Experimental analysis of the prey catching behavior ofHydromantes italicus Dunn (Amphibia,Plethodontidae)

Summary 1. The Italian bolitoglossine salamanderHydromantes italicus shows a periodic cave life. In spring and in the fall it leaves the caves after which it lives under stones, in leaves and crevices. Inside the limestone caves,H. i. can be found both in the zone of dim light near the entrance and in total darkness. 2. Corresponding to these two environments there are two guidance systems of the prey catching behavior: one visual and one olfactory. 3. The visually guided prey catching behavior is determined by the stimulus parameters: velocity, size, contrast, and ambient illumination. Continuously moving objects are effective within a velocity range of 0.05 to 6 cm/s with an optimum at 1.25 cm/s (Fig. 2). Stimuli moving stepwise elicit fixation of the prey and complete approach more frequently than continuously moving stimuli. The prey size which elicits prey catching ranges from 0.5 to 10 mm² with an optimum size between 2.5–5.0 mm² (Fig. 3). The prey catching behavior is hardly impaired by a decrease in ambient illumination down to 0.03 cd/m². Beyond 0.03 cd/m², the prey catching activity decreases sharply, but there are still responses at an illumination level of 0.003 cd/m² (Fig. 4). 4.H.i. also responds to stationary non-smelling visual stimuli following stimulation by smell or movement.H.i. is able to detect prey by smell only both in total darkness and in the light (Fig. 5 A). In the light, the prey catching behavior with regard to smelling objects is inhibited by the absence of visual contrast (Fig. 5B).I would like to thank Prof. Dr. R. Altevogt, University of Münster (Germany) for his generous help and constant encouragement. Furthermore, I want to express my gratitude to Prof. Dr. D.B. Wake, University of California, Berkeley (USA), Prof. Dr. P. Mancino, University of Pisa (Italy), and Prof. Dr. J.-P. Ewert, University (GH) of Kassel (Germany) for stimulating discussions and helpful comments.     

Adaptation of transient responses of a movement-sensitive neuron in the visual system of the blowfly Calliphora erythrocephala

We studied temporal response properties of the H1 neuron by extracellular recording. This neuron is a wide-field movement-sensitive element in the visual system of the blowfly (Calliphora erythrocephala). If the neuron is stimulated with a stepwise pattern displacement in its preferred direction, it responds with a burst of action potentials. By repeating the stimulus step one obtains the average of the step response: a 20ms latency time followed by a sharp increase in average firing rate and a slower decay to the resting activity. We report that the characteristic decay time of the step response depends on the stimulus history. If the stimulus moved prior to the step, the higher the pattern velocity, the faster was the decay of the step response to the resting level. In quantitative terms, for velocities in the range 0.4–100°/s, the decay time-constant varies from 300–10ms and is smaller for higher velocities. The time-constant is only weakly affected by other stimulus parameters such as modulation depth or spatial wavelength, and is set independently in different areas of the visual field where it is tuned to the local velocity. We discuss a possible advantage of this form of adaptation for the processing of visual signals: The performance of the nolinear operations that extract information from the visual input can be optimized by prefiltering signals in the individual visual columns with a time-constant that decreases with stimulus velocity. It will be shown that both the test step response and the response to continuous movement can be described reasonably well by a correlation model with input filters that adapt their time-constants.     

Key stimuli of prey for toads (Bufo bufo L.): Configuration and movement patterns

Summary The ability of the Common ToadBufo bufo (L.) to distinguish between a worm-like and an antiworm-like (e.g., a bar at right angles to its direction of motion) moving stripe is not generally altered by movement dynamics, such as velocity of motion or by particular movement patterns—so far as has been investigated. A small square stimulus, however, with its indifferent shape is more attractive as prey for the toad when displaced in a stepwise manner, rather than moved at corresponding continuous (constant) velocity. Step frequencies of 1 to 2 cps were found to have optimal releasing values.Supported by the Deutsche Forschungsgemeinschaft Ew 7/6     

Resonant frequencies of arms and legs identify different walking patterns

The present study is aimed at investigating changes in the coordination of arm and leg movements in young healthy subjects. It was hypothesized that with changes in walking velocity there is a change in frequency and phase coupling between the arms and the legs. In addition, it was hypothesized that the preferred frequencies of the different coordination patterns can be predicted on the basis of the resonant frequencies of arms and legs with a simple pendulum model. The kinematics of arms and legs during treadmill walking in seven healthy subjects were recorded with accelerometers in the sagittal plane at a wide range of different velocities (i.e., 0.3-1. 3m/s). Power spectral analyses revealed a statistically significant change in the frequency relation between arms and legs, i.e., within the velocity range 0.3-0.7m/s arm movement frequencies were dominantly synchronized with the step frequency, whereas from 0.8m/s onwards arm frequencies were locked onto stride frequency. Significant effects of walking speed on mean relative phase between leg and arm movements were found. All limb pairs showed a significantly more stable coordination pattern from 0.8 to 1.0m/s onwards. Results from the pendulum modelling demonstrated that for most subjects at low-velocity preferred movement frequencies of the arms are predicted by the resonant frequencies of individual arms (about 0.98Hz), whereas at higher velocities these are predicted on the basis of the resonant frequencies of the individual legs (about 0.85Hz). The results support the above-mentioned hypotheses, and suggest that different patterns of coordination, as shown by changes in frequency coupling and phase relations, can exist within the human walking mode.     

Dependence of Subjective Traverse Length on Velocity of Moving Tactile Stimuli

Two series of experiments were performed to assess the effects of stimulus velocity on human subjects' perception of the distance traversed by a moving tactile stimulus. In all experiments, constant-velocity stimuli were applied to the dorsal surface of the left forearm; velocities ranging between 1.0 and 256 cm/sec were used. In some experiments the stimuli moved from distal to proximal over the skin, and in others they moved from proximal to distal. The length of skin contacted by the moving stimulus was defined by a plate having an aperture of 4.0 × 0.5 cm.

In the first series of experiments, subjects were required to compare the distance traversed by a test stimulus delivered 2 sec after a standard stimulus, and also to report the on-locus and the off-locus of the brushing stimulus. In the second series of experiments, the subjects rated the perceived distance on the skin using a free-magnitude-estimation procedure. The data from both series of experiments defined the same relationship between stimulus velocity and perceived stimulus distance. More specifically, although the length of skin contacted by the stimulus was the same at all velocities, subjects' estimates of stimulus distance decreased with increasing stimulus velocity. In addition, the function relating estimates of stimulus distance to velocity was flat for velocities between 5 and 20 cm/sec, but possessed an appreciable negative slope at lower and higher velocities.

It is interesting that the plateau of the relationship between perceived stimulus distance and velocity occurred within the range of velocities that human subjects employ to scan textured surfaces; it also corresponded precisely with the range of stimulus velocities at which the directional sensitivity of somatosensory cortical neurons and human subjects is optimal.     

Estimation of thermal sensation using PMV and SET under high air movement conditions

1. 1. Our previous experimental results showed the thermal sensation vote was much less than the values of PMV and SET^* at air velocities above 0.5 m/s.

2. 2. The method to modify SET^* is presented from the results of subjective experiments taking account of decrease in clo value of summer clothing and decrease in skin wettedness due to increased air velocity.

3. 3. Thermal resistance under increased air movement on a standard summer clothing ensemble was measured. Basic thermal insulation of the summer ensemble was reduced by 25% at air velocity of 1.0 m/s.

4. 4. Thirty-two subjects were exposed at operative temperatures of 27 and 30°C under 1 m/s air movement in order to determine the amount of skin diffusion. Measured evaporation heat loss from skin surface was much smaller at air velocity of 1 m/s than that predicted by SET^*.

5. 5. Estimated thermal sensation vote using modified SET^* agreed well with our previous experimental results under different air velocities for the same clothing.

Minimum time required for assessing step variability during running at submaximal velocities

This study aimed to determine the minimum time required for assessing spatiotemporal variability during continuous running at different submaximal velocities and, thereby, the number of steps required. Nineteen trained endurance runners performed an incremental running protocol, with a 3-min recording period at 10, 12, 14 and 16 km/h. Spatiotemporal parameters (contact and flight times, step length and step frequency) were measured using the OptoGait system and step variability was considered for each parameter, in terms of within-participants standard deviation (SD) and coefficient of variation (CV%). Step variability was considered over six different durations at every velocity tested: 0–10 s, 0–20 s, 0–30 s, 0–60 s, 0–120 s and 0–180 s. The repeated measures ANOVA revealed no significant differences in the magnitude of the four spatiotemporal parameters between the recording intervals at each running velocity tested (p ≥ 0.05, ICC > 0.90). The post-hoc analysis confirmed no significant differences in step variability (SD and CV% of each spatiotemporal parameter at any velocity tested) between measurements. The Bland-Altman limits of agreement method showed that longer recording intervals yield smaller systematic bias, random errors, and narrower limits of agreement, regardless of running velocity. The results suggest that the duration of the recording period required to estimate spatiotemporal variability plays an important role in the accuracy of the measurement, regardless of running velocity (10–16 km/h).     

Response characteristics of wide-field non-habituating non-directional motion detecting neurons in the optic lobe of the locust,Locusta migratoria

1. Extracellular recordings from wide-field nonhabituating non-directional (ND) motion detecting neurons in the second optic chiasma of the locust Locusta migratoria are presented. The responses to various types of stepwise moving spot and bar stimuli were monitored (Fig. 1)
2. Stepwise motion in all directions elicited bursts of spikes. The response is inhibited at stimulus velocities above 5°/s. At velocities above 10°/s the ND neurons are slightly more sensitive to motion in the horizontal direction than to motion in the vertical direction (Fig. 2). The ND cells have a preference for small moving stimuli (Fig. 3).
3. The motion response has two peaks. The latency of the second peak depends on stimulus size and stimulus velocity. Increasing the height from 0.1 to 23.5° of a 5°/s moving bar results in a lowering of this latency time from 176 to 130 ms (Fig. 4). When the velocity from a single 0.1° spot is increased from 1 to 16°/s, the latency decreases from 282 to 180 ms (Figs. 5–6).
4. A change-of-direction sensitivity is displayed. Stepwise motion in one particular direction produces a continuous burst of spike discharges. Reversal or change in direction leads to an inhibition of the response (Fig. 7).
5. It shows that non-directional motion perception of the wide-field ND cells can simply be explained by combining self-and lateral inhibition.

     

Peripheral and central nervous responses evoked by small water movements in a cephalopod

Potentials were recorded from the epidermal head lines and from the CNS of young cuttlefish, Sepia officinalis, in response to weak water movements. 1. Within the test range 0.5-400 Hz a sinusoidal water movement elicits up to 4 components of response if the electrode is placed on a headline: (i) a positive phasic ON response; (ii) a tonic frequency-following microphonic response; (iii) a slow negative OFF response; and (iv) compound nerve impulses. 2. The amplitude of both the ON wave and the microphonic potential depends on stimulus frequency, stimulus amplitude and stimulus rise time. Frequencies around 100 Hz and short rise times are most effective in eliciting strong potentials. The minimal threshold was 0.06 microns peak-to-peak water displacement at 100 Hz (18.8 microns/s as velocity). 3. Change of direction of tangential sphere movement (parallel vs. across the head lines) has only a small effect on the microphonic and the summed nerve potentials. 4. Frequency and/or amplitude modulations of a carrier stimulus elicit responses at the onset and offset of the modulation and marked changes in the tonic microphonic response. 5. Evoked potentials can be recorded from the brain while stimulating the epidermal lines with weak water movements. The brain potentials differ in several aspects from the potentials of the head lines and show little or no onset or offset wave at the transitions of a frequency and amplitude modulation.     

Response of the Praying Mantis,Sphodromantis Viridis,to Target Change in Size and to Target Visual Occlusion

Mantises (Mantodea, Mantidae) visually detect insect prey and capture it by a ballistic strike of their specialized forelegs. We tested predatory responses of female mantis, Sphodromantis viridis, to computer generated visual stimuli, to determine the effects of (i) target size and velocity (ii) discrete changes in target size and (iii) visual occlusion. Maximal predatory responses were elicited by stimuli that (i) subtended ~20°–23° horizontally and ~16°–19° vertically, at the eye, and moved across the screen at angular velocities of ~46°–119°/s, (ii) increased in size in a stepwise manner, with step duration ≥0.8 s, while stimuli decreasing in size elicited only peering movements, (iii) Stimuli disappearing gradually behind a virtual occlusion elicited one or more head saccades but not actual interception.     

Vitrification of bovine blastocysts obtained by in vitro culture of oocytes matured and fertilized in vitro.

Two experiments were conducted to assess the viability of bovine blastocysts obtained by in vitro fertilization of oocytes matured in vitro (IVM-IVF) and cryopreserved by vitrification. In Expt 1, the optimal concentrations of glycerol and 1,2-propanediol in the basic medium (modified TCM199) for cooling and warming without formation of ice crystals were determined by plunging the solution into liquid nitrogen and then warming it in a water bath at 15 degrees C; when both glycerol and 1,2-propanediol were present in the solution (> 45% v/v), vitrification of the medium was observed. In Expt 2, IVM-IVF blastocysts were equilibrated to the mixture of glycerol and 1,2-propanediol (0% to 45%) at 15 degrees C in a stepwise manner as follows: (i) in one step, for 18 min to the final vitrification solution; (ii) in two steps, for 8 min in the first step and 10 min in the second step; (iii) in four steps, for 4 min in the first three steps and 6 min in the last step; (iv) in eight steps, for 2 min in each step, but 4 min in the last step; and (v) in 16 steps, for 1 min in each step, but 3 min in the last step. After removal of cryoprotectants, the blastocysts were cultured for 24 h in vitro. The survival rates for the embryos equilibrated in 1, 2, 4, 8 and 16 step(s) were 56, 89, 100, 100 and 100%, respectively. The blastocysts equilibrated in 1, 2, 4, 8 and 16 steps were vitrified by plunging the straws containing them into liquid N2, thawed and cultured in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)     

Visual acuity and contrast sensitivity of adult zebrafish

Background

The aim of this study was to evaluate the visual acuity of adult zebrafish by assessing the optokinetic reflex. Using a modified commercially available optomotor device (OptoMotry?), virtual three-dimensional gratings of variable spatial frequency or contrast were presented to adult zebrafish. In a first experiment, visual acuity was evaluated by changing the spatial frequency at different angular velocities. Thereafter, contrast sensitivity was evaluated by changing the contrast level at different spatial frequencies.

Results

At the different tested angular velocities (10, 15, 20, 25, and 30 d/s) and a contrast of 100%, visual acuity values ranged from 0.56 to 0.58 c/d. Contrast sensitivity measured at different spatial frequencies (0.011, 0.025, 0.5, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.55 c/d) with an angular velocity of 10 d/s and 25 d/s revealed an inverted U-shaped contrast sensitivity curve. The highest mean contrast sensitivity (±SD) values of 20.49?±?4.13 and 25.24?±?8.89 were found for a spatial frequency of 0.05 c/d (angular velocity 10 d/s) and 0.1 c/d (angular velocity 25 d/s), respectively.

Conclusions

Visual acuity and contrast sensitivity measurements in adult zebrafish with the OptoMotry? device are feasible and reveal a remarkably higher VA compared to larval zebrafish and mice.     

Modelling the Effects of Current on Prey Acquisition in Planktivorous Fishes

Many planktivorous fishes forage in currents, where they actively maintain position and visually strike at current-entrained zooplankton. In general, the zooplankton are wafted by the foraging fish at a rate equivalent to the current velocity. From a fish's viewpoint the plankton approach either head-on or offset at varied distances from the fish's position. We present a model that describes the relative motion of particles as they approach and pass a foraging fish at different offset distances, and the rate of change in apparent size as they close on a fish. In addition, a series of experiments of fish feeding on plankton in a flume at increasing current velocities revealed that two basic tactics are utilized. At low current velocities (<10-14 cm s m 1), the fish swims toward the prey, whereas at higher current velocities the fish tends to fall back with the current to capture a prey item. The model and experimental results are discussed in terms of the visual problems associated with the detection and tracking of items in motion.     

A cross-bridge model that is able to explain mechanical and energetic properties of shortening muscle.

The responses of muscle to steady and stepwise shortening are simulated with a model in which actin-myosin cross-bridges cycle through two pathways distinct for the attachment-detachment kinetics and for the proportion of energy converted into work. Small step releases and steady shortening at low velocity (high load) favor the cycle implying approximately 5 nm sliding per cross-bridge interaction and approximately 100/s detachment-reattachment process; large step releases and steady shortening at high velocity (low load) favor the cycle implying approximately 10 nm sliding per cross-bridge interaction and approximately 20/s detachment-reattachment process. The model satisfactorily predicts specific mechanical properties of frog skeletal muscle, such as the rate of regeneration of the working stroke as measured by double-step release experiments and the transition to steady state during multiple step releases (staircase shortening). The rate of energy liberation under different mechanical conditions is correctly reproduced by the model. During steady shortening, the relation of energy liberation rate versus shortening speed attains a maximum (approximately 6 times the isometric rate) for shortening velocities lower than half the maximum velocity of shortening and declines for higher velocities. In addition, the model provides a clue for explaining how, in different muscle types, the higher the isometric maintenance heat, the higher the power output during steady shortening.     

Temperature effects on ballistic prey capture by a dragonfly larva

Understanding the effects of temperature on prey–predator interactions is a key issue to predict the response of natural communities to climate change. Higher temperatures are expected to induce an increase in predation rates. However, little is known on how temperature influences close‐range encounter of prey–predator interactions, such as predator's attack velocities. Based on the speed–accuracy trade‐off concept, we hypothesized that the increase in predator attack velocity by increasing temperature reduces the accuracy of the attack, leading to a lower probability of capture. We tested this hypothesis on the dragonfly larvae Anax imperator and the zooplankton prey Daphnia magna. The prey–predator encounters were video‐recorded at high speed, and at three different temperatures. Overall, we found that (1) temperature had a strong effect on predator's attack velocities, (2) prey did not have the opportunity to move and/or escape due to the high velocity of the predator during the attack, and (3) neither velocity nor temperature had significant effects on the capture success. By contrast, the capture success mainly depended on the accuracy of the predator in capturing the prey. We found that (4) some 40% of mistakes were undershooting and some 60% aimed below or above the target. No lateral mistake was observed. These results did not support the speed–accuracy trade‐off hypothesis. Further studies on dragonfly larvae with different morphological labial masks and speeds of attacks, as well as on prey with different escape strategies, would provide new insights into the response to environmental changes in prey–predator interactions.     

Directionality of antennal sweeps elicited by water jet stimulation of the tailfan in the crayfish Procambarus clarkii

Summary Directionality and intensity dependence of antennal sweeps elicited by water jet stimulation of the tailfan in tethered, reversibly blinded adult and juvenile crayfish (Procambarus clarkii) were analyzed.Resting crayfish keep their antennae at about 50° symmetrically to the longitudinal body axis (Figs. 2 bottom, and 3).In adults, tailfan stimulation elicits synchronous backward sweeps of both antennae, which increase for more caudal stimulus directions (Figs. 2–4 and 5A). Directions differing by 30°–60° are significantly distinguished (Fig. 4). The mean sweep of the ipsilateral antenna significantly overrides that of the contralateral antenna for rostrolateral stimulation at 40–200 mm/s stimulus velocity and lateral to caudolateral stimulation at 40 mm/s and thus lateralization of the stimulus is revealed (Figs. 2 top, 4 and 5A). Mean antennal sweeps at a given stimulus direction and distance increase with increasing stimulus velocity (40–250 mm/s, Fig. 5A).In juveniles, the directional dependence of antennal sweeps is reduced compared to that of adults, while a similar intensity dependence is found (Fig. 5B).The pronounced directionality of the antennal response in adult crayfish vanishes and response latencies increase after reversibly covering the tailfan with a small bag or the telson with waterproof paste (Figs. 6 and 7). Thus, tailfan and especially telson mechanoreceptors play an important role in the localization of water movements elicited by predators or prey behind the crayfish.     

Responses of the spider Argiope aurantia to low frequency phasic and continuous vibrations

I maintained female orb-weaving spiders, Argiope aurantia, in the laboratory and subjected them to low frequency phasic and continuous transverse vibrations of known frequency, amplitude, and position on the web. Stimuli were administered for periods of 1 min and all behaviour recorded. Pumping was associated with tactile contact to the spider's abdomen. Pumping increased with increasing frequency of phasic stimuli delivered to the web surface. Attack increased linearly with the log of stimulus frequencies between 0 and 14 Hz for stimuli delivered to the catching spiral. Dropping occurred only when high frequencies and amplitudes of stimuli were administered to the spider's abdomen and the hub of the web.     

Velocity and dominance affect prey capture and microhabitat selection in juvenile Chinook (<Emphasis Type="Italic">Oncorhynchus tshawytscha</Emphasis>)

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.     

The chemo-mechanical coupling relation in the oscillatory contraction-relaxation cycles of insect fibrillar muscle.

The mechanical properties and the activity of the myofibrillar ATPase have been investigated at 21 degrees C on glycerinated back muscle from the water-bug Lethocerus colossicus. When the fibres were held under isometric conditions after stretching them by 0.5--4%, the ATPase required to maintain a given tension increases from 19 to 39 p-moles ATP split for each mg of tension developed as the Ca2+ level is increased from 10(-7) to up to 10(-5) M. The mechanical properties and the ATPase activity have been determined for Ca2+-activated fibres using sinusoidal frequencies of 1--30 HZ and oscillatory amplitudes of 0.5--6% peak-to-peak. In this way the R.M.S. velocity of sinusoidal movement was varied between 0.1-10 mm/sec. The rate of ATP splitting associated with oscillatory tension development, the dynamic tension cost, increases both with Ca2+ and with frequency of oscillation (at 1% peak-to-peak amplitude), becoming as high as four times the isometric value. The oscillatory power output which can be obtained is increased when the Ca2+ level is raised from 10(-7) to 10(-5) M or towards higher amplitudes of oscillation. The chemo-mechanical coupling efficiency increases proportionally with the R.M.S. velocity of muscle movement. In presence of 10(-5) M Ca2+ optimal efficiencies of 5.5--6.2 kcal work per mole ATP split are obtained at R.M.S. velocities of 1.3--2 muscle lengths/sec. The ability of the muscle fibres to perform osciillatory work at the higher frequencies was much reduced at lower Ca2+ levels of 10(-6) or 10(-7) M and the maximal efficiencies never exceeded 2.2 kcal/mole.     

Downstream migration and critical water velocities in stream channels for fry of four salmonid species

Fry of brown trout, Atlantic salmon, brook trout and lake trout were tested for downstream migration and critical velocities with a method of stepwise increasing water velocities. Each velocity was tested for 15 min before increase to the next step. Critical velocities for fry entering the free-feeding stage, defined as the stage when the fry has resorbed its yolk sac and will have to ascend from the bottom gravel to catch food, were between 0.10 and 0.25 m s⁻¹, varying among individuals and depending on species and water temperature. Downstream displacement started at lower velocities. Lake trout had the lowest critical velocity. Temperature influenced swimming performance considerably. On average, a 7°C increase in temperature resulted in a 0.05 m s⁻¹ increase in critical velocity. The fry actively searched out the low-velocity niches in the channels. Flow-sensivity gradually decreases with fry development; when the fry had reached a length of 40–50 mm they were able to tolerate water velocities higher than 0.50 m s⁻¹.