Patch Choice Decisions among Ifaluk Fishers

Studies of patch choice decisions among human foragers have failed to explain why foragers do not exclusively exploit the patch with the highest mean profitability. One possible explanation is that profitability rankings are likely to vary daily; however, this instability is not captured when profitabilities are calculated as a sampled average over a longer time span. Here I present data on the patch choice decisions of Ifaluk fishers to evaluate whether men are responding to daily variation in the profitability of their primary fishing patch. Results show that men choose to fish most frequently in the patch with the highest mean profitability. Men fish in alternative patches (alternative from the most profitable patch) when, on that morning or the previous day, return rates in the most profitable patch are lower than the overall mean per capita return rate of alternative patches. Results also indicate that when fishers pursue alternative patches after fishing in the patch with the highest profitability, their mean per capita return rates are generally higher in the alternative patches exploited. However, variance in the profitability of the most profitable patch cannot explain why men exploit two patches, the Nine-mile reef and the dogtoothed tuna patch, which on average have very low profitability. These results and directions for future research are discussed. [Keywords: human behavioral ecology, patch choice decisions, Micronesia]     

Costly signaling and torch fishing on Ifaluk atoll

In this paper I evaluate the merit of costly signaling theory (CST) as a paradigm for understanding why men of Ifaluk atoll torch fish. I argue that torch fishing is a handicap that signals men's productivity. Consistent with CST, torch fishing is observed by the predicted audience (women), energetically costly to perform, and a reliable indicator of the frequency a man fishes during the trade wind season. Contrary to expectations of who should benefit from torch fishing and consequently participate, torch fishers are not primarily young and unmarried. Torch fishers, however, are predominately from the matriline that owns the canoe on which they fish, suggesting that torch fishing also signals the productivity of a matriline. Although these results support the possibility that torch fishing is a handicap, no data are presented which demonstrate that torch fishers achieve any gains from sending the costly signal. This shortcoming and other directions for future research on Ifaluk foraging decisions are discussed.     

Foraging on patchily distributed prey by a cichlid fish (Teleostei,Cichlidae): A test of the ideal free distribution theory

Cichlid fish (Aequidens curviceps) distributed themselves and allocated their foraging time between two drift food patches in close approximation to the patch profitability ratio, as predicted by the ideal free distribution theory. The fish thereby achieved similar average feeding rates in the two patches, in two of three patch profitability ratio experiments. However, one major assumption of the ideal free model was violated, since individual fish differed in their competitive abilities for limited food resources, which resulted in unequal payoffs among individuals within each patch. Individual variation in feeding rates, and thus in competitive ability, was not related to despotism, but perhaps rather to individual differences in perceptual ability and in the ability to learn which patch was currently the more profitable. The strategy used by the fish to assess patch profitability included sampling available patches. However, individual fish switched (sampled) patches with varying frequency. Sampling had an associated cost, since high-frequency switchers had lower feeding rates on average than low-frequency switchers. Differences in foraging strategy among the fish therefore contributed to the observed in-equality in individual payoffs within patches.     

Social enhancement of foraging on an ephemeral food source in juvenile walleye pollock,Theragra chalcogramma

Synopsis The foraging effectiveness of walleye pollock juveniles, Theragra chalcogramma, was determined experimentally to test the hypothesis that social cues may facilitate the ability of individuals to exploit ephemeral food patches. Fish were tested when isolated, paired with one other fish, and in a group of six fish. Test fish exploited more food patches while in a group of six than when they were isolated. Patch exploitation by paired fish was intermediate to but not statistically different than isolate or grouped treatments. The number of pellets eaten by test fish in a group and a pair was more than 3.5 times that of when they were isolated, although the overall relationship between the amount of food eaten and group size was not statistically significant. Results support the hypothesis that juvenile walleye pollock exploit ephemeral food patches more effectively in the presence of conspecifics. In planktivores such as walleye pollock, social cues may enhance foraging on transient food sources either by facilitating detection of food patches (local enhancement) or by stimulating foraging activity when a food patch is located (social facilitation).     

Foraging behavior of an estuarine predator, the blue crab Callinectes sapidus in a patchy environment

To define general principles of predator‐prey dynamics in an estuarine subtidal environment, we manipulated predator density (the blue crab, Callinectes sapidus) and prey (the clam, Macoma balthica) patch distribution in large field enclosures in the Rhode River subestuary of the central Chesapeake Bay. The primary objectives were to determine whether predators forage in a way that maximizes prey consumption and to assess how their foraging success is affected by density of conspecifics. We developed a novel ultrasonic telemetry system to observe behavior of individual predators with unprecedented detail. Behavior of predators was more indicative of optimal than of opportunistic foraging. Predators appeared responsive to the overall quality of prey in their habitat. Rather than remaining on a prey patch until depletion, predators appeared to vary their patch use with quality of the surrounding environment. When multiple (two) prey patches were available, residence time of predators on a prey patch was shorter than when only a single prey patch was available. Predators seemed to move among the prey patches fairly regularly, dividing their foraging time between the patches and consuming prey from each of them at a similar rate. That predators more than doubled their consumption of prey when we doubled the number of prey (by adding the second patch) is consistent with optimizing behaviors ‐ rather than with an opportunistic increase in prey consumption brought about simply by the addition of more prey. Predators at high density, however, appeared to interfere with each other's foraging success, reflected by their lower rates of prey consumption. Blue crabs appear to forage more successfully (and their prey to experience higher mortality) in prey patches located within 15–20 meters of neighboring patch, than in isolated patches. Our results are likely to apply, at least qualitatively, to other crustacean‐bivalve interactions, including those of commercial interest; their quantitative applicability will depend on the mobility of other predators and the scale of patchiness they perceive.     

Patch use behaviour in benthic fish depends on their long-term growth prospects

Animals foraging in a heterogeneous environment may combine prior information on patch qualities and patch sample information to maximize intake rate. Prior information dictates the long-term expectations, whereas prior information in combination with patch sample information determines when to leave an individual food patch. We examined patch use behaviour of benthic feeding fish in their natural environment at different spatial scales to test if they could determine patch quality and if patch use behaviour was correlated with environmental quality. In seven lakes along a gradient of environmental quality (measured as maximum benthivore size), we made repeated measurements of giving-up density (GUD) in artificial food patches of different qualities. At the largest spatial scale, between lakes, we tested if giving-up densities revealed the long-term growth expectation of benthic fish. At the local scale of patches and micro patches we tested for the ability of benthic fish to assess patch quality, and how this ability depended on the patch exploitation levels between the different lakes. We found that GUD was positively related to maximum size of bream, suggesting that short-term behavioural decisions reflected long-term growth expectations. Benthic fish discriminated between nearby rich and poor patches, but not between rich and poor micropatches within a food patch. This suggests that the foraging scale of benthic fish lies between the patch and micro patch scale in our experiments. We conclude that patch use behaviour of benthic fish can provide a powerful measure of habitat quality that reveals how benthic fish perceive their environment.     

Effects of resource distribution, patch spacing, and preharvest information on foraging decisions of northern bobwhites

We investigated patch assessment by northern bobwhites (Collinusvirginianus) in an experimental arena where the distributionof resources in patches, preharvest information about thesepatches, and spacing of patches varied. We found that preharvestinformation about patch quality and a bimodal distribution ofpatch rewards allowed birds to selectively exploit patches highin resources. In contrast, uniform distribution of patch qualitiesand lack of preharvest information caused birds to forage nonselectivelyamong patches. Birds distinguished among patches of differentquality when these patches were spaced 13 m apart, but failedto react to patch quality differences when patches were 0 or3 m apart We also found a strong effect of the level of patchdepletion on foraging decisions: as resources in die arena becamescarce, birds increasingly foraged selectively in die most profitablepatches. Foraging decisions of bobwhites are biased by die waythey experience and memorize a spatially and temporally variableenvironment. The relative cost of this cognitive bias (i.e.,lost opportunity) is nonlinearty related to die mean resourcedensity in die environment and to die difference between thismean density and die resource density in die exploited patch.Cognitive bias should be considered when evaluating patch assessmentcapabilities of foragers in complex environments.     

Sex differences in foraging behaviour and oviposition site preference in an insect predator, Orius sauteri

The effects of patch quality on the foraging behaviour of an anthocorid predator Orius sauteri (Poppius) were compared between sexes. Prior experience in patches was also studied to determine whether this was a factor affecting oviposition decisions. Patch quality affected patch residence time differently for the two sexes; females stayed much longer in a patch with prey (60 Thrips palmi larvae) than a patch without prey, while males did not remain in any patch for extended periods. Most of the females remained in or moved to patches with prey, whereas males dispersed, irrespective of patch quality. Both females released in patches with prey and females released in patches without prey deposited more eggs per hour in patches with prey than in patches without prey. Females released in patches without prey laid eggs in patches with prey at higher rates than did females released in patches with prey. Causes for the sex difference in patch residence time and allocation are discussed in relation to optimal foraging theory. The significance of selective oviposition and the role of experience in oviposition decisions within heterogeneous environments are also discussed.     

Response of predators to prey abundance: separating the effects of prey density and patch size

We tested the relative and combined effects of prey density and patch size on the functional response (number of attacks per unit time and duration of attacks) of a predatory reef fish (Cheilodactylus nigripes (Richardson)) to their invertebrate prey. Fish attacked prey at a greater rate and for longer time in large than small patches of prey, but large patches had naturally greater densities of prey. We isolated the effects of patch size and prey density by reducing the density of prey in larger patches to equal that of small patches; thereby controlling for prey density. We found that the intensity at which fish attacked prey (combination of attack rate and duration) was primarily a response to prey density rather than the size of patch they occupied. However, there was evidence that fish spent more time foraging in larger than smaller patches independent of prey density; presumably because of the greater total number of prey available. These experimental observations suggest that fish can distinguish between different notions of prey abundance in ways that enhance their rate of consumption. Although fish may feed in a density dependent manner, a critical issue is whether their rate of consumption outstrips the rate of increase in prey abundance to cause density dependent mortality of prey.     

Unpacking chimpanzee (Pan troglodytes) patch use: Do individuals respond to food patches as predicted by the marginal value theorem?

The marginal value theorem is an optimal foraging model that predicts how efficient foragers should respond to both their ecological and social environments when foraging in food patches, and it has strongly influenced hypotheses for primate behavior. Nevertheless, experimental tests of the marginal value theorem have been rare in primates and observational studies have provided conflicting support. As a step towards filling this gap, we test whether the foraging decisions of captive chimpanzees (Pan troglodytes) adhere to the assumptions and qualitative predictions of the marginal value theorem. We presented 12 adult chimpanzees with a two-patch foraging environment consisting of both low-quality (i.e., low-food density) and high-quality (i.e., high-food density) patches and examined the effect of patch quality on their search behavior, foraging duration, marginal capture rate, and its proxy measures: giving-up density and giving-up time. Chimpanzees foraged longer in high-quality patches, as predicted. In contrast to predictions, they did not depress high-quality patches as thoroughly as low-quality patches. Furthermore, since chimpanzees searched in a manner that fell between systematic and random, their intake rates did not decline at a steady rate over time, especially in high-quality patches, violating an assumption of the marginal value theorem. Our study provides evidence that chimpanzees are sensitive to their rate of energy intake and that their foraging durations correlate with patch quality, supporting many assumptions underlying primate foraging and social behavior. However, our results question whether the marginal value theorem is a constructive model of chimpanzee foraging behavior, and we suggest a Bayesian foraging framework (i.e., combining past foraging experiences with current patch sampling information) as a potential alternative. More work is needed to build an understanding of the proximate mechanisms underlying primate foraging decisions, especially in more complex socioecological environments.     

Heritability of selectively advantageous foraging behaviour in a small passerine

Summary I measured the heritability of foraging patch choice in a laboratory population of zebra finches (Taeniopygia guttata). Mothers and offspring were tested for their ability to discriminate between four foraging patches which provided four different rates of energy gain. Use of a foraging patch with a high rate of energy gain has been shown to confer a selective advantage on zebra finches in a similar experimental system. In this population of zebra finches there was a large amount of variation in foraging patch choice behaviour both within and among individuals. I determined that foraging patch choice was a phenotypically labile trait with a degree of stereotypy or repeatability, much lower than those typically recorded for morphological traits. The mating behaviour of zebra finches required that heritability be determined from a mother—offspring regression, which showed that narrow sense heritability of foraging patch choice was approximately 0.346. This heritability was significantly different than zero, as was heritability when it was limited by repeatability to 0.246. Foraging patch choice, a behaviour that has a demonstrated fitness consequence, had a heritable component in this laboratory population of zebra finches.     

Plant stem density as a cue in patch choice by foraging juvenile bluegill sunfish

Synopsis Juvenile bluegill sunfish,Lepomis macrochira, are restricted to vegetated habitats by predators. Variation in plant stem density has a significant effect on bluegill foraging success. Given the mosaic nature of this habitat, plant stem density may provide a cue for selecting among patches in which to forage. In this study, juvenile bluegills were offered patches of artificial vegetation differing only in plant stem density as potential foraging sites. Three densities, 100, 250, and 500 stems m^–2 were tested. Fish were presented with a choice between patches (100:250, 250:500, or 100:500). Bluegill foraging rate in, and the number of fish choosing each patch was recorded. Juvenile bluegills showed a preference for those patches which maximized their foraging rate.     

Scombroid fish poisoning. Underreporting and prevention among noncommercial recreational fishers.

Food-borne diseases, including those caused by seafood products, are common and greatly underreported sources of morbidity. In this article we review the epidemiology of scombroid fish poisoning and its possible relationship to the noncommercial and recreational catch and sale of fish. More than 20% of all fish sold in the United States is caught by sport fishers, and outbreaks of scombroid fish poisoning have involved improperly handled fish from private catches. We report an outbreak of scombroid fish poisoning among recreational fishers in California. The unregulated sale of recreationally caught fish for consumption and the prevention of scombrotoxism are discussed from the perspectives of public health agencies, clinicians, and the fishing public. Scientific and policy issues that require further attention are high-lighted.     

Energy benefits and emergent space use patterns of an empirically parameterized model of memory‐based patch selection

Many species frequently return to previously visited foraging sites. This bias towards familiar areas suggests that remembering information from past experience is beneficial. Such a memory‐based foraging strategy has also been hypothesized to give rise to restricted space use (i.e. a home range). Nonetheless, the benefits of empirically derived memory‐based foraging tactics and the extent to which they give rise to restricted space use patterns are still relatively unknown. Using a combination of stochastic agent‐based simulations and deterministic integro‐difference equations, we developed an adaptive link (based on energy gains as a foraging currency) between memory‐based patch selection and its resulting spatial distribution. We used a memory‐based foraging model developed and parameterized with patch selection data of free‐ranging bison Bison bison in Prince Albert National Park, Canada. Relative to random use of food patches, simulated foragers using both spatial and attribute memory are more efficient, particularly in landscapes with clumped resources. However, a certain amount of random patch use is necessary to avoid frequent returns to relatively poor‐quality patches, or avoid being caught in a relatively poor quality area of the landscape. Notably, in landscapes with clumped resources, simulated foragers that kept a reference point of the quality of recently visited patches, and returned to previously visited patches when local patch quality was poorer than the reference point, experienced higher energy gains compared to random patch use. Furthermore, the model of memory‐based foraging resulted in restricted space use in simulated landscapes and replicated the restricted space use observed in free‐ranging bison reasonably well. Our work demonstrates the adaptive value of spatial and attribute memory in heterogeneous landscapes, and how home ranges can be a byproduct of non‐omniscient foragers using past experience to minimize temporal variation in energy gains.     

Testing assumptions of central place foraging theory: a study of Adélie penguins Pygoscelis adeliae in the Ross Sea

We investigated central place foraging (CPF) in the context of optimal foraging theory in Adélie penguins Pygoscelis adeliae of the southern Ross Sea by using satellite tracking and time‐depth recorders to explore foraging at two spatio‐temporal scales: within the day‐to‐day (sub‐mesoscale: single foraging trip, 10s of km²) and the entire breeding season (mesoscale: trips by multiple individuals across the collective foraging area, 100s of km²). Specifically, we examine whether three basic assumptions of the Orians–Pearson CPF model, shown to occur in other CPF species, are met: 1) within a patch, the rate of prey acquisition declines with time spent in that patch; 2) food is distributed in discrete patches and is not available between those patches; and 3) CPF species have knowledge of the potential (or average, at least) feeding rate within their universe of patches, and use this knowledge to determine their foraging strategy when planning or engaging in a foraging trip. We found that prey consumption rates did not decline with time spent in patches, and penguins foraged to some degree most of the time when at sea. Food availability, as measured by foraging dive rate, appeared to be predictable within the same day at the same location, but predictability broke down after 2 d at distances > 10 km away. We conclude that the assumptions of the Orians–Pearson CPF model are not a good fit to the circumstances of Ross Sea penguins, which clearly are central place foragers.     

Interference competition in central place foragers: the effect of imposed waiting on patch-use decisions of eastern chipmunks, Tamias striatus

When central place foragers compete aggressively for patchyresources, subordinates may be preventedfrom collecting fooduntil a dominant has departed with its load. Extensions of centralplace foraging models predict that animals forced to wait ata patch should increase their load sizes and patch times aswellas their tendency to search for and switch to alternative patches.We tested these predictions usingeastern chipmunks, Tamias striatus,hoarding sunflower seeds collected from seed/vermiculite mixturesintrays placed 5-8 m from their burrows. By using her hand toprevent access to the patch, the experimentersubjected animalsto progressively increasing waiting times at two seed densities;another series of trialsat the same seed densities monitoreda similar number of trips without imposed waiting. As predicted,patch times and load sizes were higher in sessions with imposedwaiting than in control sessions. Loadsizes increased with trialnumber in experimental sessions but decreased or remained thesame in controlsessions. Chipmunks spent more of their timesearching for alternative patches during trials with imposedwaiting than during controls. They also started searching foralternative patches at lower levels of imposed waiting whenusing poor than when using rich patches. These results indicatethat the effects of interference on foraging decisions and onspatial overlap between individuals can be predicted by simpleeconomic models. Furthermore, the results suggest how resource-defensetactics can be predicted by the economic effects of interferenceon the foraging efficiency of the opponent.     

Patch choice and risk: relative competitive ability is context dependent

The relative abilities of individual cichlids, Tilapia zillii to obtain food under scramble competition was highly repeatable between trials using a single input source, regardless of whether the input was constant or variable. However, when given a choice between two patches differing only in their temporal variability in input about an identical mean, an individual's rank based on intake in one patch was uncorrelated with either its intake in the other patch or its intake in the single-patch trials. In the two-patch trials, certain individuals both spent more time in food patches and visited patches more often than others, and overall the fish spent more time in the constant rate patch than the variable patch, leading to more items being consumed from the constant rate patch. We discuss possible causes and consequences of this dependence of relative competitive ability on the context of the foraging situation. Copyright 1999 The Association for the Study of Animal Behaviour.     

On the value of information: studying changes in patch assessment abilities through learning

Little is known about how animals acquire and use prior information, particularly for Bayesian patch assessment strategies. Because different patch assessment strategies rely upon distinct capabilities to obtain information, we analyzed whether foragers can alter their foraging strategy when they exploit predictable patches with periodic renewal. For this, we evaluated if learning contribute to increase foraging efficiency by improving patch assessment abilities in degus (Octodon degus), a diurnal caviomorph rodent from central Chile. Single degus exploited pairs of depleting patches that were renewed daily. During the initial two days of the experiment, degus exploited patches in agreement with a fixed‐time strategy, i.e. at the population level, giving‐up densities (GUD) were not distinguishable from density‐independence (i.e. consumption proportional to initial patch densities), and richer patches were under‐exploited. After day five, degus improved significantly their assessment strategy, showing agreement with Bayesian information updating. However, on day 15 and afterwards, degus foraged patches in agreement with a prescient strategy, because GUDs across patches indicated positive density‐dependence and equalization of GUDs. Although highly variable, the GUD ratio between rich and poor patches decreased significantly throughout time. Within‐subject data showed that as subjects learned patch qualities they showed a tendency toward GUD equalization and differentiation from density‐independence. By the end of the experiment, degus allocated more time to richer patches during the initial period of each trial, and allocated similar amounts of time by the end of trials. Further, the first visit of a session was significantly biased toward the rich patch by the final days of the experiment. The results suggest that assessment abilities can change when exploiting novel but predictable patches. When degus can incorporate adequate environmental information, prior and current information may become accurate enough to make animals exploit patches efficiently.     

Food consumption of yellowfin tuna,Thunnus albacares, in Sri Lankan waters

Synopsis A total of 4181 stomachs of yellowfin tuna,Thunnus albacares (22–164 cm FL), mainly caught by gillnets in the period from July 1984 to June 1986 were analyzed. Food consumption of yellowfin tuna in nature was determined using the values of the average stomach content, incorporating laboratory measurements of gastric evacuation rates in a feeding model. The diet of yellowfin tuna around Sri Lanka comprised a variety of macro zooplanktonic and nektonic organisms. Juveniles < 39 cm FL, are planktivores. Tuna > 40 cm FL gradually increase their consumption of fishes with increasing size. Among fish speciesAuxis sp. are the most important. The daily food consumption of juvenile yellowfin tuna (22–59 cm FL) range from 1.8 g to 136.2 g, increasing to about 284.7 g to 551.9 g for the medium size tuna (60–99 cm FL). The adult tuna (100–>130 em FL) consume around 513 g to 538.8 g of prey per day. Daily ration estimates increase from 2.1% to 5.5% of body weight with increasing size up to 70 cm FL, beyond which it decreases. The predatory pressure of yellowfin tuna on commercially important fishes and other species is discussed.     

Do cattle egrets gain information from conspecifics when foraging?

Summary We examined whether individual cattle egrets (Bubulcus ibis) base their decisions of where to forage, and how long to stay in a patch, on the behavior of other flock members. Cattle egrets commonly forage in flocks associated with cattle and capture prey at higher rates when they do not share a cow with another egret. Foraging egrets provide cues of the location of prey and their success in capturing prey. Therefore, there is the possibility of information transfer between egrets in a flock. We predicted that egrets should only move to occupied patches when the resident was capturing enough prey that it is profitable for the invader to share the patch or take over the patch. However, egrets did not seem to decide where to forage based on neighbors' rates of energy intake, but rather on the presence or absence of conspecifics in a patch. We also predicted that an egret should remain in a patch until its rate of energy intake dropped to or below the average rate for other egrets within the flock. However, egrets that were foraging more efficiently than the average rate for the flock switched patches sooner than less efficient foragers. Egrets did not appear to increase foraging success by gaining information on patch quality from neighbors.