The influence of diet and gastrointestinal fermentation on key enzymes of substrate utilization in marine teleost fishes

Three closely related marine teleosts with similar size, swimming mode, and habitat preference were compared to test the hypothesis that energy metabolism is linked to diet choice in the wild. Key substrate-utilization enzyme activities were assayed from white locomotory muscle and liver in a carnivore (Scorpis violaceus), an omnivore (Girella tricuspidata), and a herbivore (Kyphosus sydneyanus) collected from their natural reef habitat in northeastern New Zealand. The similar energy requirements of the study fishes were reflected in specific enzyme activities of white muscle and suggested high dependence on endogenous fuel that is independent of dietary carbohydrate intake. Clear differences were found in enzymes of hepatic carbohydrate, fat, and ketone body metabolism that appear linked to diet choice and levels of gastrointestinal fermentation. Hepatic metabolism of fat and ketone bodies was also examined in New Zealand samples of the omnivorous Girella cyanea and the herbivorous Kyphosus bigibbus, and the tropical herbivorous species Kyphosus vaigiensis and Kyphosus cinerascens collected from the Great Barrier Reef, Australia. Overall, the results suggest that, like ruminants, herbivorous fishes such as Kyphosus species that rely upon gastrointestinal fermentation preferentially use lipids as major metabolic substrates by an increased capacity for lipid metabolism, and a lower capacity for glycolysis.     

Locomotory capacity of Baltic Sea and freshwater populations of the threespine stickleback (Gasterosteus aculeatus)

Two freshwater populations and one marine population (Baltic Sea) of threespine stickeback (Gasterosteus aculeatus) from Northeastern Germany were studied with regard to locomotory capacity: sustained swimming performance, activities of key enzymes in axial muscle, pectoral fin muscle and heart, and morphology. We postulated that life history differences between migratory Baltic Sea and resident freshwater populations could have led to a divergence in their locomotory capacity. The activity of citrate synthase (CS) in pectoral muscle correlated with critical swimming speed. Critical swimming speed, aerobic and anaerobic capacity of the pectoral fin muscle were population-specific. The Baltic Sea sticklebacks had a higher locomotory capacity (activity of CS in pectoral muscle, critical swimming speed) than sticklebacks of one freshwater population. However, another freshwater population expressed a similar locomotory capacity as the Baltic Sea population. In addition, Baltic Sea sticklebacks had a greater mass and lower anaerobic capacity of the pectoral fin muscle than the freshwater sticklebacks. The results are interpreted as an indication of a proceeding divergence between marine and resident freshwater populations and between freshwater populations of G. aculeatus originating from marine ancestors. The migratory Baltic Sea sticklebacks had better morphological prerequisites for sustained swimming than both freshwater populations, but there was no general difference in the locomotory capacity between marine and freshwater sticklebacks. However, their morphology could favour a more effective locomotion in the Baltic Sea sticklebacks.     

Constraints of body size and swimming velocity on the ability of juvenile rainbow trout to endure periods without food

The hypothesis that body size and swimming velocity affect proximate body composition, wet mass and size‐selective mortality of fasted fish was evaluated using small (107 mm mean total length, L _T) and medium (168 mm mean L _T) juvenile rainbow trout Oncorhynchus mykiss that were sedentary or swimming ( c . 1 or 2 body length s⁻¹) and fasted for 147 days. The initial amount of energy reserves in the bodies of fish varied with L _T. Initially having less lipid mass and relatively higher mass‐specific metabolic rates caused small rainbow trout that were sedentary to die of starvation sooner and more frequently than medium‐length fish that were sedentary. Swimming at 2 body length s⁻¹ slightly increased the rate of lipid catabolism relative to 1 body length s⁻¹, but did not increase the occurrence of mortality among medium fish. Death from starvation occurred when fish had <3·2% lipid remaining in their bodies. Juvenile rainbow trout endured long periods without food, but their ability to resist death from starvation was limited by their length and initial lipid reserves.     

Circadian-rhythmic variations of biochemical parameters in the yearlings of indigenous perch (Perca fluviatilis L.) and nonindigenous tyulka (Clupeonella cultriventris Nordmann) from the Rybinsk Reservoir (Russia)

Diurnal variations of some parameters of protein, lipid, and water metabolisms in the underyearlings of indigenous (perch, Perca fluviatilis L.) and nonindigenous (tyulka, Clupeonella cultriventris Nordmann) fish species have been analyzed. These species form the basis of feeding aggregations of young fish in the Rybinsk Reservoir pelagial. No significant daily fluctuations in the values of protein content, water, and lipids in the studied fish species were revealed. Stable interspecific differences in mean daily and hourly values of the above-mentioned components, as well as in the contents of three lipid fractions—phospholipids (PLs), free cholesterol (FC), and triacylglycerols (TAGs)—have been determined at the level of whole organism. It is shown that the level of adipose reserves and the content of TAG in the tyulka are higher than in perch, while the contents of proteins, PL, and FC are lower. Diurnal variations of the level of each of the three fractions in both fish species are similar, but the ranges of variations are wider in tyulka than in perch. Possible mechanisms for the regulation of lipid metabolism and their similarity in young fish of different species upon adaptation to short-term changes in the environment are discussed. It is suggested that the modern state of the zooplankton community in the Rybinsk Reservoir pelagial negatively affects the feeding of perch underyearlings and parameters of lipid metabolism in their organisms.     

Fasting goldfish, Carassius auratus, and common carp, Cyprinus carpio, use different metabolic strategies when swimming

Fish need to balance their energy use between digestion and other activities, and different metabolic compromises can be pursued. We examined the effects of fasting (7days) on metabolic strategies in goldfish and common carp at different swimming levels. Fasting had no significant effect on swimming performance (U(crit)) of either species. Feeding and swimming profoundly elevated total ammonia (T(amm)) excretion in both species. In fed goldfish, this resulted in increased ammonia quotients (AQ), and additionally plasma and tissue ammonia levels increased with swimming reflecting the importance of protein contribution for aerobic metabolism. In carp, AQ did not change since oxygen consumption (MO(2)) and T(amm) excretion followed the same trend. Plasma ammonia did not increase with swimming suggesting a balance between production and excretion rate except for fasted carp at U(crit). While both species relied on anaerobic metabolism during exhaustive swimming, carp also showed increased lactate levels during routine swimming. Fasting almost completely depleted glycogen stores in carp, but not in goldfish. Both species used liver protein for basal metabolism during fasting and muscle lipid during swimming. In goldfish, feeding metabolism was sacrificed to support swimming metabolism with similar MO(2) and U(crit) between fasted and fed fish, whereas in common carp feeding increased MO(2) at U(crit) to sustain feeding and swimming independently.     

Can phenotypic plasticity facilitate the geographic expansion of the tilapia Oreochromis mossambicus?

Climate influences the distribution of organisms because of the thermal sensitivity of biochemical processes. Animals may compensate for the effects of variable temperatures, and plastic responses may facilitate radiation into different climates. The tropical fish Oreochromis mossambicus has radiated into climates that were thought to be thermally unsuitable. Here, we test the hypothesis that thermal acclimation will extend the locomotory and metabolic performance range of O. mossambicus. Juvenile fish were acclimated to 14 degrees, 17 degrees, and 22 degrees C. We measured responses to acclimation at three levels of organization: whole-animal performance (sustained swimming and resting and recovery rates of oxygen consumption), mitochondrial oxygen consumption in caudal muscle, and metabolic enzyme activities in muscle and liver at 12 degrees, 14 degrees, 17 degrees, 22 degrees, and 26 degrees C. Thermal optima of sustained swimming performance (U(crit)) changed significantly with acclimation, but acclimation had no effect on either resting or recovery oxygen consumption. Fish compensated for cold temperatures by upregulating state 3 mitochondrial oxygen consumption and increasing activity of lactate dehydrogenase in the liver. The capacity for phenotypic plasticity in O. mossambicus means that the fish would not be limited by its locomotor performance or metabolic physiology to expand its range into cooler thermal environments from its current distribution.     

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 (the compositional 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. Accepted: 3 July 1996     

Thermal stress,thermal safety margins and acclimation capacity in tropical shallow waters—An experimental approach testing multiple end-points in two common fish

Tropical organisms are predicted to be among the most impacted by increasing sea surface temperatures, particularly those from intertidal habitats. In this study, a complete thermal biology assessment was conducted for two widespread tropical Atlantic shallow reef fish: Abudefduf saxatilis (damselfish) and Scartella cristata (blenny), which make extensive use of tide pools. The main objectives were to measure the time-course changes during one month in i) thermal and oxidative stress biomarkers (in gills, muscle and skin), ii) upper thermal limits, acclimation capacity and thermal safety margins and iii) body size, condition and energy reserves (total protein and lipid contents), under two temperature treatments (control – mean summer temperature, and elevated temperature − + 3 °C, as projected by climate warming scenarios for the end of this century).Results from biomarker analyses suggest that under increased temperature, both species displayed a typical response of physiological stress characterized by the activation of molecular chaperones and antioxidant protection. Both species presented a significant acclimation potential in the long term, as shown by increased critical thermal maxima values at higher temperature. However, these species may already be at risk during summer heat waves, as thermal safety margins for both species were low. Additionally, despite acclimation, some energetic tradeoffs may exist, since specimens from both species showed smaller body sizes at higher temperature (even though maintaining body condition). Finally, temperature treatments had a significant influence not only in the total amount of energy reserves (lipid contents) but also in their rate of deposition or depletion (total proteins and lipid contents). This is the first multi-end-point holistic approach to assess the impact of warming in shallow tropical water fish and it highlights the high risk that intertidal organisms are facing in both present and future sea surface temperature conditions.     

Mutually exclusive muscle designs: the power output of the locomotory and sonic muscles of the oyster toadfish (Opsanus tau)

Animals perform a vast array of motor activities. Although it has generally been accepted that muscles are well suited to the function that they must perform, specialization for performing one function may compromise their ability for carrying out another. We examined this principle in the toadfish muscular system: slow-twitch red and fast-twitch white myotomal muscles are used for powering swimming at relatively low frequencies, while the superfast swimbladder muscle powers mating calls by contracting at 100 Hz. We measured muscle power output over a wide range of frequencies. The red and white locomotory muscles could not generate power over ca. 2.2 and 12 Hz, respectively and, hence, could not power sound production. In contrast, the swimbladder muscle has many specializations that permit it to generate power at frequencies in excess of 100 Hz. However, these specializations drastically reduce its power output at low frequencies: the swimbladder muscle generated only one-twentieth of the power of the red muscle and one-seventh of the power of the white muscle at the frequencies used during swimming. To generate the same total power needed for swimming would require unfeasibly large amounts of swimbladder muscle that could not fit into the fish. Hence, the designs of the swimbladder and locomotory muscles are mutually exclusive.     

Swimming Performance Assessment in Fishes

Swimming performance tests of fish have been integral to studies of muscle energetics, swimming mechanics, gas exchange, cardiac physiology, disease, pollution, hypoxia and temperature. This paper describes a flexible protocol to assess fish swimming performance using equipment in which water velocity can be controlled. The protocol involves one to several stepped increases in flow speed that are intended to cause fish to fatigue. Step speeds and their duration can be set to capture swimming abilities of different physiological and ecological relevance. Most frequently step size is set to determine critical swimming velocity (U_crit), which is intended to capture maximum sustained swimming ability. Traditionally this test has consisted of approximately ten steps each of 20 min duration. However, steps of shorter duration (e.g. 1 min) are increasingly being utilized to capture acceleration ability or burst swimming performance. Regardless of step size, swimming tests can be repeated over time to gauge individual variation and recovery ability. Endpoints related to swimming such as measures of metabolic rate, fin use, ventilation rate, and of behavior, such as the distance between schooling fish, are often included before, during and after swimming tests. Given the diversity of fish species, the number of unexplored research questions, and the importance of many species to global ecology and economic health, studies of fish swimming performance will remain popular and invaluable for the foreseeable future.     

Fatigue is a behavioural response in respirometer‐confined smallmouth bass

Fatigue in respirometer‐confined smallmouth bass Micropterus dolomieu was confirmed as being largely a behavioural response, and not brought about by depletion of anaerobic fuel stores. While data collected from these fish did not preclude the possibility that the behavioural response was triggered when some physiological threshold was exceeded, locomotory gait transition patterns in confined fish exposed to increasing water velocities support an alternate hypothesis. This is that fatigue is a behavioural decision made at (or near) the gait transition swimming speed, in an effort to avoid activity that, by virtue of the limited space in a typical respirometer, is energetically inefficient and biomechanically difficult.     

Morphological and histochemical characterization of the pectoral fin muscle of batoids

Batoids differ from other elasmobranch fishes in that they possess dorsoventrally flattened bodies with enlarged muscled pectoral fins. Most batoids also swim using either of two modes of locomotion: undulation or oscillation of the pectoral fins. In other elasmobranchs (e.g., sharks), the main locomotory muscle is located in the axial myotome; in contrast, the main locomotory muscle in batoids is found in the enlarged pectoral fins. The pectoral fin muscles of sharks have a simple structure, confined to the base of the fin; however, little to no data are available on the more complex musculature within the pectoral fins of batoids. Understanding the types of fibers and their arrangement within the pectoral fins may elucidate how batoid fishes are able to utilize such unique swimming modes. In the present study, histochemical methods including succinate dehydrogenase (SDH) and immunofluoresence were used to determine the different fiber types comprising these muscles in three batoid species: Atlantic stingray (Dasyatis sabina), ocellate river stingray (Potamotrygon motoro) and cownose ray (Rhinoptera bonasus). All three species had muscles comprised of two muscle fiber types (slow-red and fast-white). The undulatory species, D. sabina and P. motoro, had a larger proportion of fast-white muscle fibers compared to the oscillatory species, R. bonasus. The muscle fiber sizes were similar between each species, though generally smaller compared to the axial musculature in other elasmobranch fishes. These results suggest that batoid locomotion can be distinguished using muscle fiber type proportions. Undulatory species are more benthic with fast-white fibers allowing them to contract their muscles quickly, as a possible means of escape from potential predators. Oscillatory species are pelagic and are known to migrate long distances with muscles using slow-red fibers to aid in sustained swimming.     

A comparative analysis of parvalbumin expression in pinfish (Lagodon rhomboides) and toadfish (Opsanus sp.)

This study examines the role of a myoplasmic protein, parvalbumin, in enhancing muscle relaxation by fishes. Parvalbumin is thought to bind free Ca²⁺ during muscle contraction, thereby reducing intracellular [Ca²⁺] in muscle and speeding muscle relaxation by reducing Ca²⁺ availability to the troponin complex. We hypothesized that parvalbumin expression is ubiquitously expressed in fish muscle and that its expression levels and role in muscle relaxation would depend on the activity level and the thermal environment of a given fish species. Muscle contractile properties and patterns of parvalbumin expression were examined in pinfish (Lagodon rhomboides) and two species of toadfish (gulf toadfish, Opsanus beta, and oyster toadfish, Opsanus tau). Unlike another sparid (sheepshead), the active swimming pinfish does not express parvalbumin in its slow-twitch red muscle. However, both sheepshead and pinfish have relatively high levels of parvalbumin in their myotomal white muscle. Gulf toadfish from the Gulf of Mexico expressed higher levels of parvalbumin and had faster muscle relaxation rates than oyster toadfish from more northern latitudes. The faster muscle of gulf toadfish also expressed relatively more of one parvalbumin isoform, suggesting differences in the binding properties of the two isoforms observed in toadfish swimming muscle. Parvalbumin expression and its role in muscle relaxation appear to vary widely in fishes. There are many control points involved in the calcium transient of contracting muscle, leading to a variety of species-specific solutions to the modulation of muscle relaxation.     

Size-related effects and the influence of metabolic traits and morphology on swimming performance in fish

Energy metabolism fuels swimming and other biological processes. We compared the swimming performance and energy metabolism within and across eight freshwater fish species. Using swim tunnel respirometers, we measured the standard metabolic rate (SMR) and maximum metabolic rate (MMR) and calculated the critical swimming speed (U_crit). We accounted for body size, metabolic traits, and some morphometric ratios in an effort to understand the extent and underlying causes of variation. Body mass was largely the best predictor of swimming capacity and metabolic traits within species. Moreover, we found that predictive models using total length or SMR, in addition to body mass, significantly increased the explained variation of U_crit and MMR in certain fish species. These predictive models also underlined that, once body mass has been accounted for, U_crit can be independently affected by total length or MMR. This study exemplifies the utility of multiple regression models to assess within-species variability. At interspecific level, our results showed that variation in U_crit can partly be explained by the variation in the interrelated traits of MMR, fineness, and muscle ratios. Among the species studied, bleak Alburnus alburnus performed best in terms of swimming performance and efficiency. By contrast, pumpkinseed Lepomis gibbosus showed very poor swimming performance, but attained lower mass-specific cost of transport (MCOT) than some rheophilic species, possibly reflecting a cost reduction strategy to compensate for hydrodynamic disadvantages. In conclusion, this study provides insight into the key factors influencing the swimming performance of fish at both intra- and interspecific levels.     

The effects of feeding on the swimming performance and metabolic response of juvenile southern catfish,Silurus meridionalis,acclimated at different temperatures

To test whether the effects of feeding on swimming performance vary with acclimation temperature in juvenile southern catfish (Silurus meridionalis), we investigated the specific dynamic action (SDA) and swimming performance of fasting and feeding fish at acclimation temperatures of 15, 21, 27, and 33 °C. Feeding had no effect on the critical swimming speeding (U_crit) of fish acclimated at 15 °C (p = 0.66), whereas it elicited a 12.04, 18.70, and 20.98% decrease in U_crit for fish acclimated at 21, 27 and 33 °C, respectively (p < 0.05). Both the maximal postprandial oxygen consumption rate (VO_2peak) and the active metabolic rate (VO_2active, maximal aerobic sustainable metabolic rate of fasting fish) increased significantly with temperature (p < 0.05). The postprandial maximum oxygen consumption rates during swimming (VO_2max) were higher than the VO_2active of fasting fish at all temperature groups (p < 0.05). The VO_2max increased with increasing temperature, but the relative residual metabolic scope (VO_2max? VO_2peak) during swimming decreased with increasing in temperature. The present study showed that the impairment of postprandial swimming performance increased with increasing temperature due to the unparalleled changes in the catfish's central cardio-respiratory, peripheral digestive and locomotory capacities. The different metabolic strategies of juvenile southern catfish at different temperatures may relate to changes in oxygen demand, imbalances in ion fluxes and dissolved oxygen levels with changes in temperature.     

Physiological and metabolic adjustments of Hoplosternum littorale (Teleostei,Callichthyidae) during starvation

All animals face the possibility of limitations in food resources that could ultimately lead to mortality caused by starvation. The primary goal of this study was to characterize the various physiological strategies that allow fish to survive starvation. A multiparametric approach, including morphological biomarkers, blood plasma metabolites, oxidative stress and energy reserves, was used to assess starvation effects on the fish Hoplosternum littorale. Adult specimens were maintained at four experimental groups: control (fed ad libitum), and starved (not fed) fish for 7 and 28 days. Significant changes were observed not only after 28 days, but also after 7 days of starvation. In the shorter period, the hepatosomatic index as well as plasma triglycerides and glucose were significantly lower in starved fish than in the control ones. These results were accompanied by reduced lipid, glycogen and protein reserves in liver and diminished glycogen content in muscle, suggesting the need of these macromolecules as fuel sources. In addition, increased antioxidant enzyme activities were observed in gills, without evidence of oxidative stress in any of the evaluated tissues. Most significant differences were found in 28-days starved fish: total body weight together with the hepatosomatic index was lower when compared to control fish. The plasmatic metabolites tested (glucose, triglyceride, cholesterol and protein), all energy reserves in liver and glycogen content in muscle decreased in 28-days starved fish. Lipid oxidative damage was reported in liver, kidney and brain, and antioxidant enzymes (GST, GR, GPx and CAT) were activated in gills. According to the multivariate analysis, oxidative stress markers and metabolic parameters were key biomarkers that contributed in separating starved from fed fish. Our study allowed an integrated assessment of the fish response to this particular condition.     

Aquatic and terrestrial locomotory energetics in a toad and a turtle: a search for generalisations among ectotherms

Murray short-necked turtles were trained to walk on a motorised treadmill and to swim in a recirculating flume. Through filmed records, the frequency of limb movement and the time that thrust was directed against the substrate were measured. The animals wore masks when walking and accessed air when swimming from a ventilated capsule placed on top of the water surface. Measurement of the exhalant O(2) and CO(2) levels from these devices enabled the measurement of metabolic rates. Equivalent data were obtained from swimming and hopping cane toads, although repeatable measures of limb frequency and contact times were not obtained due to the intermittent form of locomotion in this species. Comparing the cost of transport, the energy required to transport a mass of animal over a unit distance, with other animals showed that toads do not have a cheap form of terrestrial locomotion, but turtles do; turtles use half the cost predicted from their body mass. This economy of locomotion is consistent with what is known about turtle muscle, the mechanics of their gait, and the extremely long contact time for a limb with the substrate. Swimming in toads is energetically expensive, whereas turtles, on the basis of mass, use about the same energy to transport a unit mass as an equivalent-size fish. The data were compared with the predictions of the Kram-Taylor hypothesis for locomotory scaling, and walking turtles were found to provide a numerical fit. The data show that both terrestrial and aquatic locomotory energetics in toads are generally higher than predictions on the basis of mass, whereas in turtles they are lower.     

Temperature acclimatisation of swimming performance in the European Queen Scallop

The phenotypic plasticity of muscle performance and locomotory physiology allows the maintenance of essential activity capacity in the face of environmental change, and has been demonstrated in a wide phylogenetic range of eurythermal vertebrates. This study used the scallop, Aequipecten opercularis, as a model eurythermal invertebrate. Animals caught in different seasons demonstrated marked differences in their swimming performance and the relationship between temperature and performance. When stimulated to swim at natural ranges of temperature, Winter (cold acclimatised), animals accelerated faster than autumn collected animals swimming at the same temperature (×2 at 11 °C) and attained higher velocities during jetting. The effects of acclimatisation were confined to the jetting phase and may be a mechanism for the maintenance of acceleration during predator–prey interactions. This is the first demonstration of the thermal acclimatisation of muscle performance in a mollusc and one of very few studies in invertebrates.     

Utilization of endogenous reserves and effects of starvation on the health of Prochilodus lineatus (Prochilodontidae)

Prochilodus lineatus Valenciennes (curimbatá) is an important migratory Neotropical fish. It does not feed during spawning migration, and often survives after spawning. The mobilization of energy reserves and some effects of starvation (zero to eight weeks) on fish health were experimentally evaluated. Hepatic glycogen and lipids from the perivisceral fat bodies were the main reserves mobilized during the first four weeks of fasting. During this period, somatic indices and blood parameters showed that fish health was not significantly affected. However, after five weeks of food deprivation, the main energy reserves were depleted and the fish became anaemic. The loss of muscle mass indicates that protein breakdown was an important energy source after the reduction of hepatic and perivisceral reserves. Mortality was increasingly observed from seven weeks of starvation. Prior accumulation of high amounts of reserves is essential to allow movements for long distances during spawning migrations in this species.     

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.