The effects of exercise on growth, food utilisation and osmoregulatory capacity of juvenile Atlantic salmon, Salmo salar

Aquaculture, 116 ( 1993) 233-246 Elsevier Science Publishers B.V.. Amsterdam

233

AQUA 60047

The effects of exercise on growth, food utilisation and osmoregulatory capacity of juvenile Atlantic salmon, Salmo salar Even H. Jorgensen and Malcolm Jobling The Norwegian College of Fishery Science, University of Tromss, Troms0, Norway (Accepted 4 May 1993)

ABSTR.ACT Specific growth rate (SGR), food intake (FI) and frequency of fin damage were recorded in juvenile Atlantic salmon (Salmo salar), held for 63 days in standing water (controls) and at water currents corresponding to initial swimming speeds of 1.0, 1.5 and 2.0 BI.s-‘, respectively. SGRs were significantly higher in all exercised groups than in the controls, with a maximum in the group swimming at 1.5 BI-s-‘. FI increased slightly with increasing swimming speed, and there was a more even distribution of food as swimming speed increased. The reduced inter-individual variation in FI was reflected in greater homogeneity of growth in the exercised groups than in controls reared in standing water. As compared to the controls, which had a growth/food intake ratio of 1.3 I, food utilisation was significantly higher in the fish swimming at 1.0 BI-s-’ (with a ratio of 1.56), but food utilisation of the groups swimming at the highest speeds did not differ significantly from the controls. Analysis of body composition revealed that the body water content was higher, and energy content lower in the salmon swimming at the highest speed than in the controls. Exercise affected the behaviour of the fish, appeared to result in reduced levels of agonistic activity, and the rate of fin wound healing seemed to increase with increasing swimming speed. Exercise did not, however, lead to any improvement of the osmoregulatory capacity of fish undergoing Parr-smolt transformation.

INTRODUCTION

Salmonid fish which are exposed to a moderate water current tend to hold station, and may, in that way, be exercised at defined swimming speeds for prolonged periods. Sustained exercise can result in a number of physiological and behavioural effects in salmonid fish. For example, improved growth has been reported in juvenile brown trout, Salmo trutta (Davidson and Goldspink., 1977), post-smolt and adult Atlantic salmon, Salmo salar (Kuipers, 1982; Totland et al., 1987), juvenile rainbow trout, Oncorhynchus mykiss -Correspondence to: Dr. E.H. Jorgensen, Norway.

0044-8486/93/$06.00

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0 1993 Elsevier Science Publishers

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E.H. JQRGENSEN AND M. JOBLING

(Greer Walker and Emerson, 1978; Nahhas et al., 1982; Houlihan and Laurent, 1987)) juvenile brook trout, Salvelinus funtinalis (Leon, 1986; East and Magnan, 1987) and fry and juveniles of Arctic charr, Salvelinus alpinus (Christiansen et al., 1989; Christiansen and Jobling, 1990), subjected to intermediate water velocities, at flow rates below the critical swimming speed (Beamish, 1978 ) of the fish. Improved growth appears to be associated with red and white muscle hypertrophy (Davison and Goldspink, 1977; Greer Walker and Emerson, 1978; Johnston and Moon, 1980; Totland et al., 1987), caused by the continuous physical stimulation of the muscle. There are some indications that exercising salmonids display better food conversion than fish performing routine activity (Davison and Goldspink, 1977; Kuipers, 1982; East and Magnan, 1987) and in recent experiments with Arctic charr, in which food intake was measured with great accuracy, this has been confirmed (Christiansen and Jobling, 1990; Christiansen et al., 1992). It has been suggested that fish which are exposed to moderate water currents exhibit schooling behaviour, display less agonistic encounters (East and Magnan, 1987; Christiansen and Jobling, 1990) and have a reduced tendency to form hierarchies. The development of such hierarchies may affect the patterns of growth, leading to an increased variation in fish size within the group, elevated metabolic rates and stress (Li and Brocksen, 1977; Ejike and Schreck, 1980; Jobling and Wandsvik, 198 3 ) . In general, the results from previous studies point to a potential benefit to be gained by exercising salmonid fish in captivity. In Atlantic salmon, some of the effects of exercise have been investigated in post-smolts (Kuipers, 1982; Totland et al., 1987). In Norwegian aquaculture, production of salmon smolt occurs predominantly in indoor tanks, whereas larger fish are reared in seacages. Thus, in commercial production, there are greater possibilities to exercise the fish in the pre-smolt than in the grow-out and adult stages. Consequently, the major objective of the present study was to investigate the effects of sustained exercise on growth, food utilisation and social behaviour in juvenile, pre-smolt Atlantic salmon. Abrupt transfer of salmon smolt to sea water may stress the fish (Usher et al., 199 1) , and according to Woodward and Smith ( I 98 5 ) , exercised rainbow trout responded to the stress of agitation in a more controlled and coordinated manner than unexercised fish. It was further suggested (Woodward and Smith, 1985 ) that recovery from exposure to environmental stressors might be improved by subjecting the fish to prolonged exercise regimes. Since the fish used in the current study were maintained on the different exercise regimes until they underwent the Parr-smolt transformation, it was deemed of interest to test for possible effects of exercise on the osmoregulatory capacity of the fish in the period immediately following exposure to the stress of transfer to sea water.

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MATERIALS AND METHODS

One-year-old hatchery-reared pre-smolt Atlantic salmon (Sulmo s&r) were used in the experiment. On 10.02.92, a total of 1227 fish which had been sizegraded (mean weight approx. 14 g) and individually tagged (FTF-69 Fingerling Tags, Floy Tag and Mfg. ) were divided randomly amongst 12 centrally drained circular rearing tanks with a water volume of approximately 90 litres. Until the start of the experiment on 10.03.92 lish were held under continuous light conditions and a moderate water current of 5 cm-s- ’ was maintained in all tanks. The current was created by forcing the inflowing water through vertically mounted perforated inlet pipes, as described by Christiansen and Jobling ( 1990). Thus, during the acclimation period, the fish were subjected to a moderate exercise of approximately 0.5 body lengths (BI ) *s- ’ . Water temperatures were measured daily, and were held at 8.5 ( ? 1) “C. During the experiment, which lasted 63 days from 10.03.92 to 13.05.92, triplicate groups of fish with an initial average live weight of 18.1+ 5.6 (SD) g (day 0) were subjected to water velocities of 0, 12, 17.5 and 24 cm-s-‘. Since the initial lengths of the fish were approximately 11.5 cm, the water currents corresponded to initial swimming speeds of approximately 0, 1.0, 1.5 and 2.0 BI-s-l, respectively. The water current was kept constant throughout the course of the experiment and consequently the relative swimming speeds of the exercised groups decreased as the fish grew. Relative speeds were approximately 0.8, 1.2 and 1.7 BI-s-l by the end of the experiment. Water-exchange rates were above 6 l.min-’ in all tanks and weekly measurements of oxygen saturation were performed to ensure that saturation remainmedabove 90%. At the start of the experiment, stocking density was approximately 20 kg*m-3, and this increased during the experiment to a maximum of 39 kg*m-3. All groups were fed commercial dry food (FK Start, 4 G), supplied continuously during a 9-h period every day using automatic disc feeders. Food was provided in excess, corresponding to approximately 3.0% of the tank biomass daily. Every 3 weeks (on days 0,2 1,42 and 63) individual live weight and body length were measured. At the same time the numbers of fish showing signs of fin damage (bacterial fin-rot, splitting or bite wounds) were noted. Daily food intake was recorded on days 2 1, 42 and 63, using an adaptation of the radiographic method originally described by Talbot and Higgins ( 1983 ) . During the 9-.h period for which food intake was to be determined, ordinary food was replaced with a diet that differed from the normal diet only in that it contained 1.O% by weight of glass beads (Ballotini size 10.0; Jencon’s Ltd., Leighton Buzzard). Following the feeding period, the fish were anaesthetised in benzocaine (60 ppm) and X-ray photographed (Nanodor 2 X-ray machine, AGFA Structurix D7 film). The X-ray plates were then developed and

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the numbers of glass beads in the gastrointestinal tract of the fish counted. Food intake was determined using calibration curves determined for the relationship between the dry weight of the food and number of beads. At the end of the experiment (day 63 ) , 10 fish were sampled from each of the 3 control groups (0 B1.s ’ ) and the 3 groups which had been exercised at 2.0 B1.s~‘. The fish were killed by a blow to the head and immediately frozen and stored at - 70” C for later analysis of moisture content, amount of visceral fat and carcass energy content. Visceral fat (which surrounded the pyloric caeca and intestine) was determined by careful removal by dissection. Moisture content was determined by drying whole fish to a constant weight at 65 “C (approx. 48 h), and energy content was determined by bomb calorimetry (Phillipson Microbomb Calorimeter) using triplicate subsamples of minced, dry fish. One week after termination of the experiment, all groups were exposed to full-strength sea water (33 ppt ) for 24 h. During sea water exposure, the stock supply of freshwater was switched to sea water, with the inflow being arranged in such a way that no marked circumferential water currents were generated. The fish had been deprived of food for 4 days prior to exposure to sea water and they were not fed during the seawater challenge test. The temperature of the sea water was 8 ‘C. Immediately before and after the 24-h exposure, approximately 10 fish were removed from each tank and killed by a blow to the head. The fish selected had weights which were close to the group means. Immediately after the fish had been killed, blood was sampled by cauda1 severance and transferred to a heparinized plastic tube. After centrifugation ( 5 min, 5500 rpm), plasma was removed and stored at - 20°C until analysed for osmolality ( Wescor 5500) and Cl- concentration (Radiometer CMT 10). Analyses of growth rates, food intake and condition factor were carried out using only the data collected from fish which retained their tags throughout the whole experiment (n, Table 1). Specific growth rates (SGR, %-day- ’ ) were calculated using the formula: [

(lnwr-lnw,)*(T-

t)-‘1.100,

where W, and W, are fish weights at the start and end of the experiment, respectively, and T - t is number of days between weighings. Average daily food intake (FI, mg food-g fish-’ -day- ’ ) for each fish was calculated as the mean for the three measurements conducted on days 21, 42 and 63. Food utilisation was calculated as: SGR.FI- ’ where SGR is growth rate and FI is food intake expressed in terms of percent of fish body weight per day. Coefficients of variation (CV ) were calculated for the specific growth rates

EFFECTS OF EXERCISE ON JUVENILE ATLANTIC SALMON

237

TABLE 1 Coefftcents of variation of specific growth rates and food intake (FI). and condition factors on day 0 and 6 3 of juvenile Atlantic salmon (S&no salur) exposed to water currents corresponding to initial swimmng speeds of 0, 1.0. 1.5 and 2.0 BI-s-’ Exerciss:

0.0 1.0 1.5 2.0

n

260 289 290 271

Coefficient of variation

Condition factor

Fl

SGR (To-T, )

day 0

day 63

29.6 22.6 25.6 22.8

31.2 23.3 24.5 29.2

1.16?0.004 1.17*0.005 1.13+0.004 1.16f0.004

1.15i 0.004 1.21 kO.004 1.21 kO.004 1.22+0.005

All values are means * s.e. n indicates the number of fish in each treatment group which retained their tags throughout the whole experiment.

and food intake data of the groups of fish subjected to the different experimental treatments. CV was calculated at ( lOO*SD) *mean- ‘, in which SD is the standard deviation of the mean of individual SGRs or FIs. Condition factors of individual fish were calculated as: (100. W)C3 where W is weight in g and L is fork length in cm. Group means were calculated for each replicate within treatments and data are presented as arithmetic means + s.e. Data relating to frequency of fin damage, plasma osmolality and Cl- are also presented as group means ? s.e. A one-way ANOVA was used to test for possible differences between treatments in growth rates, food intakes, food utilisations and blood plasma concentration of Cl- and osmolality. A two-way ANOVA was used to test for possible temporal effects on differences between treatments in condition factor and frequency of fin damage. Data expressed as percentages were arcsinus-transformed before the test was performed. Preliminary analyses revealed that visceral fat, whole body moisture and energy content were significantly affected by the body weight of the fish. Consequently linear regressions were plotted and covariance analyses performed in order to examine possible treatment effects. All statistical analyses were carried out using a StatViewTM (Brainpower Inc.) programme. Differences at P-c 0.05 were considered significant. RESULTS

No fish mortality was registered in any of the treatment groups during the coursse of the experiment. Specific growth rates, food intake and food utilisation of the fish subjected

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to the different water currents are shown in Fig. 1. There were significant differences in growth between treatments, with the growth rates of the fish subjected to standing water being significantly lower (P-C 0.05 ) than those of exercised fish. Among exercised fish, the growth rates of the fish held at the highest water current (corresponding to a swimming speed of 2.0 B1.s~’ ) were significantly lower (P-C 0.05 ) than those of fish swimming at intermediate speeds, but fish held at water currents corresponding to initial swimming speeds of 1.O and 1.5 B1.s~ ’ did not differ significantly (P> 0.05 ). Daily food intakes appeared to increase with increasing current speeds, but the increase was found to be significant (PC 0.05 ) only between fish kept in standing water and the fish forced to swim at 1.5 and 2.0 B1.s~ I. Food utilisation, calculated as the ratio between specific growth rate and food intake (both expressed in % of fish weight), differed between treatments. As is evident from Fig. 1, the poorest food utilisation was found for

6 F zj

15

10

F 3 8 F

05

0 r 0 SWIMMING

10 SPEED

1.5

20 (El .sec-1)

Fig. 1. Specific growth rates (A), food intake (mean of the values recorded on days 2 I, 42 and 63) (B) and food utilisation (g body weight increase per g food eaten) (C) of juvenile Atlantic salmon (S&no s&r) exposed to different water currents. All values are means 2 s.e.

EFFECTS OF EXERCISE ON JUVENILE ATLANTIC SALMON

239

the fish swimming at 2.0 BI*s-’ ( 1.25 ?0.02), but food utilisation did not differ significantly from that of fish held in standing water. The best food utilisation was displayed by fish swimming at 1.O B1.s’ ( 1.56 2 0.02) and these fish had a food utilisation that was significantly higher (PC 0.05) than both fish held in standing water and those swimming at the highest speed. Food utilisation by fish forced to swim at 1.5 BI*s- ’ was also high, but did not differ significantly (P> 0.05) from any other group. Coefficients of variance of specific growth rates and food intake are shown in Table 1. The CVs were significantly higher (P= 0.05 ) for the fish reared in standing water than for the groups of fish exercised at 1.O and 1.5 B1.s~‘. CVs of specific growth rates for the fish subjected to the strongest current (2.0 BI-s-l ) were higher than for the groups held at intermediate speeds and close to those for control groups, but the differences were not statistically significant (I’> 0.05 ) . There were no significant differences (P> 0.05 ) in condition factor between fish in the different treatments at the start of the experiment (day 0, Table 1). During the course of the experiment, condition factors of the fish held i:n standing water decreased slightly (although not significantly, P> 0.05 ), while some increase in condition factor was seen in all exercised groups. For the groups forced to swim at 1.5 and 2.0 BI-s-l, the increase was significant (P 0.05 ). During the course of the experiment, the

day swlmmlng speed

0

63 o

0

63 1.0

0

63 1.5

0

63 2.0

Fig. 2. Frequencies of fin damage in groups of juvenile Atlantic salmon (Sulmo salav), exposed to water currents corresponding to initial swimming speeds of 0, 1.O, 1.5and 2.0 B1.s~‘.

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E.H. J0RGENSEN

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frequencies of fish with Iin damage decreased in all groups, and this change was significant, with respect to both treatment and time (PC 0.05). Analyses of visceral fat, whole body moisture content (%) and energy content (kJ*g wet vv-’ ) were carried out on fish held at the highest swimming speed (mean fish weight 37.1 g, weight range 10.5-62.5 g) and on unexercised fish (mean fish weight 29.2 g, weight range 11.O-53.0 g). The regression equations for the relationships between these parameters and the body weight of the fish in these two groups are presented in Table 2. In both exercised and unexercised lish, all parameters were significantly related to fish body weight. For both visceral fat and energy content a positive correlation with body weight was seen, while moisture content decreased with increasing body weight. There were no significant differences (P> 0.05 ) between the slopes of the regression lines for the exercised and unexercised fish, but intercepts calculated on the basis of a common slope were significantly different (PC 0.05 ) for moisture and energy content. Thus, exercise at 2.0 BI-s-’ led to a significant increase in whole body moisutre and a significant decrease in energy content compared to controls reared in standing water. Comparison of regression lines failed to reveal any significant differences with respect to visceral fat. Blood plasma concentrations of Cl- ’ and osmolality before and after the 24-h sea water challenge, and details of the weights of the fish sampled, are presented in Table 3. Since the fish sampled in connection with the seawater challenge test were selected to be close to the respective group means in weight, the weights of the fish from the different groups were significantly different (PC 0.05, Table 3). This was the case for both the fish sampled prior to and TABLE 2 Effect of body weight on day 63 on visceral fat (% body weight), body moisture (%) and energy content (kJ.g wet W-i) of control and exercised (2.0 B1.s’) groups of juvenile Atlantic salmon (Salmo salar) n

a

Visceral fat Unexercised Exercised

30 31

0.127 -0.050

Moisture content Unexercised Exercised

30 31

72.45 74.54

Energy content Unexercised Exercised

30 31

6.624 5.883

b

Slope comparison

Common slope

Intercept with common slope

0.019 0.026

ns

- 0.070 -0.093

ns

-0.071

72.5 73.7

(p=O.OOl)

16.10 21.85

ns

20.08

6.508 6.170

(p=O.O13)

Linear regressions are plotted as y= a+ b&h

0.024

0.007

weight. n indicates the numbers of fish.

(ns)

EFFECTS OF EXERCISE ON JUVENILE ATLANTIC SALMON

241

TABLE .3 Body weight (g), blood plasma concentrations of Cl- (mmol.l-‘) and osmolalities (mosmol) juvenile Atlantic salmon (Sulmo sakzr) sampled prior to and after 24 h sea water (SW) exposure Exercise

0.0 I.0 1.5 2.0

Freshwater

of

After 24 h SW exposure

Fish weight

CI-

Osmolality

Fish weight

Cl-

34.7kO.9 39.st0.9 39.5& 1.3 40.4+ 1.3

131.7kO.7 330.2+0.8 132.6kO.8 133.8t0.9

300.9+ 303.9& 302.62 305.8+

36.8k0.9 42.8? 1.2 42.5? 1.2 42.4+ 1.2

150.95 148.2+ 144.6+ 147.7f

(27) (28) (29) (28)

1.3 1.6 1.4 1.8

(27) (25) (23) (21)

Osmolality 1.2 1.3 1.0 1.5

332.5+ 1.8 332.2k2.1 327.9k2.1 331.2k2.8

Fish had been exposed to water currents corresponding to swimming speeds of 0, 1.0, 1.5 and 2.0 B1.s~’ for 70 days prior to the sewater challenge test. Values are means ks.e. and numbers in parentheses indicate the numbers of fish in each sample.

after exposure to sea water. Nevertheless the weight difference did not appear to have any influence on blood plasma Cl- concentration or osmolality (analysis of variance). Blood samples taken from fish prior to sea water exposure did not show any significant differences between treatments (P> 0.05), nor were there any significant differences between the blood plasma Cl- concentrations and osmolalities of exercised and unexercised fish after 24 h in sea water (P> 0.05 ) . No mortalities were observed during the course of the sea water challenge. DISCUSSION

With the onset of uniform water currents in the tanks there was a change in the behaviour of the fish. Fish held in the tanks lacking circumferential flow tended. to remain close to the tank bottom, whereas the exercised fish held station against the water current and distributed themselves evenly in the water column. However, some of the fish in the groups exposed to the lowest flow, corresponding to a swimming speed of 1.O B1.s~ ‘, maintained station on the bottom by extending their pectoral fins in contact with the bottom of the tank. These fish could, thus, hold position for long periods without displaying pronounced swimming movements. The results of the present study confirm that prolonged exercise at moderate speeds enhances growth performance. This is in accord with previous findings for a range of salmonid species (Davison and Goldspink, 1977; Greer Walker and Emerson, 1978; Kuipers, 1982; Nahhas et al., 1982; Leon, 1986; East and Magnan, 1987; Houlihan and Laurent, 1987; Totland et al., 1987; Christi.ansen et al., 1989; Christiansen and Jobling, 1990). Thus, for the presmolt Atlantic salmon in the present study, growth rates of all exercised groups were higher than controls (Fig. IA), best growth was recorded in fish swim-

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E.H. JBRGENSEN AND M. JOBLING

ming at 1.5 BI-s-r, and these fish grew 30% better than those held in tanks lacking circumferential flow. Previous studies of the effects of sustained exercise on salmonids have been somewhat inconsistent with regard to whether increased growth results from increased food intake or improved food utilisation (Davison and Goldspink, 1977; Leon, 1986; East and Magnan, 1987). Studies with Arctic charr clearly showed that the growth enhancement in the exercised fish was accompanied by improved food utilisation, rather than being due to an increase in food intake (Christiansen and Jobling, 1990; Christiansen et al., 1992). In the present study with salmon, food intake tended to increase with increasing swimming speed (Fig. 1B), and higher food intakes were accompanied by improvements in food utilisations in the groups exercised at 1.O and 1.5 B1.s’, as compared to the controls. Swimming at high speed (2.0 BI*s- ’ ), however, tended to reduce the growth/food intake ratio to a level below that of the control fish. Thus, exercise seems to stimulate food intake, but the growth enhancement seen at moderate swimming speeds in juvenile Atlantic salmon is accompanied by improved food utilisation. This improvement in food utilisation in fish exposed to moderate water currents is in agreement with the findings for Arctic charr (Christiansen and Jobling, 1990; Christiansen et al., 1992), and with results reported for brown trout (Davison and Goldspink, 1977), post-smolt Atlantic salmon (Kuipers, 1982) and brook charr (East and Magnan, 1987 ) . The mechanisms underlying the differences in growth/food intake ratios in groups of fish exposed to different water currents are, as yet, unidentified. It is well known that rates of energy expenditure (oxygen consumption) increase with increasing swimming activity (Brett, 1964; Beamish, 1978, 1980; Nahhas et al., 1982; Weatherley et al., 1982)) a relationship which should not favour improved growth/food intake ratios when fish are held in flowing water. However, energy expenditures of fish stocked together in standing water may be substantial, and in a study with Arctic charr, Christiansen et al. (1991b) found that fish exercising at 1.3-1.6 BI*s-’ did not consume any more oxygen than did those held in standing water. Oxygen consumption in groups of fish performing routine activity may thus be elevated due to spontaneous, agonistic activity (Brett, 1964, 1973), and social stress (Vijayan and Leatherland, 1990)) resulting in lower food utilisations than expected. On the other hand, the reduced food utilisation shown by the Atlantic salmon swimming at 2.0 BI.s- ’ in the present study may partly be due to the high energetic costs associated with swimming at this relatively high speed. Another factor that must be taken into consideration when assessing growth/food intake ratios is the possible influence of different rearing regimes on body composition. In some studies, sustained exercise at moderate speeds has been reported to promote deposition of both lipid and protein in swimming muscle. This has been found for juvenile rainbow trout (Nahhas

EFFECTS OF EXERCISE ON JUVENILE ATLANTIC SALMON

243

et al., 1982), brown trout (Davison and Goldspink, 1977) and brook charr, in which the amounts of lipid in the white muscle were higher in exercised than in unexercised fish (East and Magnan, 1987). Such changes in the deposition of lipid and protein would be expected to lead to a higher energy content in the muscle tissue of exercising fish (as confirmed in the study of Nahhas et al., 1982 ). The question of the effect of enforced exercise on changes in body composition in fish remains controversial and in a study with Arctic charr there was found to be a lower percentage lipid, and a higher whole body protei-n level in exercised fry than in controls (Christiansen et al., 1989). This was also found to be the case for juvenile chinook salmon, Oncovhynchus tshawytscha (White and Li, 1985 ), in which body composition was shown to be influenced by both swimming speed and ration size. The results of the present study with salmon showed that body water content was higher, and energy content lower, in the fish held in water currents corresponding to a swimrning speed of 2.0 B1.s~’ than in the control fish. All other factors being equal, such changes would result in values of food utilisation, expressed on a weight-weight basis, being enhanced and the high food utilisations seen in the groups exercised at 1.0 and 1.5 BI*s-’ may indicate that similar reductions in tissue energy content can also have occurred in these fish. At the start of the experiment, the frequencies of fish showing signs of fin damage (mostly tin rot) were high, indicating that the previous rearing conditions had been sub-optimal. During the course of the experiment, there was seen to be a reduction in fin damage in all groups, irrespective of exercise treatment, but the degree of wound healing was positively correlated with swimming speed. If the incidence of tin damage is taken as an indirect measure of levels of agonistic activity (Christiansen and Jobling, 1990), then it seems likely that the level of aggressive interactions during the course of the experiment was low, even in the control groups of fish. The possible effect of exercise on agonistic behaviour and social hierarchy formation is thus difficult tcl evaluate even though by the end of the experiment there was a clear negative correlation between tin damage and swimming speed. Hierarchy formation in groups of salmonid fish may lead to the observation of high coefticients of variation (CV), with respect to both daily feeding (McCarthy et al., 1992) and growth rates (Jobling and Wandsvik, 1983). In the present study, the CVs for food intake and growth were higher in the control group than in the groups held at intermediate swimming speeds. Thus, the three indices (fin damage, CVs of food intake and growth) support the conclusion that agonistic activity was reduced in the fish exposed to water currents. Results from studies conducted on other salmonids provide additional evidence that moderate exercise leads to increased homogeneity in growth and size distributions (East and Magnan, 1987; Christiansen and Jobling, 1990; Christiansen et al., 199 1b ), something which would be considered advantageous from the production standpoint.

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Frequencies of fish showing fin wounds decreased with the passage of time, but wound healing appeared to be improved in the salmon exercised at the highest swimming speeds. In Arctic charr, exercised fish were found to display an altered epidermal morphology and a higher concentration of epiderma1 mucous cells compared with controls (Christiansen et al., 1991a) and the capacity for mucus production and secretion may also have been increased. Mucus plays an important role in protecting the fish against microorganisms, both by physical means and due to the presence of antimicrobial substances (Willoughby, 1989). It is possible, therefore, that exercise-induced changes in the epidermal tissue may have contributed to the improved healing with increasing swimming speed seen in the present study. During the course of the experiment, most of the fish appeared to undergo the Parr-smolt transformation. This was observed as a disappearance of the parr marks and a more silvery appearance towards the end of the experiment. A decline in condition factor is frequently observed in salmonids undergoing Parr-smolt transformation (Hoar, 1988). Whilst such a decrease was seen in the control fish, there was a significant increase in condition factor in all exercised groups during the course of the experiment. Thus, the results of the present study appear to be contrary to the expectation of a decline in condition factor accompanying Parr-smolt transformation, and also differ from the findings reported by Kuipers ( 1982) who claimed that sustained exercise (between 0.8 and 2.0 BI-s-’ ) in post-smolt Atlantic salmon resulted in a streamlined fish with a lower condition factor than fish held in standing water. Prolonged swimming at moderate speeds has been shown to influence the physiological responses shown by fish swum to exhaustion at high speed (Woodward and Smith, 1985; Davison, 1989 ), but such exercise regimes may also enhance the recovery of the fish following exposure to other stressors such as handling (Lackner et al., 1988). The transfer of salmon smolt from fresh to sea water may be considered to impose a stress on the fish, with physiological effects being visible for a period of several days (Usher et al., 199 1) . Based upon the findings of Blackburn and Clarke ( 1987), plasma ionic concentrations would have been expected to reach a maximum within 24 h of sea water exposure at the temperature used in the present experiment (8.5”C). All the values recorded were, however, well within those considered to be normal for a functional salmon smolt (Stefansson et al., 1990)) irrespective of previous treatment (Table 3 ) . Thus, exercise appeared neither to improve nor to impair the osmoregulatory capacity of the fish. In conclusion, the results of the present investigation show that continuous swimming at moderate speeds leads to improvements in growth and food utilisation, and probably leads to reductions in agonistic behaviour. Exercise did not, however, appear to lead to any improvement of the osmoregulatory capacity of fish undergoing Parr-smolt transformation.

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ACKNOWLEDGEMENTS

We thank Helge K. Johnsen and Helge Tveiten for assistance during the experiment. The work was supported by the Norwegian Council of Fisheries Research, project no. 1402 500.090.

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