Trout Swimming Research Articles

Muscle activity and hydrodynamic function of pelvic fins in trout (Oncorhynchus mykiss)

Contrary to the previous premise that pelvic fins lacked obvious function, recent work on three-dimensional fin motions suggests that pelvic fins actively control stability and speed in slowly swimming trout. This study used electromyography to measure pelvic fin muscle activity and particle imaging velocimetry to quantify flow along the ventral body region to test this hypothesis. Fish swam at slow speeds (0.13-1.36 BL s(-1)) while being filmed with three high speed cameras. Three-dimensional kinematics were captured for all trials. During EMG trials pelvic fin muscle activity was synchronized to kinematic motion, during particle imaging velocimetry trials, a laser light-sheet was used to visualize the flow surrounding the ventral aspect of the fish. Four main conclusions are reached: first, pelvic fins are actively oscillated during slow-speed swimming; antagonistic abductor and adductor muscles contracted simultaneously, their collective action producing a unique contralateral oscillating behaviour in the fins. Second, pelvic fins slow the flow along the ventral side affecting pitch and yaw instabilities; flow upstream of the pelvic fins is slowed by 0.02 m s(-1) and flow downstream of the pelvic fins is slowed by 0.034 m s(-1) compared with free stream flow. Third, pelvic fin wake influences anal fin angle of attack; flow angle in the wake of the pelvic fin was 33.84+/-2.4 deg. (max) and -11.83+/-11.2 deg. (min) compared with the free stream flow angle of 1.27+/-0.1 deg. Fourth, pelvic fins appear to actively damp body oscillation during slow-speed swimming, providing drag to help control speed and stabilize the body position during slow-speed swimming.

Escaping Flatland: three-dimensional kinematics and hydrodynamics of median fins in fishes

Fish swimming has often been simplified into the motions of a two-dimensional slice through the horizontal midline, as though fishes live in a flat world devoid of a third dimension. While fish bodies do undulate primarily horizontally, this motion has important three-dimensional components, and fish fins can move in a complex three-dimensional manner. Recent results suggest that an understanding of the three-dimensional body shape and fin motions is vital for explaining the mechanics of swimming, and that two-dimensional representations of fish locomotion are misleading. In this study, we first examine axial swimming from the two-dimensional viewpoint, detailing the limitations of this view. Then we present data on the kinematics and hydrodynamics of the dorsal fin, the anal fin and the caudal fin during steady swimming and maneuvering in brook trout, Salvelinus fontinalis, bluegill sunfish, Lepomis macrochirus, and yellow perch, Perca flavescens. These fishes actively move the dorsal and anal fins during swimming, resulting in curvature along both anterio-posterior and dorso-ventral axes. The momentum imparted to the fluid by these fins comprises a substantial portion of total swimming force, adding to thrust and contributing to roll stability. While swimming, the caudal fin also actively curves dorso-ventrally, producing vortices separately from both its upper and lower lobes. This functional separation of the lobes may allow additional control of three-dimensional orientation, but probably reduces swimming efficiency. In contrast, fish may boost the caudal fin's efficiency by taking advantage of the flow from the dorsal and anal fins as it interacts with the flow around the caudal fin itself. During maneuvering, fish readily use their fins outside of the normal planes of motion. For example, the dorsal fin can flick laterally, orienting its surface perpendicular to the body, to help in turning and braking. These data demonstrate that, while fish do move primarily in the horizontal plane, neither their bodies nor their motions can accurately be simplified in a two-dimensional representation. To begin to appreciate the functional consequences of the diversity of fish body shapes and locomotor strategies, one must escape Flatland to examine all three dimensions.

Myosin regulatory light chain expression in trout muscle

Muscle's contractile properties can vary along different trajectories, including between muscle fiber types, along the body (within a muscle fiber type), and between developmental stages. This study explores the role of the regulatory myosin light chain (MLC2) in modulating contractile properties in rainbow trout myotomal muscle. Rainbow trout show longitudinal variations in muscle activation and relaxation, with faster contractile properties in the anterior myotome. The expression of two muscle proteins, troponin T and parvalbumin, vary along the length of trout in concert with shifts in muscle activation and relaxation. However, there is no longitudinal variation in myosin heavy chain in trout. This study explores the role of MLC2 (or regulatory light chain), part of the myosin hexamer, in contributing to longitudinal variations in contractile properties of trout swimming muscle. We cloned and sequenced two isoforms of MLC2 from trout muscle and used real-time quantitative polymerase chain reaction to assess the relative expression of these two isoforms in red and white muscle from different body positions of two ages of rainbow trout: parr and smolt. Longitudinal variations in slow (sMLC2) but not fast (fMLC2) regulatory light chain isoforms were observed in young trout parr but not older trout smolts. The differences in sMLC2 expression correlated with shifts in muscle contractile properties in the parr.

Parvalbumin expression in trout swimming muscle correlates with relaxation rate

Rainbow trout (Oncorhynchus mykiss) display longitudinal and developmental shifts in muscle relaxation rate. This study aimed to determine the role of variations in parvalbumin content in modulating muscle relaxation. Parvalbumin is a low molecular weight protein that buffers myoplasmic Ca2+ and enhances muscle relaxation. In some fish, longitudinal variations in muscle relaxation have been linked to variations in the total amount of parvalbumin present in muscle and in the relative expression of two parvalbumin isoforms. We have demonstrated previously that anterior slow-twitch or red myotomal muscle relaxes more rapidly than that from the posterior for both rainbow and brook trout. Further, younger rainbow trout parr have faster red muscle relaxation rates than older smolts. Here we report similar results for fast-twitch or white muscle. We quantified the parvalbumin expression in red and white muscle from different body positions of rainbow trout parr and smolts and for brook trout (Salvelinus fontinalis) adults. There was a significant shift in total parvalbumin content of muscle: the faster muscle from the anterior myotome contained greater amounts of parvalbumin. For brook trout, longitudinal variation in relaxation rate was also associated with shifts in the relative expression of the two parvalbumin isoforms. The faster muscle of parr contained more parvalbumin. Lastly, trout white muscle tended to have higher levels of parvalbumin and greater levels of the Parv2 (relative to Parv1) isoform as compared to red muscle. Parvalbumin expression correlated with muscle relaxation rate in trout, although there were species-specific differences in the importance of altering total parvalbumin content versus shifts in relative parvalbumin isoform expression.

The Poetics of In and Out

Pilkington continuedfrom previous page investigation. From time to time the man recalls for the boy people and events from his past life. Three different times he remembers seeing lovely trout swimming in clear mountain streams only to acknowledge that there are now no trout and those streams are part of a blasted world. He recognizes that one ofthe things that separate him from his son is that terrible day of a decade ago: He turned and looked at the boy. Maybe he understood for the first time that to the boy he was himself an alien. A being from the planet that no longer existed. The tales ofwhich were suspect. He could not construct for the child's pleasure the world he's lost without constructing the loss as well and he thought perhaps the child had known this better than he. Clearly The Road resurrects the "wasteland theme" of early twentieth-century European and American literature. McCarthy's indebtedness to Hemingway has been commented on to the point of cliché, but there is no other way of describing his prose style in the novel except Hemingwayesque. Many passages consist of detailed descriptions of tasks necessary for survival, tasks he is also teaching his son in preparation for a time he will no longer be there to take care of him. McCarthy's spare lyricism and existential anguish are reminiscent notjust of Hemingway, but even more perhaps of Samuel Beckett: He walked out in the gray light and stood and saw for a brief moment the absolute truth of the world. The cold relentless circling of the intestate earth. Darkness implacable. The blind dogs of the sun in their running. The crushing black vacuum ofthe universe. And somewhere two hunted animals trembling like groundfoxes in their cover. Borrowed time and borrowed world and borrowed eyes with which to sorrow it. The Road is powerful fiction. Blood Meridian is still McCarthy's masterpiece, but the new novel is not far behind. It is dark, humorless, brooding—and completely plausible. McCarthy is not bullish on human nature. He seems to think if the natural order can be disrupted, perhaps irreparably, we will find a way to do it. The only redeeming feature of this bleak picture is the tiny sliver of hope at the end of the Road. It's not much, but at least it's something. Tom Pilkington is University Scholar at Tarleton State University in Stephenville, Texas. He is the author, most recently, of State of Mind: Texas Literature and Culture. The Poetics of In and Out Douglas Messerli The Bitter Half Toby Olson FC2 http://fc2.org 248 pages; paper, $19.95 Before memories of high school humor overwhelm any of my readers, let me assure them that, despite any associations evoked by this essay's title, my subject, Toby Olson's new novel, The BitterHalf, portrays no acts ofsex. The sexual activities ofwhich it hints take place offstage in this dark comedy. The in and outs of my title might be said to relate to everything except the act of sex. The major character of this fiction, Chris Pollard (whose last name is defined through dictionary quotes in the frontispiece ofthe book), is a consultant in the field of prison escapes—a job which Pollard has invented at a time in the Great Depression when any job, let alone newly created ones, is as sparse as the vegetation around the Pearce, Arizona, border prison where the fiction begins. Pollard has been asked out to evaluate the prison for flaws, particularly since its population ofmostly Mexican men has escaped on a regular basis and, most importantly, because the prison now houses a young man, little more than a boy dubbed by authorities as "the kid," who is a legend to inmates because of his escape from the most notorious of prisons. Pollard and the boy exchange only a few words, but within those moments an unspoken relationship between the two is established. Pollard has several suggestions for prison security, but recognizes that the job is primarily a "boondoggle," a political formality to protect the authorities when "the kid" makes his move, which, the moment the inspector has completed...

Peripherally administered growth hormone increases brain dopaminergic activity and swimming in rainbow trout

There is increasing evidence that growth hormone (GH) has important behavioral effects in fish, but the underlying mechanisms are not well understood. To investigate if peripherally administered GH influences the monoaminergic activity of the brain, and how this is correlated to behavior, juvenile rainbow trout were implanted intraperitoneally with ovine GH. Fish were either kept isolated or in groups of five. The physical activity and food intake of the isolated fish were observed after 1 and 7 days, when brains were also sampled. The content of serotonin, dopamine, and noradrenaline and their metabolites in hypothalamus, telencephalon, optic tectum, and brain stem was then analyzed. For fish kept isolated for 7 days following implant, GH increased swimming activity and the levels of the dopamine metabolite 3, 4-hydroxy-phenylacetic acid (DOPAC) were higher in all brain parts examined. In the optic tectum, the levels of the dopamine metabolite homovanillic acid (HVA) were lowered by the GH treatment. One-day GH implant did not affect behavior or monoamine levels of isolated fish. In the fish kept in groups, a 7-day GH implant increased the hypothalamic levels of DOPAC, but not in the other brain parts examined, which may indicate an effect on the brain dopaminergic system from social interactions. It can be concluded that peripherally administered GH may function as a neuromodulator, affecting the dopaminergic activity of the rainbow trout brain, and this is associated with increased swimming activity.

Measuring cultured fish swimming behaviour: first results on rainbow trout using acoustic telemetry in tanks

An experiment was conducted to measure swimming activity of rainbow trout (254.0±33.7 g) held in outdoor tanks under three culture conditions with stocking density used to generate differences: 27 (Culture Density 1, CD1), 80 (CD2) and 136 kg m −3 (CD3). Fish were fed dry pellets at 1% of tank biomass from 9h00 to 13h00 with automatic feeders. Using acoustic telemetry, fish positions were monitored every 5 s during 48 h for nine fish (three in each treatment). Swimming behaviour was analysed in terms of trajectories: they varied between (i) holding position (high turning angles), (ii) chaotic trajectories (equal use of all turning angles) and (iii) circular swimming (average turning angle of 60°). Space utilisation differed for each culture condition: CD3 conditions induced a prolonged residence time in the central zone of the tank. Rainbow trout swimming activity patterns and levels differed depending on both day–night alternation and culture conditions. All fish reared at CD1 and one reared at CD2 were day-active. All the other fish showed high activity pattern variability and a higher swimming level was observed under CD3. These results illustrate that monitoring fish swimming activity is feasible even under high densities and provide relevant insights on fish activity which can lead to behavioural welfare indices in relation to constraints imposed by culture conditions.

The Kármán gait: novel body kinematics of rainbow trout swimming in a vortex street

Most fishes commonly experience unsteady flows and hydrodynamic perturbations during their lifetime. In this study, we provide evidence that rainbow trout Oncorhynchus mykiss voluntarily alter their body kinematics when interacting with vortices present in the environment that are not self-generated. To demonstrate this, we measured axial swimming kinematics in response to changes in known hydrodynamic wake characteristics. We compared trout swimming in the Kármán street behind different diameter cylinders (2.5 and 5 cm) at two flow speeds (2.5 and 4.5 L s(-1), where L is total body length) to trout swimming in the free stream and in the cylinder bow wake. Trout swimming behind cylinders adopt a distinctive, previously undescribed pattern of movement in order to hold station, which we term the Kármán gait. During this gait, body amplitudes and curvatures are much larger than those of trout swimming at an equivalent flow velocity in the absence of a cylinder. Tail-beat frequency is not only lower than might be expected for a trout swimming in the reduced flow behind a cylinder, but also matches the vortex shedding frequency of the cylinder. Therefore, in addition to choosing to be in the slower flow velocity offered behind a cylinder (drafting), trout are also altering their body kinematics to synchronize with the shed vortices (tuning), using a mechanism that may not involve propulsive locomotion. This behavior is most distinctive when cylinder diameter is large relative to fish length. While tuning, trout have a longer body wavelength than the prescribed wake wavelength, indicating that only certain regions of the body may need to be oriented in a consistent manner to the oncoming vortices. Our results suggest that fish can capture energy from vortices generated by the environment to maintain station in downstream flow. Interestingly, trout swimming in front of a cylinder display lower tail-beat amplitudes and body wave speeds than trout subjected to any of the other treatments, implying that the bow wake may be the most energetically favorable region for a fish to hold station near a cylinder.

Red muscle function during steady swimming in brook trout, Salvelinus fontinalis

Red muscle function during steady swimming in brook trout was studied through both in vivo swimming and in vitro muscle mechanics experiments. In the swimming experiments, red muscle activity was characterized through the use of electromyography and sonomicrometry, allowing the determination of several parameters such as tailbeat frequency, EMG burst duration, muscle length change patterns and relative phase of EMG activity and length change. Brook trout do show some shifts in these variables along their length during steady swimming, but the magnitude of these shifts is relatively small. In the muscle mechanics experiments, the in vivo muscle activity data were used to evaluate patterns of power production by red muscle during swimming. Unlike many fish species, the red muscle along the length of brook trout shows little change in isometric kinetic variables such as relaxation rate and twitch time. Furthermore, there is no rostral–caudal shift in red muscle mass-specific power output during steady swimming. This last result contrasts sharply with rainbow trout and with a variety of other fish species that power steady swimming primarily with the posterior red myotome.

Non‐invasive recording of heart rate and ventilation rate in rainbow trout during rest and swimming. Fish go wireless!

Resting heart rates and ventilation rates in rainbow trout Oncorhynchus mykiss at 15°C are 31·8±1·8 beat min−1 and 53·1±3·7 breaths min−1, respectively. The non‐invasive recording system picked up the bioelectric potentials generated by the fish in the water and was based on an array of six silver‐silver chloride electrodes covered with agar‐gel, which provided a better signal‐to‐noise ratio than in previously described systems, and allowed the determination of heart rate and ventilation rates at different swimming speeds up to 21 s−1. In concert with the lower rates, the scope for changes in heart rate and ventilation rate during swimming was also considerably larger than in earlier studies (2·4‐ and 2·0‐fold, respectively). Two main conclusions result from this work: (i) short recovery times under 48 h after anaesthesia and surgery are unlikely to provide truly resting heart rates and ventilation rates in trout at 15°C; (ii) heart rate regulation during exercise is more important than previously thought and might account for a larger proportion of the increase in cardiac output observed in swimming trout.

Influence of body weight, swimming characteristics, and water temperature on the cost of swimming in brook trout (Salvelinus fontinalis)

We performed respirometry experiments to estimate the spontaneous swimming costs of brook trout (Salvelinus fontinalis) for 24 combinations of fish weight (3.5, 17, and 32 g), water temperature (4, 12, and 18°C), and respirometer size (27, 54, and 108 L). Fish swimming characteristics were estimated for each experiment using videocamera recordings and image analysis. Under our experimental conditions, average swimming characteristics of fish, such as swimming speed and turning and acceleration rates, varied from 2.5- to 29-fold. Our data, alone or combined with similar published results on brook trout weighing 1 g, indicated that fish weight was the only variable that could explain a statistically significant proportion of the variations of spontaneous swimming costs for that species (r2 = 0.91). Our work confirms, with a wider range of experimental data, that spontaneous swimming costs of fish are 3- to 22-fold (8-fold average difference) more energy demanding than predicted by forced swimming models developed using fish swimming at constant speeds and directions in flumes.

Power production during steady swimming in largemouth bass and rainbow trout.

Steady swimming in fishes is powered by the aerobic or red muscle, but there are conflicting theories on the relative roles of the anterior and posterior red muscle in powering steady swimming. To examine how red muscle is used to power steady swimming in rainbow trout (Oncorhynchus mykiss), electromyographic (EMG) and sonomicrometry recordings were made of muscle activity in vivo. These data were used in in vitro work-loop studies of muscle power production. Data on in vitro power production were also collected for largemouth bass (Micropterus salmoides) red muscle from previously published data on in vivo muscle activity. The in vivo data collected from swimming trout were similar to those for other species. The anterior red muscle of these fish has the longest duty cycle, the smallest phase shift between the onset of EMG activity and maximum muscle length during each tailbeat and undergoes the smallest strain or length change. For both trout and largemouth bass, work-loop experiments indicate that the majority of power for steady swimming is generated by the posterior muscle, as has been observed in other species.

Influence of body weight, swimming characteristics, and water temperature on the cost of swimming in brook trout (<i>Salvelinus fontinalis</i>)

Influence of body weight, swimming characteristics, and water temperature on the cost of swimming in brook trout (<i>Salvelinus fontinalis</i>)

CO 2 transport and excretion in rainbow trout ( Oncorhynchus mykiss) during graded sustained exercise

A quantitative analysis of CO 2 transport and excretion was conducted in seawater acclimated rainbow trout ( Oncorhynchus mykiss) swimming at different sustained swimming velocities. CO 2 excretion increased linearly with cardiac output during exercise but arterial P CO 2 (Pa CO 2 ) and total CO 2 levels also increased indicating a diffusion limitation to CO 2 excretion. The elevated Pa CO 2 was not accompanied by a decrease in pH, indicating that the acid–base compensation was rapid. Mixed-venous P CO 2 increased to a greater extent than Pa CO 2 resulting in a large increase in the venous–arterial difference in P CO 2 (Pv CO 2 −Pa CO 2 ). The Pv CO 2 −Pa CO 2 difference was used to calculate the proportion of total CO 2 excreted comprised of dissolved CO 2 which accounted for less than 1% of total CO 2 excreted in fish swimming at 11 cm sec −1 but increased to about 9% at the greatest swimming velocity indicating that the pattern of CO 2 excretion changes during exercise. There was no effect of exercise on the proportion of CO 2 excreted which was dependent upon HCO 3 −/Cl − exchange (54%) or that which was dependent upon the dehydration of HCO 3 − that resided within the red cell prior to gill blood entry (42%). The large proportion of total CO 2 excreted that was dependent upon HCO 3 −/Cl − exchange is significant because this is thought to be the rate limiting step in CO 2 excretion.

Initial recruitment of anaerobic metabolism during sub-maximal swimming in rainbow trout (Oncorhynchus mykiss)

Lactate and phosphocreatine concentrations were monitored in the white muscle of rainbow trout Oncorhynchus mykiss following swimming at 70, 80 and 100 % of critical swimming speed (Ucrit) using 31P-nuclear magnetic resonance (NMR) spectroscopy. Lactate was detected following swimming at all speeds, but its concentration was greatest following swimming at 80 and 100 %Ucrit. Phosphocreatine stores were reduced following swimming at 70 and 80 %Ucrit, but were further depleted following swimming at 100 %Ucrit. Following swimming at 70 and 80 %Ucrit, lactate concentrations were higher in the rostral, relative to the caudal, musculature, whereas phosphocreatine stores were least depleted in the mid, relative to the rostral and caudal, musculature. There were no differences among muscle locations in concentrations of lactate and phosphocreatine following swimming to 100 %Ucrit. Our results indicate that anaerobic metabolism is required to support swimming at speeds equal to and greater than 70 %Ucrit. Estimates of the relative cost of anaerobic metabolism during sub-maximal swimming are presented for swimming at 70 %, 80 % and 100 %Ucrit, and the implications of these results to energy budgets are discussed.

Going against the flow: an examination of the propulsive movements made by young brook trout in streams

We examined the propulsive movements and behaviour of young-of-the-year (YOY) brook trout (Salvelinus fontinalis) swimming in their natal streams. Our findings demonstrated that swimming performance was influenced by temporal and spatial heterogeneity in water flow. Pectoral fins were used commonly, even by individuals swimming in fast flowing water. There also was spatial variation in the speed attained for a given tail-beat frequency and amplitude. After controlling statistically for variation in spatial location, fork length, and tail-beat amplitude, the swimming speeds brook trout attained for a given tail-beat frequency were lower than values expected from laboratory studies of steady swimming but higher than values expected from laboratory studies of unsteady swimming in standing water. Trout holding station made short-term adjustments in tail-beat frequency also suggesting a degree of unsteady swimming. A field experiment demonstrated that introduction of a current-velocity refuge reduced swimming costs by 10%, on average, without affecting the frequency of foraging attempts made.

Effects of training on respiratory gas exchange, nitrogenous waste excretion, and fuel usage during aerobic swimming in juvenile rainbow trout (Oncorhynchus mykiss)

Fuel utilization during aerobic exercise was determined in juvenile rainbow trout that had previously undergone 2 weeks of continuous aerobic training at 25% of their maximum sustainable speed (Ucrit) and was compared with that in untrained trout. Instantaneous fuel usage was calculated from simultaneous measurements of oxygen consumption ( MO2), carbon dioxide excretion ( MCO2), and nitrogenous waste excretion (MN). Over 58 h of sustained aerobic exercise at 55% of Ucrit, MO2and MCO2remained virtually constant in the trained fish, in contrast to the significant fall over time in untrained fish, but total gas exchange was similar in the two groups. Aerobic respiratory quotient remained constant in the two treatments. MNwas lower in the trained fish, resulting in a much lower (and stable) nitrogen quotient; the nitrogen quotient increased with swimming duration in the untrained fish. Lipid was the major fuel source powering aerobic exercise in both groups. Carbohydrate also played a significant role, whereas protein was of lowest quantitative importance. Training resulted in a decrease in reliance on protein and an increase in reliance on lipid metabolism. The role of protein oxidation in supporting aerobic swimming in fish, which is smaller than commonly believed, is further reduced by training.

Effects of training on respiratory gas e×change, nitrogenous waste e×cretion, and fuel usage during aerobic swimming in juvenile rainbow trout (<I>Oncorhynchus mykiss</I>)

Effects of training on respiratory gas e×change, nitrogenous waste e×cretion, and fuel usage during aerobic swimming in juvenile rainbow trout (<I>Oncorhynchus mykiss</I>)

Respiratory gas exchange, nitrogenous waste excretion, and fuel usage during aerobic swimming in juvenile rainbow trout

The types of fuel burned by juvenile rainbow trout (17 g) during a 58-h period of aerobic sustained exercise were studied by respirometry. Attempts to measure fuel usage by depletion (thecompositional approach) in these same fish were unsuccessful due to lack of detectable changes in proximate body composition. O2 consumption, CO2 excretion, and nitrogenous waste excretion (ammonia-N plus urea-N) were measured in individual fish swum continuously at 55% and 80% of maximum sustainable swimming speed and in non-swimming controls. O2 consumption and CO2 excretion increased with swimming speed, and decreased over time. Absolute rates of N excretion were independent of swimming speed and time.Instantaneous aerobic fuel use, as calculated from the respiratory quotients and nitrogen quotients, was approximately 47% lipid, 30% protein, and 23% carbohydrate in non-swimmers at the start of the experiment. With increased swimming speed there was no change in absolute rates of protein oxidation, while lipid and carbohydrate oxidation both increased. Therefore, the relative protein contribution decreased with increasing speed but increased with swimming duration as the oxidation of other fuels declined over time. However, lipid oxidation predominated at all speeds and at all times. The relative contribution of carbohydrate increased with swimming speed and decreased over time. These results suggest that swimming becomes more efficient over time and help resolve uncertainties in the literature. We conclude that lipid is the main fuel of aerobic exercise, that protein catabolism is kept at minimum levels necessary for maintenance, and that carbohydrate oxidation becomes more important with increased white muscle recruitment at higher speed.

Cardiac Output in Swimming Rainbow Trout,Oncorhynchus mykiss,Acclimated to Seawater

This is the first study to report absolute measurements of cardiac output ($\dot{Q}$ ) with Transonic flow probes during prolonged swimming in a salmonid. Simultaneous measurements of hematocrit and arterial blood O₂ content in seawater-acclimated rainbow trout (Oncorhynchus mykiss) (body mass 401-1,025 g) at 10° ± 1° C indicated that the fish were in good physiological condition, while measurements of resting heart rate, arterial blood pressure, and systemic resistance suggested relatively low levels of stress. Maximum $\dot{Q}$ $ (\dot{Q}_{max})$ (48. 7 mL · min⁻¹ · kg⁻¹) was reached at 97.3% of the maximum prolonged swimming speed and is similar to the $\dot{Q}_{max}$ estimated by the Fick principle during swimming in freshwater rainbow trout. Maximum $\dot{Q}$ was 84% greater than the resting $\dot{Q}$ ($\dot{Q}_{rest}$ ) (26.6 mL · min⁻¹ · kg⁻¹) and was brought about by a 38% increase in heart rate ($f_{H}$ ) and a 25% increase in stroke volume ($SV_{H}$ ). Correlation analysis showed that individua...