Interactive Effects of Seasonal Temperature and Low pH on Resting Oxygen Uptake and Swimming Performance of Adult Brown Trout Salmo Trutta

The effects of naturally occurring environmental factors, such as temperature and oxygen availability, on the swimming performance of fish have been reasonably well studied (Beamish, 1978). However, as Beamish (1978) points out, studies on the influence of factors introduced into the environment directly or indirectly by man have largely been restricted to the determination of concentrations that are lethal to inactive animals, while only a few of the many factors that may contribute to disturbances in the environment have been examined for their influence on swimming performance. This is rather strange, because scope for activity has been recommended for use as a criterion in the determination of the sublethal effects of pollutants on fish (Sprague, 1971).

Sustained swimming activity, e.g. during migration, is aerobic, with little accumulation of lactic acid in the blood (Driedzic and Kiceniuk, 1976; Butler et al. 1986), although there may be temporary accumulation of lactate in the muscles at the onset of relatively high sustainable swimming speeds in rainbow trout (Wokoma and Johnston, 1981). These authors suggested that this lactate may actually be oxidized by the red fibres, although this seems not to be the case (Weber, 1991). In addition to problems relating to oxygen supply and the degree of net anaerobic metabolism during exercise, there are also increased passive fluxes of ions and water across the gills of freshwater fish when swimming (Wood and Randall, 1973a, b).

In rainbow trout Oncorhynchus mykiss (Walbaum), acid water (pH4.0–6.0) causes decreases in plasma concentrations of Na⁺ and Cl^-, to a greater or lesser extent, depending on water hardness (McDonald et al. 1980; Playle et al. 1989), together with increases in haematocrit and plasma protein concentration (McDonald et al. 1980). The latter two effects are the result of at least three factors: (a) movement of water from the extracellular to the intracellular space (itself a consequence of branchial ion loss); (b) swelling of the red blood corpuscles; and (c) an increase in the number of red blood cells. Such haemoconcentration causes a large increase in blood viscosity, which may eventually lead to circulatory failure (Milligan and Wood, 1982). Exposure to soft, acid water also causes an increase in blood lactate concentration (McDonald et al. 1980), which may be indicative of impaired oxygen transport. In very soft water there is little or no blood acidosis (Wood, 1989).

The synergistic effect of acid water and strenuous exercise on rainbow trout has been demonstrated by Graham et al. (1982). Severe (i.e. burst) exercise caused approximately 40 % mortality during the 12 h recovery period when the fish were at near neutral pH (approximately 7.5). Severe exercise in soft water at a pH of approximately 4.5 (which in resting fish caused no mortality over a 12 h period) increased mortality during the 12 h period following exercise to 80%. These authors conclude that their data indicate ‘that physiological tests on resting fish may underestimate or misinterpret the toxic effects of acid in the wild where fish are intermittently very active’.

It is certainly true that fish may be very active for short periods when escaping from predators or capturing prey, but sustained, aerobic swimming is also an important aspect of the lives of some fishes, particularly during migration. There are three reports on the effect of acid water on the swimming ability of rainbow trout. Graham and Wood (1981) found that critical swimming speed (U_crit; Brett, 1964) of fingerlings was significantly depressed at pH values below 4.6 in soft water. The fish were not acclimated to the test pH and no physiological measurements were made. The authors suggested that impairment of gas exchange and/or of oxygen transport were the major factors affecting the fishes’ swimming performance. More recently, Ye and Randall (1991) found that 24h of exposure to high pH (>pH9) or low pH (<pH6) reduced swimming performance in adult trout. In an accompanying paper, Ye et al. (1991) suggested (but did not demonstrate) that exposure to low pH reduced the oxygen content of arterial blood and, therefore, that oxygen transport capacity was impaired. Waiwood and Beamish (1978) found that exposure to water at pH 6 had no effect on the swimming performance of fingerling trout.

Environmental acidification is essentially unique to soft water (Wood and McDonald, 1982). Acute exposure to low pH (4.0–4.5) is not uncommon in natural soft waters in Northern Europe as a result of snow melt in the spring or highly acidic run-off in the summer and autumn (Wood, 1989). Because temperature affects the survival of fingerling rainbow trout exposed to lethal, low pH (Kwain, 1975), the sublethal values of pH for adult trout may be different in winter and summer. There is no doubt that, in many field situations, metals such as aluminium and copper are mobilized by acidified soft water. However, it is likely that acidity itself is the dominant toxic agent under acute conditions (Wood, 1989).

The objective of the present study was to determine the effects of exposure to sublethal (acid) pH on the resting oxygen uptake and sustainable swimming capabilities of adult brown trout in soft water, and at summer and winter temperatures. Physiological data were obtained in order to determine whether impaired gas exchange and/or oxygen transport, or circulatory failure resulting from excessive ion loss, could be the cause of any observed differences in swimming performance.

Brown trout, Salmo trutta (L.) (total length 30–46cm, mass 320–520g) were obtained from the Leadmill trout farm, Hathersage, Derbyshire (water composition, μmoll^-1: Ca²⁺, 245–290; Na⁺, 330–370; K⁺, 40–90), and kept for 2–4 weeks in large (14001) circular glass fibre tanks through which dechlorinated Birmingham tapwater (composition, μmoll^-1: Ca²⁺, 130–200; Na⁺, 260–400; K⁺, 6–26) was flowing at a rate of 120 lh^-1. The water was aerated vigorously and circulated around the tank using a pump and spray bar which produced jets of water that were nearly horizontal when they hit the water surface. The fish were provided with plastic tubes (10 cm in diameter 55 cm in length) suspended in mid water, in which they could position themselves. Because the water was circulating around the tank (at approximately 0.25 m s^-1) and through the tubes, the fish had to swim to maintain position.

Following the initial acclimation period, the fish were transferred into a similar tank containing artificial lakewater (Dalziel et al. 1985), also circulating at 0.25 ms^-1 and of the following composition (μmol I^-1): Ca²⁺, 25; Na⁺, 50; K⁺, 5; Mg²⁺, 40; Cl^-, 100; SO₄^2-, 65; NO₃^-, 5. This concentration of Ca²⁺ is similar to that found in some areas of the UK, e.g. Galloway (Harriman et al. 1987), Mid Wales and North Wales (Turnpenny et al. 1987). Dechlorinated water was passed through a deioniser (Permutit TS100) at a rate of 120 1 h^-1 and into a mixing tank. A concentrated solution of the above ions was then added to the deionized water at a rate known to give the required composition. Titanium cooling coils and small aquarium heaters maintained the temperature of the water in the two large holding tanks at the required level, which was 5°C in winter (November to March inclusive, 4–7°C at the trout farm) and 15°C in summer (June to mid-September, 12–16°C at the trout farm). The pH of this water was maintained at 7±0.1 (range) using an Analytical Instruments controller, a Russell CT757 low-conductivity pH electrode and a CP Instruments variable-speed peristaltic pump which delivered 0.25 mol l^-1 NaOH when required. The fish were kept under these conditions for a further 2–4 weeks after the initial period in Birmingham tapwater. Throughout the acclimation period the fish were exposed to the natural photoperiod and were fed daily on floating pellets [Mainstream trout diet, B.P. Nutrition (UK) Ltd], All uneaten pellets were removed from the tank an hour after feeding. No food was given the day before transfer of the fish to an experimental tank or during the experimental period.

The first task was to determine the sublethal pH for the fish. Following acclimation to artificial lakewater, the dorsal aorta was catheterized in five fish, which were anaesthetized in buffered MS222 as described by Butler et al. (1986). The five fish were placed in a smaller circular plastic tank (2801) through which artificial lakewater (pH 7) at the experimental temperature was flowing at 120lh^-1. (Note: the water was not circulated around this tank.) The fish were allowed to recover for 2 days. They were then kept at a low pH for a further 4 days. Initially the pH was set at a level which was thought to be close to, but above, the lethal level (Grande et al. 1978; Wood et al. 1988), so that all the fish survived. Successive batches of five fish were placed in water of progressively lower pH (pH was reduced by 0.25 units for each batch) until one or more fish died during the 4-day exposure period. The pH immediately above that value was designated the sublethal level. This proved to be a very repeatable test, and at 5°C the sublethal pH was found to be 4, while at 15°C it was 4.5. Similar tests were performed on fish that had been anaesthetized, but not cannulated. The sublethal pH values in this case were 3.75 at 5°C and 4.25 at 15°C. pH was controlled to an accuracy of ±0.05 units (range) by a Radiometer TTT2 titrator, a Russell CT757 low-conductivity electrode and a CP Instruments variable-speed peristaltic pump which delivered NaOH (0.25 mol l^-1) or H₂SO₄ (1 moll^-1), as required. The accuracy of the pH measurement was tested against a system selected for use with solutions of low ionic strength: a Radiometer PHM71 Mk2 acid-base analyser with a Corning 00311101J triple-purpose glass electrode and Beckman Futura Plus reference electrode (Davison and Woof, 1985).

Resting oxygen uptake and critical swimming speed (U_crit) were determined in the anaesthetised but uncannulated fish following 2 days of recovery and 4 days of exposure to artificial lakewater at neutral pH or at the sublethal pH relevant to the particular temperature. For determination of fish were placed individually in a darkened respirometer tube either 40 cm or 50 cm in length and 9 cm in diameter. Water flowed through the tube at 300–400 ml min^-1 at 5°C and at 400–600 ml min^-1 at 15°C, so that the difference between the of incurrent water and that of excurrent water was approximately 2 kPa. The fish were placed in the respirometer at approximately 11:00 h on day 1 and was determined 3–4 times during a 60-to 90-min period at approximately 18:00 h, and then in the same way and at the same times (ll:00h and 18:00 h) on each of the successive 6 days. The mean of the 3–4 determinations was calculated for each sampling time. Because the respirometer tubes were darkened to avoid disturbing the fish, it was not always possible to determine the level of activity of the animals. However, when it was obvious that a fish was active, data obtained during that period were not incorporated into the subsequent analysis. For fish at neutral pH, declined over the first 2–3 days, so the mean resting values were calculated from the lowest four data points obtained after the settled rate had been reached. For fish initially at neutral pH and subsequently transferred to sublethal pH, the mean of the last two data points (see Fig. 1A,B) was taken to represent ⁠, at sublethal pH.

U_crit (Brett, 1964) was determined in a variable-speed, Blazka-type water channel (see Axelsson and Nilsson, 1986, for details) of total volume 2001 with a test section 65 cm long and 15 cm in diameter. Water flow through the complete system was at 40 lh^-1 and water speed through the test section could be varied between 0 and 1ms^-1 and was determined by a revolution counter calibrated against a Braystoke BFM 002 current flowmeter. Each fish was swum for 15 min at 0.2 or 0.3 m s^-1 and then for the same time at increasing 0.1m s^-1 increments until it ceased to swim and rested on the back grid. The only attempts made to encourage the fish to swim were to cover the anterior portion of the swim chamber while brightly illuminating the rear and to reduce the water speed slightly after the fish had stopped swimming. Invariably it began to swim again and the water speed was returned to its previous value. This usually gave sufficient time for a blood sample (2–3 ml) to be taken from cannulated fish (see next section) before they stopped swimming again. This was taken as the U_crit (Butler et al. 1986) and was not corrected for the presence of the fish in the water channel (see Jones et al. 1974, for a discussion).

After acclimation, fish were anaesthetised in buffered MS222 and the dorsal aorta was cannulated. The animals were placed into a Blazka-type water channel and left to recover for 2 days. They were then left for a further 4 days at neutral pH or at the sublethal pH relevant to the particular temperature. The pH of the experimental water was controlled to an accuracy of ±0.05 unit by a Radiometer PHM82 standard pH meter, with a TTT80 titrator, a Russell CT757 combination electrode and a CP Instruments variable-speed peristaltic pump, as described above. Experiments were performed alternately at neutral and sublethal pH to avoid seasonal bias. At the end of the exposure period, data were collected from resting fish or from fish that had been swum up to their U_crit. Dorsal aortic blood pressure was recorded on a two-channel pen recorder (Lectromed Ltd), from which heart rate could also be determined. 2–3 ml of arterial blood was removed from the dorsal aorta (see previous section) for the determination of partial pressure of oxygen ⁠, using a Radiometer BMS3 blood micro system and a PHM 73 pH/blood gas monitor, oxygen content ⁠, using a Lexington Instruments Lex-O₂-Con, haematocrit (Het), using a Hawksley microhaematocrit centrifuge, concentrations of plasma Na⁺ and K⁺, using a Pye Unicam SP9 atomic absorption spectrophotometer, and of plasma Cl^-, using an Aminco chloride titrator. Haemoglobin concentration [Hb] and plasma protein concentration were determined with Sigma kits no. 525 and no. P5656, respectively. Water in the holding tanks and water channel was monitored weekly for ⁠, CO₂ content and total ammonia content (Verdouw et al. 1978). The first was always greater than 20 kPa, the second less than 0.1 mmol l^-1 and the third less than 20 μmol l^-1 in the holding tanks and less than 10μmoll^-1 in the water channel.

All values are given as the mean±s.E. and the significance of the differences between mean values was assessed using Student’s t-test, with P<0.05 being taken as the probability level for significance.

Seasonal temperature had small but significant effects on resting ⁠, and U_crit. Resting at neutral pH was 22 % lower at 5 °C than at 15 °C, and U_crit (BLs^-1) was 15 % lower at 5 °C. Exposure to sublethal pH had much larger effects, which are highly significant at both temperatures. At 15°C, resting at sublethal pH was 40% higher than that at neutral pH, whereas U_crit (BLs^-1) was 35 % lower. At 5 °C, resting at sublethal pH was 38 % higher than that at neutral pH, and U_crit (BLs^-1) was 31% lower. After exposure to sublethal pH, there was an immediate increase in and these levels were maintained for approximately 3 days before declined somewhat (Fig. 1A,B).

Although the sublethal pH was 0.25 units higher at both temperatures in the cannulated fish compared with that for the uncannulated animals, there were no significant differences between any of the U_crit values between the two groups (compare values in Tables 1 and 2). At neutral pH, temperature and exercise both had significant effects on heart rate. Resting heart rate at 5°C was 42 % lower than that at 15°C, and heart rate at U_crit was 2.1 times that at rest at 15°C and 2.4 times that at rest at 5°C. At sublethal pH, resting heart rate was 41%, but not significantly, higher than that at neutral pH at 15 °C whereas it was a significant 25% higher at 5 °C. At sublethal pH and heart rate was 29%, but not significantly, higher than that at rest at 15 °C but a significant 57 % higher at 5 °C. There were no significant effects of low pH or of exercise on mean dorsal aortic blood pressure at 15°C. At 5°C and neutral pH, mean pressures were significantly lower than those at 15°C in resting, but not in swimming, fish. Mean dorsal aortic pressure was, however, significantly higher in resting fish at sublethal pH at 5°C than in those at neutral pH. There were no significant differences between dorsal aortic blood pressures in fish at sublethal pH at either temperature.

Despite a significant reduction in at 15 °C when swimming at U_crit and at neutral pH (Fig. 2A), there was a significant increase in (Fig. 2B). This was the result of a significant increase in [Hb] (Fig. 2D), which also occurred in response to exposure to sublethal pH at both temperatures.

At neutral pH, temperature had little effect on plasma ion concentrations. Exposure to sublethal pH caused significant reductions in plasma Na⁺ (Fig. 3A) and Cl^- (Fig. 3B) concentrations at both temperatures, although these were more substantial at 5 °C than at 15°C. There were no effects on plasma K⁺ concentration (Fig. 3C). Swimming at U_crit had no significant effect on plasma concentrations of Na⁺, K⁺ and Cl^- under any conditions, except at sublethal pH at 5 °C when there were significant reductions in all three (Fig. 3A,B,C).

Plasma protein concentration was not influenced by temperature or swimming at U_crit (Fig. 3D). It was significantly higher than that at neutral pH after exposure to sublethal pH at 15 °C, but there was no such effect at 5 °C.

The major finding of this study is that, when exposed to their sublethal pH for 4 days, the swimming ability of adult brown trout, as indicated by the U_crit, is impaired at both 15 °C and 5°C. Although cannulation of the dorsal aorta does have an effect on sublethal pH, the effect of sublethal pH on swimming performance is the same in cannulated and uncannulated fish. This effect of low pH on the swimming performance of trout contrasts with the finding of Waiwood and Beamish (1978), but is in agreement with the findings of Graham and Wood (1981) and Ye and Randall (1991). The first two groups studied fingerling rainbow trout, and the first group exposed their fish to pH 6, whereas the last two groups exposed their animals to pH <4.6.

There are no signs of a reduction in arterial oxygen content (cf. Ye et al. 1991) or of a substantial rise in metabolic rate in response to exposure to sublethal pH for 4 days, indicating that neither of these factors was the likely cause of the reduced U_crit. Although previous data on the effect of low pH on metabolic rate are contradictory (Wood and McDonald, 1982), Rosseland (1980) did report an increase in standard oxygen consumption in brown trout exposed to pH 4.5 in brookwater (Tovdal), although he concluded that this effect was not due to low pH alone. In addition, both Hargis (1976) and Waiwood and Beamish (1978) described an increase in oxygen consumption at low swimming speed in rainbow trout exposed to low pH. In contrast, Ye et al. (1991) reported no change in oxygen uptake of rainbow trout exposed to pH 4 for 24 h. The causes of the increased resting and heart rate at sublethal pH in the present experiments are unknown. Milligan and Wood (1982) conclude that the latter effect is the result of increased adrenergic stimulation, which would indicate an increased level of stress. The increase in may be another manifestation of this stress, which may be associated with increased low-level activity of the fish although, as stated earlier, this was not determined in the present experiments. It is interesting to note that there was no significant increase in levels of plasma catecholamines in rainbow trout following 24 h of exposure to pH 4 (Ye et al. 1991).

Bearing in mind that rainbow trout swimming at U_crit increase their ⁠, by 7.5–9 times above resting (Webb, 1971; Kiceniuk and Jones, 1977), it could be argued that the relatively small increase in resting of brown trout exposed to sublethal pH would not in itself have a major effect on U_crit. This conclusion is certainly consistent with the data from Waiwood and Beamish (1978), who showed that exposure of rainbow trout to low pH caused a 25 % increase in ⁠, at very low swimming speeds, but had no ‘appreciable’ effect on U_crit. However, the metabolic cost of exposure to low pH may increase as swimming speed increases (cf. Febry and Lutz, 1987), thus reducing the fishes’ scope for activity.

It is known that exposure to low environmental pH causes an imbalance in the fluxes of Na⁺ and Cl^-, leading to net losses of both of these ions (Wood, 1989), although the precise mechanisms may be different in fish exposed to lethal levels of pH compared to those in fish chronically exposed to sublethal pH (McDonald and Wood, 1981; McDonald, 1983; McDonald et al. 1983; Audet et al. 1988). Ion regulatory failure is thought to lead to the eventual death of rainbow trout exposed to water of lethal low pH by causing haemoconcentration and ensuing circulatory failure (McDonald et al. 1980; Milligan and Wood, 1982). In the present experiments, reductions in plasma concentrations of both Na⁺ and Cl^- occurred in fish exposed to sublethal pH at both temperatures, reaching lower levels at 5°C than at 15 °C. Associated with these reductions in plasma ion concentrations were signs of haemoconcentration, although these were different at the two temperatures.

There were substantial increases in [Hb] and plasma [protein] but not in dorsal aortic blood pressure in resting fish at sublethal pH at 15°C. At 5 °C, however, there was a smaller increase in [Hb] and no change in plasma [protein], whereas dorsal aortic pressure was elevated in resting fish at sublethal pH. Thus, although neither ⁠, nor heart rate was significantly lower at U_crit in fish exposed to sublethal pH compared with those at neutral pH, the possible impairment of oxygen transport to the red muscles of swimming trout when exposed to low pH cannot be ruled out. An increase in blood viscosity, by whatever cause, may have had subtle, adverse effects on the local circulation in the red muscles. This may have contributed to the impaired swimming performance of fish at sublethal pH. It is interesting to note that swimming at U_crit had no effect on the plasma concentrations of Na⁺ and Cl^- under most conditions (cf. Wood and Randall, 1973a), the exception being fish at 5°C and sublethal pH.

Whatever the underlying cause(s), the effect of sublethal pH on swimming performance of brown trout is clear (see also Nelson, 1989, for yellow perch, Perca flavescens). Exposure to sublethal levels of other pollutants, such as copper (Waiwood and Beamish, 1978) and the water-soluble fraction of crude oil (Thomas and Rice, 1987), also reduces the swimming performance of salmonid fish, although this is not the case with all pollutants (Webb and Brett, 1973). So, although resting fish are able to survive certain levels of environmental pollutants, their ability to swim under such conditions may be diminished. This could have severe consequences on the ability of active fish, such as salmonids, to maintain station in fast-running rivers, especially at the ‘swim up’ stage (Heggenes and Traaen, 1988), or to migrate.

It was not the intention of the present study to determine the separate effects of temperature and season on the responses being studied. It was, however, intended to investigate whether the responses to sublethal pH are different in fish kept at a low (seasonal) temperature in winter compared with those kept at a higher (seasonal) temperature in summer.

In terms of the aims of the present study, the important finding is that brown trout can survive, for 4 days at least, at a lower pH (by 0.5 units) at 5°C in winter than they can at 15°C in summer. Also, some of the physiological responses to exposure to sublethal pH and to swimming in water at sublethal pH are different at the lower temperature. These observations indicate that seasonal temperature should be taken into account in studies of the influence of sublethal pH on fish. In addition, it is of some interest and worthy of discussion that seasonal temperature had only minor influences on resting and U_crit of fish kept at neutral pH.

Most earlier studies on trout, which were not specifically investigating the effect of temperature, have been performed on rainbow trout acclimated to 10–15°C at variable (unreported) times of the year and photoperiod. The measurements obtained in the present study from the brown trout at 15 °C and in water at neutral pH are very similar to those obtained during previous studies on rainbow trout. The values of resting (standard) and of U_crit are similar to those reported by Webb (1971) and Bushnell et al. (1984) for slightly smaller rainbow trout at 15 °C. The decline in during swimming has previously been reported by Kiceniuk and Jones (1977) and Thomas et al. (1987), the latter authors also noting that, as in the present study, is maintained or even increased as a result of an associated increase in [Hb], Priede (1974) noted a doubling of heart rate (from 45 beats min^-1) in rainbow trout exercising normally at 15 °C. Mean dorsal aortic pressure and haematocrit in the resting brown trout are similar to those recorded in rainbow trout at approximately 10°C (Kiceniuk and Jones, 1977) and 14°C (Milligan and Wood, 1982) and, as in the former of these two studies, mean dorsal aortic pressure at U_crit in the brown trout was no different from that at rest.

There have been many studies specifically investigating the effect of temperature on metabolism and swimming performance of fish. In a study of young sockeye salmon Oncorhynchus nerka, Brett (1964) showed that, although standard metabolic rate was 43 % lower at 5 °C than at 15 °C, U_crit was only 20 % lower. Griffiths and Alderdice (1972) working on coho salmon, O. kisutch, also indicated that there is a seasonal factor involved, so that swimming performance is improved at low temperature during winter. Data from ‘wild’ fish are somewhat contradictory. Jones et al. (1974) studied 17 species of freshwater fish and found no significant influence of acclimation temperature (between 7 and 20°C) on their swimming performance. In contrast, the water speed against which juvenile rainbow trout could hold position in winter (5°C, 10.5h:13.5h L:D) was significantly (40%) lower than that in summer (15°C, 14h:10h L:D) (Facey and Grossman, 1990).

There is no doubt that, in many species of fish, acclimation to low temperature causes a number of anatomical and biochemical adaptations that would enhance swimming performance. In cold-acclimated goldfish Carassius auratus, crucian carp C. carassius and striped bass Morone saxatilis, there are increases in the proportion of aerobic ‘red’ and ‘pink’ muscle fibres, in mitochondrial density (and hence in the activity of aerobic enzymes) and in capillary supply, and a decrease in diffusion path between the sarcoplasmic and mitochondrial compartments (Johnston and Lucking, 1978; Johnston and Maitland, 1980; Sidell, 1980; Johnston, 1982; Jones and Sidell, 1982; Egginton and Sidell, 1989). There appear to have been no such studies on salmonid fish. In addition to the above adaptations to low temperature, there are increases in myofibrillar ATPase activity in both ‘red’ and ‘white’ muscles in cyrinids (Johnston et al. 1975; Sidell, 1980), but not in ‘white’ muscle of trout (Penney and Goldspink, 1981a,b). These authors point out that this discovery is somewhat surprising in view of the active life style of salmonids. Like the striped bass (Sisson and Sidell, 1987), salmonids are presumably able to enhance their swimming ability at low temperature by increasing the proportion of ‘red’ fibres, but this remains to be demonstrated. There is certainly hypertrophy of cardiac muscle in rainbow trout at winter temperature, resulting in stroke volume being the same as, or even greater than, that in trout at summer temperature (Graham and Farrell, 1989). These authors point out that such compensation may be important in enabling the fish to maintain a high level of swimming performance during the winter.

It is also of some interest that continued swimming activity (training) of trout improves their U_crit, causes hypertrophy of ‘red’ and ‘white’ skeletal muscle and of cardiac muscle and causes an increase in standard (Davison and Goldspink, 1977; Greer Walker and Emerson, 1978; Nahhas et al. 1982; Farrell et al. 1990). The brown trout used in the present study were kept active for the entire holding period in Birmingham and, if the effects of activity were relatively more pronounced at the lower temperature, this could explain the relatively small differences in resting ⁠, and U_crit at neutral pH during summer and winter. Interestingly, seasonal temperature had no effect on U_crit at sublethal pH.

This work was supported by the Natural Environmental Research Council.