Optimal foraging as the criteria of prey selection by two centrarchid fishes

The nature of prey selection by two centrarchids (white crappie and bluegill) is presented as a model incorporating optimal foraging strategies. The visual field of the foraging fish as represented by the reactive distance is analysed in detail to estimate the number of prey encounters per search bout. The predicted reactive distances are compared with experimental data. The energetic cost associated with fish foraging behaviour is calculated based on the sequence of events that takes place for each prey consumed. Comparisons of the relative abundance of prey species and size categories in the stomach to the lake environment indicated that both white crappie and bluegill (length < 100 mm) strongly select prey utilising an energy optimization strategy. In most cases, the fish exclusively selected large Daphnia ignoring evasive prey types (Cyclops, Diaptomids) and small cladocera. This selectivity is the result of fish actively avoiding prey with high evasion capabilities even though they appear to be high in energetic content and having translated this into optimal selectivity through capture success rates. The energy consideration and visual system, apart from the forager's ability to capture prey, are the major determinants of prey selectivity for large-sized bluegill and white crappie still at planktivorous stages.     

Seasonal differences in rainfall,food availability,and the foraging behavior of Tropical Kingbirds in the southern Amazon Basin

ABSTRACT Little is known about the relationship between seasonal food availability and the foraging strategies of insectivorous Neotropical birds. We studied a population of Tropical Kingbirds (Tyrannus melancholicus), a primarily insectivorous species, in eastern Bolivia to examine relationships between rainfall, food availability, and foraging strategies throughout the year. Our study site in the southern Amazon Basin was characterized by strong seasonal variation in the abundance of the kingbird's main insect prey (coleopterans and hymenopterans), with reduced abundance during the nonbreeding season which largely overlaps the dry season. Overall, mean search times for insect prey by Tropical Kingbirds during the breeding (96.9 ± 85.6 [SD] sec) and nonbreeding (83.7 ± 91.2 sec) seasons did not differ (P= 0.23). However, during the nonbreeding season, kingbird search times were negatively, but nonsignificantly, correlated with coleopteran abundance (r²= 0.43, P= 0.16) and significantly and negatively correlated with hymenopteran abundance (r²= 0.72, P= 0.03). Although insect abundance differed seasonally, kingbird search times did not, perhaps because kingbirds forage on a greater variety of insects during the nonbreeding season or, during the breeding season, kingbird search times may be influenced by the need to monitor and defend nests as well as constraints on the types of prey that can be fed to nestlings. However, the reduced abundance of their primary insect prey and negative relationships between the abundance of those prey and search times during the dry, nonbreeding season suggest that Tropical Kingbirds in southern Amazonia may be food limited, potentially explaining why some migrate and spend that season elsewhere.     

The ontogeny of search behavior in the white crappie,Pomoxis annularis

Synopsis Animals that forage for discrete, isolated resources are often characterized as either ambush (sit-and-wait) or cruise (active) searchers. Juvenile white crappie, Pomoxis annularis, search for zooplankton prey using a saltatory search (SS) strategy. Unlike ambush and cruise search, SS involves scanning for prey only during the brief stationary periods that punctuate repositioning movements. If prey are not found, these fish swim a short distance, stop, and scan again. In this paper, we describe the ontogeny of prey search in the white crappie and compare the search pattern that they employ with that of juveniles. White crappie larvae searched for prey throughout the search space and only during the pauses that punctuated swimming movements. Prey location distances increased with fish size, as did several other components of the predation cycle. We conclude that white crappie larvae employ a search strategy similar to that exhibited by juveniles. We emphasize that, to obtain an accurate assessment of the feeding ecology of early life history stages, the search pattern that they employ must be characterized, and its components quantified.     

Prey selection and foraging period of the predaceous rocky intertidal snail,Acanthina punctulata

Summary The diet and foraging period of the neogastropod Acanthina punctulata were investigated in order to test various aspects of recent optimal foraging strategy models. This intertidal snail is an actively searching predator which preys on snails and barnacles by boring a hole in the shell and rasping out the flesh. Unlike many gastropod predators, Acanthina drill its gastropod prey at a very specific location on the columella, the thickest portion of the shell. Acanthina's foraging period can be interpreted as a compromise between maximizing the energy obtained by feeding and minimizing risk of mortality from exposure to wave action. That foraging period minimizing risk of being dislodged by waves appears to be during low tide when the predators can be in shallow pools. However, prey cannot be captured and consumed during one low tide. Thus Acanthina must be exposed during some high tides, and its strategy appears to be to restrict movement while exposed. Thus search is not initiated during high tide, but drilling and prey consumption are continued during that time. A snail not drilling or consuming prey seeks the protection of crevices or large anemones during high tide. A model is presented to indicate the relative amounts of risk and net energy for Acanthina at successive low and high tides. Predictions from the model, e.g., minimizing search time to avoid being exposed for an additional high tide and no movement during high tide are supported by field data. Acanthina commences foraging at the beginning of low tide, searches initially for preferred prey, but if unsuccessful, settles for a less preferred prey and begins drilling this prey before the end of low tide. Drilling and ingestion of prey occur during the following high and sometimes low tides. These handling times take 95% of the total foraging time in the field, while search time takes only 5% (pursuit time is negligible). Drilling alone accounts for 48–70% of the total drilling and eating time. In the laboratory, drilling and eating time for littorine food ranged from 15–60 hrs per item. The time to drill and eat a littorine increases exponentially with prey length. Since handling and processing prey items represents such a large investment of time, Acanthina would be expected to be very selective with respect to choice of prey items. Electivity coefficients from field data suggest that littorines are preferred over barnacles. Acanthina in the laboratory optimizes the amount of biomass ingested per time by choosing larger littorines over smaller ones and by preferring the more readily drilled species.It is suggested that Acanthina obtains information about the range of prey available initially by encountering and evaluating quite a few prey before making a selection, but usually by comparing an item of prey encountered to the prey it recently ingested. This latter method should provide a basis for evaluating prey encountered and has the advantage of reducing search time, the total amount of time spent feeding and thus the high-tide time exposed to wave action.In a similar manner, the decrease in the level of acceptability of prey as search time increases represents a compromise between maximizing energy obtained and minimizing risk from mortality.     

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.     

Cardiac output and stroke volume in swimming harbor seals

Summary Cardiac output was measured by the thermodilution method in three young harbor seals, at rest and while swimming up to the maximum effort for which they could be trained. Stroke volume was determined by counting heart rate simultaneously with determination of cardiac output. Cardiac outputs varied widely between surface breathing (7.8 ml · kg^–1 · s^–1) and breath-holding while swimming under water (1.8 ml · kg^–1 · s^–1). Stroke volume while at the surface was almost twice the volume white submerged. Surface cardiac output was always near maximal despite work effort, whereas submerged cardiac output gradually increased at higher work efforts. The cardiovascular performance of seals at the maximum MO₂ we could induce from them is equivalent to that of the domestic goat.Abbreviations CO Cardiac output - HR Heart rate - SV Stroke volume - MO ₂ Metabolic rate - FS Forced sumersion - V Velocity - C _DF Frontal drag coefficient - CV Cardiovascular Present address: Institute of Marine Science, University of Alaska, Fairbanks, AK, USA     

Ecological implications of unprecedented warm water anomalies on interannual prey preferences and foraging areas of Guadalupe fur seals

The Guadalupe fur seal (GFS) currently is recovering from near extinction and prey availability is an important factor. Scat and stable isotope analyses (SIA) were used to assess GFS foraging on Guadalupe Island during 2013–2016. This period was characterized by normal (2013) ocean temperatures followed by warm conditions (2014–2016). Scat samples (~40/year) were collected for prey identification. Additionally, lanugo samples from 1-month-old pups (50/year) were processed for SIA (δ¹³C/δ¹⁵N). Interannual prey preferences were identified (ANOSIM, p < .05), including significant differences between 2013 and the anomalous years. The most important prey species among these years was the jumbo squid; followed by the neon flying squid (2014–2016). The GFS diet was more specialized in 2013–2015 and more generalized in 2016. Prey from higher trophic levels (determined by scat analysis) were consumed during 2014–2016, but with significantly lower δ¹³C/δ¹⁵N values than in 2013 (ANOVA, p < .05 for both isotope ratios). This pattern may indicate more northerly or offshore foraging areas, with the presence of oceanic prey (neon flying squid). The widest isotopic niche was observed in 2015 (2.2‰²), reflecting a broader foraging area. Our findings are an important step toward better understanding the impacts of climate change on the recovery of GFS.     

Killer whales (Orcinus orca) feeding on lumpfish (Cyclopterus lumpus) in northern Norway

Killer whales (Orcinus orca) in Norwegian waters have long been known to rely on Atlantic herring (Clupea harengus) as a main prey resource. However, research almost exclusively conducted at seasonal herring grounds may have biased studies away from detecting other potentially significant prey species. Since 2013, dedicated research efforts have focused on monitoring killer whale occurrence and foraging ecology throughout the year in northern Norway. This study presents results on site-fidelity of photographically identified individuals, predation records and behavioral patterns from five spring seasons (March–April) in 2014–2018 in Andfjord, northern Norway. A minimum number of 75 adult and subadult killer whales (out of a catalog of 971 individuals) returned seasonally to the study area for foraging and residency for up to six weeks. Lumpfish (or lumpsucker, Cyclopterus lumpus) was the only type of prey identified (based on molecular or visual identification) on 22 predation events from 2016 (n = 4), 2017 (n = 2) and 2018 (n = 16). Spatial group cohesion observed when foraging was a potential adaptation for efficiently hunting this prey species. These whales were also encountered at herring wintering grounds the same years, but with different group sizes. Such behavioral adaptations suggested intraannual switching between prey resources and foraging strategies.     

Der Diffusionswiderstand der Spaltöffnungen; ein Vergleich des CO2-Gaswechsels von Blättern mit und ohne Epidermis

Summary The lower epidermis from leaves of Primula palinuri can be stripped off. Light-saturation curves of the CO₂-exchange were measured at 20°C and 300 ppm CO₂. Whereas the normal leaf reaches light-saturation at 0.3 cal cm^-2 min^-1, even 0.6 cal cm^-2 min^-1 is not sufficient to saturate the stripped leaf. Transpiration, apparent CO₂-uptake and leaf-temperature were measured simultaneously. The data were used to calculate the diffusion resistances for CO₂ with the usual methods, that is, from the diffusion resistances for water-vapour transport. The comparison of the CO₂-exchange of stripped and normal leaves makes it possible to determine the resistances—in particular those of the stomata—directly from the CO₂-exchange. Both methods agree well. When CO₂ exchanges only through the lower surface of the leaf the epidermis is—even with opened stomata—a considerable diffusion resistance. It lowers the CO₂-concentration in the intercellular system to 160 ppm and limits the CO₂-uptake.     

Zeaxanthin Formation and Energy-Dependent Fluorescence Quenching in Pea Chloroplasts under Artificially Mediated Linear and Cyclic Electron Transport

Artificially mediated linear (methylviologen) and cyclic (phenazine methosulfate) electron transport induced zeaxanthin-dependent and independent (constitutive) nonphotochemical quenching in osmotically shocked chloroplasts of pea (Pisum sativum L. cv Oregon). Nonphotochemical quenching was quantitated as Stern-Volmer quenching (SV_N) calculated as (F_m/F′_m)-1 where F_m is the fluorescence intensity with all PSII reaction centers closed in a nonenergized, dark-adapted state and F′_m is the fluorescence intensity with all PSII reaction centers closed in an energized state. Reversal of quenching by nigericin and electron-transport inhibitors showed that both quenching types were energy-dependent SV_N. Under light-induced saturating ΔpH, constitutive-SV_N reached steady-state in about 1 minute whereas zeaxanthin-SV_N continued to develop for several minutes in parallel with the slow kinetics of violaxanthin deepoxidation. SV_N above the constitutive level and relative zeaxanthin concentration showed high linear correlations at steady-state and during induction. Furthermore, F_o quenching, also treated as Stern-Volmer quenching (SV_O) and calculated as (F_o/F′_o)-1, showed high correlation with zeaxanthin and consequently with SV_N (F_o and F′_o are fluorescence intensities with all PSII reaction centers in nonenergized and energized states, respectively). These results support the view that zeaxanthin increases SV_N above the constitutive level in a concentration-dependent manner and that zeaxanthin-dependent SV_N occurs in the pigment bed. Preforming zeaxanthin increased the rate and extent of SV_N, indicating that slow events other than the amount of zeaxanthin also affect final zeaxanthin-SV_N expression. The redox state of the primary electron acceptor of photosystem II did not appear to determine SV_N. Antimycin, when added while chloroplasts were in a dark-adapted or nonenergized state, inhibited both zeaxanthin-SV_N and constitutive-SV_N induced by linear and cyclic electron transport. These similarities, including possible constitutive F_o quenching, suggest that zeaxanthin-dependent and constitutive SV_N are mechanistically related.     

Individual Flexibility and Tempo in the Ant, Pheidole dentata, the Influence of Group Size

This study investigates individual flexibility of foraging ants (Pheidole dentata) when the number of nestmates is altered by establishing broodless and queenless colony fragments all originating from a single big colony. Scouts from small groups (5 to 15 ants) behave like solitary foragers. They feed for long periods of time, they return slowly into the nest, and they recruit weakly. The ingested food is distributed by trophallaxis. Scouts from larger (20- to 30-ant) fragments forage more socially. Feeding and return times are short and recruitment is strong. Later the food is always transported into the nest. Two alternative mechanisms are discussed to explain the differences in individual foraging behavior. For the first—individual flexibility—assumptions have to be made about the capabilities of the individual, its work repertoire, and decision making outside the nest. The second mechanism takes into account that ants are capable of perceiving CO ₂ concentration differences and that ant groups are more active at higher CO ₂ concentrations. The organizational differences at the group level are explained simply by tempo differences in individual ants without making assumptions about individual capabilities.     

Cover Caption

It is often assumed that learning improves foraging, but its direct benefits are not often examined. We demonstrate the contribution of learning to the foraging success of desert ants when a trained colony successfully competes against a naïve one. The outcome also depends on the relative group size. Desert Cataglyphis ants search individually for food (mostly dead prey) without laying pheromone trails (see pages 241-250). The cover photo shows a Cataglyphis niger worker searching for food in its typical sandy habitat. Photo provided Arik Dorfman.     

A stochastic foraging model with predator training effects: I. Functional response,switching, and run lengths

Training effects are changes in a predator's behavior while it is searching for the next prey to eat which are caused by the predator's experience with the last prey encountered. A stochastic foraging model is formulated incorporating several specific types of training effects, and their impact on the functional response shape, switching, and mean prey run lengths is evaluated. The main result is that training effects such as search image formation can cause sigmoid functional responses and switching, and can result in runs of prey captures longer than expected in the absence of training.     

Optimization of foraging and diet by the piscivorous Othos dentex (Serranidae)

The aim of this study was to determine the dietary characteristics and mouth morphology of Othos dentex and to use these data, together with in situ observations of feeding behaviour, to elucidate how foraging and diet are optimized by this piscivorous serranid. Seasonal spear and line fishing over reefs in south‐western Australia yielded 426 O. dentex (total length, L_T, 183–605 mm), among which the stomachs of 95 contained food. The food in the stomachs of 76 fish was sufficiently undigested to be seen to contain, almost invariably, a single fish prey, which was typically identifiable to family and often to species. The prey of O. dentex, which were measured (L_T), represented 10 families, of which the Labridae and Pempheridae constituted nearly two‐thirds of the prey volume. Two‐way crossed analysis of similarities of volumetric data for stomach contents showed that the dietary compositions of the different length classes of O. dentex in the various seasons were significantly related to length class of prey, but not to prey family, length class within the various prey families or season. Furthermore, an inverse (Q‐mode) analysis, including one‐way analysis of similarities, showed that the patterns in the prey consumed by the different length classes of O. dentex in the various seasons were related more strongly to length class than prey family. The former trend is exemplified in a shade plot, by a marked diagonality of the length classes of prey with increasing predator size. The ingestion of typically a single teleost prey, whose body size increases as that of O. dentex increases, reduces the frequency required for seeking prey, thus saving energy and reducing the potential for intraspecific competition for food. The ability of O. dentex to ingest large prey is facilitated by its possession of a very large gape, prominent recurved teeth, dorsal and independently‐moveable eyes, cryptic colouration and effective ambush behaviour. Othos dentex has thus evolved very cost‐effective mechanisms for optimizing its foraging and diet.     

Foraging modes of stream benthic fishes in relation to their predation effects on local prey density

Habitat use and foraging behavior of two benthic insectivorous gobies, Rhinogobius sp. CO (cobalt type) and Rhinogobius sp. DA (dark type), were examined in relation to their predation effects on local prey density in a small coastal stream in southwestern Shikoku, Japan. Correlations among the foraging range, frequency of foraging attempts and current velocity indicated that individuals using fast-current habitats had small foraging ranges and infrequently made foraging attempts while those in slow currents frequently foraged over large areas. The former and the latter were recognized as ambush and wandering foragers, respectively. Interspecific comparisons of habitat use, foraging behavior and prey preference suggested that Rhinogobius sp. CO selectively forage mobile prey by ambushing in fast currents, whereas Rhinogobius sp. DA randomly forage available prey by wandering in slow-current habitats. A cage experiment was conducted to assess prey immigration rate and the degree of predation effects on local prey density in relation to current velocity. The results of the experiment support, at least in part, our initial predictions: (1) prey immigration rates increase with current velocity and (2) the effects of fish predation on local prey density are reduced as current velocity increases. Overall results illustrated a link between the foraging modes of the stream gobies and their predation effects on local prey density: fish adopt ambush foraging in fast currents, where the decrease in prey density tends to be less, whereas fish actively forage over large areas in slow currents, where the decrease in prey is relatively large.     

Hunger-induced foraging behavior of two cyprinid fish: Pseudorasbora parva and Rasbora daniconius

The feeding and swimming behaviors of Pseudorasbora parva and Rasbora daniconius (Cyprinidae) with two different prey types (Daphnia pulex and Artemia salina) at different densities (0.5, 1, 2, 5, 10, or 25 per l) were studied after 36 h of food deprivation. Full satiation was defined as the cumulative number of attacks performed until fish attain a constant attack rate which for P. parva was 425 and R. daniconius was 390 attacks. Initial feeding rates showed marked variation with prey availability. Feeding rates of fish in high prey concentrations were higher at the beginning of the experiment and decreased faster than in low prey densities. Decreases in the feeding rate at high prey densities were due to faster attainment of satiation. Feeding rates of fish across high prey densities reached a steady level after satiation. Swimming speeds of fish were inversely proportional to prey density. Moreover, the change in swimming speeds was directly related to the level of satiation. The ratios of the attack rate and the encounter rate against prey density of both fish reveal that the search for prey triggered swimming and thereby feeding during the transition from hungry to satiation. The findings of this study demonstrate that satiation plays an important role in fish foraging that should be considered a significant factor in foraging analysis.     

Prey to predator size ratio influences foraging efficiency of larval Aeshna juncea dragonflies

We investigated foraging behaviour of larval dragonflies Aeshna juncea in order to examine the significance of prey density and body size in predator-prey dynamics. A. juncea were offered separately three size-classes of Daphnia magna at low and high densities. The data were collected with direct observations of the foraging individuals. We found that large A. juncea larvae could better enhance their intake of prey biomass as prey size and prey density increased than their smaller conspecifics. However, increasing feeding efficiency of both larval instars was constrained by declining attack success and search rate with increasing prey size and density. With small D. magna, in contrast to large A. juncea, small A. juncea increased their searching efficiency as prey density increased keeping D. magna mortality rate at a constant level. In a predator-prey relationship this indicates stabilizing potential and feeding thresholds set by both prey density and prey-predator size ratio. Attack success dropped with prey size and density, but did not change in the course of the foraging bout. For both A. juncea sizes prey handling times increased as more medium and large prey were eaten. The slope of the increase became steeper with increasing prey-predator size ratio. These observations indicate that components of the predator-prey relationship vary with prey density, contrary to the basic assumptions of functional response equations. Moreover, the results suggest that the effects of prey density change during the ontogeny of predators and prey.     

Prey Odor Discrimination in the Varanoid Lizards Heloderma suspectum and Varanus exanthematicus

Use of chemical senses to detect prey is believed to be an important component of foraging behavior in actively foraging lizards. Ability to detect prey odors and discriminate them from control odors by tongue-flicking was studied in representatives of two families of lizards having highly forked, elongated, retractile tongues. Responses of gila monsters (Heloderma suspectum) and savannah monitors (Varanus exanthematicus) to deionized water, a control for pungency (cologne), and mouse odor on cotton swabs were studied in experiments using repeated-measures designs and employing the tongue-flick attack score (TFAS) as the primary measure of response strength. TFAS differed among treatments for gila monsters and monitors. Both species had greater TFAS to mouse odors than to either of the control stimuli, but responses to cologne were not statistically distinguishable from those to water. Numbers of tongue-flicks elicited by prey odors were greater than those for control stimuli in V. exanthematicus, but not for H. suspectum. Gila monsters, but not savannah monitors, bit in a significantly greater proportion of tests with prey odors than with control stimuli. Details of responses, including frequency of biting, apparent search behavior in the vicinity of applicators bearing mouse odors, and responses to control stimuli are discussed in relation to the foraging behavior of these two species and their relatives. Evidence from this and other studies suggests that the vomeronasal sense (and perhaps other chemical senses) is an important means of locating and recognizing prey in widely foraging autarchoglossan lizards.     

Group hunting in a ponerine ant,Leptogenys nitida Smith

Field observations on the emigration and foraging behaviours of the southern African ponerine ant, Leptogenys nitida, were undertaken at Mtunzini, Natal, South Africa. These colonies have a single ergatoid queen and 200–1000 workers. The nest sites are found in the leaf litter and these nests are moved frequently over distances ranging from 0.5 to 5 m. Leptogenys nitida is a diurnal predator of arthropods dwelling in the leaf litter. Up to 500 workers participate in each foraging trail, and are not led by definite scouts. Ants form clear trunk trails and fan out at various intervals to search for prey. The prey is searched for and retrieved cooperatively. From laboratory tests it was determined that ants will follow pygidial gland extracts, with the poison gland extract eliciting a limited response. The type of army ant behaviour observed in L. nitida seems to be different to that observed in other ponerine ants.     

Evaluation of the sample size used for the rapid bioassessment of rivers using macroinvertebrates

Experiments were designed to investigate selective predation by medium (40–55 mm carapace width: CW) and large (55–70 mm CW) Carcinus maenas when feeding on four bivalves of contrasting shell morphology. Size-selection was examined by presenting individual crabs with a wide size range of Mytilus edulis, Ostrea edulis, Crassostrea gigas and Cerastoderma edule. Medium-sized crabs preferred mussels 5–15 mm shell length (maximum shell dimension: SL) and cockles 5–10 mm SL, whereas large crabs preferred mussels 15–25 mm and cockles 10–20 mm SL. Crabs generally showed no preference for any particular size of either oyster species. Species-selection was examined by presenting individual crabs with paired combinations of the four bivalves in various proportions. When offered mussels and oysters simultaneously, both size categories of crabs consistently selected mussels, and food choice was independent of prey relative abundance. By contrast, C. maenas selected mussels and cockles as expected by the frequency in which each size category of crab encountered the preferred size ranges of prey. Crab preference clearly paralleled the rank order of prey profitability, which in turn was mainly determined by prey biomass, suggesting that active selection takes place at some point of the predation cycle. Experiments with epoxy resin models showed that initial reluctance of crabs to attack oysters was not associated with the ultimate energy reward. Moreover, they suggest that foraging decisions are partly based on evaluations of overall prey shape and volume, and that the minimum dimension of the shell constitutes an important feature which crabs recognise and associate with prey value.