Preference Curves Research Articles

Habitat preferences of European grayling in a medium size stream, the Ain river, France

The three grayling Thymallus thymallus age‐classes had strong preferences for their local habitat in the Ain river. All age‐classes preferred high water velocities between 70 and 110 cm s‐1, confirming that grayling is a typical rheophilic species. An important intraspecific segregation regarding depth occurred, as large individuals preferred deeper water than small ones (optimal ranges=50–60, 80–120 and 100–140 cm for 0+, 1+ and adults respectively). The three age‐classes had similar preferences for small substratum articles, with optimal values between 0·5 and 16·0 mm. Preference curves were generally comparable between the two sites, despite important differences in habitat availability. It is emphasized that grayling needs various habitat conditions to achieve its entire life cycle. The increasing scarcity of this species underlines the urgency of protecting lotic habitat integrity and diversity.

Microhabitat Use by Age-0 Brown Trout and Grayling: Seasonal Responses to Streambed Restoration under Different Flows

We examined microhabitat selection by age-0 brown trout Salmo trutta and grayling Thymallus thymallus in experimental flumes with channelized (monotonic configuration and low amount of stones) or restored (highly heterogenous substrate) streambed structures at low and high flows and at different times of year. Both brown trout and grayling used higher water velocities in summer high flows. Both species also occupied higher-velocity microhabitats in channelized than in restored flumes, but for grayling this difference was significant only in summer. Overall, brown trout used lower water velocities than grayling. In winter and in high-flow treatments, brown trout were more susceptible to downstream displacement in channelized compared with restored flumes. For grayling, the effect of flume type occurred only in winter when fish were located further downstream in channelized flumes. Both species were more aggregated in winter. Brown trout also exhibited a different spatial pattern among the flume types, being more aggregated in channelized flumes. The flume type did not affect the spatial pattern of grayling, but they were more contiguously distributed in high flows. Because both brown trout and grayling used lower velocities in winter, the potential for interspecific competition may increase in winter. The spatiotemporal differences in fish habitat preferences suggest that assessment of the physical carrying capacity of a stream in relation to species-specific habitat requirements is a prerequisite for effective management of brown trout and grayling populations. Preference curves indicated that the habitat requirements were narrowest in winter and that the crucial habitat factor was the availability of flow refuges; thus, the curves helped identify the timing of, and the key factor underlying, a potential habitat bottleneck in northern boreal streams.

Evaluation of Therapeutic Strategies: A New Method for Balancing Risk and Benefit

Objective A patient-specific drug safety-efficacy index was developed that combined objective clinical trial information about dose-related efficacy and toxicity with subjective perspectives on efficacy-toxicity trades. Methods Patient preferences were systematically assessed using the probability tradeoff technique (PTT). Toxicity ranges over which a drug's efficacy exceeded the patient's minimally acceptable efficacy represented ranges of “surplus efficacy.” These can be related to the dose interval in which a drug delivers this surplus efficacy. Seventy surplus efficacy functions (for 7 hypothetical drugs and 10 hypothetical preference curves) were simulated. Results The analysis showed that index values change markedly by dose and patient preference, suggesting that different patients will benefit from different drugs depending on the dose prescribed and each patient's subjective assessment of the efficacy/toxicity tradeoff. In most situations, drugs achieve positive surplus efficacy only over limited dose ranges. The model was sensitive to different preference curves and discriminated well among drugs with different efficacy or safety profiles. Conclusion This index provides a new, systematic approach to choosing a specific therapeutic intervention and dosage, when known risks and benefits are reconciled against patient-specific preferences among an array of therapeutic alternatives.

Densities of juvenile brown trout (Salmo trutta) in two subarctic rivers: assessing the predictive capability of habitat preference indices

We examined the applicability of habitat preference indices for predicting density variations of age-0 brown trout (Salmo trutta) in two rivers in northern Finland. Trout densities in these rivers were monitored for 7 or 8 years. Habitat suitability for trout fry was assessed using summer and winter preference curves for water velocity, depth, and substrate. Substrate suitability indices based on summer preference curves explained 21-74% of the among-site variation in trout densities. The negative effect of high discharge on trout abundance was best predicted by the composite depth × substrate index. Sites with the highest "apparent survival" (density of age-1 trout in year t versus density of age-0 trout in year t - 1) produced high indices when winter substrate curves for age-0 trout were used, indicating high immigration rates to these sites. Our study shows that when preference indices are used for predictive purposes, the mechanisms underlying habitat "bottlenecks" must be known. In boreal areas, winter presents a bottleneck period for juvenile salmonids, and we stress the importance of using winter habitat curves when habitat hydraulic models are applied to areas with severe winter conditions.

Preference and the contextual basis of ideals in judgment and choice.

Four experiments assessed preferences for schematic faces. In Experiment 1, eye gap and nose width were manipulated separately, and effects of shifting the range of values were assessed. Descriptive ratings of width showed contrast effects in accordance with A. Parducci's (1995) range-frequency theory. Evaluative ratings of pleasantness showed reversals of preference ordering that were modeled as shifts in ideal points toward the means of the contextual distributions. In Experiments 2 and 3, similar effects of context on preference were demonstrated in a trinary-choice task in which faces varied only in eye gap. In Experiment 4, eye gap and nose width were manipulated together, resulting in systematic contextual shifts of the ideal face within the 2-dimensional attribute space. The results demonstrated the pervasive effects of context on the construction of ideals determining preference and underlying attitudes. In many preference domains, more is better. For example, when all other attributes are held constant, increasing monetary gains for an alternative increases preference strength for that alternative. In such domains there is a monotonic relationship between degree of preference and the values on the underlying stimulus dimension. However, a nonmonotonic relationship between preference and value is observed in many other domains, often characterized by a single-peaked preference curve in which the ideal lies at an intermediate value between extremes. For example, with increasing sugar concentration, the pleasantness of the tastes of soft drinks initially increases and then decreases (Moskowitz, Kluter, Westerling, & Jacobs, 1974), capturing the idea that a drink can be too sweet or not sweet enough. Such single-peaked preference curves are described by Coombs's (1964) ideal-point theory of preference, in which preference is determined by the similarity of the stimulus to an ideal. Monotonic preference curves occur when the ideal is at one extreme or the other, and single-peaked curves occur when the ideal is located at an intermediate value

河川における魚類生息場の評価法に関する研究

This study proposes a new method to evaluate fish habitat in rivers. IFIM established in the United States is the most popular method but there are some problems in its application to rivers in Japan. The problems are owing to the difference of river environments between Japan and the United States. Compared to the United States, rivers are short, river environments change rapidly, and river ecosystem is complicated in Japan. So the evaluation method of fish habitat must be different. Proposed new method includes two main processes. The first is to make bar graphs from fish preference curves. The second is to evaluate the river environment not for only one species but for several species of fish. At the end of this study, one concept is proposed about how to take the river ecosystem in consideration in river planning.

PREDICTING FISH HABITAT USE TO CHANGES IN WATER FLOW: MODELLING CRITICAL MINIMUM FLOWS FOR ATLANTIC SALMON,SALMO SALAR, AND BROWN TROUT,S. TRUTTA

To identify possible critical minimum flows, habitat availability and habitat selection by young Atlantic salmon and brown trout on selected stream reaches were studied in a spatially and temporally heterogeneous Norwegian west coast river. Based on direct underwater observations of 1768 individuals across transects on each reach, unimodal habitat suitability curves for water depth, water velocity and substrate were produced for both species. There was considerable spatial niche overlap between the two species, suggesting niche competition. Rather than narrow optima, they selected habitats within broad ranges of values of the measured microhabitat variables. Atlantic salmon used the broader range. Spatial variation in habitat use was attributed to different habitat availabilities, whereas temporal variations were partly explained by varying water flows and temperature. Variation in hydrophysical conditions, i.e. habitat availability, was quantified by measuring in the same transects at different water flows. A new hydraulic simulation model, the RIMOS (RIver MOdelling System), was developed to allow for hydraulic simulation of habitat availability at a spatial and temporal scale relevant to the fish. By comparing spatial variations in habitat use and availability, habitat preference curves were calculated and applied in the model. Modelling of the availability of suitable habitats from extreme summer minimum flows to 30 year high flows identified critical minimum flow levels, below which suitable habitat was drastically reduced. These values depended on the habitat variable, fish species, temperature and spatial heterogeneity in the habitat. The negative effects of reduced water flow on fish habitat suitabilities were most pronounced for water velocities and for Atlantic salmon. Thus through hydraulic modelling combined with fish habitat preference data, it was possible to predict a likely change in species composition. Habitat–hydraulic modelling also indicated that resilience towards reduced water flows depended on the in situ stream structure and was much greater in pool-like stream reaches than in riffle/rapids areas. Modelling also allowed for time series analysis of fish habitat suitability to identify temporal bottlenecks in available spatial niches. Furthermore, some exploratory modelling was carried out to identify landscape phenomena in fish habitat use. Habitats were considerably more fragmented in riffle/rapids habitats than in pools, where three-dimensional connectivity tended to be complete over a wider range of flows. It is concluded that fish habitat selection data combined with hydraulic modelling at a scale relevant to the fish can be a useful tool in stream management. However, there is no such a thing as ‘the’ suitable minimum flow; the effect of reduced flows will vary with stream structure, the hydro-physical variables in question and the fish species. More studies are needed to elucidate possible spatial and in particular temporal variations in fish habitat selection. Care must be taken in aggregating habitat suitability data into single-valued functions.

Habitat requirements of black mudfish (Neochanna diversus) in the Waikato region, North Island, New Zealand

Abstract Black mudfish (Neochanna diversus) were found at 39 of 80 sites in the Waikato region, New Zealand, ranging from large wetlands to small swampy streams. Of the sites with mudfish, 87% were dry at some time during summer. Sites with mudfish also generally had emergent and overhanging vegetation and tree roots, and showed low to moderate human impact. Black mudfish coexisted at some sites with juvenile eels or mosquitofish, but were absent from all sites with common bullies (Gobiomorphus cotidianus) or inanga (Galaxias maculatus). Sites with mudfish had almost exclusively semi‐mineralised substrates or peat; only one site had mineralised substrate. Geometric mean catch rate for the 39 sites with mudfish was 0.70 fish per trap per night. Mean summer water depth was only 2.1 cm at sites with mudfish, compared to 22.6 cm at 41 sites without. Winter and maximum water depths were also less at sites with mudfish than at sites without mudfish. Mean turbidity was 11.5 nephelometric turbidity units (NTU) at sites with mudfish, but 21.3 NTU at sites without mudfish. Mudfish catch rates were negatively correlated with summer water depth, winter water depth, disturbance scale rating, and turbidity. A discriminant function model based on these variables successfully predicted 95% of the sites with mudfish. Habitat preference curves are also presented.

PREDICTING FISH HABITAT USE TO CHANGES IN WATER FLOW: MODELLING CRITICAL MINIMUM FLOWS FOR ATLANTIC SALMON, SALMO SALAR, AND BROWN TROUT, S. TRUTTA

To identify possible critical minimum flows, habitat availability and habitat selection by young Atlantic salmon and brown trout on selected stream reaches were studied in a spatially and temporally heterogeneous Norwegian west coast river. Based on direct underwater observations of 1768 individuals across transects on each reach, unimodal habitat suitability curves for water depth, water velocity and substrate were produced for both species. There was considerable spatial niche overlap between the two species, suggesting niche competition. Rather than narrow optima, they selected habitats within broad ranges of values of the measured microhabitat variables. Atlantic salmon used the broader range. Spatial variation in habitat use was attributed to different habitat availabilities, whereas temporal variations were partly explained by varying water flows and temperature. Variation in hydrophysical conditions, i.e. habitat availability, was quantified by measuring in the same transects at different water flows. A new hydraulic simulation model, the RIMOS (RIver MOdelling System), was developed to allow for hydraulic simulation of habitat availability at a spatial and temporal scale relevant to the fish. By comparing spatial variations in habitat use and availability, habitat preference curves were calculated and applied in the model. Modelling of the availability of suitable habitats from extreme summer minimum flows to 30 year high flows identified critical minimum flow levels, below which suitable habitat was drastically reduced. These values depended on the habitat variable, fish species, temperature and spatial heterogeneity in the habitat. The negative effects of reduced water flow on fish habitat suitabilities were most pronounced for water velocities and for Atlantic salmon. Thus through hydraulic modelling combined with fish habitat preference data, it was possible to predict a likely change in species composition. Habitat–hydraulic modelling also indicated that resilience towards reduced water flows depended on the in situ stream structure and was much greater in pool-like stream reaches than in riffle/rapids areas. Modelling also allowed for time series analysis of fish habitat suitability to identify temporal bottlenecks in available spatial niches. Furthermore, some exploratory modelling was carried out to identify landscape phenomena in fish habitat use. Habitats were considerably more fragmented in riffle/rapids habitats than in pools, where three-dimensional connectivity tended to be complete over a wider range of flows. It is concluded that fish habitat selection data combined with hydraulic modelling at a scale relevant to the fish can be a useful tool in stream management. However, there is no such a thing as ‘the’ suitable minimum flow; the effect of reduced flows will vary with stream structure, the hydro-physical variables in question and the fish species. More studies are needed to elucidate possible spatial and in particular temporal variations in fish habitat selection. Care must be taken in aggregating habitat suitability data into single-valued functions.

5481873 Hydraulic actuating system for a fluid transfer apparatus

Network planning models, which forecast the profitability of airline schedules, support many critical decisions, including equipment purchase decisions. Network planning models include an itinerary choice model that is used to allocate air total demand in a city pair to different itineraries. Multinomial logit (MNL) models are commonly used in practice and capture how individuals make trade-offs among different itinerary attributes; however, none that we are aware of account for price endogeneity. This study formulates an itinerary choice model that is consistent with those used by industry and corrects for price endogeneity using a control function that uses several types of instrumental variables. We estimate our model using a database of more than 10 million passenger trips provided by the Airlines Reporting Corporation. Results based on Continental U.S. markets for May 2013 departures show that models that fail to account for price endogeneity overestimate customers’ value of time and result in biased price estimates and incorrect pricing recommendations. The size and comprehensiveness of our database allows us to estimate highly refined departure time of day preference curves that account for distance, direction of travel, number of time zones traversed, departure day of week and itinerary type (outbound, inbound or one-way). These time of day preference curves can be used by airlines, researchers, and government organizations in the evaluation of different policies such as congestion pricing.

A STUDY ON METHODOLOGY OF FISHES' PHYSICAL HABITAT

This paper presents a new method for evaluating fish habitat environments. The proposal method is especially effective in the region like Japan where environments are various in each section along the river, and there are various fish species. Filed observation data was analyzed by the principal component analysis method, mutual relation between environmental factors and the amount of fish habitat was clarified. And, this analysis result explains fishes' actual modes of life of each fish species. That is, how which environmental factors exert the influence on the amount of fish habitat is shown. Authors adapt the characteristic of such a principal component analysis for our proposal method by weighting factors according to the size of the vector of the factor loading. The method of fish habitat potential was calculated by using the weighted principal factor method and fishes' preference curves which are used also with the IFIM. In addition, a methodology of Choquet integral is able to evaluate fish habitat environments comprehensively by regarding the factor loading as the fuzzy measure.

The influence of potential predators on the habitat preferenda of emerging brown trout

Much research has focused on the developmental behaviour of fish and it has been shown that their sensory and physical capabilities evolve very quickly during their early life. Thus, ontogenesis could influence fishes preferences for particular environmental factors. Little is known about the habitat preferences of trout during the post‐emergence phase and it is not known if they correspond to the preference curves established by the PHABSIM method for the‘ alevin phase‘. Here, the downstream movement and habitat preferences of young emerging trout were studied in a flume. In the absence of predators, alevins preferred a water depth of 20 to 30 cm and pebble rather than gravel substratum. When emergence occurred in an area with 1+ trout and sculpin, Cottus gobio, almost all the emergent trout remained cryptic. When visible, most of them were in the shallowest area (10cm depth) where their preference for pebble substratum was less marked. The presence of 1+ trout and sculpin increased the movement downstream of young trout by 20% without changing the general and diel patterns of catches. Their presence also reduced the initial growth of 0+ trout.

Periphyton responses to a hydraulic gradient in a regulated river in New Zealand

SUMMARY1. Periphyton species composition, chlorophyll a concentration, organic matter biomass, and metabolic activity were analysed at a site in a regulated river with low nutrient concentrations to investigate population and community level responses to a spatial gradient in hydraulic conditions. The communities were dominated by diatoms over the full hydraulic range [0.1–0.5m depth, 0.1–1.5ms−1 velocity, and 0.01–1.5 Froude number (Fr)] with Cymbella kappii, Synedra ulna, and Gomphoneis herculeana having the highest relative biovolumes.2. Unexpectedly, Cymbella kappii and Synedra ulna were abundant or co‐dominant at all levels of velocity and Fr. Gomphoneis herculeana was most abundant within the mid‐velocity range (velocity = 0.8–1.2ms−1).3. The physiognomy of the communities changed with a change in hydraulic conditions. There was progressively more diatom mucilage as velocity and Fr increased which resulted in a macroscopic change from relatively thin films (1–2mm) at low velocities and Fr (in pool habitats), to thick (approximately 10mm) mats at higher velocities and Fr (in riffle habitats). Associated with this, ash‐free dry mass (APDM) increased strongly, but chlorophyll a concentration did not, resulting in a decrease in % chlorophyll a over the gradient.4. The results of experiments conducted in an in situ benthic chamber showed no significant differences in gross primary production between two communities of different biomass, but indicated a slightly decreasing trend with increasing velocity (0.14 to 0.38 ms−1). In contrast, community respiration increased greatly with mat biomass and also as a function of increased water velocity. The combined AFDM, chlorophyll a and metabolic results indicated that the zone of photosynthesis was maintained at the surface of the mats, with variable amounts of mucilage being secreted below, depending on hydraulic conditions.5. The diatom community had considerable physiognomic plasticity through the accumulation of mucilage. It is suggested that this high mucilage secretion may be an active feedback mechanism to help ameliorate environmental stress. If so, present theory on velocity control of periphytic algal development in streams, which is essentially based on passive responses, needs expanding. It is concluded that the response of periphyton to spatial differences in habitat hydraulics in streams is highly complex and it may be difficult to define clear hydraulic habitat preference curves for periphyton communities in nutrient‐poor streams.

Predicting Microdistributions of Steelhead (Oncorhynchus mykiss) Parr from Depth and Velocity Preference Criteria: Test of an Assumption of the Instream Flow Incremental Methodology

We tested an assumption of the Physical Habitat Simulation of the Instream Flow Incremental Methodology (IFIM) that fish select microhabitats based on the quality of one or several hydraulic conditions. We developed preference curves for juvenile steelhead (Oncorhynchus mykiss) in Morse Creek, Washington, USA, that accounted for availability of depths and velocities and their utilization by steelhead parr. To allow comparison of intervals among preference curves from different studies, we developed preference indices. We then evaluated the relationship between steelhead parr density and preference or preference indices for depth, velocity, and depth and velocity combined using an independent data set from a different year and an adjacent location in Morse Creek; these indices reflected observed densities of steelhead parr. There was a significant rank correlation between steelhead parr density and preferences or preference indices of steelhead parr for velocity alone and for depth and velocity combined, but not for depth alone. Steelhead parr strongly avoided habitat in which depth preference was 0.0, but velocity preference appeared to influence use of habitat where depth preference was not 0.0. Steelhead parr avoided cells with low preference indices and preferred cells with high preference indices. These relationships support an assumption of the IFIM.

Flow preferences of larval Chironomidae (Diptera) in Tongariro River, New Zealand

Abstract Flow preferences of five major chironomid taxa (Cricotopus, Orthocladiinae sp. a, Eukiefferiella, Tanytarsus vespertinus, and Maoridiamesa) were investigated in December 1990 at three sites in Tongariro River, central North Island, New Zealand. Exponential polynomial preference curves indicated that most taxa were found predominantly in water ≤ 0.5 m deep when data from all sites were combined (range of conditions sampled = 0.1–1.5 m). Cricotopus and Orthocladiinae sp. a appeared to have broad velocity preferences over the range sampled (0.1–1.8 m s−1 ). In contrast, T. vespertinus had a velocity optimum of 0.1 m s−1, and Maoridiamesa had an optimum > 1.0 m s−1 at all sites. Abundances of Eukiefferiella were greatest over an intermediate range of velocity conditions (0.7–1.5 m s−1 ). Mechanisms leading to differences in velocity optima amongst chironomid taxa may include differential oxygen requirements, and changes in food supply. When determining optimal minimum flows for benthic chironomid communities, it may be sufficient to focus on the requirements of taxa such as Maoridiamesa and Tanytarsus vespertinus which appeared to have well‐defined and disparate velocity preferences in the main river.

A note on the electorate's most preferred inflation rate

Nordhaus (1975) and MacRae (1977) have articulated theories of the political business cycle which allow one to estimate an inflation rate that is optimal in the sense that it will maximize the chance of a political party remaining in power. Central to the notion of a political business cycle is a preference function or iso-vote loss curve which reflects the willingness of the electorate to endure extra unemployment for the sake of a lower inflation rate. The authors assume that this tradeoff function is well behaved and can be approximated by a parabola or half circles. The electorate's most preferred inflation rate can be defined as that rate which will maximize the chance of an incumbent political party getting reelected for any possible variation in the unemployment rate. It is also the inflation rate which would presumably prevail in a democratic society if unemployment were not a problem. In constructing their voter preference curves involving a tradeoff between inflation and unemployment Nordhaus and MacRae both assumed that price stability, other things equal, is preferred by the electorate to either inflation or deflation. Alan Greenspan, has indicated that he and other governors of the Federal Reserve share this view and are willing to support a bill sponsored by Representative Stephen Neal which would require the Fed to pursue polices aimed at eliminating inflation within five years (Wessel, 1989). When parabolas representing a non "turnover contour" curve are fit to the election year inflation and unemployment rates which encompass all those cases where the incumbent political party did not loose the presidential election, however, the conclusion is that the most preferred inflation rate is positive and has been trending upward over time. The basic data on inflation, unemployment and the outcome of presidential elections from 1920-88 are presented in Table 1. For the entire period one can encompass all those non turnover cases where the incumbent political party did not loose the election by fitting a simple parabola to the data points for 1928, 1936 and 1988. Where U is the unemployment rate and P is the inflation rate,

Temporal shifts in physical habitat of the crayfish, Orconectes neglectus (Faxon)

A total of 300 samples was collected from February 1985 to August 1986 in a medium order Ozark Mountain stream. Physical habitat measurements of temperature, mean water column velocity, depth, and substrate character were recorded for each of the 25 monthly samples along with length and sex of all individuals of Orconectes neglectus (Faxon). Analysis of habitat utilization and suitability (or preference) was conducted using exponential polynomial models of hydraulic stress models. There appeared to be equal preference for depth over the range measured. Both substrate and velocity preference curves were bimodal with each mode designating certain crayfish size classes. Young-of-the-year were found primarily in cobbled, high velocity areas while adults were found in low velocity, macrophyte beds. Utilization curves for laminar sublayer thichness also reflected size-dependent phenomena where young-of-the-year were found in thin sublayer areas and adults were found primarily in thick sublayers. When separated by time and size, adults were found to occupy higher velocity, cobbled habitats during at least two months. This time period corresponded with the time of egg-bearing and further analysis yielded a time-dependent habitat suitability surface which accounted for this movement pattern. We suggest that the application of these suitability surfaces, which reflect habitat changes during the annual life cycle, will produce more accurate predictions of density and will allow better habitat management decisions under various regulated flow scenarios.

Status of Instream Flow Legislation and Practices in North America

Abstract This paper presents the results of two nonstatistical surveys (completed in 1981 and 1986) that solicited information from state and federal agencies concerning instream flow issues and practices in North America. Forty-six states and 12 Canadian provinces responded to the survey. Fifteen of the 46 states reported legislative recognition of instream flows for fisheries protection. In Canada, individual provinces generally lacked instream flow legislation, although federal legislation existed that could be used. The most commonly applied method (in use in 38 states or provinces) for assessing instream flow requirements, as reported in the survey, was the Fish and Wildlife Service Instream Flow Incremental Methodology (IFIM). Major research needs cited by survey respondents included (1) more species habitat information and preference curves, (2) techniques for determining instream flows for atypical conditions, and (3) testing of fish habitat:flow: production relationships.

Prey size selection by carnivorous stoneflies1

To investigate prey size preference by carnivorous stoneflies, we conducted choice experiments and directly observed predatory behavior in laboratory streams. Small, medium, and large size classes of Hesperoperla pacifica (Perlidae), small Kogotus modestus, and medium and large Megarcys signata (both of the Perlodidae) were offered an array of sizes of the mayfly Baetis as prey. Small predators displayed strong positive selection for the smallest size class of prey, and preference declined steadily with increasing prey size. Large predators displayed hump‐shaped preference curves, exhibiting strongest preference for intermediate size classes of prey. Thus size selectivity was demonstrated and varied with the size class of predator.Direct observations of encounters between predators and their prey revealed that percent attacks/ encounters by small predators was strongly biased toward small prey and by large predators was weakly biased toward large prey. Capture success always was greater and handling times shorter with small prey compared to large prey.

Prey Preference of Stoneflies: Sedentary vs Mobile Prey

We investigated the effects of prey size and type (sedentary vs mobile) on prey preference in a predaceous stonefly, based on choice experiments and observations of the predator-prey interaction. We presented three size classes of black fly larvae (Prosimulium) to various sizes of the perlid stonefly Hesperoperla pacifica in laboratory experiments. Analysis by preference indices (a, e) indicated that small predators preferred small prey, large predators preferred large prey, and H. pacifica of intermediate size exhibited a weak, non-significant trend toward preferring prey of intermediate size. These results using sedentary prey, black fly larvae, are compared to previous results using a mobile prey, the mayfly Baetis. Encounter, attack, capture rates and handling times differ as a function of predator size, prey size, and prey type. The results of preference trials generally can be predicted from differential encounter, attack and capture rates, and similar preference curves may have different underlying causes. Small H. pacifica preferred small prey, mainly due to a much higher capture success. Large H. pacifica preferred large prey, due principally to higher encounter and attack rates. Offered a choice between Baetis and Prosimulium, stoneflies exhibited lower encounter and attack rates with Prosimulium, but much higher capture success, resulting in a net preference for black fly larvae. We suggest that laboratory results for attacks, captures and handling time can reasonably be extrapolated to animals in nature, thus lending insight into preference estimates based on gut analysis. However, encounter rates inevitably are influenced by details of the laboratory design and may bear little relation to nature.