Trout Muscle Research Articles

Muscle remodeling in relation to blood supply: implications for seasonal changes in mitochondrial enzymes

We investigated if seasonal changes in rainbow trout muscle energetics arise in response to seasonal changes in erythrocyte properties. We assessed if skeletal muscle mitochondrial enzymes changed (1) acutely in response to changes in erythrocyte abundance, or (2) seasonally when we altered the age profile of erythrocytes. Rainbow trout were treated with pheynylhydrazine, causing a 75% reduction in hematocrit within 4 days. After erythropoiesis had returned hematocrit to normal, treated and control fish were subjected to a seasonal cold acclimation regime to assess the impact of erythrocyte age on skeletal muscle remodeling. Anemia (i.e. phenylhydrazine treatment) did not alter the specific activities (U g(-1) tissue) of mitochondrial enzymes in white or red muscle. Anemic pretreatment did not alter the normal pattern of cold-induced mitochondrial proliferation in skeletal muscle, suggesting erythrocyte age was not an important influence on seasonal remodeling of muscle. Anemia and cold acclimation both induced a 25-30% increase in relative ventricular mass. The increase in relative ventricular mass with phenylhydrazine treatment was accompanied by a 35% increase in DNA content (mg DNA per ventricle), suggesting an increase in number of cells. In contrast, the increase in ventricular mass with cold temperature acclimation occurred without a change in DNA content (mg DNA per ventricle), suggesting an increase in cell size. Despite the major increases in relative ventricular mass, neither anemia nor seasonal acclimation had a major influence on the specific activities of a suite of mitochondrial enzymes in heart. Collectively, these studies argue against a role for erythrocyte dynamics in seasonal adaptive remodeling of skeletal muscle energetics.

Troponin T expression in trout red muscle correlates with muscle activation

Red or aerobic muscle from the anterior of rainbow trout Oncorhynchus mykiss activates (generates force) more quickly than that from the posterior. TnT is a component of the troponin complex that modulates muscle activation once Ca(2+) is bound. Since trout express at least two forms of TnT in their red muscle (S1 and S2), the differential expression of these two forms was predicted to explain variations in contractile properties. TnT isoforms from trout muscle were identified through hydroxy-apatite chromatography of purified myofibrillar proteins followed by SDS-PAGE. Western blots employing a mammalian anti-troponin T monoclonal antibody were used to identify TnT isoforms. The relative expression of the two isoforms of TnT was then examined at seven longitudinal positions from each of three fish using SDS-PAGE and densitometry on the silver-stained TnT bands. A significant shift in expression was detected from anterior to posterior in all three fish with TnT S1 becoming more dominant in the posterior red muscle. As predicted, a shift in TnT expression was associated with the decrease in activation rate along the length of the fish. This study was then extended to include a different species of salmonid, brook trout Salvelinus fontinalis, to explore the generality of TnT modulation of muscle activation. Muscle contractile properties were determined from anterior and posterior muscle, and relative expression of S1 and S2 was determined. Unlike rainbow trout, there is no consistent longitudinal pattern of muscle activation in brook trout: some fish have kinetically faster muscle in the anterior, some in the posterior. Similarly, there is no consistent pattern of TnT expression. Individual analysis of muscle activation and TnT expression in brook trout provides insight into the role of TnT in modulating muscle activation in slow fish muscle.

The calpastatin/calpain system in trout Salmo trutta trutta muscle

Many recent reports suggest that the calpastatin/calpain system plays a role in cellular growth and differentiation. Defects of the calpastatin/calpain system have been linked to cellular dysfunctions, apoptosis, myocardial infarct, and dystrophies. The calpastatin/calpain system has also been implicated in post‐mortem tenderization of skeletal muscle through degradation of key myofibrillar and associated proteins, a process of key importance to meat quality. In the present study we investigate the presence and activity of the calpastatin/calpain system in trout muscle samples, collected at 0, 3, 18 and 28 h post‐mortem, by immunohistochemistry method. Calpastatin is a specific endogenous enzyme of cytosol, modulating the ubiquitous calpains. Calpastatin was found in samples obtained in vivo and immediately post‐mortem, but its concentration declined rapidly in samples obtained 3, 18 and 28 h post‐mortem. The ubiquitous m e m‐calpains, which are localized on Z line proteins and activated by intracellular Ca2+ increase, showed a rapid decline within 3 h post‐mortem. By contrast p94 calpain, which is specific to skeletal muscle, showed a slow decrease post‐mortem which was independent of intracellular Ca2+ increase. Our results suggest that the mechanism of activation and activity of the calpastatin/calpain system in trout is similar to that described in mammals.

Assessment of metal contamination in the upper reaches of the Tichá Orlice River

The aim of the present study was to assess metal contamination in the same reaches of the river, and thus to help explain unsatisfactory reproduction results in the reproduction of salmonoid fish. The contamination assessment was based on measuring metal concentrations in the brown trout (Salmo trutta morpha fario) and some bottom sediment samples. The samples were collected in June 2000 and 2001 at two collection sites from theTicháOrliceRiver (Červená Voda – 103rd river km; Lichkov – 93rd river km) and its tributary Kralický Brook (100th river km). At each of the sites, 14 brown trouts were collected in each of the periods of monitoring. The AAS method was used to determine the total mercury, arsenic, cadmium, lead, copper, zinc, chromium and nickel contents in the muscle tissue of the fish and in aquatic sediments. The Kralický Brook is an important anthropogenic source of metal contamination (particularly of mercury and copper) for theTicháOrliceRiver. The highest concentrations of mercury in muscles of brown trout were found at the Kralický Brook (0.37 ± 0.08 and 0.40 ± 0.08 mg/kg) and Lichkov (0.41 ± 0.10 and 0.34 ± 0.07 mg/kg) in 2000 and 2001 and the lowest concentration at Červená Voda (0.017 ± 0.02 mg/kg in the same years). Significantly higher concentrations of arsenic (in 2000: 0.30 ± 0.08 mg/kg; in 2001: 0.38 ± 0.07 mg/kg) were found in the muscle tissue of the brown trout collected at Červená Voda than at the downstream site Lichkov (in 2000: 0.18 ± 0.09 mg/kg;in 2001: 0.14 ± 0.07 mg/kg). The authors hypothesize that the difference was due to different conditions (principally water temperature). It seems reasonable to assume that unsatisfactory results in the reproduction of fish from the upper reaches of theTicháOrliceRiver are due not only to organic pollutants but also to mercury compounds that are classified among suspect endocrine disruptors.  

Two-Dimensional Gel Electrophoresis Detection of Protein Oxidation in Fresh and Tainted Rainbow Trout Muscle

Protein oxidation is evaluated in rainbow trout muscle by labeling protein carbonyls with 2,4-dinitrophenyl hydrazine (DNPH) followed by immunoblotting of proteins separated by SDS-PAGE or two-dimensional gel electrophoresis (2D-GE). The carbonylation level is accessed on proteins in a whole muscle homogenate or proteins soluble in a high-salt or low-salt buffer. Spoilage-related changes in carbonylation are followed in the high-salt-protein and low-salt-protein fractions by 2D immunoblotting, which reveals increases regarding total number and intensity of carbonylation in both protein fractions for fish kept at room temperature for 48 h. The major amount of carbonylated proteins is found among the high-salt-soluble proteins, and this protein fraction is also responsible for the biggest increase in carbonylation during fish tainting. The results give an estimate of the level of protein carbonylation in rainbow trout and reveal that oxidation increases for a distinct number of proteins during tainting.

ISOLATION AND CHARACTERIZATION OF CALPAINS FROM RAINBOW TROUT MUSCLE AND THEIR ROLE IN TEXTURE DEVELOPMENT

ABSTRACT A single anion‐exchange column was used to resolve calpain I and II and their endogenous inhibitor, calpastatin, from the white muscle of rainbow trout (Oncorhynchus mykiss). Calcium requirement assays showed that calpain I and II required approximately 0.2 mM and 2.5 mM, respectively, for half maximal activities. Isolation of calpain I and calpastatin were confirmed by Western blot. Incubation of trout muscle for 12 h postmortem, in a buffer containing CaCl2, decreased (P < 0.0001) shear force compared to samples incubated in a buffer containing EDTA. This effect indicates that softening of fish muscle can be accelerated by activation of calpains with exogenous calcium.

Heavy Metal Contamination of European Eel (Anguilla anguilla) and Brown Trout (Salmo trutta) Caught in Wild Ecosystems in Spain

In this study we analyzed the pattern of copper (Cu), cadmium (Cd), lead (Pb), and mercury (Hg) accumulation in liver (as a detoxifying organ) and muscle (as the most important tissue for human consumption) of brown trout (Salmo trutta) and European eel (Anguilla anguilla) caught in two wild Spanish rivers where both species are usually angled for human consumption. Cd, Pb, and Cu accumulated preferentially in the liver of both species. Hg accumulated both in the liver and muscle in brown trout, whereas it accumulated preferentially in muscle in European eel. Both high metal content and preferential accumulation of Hg in muscle suggest that European eel is more harmful than brown trout for human consumption.

Long depletion time of enrofloxacin in rainbow trout (Oncorhynchus mykiss).

The international production of farmed fish has been growing continuously over recent years. Until now few veterinary drugs have been approved by the European Union for use in aquaculture, and this has favored the off-label use of products authorized for use in food-producing animal species different from fishes among fish farmers. Adequate field studies are lacking, especially for those species called minor species which are consumed extensively only in some European countries. In the present investigation we studied the depletion of the fluoroquinolone antibacterial enrofloxacin over time in a minor species, the rainbow trout (Oncorhynchus mykiss), reared on a real fish farm and treated with medicated feed (10 mg kg of trout body weight(-1) day(-1)). Edible tissue samples (muscle plus skin in natural proportions) and fish bone samples were analyzed for enrofloxacin and for its major metabolite, ciprofloxacin, by high-performance liquid chromatography with fluorescence detection at different times after the end of treatment. Our results show that at 500 degrees C-day (in which degree-days are calculated by multiplying the mean daily water temperature by the total number of days on which the temperature was measured), which is the minimum withdrawal period established by European Economic Commission Directive No. 82/2001 for any type of product administered off-label, edible trout tissues might still contain about 170 microg of enrofloxacin kg(-1), whereas the maximum residue level for enrofloxacin plus ciprofloxacin is set at 100 microg kg(-1). To our knowledge, no studies of the depletion of enrofloxacin in rainbow trout have been performed. On the basis of the data obtained in the present study, we suggest a more appropriate withdrawal time of 816 degrees C-day for the sum of enrofloxacin plus ciprofloxacin levels in rainbow trout muscle plus skin tissues.

The impact of feeding β2‐adrenergic agonists on rainbow trout muscle β2‐adrenoceptors and protein synthesis

The β2‐adrenergic agonists (β2‐AAs) clenbuterol (CLEN) and ractopamine (RACT) were fed to rainbow trout Oncorhynchus mykiss for 30–37 days and red and white muscle β2‐adrenoceptor (β2‐AR) binding characteristics and mRNA expression were assessed in parallel with fractional protein synthetic rates. Feeding CLEN or RACT had no significant effect on any body or physiological variables measured. There were no significant differences in the number of binding sites (Bmax) while a significant increase in the Kd was observed for the β‐ARs of red and white muscle membranes isolated from β2‐AA‐fed rainbow trout. No change in β2‐AR mRNA levels was observed with β2‐AA feeding, implying that these β2‐AAs do not act at the transcriptional level at least for the β2‐AR. The β2‐AA treatments, however, did significantly increase red and white muscle fractional protein synthesis rates in whole protein, myofibrillar protein and sarcoplasmic soluble protein fractions. Although not conclusive, this study supports a direct link between rainbow trout muscle β2‐ARs and muscle protein synthesis.

Hormonal regulation of glycogen metabolism in white muscle slices from rainbow trout (Oncorhynchus mykiss Walbaum).

To test the hypothesis that cortisol and epinephrine have direct regulatory roles in muscle glycogen metabolism and to determine what those roles might be, we developed an in vitro white muscle slice preparation from rainbow trout (Oncorhynchus mykiss Walbaum). In the absence of hormones, glycogen-depleted muscle slices obtained from exercised trout were capable of significant glycogen synthesis, and the amount of glycogen synthesized was inversely correlated with the initial postexercise glycogen content. When postexercise glycogen levels were <5 micromol/g, about 4.3 micromol/g of glycogen were synthesized, but when postexercise glycogen levels were >5 micromol/g, only about 1.7 micromol/g of glycogen was synthesized. This difference in the amount of glycogen synthesized was reflected in the degree of activation of glycogen synthase. Postexercise glycogen content also influenced the response of the muscle to 10(-8) M epinephrine and 10(-8) M dexamethasone (a glucocorticoid analog). At high glycogen levels (>5 micromol/g), epinephrine and dexamethasone stimulated glycogen phosphorylase activity and net glycogenolysis, whereas at low (<5 micromol/g) glycogen levels, glycogenesis and activation of glycogen synthase activity prevailed. These data clearly indicate not only is trout muscle capable of in situ glycogenesis, but the amount of glycogen synthesized is a function of initial glycogen content. Furthermore, whereas dexamethasone and epinephrine directly stimulate muscle glycogen metabolism, the net effect is dependent on initial glycogen content.

Age dependence of the accumulation of organochlorine pollutants in brown trout ( Salmo trutta) from a remote high mountain lake (Redó, Pyrenees)

Polychlorobiphenyls (PCBs), hexachlorocyclohexanes (HCHs), hexachlorobenzene (HCB) and DDT were examined in the muscle of brown trout ( Salmo trutta) from a high mountain lake located in the Pyrenees (Catalonia, Spain) that was used as a model of these lacustrine environments. Results indicate that fish age is the main factor of variability among specimens in this population that is subjected to atmospheric inputs of the organochlorine compounds (OC). Increases of 2- and 20-fold between fish aged 1 year and 15 years old are found. The observed pattern cannot be explained in terms of fish size, condition factor, or muscle lipid content. Higher molecular weight compounds (higher lipophilicity) are better correlated with fish age than low molecular weight compounds. A transformation from 4,4′-DDT to 4,4′-DDE occurs in fish after ingestion; this results in amplified age-dependent signals, especially in male specimens. In contrast, PCB congener #180 has lower age dependence than the general OC group, which could be due to its high hydrophobicity (log K ow > 7). In any case, selective accumulation of hydrophobic compounds is already observed among younger fish (age, 1 year). Due to this effect, the relative OC composition does not reflect the main OC pollutants in the lake waters.

Determination of malachite green residues in rainbow trout muscle with liquid chromatography and liquid chromatography coupled with tandem mass spectrometry

A method for the determination of malachite green and its major metabolite leucomalachite green in rainbow trout muscle is reported with limits of detection of 0.8 and 0.6 μg kg−1, respectively. Residues were extracted with an acetonitrile–acetate buffer mixture and partitioned into methylene chloride. Clean-up of the extracts was performed on alumina and propylsulfonic acid solid-phase extraction columns using the automated solid-phase extraction system. The chromatographic separation of malachite green and leucomalachite green was achieved on a Chromspher 5B column using an acetonitrile–acetate buffer mobile phase. Leucomalachite green was converted to malachite green by post-column oxidation before spectrophotometric detection at 600 nm. The mean recoveries of malachite green and leucomalachite green from control rainbow trout muscle spiked at 2–50 μg kg−1 were 65% (range 63.4–65.9%, relative standard deviation 3.9–16.1%) and 74% (range 58.3–82.6%, relative standard deviation 3.3–11.4%), respectively. Qualitative confirmation of the determined residues was performed with liquid chromatography coupled with tandem mass spectrometry detection with limits of detection of 2.5 and 1 μg kg−1 for malachite green and leucomalachite green, respectively.

Growth hormone differentially regulates muscle myostatin1 and -2 and increases circulating cortisol in rainbow trout ( Oncorhynchus mykiss)

Myostatin (MSTN) negatively regulates muscle growth in vertebrates. Salmonids produce two myostatin transcripts from separate genes. Surprisingly, quantitative analyses indicate different regulatory mechanisms for the two myostatin genes in rainbow trout. MSTN1 mRNA levels were elevated 26% following recombinant bovine growth hormone (rbGH) treatment, while MSTN2 mRNA levels were reduced 74% compared to controls. MSTN precursor protein (42 kDa) levels were elevated in rbGH treated fish compared to controls. In addition, circulating cortisol levels were elevated 71% following rbGH treatment compared to controls. In treated and control fish, cortisol levels were elevated 245% at day 0 compared to subsequent days. Treated fish exhibited cortisol levels 207% higher than controls at 0.5 day, and remained at least 50% higher for 7 days following treatment. This pattern of change was positively correlated to MSTN1 mRNA levels. This is the first time a direct relationship has been reported between GH, cortisol, and myostatin. In addition, following rbGH administration, myosin protein concentrations in skeletal muscle samples increased, suggesting that GH regulates expression of the most abundant muscle protein. These results indicate the two myostatin genes are differentially regulated and may possess different functions in rainbow trout muscle, and suggests a possible interaction between GH, cortisol, and muscle growth.

Optimization of Incorporation of Low-molecular-weight Cryoprotectants into Intact Fish Muscle

ABSTRACT: Chunks of freshwater trout muscle were immersed in sorbitol solutions (0% to 60%), under different vacuum conditions, for up to 30 min at 5 °C. Moisture loss, weight change, and sorbitol uptake were measured or calculated by mass balance, and cryoprotection during subsequent freezing and thawing was monitored as change in myosin Ca2+ ATPase activity. Vacuum treatment had no effect on measured parameters. Initial sorbitol uptake and weight loss were greater at higher sorbitol concentrations, but adequate cryoprotection was achieved by all treatments when diffusion time following immersion was extended sufficiently. Injection of 60% sorbitol was faster in achieving desired levels of sorbitol in fish meat and induced excellent cryoprotection.

Autoproteolysis rate of rainbow trout muscle proteins.

Effect of some physicochemical agents (pH, temperature and incubation time) and iced storage of farmed rainbow trout (Oncorhynchus mykiss) on autoproteolysis of muscle proteins and quality (sensory assessment, instrumental texture, total bacterial counts) were determined. The results showed that the autoproteolysis rate proceeded with its highest intensity at pH 4.2 and temperature about 55 degrees C. The rate of protein hydrolysis and protease activity, as well as microbial contamination, gradually increased with prolonged periods of iced storage of fish. Properly iced trout immediately after its evisceration was acceptable in quality for up to 10 days.

Metabolic and mitogenic effects of IGF-I and insulin on muscle cells of rainbow trout.

The relative function of IGF-I and insulin on fish muscle metabolism and growth has been investigated by the isolation and culture at different stages (myoblasts at day 1, myocytes at day 4, and myotubes at day 10) of rainbow trout muscle cells. This in vitro model avoids interactions with endogenous peptides, which could interfere with the muscle response. In these cells, the effects of IGF-I and insulin on cell proliferation, 2-deoxyglucose (2-DG), and l-alanine uptake at different development stages, and the use of inhibitors were studied and quantified. Insulin (10-1,000 nM) and IGF-I (10-100 nM) stimulated 2-DG uptake in trout myocytes at day 4 in a similar manner (maximum of 124% for insulin and of 142% for IGF-I), and this stimulation increased when cells differentiated to myotubes (maximum for IGF-I of 193%). When incubating the cells with PD-98059 and especially cytochalasin B, a reduction in 2-DG uptake was observed, suggesting that glucose transport takes place through specific facilitative transporters. IGF-I (1-100 nM) stimulated the l-alanine uptake in myocytes at day 4 (maximum of 239%), reaching higher values of stimulation than insulin (100-1,000 nM) (maximum of 160%). This stimulation decreased when cells developed to myotubes at day 10 (118% for IGF-I and 114% for insulin). IGF-I (0.125-25 nM) had a significant effect on myoblast proliferation, measured by thymidine incorporation (maximum of 170%), and required the presence of 2-5% fetal serum (FBS) to promote thymidine uptake. On the other hand, insulin was totally ineffective in stimulating thymidine uptake. We conclude that IGF-I is more effective than insulin in stimulating glucose and alanine uptake in rainbow trout myosatellite cells and that the degree of stimulation changes when cells differentiate to myotubes. IGF-I stimulates cell proliferation in this model of muscle in vitro and insulin does not. These results indicate the important role of IGF-I on growth and metabolism of fish muscle.

Screening for peptides from fish and cheese inhibitory to prolyl endopeptidase.

Peptides from hydrolysates of fish proteins and from cheeses were analysed for inhibition of prolyl endopeptidase (PE) isolated from porcine muscle. Muscles of cod, salmon, and trout were homogenised and incubated at pH 4.0 with pepsin and then at pH 7.5 with trypsin to obtain fish protein hydrolysates. Homogenates were incubated without exogenous enzymes at pH 4.0 and 7.5 to obtain fish protein autolysates. Water-soluble extracts from "rakfisk" (a Norwegian fermented/autolysed trout muscle dish) and water-soluble extracts from Cheddar, Norvegia, Jarlsberg, and Blue cheese were also prepared. Peptides in the supernatants obtained after heat-treatment of fish hydrolysates, autolysates and water-soluble extracts of rakfisk and cheeses at 95 degrees C for 15 min were analysed for inhibition of PE. Inhibition was also measured in peptide fractions separated by reversed-phase high-performance chromatography and by gel permeation chromatography. The peptide fractions from fish hydrolysates, fish autolysates, and water-soluble extracts of cheeses inhibited PE in hydrolysing Z-Gly-Pro-amidomethylcoumarin. Inhibition by peptides from rakfisk was negligible. Pepsin + trypsin hydrolysates from the three fish species contained PE inhibitory peptides with a broad range of apparent hydrophobicity and apparent molecular mass. Autolysates from muscles of the 3 fish species contained narrow peptide peaks of different molecular mass and different apparent hydrophobicity with strong PE inhibitory activity. The content of hydrophilic inhibitory peptides was lower in cheeses than in pepsin + trypsin hydrolysates of fish muscle.

Cooking Enhances the Antioxidant Properties of Polyphosphates

ABSTRACT: Raw and cooked brook trout muscle (BTM) samples were treated with phosphates and other antioxidants. Sodium tripolyphosphate (STPP), added before or after cooking, significantly reduces the formation of thiobarbituric acid‐reactive substances (TBARS) only in cooked BTM. Fish processed during rigor mortis had higher TBARS. The effect of different types of phosphates depended also on cooking BTM. When copper was added to accelerate oxidation, only EDTA completely prevented TBARS formation. Sodium citrate (SCit) was more concentration‐dependent and slightly less efficient than STPP. Higher levels of orthophosphates were found in raw BTM. Because cooking enhanced the antioxidant activity of STPP, SCit, and EDTA, the results suggest that phosphates are good chelators, provided cooking has eliminated phosphatase hydrolysis.

Comparison of fatty acid composition in some tissues of rainbow trout ( Oncorhynchus mykiss) living in seawater and freshwater

The effects of salinity on the fatty acid profiles of liver, gonad, muscle and adipose tissues of rainbow trout ( Oncorhynchus mykiss) kept in cages suspended in a seawater (SW) and freshwater (FW), respectively, and fed with the same commercially available diets before marketing, were investigated. In muscle tissue, the effect of salinity (0.17%) was found to be insignificant in terms of the total monounsaturated fatty acid (MUFA), total polyunsaturated fatty acid (n-3, n-6 PUFA) and total saturated fatty acid (SFA), but significant in terms of the eicosapentaenoic acid (EPA)/docosahexaenoic acid (DHA) ratio. In gonad and liver tissues, MUFA and n-3 PUFA were found to be significantly affected by salinity, but not SFA or n-6 PUFA. In adipose tissues, all the parameters were significantly affected by salinity. Also n-3/n-6 PUFA and EPA/DHA ratios were characteristic for FW and SW fish. While the salinity significantly affected the n-3/n-6 ratio in the adipose and liver tissues, it did not have any significant effect in the muscle and gonads. Additionally, the EPA/DHA ratio was found to be significantly affected by the salinity in all the tissues examined.

Red muscle recruitment during steady swimming correlates with rostral–caudal patterns of power production in trout

Rainbow trout ( Oncorhynchus mykiss) and brook trout (or charr, Salvelinus fontinalis) display different rostral–caudal patterns of power production by the red or aerobic muscle during steady swimming. The anterior muscle of rainbow trout produces much less power for swimming than the posterior, while in brook trout there is no variation in power output. To determine if red muscle recruitment is associated with anterior–posterior patterns of power production, electromyography (EMG) was used to record red muscle activity at three body positions across a range of swimming speeds in fish of each species. The initial recruitment of the anterior red muscle in swimming rainbow trout was predicted to lag behind, i.e. occur at higher speeds, that of the posterior due to the variation in power production, but no variation in recruitment was expected for brook trout. Burst of red muscle EMG activity occurring with each tailbeat was analyzed for frequency (tailbeat frequency), duty cycle (DC) (duration of burst relative to the period of the tailbeat) and burst intensity (BI) (magnitude of the measured EMG activity). Brook trout swam with higher tailbeat frequencies and longer values of DC than rainbow trout. Both species showed a pattern of longitudinal variation in DC, with longer DC values in the anterior red muscle. BI also differed significantly along the length of rainbow trout but not brook trout. In the former, BI of anterior muscle was significantly less than the posterior at lower steady swimming speeds. The EMG data suggest that power production and muscle recruitment are related. In rainbow trout, where there is longitudinal variation in muscle power output, there are also significant rostral–caudal differences in red muscle recruitment.