Minnow predation on vendace larvae: intersection of alternative prey phenologies and size-based vulnerability

Time-intensive sampling was used to study minnow Phoxinus phoxinus density and foraging activity in the littoral area of Lake Lentua at a time of high vendace Coregonus albula larval abundance. Minnow activity and foraging during the late spring-early summer period at low temperatures was found to be mainly nocturnal and quite consistent with features reported in the literature, with the exception of feeding on vendace larvae. The absence of the latter finding from previous studies may be due to previous seasonally limited daytime samplings. The temporal or spatial scale of sampling is decisive when studying foraging on food animals with a brief period of vulnerability and seasonal phenology. However, minnows in Lake Lentua seemed to prefer Bosmina longispina whenever available and low B. longispina density during the first weeks after the break-up of ice directed minnow predation towards the vendace larvae. Estimated gross predation values showed that the minnow has the potential to become a remarkable predator on fish larvae. Slight variations in the vernal timing of the vendace hatching and larval development with respect to minnow activity, both of which are obviously temperature related, may be critical to vendace larval survival in the nearshore zone. However, the predation on the larvae is probably a strong factor only for a short period and the survival of the vendace larvae may be more related to the availability of suitably sized food resources than to predation by the minnow.     

Predation of perch on vendace larvae: diet composition in an oligotrophic lake and digestion time of the larvae

The diet of perch Perca fluviatilis was studied to reveal possible predation on vendace Coregonus albula larvae in an oligotrophic lake. Perch diet changed with the size of the fish: small perch ate mainly zooplankton and the diet shifted more to benthic invertebrates and fishes in larger perch. There were also annual and spatial differences in the diet, probably reflecting differences in the availability of prey animals. Perch predation on vendace larvae was only observed in the area with high availability of the larvae. The result suggested strengthened predation when the density of the larvae increases. According to bioenergetics modelling, the perch population increased natural mortality of vendace larvae only marginally. Food intake of spawning female perch was slightly reduced, whereas spawning males fed similarly to non-spawning males. Hence, the spawning period of perch was only a minor refuge for vendace larvae. Laboratory experiments of perch digestion rate demonstrated that, due to rapid digestion of the small fish larvae, diet sampling interval should not be >2 h in the field.     

Morphology dependent foraging efficiency in perch: a trade‐off for ecological specialization?

Trade-offs in foraging efficiency leading to divergent natural selection between and within populations exploiting different resources are thought to be a primary cause of trophic polymorphism. In this study we focused on the trade-offs in foraging efficiency and growth in a polymorphic perch population. Specifically, we related habitat-specific growth and diet of perch to perch morphology. In a subsequent laboratory study we experimentally tested the trade-off by testing the efficiency of perch with different morphology feeding on pelagic ( Daphnia sp., Chaoborus sp.) and littoral (mayfly larvae) food resources. The feeding performance was tested in different physical environments to see if we could predict growth patterns in the field based on foraging rate and behavior of perch.
In the field study, we found that the perch from the littoral and the pelagic zones differed in both morphology and diet. Within the littoral zone the deeper-bodied individuals grew faster compared to the more streamlined individuals, whereas the opposite pattern was found in the pelagic zone. In the aquarium experiments, perch from the littoral zone had higher capture rates on the pelagic prey types in vegetation trials and on mayfly larvae in both open water and vegetation trials. The pelagic perch had higher capture rates on the pelagic prey types in open water trials. The littoral perch had lower search velocity than the pelagic perch in open water trials whereas the opposite pattern was found in vegetation trials. The attack velocity of the pelagic perch was also higher than that of the littoral perch independent of vegetation structure. Our results suggest that there is a functional trade-off between performance in alternate habitats and general body form in perch. Such trade-offs may promote divergent natural selection and could be the mechanism that give rise to and upholds the pattern in the field.     

Foraging in yellow perch,Perca flavescens: biological and physical factors affecting diel periodicity in feeding,consumption, and movement

Synopsis We examined the diel chronology in food consumption and dietary composition, and in the local distribution of yellow perch,Perca flavescens, at 3 h intervals over two 24 h periods during the summer at Baptiste Lake, Alberta. Feeding intensity, as indicated by changes in stomach fullness, increased throughout the day, peaked in late evening, and almost ceased after sunset. Changes in perch densities at the sampling site reflected the pattern of feeding intensity, indicating that movements into the littoral zone are correlated with foraging. Perch diet composition, analyzed as percent occurrence, relative numerical abundance, and percent contribution by weight, differed significantly between times of day, particularly between day and night, and between sampling periods. These differences could be related both to prey behavior and distribution, and to changes in perch foraging behavior in response to light intensity. In terms of biomass, forage fish, amphipods, chironomids, and trichoptera were the most important food items on both sampling dates.     

Indirect trophic interactions with an invasive species affect phenotypic divergence in a top consumer

While phenotypic responses to direct species interactions are well studied, we know little about the consequences of indirect interactions for phenotypic divergence. In this study we used lakes with and without the zebra mussel to investigate effects of indirect trophic interactions on phenotypic divergence between littoral and pelagic perch. We found a greater phenotypic divergence between littoral and pelagic individuals in lakes with zebra mussels and propose a mussel-mediated increase in pelagic and benthic resource availability as a major factor underlying this divergence. Lakes with zebra mussels contained higher densities of large plankton taxa and large invertebrates. We suggest that this augmented resource availability improved perch foraging opportunities in both the littoral and pelagic zones. Perch in both habitats could hence express a more specialized foraging morphology, leading to an increased divergence of perch forms in lakes with zebra mussels. As perch do not prey on mussels directly, we conclude that the increased divergence results from indirect interactions with the mussels. Our results hence suggest that species at lower food web levels can indirectly affect phenotypic divergence in species at the top of the food chain.     

Diet changes in prey size selectivity in larvae of perch <Emphasis Type="Italic">Perca fluviatilis</Emphasis>

A hypothesis on change in prey size selectivity in relation to illumination level was tested on the basis of data on weight and size composition of the content of the digestive tract of larvae of perch Perca fluviatilis and on zooplankton in the layer 0–6 m (Wallersee Lake, Austria). Larvae foraging in the twilight-night period had almost two times more food in the intestine than those foraging in the daytime. The size composition of perch larvae and concentration and size composition of zooplankton hardly differed in the daytime and twilight-night samples. For the first time, it is shown on field material that more intensive feeding of larvae at twilight is related to selection of significantly larger prey than in daytime feeding. In the day-time, the larvae consumed more prey but their maximum size did not exceed 0.6 of the diameter of mouth opening of the fish; at twilight it was over 0.8. In case of feeding on so large prey, not only the weight of the consumed feed increases but the time used for capture and swallowing also considerably increases. The larvae which in the period of investigation did not yet form schools, which perform the principal defensive function, were especially vulnerable for predators feeding on relatively large prey. The decrease of the part of small-sized prey at twilight is not related to their lesser availability due to a low illumination. It is assumed that feeding on energetically more valuable but less available prey is shifted to the period of low illumination when the larvae are less exposed to predation risk. The obtained results are discussed from positions of the triotroph concept (Manteifel’, 1961).     

Water Transparency Drives Intra-Population Divergence in Eurasian Perch (Perca fluviatilis)

Trait combinations that lead to a higher efficiency in resource utilization are important drivers of divergent natural selection and adaptive radiation. However, variation in environmental features might constrain foraging in complex ways and therefore impede the exploitation of critical resources. We tested the effect of water transparency on intra-population divergence in morphology of Eurasian perch (Perca fluviatilis) across seven lakes in central Sweden. Morphological divergence between near-shore littoral and open-water pelagic perch substantially increased with increasing water transparency. Reliance on littoral resources increased strongly with increasing water transparency in littoral populations, whereas littoral reliance was not affected by water transparency in pelagic populations. Despite the similar reliance on pelagic resources in pelagic populations along the water transparency gradient, the utilization of particular pelagic prey items differed with variation in water transparency in pelagic populations. Pelagic perch utilized cladocerans in lakes with high water transparency and copepods in lakes with low water transparency. We suggest that under impaired visual conditions low utilization of littoral resources by littoral perch and utilization of evasive copepods by pelagic perch may lead to changes in morphology. Our findings indicate that visual conditions can affect population divergence in predator populations through their effects on resource utilization.     

Influence of visual conditions on foraging and growth of juvenile fishes with dissimilar sensory physiology

The influence of turbidity on foraging and growth of young-of-the-year (YOY) of two percid species with relatively dissimilar sensory physiology, perch Perca fluviatilis and pikeperch Sander lucioperca was tested. A littoral mysid, Neomysis integer , was used as prey. Functional response was investigated indoors at two levels of turbidity, 3 NTU (clear) and 25 NTU (turbid). Growth and consumption rate were studied both during day and night in outdoor experiments at similar turbidity levels. Pikeperch were not affected by turbid conditions in either of the experiments. Foraging of perch, on the other hand, was significantly negatively affected by higher turbidity in the functional response experiments and during the night-time in the outdoor experiments. As opposed to pikeperch, consumption rates of perch decreased markedly during nights. Perch also grew more slowly in the treatments with turbid water. The dissimilar reactions of the two species indicate that sensory physiological adaptations and foraging behaviour are important factors that partly can explain disparate reactions of YOY fishes to the level of turbidity.     

Search strategies of tyrant flycatchers

Aspects of searching behaviour among free-living South American flycatchers (Aves: Tyrannidae) are compared quantitatively. Flycatchers forage with stationary searching periods, followed either by an attempted prey capture (sally) or a ‘give-up’ flight to a new perch. Search times are proportional to body size within each of three categories of foraging behaviour: aerial hawking, sally-gleaning, and perch-gleaning. Over the family as a whole, search times are directly proportional to the size of the visual field scanned during the search. Intraspecific variations in search times are caused by local variations in prey density or visual complexity of the habitat. Between foraging modes, differences in searching and movement patterns are related to prey dispersion characteristics. Aerial hawkers regularly return to favoured perches, but foliage gleaners, which reduce the resources surrounding a perch by sallying only once, rarely return to a perch. In contrast to aerial hawkers, foliage gleaners appear to follow an organized scanning procedure on each perch, by searching nearby surfaces before they examine more distant prey substrates. Throughout the family, the median flight distance after a perch is abandoned is approximately twice the median search radius. Comparisons of search time distributions preceding sallies with those preceding give-up flights suggest that there is no single, optimal give-up time in a given habitat. Foliage-gleaning species appear to assess the amount of search time each perch warrants, presumably based on the degree of complexity of the search area. They either sally at prey before that time, or give-up when the allotted time has elapsed.     

Is coarse woody debris in lakes a refuge or a trap for benthic invertebrates exposed to fish predation?

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Effects of prey size structure and turbulence on feeding and growth of anchovy larvae

Foraging processes in plankton and planktivorous fish are constrained by relative prey and predator size and therefore, these are important variables to include in a foraging model. The distribution of prey biomass across different size classes can be characterized by a size spectrum slope. We present a foraging model for anchovy larvae including the most relevant processes such as prey encounter, capture- and pursuit success, all influenced by light, turbulence and prey characteristics. We modelled ingestion rates and specific growth rate by coupling the foraging model with an existing bioenergetic model, and performed a sensitivity analysis of prey ingestion in turbulent environments assuming either hemispherical or conical perceptive volume. Our results suggest that turbulence has no positive effect because of the low capture ability, small prey size and small visual volume for anchovy larvae. The predicted ingestion is too low to sustain the growth potential of larvae when assuming conical perceptive volume even under prey densities substantially higher than normally found in the field. Ingestion rate is sensitive to the total biomass and the slope of the prey size spectra, specifically because it determines the abundance of prey around the optimal size for the larvae. The model also suggests that small larvae benefit from a prey size structure with steep prey size-spectra slope while a large larva benefit from less steep slopes. The model can act as a link between size-spectra measurements from the field and the foraging conditions of larval anchovies.     

Dispersion of vendace eggs and larvae around potential nursery areas reveals their reproductive strategy

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Seasonal variations of lipid fatty acids of boreal freshwater fish species

1. The fatty acid levels of muscle and liver lipids of perch, vendace and rainbow trout (Salmo gairdneri) cultivated in the same area (for comparison) were monitored. 2. The total lipid content in the muscle of perch (Perca fluviatilis) and vendace (Coregonus albula) was less than 50% of that in rainbow trout and a seasonal variation was clear only in vendace. 3. The relative amounts of polyunsaturated fatty acids as well as omega-3 acids were higher in vendace and perch than in cultivated rainbow trouts. Arachidonic acid content was much higher in vendace and perch than in rainbow trout. The content of monoenes was considerably higher in rainbow trout than in free freshwater fish. 4. The seasonal variations in the degree of unsaturation were small in fish muscle. 5. In the muscle of rainbow trout the relative amount of polyunsaturated fatty acids diminished with the increase of total lipids.     

Vulnerability of larval white perch,Morone americana,to fish predation

Synopsis The vulnerability of white perch, Morone americana, larvae to yearling bluegill, Lepomis macrochira, predators was examined in relation to larval size, nutritional condition and relative abundance of alternative prey. Short-term (15 min) predation trials were conducted in 381 tanks in the laboratory. Larval vulnerability was measured as the proportion of larvae killed and the proportion of successful attacks per predator in each 15 min trial. No significant differences in vulnerability were apparent among larvae regardless of feeding history at sizes < 6 mm SL. At larval lengths > 6 mm SL, size of larvae was the crucial determinant of their vulnerability. Percentage of larvae killed in 15 min was nearly 100% at sizes < 6 mm SL, decreased to 30% at a length of 12.0 mm SL and dropped to 18% at 14.0 mm SL. Larvae initially feeding at low food levels for 2–4 d exhibited decreased growth of 13–25% over the first 3 wks of life, and simulations based on laboratory results indicated that these growth deficits could result in 5- to 68-fold decreases in survivorship at 38 days after hatching (DAH). The relative abundance of alternative prey also had a pronounced effect on mortality of larvae. A 10-fold increase in alternative prey (Daphnia magna) abundance decreased bluegill predation rates on white perch larvae by 10–20%, while a 100-fold increase in Daphnia density decreased larval mortality by 75–90%.     

Effects of light and prey availability on nocturnal, lunar and seasonal activity of tropical nightjars

Nightjars and their allies represent the only major group of visually hunting aerial insectivores with a crepuscular and/or nocturnal lifestyle. Our purpose was to examine how both light regime and prey abundance in the tropics, where periods of twilight are extremely short, but nightjar diversity is high, affect activity across different temporal scales. We studied two nightjar species in West African bush savannah, standard‐winged nightjars Macrodipteryx longipennis Shaw and long‐tailed nightjars Caprimulgus climacurus Vieillot. We measured biomass of potential prey available using a vehicle mounted trap and found that it was highest at dusk and significantly lower at dawn and during the night. Based on direct observations, both nightjars exhibit the most intense foraging behaviour at dusk, less intense foraging at dawn and least at night, as predicted by both prey abundance and conditions for visual prey detection. Nocturnal foraging was positively correlated with lunar light levels and ceased below about 0.03 mW m^?2. Over the course of a lunar cycle, nocturnal light availability varied markedly, while prey abundance remained constant at dusk and at night was slightly higher at full moon. Both species increased twilight foraging activity during new moon periods, compensating for the shorter nocturnal foraging window at that time. Seasonally, the pattern of nocturnal light availability was similar throughout the year, while prey availability peaked shortly after onset of the wet season and then slowly decreased over the following four months. The courtship and breeding phenology of both species was timed to coincide with the peak in aerial insect abundance, suggesting that prey availability rather than direct abiotic factors act as constraints, at least at the seasonal level. Our findings illustrate the peculiar constraints on visually orienting aerial nocturnal insectivores in general and tropical nightjars in particular and highlight the resulting nocturnal, lunar and seasonal allocation of activities.     

Aggregative response in bats: prey abundance versus habitat

In habitats where prey is either rare or difficult to predict spatiotemporally, such as open habitats, predators must be adapted to react effectively to variations in prey abundance. Open-habitat foraging bats have a wing morphology adapted for covering long distances, possibly use information transfer to locate patches of high prey abundance, and would therefore be expected to show an aggregative response at these patches. Here, we examined the effects of prey abundance on foraging activities of open-habitat foragers in comparison to that of edge-habitat foragers and closed-habitat foragers. Bat activity was estimated by counting foraging calls recorded with bat call recorders (38,371 calls). Prey abundance was estimated concurrently at each site using light and pitfall traps. The habitat was characterized by terrestrial laser scanning. Prey abundance increased with vegetation density. As expected, recordings of open-habitat foragers clearly decreased with increasing vegetation density. The foraging activity of edge- and closed-habitat foragers was not significantly affected by the vegetation density, i.e., these guilds were able to forage from open habitats to habitats with dense vegetation. Only open-habitat foragers displayed a significant and proportional aggregative response to increasing prey abundance. Our results suggest that adaptations for effective and low-cost foraging constrains habitat use and excludes the guild of open-habitat foragers from foraging in habitats with high prey abundance, such as dense forest stands.     

Distribution of perch (Perca fluviatilis L.) during their first year of life in Lake Constance

The distribution and behaviour of larval and juvenile perch (Perca fluviatilis L.) were studied for two years in large, deep Lake Constance. After hatching larvae were transported by water currents to the open water. The majority of larvae remained in the pelagic zone for about one month. In both years, their return to the littoral zone coincided with the decline of pelagic zooplankton abundance. After returning to the littoral zone, juveniles stayed among submerged macrophytes within 5 m depth and lived apart from larger perch which lived at depths of 6–20 m. By late summer, juveniles changed their distribution pattern: during the day they stayed intensively close to piers and ports, but increased their swimming activity at dusk, cruising among shallow and deep waters and feeding on zooplankton, and rested on the bottom at night. This behaviour appears to be related to the decrease of inshore food resources and to the presence of predators in deeper water. 0⁺ perch left the littoral zone and moved into deep waters when autumnal mixing began in late October. They overwintered near the bottom at depths of more than 30 m. During most of the year, juvenile and adult perch were separated from each other. But as soon as they occupied the same habitat, the occurrence of cannibalism increased.     

The predation on Asellus aquaticus (L.) by perch,Perca fluviatilis (L.), in a small forest lake

Asellus aquaticus (L.) was the most important benthic food item for perch, Perca fluviatilis (L.), in a small, extremely humic forest lake in southern Finland. The proportion of A. aquaticus in the diet of perch varied according to the former's availability, which, in turn, depended on its life cycle. Perch 11.0–12.9 cm (total length) most frequently fed on A. aquaticus; smaller and larger perch fed more frequently on crustacean zooplankton and aquatic insect larvae, respectively. The high density of the perch population, the importance of A. aquaticus in the benthos of the study lake and the general high activity level of the prey resulted in a high predation (0.1–1.8% per day). The population of A. aquaticus was also limited by the scarcity of macrophytic vegetation and the small area of oxygenated littoral in the lake.     

Unpredictability of fish recruitment: interannual variation in young–of–the–year abundance

In 1989–1998, vendace larvae Coregonus albula were sampled in Finnish lakes following a stratified random sampling design. The abundance of young–of–the–year vendace after the first growing season was estimated using catch–per–unit–effort statistics. The number or total area of nursery places hardly limited the recruitment of vendace in the study lakes. The major proportion of prerecruit mortality of vendace occurred in the larval phase and larval sampling produced significant information on young–of–the–year survival. Although larval abundances and recruitment were clearly associated only in Lake Onkamo, generally high larval abundance was needed to produce high number of recruits.     

Factors affecting the early life history of yellow perch,Perca flavescens

Synopsis From 1979 to 1981 we followed the movement, diet, and growth of yellow perch,Perca flavescens, for their first 70 days after hatching in Lake Itasca, Minnesota. Perch spawned inshore during early spring; hatching occurred 10–20 days after spawning. Newly hatched perch were 5.6–6.2 mm total length (TL). Soon after hatching the larvae moved into the limnetic zone where they began feeding. This movement is probably a mechanism to escape intense predation in the littoral zone. Normally the first food of perch was immature copepods, but within a week they incorporated all common zooplankters into their diet. When the perch reached 25 mm TL (about day 40) they returned to the littoral zone, where they ate larger and more abundant prey than was present in the limnetic habitat. There is no correlation between growth rates and zooplankton abundances, which suggests that food quantity is not a limiting factor in the early life history of perch in Lake Itasca.