The Scaling of Aerobic and Anaerobic Muscle Power in Rainbow Trout (Salmo Gairdneri)

A fundamental issue in the study of vertebrate swimming is the relationship between muscle power production and the power required to achieve a particular level of performance (Gray, 1936; Bainbridge, 1961; Webb, 1978). The scaling of power requirements has been the subject of considerable analysis (Vlymen, 1974; Weihs, 1977; Webb et al. 1984) and, in general, displays positive allometry for both sustained and burst swimming. Less attention has been given to the scaling of muscle power output, but it appears that the mechanical limitations for maximum power during a single muscle contraction are not size-dependent (Schmidt-Nielsen, 1977; Webb & Johnsrude, 1988).

The scaling of muscle power output in fish is complicated by its differentiation into aerobic red muscle tissue (used for sustained swimming) and anaerobic white muscle tissue (used for high-speed burst or sprint swimming). It has generally been assumed that the scaling of red muscle mass and mass-specific anaerobic metabolism are isometric. Limited data on the scaling of red muscle tissue among specialized scombrids (Graham et al. 1983) suggest that red muscle mass may increase more rapidly with size than expected for isometric scaling. The results for endothermic species, however, indicate the opposite relationship and therefore the general situation for fish remains unclear.

Intraspecific studies on the scaling of glycolytic enzyme activity in white muscle tissue suggest that the potential for anaerobic metabolism in fish may also be positively allometric (Somero & Childress, 1980). Expressed as a function of body mass (y = aM^b), mean mass-specific lactate dehydrogenase (LDH) and pyruvate kinase (PK) activities increase with M^0·35 and M^0·21, respectively. It is not known if this allometric scaling of enzyme activity translates into the rate of anaerobic energy production or total capacity during burst activities.

The main purpose of the present study was to quantify the scaling of red muscle tissue and anaerobic metabolism (lactate production) in the rainbow trout (Salmo gairdneri) and compare these results with theoretically predicted power requirements during prolonged and burst swimming, respectively.

Rainbow trout were obtained from the Fisheries Laboratory of the Ministry of Agriculture, Fisheries and Food at Lowestoft, England. The fish were fed a commercial trout food and maintained in flowthrough tanks (15±1°C) using dechlorinated fresh water.

The rate of white muscle lactate production after 1, 2 and 6 min of maximal burst activity was determined for small (8·0 ±0·15 cm; X±S.E.) and large (28·1 ± 0·55 cm) fish. Four, five or six fish of each size group were sampled at each time interval. Individual fish were chased by net in a way that produced sequential bursts rather than continuous swimming. Tanks of the same relative size, approximately 3×4 body lengths, were used for each size of fish. As a measure of total anaerobic capacity, 21 additional fish (7·0–40·5 cm; 3·5–1225 g) were exercised for a period of 6 min and analyzed for white muscle lactate production. Most of the fish were physically exhausted after 3 or 4 min of activity, and by the end of 6min all the fish appeared fatigued.

An attempt was made to obtain resting levels of white muscle lactate in fish ranging in size from 7·2 to 37·9cm (3·8–649g). Individual fish were isolated in separate tanks for 36–42 h prior to sampling to minimize their level of excitement. The anesthetic 2-phenoxyethanol was introduced into each tank to minimize activity, and then the fish were quickly removed and sampled. Although intended as a resting sample, these lactate levels unavoidably reflect an approximately 30 s period of stress (i.e. 20s for anesthesia and 10s for sampling).

An approximately 500 mg sample of white muscle tissue was excised from each fish below the dorsal fin and immediately immersed in liquid nitrogen. The sample was pressed against the side of the container to facilitate freezing. All tissue samples were homogenized in cold 8 % perchloric acid using a Polytron tissue grinder and centrifuged for 10 min at 1500 g. Lactate was quantified spectrophoto-metrically at 340 nm using lactate dehydrogenase and nicotinamide adenine dinucleotide (Sigma, 1984).

The problems associated with obtaining accurate muscle concentrations of lactate are well documented (Adcock & Dando, 1983; Wieser et al. 1986). They are related to the rate of tissue cooling which, even when using liquid nitrogen, may be affected by the size of the tissue sample. As a check on this source of error, and the variability of the procedure used here, a 27 cm fish was exercised to increase its muscle lactate level (6min) and then sampled in replicate. Six adjacent tissue samples ranging from 0·50 to 2·00g were excised, frozen and analyzed for lactate. White muscle lactate concentrations following activity have been shown-not to vary with location on the body (Wieser et al. 1986) or to vary by less than 10% (Schwalme & Mackay, 1985).

The values of replicate lactate samples obtained from a single fish were not affected by the size of muscle tissue sampled (P< 0·40). Tissue samples weighing 0·56, 0·63, 0·72,1·03,1·70 and 2·02 g had lactate concentrations of 46·6, 43·9,44·1, 44·8, 43·1 and 44·5 μmolg⁻¹, respectively.

The concentration of white muscle lactate from anesthetized rainbow trout increased with the size of the fish (P>0·001; r² = 0·72). Lactate levels of the largest fish were approximately threefold higher than those of the smallest (Fig. 1). The behavior of the fish during anesthesia suggests that the elevated lactate concentrations were the result of stress and activity, and that they do not accurately reflect the resting values for large fish. The typical response of fish as they lost equilibrium involved slow periodic body/caudal fin movements, and in a few instances the fish first moved to a new location in the tank as the presence of the anesthetic was sensed. Furthermore, if the observed rate of increase in resting lactate concentration were accurate it would indicate the unlikely possibility that large fish have no anaerobic scope. The actual resting white muscle lactate concentration is probably between 0·0 and 7·0 μmol g⁻¹ (Black et al. 1962; Wieser et al. 1986). The lowest concentration observed in the present study was 5·6μmol g⁻¹ (8·2cm fish), and this value was used to estimate conservatively the lactate production of exercised fish.

The initial rate of muscle lactate production during maximal burst activity was much higher for large fish (28-1 cm) than for small fish (8·0cm; Fig. 2). Larger fish reached maximum muscle lactate concentrations after approximately 1 min rather than 2 min, and also produced twice as much lactate per gram of tissue after 6 min of activity.

The 6-min lactate concentrations from the time series study above are plotted in Fig. 3 together with data for intermediate and larger-sized fish following 6 min of activity (N=32). The overall relationship between fish size in length (cm) and muscle lactate concentration (μmolg⁻¹) after 6 min of activity is significant (P< 0·001; r² = 0·67) and best described by the equation:

The cross-sectional area of red muscle normalized for fish length (Q_n) also increased in larger fish (Fig. 4B). These values were not influenced by changes in the quantity of white muscle, and are therefore a measure of the absolute amount of red muscle present at each section of the fish’s length.

Expressed in terms of mass, red muscle tissue increased linearly with increased fish length from approximately 1·0 to 3·0 % of total body mass (Fig. 5; r² = 0·88). The relationship between red muscle mass and body mass was described by a power function (Fig. 6; r²= 0·98), and as a function of length red muscle mass increased as:

The results of this study indicate that both the rate and the total capacity of maerobic metabolism increase with size in rainbow trout. A possible source of srror in the estimate of total anaerobic capacity using 6-min muscle lactate production is the loss of lactate into the blood. The data of Turner et al. (1983a) on rainbow trout permit an evaluation of the magnitude of this error. In that study,white muscle lactate concentrations following 6 min of activity reached approximately 43 mequiv kg⁻¹ of muscle cells. For a 1-kg fish with approximately 50% white muscle (Webb & Johnsrude, 1988; the present study), this is equivalent to 21·5 mequiv of muscle lactate production. Blood lactate levels are known to reach maximum levels several hours after exhaustive activity (Wood & Perry, 1985), and after 6 min increased to only 6·5 mequiv I⁻¹ in the rainbow trout (Turner et al. 1983a). With a blood volume of approximately 50ml kg⁻¹ (Smith, 1966; Stevens, 1968), the total blood lactate of a 1-kg fish after 6 min of activity (0·325 mequiv) would be just 1·5 % of muscle lactate production. The 6-min lactate data, therefore, appear to be realistic estimates of total white muscle lactate production.

In the present study a much higher rate of lactate production was observed in larger fish during maximal burst activity (Fig. 2). This increased potential for anaerobic metabolism by larger fish was even apparent in the lactate concentration of anesthetized (resting) fish. These results are consistent with the intraspecific scaling of glycolytic enzyme activity in fish, which is also characterized by positive allometry (Somero & Childress, 1980). The intraspecific scaling of whole-body LDH activity, averaged for several species, increases as L⁴·⁶ (Somero & Childress, 1980). The several-fold difference in rate of lactate production between 8·0 and 28·1 cm fish is of approximately the same magnitude as that for LDH activity over the same size range. The maximum concentrations of lactate accumulated per gram of white muscle tissue were also higher in larger fish after exhaustive swimming. Interspecific reports on the scaling of anaerobic metabolism (Castellini & Somero, 1981) suggest that the allometry in anaerobic potential seen in the present study may be accompanied by changes in muscle buffering capacity.I know of no studies, however, that have examined the intraspecific scaling of muscle or blood buffering capacity.

The scaling of anaerobic energy availability is estimated from the total anaerobic capacity of each size of fish (equation 6). A conversion value of 558 J g⁻¹ of glycogen metabolized is used (Wardle, 1975), which assumes an ATP energy equivalence of 12·0 kcal mol⁻¹. The burst speeds that are possible from the energy available to fish of a certain size will, of course, depend on the rate at which energy is used. Rates of power output were calculated for each size of fish as the ratio of total anaerobic energy available to the duration of activity for periods ranging from 0·5 to 6·0 min. These rates of power output were entered into equation 7 for each size of fish and the equation was solved for velocity.

If the same stamina is assumed for fish of all sizes, these calculations indicate that a large decrease in length-specific burst performance should occur with increased size, and that this decrease in performance should be most severe during short swimming episodes (Fig. 7). However, the time series data for lactate production (Fig. 2) suggest that the rate of depletion of anaerobic reserves is faster for larger fish and, hence, they would exhibit less stamina at higher sprint speeds. In fact, if the commonly accepted velocity of 10Ls⁻¹ is considered, stamina is predicted to fall from 4 min to 30 s for fish of 5 and 40 cm, respectively. When the observed staminas of large and small fish (approximately 1 and 2 min, respectively) are plotted onto Fig. 7, their location suggests that, in addition to decreased stamina, burst swimming velocities also fall from near 12 to 9Ls⁻¹ with an increase in size from 8·0 to 28·1 cm.

Previous studies of red muscle tissue in fish have typically expressed its mass as a percentage of total muscle or fish cross-sectional area (Greer-Walker & Pull, 1975; Graham et al. 1983). The occurrence of maximum values near the caudal peduncle has usually been explained by the observation that this is the point of highest flexion (maximum amplitude and lateral velocity). Because of the small total muscle area at the caudal peduncle, however, the absolute forces and red muscle mass required at this point would be relatively small. A more accurate measure of the longitudinal distribution of red muscle power development would be the absolute area (per unit length) of red muscle tissue (Q_n). Considering all sizes of fish, this value is highest at about 50 % along the length of the muscle mass. At this point along the fish the lateral velocity would be intermediate in magnitude but the total muscle area would still be large. There is a general tendency, with increased fish size, for the section where Q_n is at a maximum to move posteriorly. This may reflect the scaling of length-specific body wavelength (flexibility) which has been reported for rainbow trout (Webb et al. 1984).

The magnitude of allometric scaling for red muscle mass (L³’⁶²) is greater than the scaling of power requirements during sustained aerobic swimming predicted from hydrodynamic theory. Scaling functions predicted from the drag model (Bainbridge, 1961) for the rate of work during aerobic swimming are L²’⁷⁵ and L³’², assuming laminar and turbulent flow, respectively. These relationships are based on sustained swimming performance (Ls⁻¹) increasing as L^0·5 (Wu, 1977; Beamish, 1978). The bulk momentum model of Lighthill (1970) predicts a scaling function of L^3·52 for the same level of performance (Webb, 1977).

These direct comparisons of the scaling function for red muscle mass and the power requirements during aerobic swimming assume that the mass-specific power output of red muscle tissue is not influenced by size. Because we are considering sustainable swimming speeds, however, it must be that the continuous aerobic production of energy by red muscle tissue decreases with increased size. This type of scaling relationship is true for both whole-body metabolic rate and, based on enzymatic data, locomotor muscle tissue (Somero & Childress, 1980). The sustainable mass-specific power output of larger fish, therefore, is likely to be lower according to the scaling of aerobic respiration (i.e. M^0·81; Winberg, 1960). In addition, the average mass-specific power output of red muscle will be proportional to its frequency of contraction. Although no information appears to be available on the scaling of contraction time for fish red muscle, data for white muscle contraction time (Wardle, 1975) and tail-beat frequency during aerobic swimming (Webb et al. 1984) suggest that mass-specific power output for red muscle should be greater for small fish. The observed allometry in red muscle growth would provide some compensation for a decrease in mass-specific power output with increased size.

Interspecific studies of anaerobic potential in fish suggest that some relationship exists between anaerobic and aerobic metabolism. Extremely high LDH activities (Guppy & Hochachka, 1978) and lactate production (Pritchard et al. 1971) are seen among the scombrids; fish with very high aerobic scope. In contrast, sluggish species, such as the flathead sole, appear to show much lower anaerobic capacity (Turner et al. 1983b). This relationship could exist because fish which generate high levels of lactate must also have high aerobic capacity to metabolize the lactate and resynthesize glycogen. Alternatively, it may not be whole-body aerobic capacity per se but rather the amount of red muscle present that is important; red muscle tissue is believed to be involved in the metabolism of anaerobic endproducts (Wittenberger et al. 1975). These two possibilities cannot be resolved in interspecific comparisons since aerobic capacity and red muscle mass are usually correlated. However, in the present (intraspecific) study, increased mass-specific anaerobic potential was observed in large fish in spite of the decrease in aerobic metabolism which occurs with increased size. The similar pattern of scaling for anaerobic metabolism and red muscle mass in the present study suggests, therefore, that the general association between aerobic and anaerobic metabolism may be related more to increased red muscle metabolism than to overall aerobic capacity.

This study was conducted while the author was a National Science Foundation Postdoctoral Fellow of Environmental Biology (BSR-8700153). I wish to express my appreciation to all of those at the Fisheries Laboratory of the Ministry of Agriculture, Fisheries and Food (Lowestoft, England) who provided facilities and fish for these studies. I also thank Dr Paul W. Webb for contributions made throughout these studies.