Article

Mechanisms of drift-feeding behavior in juvenile Chinook salmon and the role of inedible debris in a clear-water Alaskan stream

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Abstract

Drift-feeding fish are challenged to discriminate between prey and similar-sized particles of debris, which are ubiquitous even in clear-water streams. Spending time and energy pursuing debris mistaken as prey could affect fish growth and the fitness potential of different foraging strategies. Our goal was to determine the extent to which debris influences drift-feeding fish in clear water under low-flow conditions when the distracting effect of debris should be at a minimum. We used high-definition video to measure the reactions of drift-feeding juvenile Chinook salmon (Oncorhynchus tshawytscha) to natural debris and prey in situ in the Chena River, Alaska. Among all potential food items fish pursued, 52 % were captured and quickly expelled from the mouth, 39 % were visually inspected but not captured, and only 9 % were ingested. Foraging attempt rate was only moderately correlated with ingestion rate (Kendall’s τ = 0.55), raising concerns about the common use of foraging attempts as a presumed index of foraging success. The total time fish spent handling debris increased linearly with foraging attempt rate and ranged between 4 and 25 % of total foraging time among observed groups. Our results help motivate a revised theoretical view of drift feeding that emphasizes prey detection and discrimination, incorporating ideas from signal detection theory and the study of visual attention in cognitive ecology. We discuss how these ideas could lead to better explanations and predictions of the spatial behavior, prey selection, and energy intake of drift-feeding fish.

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... Chief among these was that the model overestimated both prey capture rate and gross energy intake (GEI) by a factor of two, due largely to poor predictions of prey detection probabilities. This finding has subsequently been supported by other researchers (Piccolo et al. 2008a;Neuswanger et al. 2014). Hence improving our understanding of prey detection by drift-feeding fish is an important direction for further theoretical work. ...
... Hughes et al. (2003) opined that a maneuver model might be essential for realistically modelling prey detection and interception; Hughes and Kelly's (1996) study was a first step in that direction. Hughes et al.'s (2003) model is now over 10 years old, and there is a need to revise it using recent findings (e.g., Piccolo et al. 2007Piccolo et al. , 2008aPiccolo 2011, 2012;Neuswanger et al. 2014). At the time of his premature death N. Hughes had made substantial progress, working with colleagues R. Dukas and L. Dill, in developing new models of prey detection (incorporating information processing limitations) and the geometry, dynamics, and energetics of prey interception, to remedy the deficiencies in current drift-foraging models. ...
... More recently, experimental studies have been undertaken to specifically address limitations and assumptions of models like Hughes and Dill (1990) and Hill and Grossman (1993). These include experiments and model development on the effects of the following on prey detection and interception: water depth (Piccolo et al. 2007), velocity and suspended debris/detritus (O'Brien and Showalter 1993;O'Brien et al. 2001;Piccolo et al. 2008a;Neuswanger et al. 2014), light intensity (Aksnes and Giske 1993;Aksnes and Utne 1997;Metcalfe et al. 1997), turbidity (Barrett et al. 1992;Gregory and Northcote 1993;Sweka and Hartman 2001;Taylor and Rosenfeld 2003;Zamor and Grossman 2007), and temperature (Watz and Piccolo 2011;Watz et al. 2012). Advances in understanding swimming costs associated with swimming maneuvers and in turbulent flow are also relevant for improving drift-foraging models (Krohn and Boisclair 1994;Hughes and Kelly 1996;Enders et al. 2005;Piccolo et al. 2008b). ...
Article
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In this paper we review drift-feeding models for stream salmonids. We assess their historical development and current state, and we propose areas for future research. Drift-feeding models serve as the critical input for energetics-based habitat selection and habitat quality models, which have recently begun to see widespread use for predicting salmonid distribution, growth and abundance. We use a bibliometric approach to find drift-feeding model publications, especially those citing three landmark papers that began the quantification of drift feeding by stream fish (Fausch 1984; Hughes and Dill 1990; Hill and Grossman 1993). Subsequent drift-feeding models have largely been built upon these models. Research effort has focused on model development and applications but model testing has been neglected. To date, the only rigorous test of a drift-feeding model (Hughes et al. 2003) identified several limitations and violations of model assumptions. The most important limitation was that prey capture- and gross energy intake rates were overestimated by a factor of two, due largely to poor predictions of prey detection probabilities. Consequences of error in drift-feeding models, and consequently in the habitat selection/quality models that employ them, are greater for applications aimed at predicting growth and abundance than they are for predicting distribution. Research effort on a broad front is needed to advance both drift-feeding models and habitat selection/quality models, including: further development of drift-foraging theory, revision and testing of drift-feeding models (specifically new, functional prey detection and interception sub-models), and revision of habitat selection/quality models to incorporate spatial, temporal, and flow-dependent variation in drift concentration.
... Chief among these was that the model overestimated both prey capture rate and gross energy intake (GEI) by a factor of two, due largely to poor predictions of prey detection probabilities. This finding has subsequently been supported by other re- searchers ( Piccolo et al. 2008a;Neuswanger et al. 2014). Hence improving our understanding of prey detection by drift-feeding fish is an important direction for further theoretical work. ...
... Hughes et al. (2003) opined that a maneuver model might be essential for realistically modelling prey detec- tion and interception; Hughes and Kelly's (1996) study was a first step in that direction. Hughes et al.'s (2003) model is now over 10 years old, and there is a need to revise it using recent findings (e.g., Piccolo et al. 2007Piccolo et al. , 2008aPiccolo 2011, 2012;Neuswanger et al. 2014). At the time of his premature death N. Hughes had made substantial prog- ress, working with colleagues R. Dukas and L. Dill, in developing new models of prey detection (incorporating information processing limitations) and the geometry, dynamics, and energetics of prey interception, to reme- dy the deficiencies in current drift-foraging models. ...
... More recently, experimental studies have been un- dertaken to specifically address limitations and assump- tions of models like Hughes and Dill (1990) and Hill and Grossman (1993). These include experiments and model development on the effects of the following on prey detection and interception: water depth ( Piccolo et al. 2007), velocity and suspended debris/detritus (O'Brien and Showalter 1993;O'Brien et al. 2001;Piccolo et al. 2008a;Neuswanger et al. 2014), light intensity (Aksnes and Giske 1993; Aksnes and Utne 1997; Metcalfe et al. 1997), turbidity ( Barrett et al. 1992;Gregory and Northcote 1993 Wankowski's (1979) gill raker spacing constraint for small Atlantic salmon. However, in a similar, unpublished test, large rainbow trout in a New Zealand river ate smaller prey than predicted by the same model (J. ...
Conference Paper
Estimating net energy intake (NEI) of is a key requirement in a new suite of models being developed to assess habitat quality for stream fish. To estimate NEI, habitat quality models use a drift-foraging sub-model, typically based on Hughes and Dill’s (1990, CJFAS) well-known model. The Hughes and Dill model estimates the energetic costs and benefits of a fish’s position in the stream based upon swimming costs and prey capture success. The model includes a number of unrealistic assumptions about prey detection and capture, and swimming costs, however, some of which might be addressed through lab or field experiments. Here we present the results of some recent experiments on the effects of water depth and velocity, and cold temperatures, on the foraging success of juvenile salmonids. We demonstrate that prey capture success is reduced by both faster velocities and colder temperatures, and that swimming costs appear to play a minor role in estimating NEI. We also report on the effects of fish species and size. In general, much experimental work remains to be done in the area of drift foraging theory, however, and we will discuss ongoing research and future needs.
... Chinook Salmon embryos and alevins remain in the gravel until early spring when fry emerge to initiate feeding. In the Chena River basin, juveniles are known to rear in main stem habitats, often in the presence of accumulations of large woody debris (i.e., logjams; Perry 2012; Neuswanger et al. 2014;Wipfli et al. 2014). However, evidence suggests that small tributaries (e.g., > 20 km 2 contributing area; State of Alaska 2017) and main stem offchannel habitats (B. ...
... Chinook Salmon embryos and alevins remain in the gravel until early spring when fry emerge to initiate feeding. In the Chena River basin, juveniles are known to rear in main stem habitats, often in the presence of accumulations of large woody debris (i.e., logjams; Perry 2012; Neuswanger et al. 2014;Wipfli et al. 2014). However, evidence suggests that small tributaries (e.g., > 20 km 2 contributing area; State of Alaska 2017) and main stem offchannel habitats (B. ...
... Huntsman, UAF unpublished data) are used extensively by juveniles but not by spawning adults. We chose to focus on tributaries because there is incomplete information for the Chena River basin on the extent of small tributary use by juvenile Chinook salmon and because samples collected for eDNA analysis would not be confounded by the presence of adult Chinook Salmon at sampling sites or upstream (State of Alaska 2017; Neuswanger et al. 2014). ...
Article
Identification and protection of water bodies used by anadromous species are critical in light of increasing threats to fish populations, yet often challenging given budgetary and logistical limitations. Noninvasive, rapid‐assessment, sampling techniques may reduce costs and effort while increasing species detection efficiencies. We used an intrinsic potential (IP) habitat model to identify high‐quality rearing habitats for Chinook Salmon Oncorhynchus tshawytscha and select sites to sample throughout the Chena River basin, Alaska, for juvenile occupancy using an environmental DNA (eDNA) approach. Water samples were collected from 75 tributary sites in 2014 and 2015. The presence of Chinook Salmon DNA in water samples was assessed using a species‐specific quantitative PCR (qPCR) assay. The IP model predicted over 900 stream kilometers in the basin to support high‐quality (IP ≥ 0.75) rearing habitat. Occupancy estimation based on eDNA samples indicated that 80% and 56% of previously unsampled sites classified as high or low IP (IP < 0.75), respectively, were occupied. The probability of detection (p) of Chinook Salmon DNA from three replicate water samples was high (p = 0.76) but varied with drainage area (km2). A power analysis indicated high power to detect proportional changes in occupancy based on parameter values estimated from eDNA occupancy models, although power curves were not symmetrical around zero, indicating greater power to detect positive than negative proportional changes in occupancy. Overall, the combination of IP habitat modeling and occupancy estimation provided a useful, rapid‐assessment method to predict and subsequently quantify the distribution of juvenile salmon in previously unsampled tributary habitats. Additionally, these methods are flexible and can be modified for application to other species and in other locations, which may contribute towards improved population monitoring and management.
... Consistent with Neuswanger et al. (2015), our results suggest increased water discharge in natal streams during the summer following emergence is associated with lower juvenile survival (Figure 3a,b), although discharge also had a low inclusion probability (Figure 3c,d). Neuswanger et al. (2015) hypothesized this negative association with arise from reduced foraging efficiency of juvenile Chinook as a result of increased water turbidity during high flows, though the exact mechanism(s) remain unclear (Neuswanger, Wipfli, Rosenberger, & Hughes, 2014). ...
Article
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Understanding how species might respond to climate change involves disentangling the influence of co‐occurring environmental factors on population dynamics, and is especially problematic for migratory species like Pacific salmon that move between ecosystems. To date, debate surrounding the causes of recent declines in Yukon River Chinook salmon (Oncorhynchus tshawytscha) abundance has centered on whether factors in freshwater or marine environments control variation in survival, and how these populations at the northern extremity of the species range will respond to climate change. To estimate the effect of factors in marine and freshwater environments on Chinook salmon survival, we constructed a stage‐structured assessment model that incorporates the best available data, estimates incidental marine bycatch mortality in trawl fisheries, and uses Bayesian model selection methods to quantify support for alternative hypotheses. Models fitted to two index populations of Yukon River Chinook salmon indicate that processes in the nearshore and marine environments are the most important determinants of survival. Specifically, survival declines when ice leaves the Yukon River later in the spring, increases with wintertime temperature in the Bering Sea, and declines with the abundance of globally enhanced salmon species consistent with competition at sea. In addition, we found support for density‐dependent survival limitations in freshwater but not marine portions of the life cycle, increasing average survival with ocean age, and age‐specific selectivity of bycatch mortality in the Bering Sea. This study underscores the utility of flexible estimation models capable of fitting multiple data types and evaluating mortality from both natural and anthropogenic sources in multiple habitats. Overall, these analyses suggest that mortality at sea is the primary driver of population dynamics, yet under a warming climate Chinook salmon populations at the northern extent of the species’ range may be expected to fare better than southern populations, but are influenced by foreign salmon production. This article is protected by copyright. All rights reserved.
... VidSync software allows calculating 3D positions from stereocamera footages taken in the wild, organizing measurements according to object (e.g., individual fish observation) and associated measurements (e.g., spatiotemporal position or foraging attempt; see Neuswanger et al. 2014 for more detail). Movement and behaviour of every fish within the field of view was monitored until the end of the video session. ...
Article
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Fine-scale space-use of drift-feeding fish is underpinned by an energetic trade-off that makes spatial positioning paramount for fitness, especially in a group context. However, methodologies used to study the space-use of drift-feeding fish are mainly based on direct visual observation, and the accuracy of such an approach can be questionable. Furthermore, previous studies mainly focused on the space-use of territorial salmonids, as they are relatively easy to observe. Here, we use a digital imaging technique to manually extract the spatial position of fish and feeding events in three dimensions (3D), at a very fine spatiotemporal scale, from in situ stereo-video footages. We use a motion model to quantify individual space-use strategies in two dimensions (2D) through the horizontal plane (perpendicular to gravity) and through the vertical plane (parallel to the flow). We study territorial (brown trout, Salmo trutta) and nonterritorial (roundhead galaxiid, Galaxias anomalus) juvenile drift-feeding fish to test this methodology over a broad spectrum of drift-feeding fish space-use. Results show that the methodology used permits the reliable quantification of space-use by territorial and nonterritorial drift-feeding fish and could be used to reveal relevant insights on their respective behavioral ecologies. © 2015, National Research Council of Canada. All rights reserved.
... Water discharge rates at Keswick Dam were found to negatively influence survival of mainstem spawning wild fall-run Chinook, and water discharge in Deer Creek was found to reduce survival of the Deer Creek spring-run population although to a lesser extent (Figure I.7 and Table I.6). While it is reasonable to assume that higher discharge rates could lead to greater access to valuable off-channel rearing habitat, water flow conditions additionally have the potential to influence foraging ability by juveniles through the availability of drifting food sources (Neuswanger et al. 2014). Nonetheless the finding that fall-run Chinook survival was negatively influenced by increased water flow contradicts findings by Stevens and Miller (1983) and Newman and Rice (2002). ...
Technical Report
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This project developed a stage-structured life history model of fall, spring and winter run Chinook salmon, fit this model to available data on salmon stock abundance and environmental conditions, and estimated the impact of the environmental conditions on survival of the different stocks of Chinook salmon. This model was then used to forecast how differences in future climate change, marine conditions or productivity, and water exports would affect the survival of the different stocks of Chinook salmon. We used several statistical techniques to evaluate the relative importance of environmental variables on the survival including both information theoretic approaches and Bayesian approaches. Due to the large number of potential explanatory covariates (59) and the inability to fit all combinations of these covariates, we used Akaike Information Criterion for small sample size (AICc) and a novel method for exploring the model space. The approach used a forward stepwise model building with AICc as the selection criteria. The steps were: 1) fit a null model without any covariate effects to the available data; 2) construct a proposal model by selecting a covariate at random from amongst the set of 59 possible covariates; 3) fit the proposed model to the data; 4) compare the proposal model to the null model; 5) keep proposal model if reduction in AICc value is greater than 2 units; 6) repeat sampling covariates without replacement, fitting the model to data, and evaluating AICc i.e. until all covariates have been tested. Using the information theoretic approaches we found support for environmental impacts of 14 variables including flow, temperature, sediment concentration, export inflow ratios, exports, ocean upwelling, curl and PDO. The top three environmental drivers affecting fall run were export to inflow ratio, spring upwelling south of the Farallon Islands, and the delta gross channel depletion. The top three drivers affecting spring run were size at Chipps Island, export levels, and sediment concentration at Freemont. The three main factors affecting winter-run were minimum flow during fry rearing, temperatures during egg incubation, and spring upwelling south of the Farallon Islands. We then conducted a Bayesian analysis using these 14 variables to calculate the posterior distribution of the impact of these variables on survival. We conducted forward simulations under four different export regimes to understand how management of exports would affect each of the races. Furthermore, we evaluated export management under two different climate scenarios and two ocean productivity scenarios to understand how climate variability and ocean productivity may act in concert with management of exports to affect the three Chinook runs. We developed a harvest model that reflected current management of the Central Valley Chinook stocks in which low levels of winter run escapement can reduce fall run harvest. We found that both climate and exports affected projected survival and the potential recruits per spawner for wild populations. Under current export levels all stocks of spring run would increase across all climate scenarios tested. Winter run would increase except under the most pessimistic of the four climate conditions we evaluated. Mainstem Fall run would have recruits per spawner greater than 1 under the two optimistic climate scenarios and less than 1 under the two pessimistic climate scenarios although the future trend in mainstem fall chinook could be heavily influenced by straying from hatcheries and thus hard to predict. A 30% increase in exports decreased spring and fall stock survival to the point where they would all decline regardless of the climate scenario. A 30% decrease in exports improved survival and recruits per spawner for all stocks. We found spring Chinook stocks to be most sensitive to exports and less sensitive to climate conditions, whereas winter Chinook were more sensitive to climate conditions than exports. We did not evaluate alternative ocean harvest scenarios, although reduction or elimination of ocean harvest would increase survival to spawning and thus contribute to rebuilding in the same way as better climate or reduced exports.
... This foundational work on arctic grayling (Thymallus arcticus) has served as the basis for many subsequent drift-foraging models (see Piccolo et al. 2014). More recent work has focused on incorporating three-dimensional prey reaction volumes, the role of inedible debris, and incorporating prey detection functions thought to be important factors in development of more accurate drift-foraging models (Hughes et al. 2003;Piccolo et al. 2008;Neuswanger et al. 2014). While we have shown that variants of Hughes and Dill's model are sensitive to alternate representations of prey size, future driftforaging models will be similarly sensitive. ...
Article
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Drift-foraging models offer a mechanistic description of how fish feed in flowing water and the application of drift-foraging bioenergetics models to answer both applied and theoretical questions in aquatic ecology is growing. These models typically include nonlinear descriptions of ecological processes and as a result may be sensitive to how model inputs are summarized because of a mathematical property of nonlinear equations known as Jensen’s inequality. In particular, we show that the way in which continuous size distributions of invertebrate prey are represented within foraging models can lead to biases within the modeling process. We begin by illustrating how different equations common to drift-foraging models are sensitive to invertebrate inputs. We then use two case studies to show how different representations of invertebrate prey can influence predictions of energy intake and lifetime growth. Greater emphasis should be placed on accurate characterizations of invertebrate drift, acknowledging that inferences from drift-foraging models may be influenced by how invertebrate prey are represented.
... individual fish observation) and associated measurements (e.g. spatiotemporal position or foraging attempt; see Neuswanger et al. (2014) for details). Movement and behavior of every fish within the field of view was monitored until the end of the video session. ...
Article
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Methodologies enabling the monitoring of animal movement and behavior in 3-dimensions (3D; x, y, z, the latter accounting for the vertical dimension) are becoming increasingly accessible and can be deployed on entire groups of animals inhabiting 3D habitats. When 2-dimensional (2D; x, y) space-use analyses are used on such groups, their spatial organization is represented as a planar projection of individuals’ space-use. Movement on the vertical dimension is ignored and could biased ecological inference made from the spatial structure of the group. We used a digital imaging technique to track movements and feeding behavior of individual animals within a free-range aggregation of juvenile drift-feeding fish (Galaxias anomalus) in 3D and at fine spatiotemporal scales. We estimated spatiotemporal overlap of space-use and feeding territories between group-members using 2D (x, y) and 3D spatial analysis to: (1) describe the spatial structure of the group, (2) identify patterns of resource partitioning, and (3) investigate the relationship between space-use overlap and feeding behavior. We found that overlapping ratios of space-use and feeding territories were over-estimated in 2D, while 3D analysis of space-use provided evidence of spatial partitioning between group-members. We also found that, regardless of the computation used, the overlapping ratios of space-use were positively correlated with overlapping ratios of feeding territories while no effect was found on feeding activity. In conclusion, whilst 3D analysis provided valuable information on the spatial structure of a group, inferences on the ecological function of space-use can also be obtained from 2D analysis.
... For example, during higher water velocities in the Sacramento River, juvenile Chinook salmon were reported to display a more directional migration, suggesting that this was an energy saving behaviour (Steel, Sandstrom, Brandes, & Klimley, 2012). Other studies have reported a negative effect of high river discharge on foraging success because of increased turbidity (Arnekleiv, Finstad, & Rønning, 2006;Enders, Buffin-Bélanger, Boisclair, & Roy, 2005;Neuswanger, Wipfli, Rosenberger, & Hughes, 2014;Neuswanger et al., 2015). In our analysis, however, river discharge did not play an important role accounting for variability in growth rate among any of the data sets. ...
Article
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Estuarine habitats provide rearing opportunities for the juvenile life stage of anadromous fishes. Because survival is positively correlated with juvenile performance, these estuarine habitats play an important role in population abundance and productivity. To provide information for the recovery of several depressed stocks of Chinook salmon in the Columbia River Basin, we sought to identify the factors that explain variability in performance. Using otolith‐derived estimates of juvenile somatic growth rate as an index of recent performance, we observed a negative nonlinear relationship between growth rate and day of year, and a decreasing and increasing trend of growth rate over the 8 years of this study and distance from the river mouth respectively. Using a generalised linear modelling approach, we found that variability in juvenile somatic growth rate was best explained by where and when individuals were collected, their body size, contaminant loads, stock of origin, and whether a fish was hatchery produced or unmarked. Lastly, we argue that a considerable improvement to the growth rate of juveniles in estuarine habitats is physiologically possible. The results of this 8‐year study provide a baseline of the performance of juvenile Chinook salmon to evaluate habitat restoration programs and to compare against future anthropogenic conditions.
... i.e., simulations assume all added prey would be captured. This assumption is probably unrealistic given prior empirical work (Piccolo, Hughes & Bryant 2008;Neuswanger et al. 2014) and given that our exploitation efficiency estimates indicated a significant reduction in the proportion of prey flux consumed at low pool proportions (see Results). To address this issue, we modelled three potential scenarios: first, where no additional prey capture was allowed in enclosures where less than 95% of the observed prey was consumed (i.e., assuming strong constraints on capture success in riffle-dominated enclosures); second, where additional prey was allowed to be consumed at the observed efficiency; and third, where all additional prey was consumed. ...
Article
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1.Increasing habitat availability (i.e. habitat suitable for occupancy) is often assumed to elevate the abundance or production of mobile consumers; however, this relationship is often nonlinear (threshold or unimodal). Identifying the mechanisms underlying these nonlinearities is essential for predicting the ecological impacts of habitat change, yet the functional forms and ultimate causation of consumer‐habitat relationships are often poorly understood. 2.Nonlinear effects of habitat on animal abundance may manifest through physical constraints on foraging that restrict consumers from accessing their resources. Subsequent spatial incongruence between consumers and resources should lead to unimodal or saturating effects of habitat availability on consumer production if increasing the area of habitat suitable for consumer occupancy comes at the expense of habitats that generate resources. However, the shape of this relationship could be sensitive to cross‐ecosystem prey subsidies, which may be unrelated to recipient habitat structure and result in more linear habitat effects on consumer production. 3.We investigated habitat‐productivity relationships for juveniles of stream‐rearing Pacific salmon and trout (Oncorhynchus spp.), which typically forage in low‐velocity pool habitats, while their prey (drifting benthic invertebrates) are produced upstream in high‐velocity riffles. However, juvenile salmonids also consume subsidies of terrestrial invertebrates that may be independent of pool‐riffle structure. 4.We measured salmonid biomass production in 13 experimental enclosures each containing a downstream pool and upstream riffle, spanning a gradient of relative pool area (14‐80% pool). Increasing pool relative to riffle habitat area decreased prey abundance, leading to a nonlinear saturating effect on fish production. We then used bioenergetics model simulations to examine how the relationship between pool area and salmonid biomass is affected by varying levels of terrestrial subsidy. Simulations indicated that increasing terrestrial prey inputs linearized the effect of habitat availability on salmonid biomass, while decreasing terrestrial inputs exaggerated a ‘hump‐shaped’ effect. 5.Our results imply that nonlinear effects of habitat availability on consumer production can arise from trade‐offs between habitat suitable for consumer occupancy and habitat that generates prey. However, cross‐ecosystem prey subsidies can effectively decouple this trade‐off and modify consumer‐habitat relationships in recipient systems. This article is protected by copyright. All rights reserved.
... The low abundance of juvenile Chinook salmon in the high productivity off-channels, particularly when stream temperatures were cold, may also be influenced by the efficiency by which salmon can extract available resources between habitats. Juvenile Chinook salmon typically drift forage (Neuswanger, Wipfli, Rosenberger, & Hughes, 2014) and lack of flow in off-channel habitat would require salmon to become more active in foraging pursuits. This would not only force juvenile salmon to adopt an alternative foraging strategy (floaters; Nielsen, 1992), but also expose salmon to greater predation risk. ...
Article
• Poor growth and survival in freshwater and marine environments have been implicated as responsible for Chinook salmon (Oncorhynchus tshawytscha) declines across Alaska. • Lateral connectivity of river main stems with off‐channel habitats may play an integral role in sustaining Alaskan salmonid populations because off‐channel habitats commonly provide greater growth opportunities than main stem habitats through greater macroinvertebrate productivity and warmer water temperatures. However, off‐channel habitats may impose greater mortality risks to juvenile salmonids, as these habitats are typically more susceptible to drying and are often occupied by potential predators. • We used a hierarchical Bayesian count model to describe juvenile Chinook salmon distributions throughout the Chena River, Alaska in main stem and off‐channel habitats and employed diet, prey availability, and bioenergetic analyses to explain these habitat selection decisions from data collected in the summer of 2015. • We found salmon to be most abundant in off‐channel habitats as summer temperature increased, which suggested that salmon dispersed to off‐channel habitats to take advantage of energetically favourable growth conditions as indicated by the higher prey biomass in benthic and diet samples collected within off‐channel habitats. • Our results could have significant implications for juvenile salmon under a warming Alaskan climate as access to productive off‐channel habitats may be important to offset increased energetic costs as temperature warms.
... To determine the effects of predation risk and resource abundance on energy distribution among individuals and the extent of resource monopolization, we compared the frequency distributions of the total foraging attempts observed across body lengths (mm) before versus after predator exposure and among resource treatment combinations. While foraging attempts as a metric of absolute energy intake is inappropriate given many attempts are likely unsuccessful (Neuswanger et al. 2014), we assume it is a reasonable proxy for the relative energy intake among individuals. We specifically tested the prediction that predation risk and resource abundance should lead to more positively skewed foraging attempt-body size distributions, indicating a higher relative energy intake by smaller individuals. ...
Article
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Dominance hierarchies and the resulting unequal resource partitioning among individuals are key mechanisms of population regulation. The strength of dominance hierarchies can be influenced by size‐dependent tradeoffs between foraging and predator avoidance whereby competitively inferior subdominants can access a larger proportion of limiting resources by accepting higher predation risk. Foraging–predation risk tradeoffs also depend on resource abundance. Yet, few studies have manipulated predation risk and resource abundance simultaneously; consequently, their joint effect on resource partitioning within dominance hierarchies are not well understood. We addressed this gap by measuring behavioural responses of masu salmon Oncorhynchus masou ishikawae to experimental manipulations of predation risk and resource abundance in a natural temperate forest stream. Responses to predation risk depended on body size and social status such that larger fish (often social dominants) exhibited more risk‐averse behaviour (e.g. lower foraging and appearance rates) than smaller subdominants after exposure to a simulated predator. The magnitude of this effect was lower when resources were elevated, indicating that dominant fish accepted a higher predation risk to forage on abundant resources. However, the influence of resource abundance did not extend to the population level, where predation risk altered the distribution of foraging attempts (a proxy for energy intake) from being skewed towards large individuals to being skewed towards small individuals after predator exposure. Our results imply that size‐dependent foraging‐predation risk tradeoffs can weaken the strength of dominance hierarchies by allowing competitively inferior subdominants to access resources that would otherwise be monopolized. This article is protected by copyright. All rights reserved.
... There are variations on this theme, with some fish foraging higher in the water column and at the surface, especially when drift is abundant (e.g. when terrestrial invertebrates are abundant in summer, and when aquatic invertebrates are emerging at dusk). Juvenile Chinook salmon tend to drift feed higher in the water column more often than trout (Glova 1995;Neuswanger et al. 2014). ...
... However, observing predation events in situ is often nearly impossible and, as a result, is often measured in laboratory settings. Predation events that are observed in situ are generally at extremely small spatial or temporal scales (e.g., Neuswanger et al. 2014), making landscape-scale predictions challenging (Hunsicker et al. 2011). ...
Article
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Predator–prey dynamics can have landscape‐level impacts on ecosystems, and yet, spatial patterns and environmental predictors of predator–prey dynamics are often investigated at discrete locations, limiting our understanding of the broader impacts. At these broader scales, landscapes often contain multiple complex and heterogeneous habitats, requiring a spatially representative sampling design. This challenge is especially pronounced in California’s Sacramento–San Joaquin River Delta, where managers require information on the landscape‐scale impacts of non‐native fish predators on multiple imperiled native prey fish populations. We quantified relative predation risk in the southern half of the Delta (South Delta) in 2017 using floating baited tethers that record the exact time and location of predation events. We selected 20 study sites using a generalized random tessellation stratified survey design, which allowed us to infer relationships between key environmental covariates and predation across a broader spatial scale than previous studies. Covariates included distance‐to‐nearest predators, water temperature, turbidity, depth, bottom slope, bottom roughness, water velocity, and distance‐to‐nearest riverbank and nearest aquatic vegetation bed. Model selection determined the covariates that best predicted relative predation risk: water temperature, time of day, mean predator distance, and river bottom roughness. Using this model, we estimated predation risk for the South Delta landscape at a 1‐day and 1‐km resolution. This effort identified hot spots of predation risk and allowed us to generate predicted survival for migrating fish transiting the South Delta. This methodology can be applied to other systems to evaluate spatio‐temporal dynamics in predation risk, and their biotic and abiotic predictors.
... Moderate increases in flow also allow juveniles to access off-channel habitats, which can confer advantages for growth and survival due to favorable temperatures, high invertebrate production, and cross-ecosystem resource subsidies (Baldock, Armstrong, Schindler, & Carter, 2016;Huntsman & Falke, 2019;Rine, Wipfli, Schoen, Nightengale, & Stricker, 2016;Sommer, Nobriga, Harrell, Batham, & Kimmerer, 2001). At the extreme, however, heavy rains and associated high flows likely increase water velocity, turbidity, or inedible debris densities to levels that reduce drift foraging efficiency (Donofrio, Simon, Neuswanger, & Grossman, 2018;Gregory & Northcote, 1993;Neuswanger, Wipfli, Rosenberger, & Hughes, 2014). ...
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... Second, consider the scenario where ϕ F ki F kj ¼ 0, implying that the feeding rate on j has no impact on the feeding rate on i as might occur for drift-feeding fish that are unlikely to pass up an opportunistic encounter with i even shortly after ingesting j (e.g. Neuswanger et al., 2014). Alternatively, this could arise whenever F kj is effectively Type I over the range of experienced abundances (Novak, 2010). ...
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... While these aspects are certainly important, the visual system and information processing presents an additional layer in our understanding of the dynamics driving foraging success. Work examining the effects of debris on drift-foraging has alluded to the importance of limited visual attention due to limits on cognitive processing and incorporating ideas from the signal processing literature (O'Brien and Showalter 1993;Neuswanger et al. 2014). More fully incorporating these ideas into future work may help to explain patterns in prey selection and further the development and validation of drift-foraging models. ...
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Chapter
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Chapter
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Publication summarises recent issues concerning spawning grounds of rheophilic fish species in Czech rivers. Practical solutions for river managements authorities are offered. It is essential to start dealing with causes of decrease in rheophilic species including support of natural population recruitment using technical solutions on the spawning ground. Publikace shrnuje současné problémy týkající se vhodného prostředí pro rozmnožování reofilních druhů ryb v řekách ČR. Nabízí praktická řešení pro management v povodí, kde se druhy vyskytují. Zejména zdůrazňuje nutnost řešení příčin úbytku reofilních druhů včetně podpory přirozené obnovy populací pomocí technických opatření na trdlišti.
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Assumes that a fish chooses the position that maximizes its net energy intake rate. The fish's habitat is represented as a series of stream cross-profiles, each divided into vertical strips characterized by water depth and velocity. The fish may select a focal point in any of these strips, and include several neighbouring strips in its foraging area. Number of prey the fish encounters depends on its reaction distance to prey, water depth, and water velocity; proportion of detected prey the fish is able to capture declines with water velocity. The fish's net energy intake rate is its gross energy intake rate from feeding minus the swimming cost calculated by using water velocity at the fish's focal point. -from Authors
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In a typical visual search experiment, observers look through a set of items for a designated target that may or may not be present. Reaction time (RT) is measured as a function of the number of items in the display (set size), and inferences about the underlying search processes are based on the slopes of the resulting RT 〈 Set Size functions. Most search experiments involve 5 to 15 subjects perform- ing a few hundred trials each. In this retrospective study, I examine results from 2,500 experimental sessions of a few hundred trials each (approximately 1 million total trials). These data represent a wide variety of search tasks. The resulting picture of human search behav- ior requires changes in our theories of visual search.
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This paper describes an individual-based model of sympatric populations of brown and rainbow trout in a stream habitat. The model provides a tool for projecting flow and temperature effects on trout populations by linking the hydraulic component of the instream flow incremental methodology/physical habitat simulation system (IFIM/PHABSIM) to an individual-based population model. PHABSIM simulates the spatial distribution of depth and velocity at different flows, and indirectly, the availability of spawning habitat, cover and feeding station. The individual-based model simulates reproduction, growth and mortality of individual trout as a function of flow and temperature. Population dynamics arise from the survival and reproduction of individual trout. The spatially explicit nature of the model permits evaluation of behavioral responses used by fish to changes in physical habitat. The model has been calibrated to a stream segment in the North Fork Middle Fork Tule River, California. Selected parameters were adjusted to calibrate the model for length and abundance (including production of a new year class) at the end of 1-year simulations for each of 9 years. Predicted and observed lengths were in good agreement, although neither varied appreciably among years. Predicted and observed abundances were not in as good agreement, and differed considerably for some years. These differences reflect a combination of uncertainties in the field data and uncertainties in the model structure and parameter values. Fifty-year simulations indicated that model projections of length and abundance were stationary, although abundance values fluctuated considerably. Seven advantages for using simulation models of this type are emphasized. How to most effectively interpret results from such simulation models as part of instream flow environmental assessments remains a challenge. Variability and uncertainty in both field data and replicate model simulations are realities that have implications for scientists, resource managers, and regulators in projecting growth and abundance responses of fish populations to alternative flow or temperature regimes.
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We developed and tested a combined foraging and bioenergetics model for predicting growth over the lifetime of drift-feeding brown trout. The foraging component estimates gross energy intake within a fish- and prey size-dependent semicircular foraging area that is perpendicular to the flow, with options for fish feeding across velocity differentials. The bioenergetics component predicts how energy is allocated to growth, reproduction, foraging costs, and basal metabolism. The model can reveal the degree to which growth is limited by the density and size structure of invertebrate drift within the physiological constraints set by water temperature. We tested the model by predicting growth based on water temperature and on drift density and size structure data from postemergence to age 12, and we compared the predictions with observed size at age as determined from otoliths and scales for a New Zealand river brown trout population. The model produced realistically shaped growth curves in relation to the observed data, accurately predicting mean size at age over the lifetime of the trout, assuming 24-h maximum rations and including diurnal drift-foraging costs (predicted versus observed weight r = 0.94; length r = 0.97). The model predicted that, within a given water-temperature regime, growth is limited primarily by reproduction costs but also by increasing foraging costs as trout grow (a phenomenon that is associated with the increasing foraging time that is required in order to feed to satiation on small invertebrate drift prey). Invertebrate drift size structure significantly influenced predicted growth, especially maximum size, through its effect on foraging time. The model has potential in terms of the exploration of growth-limiting factors and has associated use as an environmental-impact tool and as an aid for hypothesis generation in studies of salmonid growth processes.
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To assess the costs and benefits of young fish adopting different behavioural tactics, field studies of juvenile salmonines have assumed that (but did not test whether) the rate of foraging attempts predicts ingestion rate. We tested this assumption by quantifying capture, ingestion, and rejection rates of potential prey items for individual young-of-the-year brook trout (Salvenlinus fontinalis) in a lake. Overall, capture rate (a conservative estimate of the rate of foraging attempts) was only a fair predictor of overall ingestion rate (Kendall's tau=0.54) and only 46% of captured items (number/minute) were ingested. Surface capture rate was a poor predictor of surface ingestion rate (tau=0.27) and only 1% of captured items were ingested. In contrast, subsurface capture rate was an excellent predictor of subsurface ingestion rate (tau=0.75) and 93% of captured items were ingested. No benthic prey captures were observed. Fish that ingested a low proportion of captured items spent a greater proportion of time moving, moved faster, and pursued prey further than fish that ingested a higher proportion of captured items. Rejection of captured items can represent a significant and little appreciated component of the foraging cycle for young salmonid fishes.
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Masquerading organisms appear to closely resemble inedible and generally inanimate objects, such as twigs, leaves, stones, and bird droppings. It has recently been demonstrated that masquerading prey gain protection from predation by being misclassified as inedible objects by their predators. Here, we present the first experimental test of the requirements of effective masquerade. Specifically, we explore whether masquerading prey need to be very similar in size to the “model” objects that they appear to resemble. Using domestic chicks as predators of twig-mimicking caterpillars, we find that matching a model object in size increases protection from predation; however, similarity of appearance without size matching still affords some protection. This study helps to explain why masquerading prey often resemble objects that are inherently variable in size (e.g., twigs, leaves, and stones) and has important implications for the evolution of masquerade as an antipredator defense.
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We developed models to predict the effect of water velocity on prey capture rates and on optimal foraging velocities of two sympatric juvenile salmonids, coho salmon and steelhead. Mean fish size was ~80mm, the size of age I+ coho and steelhead during their second summer in Southeast Alaska streams, when size overlap suggests that competition might be strongest. We used experimentally determined prey capture probabilities to estimate the effect of water velocity on gross energy intake rates, and we modeled prey capture costs using experimental data for search and handling times and published models of swimming costs. We used the difference between gross energy intake and prey capture costs to predict velocities at which each species maximized net energy intake rate. Predicted prey capture rates for both species declined from ~75 to 30–40 prey/h with a velocity increase from 0.30 to 0.60m·s−1. We found little difference between coho and steelhead in predicted optimum foraging velocities (0.29m·s−1 for coho and 0.30m·s−1 for steelhead). Although prey capture ability appears to be more important than are prey capture costs in determining optimum foraging velocities, capture costs may be important for models that predict fish growth. Because coho are assumed to pay a greater swimming cost due to a less hydrodynamic body form, we also modeled 10 and 25% increases in hydrodynamic drag to assess the effect of increased prey capture costs. This reduced optimum velocity by 0 and 0.01m∙s−1, respectively. Habitat segregation among equal-sized coho and steelhead does not appear to be related to the effects of water velocity on their respective foraging abilities.
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We hypothesize that foraging stream salmonids move during summer because (1) they monitor habitat conditions at a reach scale (100s of m), and (2) dominant fish move when conditions in their present foraging location become sub-optimal relative to conditions at other locations in the reach. To test these ideas, we quantified temporal variation in foraging habitat quality between late spring and early fall in a reach of a small Rocky Mountain brook charr, Salvelinus fontinalis, stream, predicted optimal-foraging fish distributions within the reach, and experimentally manipulated access to foraging sites and measured fish responses. Our results show that high-quality foraging sites were located at certain places in the reach during one period, but at different places during others, consistent with the hypothesis that fish movement is required if dominant fish are to occupy high-quality foraging sites throughout summer. The optimal foraging model was able to predict foraging locations within study pools, but not the exact location of individual fish within the pools or the reach. However, empirical evidence suggests that fish were distributed in order to maximize energy intake at the reach scale. Finally, dominant fish excluded from their preferred foraging location either left the pools (three of six cases), or began to occupy focal points of the next largest fish which, in turn, exited the pool (two of six cases). If habitat selection was occurring only within habitat units, then large fish, when excluded from their preferred locations, would select the next best locations within the pool. Taken together, these results suggest that charr use summertime movements to both monitor habitat conditions at a large spatial scale, and to gain access to optimal foraging locations even as conditions change temporally.
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Rainbow trout fry Salmo gairdneri, feeding during the day in a large river below a lake, far more frequently attacked drifting organisms than substrate-associated organisms. Recently emerged fry struck at prey more frequently than did older fry. Zooplankton from the lake upstream were the prey most frequently consumed by small fry, whereas larger aquatic and terrestrial insects were the major food of larger fry. Of those zooplankters fed upon by small fry, large calanoid copepods (mainly Epischura nevadensis) and Daphnia galeata mendotae were selectively consumed and smaller Bosmina longirostris rarely consumed, as would be expected of size-selective predators, but large Cyclops bicuspidatus were also under-represented in fry stomachs. There was a seasonal increase in size of prey consumed. The relative prey size (size of prey consumed divided by fry size) increased throughout the year, except from fall to winter when fry growth was slow. It is suggested that the sequential change in feeding patterns increased the efficiency of energy intake for the young rainbow trout, conserved their energy expenditure, and thereby improved their likelihood for survival.
Article
We developed and tested a combined foraging and bioenergetics model for predicting growth over the lifetime of drift-feeding brown trout. The foraging component estimates gross energy intake within a fish- and prey size-dependent semicircular foraging area that is perpendicular to the flow, with options for fish feeding across velocity differentials. The bioenergetics component predicts how energy is allocated to growth, reproduction, foraging costs, and basal metabolism. The model can reveal the degree to which growth is limited by the density and size structure of invertebrate drift within the physiological constraints set by water temperature. We tested the model by predicting growth based on water temperature and on drift density and size structure data from postemergence to age 12, and we compared the predictions with observed size at age as determined from otoliths and scales for a New Zealand river brown trout population. The model produced realistically shaped growth curves in relation to the observed data, accurately predicting mean size at age over the lifetime of the trout, assuming 24-h maximum rations and including diurnal drift-foraging costs (predicted versus observed weight r ² = 0.94; length r ² = 0.97). The model predicted that, within a given water-temperature regime, growth is limited primarily by reproduction costs but also by increasing foraging costs as trout grow (a phenomenon that is associated with the increasing foraging time that is required in order to feed to satiation on small invertebrate drift prey). Invertebrate drift size structure significantly influenced predicted growth, especially maximum size, through its effect on foraging time. The model has potential in terms of the exploration of growth-limiting factors and has associated use as an environmental-impact tool and as an aid for hypothesis generation in studies of salmonid growth processes.
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Foraging tactics of brown trout (Salmo trutta) from a wild population were observed in a laboratory stream. Floating tent caterpillars (Malacosoma americanum) and mealworm larvae (Tenebrio molitor) were introduced to the stream by conveyor belt and the ratio of prey species was changed over an 11 d period from 0:1-5:1. Two of five individuals appeared to switch to the most abundant prey species. Preference values based on a model of switching (Murdoch, 1969) differed among individuals and varied as much as five-fold among 5 min intervals. In some individuals diet preferences reversed during 30 min bouts; short-term changes in feeding preference were not generally predictive of longer-term trends. Individual differences were detected in the expression of negative bias toward tent caterpillars. Some effects of exposure to relatively unpalatable prey appeared to extend over at least a 24 d period. Studies of foraging variability should contribute to an understanding of the role of switching in stabilizing prey communities and to the successful application of optimal foraging theory to stream fishes.
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Signal detection theory, speed–accuracy trade-offs, and attentional allocation trade-offs all describe trade-offs between different components of performance in a detection task; however, these phenomena have generally been considered independently and their relationships are unclear. In this article, we expand the classical signal detection model in a way that allows us to incorporate speed, accuracy, and attention into a single unifying framework. Classical signal detection theory generally assumes fixed overlapping distributions of the perceived stimuli generated by desirable and undesirable objects. The variability of these distributions is typically assumed to be attributable either to the true variation among objects or perceptual error. Our new framework considers how investment in learning about the signal being emitted by encountered objects (sampling) might reduce one component of this variability, namely that generated by perceptual error. First, we identify the optimal sampling strategy, based on the payoff-maximizing time or attention a receiver should allocate to a given object. Next, we show how this optimal strategy can vary with parameters such as the ratio of desirable to undesirable objects and the initial perceptual error. Finally, we highlight the consequences of these optimal sampling strategies, using Batesian mimicry as a central example. The implications of the ability of receivers to reduce perceptual error by allocating more time or attention are potentially far reaching. For instance, snap decisions by predators will arise when predators do not gain from allocating more time to make better informed decisions, and under some conditions, this behavior will allow more imperfect mimicry to persist.
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Wild brown trout Salmo trutta in a fertile, high-conductivity stream in central Pennsylvania were observed from camouflaged towers for three consecutive years in order to quantify the diurnal feeding and social behavior of undisturbed adults. The foraging behavior observed was characterized in general as one of net energy maximization effectuated principally by cost minimization. Individuals ranging in age from young of the year to 8 years spent 86% of foraging time in a sit-and-wait search state, used discrete, energy-saving foraging sites year after year, and fed mainly off drift, taking less than 15% of their food items directly off the bottom. Feeding rates decreased with age, were highest in spring and fall, and showed little effect of time of day except for short peaks at dusk in May and June. The home range of most individuals was established in the first or second year of life and changed little thereafter. The mean size of the home range of individuals was 15.6 m and decreased slightly during the first 4 years of growth. No individual had exclusive use of any home range and no clearly defined territory could be described for any fish. Rather, the social structure evidenced is best described as a cost-minimizing, size-dependent, linear dominance hierarchy of individuals having overlapping home ranges. There was no apparent correlation between dominance and site selection with respect to distance to cover or feeding rate. Use of overhead cover ranged from 17% or less of daylight hours for wild brown trout of age-group 2 to no more than 43% for age-group 5. Length was asymptotic at 40 cm. A rectangular hyperbola described well the overall growth curve of fish in this population, half of the asymptotic length being attained at the age of 23 months. Hatchery brown trout, introduced for experimental purposes, fed less, moved more, and used cost-minimizing features of the substrate less than wild trout. It is postulated that high energy cost is a major cause of mortality among hatchery-reared brown trout stocked in streams, that at high population densities foraging sites are limiting factors, and that growth rate of drift-feeding salmonids is density-independent.Received January 10, 1983 Accepted November 6, 1983
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While the concept of cumulative effects is prominent in legislation governing environmental management, the ability to estimate cumulative effects remains limited. One reason for this limitation is that important natural resources such as fish populations may exhibit complex responses to changes in environmental conditions, particularly to alteration of multiple environmental factors. Individual-based models hold promise for estimating cumulative effects in these situations. We present an example application of an individual-based model of stream trout to the problem of estimating the cumulative effects of multiple environmental changes: elevated wet-season turbidity, elevated dry-season stream temperature, and reduced pool frequency. Each of these physical changes had multiple consequences for individual fish in the model, reflecting existing information. The simulations exhibited non-linear and non-multiplicative population responses to the multiple stressors. The results indicate the value of the individual-based approach for estimating cumulative effects and challenge the assumption that consequences for animal populations of increasing or multiple environmental changes are readily estimated from responses to modest changes in single factors.
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Relationships between environmental variables, ecosystem metabolism, and benthos are not well understood in sub-arctic ecosystems. The goal of this study was to investigate environmental drivers of river ecosystem metabolism and macroinvertebrate density in a sub-arctic river. We estimated primary production and respiration rates, sampled benthic macroinvertebrates, and monitored light intensity, discharge rate, and nutrient concentrations in the Chena River, interior Alaska, over two summers. We employed Random Forests models to identify predictor variables for metabolism rates and benthic macroinvertebrate density and biomass, and calculated Spearman correlations between in-stream nutrient levels and metabolism rates. Models indicated that discharge and length of time between high water events were the most important factors measured for predicting metabolism rates. Discharge was the most important variable for predicting benthic macroinvertebrate density and biomass. Primary production rate peaked at intermediate discharge, respiration rate was lowest at the greatest time since last high water event, and benthic macroinvertebrate density was lowest at high discharge rates. The ratio of dissolved inorganic nitrogen to soluble reactive phosphorus ranged from 27:1 to 172:1. We found that discharge plays a key role in regulating stream ecosystem metabolism, but that low phosphorous levels also likely limit primary production in this sub-arctic stream.
Article
Prey availability for site-attached planktivorous fish is delimited not only by the density of their prey, but also by the speed of the waters that carry the prey. The effects of both flow speed and prey density on the feeding rates of coral reef dwelling planktivorous fish were examined under controlled laboratory conditions. Fish placed inside a large recirculating flow chamber exhibited a limited enhancement of feeding rates following the increase of flow speed from 3 to 18 cm/s. Feeding rates peaked between 6 and 12 cm/s and appeared to decline upon subsequent increases in speed. Saturation of feeding rates did not correspond directly with prey flux, as higher rates were achieved at 'higher prey density-lower flow speed' treatments of equal flux. Video analysis of fish movements while foraging, together with experimentation involving fish feeding under restricted spatial conditions, indicated the narrowing of the fishes' reactive volume with increasing flow speed. The dependence of reactive volume size on flow speed was incorporated into the Holling disk equation and shown to reproduce much of the observed trends. Disparities between the observed and predicted dependence of feeding rates on flow speed suggest additional effects of flow speed on search efficiency parameters, such as prey detectability and capture success.
Article
Spatial and temporal variation in growth conditions for young juveniles may determine the ultimate success of salmonid populations. To assess this aspect of habitat quality, we developed a spatially explicit bioenergetics model to predict age-0 Atlantic salmon Salmo salar growth rate potential (GRP) in rearing streams of the Connecticut River, from the time of stocking in the spring, to the end of the summer. During the first month after stocking, there appears to be a paucity of suitable habitat. Most available habitat is predicted to result in low foraging success of small fish and to be energetically stressful because of the combination of high spring discharge and low water temperature. Although less than 14% of available habitat was predicted to support positive growth in the spring, 47% of the fish we observed occupied microhabitats predicted to yield positive growth, indicating the importance of habitat selection. In contrast, from mid-June to August, 67% of available habitat was predicted to yield positive growth, and 92% of all fish occupied positive growth microhabitats. Consistent with these results, sites with higher salmon GRP in the early season, but not in the mid- or late season, had higher final salmon densities by the end of August. Hydroclimatic regimes characteristic of more southerly rearing streams in the Connecticut River basin were predicted from our model to cause a potential shift from early-season to late-season habitat-related growth constraints along this environmental gradient. This work demonstrates the value of applying a bioenergetics approach to issues related to conservation of Atlantic salmon and provides a framework for future research on early life history energetic determinants of habitat quality.
Article
We videotaped Arctic grayling Thymallus arcticus feeding on large Daphnia middendorffiana drifting at different water velocities in an experimental stream with and without stream debris. The angle and distance at which fish first located each prey was determined from the videotapes. Both measures were affected by stream velocity and added debris. Location distance was unchanged at the lower velocities (11.6 and 32.3 cm/s) but declined at higher velocities. However, prey encounter rate increased up to water velocities of 45.8 cm/s, and thus water velocity compensated for reduced search area. Added debris always shortened location distance and decreased location angle. These findings have implications for position choice in streams and search strategies.
Article
The results of a test of a mechanistic encounter model that predicts the size-frequency composition of the diet of drift-feeding coho salmon (Oncorhynchus kisulch, Walbaum) are described. When all taxa in the drift were included in the model the predicted and actual diets differed significantly, although the model explained most of the variance between the diet and the predictions of a null model. When non-consumed taxa, including the distasteful and aposematic taxon Hydracarina, were excluded from the drift composition there was no significant difference between predicted and actual diets. The model's goodness of fit increased when it was modified to predict the biomass size-frequency diet composition, explaining 99% of the weighted sum of squared deviations between the diet and a null model. A number of alternate models are tested and a potentially useful simplification of the model is identified.
Article
Visual search is a common task both in naturalistic settings and in the laboratory. Outside the laboratory, one might look for a car in a parking lot, a name in text, or a navigation marker on the horizon. In the laboratory, search is simplified in several ways; commonly, the subject views a set of distinct objects and is asked to detect the presence of a particular object (the target) among a set of distracters. Two examples are shown in Figure 1. The top two panels illustrate the contrast increment task, in which the target is a disk of high luminance and the distractors are disks of lower luminance; the bottom two panels illustrate the line bisection task, in which the target is a rotated L and the distractors are rotated Ts. One of the most studied aspects of visual search is the effect on performance of the number of objects, here referred to as the display set size. Display set sizes of 2 and 24 are illustrated in Figure 1. In the top panels, the target ''pops out''-even for a large display set size. More precisely, display set size has little or no effect on search time or accuracy when the target is much brighter than the distracters. In contrast, in the bottom panels, finding the target requires ''scrutiny'' for the large set size. Display set size has a large effect on both search time and accuracy when the target and the distracters are these different rotated characters. These variations in the magnitude of set-size effects pose a central question for research on visual search. In this article, I use signal detection theory to analyze set size effects on search accuracy. It remains to be seen how this analysis will extend to the more commonly studied set-size effect on search time.
Article
Signal detection theory describes how an observer makes decisions about weak, uncertain, or ambiguous events or signals. It is widely applied in psychology, medicine, and other related fields. This book describes the theory, explains its mathematical basis, and shows how to separate the observer's sensitivity to a signal from his or her tendency to say "yes" or "no." Both detection of an event and discrimination between two events are treated. Chapters 1-4 describe the basic form of the signal-detection model and how to use it; Chapters 5-7 extend the model to different procedures such as identification of a signal; Chapters 8-10 expand it to other methods and distributions; and Chapter 11 describes the statistical treatment of detection data.
Article
A model is developed to predict potential net energy gain for salmonids in streams from characteristics of water velocity and invertebrate drift. Potential net energy gain, or potential profit, is calculated for individuals of three species of juvenile salmonids in a laboratory stream aquarium, based on the energy available from drift minus the cost of swimming to maintain position. The Michaelis–Menten or Monod model is used to describe the relationship between potential profit and specific growth rate. Potential profit was a better predictor of specific growth rate for coho salmon (Oncorhynchus kisutch) than for brook trout (Salvelinus fontinalis) or brown trout (Salmo trutta). Coho salmon always achieved higher specific growth rates than either brook trout or brown trout in concurrent experiments, and maintained growth to lower resource thresholds. In each experiment, fish established intraspecific hierarchies and dominant fish held positions affording maximum potential profit. The use of potential profit as an optimal foraging model was tested by predicting the potential for net energy gain at coho salmon positions from the overall pattern of flow and invertebrate drift in the stream aquarium, and ranking these positions from highest to lowest potential profit. This predicted ranking was nearly identical to the rank observed in the linear dominance hierarchy. The results of experiments confirm ideas of other investigators about mechanisms of microhabitat selection by stream salmonids.
Article
In this paper, we describe a model that allows estimation of the energy expenditure of a maneuvering fish from a knowledge of its changes in speed and direction. This model utilizes speed-time data and measurements of the timing and radii of turns. We use hydrodynamic theory to estimate the swimming thrust produced by fish during the accelerations, decelerations, coasting, and turns involved in a maneuver, expressing this thrust in terms of the speed at which it would carry a fish swimming at a constant speed in a straight line. This allows us to estimate energy expenditure by referencing well-established, empirically derived relationships between fish size, water temperature, swimming speed, and oxygen consumption. Application of this model to prey interception maneuvers by drift-feeding Arctic grayling (Thymallus arcticus) gave estimates of energy expenditure ranging from 2.6 to 10 (mean 4.7) times higher than the traditional method which assumes fish pay a cost equivalent to steady swimming at their average speed. These results compare favorably with recent respirometry studies showing that actual oxygen consumption by maneuvering fish can reach as much as 6-14 times the values predicted by the traditional method.
Article
Consumption of three species of prey by brown trout (Salmo trutta) in a laboratory stream was studied during 7-d experiments. Two drift rates (5 and 10 organisms/min) and three ratios (1:1, 2:1, 5:1) of small:large alternative prey were employed. Responses to prey species stabilized after 4–6 d and 800–1200 prey captures, but no prey was completely excluded from the diet. Size-selective predation was a dominant characteristic of the response. The fish appeared to alter the area (depth) searched in response to prey density; electivity was greatest when prey densities were high. Disproportionate predation on abundant prey ("switching") was a temporary phenomenon, which may have been masked by prey size. Brown trout ultimately achieved 54–91% of a hypothetical diet in which prey are ranked in order of size (energy content). Deviations from an optimal diet may be explained in terms of a feeding strategy that deals with heterogeneous distribution of prey, as well as with the behavioral capabilities of the predator. Key words: behavior, fish, invertebrate drift, optimal foraging, predation, prey size and abundance, Salmonidae, search image, streams
Article
The food selected by small (~ 3 g) and large (~ 45 g) hatchery rainbow trout (Salmo gairdneri), studied over a 28-h period, showed that the fish fed selectively, but often on different prey organisms. Feeding activity was highest during daylight hours but was only loosely associated with increases in invertebrate drift density. The majority of large trout exploited adult chironomids on the surface, whereas small trout fed primarily on midwater drift. At night when drift densities were low the limited feeding that took place apparently shifted to bottom foraging. Prey size was the most important factor affecting vulnerability to predation at all hours. Both large and small fish rarely consumed invertebrates < 2 mm long. Selection of larger individuals among certain prey taxa occurred, and in two important groups (Trichoptera and Chironomidae) large trout ate significantly larger prey than did small trout. By being size selective, the trout lost the opportunity to exploit smaller organisms, particularly Collembola, which constituted the bulk of the total drift. Key words: diel habits, drift, predation, rainbow trout, size selection
Article
Analyses of stomach contents showed that the kinds of prey eaten by brook trout (Salvelinus fontinalis), cutthroat trout (Salmo clarki), and rainbow trout (Salmo gairdneri) were seldom distributed at random among the individuals. Repeated observation of food eaten by individuals in a stream and ponds showed that prey types were eaten in proportions which were characteristic for an individual.Specialization occurred on several different kinds of prey. Although the degree of specialization was higher during shorter intervals, the data suggested that some specialization persisted for half a year. There were no striking correlations between degree of specialization and other individual properties such as size, growth rate, weight of food, number of food items, previous specialization, or area of recapture.In addition to the observations on trout in relatively undisturbed habitats, a field experiment was conducted using laboratory-reared rainbow trout held in small ponds. The food of each trout in the experiment was sampled repeatedly. In analysis of variance, interaction among the individuals and kinds of prey eaten showed that food specialization occurred. Both the absolute and relative abundance of potential prey were constant during the experiment.
Article
We tested the assumptions and predictions of a foraging model for drift-feeding fish. We used three-dimensional videography to describe the foraging behavior of brown trout, Salmo trutta, mapped water depth and velocity in their foraging area, sampled invertebrate drift to determine length class specific drift densities, and captured trout to determine the size composition of their diet. The model overestimated the fish's prey capture rate and gross energy intake rate by a factor of two. Most of this error resulted from the fact that prey detection probabilities within the fish's foraging area averaged only half the expected value. This was the result of a rapid decrease in capture probability with increasing lateral distance from the fish's focal point. Some of the model's assumptions were accurate: equations for predicting reaction distance and minimum prey size supported reliable predictions of the shape and size of the fish's foraging area and the size composition of the diet. Other assumptions were incorrect: fish detected prey within the predicted reaction volume, not on its upstream surface as expected, fish intercepted prey more slowly than the expected maximum sustainable swimming speed, and fish captured about two-thirds of their prey downstream of their focal point, rather than upstream.
Article
We demonstrated the ability of a mechanistic habitat selection model to predict habitat selection of brown trout (Salmo trutta) and mountain whitefish (Prosopium williamsoni) during summer and winter conditions in the Blacksmith Fork River, Utah. By subtracting energy costs and losses from the gross energy intake rate (GEI) obtained through simulation of prey capture, the model calculates the potential net energy intake rate (NEI) of a given stream position, which is essentially the rate of energy intake available for growth and reproduction. The prey capture model incorporates the size, swimming speed, and reaction distance of the fish; the velocity, depth, temperature, and turbidity of the water; and the density and size composition of the drifting invertebrates. The results suggest that during both summer and winter, the brown trout and mountain whitefish in our study reach avoided locations providing low NEI and preferred locations providing a high ratio of NEI to the swimming cost (SC) at the focal position of the fish (NEI/SC). This supports the idea that the drift-feeding fish in this study selected stream positions that provided adequate NEI for the least amount of swimming effort.
Article
We used a bioenergetic model to determine if cutthroat trout (Oncorhynchus clarkii bouvieri) abundance was related to net energy intake rates (NEI) and the proportion of suitable habitat and to evaluate potential changes in habitat quality due to climate change and stream fertilization efforts. We conducted monthly sampling of cutthroat trout, invertebrate drift, and physical habitat features in pool and riffle habitats. Fish in this Study selected foraging positions that enabled them to maximize NEI, and most fish were capable of sustaining high growth rates from July to September. Mean NEI and the proportion of suitable habitat at sites were greater in pools relative to riffle habitats and declined from July to October, primarily due to a decline in temperature over the four months. Cutthroat trout biomass was significantly related to NEI and the proportion of suitable habitat at a site. Model simulations indicated that climate change might reduce habitat quality for small-bodied trout, while extending the growing season for larger fish. Increased food abundance provided only marginal changes to model outcomes, whereas reductions in food significantly reduced habitat quality.
Article
The means by which an increase in predation risk would affect the attention devoted to food by juvenile salmon was investigated by determining whether the ability of salmon to distinguish between 2 types of food varies with predation risk. Results suggest that an increase in predation risk leads to a decrease in the attention paid to the selection of food items. (PsycINFO Database Record (c) 2012 APA, all rights reserved)
Article
In an earlier study (Holling, 1959) the basic and subsidiary components of predation were demonstrated in a predator-prey situation involving the predation of sawfly cocoons by small mammals. One of the basic components, termed the functional response, was a response of the consumption of prey by individual predators to changes of prey density, and it appeared to be at least theoretically important in population regulation: Because of this importance the functional response has been further examined in an attempt to explain its characteristics.
Article
1. The chief objective of the present study was to develop a functional model for the daily change in the total energy content of a brown trout, Salmo trutta , (equivalent to growth when positive) in relation to the difference between energy intake (energy content of food) and energy losses (metabolism + losses in faeces and excretory products). Energy budgets for individual fish were obtained in earlier experiments with 210 hatchery trout (live weight = 11–270 g) kept at fairly constant temperatures (mean values ranging from 3.6 to 20.4 °C), but without strict control of temperature or oxygen, and in later experiments, with 252 trout (1–300 g) bred from wild parents and kept at five constant temperatures (5, 10, 13, 15 and 18 °C) and 100% oxygen saturation. Each trout was fed a fixed ration of shrimps, Gammarus pulex, the ration level varying between zero and maximum.
Article
The size distributions of invertebrates in drift samples and stomachs of juvenile brook charr, Salvelinus fontinalis, were compared to test Dunbrack & Dill's (1983) model which predicts the size composition of the diet of stream salmonids. Prey less than 0.4 mm wide were less common in the diet than in the drift, while prey greater than 0.6 mm wide were more common. The behavioural model of Dunbrack & Dill (1983) predicted the actual diet reasonably well, although there were significant differences between the predicted and observed diets in three of four cases.
Article
Abstract – There is substantial need for models that accurately predict habitat selection by fishes for purposes ranging from the elaboration of ecological theory to the preservation of biodiversity. We have developed a new and highly tractable optimal foraging model for drift-feeding fishes that is based on the profitability of occupying varying focal-point velocities in a stream. The basic model can be written as: Ix = (Ex * Px) = {(D * A * V) * [1/(1 + e(b + cV))]} − Sx, where: (1) Ix is the net energy intake at velocity x; (2) E is prey encounter rate; (3) P is prey capture success rate which can be modelled as 1/(1 + e(b + cV)) where b and c are fitting constants from the prey capture success curve; (4) D is the energy content of prey (J/m3) in the drift; (5) A is the visual reactive area of the fish; (6) V is velocity (cm/s); and (7) S is the cost of maintaining position (J/s). Given that D, A and S can be considered constant over the range of velocities occupied by these fishes, the model reduces to e(b + cV) = 1/(cV − 1) which we solved iteratively to yield an optimal focal-point velocity for species in each sample. We tested the model by comparing its predictions to the mean focal-point velocities (i.e. microhabitats) occupied by four species of drift-feeding minnows in two sites in a stream in North Carolina, USA. The model successfully predicted focal-point velocities occupied by these species (11 out of 14 cases) in three seasonal samples collected over 2 years at two sites. The unsuccessful predictions still were within 2 cm/s of the 95% confidence intervals of mean velocities occupied by fishes, whereas the overall mean deviation between optimal velocities and mean fish velocities was small (range = 0.9 and 3.3 cm/s for the warpaint shiner and the Tennessee shiner, respectively). Available focal-point velocities ranged from 0–76 to 0–128 cm/s depending on site and season. Our findings represent one of the more rigorous field tests of an optimal foraging/habitat selection model for aquatic organisms because they encompass multiple species and years, and for one species, multiple sites. Because of the ease of parameterization of our model, it should be readily testable in a range of lotic habitats. If validated in other systems, the model should provide critical habitat information that will aid in the management of riverine systems and improve the performance of a variety of currently used management models (e.g. instream flow incremental methodology (IFIM) and total maximum daily load calculations (TMDL)). 1. Existe una grave necesidad de modelos que predigan con precisión la selección de hábitat por parte de los peces con fines que van del desarrollo de la teoría ecológica a la conservación de la biodiversidad. Nosotros hemos desarrollado un modelo nuevo y de fácil manejo de alimentación óptima para peces que se alimentan de la deriva que se fundamenta en los diferentes beneficios energéticos derivados de ocupar velocidades focales distintas en un río. 2. El modelo básico puede formularse como: Ix = (Ex * Px) = {(D * A * V) * [1/(1 + e(b + cV))]} − Sx, donde: (1) Ix es el energía neta obtenida a la velocidad, x; (2) V es la velocidad (cm/s); (3) A es el area visual de reacción del pez; (4) D es la energía contenida en las presas (J/m3) en la deriva; (5) E es la tasa de encuentro de presas; (6) P es la probabilidad de captura de la presa, que puede representarse como 1/(1 + e(b + cV)) donde b y c son constantes; y (7) S es el coste de nadar para mantener la posición en la corriente (J/s). Puesto que D, A y S pueden considerarse constantes en el rango de velocidades que ocupan estos peces, el modelo se reduce a e(b + cV) = 1/(cV − 1) que resolvimos iterativamente para obtener una velocidad focal óptima para cada especie en cada muestreo. 3. Probamos el modelo comparando su predicciones con la velocidades focales medias (i.e. microhabitats) ocupadas por cuatro especies de ciprínidos que se alimentan de la deriva en un río de Carolina del Norte. El modelo predijo con éxito las velocidades focales ocupadas por estas especies (11/14 casos) en tres muestreos estacionales llevados a cabo a lo largo de dos años en dos estaciones. Incluso las predicciones fallidas se diferenciaron en menos de 2 cm/s del límite de confianza al 95% CIs de las velocidades medias ocupadas, y la diferencia media entre predicciones y observaciones fue pequeña (rango = 0.9 cm/s warpaint shiner, a 3.3-cm/s Tennessee shiner). El rango de las velocidades focales medias disponibles fue de 0–76 cm/s a 0–128 cm/s dependiendo de la localidad y estación del año. 4. Nuestros resultados son una de las pruebas de campo más rigurosas de un modelo de alimentación óptima/selección de hábitat para organismos acuáticos puesto que incluyen diversas especies, años y, para una de las especies, localidades. La facilidad de la estima de los parámetros del modelo hace que sea fácil probarlo en diversos hábitats lóticos. Si es validado en ellos, el modelo debería proporcionar información valiosa que ayudará a la gestión de los sistemas fluviales y mejorará los resultados obtenidos a través de varios modelos usados actualmente para la gestión (p.e. IFIM y cálculos TMDL).
Article
Abstract – We examined the prey capture success of recently emerged brook charr (Salvelinus fontinalis) foraging in shallow, clear, still-water pools along the edges of streams. Fewer than 42% of attacks ended with ingestion either because of difficulty distinguishing suitable prey from unsuitable items or because of difficulty capturing evasive prey. Probabilities of capture upon attack and ingestion upon capture depended upon where attacks were directed in the water column, the fish's level of activity at the time of attack and its fork length, and the sampling date. In general, success was higher for larger, sedentary fish attacking prey in the lower portion of the water column than for smaller, active fish attacking prey at the water surface. The frequency of items attacked was only a moderate predictor of the frequency of prey ingested. Poor capture success is an important aspect of the early life history of brook charr in particular and probably of young salmonines in general.†
Article
1. Habitat degradation and biological invasions are important threats to fish diversity worldwide. We experimentally examined the effects of turbidity, velocity and intra- and interspecific competition on prey capture location, reactive distance and prey capture success of native rosyside dace (Clinostomus funduloides) and invasive yellowfin shiners (Notropis lutipinnis) in Coweeta Creek, North Carolina, U.S.A. 2. Increased turbidity and velocity produced significant decreases in the number of prey captured forward of the fish’s location. It is possible that this represents an increase in the amount of energy expended per prey captured. 3. We used Akaike’s Information Criterion (AIC) to evaluate competing explanatory models for reactive distance (10 generalised linear models, GLM) and prey capture success (9 generalised linear mixed models, GLMM). 4. Reactive distance decreased by 12% with an increase from 2 to 4 conspecifics, whereas a 10 NTU increase in turbidity reduced reactive distance by 9%. Capture success was affected by velocity, dominance and competition, and varied among species. A 6 cm s−1 increase in velocity produced a 28% decline in capture probability; however, dominant fish were 3.2 times more likely to capture a prey item than non-dominant fish. Yellowfin shiners only were 0.62 times as likely to capture a prey item as rosyside dace. Both intra- and interspecific competition reduced capture probability, and fish in high density intraspecific or interspecific trials were 0.46 times and 0.44 times as likely to capture prey, respectively, as fish in two fish intraspecific trials. 5. These results suggest behavioural variables are as important as physical factors in determining reactive distance and capture probability by these minnows.
Article
1. ,One of two things can happen to allochthonous material once it enters a stream: it can be broken down or it can be transported downstream. The efficiency with which allochthonous material is used is the result of these two opposing factors: breakdown and transport. 2. ,The present synthesis of new and published studies at Coweeta Hydrologic Laboratory compares biological use versus transport for four categories of particulate organic material: (1) large wood (logs); (2) small wood (sticks); (3) leaves; and (4) fine particulate organic matter (FPOM). 3. ,Over 8_years, logs showed no breakdown or movement. 4. ,The breakdown rate of sticks (≤3_cm diameter) ranged from 0.00017 to 0.00103_day−1, while their rate of transport, although varying considerably with discharge, ranged from 0 to 0.1_m_day−1. 5. ,Based on 40 published measurements, the average rate of leaf breakdown was 0.0098_day−1. The leaf transport rate depended on stream size and discharge. 6. ,The average respiration rate of FPOM was 1.4_mg_O2_g_AFDM−1_day−1 over a temperature range of 6–22_°C, which implies a decomposition rate of 0.00104_day−1. Transport distances of both corn pollen and glass beads, surrogates of natural FPOM, were short (<_10_m) except during high discharge. 7. , Estimates of transport rate were substantially larger than the breakdown rates for sticks, leaves and FPOM. Thus, an organic particle on the stream bottom is more likely to be transported than broken down by biological processes, although estimates of turnover length suggest that sticks and leaves do not travel far. However, once these larger particles are converted to refractory FPOM, either by physical or biological processes, they may be transported long distances before being metabolized.
Article
Direct observations of young-of-the-year brook charr, Salvelinus fontinalis, in a second-order woodland stream indicated that most of their feeding effort was directed toward sub-surface, drifting prey (83% of feeding time). Feeding from the substrate and water surface was much less frequent (17% of feeding time). Comparisons of gut contents to drift net and substrate fauna samples corroborated that the most commonly consumed prey (chironomid and trichopteran larvae, ostracods, and ephemeropteran nymphs) were captured primarily from sub-surface, invertebrate drift. The disproportionate numbers of some prey species in the guts of several fish indicate that some prey selection occurred. Territories appeared to be cardioid-shaped, and were often contiguous, with dominance hierarchies evident among the residents. Agonistic interactions were frequent. Charges and chases predominated (91% of interactions) while lateral displays were infrequent (9% of interactions). Overall, these fish spent most of the daylight hours station-holding (77%) and feeding (18%). While only 3% of total time was spent in aggression, this amounted to 14% of the time a fish spent away from its station. There was some indication that territories were defended at a cost of feeding time.
Article
We present an overview of a process-based modelling approach for predicting how change in flow affects drift density, net rate of energy intake (NREI) and numbers of drift-feeding salmonids. It involves linking an existing two-dimensional flow model (River2D) with models of invertebrate drift transport and drift-foraging which we have developed. We describe, demonstrate and partially test our models in an application on a 80 m × 20 m pool on a New Zealand river. We show how these models realistically capture hydraulic, drift dispersion and bioenergetics drift-foraging processes to predict the relationship between stream flow, habitat quality and quantity (in terms of NREI), and carrying capacity for drift-feeding salmonids. Overall, the 2D hydraulic model made good predictions of water levels, depths and water velocity at the calibration flow and a lower (validation) flow. The drift transport model made good predictions of the spatial distribution of invertebrate drift density throughout the pool at low flow after it was calibrated against observed drift density at the higher flow. The model correctly predicted that drift density would decline downstream and into the margins due to the process of settling dominating over entry from the stream bed, and that drift would be carried further downstream and laterally as flow increased. The foraging model made a reasonable prediction (6–7) of the numbers of 0.5 m adult brown trout observed (5) in the pool. It accurately predicted that trout should be distributed down the thalweg where net rate of energy intake (NREI) was highest, but when NREI was adjusted for depletion by feeding fish the predicted drift-feeding locations were more closely spaced (bunched) than observed fish locations. Our process-based modelling approach has important implications for improving biological realism in predictions of the response of drift-feeding fishes to flow change within the context of the IFIM.
Article
This review focuses on covert attention and how it alters early vision. I explain why attention is considered a selective process, the constructs of covert attention, spatial endogenous and exogenous attention, and feature-based attention. I explain how in the last 25 years research on attention has characterized the effects of covert attention on spatial filters and how attention influences the selection of stimuli of interest. This review includes the effects of spatial attention on discriminability and appearance in tasks mediated by contrast sensitivity and spatial resolution; the effects of feature-based attention on basic visual processes, and a comparison of the effects of spatial and feature-based attention. The emphasis of this review is on psychophysical studies, but relevant electrophysiological and neuroimaging studies and models regarding how and where neuronal responses are modulated are also discussed.
Article
Visual search attracted great interest because its ease under certain circumstances seemed to provide a way to understand how properties of early visual cortical areas could explain complex perception without resorting to higher order psychological or neurophysiological mechanisms. Furthermore, there was the hope that properties of visual search itself might even reveal new cortical features or dimensions. The shortcomings of this perspective suggest that we abandon fixed canonical elementary particles of vision as well as a corresponding simple to complex cognitive architecture for vision. Instead recent research has suggested a different organization of the visual brain with putative high level processing occurring very rapidly and often unconsciously. Given this outlook, we reconsider visual search under the broad category of recognition tasks, each having different trade-offs for computational resources, between detail and scope. We conclude noting recent trends showing how visual search is relevant to a wider range of issues in cognitive science, in particular to memory, decision making, and reward.