Variation in plant stem density and its effects on foraging success of juvenile bluegill sunfish

Synopsis Juvenile bluegill sunfish, Lepomis macrochirus, are known to use beds of aquatic vegetation as a refuge from predators. This study examines the effects of increasing plant stem density on juvenile bluegill foraging. Three stem densities (100, 250 and 500 stems m⁻²), varying in their refuge potential for bluegills from predators, were tested. Results demonstrate that stem densities chosen as a refuge from predation (i.e. 500 stems m⁻²) significantly reduced bluegill foraging success and increased time required to capture prey. Therefore, juvenile bluegills seeking safety in vegetation may be faced with a trade-off between foraging success and effective refuge from predation when choosing among plant stem densities.     

Differences in learning by foraging juvenile pumpkinseed and bluegill sunfish in a structured habitat

Synopsis We investigated the ability of two congeneric species of sunfish to learn to forage on a novel prey item in feeding arenas containing structured habitats. Eight bluegill sunfish and eight pumpkinseed sunfish were given the opportunity to forage on whiteworms daily for 10 days. Each day, several behavioural measures were recorded for each fish. Both species of sunfish learned to feed over the 10-day period but the bluegill sunfish learned to feed more quickly than the pumpkinseed sunfish. Pumpkinseeds, however, attained a higher level of foraging efficiency. The differences in learning and foraging efficiency were related to body morphology.     

Foraging and predator avoidance: a test of a patch choice model with juvenile bluegill sunfish

Summary In situations where foraging sites vary both in food reward and predation risk, conventional optimal foraging models based on the criterion of maximizing net rate of energy intake commonly fail to predict patch choice by foragers. Recently, an alternative model based on the simple rule when foraging, minimize the ratio of mortality rate (u) to foraging rate (f) was successful in predicting patch preference under such conditions (Gilliam and Fraser 1987). In the present study, I compare the predictive ability of these two models under conditions where available patches vary both in predation hazard and foraging returns. Juvenile bluegill sunfish (Lepomis macrochirus) were presented with a choice between two patches of artificial vegetation differing in stem density (i.e. 100, 250, and 500 stems/m²) in which to forage. Each combination (100:250, 250:500, or 100:500) was presented in the absence, presence, and after exposure to a bass predator (Micropterus salmoides). Which patch of vegetation bluegills chose to forage in, and foraging rate within each patch were recorded. Independent measurements of bluegill foraging rate and risk of mortality in the three stem densities provided the data for predicting patch choice by the two models. With no predator, preference between plots was consistent with the maximize energy intake per unit time rule of conventional optimality models. However, with a predator present, patch preference switched to match a minimize u/f criterion. Finally, when tested shortly after exposure to a predator (i.e. 15 min), bluegill preference appeared to be in a transitional phase between these two rules. Results are discussed with respect to factors determining the distribution of organisms within beds of aquatic vegetation.     

Ecological and morphological differentiation of pumpkinseed sunfish in lakes without bluegill sunfish

Summary Over the last three decades, sunfish of the familyCentrarchidae have become recognized as a model system in which the ecological consequences of species interactions can be observed and tested. The evolutionary consequences of species interactions in sunfish have received less attention. Bluegill (Lepomis machrochirus) and pumpkinseed (Lepomis gibbosus) sunfish are two common and well-studied species that occupy separate ecological niches. Adult bluegill are generalists that feed in the open water on zooplankton during much of the year, while adult pumpkinseeds specialize on crushing hard-bodied prey such as snails. These species coexist over much of their geographical ranges, but bluegill are historically absent from several large drainage basins in the northeastern US. Here we show that pumpkinseeds from an Adirondack lake without bluegills have differentiated into two morphological forms, one of which is planktivorous. Differentiation is independent of sex and occurs over a broad range of sizes. Thus, the ecological diversity that exists between the bluegill and pumpkinseeds in sympatry has been replaced by a comparable degree of diversity within pumpkinseeds in allopatry.     

Phenotypic variation and vulnerability to predation in juvenile bluegill sunfish (<Emphasis Type="Italic">Lepomis macrochirus</Emphasis>)

Bluegill sunfish (Lepomis macrochirus) are known to diversify into two forms specialized for foraging on either limnetic or littoral prey. Because juvenile bluegills seek vegetative cover in the presence of largemouth bass (Micropterus salmoides) predators, natural selection should favor the littoral body design at size ranges most vulnerable to predation. Yet within bluegill populations, both limnetic and littoral forms occur where vegetation and predators are present. While adaptive for foraging in different environments, does habitat-linked phenotypic variation also influence predator evasiveness for juvenile bluegills? We evaluate this question by quantifying susceptibility to predation for two groups of morphologically distinct bluegills; a limnetic form characteristic of bluegills inhabiting open water areas (limnetic bluegill) and a littoral form characteristic of bluegills inhabiting dense vegetation (littoral bluegill). In a series of predation trials, we found that bluegill behaviors differed in open water habitat but not in simulated vegetation. In open water habitat, limnetic bluegills formed more dense shoaling aggregations, maintained a larger distance from the predator, and required longer amounts of time to capture than littoral bluegill. When provided with simulated vegetation, largemouth bass spent longer amounts of time pursuing littoral bluegill and captured significantly fewer littoral bluegills than limnetic fish. Hence, morphological and behavioral variation in bluegills was linked to differential susceptibility to predation in open water and vegetated environments. Combined with previous studies, these findings show that morphological and behavioral adaptations enhance both foraging performance and predator evasiveness in different lake habitats.     

Feeding-patch choice by red deer in relation to foraging efficiency

We tested the idea that ruminants allocate their feeding time to habitat patches in relation to foraging efficiency. We used five tame red deer (Cervus elaphus) in an enclosure planted with four treatment of timothy grass (Phleum pratense) differing in their stage of growth. Older swards offered higher biomass but lower nutritional quality than younger swards. We observed time spent feeding in each treatment during each of seven trials. We measured goodness-of-fit between observed times and predictions from two alternative hypotheses differing in optimization strategy (maximizing versus matching), and a third, null hypothesis. We tested the hypotheses using two alternative currecies: digestible protein, and digestible dry matter or energy. Although digestible protein concentration and dry-matter digestibility were highly correlated (r=0.763, P<0.001), the wider range of digestible protein made it the much more sensitive measure of forage quality. Distributions of feeding time closely matched estimated intake rates of digestible protein (R _infPred ^sup2 =0.899) across all animals and trials. The other hypotheses were rejected. The results have important ecological implications in showing the underlying role of food in the selection of habitat by ruminants, and that simple, mechanistic models of forage intake and digestion can be scaled up to the level of animal behavioural choices.     

The effect of parthenogenic eggs in Daphnia magna on prey location by the bluegill sunfish (Lepomis macrochirus)

The distance at which the bluegill sunfish (Lepomis macrochirus) can locate Daphnia magna with parthenogenic eggs is shown to be significantly greater than the reactive distance for non-gravid Daphnia of the same size. This difference is due to greater visibility of the gravid prey and not to different locomotor behavior, since there was no correlation between the number of eggs carried by a Daphnia and the antennal beat frequency. Based on this experiment and other observations, an explanation is given for selective predation of polymorphic cladoceran populations.     

Effects of Diazinon on bluegill sunfish, Lepomis macrochirus, gills: scanning electron microscope observations

Gills of bluegill sunfish, Lepomis macrochirus, exhibited varied degrees of structural damage following a 24-h exposure to sublethal concentrations (15 μg/l, 30 μg/l, 45 μg/l, 60 μg/l and 75 μg/l) of Diazinon [O,O-diethyl-O-(2-isopropyl-6-methyl-4 pyrimidinyl ester or phosphorothioate]. Exposure to 15 μg/l and 30 μg/l resulted in exocytosis of some material to the cell surface and perforations of the microridges. At higher doses (above 45 μg/l), the extrusion was reduced and the cells were swollen. Compared to control values, the thickness of the microridge on the gill arch and on the gill filament generally increased with exposure to Diazinon. Also, the distance between microridges decreased with increased exposure concentrations. At 60 μg/l, gill arch microridges fused and some ridges of gill filaments disappeared. At 75 μg/l exposure, epithelial cells of the gill arch became obscured with severe cellular extrusions and the lamellar surfaces swelled. The mucus extrusion, lamellar swelling and reduced microridges may be related to a defence mechanism which reduces the water surface around the gill and increases the barrier distance for diffusion of toxicants from outside to the blood capillaries. Although this mechanism protects the fish from toxicants, it also reduces the oxygen supply which leads to suffocation of the fish.     

Effects of location of food delivery and social status on foraging-site selection by juvenile Atlantic salmon

Synopsis Experiments were conducted to investigate the response of juvenile Atlantic salmon,Salmo salar, to changes in the location at which food entered a stream tank. Experience with the location of food input into the system significantly influenced foraging-site selection by the dominant fish. Dominant fish changed location of their foraging site in response to a change in the location of food input, and occupied, aggressively defending, sites just downstream of the location of food introduction. In contrast, subordinate fish occupied foraging sites at the downstream end of the stream tank, regardless of location of food input. As a result of both site selection and social status, dominant fish captured significantly more prey than subordinates. Our results support the contention that salmonids choose foraging sites to maximize foraging opportunities. Our results also provide a possible explanation for the use of atypical foraging sites by individual fish within their home range over the course of a single day, as observed in a number of salmonid species in the field.     

Optimal foraging: food patch depletion by ruddy ducks

Summary I studied the foraging behavior of ruddy ducks (Oxyura jamaicensis) feeding on patchily distributed prey in a large (5-m long, 2-m wide, and up to 2-m deep) aquarium. The substrate consisted of a 4x4 array of wooden trays (1.0-m long, 0.5-m wide, and 0.1-m deep) which contained 6 cm of sand. Any tray could be removed from the aquarium and loaded with a known number of prey. One bird foraged in the aquarium at a time; thus, by removing a food tray after a trial ended and counting the remaining prey, I calculated the number of prey consumed by the bird. I designed several experiments to determine if ruddy ducks abandoned a food patch in a manner consistent with the predictions of a simple, deterministic, patch depletion model. This model is based on the premise that a predator should maximize its rate of net energy intake while foraging. To accomplish this, a predator should only remain in a food patch as long as its rate of energy intake from that patch exceeds the average rate of intake from the environment. In the majority of comparisons, the number of food items consumed by the ruddy ducks in these experiments was consistent with the predictions of the foraging model. When the birds did not forage as predicted by the model, they stayed in the patch longer and consumed more prey than predicted by the model. An examination of the relation between rate of net energy intake and time spent foraging in the food patch indicated that by staying in a patch longer than predicted, the ruddy ducks experienced only a small deviation from maximum rate of net energy intake. These results provided quantitative support for the prediction that ruddy ducks maximize their rate of net energy intake while foraging.     

Short-term cues used by foraging trout in a California stream

Stream salmonids choose foraging locations to maximize the energy benefit of foraging within the constraints of size-mediated dominance hierarchies and predation risk. But, because stream habitats are temporally variable, fish must use a search process to monitor changing habitat conditions as a means of locating potentially-better foraging locations. I explored the cues used by the cutthroat trout, Oncorhynchus clarki clarki, when searching for food at the pool scale by artificially increasing prey availability at different locations by using special feeders and by manipulating pool velocities. Behavior of individually marked fish was monitored from stream bank platforms under unmanipulated control conditions and under seven experimental sets of conditions involving different combinations of feeder location and velocity manipulation. Under natural conditions fish elected to forage in the deepest (>50 cm), fastest (0.10–0.25 m s⁻¹) locations and within 1 m of structure cover, but would readily move to shallower (<30 cm) water away from cover if velocities were manipulated to be highest there. Although fish did not locate feeders unless they were placed in high-velocity areas, when high velocity was provided fish would move into very shallow water (<20 cm) if prey were delivered there. Responses of individual trout to manipulations indicated that water velocity was the main physical cue used by fish to decide where to forage, and that fish could also learn about new food sources by observing conspecifics. Overall, results indicated fish were not “perfect searchers” that could quickly locate new food resources over short time scales, even when the new resources were within a few meters of the fish’s normal foraging location. When given the correct cues, however, fish could detect new food sources and defend them against subordinate fish. Movement of new fish into and out of the study pools during the ten-day observation period was common, consistent with the idea that trout used movement as a means of exploring and learning about habitat conditions at the reach scale.     

Reproductive investment and somatic growth rates in longear sunfish

Synopsis Allocation of energy to current reproduction at the expense of other functions, such as growth, can limit future reproductive potential. This cost of reproduction is a central concept of life history theory but has been difficult to verify in comparative field studies. Three levels of comparison of growth rates and reproductive investments were evaluated within and among populations of longear sunfish,Lepomis megalotis. All three demonstrated high levels of reproductive investment associated with reduced somatic growth. Within populations of central longear sunfish there are precociously mature sneaker makes and later maturing parental makes; sneakers have greater gonadosomatic index (GSI) values and slower somatic growth rates than parental makes. Between subspecies of longear sunfish grown under common conditions, there are differences in age at maturity and in the level of physiological reproductive investment that are associated with distinct differences in growth rates. Between populations of central longear sunfish inhabiting different sites, there are differences in the level of reproductive investment that are also associated with differences in somatic growth. Each comparison produced evidence that trade-offs occur between these life history traits, supporting the hypothesis that there is a cost of reproduction among male sunfish and suggesting that differences in strategies of reproductive investment contribute to variation in somatic growth.     

Early oviposition experience affects patch residence time in a foraging parasitoid

Parasitoids learn olfactory and visual cues that are associated with their hosts, and use these cues to forage more efficiently. Classical conditioning theory predicts that encounters with high-quality hosts will lead to better learning of host-associated cues than encounters with low-quality hosts. We tested this prediction in a two-phase laboratory experiment with the parasitoid Trichogramma thalense Pinto & Oatman (Hymenoptera: Trichogrammatidae) and the host Anagasta kuehniella Zeller (Lepidoptera: Pyralidae).Host quality during the first exposure to hosts affected later foraging behavior for some experimental treatments, as predicted. We used a learning model, followed by patch-time optimization, to interpret our findings. We first simulated the parasitoids' host encounters during the experiment, and predicted their estimate of patch quality after each encounter. We then used dynamic optimization to predict the parasitoids' optimal patch residence times. The model reproduces the trends of the experimental results.     

Biotic and abiotic factors affecting survival of early life history intervals of a stream-dwelling sunfish

Synopsis Biotic interactions, such as competition and predation, and abiotic events, such as floods, both have important consequences for organisms inhabiting warmwater streams. Empirical studies can lead to better understanding of how these factors interact to affect distinct life history intervals of stream fishes. Survival of eggs, embryos and larvae in nests of longear sunfishLepomis megalotis was monitored in a section of Jordan Creek for four years. During two years characterized by low, relatively stable flow, nest failures could be attributed to biotic interactions. During two years with more variable flow, most brood losses occurred during floods. Flooding led to nest desertion and loss of offspring regardless of nest location. The number of nests constructed tended to be lower during years with more variable flow. The proportion of nests with successful broods did not differ between colonial and solitary nests. Variation in flow regime modifies the importance of biotic effects on survival of early life history intervals of longear sunfish.     

The effect of prey type and density on the foraging efficiency of juvenile perch, (Perca fluviatilis)

The foraging efficiency of juvenile perch (Perca fluviatilis), feeding on two types of prey, was studied in laboratory experiments. Waterfleas (Daphnia magna) and phantom midge larvae (Chaoborus flavicans) were offered in a range of densities, either separately or combined. Perch fed more efficiently on each prey type separately than when both were mixed. Foraging efficiency decreased with an increase of mixed prey density with both prey types present in equal numbers, but also when the proportion of Chaoborus increased. This could be caused by the existence of different hunting techniques, each of which is fully efficient in the presence of one prey type only. In the presence of two prey types, the predator constantly has to switch from one hunting technique to another.     

Maintenance of female mimicry as a reproductive strategy in bluegill sunfish (Lepomis macrochirus)

Synopsis In species where male reproductive success is dependent on male competition and aggression, alternative reproductive patterns, thought to represent a reduction in male reproductive effort, sometimes occur. Female mimicry in bluegill sunfish (Lepomis macrochirus) is an example of an obligate alternative male strategy. Female mimics are small, sexually mature males which mimic the details of female behavior, and gain access to functional females attracted to the nests of large, aggressive territorial males. The costs of female mimicry relative to nesting male behavior are discussed and two hypotheses, deception and mutual gain, are presented to explain the tolerance and courtship of female mimics by nesting males.     

Optimal patch residence time in egg parasitoids: innate versus learned estimate of patch quality

Charnovs marginal value theorem predicts that female parasitoids should exploit patches of their hosts until their instantaneous rate of fitness gain reaches a marginal value. The consequences of this are that: (1) better patches should be exploited for a longer time; (2) as travel time between patches increases, so does the patch residence time; and (3) all exploited patches should be reduced to the same level of profitability. Patch residence time was measured in an egg parasitoid Anaphes victus (Hymenoptera: Mymaridae) when patch quality and travel time, approximated here as an increased delay between emergence and patch exploitation, varied. As predicted, females stayed longer when patch quality and travel time increased. However, the marginal value of fitness gain when females left the patch increased with patch quality and decreased with travel time. A. victus females appear to base their patch quality estimate on the first patch encountered rather than on a fixed innate estimate, as was shown for another egg parasitoid Trichogramma brassicae. Such a strategy could be optimal when inter-generational variability in patch quality is high and within-generational variability is low.     

The repellent scent-mark of the honeybeeApis mellifera tigustica and its role as communication cue during foraging

Summary Experimental evidence for flower-marking in honeybees (Apis mellifera ligustica), using pairs of workers from the same colony foraging on an artificial patch of flowers, is reported. Workers marked artificial flowers with scent and strongly rejected all flowers they had recently visited. The same rejection behavior, in a lower although significant proportion, was observed when bees visited flowers just abandoned by the other individual of the pair. The repellent nature of this scent-mark was demonstrated with the use of an air extractor connected to the patch of artificial flowers. When the apparatus was turned on, the rejection behavior disappeared and bees accepted both flowers just abandoned by themselves and flowers just abandoned by the other bee. Differences in the response level of bees to their own marks or to the partner's marks suggest that the repellent scent-mark applied by a bee during foraging would basically be a self-use signal, although it certainly has value in communicating with other workers.     

Bush selection along foraging pathways by sympatric impala and greater kudu

In order to identify the selection mechanism of two sympatric African browsers, we analysed encounter rates and selection of bushes along foraging pathways. We monitored the tracks, left overnight, by kudu and impala on an experimental plot of natural Acacia nilotica and Dichrostachys cinerea in the highveld of Zimbabwe, and recorded the number of bushes attacked in each category. Both ungulates were selective for the bush categories, but kudu were consistently more selective than impala, and showed a higher preference for the larger A. nilotica and D. cinerea bushes, which had a significantly greater number of bites which were not reachable by impala. For both kudu and impala, the probability of attacking larger bushes increased significantly with the proportion of large bushes encountered along the foraging pathways, whereas the consumption of smaller bushes was apparently unpredictable. For the most abundant food item (medium D. cinerea), the probability of attack by impala along a pathway decreased with increasing proportions of larger bushes in the experimental area, but was also dependent on impala group size and season. In addition, we found that encounter rates with larger bushes were significantly higher for kudu than for impala. Experimentally reducing the availability of the larger bushes had little effect on both impala and kudu during the following rainy season. However, during the following cool dry season, kudu showed an increased selectivity with a strong preference for the remaining large bushes (large A. nilotica), followed by a sharp decrease in selectivity in the hot dry season when they also fed from significant numbers of medium trees. Impala had little reaction to the experimental changes in the availability of bush categories in either season. We suggest that both kudu and impala selected bushes on the basis of the potential number of bites they can provide, and this resulted in different search strategies. Kudu focussed on the larger bushes which have a larger number of twigs which are out of reach of impala and kudu also probably directed their path preferentially towards the few larger bushes to maximize encounter rates with this favoured bush category. These differences in bush selection process lead to a low overlap in resource use between the two browsers in this type of savanna.     

Use of water hyacinths as shelter,foraging place,and transport by young piranhas,Serrasalmus spilopleura

Synopsis The water hyacinth, Eichhornia crassipes, plays an important role in the early life of the piranha, Serrasalmus spilopleura in southeastern Brazil. Larvae and early juveniles are found by both day and night among the roots of this free floating waterweed, thus gaining shelter, a rich foraging place, and potential rafting dispersal. Piranha larvae up to 19 mm SL feed mainly on small aquatic arthropods, slowly searched for inside the root tangle; larger juveniles tend to leave the plants and patrol more open areas. At 24 mm SL young piranhas begin to clip out pieces from fins of other fishes and seek shelter in water hyacinths only at night. About 30% of the rafting clumps of water hyacinths may harbour one to three piranha larvae, providing dispersal during floods.