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Predators exert strong regulating forces on lower trophic levels through predation. As most fish are visual foragers, visual conditions in the water may alter the strength of this regulation. We evaluated effects of turbidity and humic water on foraging efficiency and prey-size selectivity in Northern pike (Esox lucius) feeding on roach (Rutilus rutilus). Encounter rates decreased in both turbid and humic water but were not counteracted by increased searching activity. Capture success was unaffected by turbidity but was nonlinearly affected by humic water by being high in clear and highly humic water but low in less humic water. In highly humic water, the visual range approached pike's strike distance and, together with its cryptic colours, pike may have initiated its attack before the prey detected it, limiting the possibility for prey evasive manoeuvres. Prey-size selectivity towards small prey in clear water disappeared in turbid water but was maintained in humic water. Owing to its optical properties, turbidity degrades the quality of the visual information more through scattering than humic water does through absorption. We show that the effect of visual degradation on foraging depends on the cause of visual degradation, which has not previously been acknowledged in the visual foraging literature. Par la predation, les predateurs exercent de grandes forces regulatrices sur les niveaux trophiques inferieurs. Comme la plupart des poissons se servent de leur vision pour s'alimenter, les conditions de l'eau qui ont une incidence sur la vision peuvent moduler l'intensite de cette regulation. Nous avons evalue les effets de la turbidite et de l'eau humique sur l'eficacite d'alimentation et la selectivite de la taille des proies chez des grands brochets (Esox lucius) se nourrissant de gardons (Rutilus rutilus). La frequence des rencontres diminuait tant dans l'eau turbide que dans l'eau humique, mais cette diminution n'etait pas compensee par une activite de quete accrue. Si le succes de capture n'etait pas influence par la turbidite, il l'etait de maniere non lineaire par l'eau humique; ainsi ce succes etait eleve quand l'eau etait limpide ou tres humique, mais faible dans l'eau moins humique. Dans l'eau tres humique, la portee visuelle s'approchait de la distance de frappe des brochets et, en profitant egalement de leur coloration cryptique, ces derniers pouvaient lancer leur attaque avant que la proie ne detecte leur presence, limitant ainsi l'utilisation possible de manoeuvres d'evasion. La selectivite de la taille des proies qui penche vers les petites proies en eau limpide n'etait plus observee en eau turbide, mais demeurait dans l'eau humique. En raison de ses proprietes optiques, la turbidite cause une plus grande degradation de l'information visuelle par diffusion que l'eau humique ne le fait par absorption. Nous demontrons que l'effet de la degradation visuelle sur l'alimentation depend de la cause de cette degradation, un lien qui n'avait pas ete reconnu dans les etudes anterieures sur l'alimentation visuelle. [Traduit par la Redaction], Introduction Predators are known to exert a strong regulating force on lower trophic levels by reducing prey densities (Carpenter et al. 1985; Northcote 1988). However, the strength of this regulation [...]

Partial migration is a common phenomenon among many animals and occurs in many types of ecosystems. Understanding the mechanisms behind partial migration is of major importance for the understanding of population dynamics and, eventually, ecosystem processes. We studied the effects of food availability on the seasonal partial migration of cyprinid fish from a lake to connected streams during winter by the use of passive telemetry. Fish with increased access to food were found to migrate in higher proportion, earlier in the season, and to reside in the streams for a longer period compared to fish with decreased access to food. Furthermore, fewer unfed migrants returned to the lake, indicating higher overwinter mortality. Our results suggest that individual fish trade off safety from predation and access to food differently depending on their body condition, which results in a condition-dependent partial migration. Hence, our main conclusion is that individual decision-making is based on assessment of own condition which offers a mechanistic explanation to partial migration. Moreover, this may be of high importance for understanding population responses to environmental variation as well as ecosystem dynamics and stability. Key words: cyprinid fish; food availability; individual behavior; migration; partial migration; predation; roach; Rutilus rutilus.

To purchase or authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.2007.0030-1299.16118.x Byline: Jakob Brodersen, Emma Adahl, Christer Bronmark, Lars-Anders Hansson Abstract: Migration is a widespread phenomenon in many ecosystems. Most often, studies on migration have focused on how migration strategies are dependent on ecological parameters, but little attention has been paid to the top-down effect of migration on ecosystem processes. Cyprinid fish in many European lakes undergo partial migration, where a part of the population leaves the lake and enters streams for the winter. In this study, we model the effect of partial migration by fish on lower trophic levels in a lake ecosystem. Our results suggest that spring phyto- and zooplankton dynamics, including occurrences of clear-water phases, can be related to the timing and magnitude of partial migration of planktivorous fish. From our results we conclude that partial migration can influence the dynamics of lower trophic levels in the ecosystem. Furthermore, we hypothesize that partial migration may affect the stability of alternative stable states and transitions between them. Article History: Manuscript Accepted 10 September 2007 Article note: J. Brodersen (jakob.brodersen@limnol.lu.se), C. Bronmark and L.-A. Hansson, Dept of Ecology/Limnology, Ecology Building, Lund Univ., SE-223 62 Lund, Sweden. - E. Adahl, Dept of Ecology/Theoretical Ecology, Ecology Building, Lund Univ., SE-223 62 Lund, Sweden.

We aimed to assess whether prey organisms with limited large-scale dispersal abilities are locally adapted to prevailing predator regimes by studying how chemical cues from predatory fish affected the behavior of Gammarus pulex (Amphipoda) from ponds with and without fish. We also examined, in the laboratory, the F1 generation from each pond by incubating them with or without cues from predatory fish. The potential benefits of a behavioral avoidance response were also assessed in an experiment in which G. pulex from the different ponds and incubations were exposed to fish predation. G. pulex from fish ponds increased their refuge use when exposed to fish cues, whereas populations from fishless ponds reduced their refuge use. The F1 generation responded similarly to their parents. Only the F1 generation from fish pond populations responded with more pronounced antipredatory behavior when raised in fish water. Moreover, both the original and the F1 generation of fish pond G. pulex survived longer when exposed to fish predation than those from fishless ponds, independent of whether they were raised in fish water or not (F1). Our results suggest that the behavioral response to predator cues in G. pulex is an inherited trait, i.e., a local adaptation to prevailing predator regimes. Key words: antipredator behavior; fish; Gammarus pulex; inherited trait; local adaptation; predation.

We investigated the effects of an exotic snail, the golden apple snail (Pomacea canaliculata) on biodiversity and ecosystem functioning in tropical wetland ecosystems. This large snail (up to 80-mm shell height) has invaded large parts of Southeast Asia during recent decades. A survey of natural wetlands in Thailand showed that high densities of the snail were associated with almost complete absence of aquatic plants, high nutrient concentrations, and high phytoplankton biomass, that is, a complete shift in both ecosystem state and function. A field experiment demonstrated that grazing by the snail can cause the loss of aquatic plants, a change toward dominance of planktonic algae, and thereby a shift toward turbid water. Estimates of biologically fixed nutrients released through snails grazing on aquatic plants revealed that phosphorus releases were sufficient to explain the recorded increase in phytoplankton biomass. Hence, our study demonstrates how an herbivore may trigger a shift from clear water and macrophyte dominance to a turbid state dominated by planktonic algae. This shift and the continuing aggressive invasion of this exotic species are detrimental to the integrity and functioning of wetland ecosystems, and to the services these provide in Southeast Asia. Key words: ecosystem functioning; exotic species; freshwater; golden apple snail; herbivory; invasive species; limnology; macrophyte; Pomacea; species richness.

Species undergoing ontogenetic diet shifts face a risk of resource competition that delays transitions between feeding stages. Such ontogenetic bottlenecks are common in piscivorous fish because competition with future prey may retard growth and prevent a size advantage. In pikeperch (Stizostedion lucioperca), year class strength of 0+ cohorts is highly variable and positively related to early onset of piscivory. To identify the causes of this pattern, we experimentally quantified size-dependent planktivorous and piscivorous foraging capacity and incorporated the data into a growth model. For any given prey type and size, foraging capacity described a hump-shaped relationship with predator size. Foraging capacity on daphnids peaked at a pikeperch length of 66 mm, suggesting a narrow scope of planktivory. The highest capacity in the piscivorous niche was reached at a predator-to-prey length ratio of 5, where the ratio was an integrated measure of predator size over several prey sizes. With the growth model, we derived size distributions of 0+ cohorts as functions of resource levels. Simulations revealed two major determinants for the year class strength of pikeperch. First, discontinuous availability of prey sizes counteracted switching to piscivory within the first growing season. This was accentuated by prey fish growth, which caused the planktivory and piscivory niches to separate over time and limited the time window when diet shift was possible. Second, the hump-shaped relationship between size and foraging capacity resulted in growth reduction when growing out of the planktivorous niche. Switching to piscivory in our model occurred along a perpendicular relationship between fish prey and zooplankton density. Zooplankton density determined whether pikeperch reached a size advantage over prey, and fish prey density affected whether the foraging return of piscivory was higher than planktivory. Individuals not reaching a size advantage over prey and failing to become piscivorous were stunted at a size when consumption balanced metabolic requirements. Piscivorous individuals, however, continued to grow fast throughout the season by feeding on the wave of the prey cohort. Our results highlight the importance for predators that shift diet to be synchronized with fluctuations in resource availability, such as the pulses of new cohorts of prey fish. Key words: growth model; ontogenetic diet shift; pikeperch; piscivory; planktivory; size-based model; size-structured population; Stizostedion lucioperca.

Laboratory estimates of bream foraging in benthivorous and planktivorous feeding niches were corroborated with observations in the wild. Size -specific competitive ability was determined, with large bream found to be more competitive benthivores, and small bream found to be more competitive planktivores.

Theoretical models for the evolution of inducible anti-predator defenses predict that there should be fitness costs associated with the defense, otherwise it should be permanent. Thus, in the absence of predators and with limited resources, defended individuals are expected to be outcompeted by individuals lacking the costly defense. Crucian carp, Carassius carassius, increase in body depth in response to chemical stimuli from piscivorous fish, and the deeper body constitutes a morphological defense against gape-limited piscivores. Here, we have performed a field experiment investigating effects of population density on the relative competitive performance of shallow-bodied and deep-bodied crucian carp. We assigned groups of six individually marked fish (the 'focal' individuals) of either morph into enclosures with either low or high crucian carp density, and monitored changes in body mass, length, and body depth of these fish for 4 mo. In addition, we determined sex, and final liver and gonadal mass, of all focal fish. At high population density, there was a reduction in food resources (zooplankters), which resulted in reduced gonad mass, reduced gonadosomatic indices, and reduced growth in fish of both morphs. Unexpectedly, the relative allocation to livers was higher at high density. There was no difference in body mass gain between morphs at the low density, whereas at the high density shallow-bodied fish gained twice as much body mass as deep-bodied fish. Hence, when predators were absent and food resources were limited, deep-bodied individuals suffered a substantial cost when competing with shallow-bodied conspecifics. This study verifies the theoretical prediction that an inducible defense results in fitness costs in the absence of predators. Further, as we found no measurable disadvantage of the deep-bodied morphology at high food availability but a strong disadvantage under intense competition for food, our results also demonstrate the importance of relating defense costs to levels of intraspecific competition. Key words: antipredator; body size; Carassius carassius; competition; cost; density dependence; field-enclosure experiment; fish; inducible defense; phenotypic plasticity; resource allocation; Sweden., INTRODUCTION Predation is an important selective force in most habitats. Through direct, lethal effects, predators may have profound effects on abundance and size structure of prey populations and on species [...]

The purpose of this study was to determine the importance of cascading trophic interactions in a freshwater, benthic food chain and if the strength of the indirect interactions depends on the degree of specialization in the predator. The effects of predation by two benthivorous fishes, tench (Tinca tinca) and Eurasian perch (Perca fluviatilis), on benthic macroinvertebrates, epiphytic algae, and submerged macrophytes were studied in a field experiment, using cages (2 m X 3 m X 0.8 m) placed in a eutrophic pond in southern Sweden. Cages were assigned to four different treatments: fishless controls, tench, perch, and tench + perch. Core samples were taken to assess the density of benthic macroinvertebrates and submerged macrophytes, and artificial substrates were used to study the dynamics of periphyton. Nonmolluscan benthic macroinvertebrates were not greatly affected by the presence of fish, whereas predation by tench dramatically reduced the biomass of snails and bivalves. Tench had an indirect, positive effect on the biomass of periphyton through a reduction of grazing pressure by snails, whereas perch had no demonstrable direct effect on molluscs or indirect effect on periphyton. Further, in the cages with low snail and high periphytic biomass (tench and tench + perch cages), growth of the dominant submerged macrophyte (Elodea canadensis) was reduced, probably due to shading by periphyton. This experiment confirms that a predator can have profound effects on interactions in benthic food chains and that the strength of the indirect interactions is dependent on the strength of the direct interactions. The strong direct effect of tench on snails indirectly affected periphyton and submerged macrophytes, whereas perch had no direct effect on benthic macroinvertebrates and, consequently, no indirect effect on periphyton and submerged macrophytes.

Theories of cascading trophic interactions provide specific predictions regarding the forces that regulate populations across trophic levels. Reducing predators in a food chain with three trophic levels should permit herbivores to increase, thus reducing primary producers. In a manipulative experiment involving a molluscivorous fish (pumpkinseed sunfish, Lepomis gibbosus), freshwater snails, and periphytic algae, we tested this prediction. With 10 cages (3 x 3 x 2 m) in each of two lakes in northern Wisconsin, we generated three treatments: exclosures (fishless), enclosures (three pumpkinseed sunfish), and cageless controls, both in Mann Lake, which had high natural densities of pumpkinseed sunfish (HDP), and in Round Lake, which had low densities (LDP). During a 16-mo experiment, we quantified snail and periphyton biomass on plastic flagging within treatments during summer at 3- and 6-wk intervals in 1986 and 1987, respectively. Predation by pumpkinseed sunfish reduced snail biomass on flagging, permitting periphyton biomass to increase, as compared to exclosures. As expected, periphyton biomass in cageless controls in Mann Lake (HDP) mirrored periphyton biomass in enclosures, whereas cageless controls in Round Lake (LDP) differed from exclosures. The periphyton assemblage changed dramatically with increasing grazing pressure. In enclosures (low grazing), filamentous algae and large, stalked diatoms dominated the periphyton assemblage, whereas in exclosures (high grazing), assemblages were dominated by small, adnate diatoms and a colonial, filamentous bluegreen alga (Gloeotrichia). In laboratory trials, snails preferential fed on periphyton from enclosures (where grazing pressure had been low). Predation reduced snail density, but indirectly increased periphyton biomass, dramatically modifying species composition of the assemblage. Thus, in mesotrophic Wisconsin takes, top-down interactions regulate the benthic, freshwater food chain., Field experiments in two Wisconsin lakes involving the tri-trophic level interactions of the molluscivorous pumpkinseed fish (Lepomis gibbosus), freshwater snails and periphytic algae were performed to test the prediction of cascading trophic interactions theories. The results showed that fish predation reduced snail density. In turn, the reduced grazer density caused an increase in periphytic algae density. Therefore, top-down interactions govern benthic, freshwater food chains in mesotrophic Wisconsin lakes.

In a field experiment where the presence or absence of piscivorous pike (Esox lucius) in ponds was manipulated, the morphology of crucian carp (Carassius carassius) diverged, such that individuals became deeper bodied in pond sections with pike. A laboratory experiment confirmed that the presence of this predator induced a change in body morphology in the carp. Estimation of prey vulnerability to predation by pike, a gape-limited predator, revealed that this increase in body depth resulted in crucian carp reaching a size that provided refuge from predation. However, this change in morphology incurs a cost through an increase in drag when the carp are swimming. Because crucian carp are limited by resources in the absence of piscivores and by the substantial cost of the defensive morph in their presence, phenotypic plasticity should be the optimal strategy for this species., Various morphological structures in prey organisms function as efficient adaptations against predation [1], and these morphological defenses could be either constitutive or environmentally induced. The evolution and maintenance of inducible [...]

In a replicated field experiment we studied the effects of natural densities of two exotic consumers, the predatory and herbivorous signal crayfish (Pacifastacus leniusculus) and the predatory rainbow trout (Oncorhynchus mykiss), on multiple trophic levels of a pond community. The goals were to: (1) determine the individual and combined effects of predators on macroinvertebrates, macrophytes, and periphytic algae; (2) evaluate the strength of direct and indirect interactions in a food web influenced by omnivores; and (3) evaluate the relative importance of direct and indirect predator effects on mortality and growth of a native frog species, Rana temporaria. The experiment showed that both signal crayfish and rainbow trout had strong effects on multitrophic levels of a littoral pond community, through direct consumption and indirect effects on lower trophic levels. Crayfish had weak but significant negative effects on the biomass of predatory invertebrates and greatly reduced the biomass of snails, the most abundant invertebrate grazers. Although the number of active herbivorous tadpoles tended to be higher in crayfish cages relative to control cages, the proportion of surviving froglets was lower in crayfish cages than in control cages, possibly due to crayfish predation on injured tadpoles. The size of surviving froglets did not differ from controls, but tadpoles in crayfish cages often suffered tail injuries. Macrophyte coverage decreased as a result of crayfish consumption and nonconsumptive fragmentation. However, the biomass of periphyton increased in crayfish cages relative to controls, probably due to reduced grazing from snails. In contrast, trout had strong negative effects on the biomass of both predatory invertebrates and insect grazers, whereas trout had less effect on snail biomass than did crayfish. Also, in contrast to crayfish cages, the number of active tadpoles in trout cages was lower than in controls, probably due to a combination of trout predation and trout-induced reduced tadpole activity. Trout had a strong negative impact on froglet survival, and froglets in trout cages metamorphosed at a smaller size and had reduced growth rates compared to froglets in crayfish and control cages. As with crayfish, the biomass of periphyton increased in trout cages relative to controls, which may be due to a combination of both density and trait-mediated trout effects on tadpole grazing. In treatments with multiple predators the effects of crayfish and trout on caged communities were independent, and there were few interactions. Mostly effects of combined predators reflected those in single predator cages. Our results demonstrate that noninteracting, introduced multiple predators can have strong direct and indirect effects on multiple trophic levels in pond communities. Trophic cascades may develop in aquatic food webs even with omnivores such as crayfish, and in complex habitats with trout. These strong indirect effects are mediated through both predation on important grazers (i.e., the crayfish--snail--periphyton link) and a combination of density and behavioral responses of grazers to predators (i.e., the trout-tadpole-periphyton link). When two noninteracting predators have strong but different effects on prey survival or activity, their combined effects on intermediate trophic levels reflect responses to the more dangerous predator. Key words: exotic species; freshwater pond; grazers; growth rate; littoral pond; omnivory; Oncorhynchus mykiss; Pacifastacus leniusculus; predators, multiple; Rana temporaria; Sweden; trophic cascade.

Presents a study which examined the effects of enrichment in three aquatic food web configurations subjected to either high or low nutrient additions. Materials and methods; Results; Discussion of the results.

Focuses on a prey's use of chemical communication to evade predators in aquatic systems. Lack of olfactory and auditory interactions between predator/prey in aquatic systems, leading to empirical studies to focus only on direct confrontations; Adaptations that allow prey to evade predators in aquatic systems, including camouflage, morphological adaptations that make it harder to capture, and behavioral alterations; Possibility that the detection of chemicals emitted by predators is used by prey as a warning.

The northern pike (Esox lucius) is an important and selective piscivore that chooses smaller prey than predicted from energy / time budgets. In a laboratory experiment, we investigated pike predatory behavior to explain this selectivity. Northern pike feeding on different prey sizes in aquaria were observed when foraging alone, when in the presence of chemical cues from similar-sized or larger conspecifics, and when in the presence of conspecifics that were allowed to interact with the focal pike. The results show that prey handling time increases with prey size and that the duration of manipulating and handling prey inflicts a risk of exposure to cannibals and kleptoparasites on the pike. Therefore, the risk of falling victim to cannibals or kleptoparasites increases with prey size. Attracting and experiencing intraspecific interactors can be regarded as major fitness costs. Chemical cues from foraging conspecifics have only minor effects on pike foraging behavior. Furthermore, the ability to strike and swallow prey head first improves pike predatory performance because failing to do so increases handling time. Our findings emphasize the increasing potential costs with large prey and explain previous contradictory suggestions on the underlying mechanisms of behavior, selectivity, and trophic effects of northern pike predation. [ABSTRACT FROM AUTHOR]