Focal Point Velocities Research Articles

Flying focus laser Wakefield scheme for efficient acceleration

The acceleration of charged particles with a short laser pulse is a revolutionary acceleration technology, but many restrictions limit their advantages. Wave breaking and dephasing are most important of these restrictions which weaken acceleration efficiency in the laser Wakefield acceleration scheme. The purpose of this paper is to demonstrate that flying focus laser Wakefield acceleration (FF-LWFA) can confirm this effectiveness. Flying focus creates in the focal region of a hyper-chromatic lens when the chirped laser pulse passing through them. Therefore, by controlling the lens chromaticity and initial chirp characteristics, the focal point velocity can be controlled and adjusted near the speed of light. For a realistic working parameter, two-dimensional simulation results show the flying focus pulse can retain its shape and energy within a few centimeters (one order of magnitude greater than Rayleigh's length). We introduce a velocity interval around velocity of light in which dephasing and wave breaking are restricted and high acceleration gradients (about a hundred of GeV/m) are achieved in the flying focus region. Moreover, in the optimized condition for negative chirp, wakefield amplitude is increased 50 %. Under this ideal situation, 0.5 % of background electrons accelerate to 2GeV in the 5 cm, where the acceleration gradient reaches to, 40GeV/m, and about 0.2 % of them accelerate up to 10GeV.

Exploitation of Velocity Gradients by Sympatric Stream Salmonids: Basic Insights and Implications for Instream Flow Management

AbstractHydraulic heterogeneity can strongly influence habitat selection by stream fishes. Velocity gradients created by channel roughness and flow obstructions may be particularly important for species that feed on drifting invertebrates, where maintaining focal points in low‐velocity microhabitats adjacent to faster water allows fish to scan a larger water volume for prey while minimizing swimming costs. However, these velocity gradients are rarely integrated into habitat suitability criteria used for defining instream flow requirements, which are generally based on mean column velocity measurements at the focal point location. It is also unclear how velocity gradient exploitation differs among sympatric drift‐feeding species. We measured the use of velocity gradients by two sympatric juvenile salmonids, Coho Salmon Oncorhynchus kisutch and steelhead O. mykiss, in a midorder cobble–boulder‐dominated river. We compared focal point velocities of fish to adjacent velocities within their foraging area and compared the magnitude of velocity and kinetic energy gradients between species. We then explored how lateral velocity gradients may bias instream flow assessments by deriving two sets of velocity habitat suitability curves (HSCs): conventional HSCs using average water column velocities measured at focal point locations and spatially averaged HSCs incorporating adjacent velocities (four body lengths from focal points). These contrasting HSCs were then used as input into the physical habitat simulation model to predict the influence of flow on habitat availability. Both species often used focal velocities that were lower than adjacent points, but the magnitude of these velocity gradients was higher for steelhead, consistent with known differences in foraging behavior, habitat selection, and physiology. Incorporating adjacent velocities into HSCs resulted in a ~40% (steelhead) and ~10% (Coho Salmon) increase in flows predicted to optimize habitat availability. Thus, small‐scale heterogeneity in velocity used by drift‐feeding fish can lead to large biases in flow assessments.

Velocity and dominance affect prey capture and microhabitat selection in juvenile Chinook (Oncorhynchus tshawytscha)

Habitat selection is an important phenomenon that may greatly affect individual fitness. Using an artificial stream, we examined the relationship between the percentage of prey captured, reactive distance, dominance, and water velocity for juvenile Chinook Salmon (Oncorhynchus tshawytscha) from the Chena River, Alaska, and tested the fitness-based microhabitat selection model of Grossman et al. (Ecol Freshw Fish 11:2–10, 2002). Recent declines in the abundance of Chinook accentuate our need for habitat selection studies on this species. We conducted three experiments: two with single fish (1st N = 27, fish SL 58–84 mm, 2nd N = 14, fish SL 49–56 mm) and one with pairs of dominant and subordinate fish (N = 10 pairs, 64–96 mm, mean difference in SL = 7 mm). We placed individual or pairs of fish in an artificial flume and recorded reactive distance and the percent prey capture with individual dead brine shrimp (Artemia spp.) as prey. Prey were presented at 10 cm/s velocity intervals ranging from 10 to 60 cm/s; velocities found in the natural habitat. Mean reactive distance in single fish experiments (henceforth SFE) averaged 33 and 29 cm respectively, and was not related to velocity. We detected a negative, curvilinear relationship between velocity and percent prey capture. Holding velocities for juvenile Chinook were significantly lower than prey capture velocities. The Grossman et al. (Ecol Freshw Fish 11:2–10, 2002) model yielded an optimal focal-point velocity prediction of 35 cm/s for juvenile Chinook, however focal-point velocities occupied by juveniles in the Chena River averaged 12 cm/s. Predicted optimal velocities were present in the Chena River; hence, this discrepancy suggests that other factors such as distraction from drifting debris or predation risk influenced habitat selection. There were no differences in reactive distances or holding velocity/capture velocity relationships for dominant and subordinate fish; however, dominants captured significantly more prey than subordinates. Being subordinate resulted in a decrease of 61% in mean percent prey capture (the difference between what was captured by the fish alone versus the difference with a dominant), whereas the mean cost to fish with dominant rank was a 21% decline between the percentage captured alone versus that with a subordinate.

Not all drift feeders are trout: a short review of fitness-based habitat selection models for fishes

Currently, there are few mechanistic fitness-based habitat selection models for stream fishes and most models used by management agencies focus on physical habitat alone. In this review, I describe the historical development and the status of mechanistic, fitness-based, habitat selection models for both water column (i.e., drift-feeding) and benthic stream fishes focusing on North America. Although the majority of drift feeders are not salmonids, most mechanistic habitat selection models have been developed and tested only in this group of fishes, likely due to their substantial economic importance. I review the fitness-based microhabitat selection model of Grossman et al. (Ecol Fresh Fish 11:2–10, 2002), which has been tested in both a salmonid and multiple cyprinid species. The model predicts optimal focal point velocities for drift feeders based on prey capture success–velocity relationships and does not include physiological costs, which are logistically difficult to quantify. In addition, I discuss mechanistic, fitness-based models used to predict microhabitat (i.e., patch) selection in benthic fishes. For both basic scientific and management/conservation perspectives, it is important to quantify habitat choice in fishes using mechanistic, fitness-based criteria.

Microhabitat use by southern brook trout (Salvelinus fontinalis) in a headwater North Carolina stream

AbstractBrook trout are the one of the only Salvelinus species native to eastern North America and range from Canada to Georgia. Very little is known, however, about the ecology of the southern form of this species. We quantified microhabitat use of southern brook trout in Ball Creek NC, a third‐order stream, during six seasonal samples (summer 2010, autumn 2010, spring 2011, summer 2011, autumn 2011 and spring 2012). In general, trout preferentially occupied deeper microhabitats with lower mean velocities and higher amounts of erosional substrata than were randomly available. Older trout (1+ and 2+) occupied deeper microhabitats with lower mean velocities than yearling trout. These microhabitats typically represent ‘plunge pools’. Southern brook trout also occupied focal point velocities that were statistically indistinguishable from optimal velocities calculated for rainbow trout in the same system and thus may chose microhabitats that maximise net energy gain. Southern brook trout are found in isolated populations, and management strategies should focus on the preservation of plunge pool habitat for conservation of this subspecies.

Ecology of Young Stream Resident Warner Sucker (Catostomus warnerensis) in Warner Basin, Oregon

We visually censused Warner sucker cohorts in a low gradient stream reach and determined microhabitat use from random availability data. For above-water visual censuses (AVC), we counted aggregations while slowly wading through Honey Creek three times both years. Underwater visual censuses (UVC) were done in 1993 by snorkeling. Mean focal point velocity (FPV) during AVC significantly differed by year but not by census date, and mean depth did not differ for either. In 1993 we found suckers used microhabitats with FPV between >3 to 6 cm/s (χ12 = 3.93, P < 0.05) and depth between >20 to 40 cm (χ12 = 4.5, P < 0.05). Suckers avoided areas where FPV exceeded 15 cm/s (χ12 = 18.7, P < 0.001) and depth between >60 to 80 cm (χ12 = 8.1, P < 0.005). Aggregate abundance was significantly related to both distance to (r64 = −0.45, P = 0.0002) and percentage (r64 = 0.44, P = 0.0003) submerged vegetation. We found 97% of suckers (N/m3) in riffle/run habitat during UVC. When mean flow was used as a continuous variable, we found suckers occupied habitats >3 cm3/s and avoided habitats <2 cm3/s in a disproportionate manner relative to availability. These results suggest young suckers select vegetated areas with moderate flow and relatively shallow depths during first few months of life. This information improves our understanding of ecological habits of early life stages of a rare western catostomid during stream residence, which could be useful for conservation management in low gradient stream reaches of Warner Basin.

Local hyperthermia with MR-guided focused ultrasound: spiral trajectory of the focal point optimized for temperature uniformity in the target region.

The objective of hyperthermia treatment is to deliver a similar therapeutic thermal dose throughout the target volume within a minimum amount of time. We describe a noninvasive approach to this goal based on magnetic resonance imaging (MRI)-guided focused ultrasound (FUS) with a spherical transducer that can be moved along two directions inside the bed of a clinical MR imager and that has an adjustable focal length in the third dimension. Absorption of FUS gives rise to a highly localized thermal buildup, which then spreads by heat diffusion and blood perfusion. A uniform temperature within a large target volume can be obtained using a double spiral trajectory of the transducer focal point together with constant and maximum FUS power. Differences between the real and target temperatures during the first spiral are evaluated in real time with temperature MRI and corrected for during the second spiral trajectory employing FUS focal point velocity modulation. Once a uniform temperature distribution is reached within the entire volume, FUS heating is applied only at the region's boundaries to maintain the raised temperature levels. Heat conduction, together with the design and timing of the trajectories, therefore ensures a similar thermal dose for the entire target region. Good agreement is obtained between theory and experimental results in vitro on gel phantoms, ex vivo on meat samples, and in vivo on rabbit thigh muscle. Edema in muscle was visible 1 hour after hyperthermia as a spatially uniform rise of the signal intensity in T(2)-weighted images.

Growth, Survival, and Migration Patterns of Juvenile Adfluvial Bonneville Cutthroat Trout in Tributaries of Strawberry Reservoir, Utah

Adfluvial Bonneville cutthroat trout Oncorhynchus clarki utah were introduced into Strawberry Reservoir after a rotenone treatment in 1990. Naturally reproduced fry either migrated to the reservoir immediately after emergence (migrants) or remained in the tributaries for 1–2 years (nonmigrants). In 1995, fry emerged from mid-August to early September. Migrants moved downstream almost exclusively between 2100 and 0300 hours. Migrants were always the same size as newly emerged fry, whereas the mean size of nonmigrant fry increased throughout the emergence period. Most fry followed the nonmigrant strategy. Nonmigrant fry grew from 0.12 g (26 mm) at emergence to an average 1.2 g (50 mm) by early November. Mean densities of nonmigrant fry in low-velocity microhabitats along the stream margins ranged from 5.5 to 11.0 fry/m2 during the 6 weeks after first emergence. Mean focal point velocity of nonmigrant fry in microhabitats was 3.3 cm/s (SD = 1.4, N = 42) and the mean focal point depth was 11.8 cm (SD = 6.5, N = 80). By late September, larger fry (>50 mm) shifted from the stream margins into the main channel. Growth and survival of migrants and nonmigrants were compared at low, ambient, and high fry densities in enclosure experiments to test whether migrants were inferior competitors in the stream. Growth and survival were similar for cohabiting migrants and nonmigrants within and among the three fry density treatments in the enclosure experiments. Growth was not affected by fry densities after 3 weeks; however, growth declined similarly for both migrant and nonmigrant fry in the high-density treatment compared with the low- and medium-density treatments after 6 weeks. Since no competitive differences were evident among the migrants and nonmigrants, emigration could be an innate response by some fraction of the population rather than the result of inferior competitors being forced downstream.

Physiological Performance and Stream Microhabitat use by the Centrarchids Lepomis megalotis and Lepomis macrochirus

Two centrarchids, Lepomis megalotis and L. macrochirus, were compared in laboratory studies of prey capture success, swimming endurance, morphology, hydrodynamic drag, and thermal tolerance, as well as field observations of focal point velocity and depth. For both species, capture of planktonic prey declined as current velocity increased, but L. megalotis was more efficient than L. macrochirus at higher current velocities. Capture of floating prey was not influenced by current velocity, but L. megalotis was more efficient overall at all velocities. Of the two species, L. megalotis was significantly more streamlined, had relatively lower hydrodynamic drag, and had higher swimming endurance in current. Both species had significantly higher critical thermal maxima (CTMax) in summer than in winter, but variance in CTMax was greater for L. megalotis than for L. macrochirus in both summer and winter. Differences between L. megalotis and L. macrochirus in performance and morphology may have direct influence on their relative abundances in small streams. Field observations showed L. megalotis was more common than L. macrochirus in faster, shallower microhabitats.

Competitive Interactions between Arctic Grayling and Brook Trout in the Big Hole River Drainage, Montana

Abstract We investigated mechanisms through which nonnative brook trout Salvelinus fontinalis could negatively influence native Arctic grayling Thymallus arcticus. We assessed occupied microhabitats, growth, and condition of Arctic grayling and brook trout in sympatric and allopatric groups in stream enclosures. Microhabitat use by both species was also measured in a natural assemblage. In stream enclosures and the natural assemblage, Arctic grayling and brook trout exhibited similar preferences for pools and runs, but they spatially segregated in microhabitats. Arctic grayling occupied faster focal point and mean column velocities closer to the surface than brook trout. Brook trout occupied slower focal point velocities in proximity to structural cover. Specific growth rates indicated that intraspecific competition operated more strongly than interspecific competition in regulating growth of both species in stream enclosures. In sympatry, Arctic grayling grew faster at a low density with a high density o...

Habitat utilization and diel behavior of juvenile bull trout (Salvelinus confluentus) at the onset of winter

Abstract— Underwater observations were used to describe habitat use and diel behavior of juvenile bull trout (Salvelinus confluentus) at the onset of winter (0.8°C) in a second‐order central Idaho stream. All fish observed during daytime counts were concealed beneath “home stones” (mean dimensions 32.3 by 21.9 cm) in primarily pool and run habitats. Focal point depths and velocities averaged 57.2 cm and 4.7 cm/s (home stone removed). Fish maintained stations over substrate with low percentages of fine (&lt; 2 mm, mean= 5.2%) sediment. Concealed bull trout were not evenly distributed across the stream channel. Significantly more home stones were distributed near the midline of the channel, compared to the channel margins. Fish size was positively correlated with home stone size. At night, bull trout exhibited a diel behavioral shift, some fish moved out of daytime concealment cover into the water column. At night, bull trout were observed feeding and resting, primarily in pool and run habitats. During both day and night, bull trout used riffles significantly less frequently than those habitats were available.

Habitat Use by Adult Rainbow Trout under Moderate Artificial Fluctuations in Flow

Adult rainbow trout Oncorhynchus mykiss were observed in a 20-m reach of river to determine habitat use (at four flow levels) and preference (at two flow levels) under daily fluctuations in discharge from a hydropower peaking operation. Maximum increase in discharge was threefold (from 1.6 to 5.1 m3/s), which is small compared with that of some hydropower peaking operations. Available habitat (based on velocity and depth) was different under low and high flows. At the low discharge level, nearly twice as much of the lowest velocity-class (0.00–0.15 m/s) and eight times more of the shallowest depth-class (0.0–0.2 m) were available. Distributions of adult habitat use revealed that fish focal point and water column velocities increased with increasing discharge. At the highest discharge levels, more fish were found in the deepest water, and the fish assumed positions closer to the streambed, than at the three lower discharges. Fish were usually associated with boulders at all discharges. Habitat preference shifted to deeper and faster water as discharge increased. Two types of individuals were identified on the basis of habitat use under various discharge levels. Pattern-1 individuals displayed strong site fidelity and used higher focal point velocities at higher discharges. Pattern-2 individuals were generally more mobile than pattern-1 individuals and showed no relationship between discharge and focal point velocity. Repeated observations made on individually marked fish indicated that description of habitat use and preference in terms of microhabitat may yield a false interpretation of optimal habitat for the population as a whole, It is likely that few individuals in a population of territorial fish occupy the optimal habitat. Interpreting the most frequently used microhabitat as optimal habitat for a population is probably incorrect and could result in erroneous predictions of available habitat based on instream flow assessment models.

Microhabitat Models of Large Drift-Feeding Brown Trout in Three New Zealand Rivers

We examined summer habitat use by 189 drift-feeding brown trout Salmo trutta, 45–65 cm long (fork length), by measuring substrate, depth, mean velocity, focal point velocity use, and adjacent velocity into which fish were feeding in three New Zealand rivers. We compared habitat used with habitat available (simulated by hydraulic modeling), and we derived habitat use, habitat preference, and logistic regression models of habitat selection. Focal points usually were associated with large substrate components including bedrock projections, boulders, and large cobbles. Depths between 0.67 and 0.86 m were most commonly used by brown trout but the preferred depth was 1.0 m. Mean velocities between 0.38 and 0.48 m/s were the most commonly used, but preferred mean velocities were 0.05 m/s higher. Optimal focal point velocities, 0.19–0.28 m/s, were lower than mean velocities. Vertical and lateral velocity shears were calculated as measures of the velocity differentials over which fish were feeding. The velocity shears most commonly used by brown trout were between 0.50 and 0.65 m/s per meter for vertical shear and 0.02 and 0.06 m/s per meter for lateral shear. Those most preferred by brown trout were between 0.50 and 1.20 m/s per meter for vertical shear and between 0.06 and 0.26 m/s per meter for lateral shear. Depth, mean velocity, and substrate were selected independently of each other. Significant differences between rivers were found for the use of substrate, depth, focal point velocity, mean velocity, and lateral velocity shear. Depth and mean velocity were consistently significant variables in logistic regression models of habitat selection, accounting for 33–85% of the explained deviances. Large substrate components accounted for 47% of the explained deviance in one river. Lateral and vertical velocity shear together accounted for 15–27% of the explained deviances. Logistic regression and joint habitat preference models were better predictors of suitable habitat in the rivers for which they were developed than were joint habitat use models. However, joint habitat use models may be more general, because, unlike logistic regression and joint habitat preference models, the predictive success of transferred joint habitat use models was similar to that of models tested on the river for which they were developed. Combined-river habitat use, habitat preference, and logistic regression models are presented for general habitat management applications when habitat criteria are not available for specific rivers.

Temporal Variation in Microhabitat Use by Age-0 Smallmouth Bass in the North Anna River, Virginia

Abstract Microhabitat use by age-0 smallmouth bass Micropterus dolomieu in the North Anna River was quantified from 10 snorkel surveys conducted between May and August in 1990 and 1991. Depth, mean velocity within the water column, substrate, and cover were quantified in 25 larval brood sites, and the same variables (plus focal-point velocity) were quantified at 634 points where dispersed individuals were located. Larvae in broods occupied a wide variety of microhabitats, but after larvae dispersed from brood sites they primarily occupied low-velocity areas with large substrate or cover. During the first 4–6 weeks after dispersal, juveniles continued to use relatively deep, low-velocity microhabitats. Thereafter, juveniles occupied shallower microhabitats with greater focal-point velocities. Fish size was generally not related to microhabitat use within a sampling period, except during two periods when there was a positive relationship between size and depth use. Microhabitat use of the age-0 cohort of sm...

Microhabitat-Specific Foraging Behavior, Diet, and Growth of Juvenile Coho Salmon

Abstract Observations of behavior were made on marked juvenile coho salmon Oncorhynchus kisutch in pools in Huckleberry Creek, Washington. Fish exhibited two distinct types of forage behavior during observations made from May to August 1988 and from June to August 1989. Some coho salmon formed dominance hierarchies wherein fish defended forage stations in ranks determined by an individual's ability or willingness to defend access to drifting food. Other coho salmon foraged as nonhierarchical floaters. Eight percent of marked individuals changed their foraging behavior class during their first summer. Focal point velocity (FPV) differentiated specific microhabitats used by these two foraging behavior classes. Coho salmon in dominance hierarchies (FPV > 0.06 m/s) fed most frequently on drifting invertebrates, which contributed 81% of their energy intake. Floaters (FPV ≤ 0.06 m/s) fed by patrolling large forage arenas, where they fed opportunistically on items delivered by aerial drop and instream drift. Ene...

The relationship between water velocity, energetic costs, and microhabitat use in four North American stream fishes

We employed field and laboratory data to test the hypothesis that microhabitat use by rainbow trout (Oncorhynchus mykiss), rosyside dace (Clinostomus funduloides), longnose dace (Rhinichthys cataractae) and mottled sculpin (Cottus bairdi) was affected by energetic constraints. Both rainbow trout and rosyside dace occupied water-column microhabitats in Coweeta Creek, North Carolina, U.S.A. These species were over-represented at velocities requiring lower rates of oxygen consumption, as predicted by an energetic cost hypothesis. Morphological and behavioral adaptations of the two benthic species (mottled sculpin and longnose dace) enabled them to hold position over a wide range of velocities without significant changes in oxygen consumption. Focal point velocities of longnose dace were not significantly different from those available, suggesting a lack of velocity preference. Mottled sculpin, however, consistently were over-represented at low velocities, even though oxygen consumption data showed that energetic costs were not decreased by occupying these microhabitats. Our results indicate that microhabitat use by rainbow trout and rosyside dace is consistent with predictions of an energetic cost hypothesis, whereas the morphological adaptations of longnose dace and mottled sculpin minimize the constraints of energetic costs in microhabitat selection.

Microhabitat Partitioning among Stream-Dwelling juvenile Coho Salmon, Oncorhynchus kisutch, and Dolly Varden, Salvelinus malma

Microhabitat use and partitioning among age 0+ and 1+ coho salmon, Oncorhynchus kisutch, and age 0+, 1+ and 2+ Dolly Varden, Salvelinus malma, was studied in small (&lt;2 m wide), natural streams on Prince of Wales Island, Alaska, and in laboratory stream channels. Coho salmon occupied midwater positions that they defended from other fish. Dolly Varden were more closely associated with the stream bottom and were seldom territorial. For each species, the depth of water, depth of focal point, and distance to nearest fish increased with fish size, whereas the distance to nearest cover decreased as fish size increased. Most fish selected focal point velocities between 0.0–9.0 cm∙s−1.Woody debris was the most frequently used cover type and most fish occurred over gravel substrates ranging from 2–100 mm particle diameter. Habitat use by each species in the laboratory was similar to the pattern observed in the field. Each species occupied similar habitats both when alone and when the other species was present. Although habitat use by juveniles of coho salmon and Dolly Varden overlapped among several key parameters, each species primarily exploited resources not readily available to or selected by the other in the natural streams we studied.

Influence of Temperature on Microhabitat Choice by Fishes in a California Stream

Abstract We used eight microhabitat variables to examine the assumption that the variables normally included in instream flow studies are adequate to discriminate among species' microhabitats. When eight variables were available in stepwise-discriminant analysis models to distinguish among the microhabitats of four fish species in a northern California stream, the variance explained ranged from 52 to 77%, and 59 to 86% of the observational records were correctly classified to species. The variables measured were temperature, total depth, focal point elevation (distance of fish from the bottom), focal point velocity (water velocity at fish's snout), mean water column velocity, surface velocity, substrate, and cover. When the number of variables available was reduced to the three normally used in instream flow studies (i.e., total depth, mean water column velocity, and substrate), the variance explained ranged from 0 to 20%, and 46 to 55% of the observational records were correctly classified to species. Wh...

Segregation by species and size classes of rainbow trout, Salmo gairdneri, and Sacramento sucker, Catostomus occidentalis, in three California streams

The hypothesis that Sacramento suckers, Catostomus occidentalis, compete with rainbow trout, Salmo gairdneri, for space in streams was examined by measuring microhabitat utilization of both species in three California streams. Two streams were similar in most respects except one contained only trout and one contained trout and a large population of suckers. The third stream, formed by the union of the first two, contained trout and a small population of suckers. The species overlapped in five of the six microhabitat variables measured: maximum depth, mean water column velocity, focal point velocity, surface water velocity, and substrate type. However, the species had strong vertical segregation; there was little overlap between species in focal point depth. Mean focal point velocities were also significantly different. Suckers roamed over and generally remained in contact with the bottom while trout held position in the water column. Microhabitat utilization by trout in the stream without suckers was similar to in the stream with a higher sucker density. Differences in microhabitat utilization by trout between the third stream and the other two was attributed to the larger size of the third stream. Both sucker and trout showed a similar within-species segregation of size classes - fish under 50 mm in length sought shallow water. Size-specific trends indicated ontogenic shifts in resource utilization which reduced overlap within species. These results suggest that competition for space between trout and suckers was not a major factor regulating microhabitat utilization of trout, although the possibility that larger suckers may displace small trout needs further study.