Piscivore-Prey Fish Interactions: Mechanisms behind Diurnal Patterns in Prey Selectivity in Brown and Clear Water

Environmental change may affect predator-prey interactions in lakes through deterioration of visual conditions affecting foraging success of visually oriented predators. Environmental change in lakes includes an increase in humic matter causing browner water and reduced visibility, affecting the behavioural performance of both piscivores and prey. We studied diurnal patterns of prey selection in piscivorous pikeperch (Sander lucioperca) in both field and laboratory investigations. In the field we estimated prey selectivity and prey availability during day and night in a clear and a brown water lake. Further, prey selectivity during day and night conditions was studied in the laboratory where we manipulated optical conditions (humic matter content) of the water. Here, we also studied the behaviours of piscivores and prey, focusing on foraging-cycle stages such as number of interests and attacks by the pikeperch as well as the escape distance of the prey fish species. Analyses of gut contents from the field study showed that pikeperch selected perch (Perca fluviatilis) over roach (Rutilus rutilus) prey in both lakes during the day, but changed selectivity towards roach in both lakes at night. These results were corroborated in the selectivity experiments along a brown-water gradient in day and night light conditions. However, a change in selectivity from perch to roach was observed when the optical condition was heavily degraded, from either brown-stained water or light intensity. At longer visual ranges, roach initiated escape at distances greater than pikeperch attack distances, whereas perch stayed inactive making pikeperch approach and attack at the closest range possible. Roach anti-predatory behaviour decreased in deteriorated visual conditions, altering selectivity patterns. Our results highlight the importance of investigating both predator and prey responses to visibility conditions in order to understand the effects of degrading optical conditions on piscivore-prey interaction strength and thereby ecosystem responses to brownification of waters.     

Foraging success of juvenile pike Esox lucius depends on visual conditions and prey pigmentation

Young-of-the-year pike Esox lucius foraging on copepods experienced different foraging success depending on prey pigmentation in water visually degraded by brown colouration or algae. Both attack rate and prey consumption rate were higher for E. lucius foraging on transparent prey in brown water, whereas the opposite was true in algal turbid water. Pigments in copepod prey may have a cryptic function in brown water instead of a photo-protective function even if prey-size selectivity was stronger than selection based on pigmentation in juvenile E. lucius.     

Turbidity amplifies the non‐lethal effects of predation and affects the foraging success of characid fish shoals

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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.     

Complex Relationships amongst Parasite Load and Escape Behaviour in an Insular Lizard

Economic escape models predict escape decisions of prey which are approached by predators. Flight initiation distance (FID, predator–prey distance when prey begins to flee) and distance fled (DF) are major variables used to characterize escape responses. In optimal escape theory, FID increases as cost of not fleeing also increases. Moreover, FID decreases as cost of fleeing increases, due to lost opportunities to perform activities that may increase fitness. Finally, FID further increases as the prey's fitness increases. Some factors, including parasitism, may affect more than one of these predictors of FID. Initially, parasitized prey may have lower fitness as well as impaired locomotor ability, which would avoid predation and/or reduce their foraging ability, further decreasing the opportunity of fleeing. For example, if parasites decrease body condition, prey fitness is reduced and escape ability may be impaired. Hence, the overall influence of parasitism on FID is difficult to predict. We examined relationships between escape decisions and different traits: parasite load, body size and body condition in the Balearic lizard, Podarcis lilfordi. Lizards that showed higher haemogregarines load had longer FID and shorter DF. Although results did not confirm our initial predictions made on the basis of optimal escape theory, our findings suggest that parasites can alter several aspects of escape behaviour in a complex way.     

Altered visual environment affects a tropical freshwater fish assemblage through impacts on predator–prey interactions

1. Increased turbidity and siltation caused by rock quarrying, mining, and deforestation are pervasive disturbances in aquatic systems. Turbidity interferes with vision for aquatic organisms, potentially altering predator–prey interactions.
2. We studied the effects of these disturbances in Trinidadian streams by surveying predators and their shared prey both in streams with versus without quarries as well as in a focal stream before and after the establishment of a quarry. Then, to evaluate whether differential foraging success in turbid water might underlie abundance patterns of predators, we experimentally induced turbidity in mesocosms and measured predator foraging success.
3. Upstream quarry presence had a dramatic effect on the benthic structure of streams, greatly increasing siltation. A substantial decrease in the abundance of a diurnal cichlid predator (Crenicichla frenata) was associated with quarry presence, while a nocturnal erytherinid predator (Hoplias malabaricus) was equally as abundant in streams with or without quarries. The density of their shared prey, the Trinidadian guppy (Poecilia reticulata) remained unchanged.
4. In mesocosm trials, Crenicichla were less successful predators with turbidity, whereas Hoplias performed equally across turbidities. These foraging success results help explain differences in demographic shifts in response to turbidity for both predators.
5. By relating short-term effects of an anthropogenically altered visual environment on species interactions to abundance patterns of predators and prey, this study helps to identify an important mechanism whereby changes to species’ visual ecology may have long-term effects on population biology.

     

Environmental Conditions and Intraspecific Interference: Unexpected Effects of Turbidity on Pike (Esox lucius) Foraging

Interference among predators decreases per capita foraging rates and has implications for both community dynamics and top-down trophic processes. Interference originates from behavioural interactions among foragers, and these behaviours could be affected by environmental conditions. In experiments on pike foraging alone or among conspecifics in different levels of water turbidity, we expected high turbidity to decrease the perceived risk of intraspecific interactions among pike, and thereby decrease the strength of interference, as turbidity would decrease the visual contact between individuals and act as a refuge from behavioural interactions. The results show that this is not the case, but suggest that interference is induced instead of reduced in high turbidity. Per capita foraging rates do not differ between pike foraging alone or in groups in our clear and moderately turbid treatments, indicating no effect of interference. As high turbidity enhances prey consumption for pike individuals foraging alone, but does not have this effect for pike in groups, high turbidity induces the relative interference effect. We suggest that future evaluations of the stabilizing effects of interference on community dynamics and its reduction of predation impact on top-down trophic cascades should consider potential unexpected effects of environmental conditions.     

Effects of crayfish size,orientation, and movement on the reactive distance of largemouth bass foraging in clear and turbid water

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Effects of degraded optical conditions on behavioural responses to alarm cues in a freshwater fish

Prey organisms often use multiple sensory cues to gain reliable information about imminent predation threat. In this study we test if a freshwater fish increases the reliance on supplementary cues when the reliability of the primary cue is reduced. Fish commonly use vision to evaluate predation threat, but may also use chemical cues from predators or injured conspecifics. Environmental changes, such as increasing turbidity or water colour, may compromise the use of vision through changes in the optical properties of water. In an experiment we tested if changes in optical conditions have any effects on how crucian carp respond to chemical predator cues. In turbidity treatments we added either clay or algae, and in a brown water colour treatment we added water with a high humic content. We found that carp reduced activity in response to predator cues, but only in the turbidity treatments (clay, algae), whereas the response in the brown water treatment was intermediate, and not significantly different from, clear and turbid water treatments. The increased reliance on chemical cues indicates that crucian carp can compensate for the reduced information content from vision in waters where optical conditions are degraded. The lower effect in brown water may be due to the reduction in light intensity, changes in the spectral composition (reduction of UV light) or to a change in chemical properties of the cue in humic waters.     

Temperature dependence of predation depends on the relative performance of predators and prey

The temperature dependence of predation rates is a key issue for understanding and predicting the responses of ecosystems to climate change. Using a simple mechanistic model, we demonstrate that differences in the relative performances of predator and prey can cause strong threshold effects in the temperature dependence of attack rates. Empirical data on the attack rate of northern pike (Esox lucius) feeding on brown trout (Salmo trutta) confirm this result. Attack rates fell sharply below a threshold temperature of +11°C, which corresponded to a shift in relative performance of pike and brown trout with respect to maximum attack and escape swimming speeds. The average attack speed of pike was an order of magnitude lower than the escape speed of brown trout at 5°C, but approximately equal at temperatures above 11°C. Thresholds in the temperature dependence of ecological rates can create tipping points in the responses of ecosystems to increasing temperatures. Thus, identifying thresholds is crucial when predicting future effects of climate warming.     

Modelling the attack success of planktonic predators: patterns and mechanisms of prey size selectivity

A mathematical model of the attack success of planktonic predators(fish larvae and carnivorous copepods) is proposed. Based ona geometric representation of attack events, the model considershow the escape reaction characteristics (speed and direction)of copepod prey affect their probability of being captured.By combining the attack success model with previously publishedhydrodynamic models of predator and prey perception, we examinehow predator foraging behaviour and prey perceptive abilityaffect the size spectra of encountered and captured copepodprey. We examine food size spectra of (i) a rheotactic cruisingpredator, (ii) a suspension-feeding hovering copepod and (iii)a larval fish. For rheotactic predators such as carnivorouscopepods, a central assumption of the model is that attack istriggered by prey escape reaction, which in turn depends onthe deformation rate of the fluid created by the predator. Themodel demonstrates that within a species of copepod prey, theability of larger stages to react at a greater distance fromthe predator results in increased strike distance and, hence,lower capture probability. For hovering copepods, the vorticityfield associated with the feeding current also acts in modifyingthe prey escape direction. The model demonstrates that the reorientationof the prey escape path towards the centre of the feeding current'sflow field results in increased attack success of the predator.Finally, the model examines how variability in the kineticsof approach affects the strike distance of larval fish. In caseswhere observational data are available, model predictions closelyfit observations.     

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.     

Piscivory of 0+ pike (Esox lucius L.) in a small eutrophic lake and its implication for biomanipulation

Stocking 0+ pike in shallow eutrophic lakes as an indirect way to facilitate clear water by increased phytoplankton grazing by zooplankton, has been used in several studies with varying success. Lack of piscivory could be one reason for poor impact of the pike stockings, on reducing the 0+ cyprinid fish population. In relation to this, we present data on food and feeding habits of 936 0+ pike (Esox lucius L.) (2–18 cm) sampled during five years in a small eutrophic lake undergoing biomanipulation. Overall, the food of 0+ pike followed a sequence of diet shifts including microcrustacea, insects, macrocrustacea and finally vertebrates (fish) with increase in pike length. Despite a high abundance of potential prey fish in the lake, in general less than 50% of the pike were piscivorous until a length of 10.1–13.7 cm. As piscivory increased, the principal prey fish (0+ roach) apparently displayed antipredatorial behaviour which decreased their availability for 0+ pike. We conclude that lack of piscivory caused by e.g. anti predator tactics by prey fish, may reduce the impact of 0+ pike stockings in lake restoration projects.

     

Effects of turbidity and risk of predation on habitat selection decisions by Fathead Minnow (<Emphasis Type="Italic">Pimephales promelas</Emphasis>)

Within aquatic ecosystems, turbid environments will have a significant impact upon predator-prey interactions if both the predator and their prey rely upon vision as their primary sense. Increasing water turbidity will reduce the probability of being detected by a predator, and once detected should provide prey with cover that is close and ubiquitous. We tested the extent that these features of a turbid environment will have in affecting the impact of predation risk on habitat quality using Fathead Minnow (Pimephales promelas) as the prey, and Yellow Perch (Perca flavescens) and Black Bullhead (Ameiurus melas) as visual and non-visual predators, respectively. Our experiments demonstrated a strong preference for turbid habitats in the absence of a predator. When a predator was present in a turbid habitat, the minnows reduced their use of this location but still preferred it to a clear habitat with no predator. These data suggest turbidity confers a benefit to feeding Fathead Minnow that more than compensates for the cost of predation risk.     

Growth of pike larvae: effects of prey, turbidity and food quality

We studied experimentally the effects of turbidity and prey composition on pike larval growth and hypothesized that pike larval growth varies with turbidity and food quality. We reared the first-feeding pike larvae (Esox lucius) in laboratory tanks with (1) clear or (2) turbid water provided with zooplankton rations from (3) an inner and (4) an outer archipelago site. The sites differ in physical features, salinity, eutrophication status, zooplankton community structure and density. Pike larvae showed the highest weight increase in clear water with zooplankton from the outer site and the poorest weight increase in turbid water with zooplankton as prey from the inner site. Our fatty acid analysis revealed that unsaturated fatty acid levels were highest in the outer site. The relative percentage of copepods was also higher in the outer site. This study supports the hypothesis that turbidity weakens the ability of pike larvae to capture certain prey. Further, zooplankton community composition matters in turbid water, but is not a primary factor in clear water.     

Foraging behaviour of planktivorous fish in artificial vegetation: the effects on swimming and feeding

In the littoral zones of lakes, aquatic macrophytes produce considerable structural variation that can provide protection to prey communities by hindering predator foraging activity. The swimming and feeding behaviour of a planktivore, Pseudorasbora parva(Cyprinidae) on its prey (Daphnia pulex) was studied in a series of laboratory experiments with varying densities (0, 350, 700, 1400, 2100 and 2800 stems m^–2) of simulated submerged vegetation. Prey availability was varied from 0.5, 1.0, 2.0, 5.0, 10.0 and 25.0 prey l^–1. As the stem density increased, the predator's swimming speed and the number of prey captured decreased relative to feeding in open water. A good relation existed between the number of successful prey captures and swimming speed with the average stem distance to fish body length ratio (D). An abrupt reduction in feeding and swimming was recorded when D was reduced to values less than one.     

Sensory cues of a top‐predator indirectly control a reef fish mesopredator

Behavioural trophic cascades highlight the importance of indirect/risk effects in the maintenance of healthy trophic‐level links in complex ecosystems. However, there is limited understanding on how the loss of indirect top–down control can cascade through the food‐web to modify lower level predator–prey interactions. Using a reef fish food‐web, our study examines behavioural interactions among predators to assess how fear elicited by top‐predator cues (visual and chemical stimuli) can alter mesopredator behaviour and modify their interaction with resource prey. Under experimental conditions, the presence of any cue (visual, chemical, or both) from the top‐predator (coral trout Plectropomus leopardus) strongly restricted the distance swum, area explored and foraging activity of the mesopredator (dottyback Pseudochromis fuscus), while indirectly triggering a behavioural release of the resource prey (recruits of the damselfish Pomacentrus chrysurus). Interestingly, the presence of a large non‐predator species (thicklip wrasse Hemigymnus melapterus) also mediated the impact of the mesopredator on prey, as it provoked mesopredators to engage in an ‘inspection’ behaviour, while significantly reducing their feeding activity. Our study describes for the first time a three‐level behavioural cascade of coral reef fish and stresses the importance of indirect interactions in marine food‐webs.     

Behavioural syndrome in a solitary predator is independent of body size and growth rate

Models explaining behavioural syndromes often focus on state-dependency, linking behavioural variation to individual differences in other phenotypic features. Empirical studies are, however, rare. Here, we tested for a size and growth-dependent stable behavioural syndrome in the juvenile-stages of a solitary apex predator (pike, Esox lucius), shown as repeatable foraging behaviour across risk. Pike swimming activity, latency to prey attack, number of successful and unsuccessful prey attacks was measured during the presence/absence of visual contact with a competitor or predator. Foraging behaviour across risks was considered an appropriate indicator of boldness in this solitary predator where a trade-off between foraging behaviour and threat avoidance has been reported. Support was found for a behavioural syndrome, where the rank order differences in the foraging behaviour between individuals were maintained across time and risk situation. However, individual behaviour was independent of body size and growth in conditions of high food availability, showing no evidence to support the state-dependent personality hypothesis. The importance of a combination of spatial and temporal environmental variation for generating growth differences is highlighted.     

State-dependent invasion windows for prey in size-structured predator-prey systems: whole lake experiments

1. In size-structured communities where individuals grow in size over their life cycle, interactions between species will shift between competitive and predatory interactions depending on size relationships. The outcome of interactions will subsequently depend on the strength of competitive and predatory interactions, respectively. 2. In a whole lake experiment including four experimental lakes, it was tested under which conditions the competing prey, roach Rutilus rutilus, could successfully recruit into systems previously occupied by the predator, perch Perca fluviatilis. Two replicated introduction experiments were carried out 3 years apart. 3. Roach were able to successfully recruit into three of the four experimental lakes of which two were also inhabited by the top predator pike Esox lucius. Resource levels were unrelated to whether roach could successfully recruit into the systems as recruiting roach in all years were feeding close to their maximum rate. 4. High population fecundity of roach and low predation pressure by perch combined were necessary ingredients for successful recruitment and the presence of only one of these conditions did not result in successful recruitment. 5. It is hypothesized that, although roach were able to successfully recruit into one lake with only perch present in addition to the two lakes that also inhabited pike, long-term coexistence of roach and perch depends on the presence of another top predator (e.g. pike) selectively preying on perch. This hypothesis was supported by data on co-occurrence of perch and roach in different lakes. 6. Overall, the results are in accordance with expectation of size-structured life-history omnivory theory suggesting that coexistence between top predator and intermediate consumer is fragile.     

The Influence of Structural Complexity on Fish–zooplankton Interactions: A Study Using Artificial Submerged Macrophytes

Aquatic macrophytes produce considerable structural variation within the littoral zone and as a result the vegetation provides refuge to prey communities by hindering predator foraging activities. The behavior of planktivorous fish Pseudorasbora parva (Cyprinidae) and their zooplankton prey Daphnia pulex were quantified in a series of laboratory experiments with artificial vegetation at densities of 0, 350, 700, 1400, 2100 and 2800 stemsm^–2. Swimming speeds and foraging rates of the fish were recorded at different prey densities for all stem densities. The foraging efficiency of P. parva decreased significantly with increasing habitat complexity. This decline in feeding efficiency was related to two factors: submerged vegetation impeded swimming behavior and obstructed sight while foraging. This study separated the effects of swimming speed variation and of visual impairment, both due to stems, that led to reduced prey–predator encounters and examined how the reduction of the visual field volume may be predicted using a random encounter model.