alaska: diet,native fish abundance and lake environment

Northern pike (Esox lucius) were introduced to the northern Susitna Basin of south-central Alaska in the 1950’s, and have since spread throughout the upper Cook Inlet Basin. Extirpations of several native fish populations have been documented in this area. It is hypothesized here that invasive pike remodel the ecology of lakes by removing vulnerable prey types and that these changes are reflected in the diet of invasive pike. Trends in pike diet suggest that pike switch to less desirable but more abundant macroinvertebrate prey as preferred fish prey are eliminated. The impacts of pike introduction were studied in detail for one species of resident fish, the threespine stickleback (Gasterosteus aculeatus). Stickleback abundance decreases as pike invasion progresses. Conductivity is a significant environmental predictor of stickleback abundance, with higher conductivity apparently mitigating population reduction. Higher conductivity water may lessen the physiological costs of developing more robust armor, which reduces vulnerability to predation. Maximum lake depth also appears to predict stickleback abundance, though this trend was only marginally significant. Deeper lakes may provide an open-water refuge from pike predation by allowing stickleback to exist outside of the pike inhabited littoral zone. These findings indicate the importance of diverse habitat types and certain chemical and physical characteristics to the outcome of invasion by fish predators.     

Analysis of sibling cannibalism among pike,Esox lucius,juveniles reared under semi-natural conditions

Synopsis Sibling cannibalism in pike, Esox lucius, larvae and juveniles living in outdoor rearing ponds was studied using stomach contents analysis. For the two initial densities tested (6 and 18 larvae m^–2, equivalent to 12 and 36 larvae m^–3), cannibalism was non-existent during the larval period (13 to 35 mm total length) and was observed only during the juvenile stages. Initial density of larvae influenced both the date of first detection of cannibalistic individuals and the rate of development of cannibalism in the population. At initial stocking densities of 18 larvae m^–2 (36 larvae m^–3), cannibalism was observed from 21 days after the start of exogenous feeding (mean total length: 60 mm) onwards. At a mean total length of 100 mm and for initial stocking densities of 6 and 18 larvae m^–2, (12 and 36 larvae m^–3), the average proportions of cannibals in the populations of juveniles were 7.8% and 41.3% and the cannibals accounted for 15.5% and 65.9% of the total pike biomass, respectively. In stomachs of cannibals, young pike were the dominant prey in terms of weight. Dry weights of invertebrate-prey were lower in cannibals than in non-cannibals of similar size. Cannibalism among pike juveniles was characterized by the prey being swallowed whole and head first in the vast majority of cases. There was a strong positive correlation between predator and prey size and the mouth size of a cannibal was found to be an important constraint determining maximum victim size. The overall mean ratio of pike prey length to pike cannibal length was 66.2% and the average ratio of prey head depth to predator mouth width amounted to 87.6%. Prey size selection could be demonstrated for several length-groups of cannibals. These results are compared with the characteristics of early cannibalism in other fish species.     

Temporal stability in size distributions and growth rates of three Esox lucius L. populations. A result of cannibalism?

The population total length ( L _T) structures and individual growth trajectories for three stream living pike Esox lucius populations were studied for 7 years. All three populations exhibited small variation in both population L _T structure and individual growth trajectories over time. These dynamics contrasted to the much more variable population L _T structure of perch Perca fluviatilis studied previously. The difference in population dynamics between the two species was related to differences in prey:predator size ratios. The pike populations in the more open and larger streams grew to larger sizes, but this difference in life history did not affect population dynamics of pike. It is concluded that (1) cannibalistic population dynamics may be predicted from individual life-history characteristics such as minimum and maximum victim:cannibal size ratios and (2) the cannibal-driven population dynamics observed in pike seems to be robust to variation in environmental conditions (system openness).     

An invasive species may be better than none: invasive signal and native noble crayfish have similar community effects

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Comparison of mouth morphology and prey size selection among three esocid taxa

Aquatic organisms, especially fishes, exhibit exceptional diversity in mouth morphology and this variation has been shown to influence foraging patterns. We compared mouth morphology among muskellunge Esox masquinongy, northern pike Esox lucius and their hybrid, tiger muskellunge E. masquinongy x E. lucius. Head and mouth size among the three taxa were similar for juveniles (<400 mm total length), but diverged with increasing length, being greater for northern pike than muskellunge. Tiger muskellunge had a head and mouth size intermediate to the two, but more similar to northern pike than muskellunge. Morphological differences among taxa were related to data examining prey size selection in laboratory and field experiments. In the laboratory, northern pike selected prey that were smaller than their maximum mouth width (widest point between outside corners of mouth), tiger muskellunge selected larger prey, and muskellunge size-selection was intermediate between the other two taxa. Among the three esocids, muskellunge had the smallest increase in handling time with increasing prey body depth relative to predator mouth width. In a common garden field experiment in three lakes containing mainly deep-bodied prey, results generally followed morphological patterns, with northern pike selecting larger prey compared to muskellunge. Although morphology predicted most of the variation in greatest body depth of prey consumed, the best predictor of prey size was a model that included predator mouth width, taxon, and interaction. Information comparing prey size selection among esocid taxa is useful for understanding how to manage esocid populations based on system-specific prey characteristics and also for understanding how variations in morphological characteristics of apex predators can influence prey vulnerability and ecosystem structure.     

Morphological convergence in a Mexican garter snake associated with the ingestion of a novel prey

Morphological convergence is expected when organisms which differ in phenotype experience similar functional demands, which lead to similar associations between resource utilization and performance. To consume prey with hard exoskeletons, snakes require either specialized head morphology, or to deal with them when they are vulnerable, for example, during molting. Such attributes may in turn reduce the efficiency with which they prey on soft‐bodied, slippery animals such as fish. Snakes which consume a range of prey may present intermediate morphology, such as that of Thamnophiine (Natricinae), which may be classified morphometrically across the soft–hard prey dietary boundary. In this study, we compared the dentition and head structure of populations of Thamnophis melanogaster that have entered the arthropod–crustacean (crayfish)‐eating niche and those that have not, and tested for convergence between the former and two distantly related crayfish specialists of the genus Regina (R. septemvittata and R. grahamii). As a control, we included the congener T. eques. Multivariate analysis of jaw length, head length, head width, and number of maxillary teeth yielded three significant canonical variables that together explained 98.8% of the variance in the size‐corrected morphological data. The first canonical variable significantly discriminated between the three species. The results show that head dimensions and number of teeth of the two Regina species are more similar to those of crayfish‐eating T. melanogaster than to non‐crayfish‐eating snakes or of T. eques. It is unclear how particular head proportions or teeth number facilitates capture of crayfish, but our results and the rarity of soft crayfish ingestion by T. melanogaster may reflect the novelty of this niche expansion, and are consistent with the hypothesis that some populations of T. melanogaster have converged in their head morphology with the two soft crayfish‐eating Regina species, although we cannot rule out the possibility of a morphological pre‐adaptation to ingest crayfish.     

The Role of gastric evacuation rate in handling time of equal-mass rations of different prey sizes in northern pike

In a laboratory experiment, northern pike Esox lucius gastric evacuation rates did not differ between equal-mass rations of small and large prey. In a comparison between intermediate and large prey, the pike were unable to fit two intermediate prey completely into the stomach at the same time, resulting in two consecutive evacuations, and changes in patterns of gastric evacuation. Thus, total gastric evacuation time was not affected by prey size composition in equal-mass rations, but patterns in evacuation rate may depend on the size ratio between predator and prey. Cumulative manipulation time between strike and complete swallowing of prey differed between equal-mass rations of small, intermediate and large prey, in that small prey took the shortest time to manipulate. Pike had problems striking and redirecting intermediate prey to swallow them head first, and the manipulation times for intermediate prey were as long as for large prey. Since an increased time manipulating prey in the mouth increases risk of predation and intraspecific interactions in pike, it is concluded that risks associated with long manipulation times, and not only energy per total handling time, determine prey value and prey size preference in this piscivore.     

Predatory functional response and prey choice identify predation differences between native/invasive and parasitised/unparasitised crayfish

Background

Invasive predators may change the structure of invaded communities through predation and competition with native species. In Europe, the invasive signal crayfish Pacifastacus leniusculus is excluding the native white clawed crayfish Austropotamobius pallipes.

Methodology and Principal Findings

This study compared the predatory functional responses and prey choice of native and invasive crayfish and measured impacts of parasitism on the predatory strength of the native species. Invasive crayfish showed a higher (>10%) prey (Gammarus pulex) intake rate than (size matched) natives, reflecting a shorter (16%) prey handling time. The native crayfish also showed greater selection for crustacean prey over molluscs and bloodworm, whereas the invasive species was a more generalist predator. A. pallipes parasitised by the microsporidian parasite Thelohania contejeani showed a 30% reduction in prey intake. We suggest that this results from parasite-induced muscle damage, and this is supported by a reduced (38%) attack rate and increased (30%) prey handling time.

Conclusions and Significance

Our results indicate that the per capita (i.e., functional response) difference between the species may contribute to success of the invader and extinction of the native species, as well as decreased biodiversity and biomass in invaded rivers. In addition, the reduced predatory strength of parasitized natives may impair their competitive abilities, facilitating exclusion by the invader.     

Piscivory and prey selection of four predator species in a whitefish dominated subarctic lake

According to logistic regressions derived for pike Esox lucius and burbot Lota lota , the probability of ingesting fishes in Lake Muddusjärvi, northern Finland, was 50% at 19·3 and 22·1 cm L _T, whereas Arctic charr Salvelinus alpinus and brown trout Salmo trutta shifted to piscivory at the lengths of 25·7 and 26·4 cm L _T. The specialist piscivores, pike and burbot, consumed more prey species and took a wider range of prey sizes than Arctic charr and brown trout. The prey length for all predators increased in relationship to predator length. Whitefish Coregonus lavaretus was the dominant prey species in the lake and in the diet of all the piscivorous species. The whitefish population was divided into three forms, of which the slow-growing, and the most numerous densely rakered whitefish form (DR), was selected by all predator species. This form also had the smallest average size and widest habitat range, utilizing both pelagic and epibenthic habitats. Two sparsely rakered whitefish forms (LSR and SSR) occupied only epibenthic habitats and had lower relative densities than DR. These forms, LSR and SSR, had a minor importance in the diet of predator species.     

Crayfish in lakes and streams: individual and population responses to predation, productivity and substratum availability

1. In a correlative study, we investigated the relative importance of fish predation, refuge availability and resource supply in determining the abundance and size distributions of the introduced and omnivorous signal crayfish (Pacifastacus leniusculus) in lakes and streams. Moreover, the biomass and food selection of predatory fish was estimated in each habitat type and stable isotopes of carbon and nitrogen were measured in perch (Perca fluviatilis), the dominant predator in the lakes, and in its potential food sources (crayfish, juvenile roach and isopods). 2. In lakes, crayfish were the most frequent prey in large perch (46%), followed by other macroinvertebrates (26%, including the isopod Asellus aquaticus) and small fish (25%). Crayfish and fish dominated the gut contents of large perch with respect to biomass. Nitrogen signatures showed that perch were one trophic level above crayfish (approx. 3.4‰) and a two‐source mixing model using nitrogen isotope values indicated that crayfish (81%) contributed significantly more to perch isotope values than did juvenile roach (19%). A positive correlation was found between the abundance of crayfish and the biomass of large perch. Crayfish abundance in lakes was also positively correlated with the proportion of cobbles in the littoral zone. Lake productivity (chlorophyll a) was positively correlated with crayfish size, but not with crayfish abundance. 3. In streams, brown trout (Salmo trutta) were the most abundant predatory fish. Gut contents of large trout in a forested stream showed that terrestrial insects were the most frequently found prey (60%), followed by small crayfish (27%) and isopods (27%). In contrast to lakes, the relative abundance of crayfish was negatively correlated with the total biomass of predatory fish and with total biomass of trout. However, abundance of crayfish at sites with a low biomass of predatory fish varied considerably and was related to substratum grain size, with fewer crayfish being caught when the substratum was sandy or dominated by large boulders. The mean size of crayfish was greater at stream sites with a high standing stock of periphyton, but neither predator biomass nor substratum grain size was correlated with crayfish size. 4. Our results suggest that bottom‐up processes influence crayfish size in lakes and streams independent of predator biomass and substratum availability. However, bottom‐up processes do not influence crayfish abundance. Instead, substratum availability (lakes) and interactions between predation and substratum grain size (streams) need to be considered in order to predict crayfish abundance.     

Home-field advantage: native signal crayfish (Pacifastacus leniusculus) out consume newly introduced crayfishes for invasive Chinese mystery snail (Bellamya chinensis)

The introduction of non-indigenous plants, animals and pathogens is one of today’s most pressing environmental challenges. Freshwater ecologists are challenged to predict the potential consequences of species invasions because many ecosystems increasingly support novel assemblages of native and non-native species that are likely to interact in complex ways. In this study we evaluated how native signal crayfish (Pacifastacus leniusculus) and non-native red swamp crayfish (Procambarus clarkii) and northern crayfish (Orconectes virilis) utilize a novel prey resource: the non-native Chinese mystery snail (Bellamya chinensis). All species are widespread in the United States, as well as globally, and recent surveys have discovered them co-occurring in lakes of Washington State. A series of mesocosm experiments revealed that crayfish are able to consume B. chinensis, despite the snail’s large size, thick outer shell and trapdoor defense behaviour. Crayfish exhibited size-selective predation whereby consumption levels decreased with increasing snail size; a common pattern among decapod predators. Comparison of prey profitability curves—defined as the yield of food (weight of snail tissue) per second of feeding time (the time taken to crack the shell and consume the contents)—suggests that small and very large snails may represent the most profitable prey choice. By contrast, previous studies have reported the opposite pattern for crayfish consumption on thin-shelled snails. For all snail size classes, we found that native P. leniusculus and invasive O. virilis consumed greater numbers of snails than invasive P. clarkii. Moreover, P. leniusculus consistently handled and consumed snails at a faster pace compared to both invasive crayfishes across the range of snail sizes examined in our study. These results suggest not only that B. chinensis is a suitable food source for crayfish, but also that native P. leniusculus may ultimately out-consume invasive crayfishes for this new prey resource.