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Abstract. We present an experiment designed to test the hypothesis that fish respond to both relative predation risk and habitat profitability in choosing habitats in which to feed. Identical populations of three size-classes of bluegill sunfish (Lepomis macrochirus) were stocked on both sides of a divided pond (29m in diameter), and eight piscivorous largemouth bass (Micropterus salmoides) were introduced to one side. Sizes of both species were chosen such that the small class of bluegills was very vulnerable to the bass, whereas the largest class was invulnerable to bass predation. We then compared mortality, habitat use, and growth of each size-class in the presence and absence of the bass.Only the small size-class suffered significant mortality from the bass (each bass consumed on average about one small bluegill every 3.8 d); the two larger size-classes exhibited similar mortality rates on both sides of the pond. In the absence of the bass, we found that habitat use of all sizeclasses was similar and that the pattern of habitat use maximized foraging return rates (Werner et al. 1983). In the presence of the bass the two larger size-classes chose habitats to maximize return rates, but the small size-class obtained a greater fraction of its diet from the vegetation habitat, where foraging return rates were only one-third of those in the more open habitats. The small size-class further exhibited a significant depression in individual growth in the presence of the bass; the growth increment during the experiment was 27% less than that for small bluegills in the absence of the bass. Because of the reduced utilization of more open habitats by the small fish in the presence of bass, resources in these habitats were released to the larger size-classes, which showed greater growth in the presence of the bass than in its absence. We develop methods to predict the additional mortality expected on a cohort due to a reduction in growth rate (because individuals are spending a longer time in vunerable sizes), and discuss the potential for predation risk to enforce size-class segregation, which leads de facto to resource partitioning.

Animals commonly choose among habitats that differ both in foraging return and mortality hazard. However, no experimental study has attempted to predict the level of increase in resources, or the decrease in mortality hazard, which will induce a forager to shift from a safer to a more hazardous (but richer) foraging area. Here we present and test a model that specifies the choice of foraging areas ("habitats") that would minimize total mortality risk while allowing collection of some arbitrary net energy gain. We tested the model with juvenile creek chubs (Semotilus atromaculatus) in an experimental field stream in which the foragers could utilize a foodless refuge and choose between two foraging areas that differed in experimentally manipulated resource densities (Tubifex spp. worms in sediments) and mortality hazard (adult creek chubs). For the case tested, the model specified a simple rule: "use the refuge plus the site with the lowest ratio of mortality rate (μ) to gross foraging rat (f)," i.e., "minimize μ./f." Independent prior measurements of mortality hazard (as a function of predator density) and gross foraging rate (as a function of resource density) allowed us to predict the resource level in the more hazardous foraging site that should induce a shift from the safer to the more hazardous site. The chubs' preferences in subsequent choice experiments agreed well with the theoretical predictions. The "minimize μ/f" rule (deaths per unit energy), perhaps in modified form, provides a simple alternative to the "maximize f" (energy per unit time) criterion that applies to long-term rate maximization when predation hazard does not differ among choices.

Leptokurtic distributions of movement distances observed in field-release studies, in which some individuals move long distances while most remain at or near their release point, are a common feature of mobile animals. However, because leptokurtosis is predicted to be transient in homogeneous populations, persistent leptokurtosis suggests a population heterogeneity. We found evidence for a heterogeneity that may generate persistent leptokurtosis. We tested individuals of the Trinidad killifish Rivulus hartii for boldness in a tank test and released them back into their native stream. Boldness in the tank test predicted distance moved in the field releases, even after effects of size and sex were removed. Further, data from a 19-mo mark-recapture study showed that individual growth correlated positively with movement in a predator-threatened river zone where the Rivulus population is spatially fragmented and dispersal is likely to be a hazardous activity. In contrast, no such correlation existed in a predator-absent zone where the population is unfragmented. These results show that a behavioral trait, not discernible from body size or sex, contributes to dispersal and that a component of fitness of surviving "dispersers" is elevated above that of "stayers," a fundamental assumption or prediction of many models of the evolution of dispersal through hazardous habitat.

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We surveyed the fish community of a tropical watershed in the Northern Range Mountains of Trinidad and conducted experiments to test the hypothesis that species interactions structure the fish community. We censused the fish community at 86 sites, measuring each fish collected and recording physical variables for each site (pool size, substrate composition, cover, etc.). One member of the fauna, a killifish, Rivulus hartii, was widely distributed in the drainage, occurring alone in each headwater but also encountering other fish species below barrier waterfalls. This pattern allowed us to use Rivulus to test four predictions derived from the species interaction hypothesis: (I) Rivulus density (in grams per square metre) would be depressed at sites with other fish species present, when compared to the density expected from physical characteristics of each site as if Rivulus were alone; (2) Rivulus density would decline at points of contact with the piscivorous fish Hoplias malabaricus; (3) local populations of Rivulus found in pools occupied by the predator would show a smaller maximal size than in pools without the predator; and (4) the growth rate of Rivulus would be lowered in the event of a local invasion by the guppy, Poecilia reticulata, another abundant and sometimes co-occurring species in the watershed. The first three predictions were tested with data from the survey, while the fourth prediction was tested by experiments in a nearby stream.We used a factor analysis to reduce the dimensionality of the physical variables to four factors. A regression analysis of Rivulus density on the four factors, using 43 Rivulus-only pools, was used to predict Rivulus density in nine pools, each in a different tributary, in which Rivulus first encountered a second species after being the only species present upstream of this pool. The expected density was based solely on the physical characteristics of the pool and represented the expected Rivulus density purely from physical attributes, as if Rivulus were the only species present. Comparison of these expected densities with the actual Rivulus densities revealed a consistent negative impact of the presence of other species on Rivulus density, in support of Prediction 1. Overall, actual Rivulus densities at points of contact with other species were about one-third of the density expected if the other species were absent.The distribution of Rivulus was essentially complementary to that of the piscivorous fish, although only one site provided an unambiguous test of Prediction 2, since Hoplias did not ascend the tributaries in this watershed. The size distribution of Rivulus in populations occurring in pools with predators was dominated by small individuals, with none larger than the median size of Rivulus in predator-free populations, supporting Prediction 3. Finally, the competition experiments revealed that adult Poecilia depressed the growth rate of young Rivulus.We conclude that interspecific interactions are important in structuring the geographical distribution and a...

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.Abstract. Utilizing optimal foraging theory and laboratory estimates of foraging costs, we predict the choice of foods and use of habitats by fish in the field. These predictions are tested with the bluegill sunfish (Lepomis macrochirus) foraging in three habitats (open water, sediments, and vegetation) in a pond. Relations describing prey encounter rates in each habitat as a function of prey size, prey density, and fish size were derived from laboratory experiments. These relations permitted us to estimate prey encounter rates based on weekly prey samples in each habitat of the pond. We then determined the optimal diet and profitability (net energy return) for each habitat through time.Predictions of optimal diet exhibited good qualitative correspondence to the actual diet of the fish in the open water and vegetation, although we consistently predicted a slightly narrower diet than the fish were choosing. The model correctly predicted the magnitude of the change in size selection on Daphnia pulex with fish size and with decline in prey density. Predictions of optimal diet in the sediments were considerably in error apparently due to a tendency for late-instar midges to burrow deep in the sediments, thereby becoming unavailable to the fish. In this case habitat profitabilities were computed simply on the basis of the actual observed diet.Predictions of optimal habitat use, i.e., when the fish should switch habitats to maximize feeding rates, showed striking correspondence to the actual habitat use of the fish; the bluegills switched from feeding in the open water column to feeding from the sediments within a few days of our predictions. The actual habitat use pattern differs dramatically from a null model of random habitat use. We indicate how this approach may be useful in studying intra-and interspecific exploitative interactions. All use subject to JSTOR Terms and Conditions 1526 EARL E. WERNER ET AL. Ecology, Vol. 64, No. 6itats. Net energy return rates for the different habitat types are then calculated, which enable us to quantify habitat profitability and predict optimal habitat use. Finally, we show that temporal habitat switches by the fish conform to our predictions based on the premise that they maximize energetic gain. These tests of optimal habitat use were performed in the context of a larger experiment evaluating the influence of foraging profitability and predation risk on habitat use by three size-classes of the bluegill. A companion paper (Werner et al. 1983) examines the bluegills' behavior under predation risk. The current paper concerns the foraging relations of the bluegill, with respect to changing resource levels. EXPERIMENTAL DESIGN AND METHODS The experime...

Using a mark-recapture technique in a small temperate stream, we described the movement of four fish species over a five-month period and developed a mathematical model that described the observed movement patterns. The movement distributions were generally leptokurtic, and two of the four species demonstrated some degree of upstream bias. There was little difference in movement among species or through time. There were no temporal correlations in probability of movement, movement direction, or distance moved. The spatial spread of the most abundant species, bluehead chubs, over a four-month period was characterized by upstream bias, diffusion-like spread, and persistent leptokurtosis. Bluehead chubs demonstrated complex relationships between probability of movement and size and growth, while creek chubs showed only an effect of size on probability of movement. Further, growth of individual bluehead chubs was correlated through time. These empirical results suggest the hypothesis that heterogeneity in phenotypic attributes, such as size and growth, is related to heterogeneity in movement behavior.A diffusion-advection model of bluehead chub movement, structured with two subgroups that dispersed at different rates (''fast fish'' and ''slow fish''), was parameterized and validated with the field data. This model with heterogeneity in movement rates generated the leptokurtic pattern observed in the field data, in contrast to the classic diffusion model without population heterogeneity, which produces a normal distribution.The results from this work suggest that heterogeneity in fitness-influencing attributes such as size and growth could explain heterogeneity in individual-level movement behavior and might underlie the leptokurtic patterns that have been observed at the population level in numerous field studies.