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Metabolic differences between Atlantic salmon (Salmo salar) parr and smolts
Aquaculture, (1985) 33-53 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
METABOLIC DIFFERENCES BETWEEN SAL/U?) PARR AND SMOLTS
ATLANTIC
SALMON
33
(SALMO
P.J. HIGGINS
1Freshwater Fisheries Laboratory, Pitlochry, PHI6 5LB (Great Britain); and Department of Zoology, University of Aberdeen, Aberdeen AB9 ZTN (Great Britain) ’ Correspondence address.
ABSTRACT Higgins, P.J., 1985. Metabolic differences between Atlantic salmon (Salmo salar) pan and smolts. Aquaculture, 45: 33-53. Measurements were made of growth, feeding, oxygen consumption and respiratory organs in sibling groups of juvenile Atlantic salmon during the first year of growth. The higher levels of growth and metabolism usually associated with smoltification were found to be a normal characteristic of the upper growth mode of the bimodal population. It is suggested that upper mode growth can be considered normal, and lower mode growth regarded as being suppressed, perhaps by changes in photoperiod in midsummer.
INTRODUCTION
Elson (1957) suggested that there is a certain critical minimum size for smolting in Atlantic salmon (Salmo suhr L.) and this idea has been supported by studies of the sizes of migrating smolts in many river systems throughout the geographical range of the species. Some of the changes in body physiology associated with migration can take place over a relatively short time prior to smoltification in the spring (e.g. salinity tolerance and migratory behaviour). However, if a minimum size threshold is to be reached by the spring, the fish must start allocating resources to somatic growth much earlier in the year. Support for this question has come from studies on the growth and development of hatchery-reared populations of sibling juvenile Atlantic salmon. Simpson and Thorpe (1976), Thorpe (1977) and Thorpe et al. (1980) have reported that in the first year of growth sibling populations developed a bimodality in both their weight and length frequency distributions. The two modes of these distributions were first distinguishable in the autumn. The fish in the upper mode developed into l-year-old smolts in the spring; those in the lower growth mode did not smoltify and remained as parr during their second year in freshwater. This bimodality is not the result of a dominance hierarchy (Simpson and Thorpe, 1976) and also appears to be consistent 0644-8486/85/$03.30
0 1985 Elsevier Science Publishers B.V.
34
with the migratory age structure of natural populations in four Scottish rivers (Thorpe, 1977). In further hatchery experiments Thorpe and Morgan (1978) demonstrated that there was also a genetic influence on the proportions of fish in the two modes, and both the growth rate and smolting rate. Bimodality has also been reported in Canadian (Bailey et al., 1980; Saunders et al., 1982) and Norwegian (Knutsson and Grav, 1976) stocks. For these growth differences to be evident in the population in the autumn, the initiation of differential growth rates must have occurred somewhat earlier. Villarreal’s (1983) evidence on the relative rates of RNA and DNA synthesis in bimodal populations suggests that this growth change is under photoperiodic control and that the time of “decision” is in a “critical photosensitive period” in June or July, Whatever changes in growth or metabolism occur during the juvenile development of the fish, an overall balance must be maintained in the energy budget. The energy value of the food consumed (C) must be accounted for in the growth (G), activity (A), respiration (R) and faecal and urinary excretion (F + U). C=G+A+R+F+U
(after Windell, 1978)
As the observed growth rates of the two modes of bimodal populations are different, the fish in those modes should also demonstrate differences in their metabolism through other components of the energy budget. In a full study of these energetic differences all components should be estimated. This is impractical if the fish are to be maintained in a standard hatchery environment and therefore, within the limitations of size, the three major metabolic parameters of growth, respiration and feeding are considered here. Growth Besides gross weight changes, the partitioning of energy into various types of growth should also be considered. Changes in the size frequency distribution may be the result of different developmental rates in individuals or the allocation (or re-allocation) of resources between different tissues. For example, Saunders et al. (1982) showed that the relative position of precocious males in the length frequency distribution changed during the period of their sexual development. To avoid such complications in the present study, a population of fish, all marked individually, was used to estimate specific growth rates. Weatherley and Rogers (1978) suggest that “rapid somatic growth in fish may be associated more with the addition of new muscle fibres than with increase in diameter of existing ones”. Weatherley et al. (1979, 1980) and Stickland (1983) also suggest that muscular growth in the rainbow trout (S&no gairdneri) is characterised by different phases in which hyperplasia (increasing fibre number) and fibre hypertrophy (increasing fibre size) change in their relative importance. Similar changes might also be associated
35
with the development of bimodality and may be related growth required to attain a threshold size for smolting.
to the rates of
Respiration Baraduc and Fontaine (1956) demonstrated a 30% elevation in the resting weight specific oxygen consumption of wild smolts when compared to parr at around 8°C. Power’s (1959) results for fish from an Arctic environment are less straightforward, with smolts having the lower oxygen consumption per unit weight below 13.5”C, but higher than parr above this temperature. His explanation is that, metabolically, smolts are more responsive to temperature change than parr. Baraduc and Fontaine (1956) also noted the link between metabolic rate and hormone production in smolts; and Withey and Saunders (1973) have also shown that manipulation of photoperiod affects standard rates of oxygen consumption and body composition. Heart and gills have a direct involvement in respiratory metabolism through the movement of blood and gas exchange: e.g., Pauly (1981) suggests that oxygen supply limits anabolism and hence growth, and therefore that rates of growth and maximum body size are limited by the size of the respiratory surface (gill lamellar surface area). Similarly, an increase in oxygen demand can be satisfied by an increase in the heart size as a function of the body mass or an increase in pumping rate (Schmidt-Nielsen, 1979). Clearly, it is worthwhile examining these organs within a general study of respiratory physiology prior to smoltification. Feeding Many authors have examined the link between feeding and growth and the topic has been discussed in major reviews (e.g., Brett and Groves, 1979). Many excellent studies have also been carried out; Elliott’s (1975a, b) studies on the relationship between ration size and growth are noteworthy as they involved a similar salmonid species (Salmo trutta). Although in a hatchery environment food availability should not limit growth, a relatively higher food intake in the more rapidly growing fish would be expected. Results of an investigation of these aspects of growth, respiration and feeding in bimodal populations of Atlantic salmon throughout the period of juvenile development prior to smoltification, are reported below. MATERIALS
AND METHODS
Stocks and maintenance All experiments were carried out on juvenile Almondbank hatchery which were the progeny
Atlantic salmon from the of adults caught from the
36
river Almond by electrofishing. Each family was the product of a pairing between a single male and a single female. From first feeding, fry were maintained in single family populations in radial flow tanks (see Thorpe, 1981 for details), and subject to normal water velocities of 24 to 26 cm/s. No attempt was made to control the natural photoperiod or ambient water temperature (e.g. Fig. 1) at the hatchery, However, as the rearing tanks are under cover, the light intensity is normally less than that of full daylight, but reaches around 20 lux at the water surface in bright sunshine. Under routine maintenance conditions the fish were fed on a pelletised commercial diet (Ewos Baker salmon starter, sizes 0 to 4). Pellets of a grade appropriate to the size of the fish were delivered by “caddymatic” feeders (see Thorpe, 1981). When size differences developed in the population an appropriate mixture of pellet sizes was delivered. The feeders were timer controlled and electrically triggered to deliver a preset amount of food at 5- to 15min intervals, 24 h a day. The amount of food delivered was always well in excess of the requirements of the fish in the tank, and was maintained for all stocks from first feeding, ensuring that the fish did not become conditioned to any unnatural feeding rhythm. Criteria for smoltification Fish were classified as “smolts” when they satisfied the criteria of complete body silvering, loss of parr marks and fin darkening. Fish in the intermediate stages of smoltification were not used. Specific growth rate Specific growth rate (SGR) was estimated on an approximately monthly basis for all the individuals within a special experimental population. In September 1982 two groups of approximately 45 sibling 0+ salmon representing each of the two modes of a bimodal population were established in a l-m diameter radial flow tank. Modal weight ranges were 0.6 to 1.9 g and 2.1 to 5.0 g. Each fish in the population was marked dorsally with a binary coded X-ray readable microtag (Northwest Marine Technology, Seattle, U.S.A.) distinguishing it from all others in the tank. Full details of the technique are provided elsewhere, as are results which demonstrate that the method causes no adverse growth effects (Higgins, 1985). The experimental population was monitored at approximately monthly intervals, and specific growth rate was calculated for each identifiable individual using the formula SGR = (log, W2 - log, IVl)/elapsed
time (days) ( after Ricker,
1979)
From these results a mean of all the individual specific growth rates within each mode was calculated. By June 1983, modal weight ranges were 1.7 to 5.1 g and 5.0 to 20.7 g. At that time approximately 53% of the fish in the upper mode could be classified as smelts.
37
Muscle histology A full histochemical description of the fibre types present will be presented elsewhere (Higgins, in prep.). However, the situation in Atlantic salmon is somewhat similar to that described for rainbow trout by Johnston et al. (1975). Although a narrow band of “intermediate” or “pink” muscle is present between the blocks of white and red fibres, the latter two fibre types comprised the bulk of the fish. All muscle samples taken from the tissue beneath the adipose fin were prepared using methods which are summarised by Stickland (1983). The lo--12~pm sections were stained using Weigert’s haematoxylin and eosin. Whole transverse sections were taken where possible, though the harder spines of the larger fish caused ripping during sectioning and in this event, half-sections (dorso-ventral) were used. Subsequent size frequency analysis was carried out on red and white fibre types using a microscope with a drawing attachment associated with an image analyser (ReichertJung “Videoplan”) and graphics tablet. The areas of around 100 fibres in each of these samples were measured for both muscle types for each fish. The total fibre number was then estimated by dividing mean fibre size into the area of the whole muscle block. Samples of two to four fish were used for each mode at approximately monthly intervals throughout the two years of juvenile development. Respirome try Respirometer. A five-cell recirculating flow respirometer was constructed for measurement of resting rates of oxygen consumption. An extensive description and diagram of the apparatus is given by Higgins (in prep.). Experimentd protocol. In order to avoid any thermal shock for the fish, measurements were made at intervals of 2.5”C between 5 and 15”C, at whatever interval was nearest the ambient temperature at the time. The results for an experimental temperature of 7.5”C are given here as they cover the period from the beginning of April to the end of May in 1982 and 1983. During this period upper and lower mode fish and smolts were available. Prior to use in experiments, all fish were maintained in radial flow tanks as above, The fish were starved for up to 2 days depending on water temperatures to allow evacuation of the gut, and left to settle for at least 8 h after introduction into the chamber before measurements were made. A continuous recording was then made for at least 24 h. As the timer-controlled cycle of the five chambers (four with fish in and one blank for comparison) took 3 h, eight results per animal were collected each day. These results were used to calculate a daily mean resting rate of oxygen consumption for each animal.
38
Hearts and gills At the end of each respiration study the fish were killed, measured and the hearts and gills dissected out. The whole of the gill assembly including gills and gill rakers was removed and cleaned of any excess tissue prior to weighing. The atrium was dissected from the heart prior to weighing the remainder of the organ as it usually contains a variable amount of blood. The ventricle, which is involved in the muscular effort of pumping the blood, has also been used in another study of heart development in Atlantic salmon (Poupa et al., 1974). No measurements were made of amounts of blood present at dissection in hearts or gills. Feeding experiments The 24-h food intake of the two modes of a bimodal population was estimated once a month between October 1983 and March 1984, using a special diet containing a small percentage of iron powder as a radio-opaque marker (Talbot and Higgins, 1983). Experimental protoco2. To avoid altering the normal feeding pattern, no periods of pre- or post-prandial starvation were imposed on the experimental fish. The adverse effects of such regimes are described by Talbot et al. (1984). The labelled diet was delivered for 24 h by simply replacing the hopper containing normal food with one containing the iron-labelled diet. When estimating a 24-h food intake on occasions when the fish had short evacuation times, the labelled food was offered in two 12-h periods separated by an interval of feeding with unlablled food which allowed evacuation of the previous labelled meal. Water contents of both food and fish were estimated in separate experiments (see Higgins and Talbot, 1985) and dry meal size was expressed as a percentage of the dry fish weight. Statistical
methods
All regression lines were fitted by the method of least-squares, and regression parameters were compared by analysis of covariance. A two-means T-test was used to establish differences between estimates of food intake. RESULTS
Growth Specific growth rate There appears to be a general association between growth, temperature and photoperiod (Fig. l), though closer examination reveals conflicting results in the temperature/growth association. Temperatures were generally
39
higher (3 to 7°C) between 25 October and 10 December than in the two subsequent intervals (10 December to 25 January, around 3.5”C; and 25 January to 1 March, around 1°C). However, specific growth rate increased through the period, following more closely the trend in photoperiod. Furthermore, growth rate increased rapidly in the spring though temperatures were generally lower than those of the preceding autumn. Again, the general trends of growth and photoperiod are similar. Throughout the study period all the microtagged individuals retained their 1600 1400 1200
Daylength (mln)
Specific Growth Rate 0 8.
(%I
0 6Q
Upper Mode
*
Lower Mode
0 1o- 20 o*
Water Temperature PC
1
I’ “I.“” 22 25 Sept. 03.
I IO Dec.
‘..I. 25 Jan.
~.I....l~.~~I~..~I 1 5 9 8 Mar Apr. May. Jun.
-4
Fig. 1. Mean of individual specific growth rates (* 1 standard error) of microtagged upper and lower mode salmon in relation to ambient water temperature (“C), and photoperiod (daylength in minutes) from September 1982 to June 1983. Water temperature is displayed as the weekly mean.
40
positions within each mode, with none of the upper mode fish dropping back into the lower mode or vice versa. This consistency is reflected in the little variability in the mean results (standard error). The upper mode fish maintained a higher growth rate than the lower mode fish throughout the period. This was most notable between 25 October and 25 January when the upper mode maintained growth whilst the lower mode showed effectively no growth at all.
Muscle his tology The initial gradual increase in white fibre number fry is maintained by both modes until a length of (March 1983) is achieved, when the upper mode rapidly (Fig. 2a). Mean fibre size follows a similar
r
WHITE
shown by the unimodal approximately 8.5 cm fibre number increases trend but without the
FIPRE NLblEIER
.
33000. 3 0000~ 27000.
.
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. 12000
_
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-
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MEAN WHITE
:
FIBREAREA
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:,
:
0
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00
.
oa
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:
.
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l
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8
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0
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l
.
:
.
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..~'
500. 300.
l”2.25
,
3.25
L.25
5.25
6.25 FISH
725
$25
9:25
Id.25
11.25
I
LENGTH (cm1
Fig. 2. (a) Changes in white fibre number with fish length. (b) Changes in mean white fibre size with fish length. = - unimodal fry; o - lower mode; l upper mode.
41
large increase above 8.5 cm. At this stage the upper mode fish are recruiting new fibres rather than increasing existing fibre size (Fig. Zb). The development of red muscle follows a similar pattern (Figs. 3a, b). Respiration Oxygen consumption Highly significant (P
T 7200 6‘00
‘1
RED FIBRE
NUMBER
I I
. .
em
,
+
t
MEAN RED FIBRE
t
--t-
+-t~t--+
--t---t+--
AREA (~1
1
:
I
. .
.
. .
I
225
325
L.25
5.25
6.25 FISH
LENGTH
7.25
8.25
925
10.25
11.25
km)
Fig. 3. (a) Changes in red fibre number with fish length. (b) Changes in mean red fibre size with fish length. n - unimodal fry; 0 -lower mode; l - upper mode.
42
+/I=‘Y!=‘/“‘JJ
J’UP”J’l”~/‘!‘~u)
1 NOIldWnSN03
N30AXO
601
NOIldWnSNO3
NJOAXO
00-1
~‘4/I=“IUD/oY)
J’4/I“‘l”=‘/“w)
NOILdWIlSN03
NOIldWflSN03
N3ShXO
N30AXO
001
“‘7
43 TABLE I Oxygen consumption at 7.5%. Equations describing the line of best fit 0, consumption (mge animal-‘. h-’ ) = a fish weight (g) b For comparison of regressions, see text l
Smolts Non-smolts Upper mode Lower mode
a
b
Obs.
P
0.0129 0.0433 0.0268 0.0477
1.2641 0.8982 1.1168 0.7554
14 54 29 25
<.OOl <.OOl <.OOl <.OOl
Gill weight Highly significant (P
Smolts Non-smolts Upper mode Lower mode
a
b
Obs.
P
0.0382 0.0334 0.0348 0.0371
0.8340 0.9316 0.8942 0.8713
19 180 67 133
<.OOl <.OOl <.OOl <.OOl
Heart weight Highly significant (P
44
(0) lHDI3R 1lID 0O-l
. . . . . .- .
I
\
. .
..
. .
\-
(0) lHOI3H 1lID 601
(0) LHOI3fl1110 e0-l
(6)
lHOI3fl
lW3H
601
(6)
lHOI3fi
lW3H
EOl
(6)
lH013M
lW3H
601
46 TABLE III Equations describing the line of best fit of heart weights Heart weight (g) = a - fish weight (g) b For comparison of regressions, see text
Smolts Non-smolts Upper mode Lower mode
a
b
Obs.
P
0.00135 0.00139 0.00149 0.00140
1.0538 1.0558 1.0337 1.0416
19 180 67 133
<.OOl <.OOl <.OOl <.OOl
exception that upper mode cept than lower mode fish.
fish had a significantly
greater
(P
Feeding Food in take On each of the five (monthly) sampling occasions the upper mode fish took a significantly (KO.01) larger meal (dry weight of food as percentage of dry body weight) than the lower mode fish (Fig. 7 and Table IV).
Oct.
Nov
Fig. 7. Food intake of upper and lower mode fish, sampled from October 1983 to March 1984. Intake estimated as dry weight of food as a percentage of dry fish weight. **differences significant at FYO.01; ***difference significant at P
47 TABLE IV 24-h food intake in feeding experiments Meal (%) = (dry weight food * dry weight fish-‘) * 100 N = number of observations; P = level of significance of differences between modes at that date Date
Temp. (“0
Mode
Length (-1
Meal (%I
S.E.
N
P
13 Oct.
7.00
upper lower upper lower upper lower upper lower
7.83 6.52 8.05 6.46 8.22 6.54 8.38 6.57 8.52 6.61 8.77 6.69
2.95
0.216 0.342
15 6
0.136 0.125
13 8
< 0.001
0.131 0.116
12 8
0.253 0.111
13 7
0.074 0.030
15 5
0.148 0.162
14 6
10 Nov.
5.38
08 Dec.
4.50
06 Jan.
3.50
08 Feb.
0.88
15 Mar.
4.00
upper lower upper lower
1.76 1.51 0.63 1.72 0.90 1.52 0.30 0.67 0.15 1.76 0.50
DISCUSSION
Growth Specific growth rate remained higher in the upper modal group throughout the study period and mirrored the time-divergence in length shown by other experimental populations (Bailey et al., 1980; Thorpe et al., 1980). Although this high growth rate was maintained through the autumn and winter, a substantial increase was not evident until March (Fig. 1). This coincided with a rapid increase in fibre number in the upper mode fish (Figs. 2a, 3a) which Weatherley et al. (1979,198O) have established as a characteristic of rapid growth in rainbow trout. This should be expected as the surface area/volume ratio of large cells (above a certain size) limits their metabolic efficiency (Von Bertalanffy, 1960). Although no measurements of swimming activity were made in the present study, a tendency towards increasing fibre number is unlikely to be the result of elevated activity levels in the upper mode fish as, according to Johnston (1981), exercise leads to fibre hypertrophy. Seasonal changes in ambient temperature and photoperiod are the main environmental factors which might be expected to influence growth and smoltification (Wedemeyer et al., 1980). Although artificial elevation of water temperature appears to accelerate growth and produce smolts in a shorter time (Wagner, 1974; Saunders, 1976), most evidence points to photoperiod being the primary influence. This is thought to operate by controlling neuroendocrine activity and metabolism and thereby growth rate
(Hoar, 1965, 1976; Saunders and Henderson, 1970) and smoltification (Saunders and Henderson, 1970; Wagner, 1974). That changing photoperiod should be the more reliable influence seems reasonable as it is not prone to the “noise” often recorded in temperature seasonality. Despite an unusually cold winter and spring, both modes of the microtagged population showed a rapid increase in specific growth rate from January onwards which seems to be associated with the seasonal change in photoperiod. The actual daylength does not seem as critical as the direction of the trend (increasing or decreasing) in photoperiod and this is in accord with the findings of Saunders and Henderson (1970), Knutsson and Grav (1976) and Komourdjian et al. (1976). A similar response has also been recorded for brown trout by Wingfield (1940) and Swift (1961), but was not found by Elliott (1975a). As both modes demonstrated the same form of growth response (but not to the same degree) they were probably responding to the same stimuli at this stage of development. Although smolts have a higher growth rate than non-smolts (Saunders and Henderson, 1970; Komourdjian et al., 1976) the initiation of this difference does not appear to be linked to spring photoperiod changes, and probably occurs at the initiation of bimodality which Thorpe et al. (1982) and Villarreal(l983) put at midsummer of the first year of growth.
Respiration Von Bertalanffy (1951) and Pauly (1981) have proposed that anabolism in fish is dependent on respiration rate, which is in turn limited by the extent of a respiratory surface (e.g. gills). Pauly discounts food supply as the main factor limiting growth as fish can to some extent store food, but this is not the case with oxygen. By drawing on the literature available on growth, oxygen consumption and gill surface area for a wide range of fish species, he puts forward a convincing argument that the “limiting respiratory surface” for growth is the gill surface area. He also argues (per-s. comm., 1982) that although the full capacity of the gills is only used during exercise, there must be a constant factor relating the area used at rest with the total surface area available. As a general trend, in the present study the fish which demonstrate the highest growth rates in winter and spring (smolts) have the highest weight-specific rates of oxygen consumption and associated gill weights. This lends broad general support to the proposition of Von Bertalanffy and Pauly. However, as oxygen consumption was relatively higher in the upper mode compared to the lower mode, we might have expected larger gills, whereas no differences were found. It may therefore be that gill size is not a factor limiting the growth of the lower mode fish and that some other process is responsible for their reduced growth rate (e.g. photoperiod acting on the neuroendocrine system as suggested earlier). More simply, gross changes in gill weight may not be a sensitive enough estimator of gill surface area. Some consideration should also be given to changes in the ionic regulation capability of the gills, as for example, Langdon and Thorpe (1985) have
49
shown that there are intermodal differences in the pattern of development of ionoregulation and that the previously observed changes in e.g. Na/K-ATPase (Saunders and Henderson, 1978) are not solely a function of smoltification. Changes of this type might be expected to have some impact on the respiratory function of the gills. The only difference between the heart weights of the groups was that upper mode fish had significantly larger hearts as a function of body weight than the lower mode fish. This points to a difference in respiratory capacity between the modes but it is perhaps surprising that this was not also found for smolts. Poupa et al. (1974) found an increase in relative heart weight after migration and throughout the marine phase. They noted no relative increase in gross heart weight in smolts but did find an increase in the thickening of the ventricular shell and compact ventricular volume in relation to the ventricle volume. The sample size of smolts for their investigation was small (6) and they did not have the opportunity to compare upper and lower modal groups. The results of the two studies may therefore complement each other, and the larger hearts found in the upper mode fish may represent a greater facility to carry out its role in respiration. Feeding Brett and Groves (1979) point out that in almost all species investigated “the higher the ration, the greater the growth”. The fish used in the feeding experiments were offered excess food and so their situation is comparable with the brown trout fed on maximum rations by Elliott (1975a). Under these conditions, Elliott found that over a wide size and temperature range the smaller the fish the higher its specific growth rate, and the higher its food intake as a function of body weight. He also demonstrated that the weightspecific maintenance ration decreased as a function of increasing fish size (Elliott, 1975b). Thus, according to Elliott’s evidence, if the modal differences are simply a function of size itself, we should expect the higher weightspecific food intake and growth rate to be found in the lower mode fish. In fact, the converse is the case (Figs. 1,6). These results can be compared with the generalised ration/temperature relationship for sockeye salmon (Oncorhynchus nerka) of an equivalent size range (Brett et al., 1969, fig. 8). The daily meal taken by the upper mode fish is approximately equivalent to the optimum ration size for a young sockeye salmon at the same temperature, though the lower mode food intake is generally in the region of or just above maintenance. One important difference, however, is that our fish were subject to a winter photoperiod whilst Brett’s sockeye experiments were conducted between spring and autumn. Feeding in juvenile Atlantic salmon is limited in periods of darkness (Talbot and Higgins, 1983; Higgins and Talbot, 1985), and thus a winter photoperiod would represent a reduced opportunity to feed. The upper mode fish therefore seem to have fed optimally despite the shorter daylength, whereas the lower mode fed sub-optimally.
50
Clearly, the higher growth rate maintained by the upper mode through the winter, which appears to be a prerequisite of smolting, is supported by the optimum size of ration under the prevailing environmental conditions. Commentary The changes in respiration recorded in the present experiments are clearly not simply associated with smoltification as they also relate to upper mode fish. Evidence will be presented elsewhere to show that modal differences in metabolism exist throughout the period of bimodal development (Higgins, in prep.). Furthermore, elevated respiration rate is not simply a function of changes in body composition recorded by Komourdjian et al. (1976), Woo et al. (1978) and others (see review by Wedemeyer et al., 1980). In separate body composition analyses of samples of fish throughout the first year of growth (Higgins and Talbot, 1985), upper mode fish maintained higher reserves of lipid and a lower proportion of body weight as water than the lower mode fish. Therefore, the elevated respiration rate in upper mode fish and smolts cannot be due solely to catabolism of lipid stores. As the results of oxygen consumption measurements presented here are generally lower than those measured by Baraduc and Fontaine (1956) and Power (1959), it seems unlikely that the results for the upper mode and smolts (around 40 to 45 mg * kg-’ - h-l) represent a state of elevated metabolism. Rather, as was argued for the results of food intake and growth, the lower mode fish may be considered as demonstrating a “suppressed” metabolic response relative to the upper mode fish. These bimodal differences, and in particular any suppression of growth and metabolism in the lower mode fish, might, as was suggested by Simpson and Thorpe (1976), be initiated and regulated by environmental influences (e.g. photoperiod as emphasised here) acting through the endocrine system. ACKNOWLEDGEMENTS
This work was carried out under an award from the Science and Engineering Research Council, held at the University of Aberdeen and the Freshwater Fisheries Laboratory, Pitlochry. I am grateful to friends and colleagues for their help and encouragement throughout the study period, and in particular to Dr. John Thorpe for assistance in the preparation of the manuscript.
REFERENCES Bailey, J.K., Saunders, R.L. and Buzeta, M.I., 1980. Influence of parental smolt age and sea age on growth and smoking of hatchery-reared Atlantic salmon (Salmo salar). Can. J. Fish. Aquat. Sci., 37: 1379-1386, Baraduc, MM and Fontaine, M., 1956. Etude comparee du metabolisme respiratoire du jeune saumon s&let&ire (Parr) et migrateur (smolt). C.R. Sot. Biol. Paris, 149: 13271329.
51 Brett, J.R. and Groves, T.D.D., 1979. Physiological energetics. In: W.S. Hoar, H.J. Randall and J.R. Brett (Editors), Fish Physiology. Vol. 8. Academic Press, New York, NY, pp. 279-352. Brett, J.R., Shelbourn, J.E. and Shoop, C.T., 1969. Growth rate and body composition of fingerling sockeye salmon, Oncorkynchus nerka, in relation to temperature and ration size. J. Fish. Res. Board Can., 26 : 2363-2394. Elliott, J.M., 1975a. The growth rate of brown trout, S&no truth L., fed on maximum rations. J. Anim. Ecol., 44: 805-821. Elliott, J.M., 1975b. The growth rate of brown trout, Solmo truth L., fed on reduced rations. J. Anim. Ecol., 44: 823-842. Elson, P.F., 1957. The importance of size in the change from parr to smolt in Atlantic salmon. Can. Fish Cult., 21: 1-6. Higgins, P.J., 1985. X-ray microtags for identification of individuals in studies of fish energetics. J. Fish Biol., 26(2): in press. Higgins, P.J. (in preparation). Growth, metabolism and feeding in juvenile Atlantic salmon. Ph.D. Thesis, University of Aberdeen, Scotland. Higgins, P.J. and Talbot, C., 1985. Feeding and growth in juvenile Atlantic salmon. In: C.B. Cowey, A.M. Mackie and J.G. Bell (Editors), Nutrition and Feeding in Fish. Academic Press, London and New York, in press. Hoar, W.S., 1965. The endocrine system as a chemical link between the organism and its environment. Trans. R. Sot. Can. Ser. IV, 3: 175-200. Hoar, W.S., 1976. Smolt transformation: evolution, behaviour and physiology. J. Fish Res. Board Can., 33: 1234-1252. Johnston, I.A., 1981. Specialization of fish muscle. In: D.F. Goldspink (Editor), Development and Specialization of Skeletal Muscle. Cambridge University Press, Cambridge, pp. 123-148. Johnston, I.A., Ward, P.S. and Goldspink, G., 1975. Studies on the swimming musculature of the rainbow trout. I. Fibre types. J. Fish Biol., 7: 451-458. Knutsson, S. and Grav, T., 1976. Seawater adaptation in Atlantic salmon (Salmo salar) at different experimental temperatures and photoperiods. Aquaculture, 8: 169-187. Komourdjian, M.P., Saunders, R.L. and Fenwick, J.C., 1976. Evidence for the role of of growth hormone as a part of a “light-pituitary axis” in growth and smoltification Atlantic salmon (Salmo salar). Can. J. Zool., 54: 544-551. Langdon, J.S. and Thorpe, J.E., 1985. The ontogeny of smoltification: developmental patterns of gill Na+/K+-ATPase, SDH, and chloride cells in juvenile Atlantic salmon, Salmo salar L. Aquaculture, 45: 83-95. Pauly, D., 1981. The relationship between gill surface area and growth performance in fish: a generalization of von Bertalanffy’s theory of growth. Meeresforschung, 28 : 251-282. Poupa, O., Gesser, H., Jonsson, S. and Sullivan, L., 1974. Coronary-supplied compact shell of ventricular myocardium in sahnonids: growth and enzyme pattern. Comp. Biochem. Physiol., 48: 85-95. Power, G., 1959. Field measurements of the basal oxygen consumption of Atlantic salmon parr and smolts. Arctic, 12: 195-202. Ricker, W.E., 1979. Growth rates and models. In: W.S. Hoar, D.J. Randall and J.R. Brett (Editors), Fish Physiology, Vol. 8. Academic Press, New York, NY, pp. 677-743. Saunders, R.L., 1976. Heated effluent for rearing of fry - for farming or release. In: 0. Devik (Editor), Harvesting Polluted Waters. Plenum Press, New York, NY, pp. 213236. Saunders, R.L. and Henderson, E.B., 1970. Influence of photoperiod on smolt development and growth in Atlantic salmon (Salmo solar). J. Fish. Res. Board Can., 27: 1295-1311. Saunders, R.L. and Henderson, E-B., 1978. Changes in gill ATPase activity and smolt status of Atlantic salmon (Salmo su2ar). J. Fish. Res. Board Can., 35: 1542-1546.
52 Saunders, R.L., Henderson, E.B. and Glebe, B.D., 1982. Precocious sexual maturation and smoltification in male Atlantic salmon (Salmo salar). Aquaculture, 28: 211-229. Schmidt-Nielsen, K., 1979. Animal Physiology: Adaption and Environment. Cambridge University Press, Cambridge, 560 pp. Simpson, T.H. and Thorpe, J.E., 1976. Growth bimodality in the Atlantic salmon. Int. Coun. Expl. Sea, CM 1976/M22, 7 pp. Stickland, N.C., 1983. Growth and development of muscle fibres in the rainbow trout (Salmo gairdneri). J. Anat., 137: 323-333. Swift, D.R., 1961. The annual growth rate cycle in brown trout (Salmo trutta Linn.) and its cause. J. Exp. Biol., 38: 595-604. Talbot, C. and Higgins, P.J., 1983. A radiographic method for feeding studies on fish using metallic iron powder as a marker. J. Fish Biol., 23: 21 l-220. Talbot, C., Higgins, P.J. and Shanks, A.M., 1984. Effects of pre- and postprandial starvation on meal size and evacuation rate of juvenile Atlantic salmon (Salmo salar L.). J. Fish Biol., 25: 551-560. Thorpe, J.E., 1977. Bimodal distribution of length of juvenile Atlantic salmon (Salmo salar L.) under artificial rearing conditions. J. Fish Biol., 11: 175-184. Thorpe, J.E., 1981. Rearing salmonids in freshwater. In: A.D. Hawkins (Editor), Aquarium Systems. Academic Press, London, pp. 325-344. Thorpe, J.E. and Morgan, R.I.G., 1978. Parental influence on growth rate, smolting rate and survival in hatchery-reared juvenile Atlantic salmon, Salmo solar L. J. Fish Biol., 13: 549-556. Thorpe, J.E., Morgan, R.I.G., Ottaway, E.M. and Miles, M.S., 1980. Time divergence of growth groups between potential l+ and 2+ smolts among sibling Atlantic salmon. J. Fish Biol., 17: 13-21. Thorpe, J.E., Talbot, C. and Villarreal, C., 1982. Bimodality of growth and smelting in Atlantic salmon, Salmo salar L. Aquaculture, 28: 123-132. Villarreal, C.A., 1983. The role of light and endocrine factors in the development of bimodality in the juvenile Atlantic salmon (Salmo solar L.). Ph.D. Thesis, University of Stirling, Scotland, 393 pp. Von Bertalanffy, L., 1951. Theoretische Biologie - Zweiter Band: Stoffwechsel, Wachsturn. A. Francke AG Verlag, Bern, 418 pp. Von Bertanlanffy, L., 1960. Principles and theory of growth. In: W.W. Nowinski (Editor), Fundamental Aspects of Normal and Malignant Growth. Elsevier, Amsterdam. Wagner, H.H., 1974. Photoperiod and temperature regulation of smolting in steelhead trout (Salmo gairdneri). Can. J. Zool., 52: 219-234. Weatherley, A.H. and Rogers, S.C., 1978. Some aspects of age and growth. In: S.D. Gerking (Editor), Ecology of Freshwater Fish Production. Blackwell Scientific Publications, Oxford, pp. 52-74. Weatherley, A.H., Gill, H.S. and Rogers, S.C., 1979. Growth dynamics of muscle fibres, dry weight, and condition in relation to somatic growth rate in yearling rainbow trout (Salmo gairdneri). Can. J. Zool., 57 : 2385-2392. Weatherley, A.H., Gill, H.S. and Rogers, S.C., 1980. Growth dynamics of mosaic muscle fibres in fingerling rainbow trout (Salmo gairdneri) in relation to somatic growth rate. Can. J. Zool., 58: 1535-1541. Wedemeyer, G.A., Saunders, R.L. and Clarke, W.C., 1980. Environmental factors affecting smoltification and early marine survival of anadromous salmonids. Mar. Fish. Rev., 42: l-14. Windell, J.T., 1978. Estimating food consumption rates in fish populations. In: T. Bagenal (Editor), Methods for Assessment of Fish Production in Fresh Waters, Blackwell Scientific Publications, Oxford, pp. 227-254. Wingfield, C.A., 1940. The effect of certain environmental factors on the growth of brown trout (Salmo trutta L.). J. Exp. Biol., 17: 435-448.
53 photoperiod regime on Withey, K.G. and Saunders, R.L., 1973. Effect of a reciprocal standard rate of oxygen consumption of postsmolt Atlantic salmon (S&no salor). J. Fish. Res. Board Can., 30: 1989-1900. Woo, N.Y.S., Bern, H.A. and Nishioka, R.S., 1978. Changes in body composition associated with smoltification and premature transfer to seawater in coho salmon (Oncorhynchus kisutch) and king salmon (0. tschawytscho). J. Fish Biol., 13: 421-428.
METABOLIC DIFFERENCES BETWEEN SAL/U?) PARR AND SMOLTS
ATLANTIC
SALMON
33
(SALMO
P.J. HIGGINS
1Freshwater Fisheries Laboratory, Pitlochry, PHI6 5LB (Great Britain); and Department of Zoology, University of Aberdeen, Aberdeen AB9 ZTN (Great Britain) ’ Correspondence address.
ABSTRACT Higgins, P.J., 1985. Metabolic differences between Atlantic salmon (Salmo salar) pan and smolts. Aquaculture, 45: 33-53. Measurements were made of growth, feeding, oxygen consumption and respiratory organs in sibling groups of juvenile Atlantic salmon during the first year of growth. The higher levels of growth and metabolism usually associated with smoltification were found to be a normal characteristic of the upper growth mode of the bimodal population. It is suggested that upper mode growth can be considered normal, and lower mode growth regarded as being suppressed, perhaps by changes in photoperiod in midsummer.
INTRODUCTION
Elson (1957) suggested that there is a certain critical minimum size for smolting in Atlantic salmon (Salmo suhr L.) and this idea has been supported by studies of the sizes of migrating smolts in many river systems throughout the geographical range of the species. Some of the changes in body physiology associated with migration can take place over a relatively short time prior to smoltification in the spring (e.g. salinity tolerance and migratory behaviour). However, if a minimum size threshold is to be reached by the spring, the fish must start allocating resources to somatic growth much earlier in the year. Support for this question has come from studies on the growth and development of hatchery-reared populations of sibling juvenile Atlantic salmon. Simpson and Thorpe (1976), Thorpe (1977) and Thorpe et al. (1980) have reported that in the first year of growth sibling populations developed a bimodality in both their weight and length frequency distributions. The two modes of these distributions were first distinguishable in the autumn. The fish in the upper mode developed into l-year-old smolts in the spring; those in the lower growth mode did not smoltify and remained as parr during their second year in freshwater. This bimodality is not the result of a dominance hierarchy (Simpson and Thorpe, 1976) and also appears to be consistent 0644-8486/85/$03.30
0 1985 Elsevier Science Publishers B.V.
34
with the migratory age structure of natural populations in four Scottish rivers (Thorpe, 1977). In further hatchery experiments Thorpe and Morgan (1978) demonstrated that there was also a genetic influence on the proportions of fish in the two modes, and both the growth rate and smolting rate. Bimodality has also been reported in Canadian (Bailey et al., 1980; Saunders et al., 1982) and Norwegian (Knutsson and Grav, 1976) stocks. For these growth differences to be evident in the population in the autumn, the initiation of differential growth rates must have occurred somewhat earlier. Villarreal’s (1983) evidence on the relative rates of RNA and DNA synthesis in bimodal populations suggests that this growth change is under photoperiodic control and that the time of “decision” is in a “critical photosensitive period” in June or July, Whatever changes in growth or metabolism occur during the juvenile development of the fish, an overall balance must be maintained in the energy budget. The energy value of the food consumed (C) must be accounted for in the growth (G), activity (A), respiration (R) and faecal and urinary excretion (F + U). C=G+A+R+F+U
(after Windell, 1978)
As the observed growth rates of the two modes of bimodal populations are different, the fish in those modes should also demonstrate differences in their metabolism through other components of the energy budget. In a full study of these energetic differences all components should be estimated. This is impractical if the fish are to be maintained in a standard hatchery environment and therefore, within the limitations of size, the three major metabolic parameters of growth, respiration and feeding are considered here. Growth Besides gross weight changes, the partitioning of energy into various types of growth should also be considered. Changes in the size frequency distribution may be the result of different developmental rates in individuals or the allocation (or re-allocation) of resources between different tissues. For example, Saunders et al. (1982) showed that the relative position of precocious males in the length frequency distribution changed during the period of their sexual development. To avoid such complications in the present study, a population of fish, all marked individually, was used to estimate specific growth rates. Weatherley and Rogers (1978) suggest that “rapid somatic growth in fish may be associated more with the addition of new muscle fibres than with increase in diameter of existing ones”. Weatherley et al. (1979, 1980) and Stickland (1983) also suggest that muscular growth in the rainbow trout (S&no gairdneri) is characterised by different phases in which hyperplasia (increasing fibre number) and fibre hypertrophy (increasing fibre size) change in their relative importance. Similar changes might also be associated
35
with the development of bimodality and may be related growth required to attain a threshold size for smolting.
to the rates of
Respiration Baraduc and Fontaine (1956) demonstrated a 30% elevation in the resting weight specific oxygen consumption of wild smolts when compared to parr at around 8°C. Power’s (1959) results for fish from an Arctic environment are less straightforward, with smolts having the lower oxygen consumption per unit weight below 13.5”C, but higher than parr above this temperature. His explanation is that, metabolically, smolts are more responsive to temperature change than parr. Baraduc and Fontaine (1956) also noted the link between metabolic rate and hormone production in smolts; and Withey and Saunders (1973) have also shown that manipulation of photoperiod affects standard rates of oxygen consumption and body composition. Heart and gills have a direct involvement in respiratory metabolism through the movement of blood and gas exchange: e.g., Pauly (1981) suggests that oxygen supply limits anabolism and hence growth, and therefore that rates of growth and maximum body size are limited by the size of the respiratory surface (gill lamellar surface area). Similarly, an increase in oxygen demand can be satisfied by an increase in the heart size as a function of the body mass or an increase in pumping rate (Schmidt-Nielsen, 1979). Clearly, it is worthwhile examining these organs within a general study of respiratory physiology prior to smoltification. Feeding Many authors have examined the link between feeding and growth and the topic has been discussed in major reviews (e.g., Brett and Groves, 1979). Many excellent studies have also been carried out; Elliott’s (1975a, b) studies on the relationship between ration size and growth are noteworthy as they involved a similar salmonid species (Salmo trutta). Although in a hatchery environment food availability should not limit growth, a relatively higher food intake in the more rapidly growing fish would be expected. Results of an investigation of these aspects of growth, respiration and feeding in bimodal populations of Atlantic salmon throughout the period of juvenile development prior to smoltification, are reported below. MATERIALS
AND METHODS
Stocks and maintenance All experiments were carried out on juvenile Almondbank hatchery which were the progeny
Atlantic salmon from the of adults caught from the
36
river Almond by electrofishing. Each family was the product of a pairing between a single male and a single female. From first feeding, fry were maintained in single family populations in radial flow tanks (see Thorpe, 1981 for details), and subject to normal water velocities of 24 to 26 cm/s. No attempt was made to control the natural photoperiod or ambient water temperature (e.g. Fig. 1) at the hatchery, However, as the rearing tanks are under cover, the light intensity is normally less than that of full daylight, but reaches around 20 lux at the water surface in bright sunshine. Under routine maintenance conditions the fish were fed on a pelletised commercial diet (Ewos Baker salmon starter, sizes 0 to 4). Pellets of a grade appropriate to the size of the fish were delivered by “caddymatic” feeders (see Thorpe, 1981). When size differences developed in the population an appropriate mixture of pellet sizes was delivered. The feeders were timer controlled and electrically triggered to deliver a preset amount of food at 5- to 15min intervals, 24 h a day. The amount of food delivered was always well in excess of the requirements of the fish in the tank, and was maintained for all stocks from first feeding, ensuring that the fish did not become conditioned to any unnatural feeding rhythm. Criteria for smoltification Fish were classified as “smolts” when they satisfied the criteria of complete body silvering, loss of parr marks and fin darkening. Fish in the intermediate stages of smoltification were not used. Specific growth rate Specific growth rate (SGR) was estimated on an approximately monthly basis for all the individuals within a special experimental population. In September 1982 two groups of approximately 45 sibling 0+ salmon representing each of the two modes of a bimodal population were established in a l-m diameter radial flow tank. Modal weight ranges were 0.6 to 1.9 g and 2.1 to 5.0 g. Each fish in the population was marked dorsally with a binary coded X-ray readable microtag (Northwest Marine Technology, Seattle, U.S.A.) distinguishing it from all others in the tank. Full details of the technique are provided elsewhere, as are results which demonstrate that the method causes no adverse growth effects (Higgins, 1985). The experimental population was monitored at approximately monthly intervals, and specific growth rate was calculated for each identifiable individual using the formula SGR = (log, W2 - log, IVl)/elapsed
time (days) ( after Ricker,
1979)
From these results a mean of all the individual specific growth rates within each mode was calculated. By June 1983, modal weight ranges were 1.7 to 5.1 g and 5.0 to 20.7 g. At that time approximately 53% of the fish in the upper mode could be classified as smelts.
37
Muscle histology A full histochemical description of the fibre types present will be presented elsewhere (Higgins, in prep.). However, the situation in Atlantic salmon is somewhat similar to that described for rainbow trout by Johnston et al. (1975). Although a narrow band of “intermediate” or “pink” muscle is present between the blocks of white and red fibres, the latter two fibre types comprised the bulk of the fish. All muscle samples taken from the tissue beneath the adipose fin were prepared using methods which are summarised by Stickland (1983). The lo--12~pm sections were stained using Weigert’s haematoxylin and eosin. Whole transverse sections were taken where possible, though the harder spines of the larger fish caused ripping during sectioning and in this event, half-sections (dorso-ventral) were used. Subsequent size frequency analysis was carried out on red and white fibre types using a microscope with a drawing attachment associated with an image analyser (ReichertJung “Videoplan”) and graphics tablet. The areas of around 100 fibres in each of these samples were measured for both muscle types for each fish. The total fibre number was then estimated by dividing mean fibre size into the area of the whole muscle block. Samples of two to four fish were used for each mode at approximately monthly intervals throughout the two years of juvenile development. Respirome try Respirometer. A five-cell recirculating flow respirometer was constructed for measurement of resting rates of oxygen consumption. An extensive description and diagram of the apparatus is given by Higgins (in prep.). Experimentd protocol. In order to avoid any thermal shock for the fish, measurements were made at intervals of 2.5”C between 5 and 15”C, at whatever interval was nearest the ambient temperature at the time. The results for an experimental temperature of 7.5”C are given here as they cover the period from the beginning of April to the end of May in 1982 and 1983. During this period upper and lower mode fish and smolts were available. Prior to use in experiments, all fish were maintained in radial flow tanks as above, The fish were starved for up to 2 days depending on water temperatures to allow evacuation of the gut, and left to settle for at least 8 h after introduction into the chamber before measurements were made. A continuous recording was then made for at least 24 h. As the timer-controlled cycle of the five chambers (four with fish in and one blank for comparison) took 3 h, eight results per animal were collected each day. These results were used to calculate a daily mean resting rate of oxygen consumption for each animal.
38
Hearts and gills At the end of each respiration study the fish were killed, measured and the hearts and gills dissected out. The whole of the gill assembly including gills and gill rakers was removed and cleaned of any excess tissue prior to weighing. The atrium was dissected from the heart prior to weighing the remainder of the organ as it usually contains a variable amount of blood. The ventricle, which is involved in the muscular effort of pumping the blood, has also been used in another study of heart development in Atlantic salmon (Poupa et al., 1974). No measurements were made of amounts of blood present at dissection in hearts or gills. Feeding experiments The 24-h food intake of the two modes of a bimodal population was estimated once a month between October 1983 and March 1984, using a special diet containing a small percentage of iron powder as a radio-opaque marker (Talbot and Higgins, 1983). Experimental protoco2. To avoid altering the normal feeding pattern, no periods of pre- or post-prandial starvation were imposed on the experimental fish. The adverse effects of such regimes are described by Talbot et al. (1984). The labelled diet was delivered for 24 h by simply replacing the hopper containing normal food with one containing the iron-labelled diet. When estimating a 24-h food intake on occasions when the fish had short evacuation times, the labelled food was offered in two 12-h periods separated by an interval of feeding with unlablled food which allowed evacuation of the previous labelled meal. Water contents of both food and fish were estimated in separate experiments (see Higgins and Talbot, 1985) and dry meal size was expressed as a percentage of the dry fish weight. Statistical
methods
All regression lines were fitted by the method of least-squares, and regression parameters were compared by analysis of covariance. A two-means T-test was used to establish differences between estimates of food intake. RESULTS
Growth Specific growth rate There appears to be a general association between growth, temperature and photoperiod (Fig. l), though closer examination reveals conflicting results in the temperature/growth association. Temperatures were generally
39
higher (3 to 7°C) between 25 October and 10 December than in the two subsequent intervals (10 December to 25 January, around 3.5”C; and 25 January to 1 March, around 1°C). However, specific growth rate increased through the period, following more closely the trend in photoperiod. Furthermore, growth rate increased rapidly in the spring though temperatures were generally lower than those of the preceding autumn. Again, the general trends of growth and photoperiod are similar. Throughout the study period all the microtagged individuals retained their 1600 1400 1200
Daylength (mln)
Specific Growth Rate 0 8.
(%I
0 6Q
Upper Mode
*
Lower Mode
0 1o- 20 o*
Water Temperature PC
1
I’ “I.“” 22 25 Sept. 03.
I IO Dec.
‘..I. 25 Jan.
~.I....l~.~~I~..~I 1 5 9 8 Mar Apr. May. Jun.
-4
Fig. 1. Mean of individual specific growth rates (* 1 standard error) of microtagged upper and lower mode salmon in relation to ambient water temperature (“C), and photoperiod (daylength in minutes) from September 1982 to June 1983. Water temperature is displayed as the weekly mean.
40
positions within each mode, with none of the upper mode fish dropping back into the lower mode or vice versa. This consistency is reflected in the little variability in the mean results (standard error). The upper mode fish maintained a higher growth rate than the lower mode fish throughout the period. This was most notable between 25 October and 25 January when the upper mode maintained growth whilst the lower mode showed effectively no growth at all.
Muscle his tology The initial gradual increase in white fibre number fry is maintained by both modes until a length of (March 1983) is achieved, when the upper mode rapidly (Fig. 2a). Mean fibre size follows a similar
r
WHITE
shown by the unimodal approximately 8.5 cm fibre number increases trend but without the
FIPRE NLblEIER
.
33000. 3 0000~ 27000.
.
.
2 i 000 21000~
a*
laooo. 0 lS”OO~
0
. 12000
_
9000
-
6000;I
-
I.
MEAN WHITE
:
FIBREAREA
:
:,
:
0
,500 O0
.“Og
900.
8o
l **
0
00
.
oa
,100.
:
:
.
1700~
,300.
l
:,
:
:
!
I,
(yn_l
19DO)
8
.
: ::
0
.$'
01,:.
.
@.‘f o o* Oo
.
4. w
3000.
2
o
.
’
l’ o
l ‘.
. .
0
l
l
.
:
.
700 pi
..~'
500. 300.
l”2.25
,
3.25
L.25
5.25
6.25 FISH
725
$25
9:25
Id.25
11.25
I
LENGTH (cm1
Fig. 2. (a) Changes in white fibre number with fish length. (b) Changes in mean white fibre size with fish length. = - unimodal fry; o - lower mode; l upper mode.
41
large increase above 8.5 cm. At this stage the upper mode fish are recruiting new fibres rather than increasing existing fibre size (Fig. Zb). The development of red muscle follows a similar pattern (Figs. 3a, b). Respiration Oxygen consumption Highly significant (P
T 7200 6‘00
‘1
RED FIBRE
NUMBER
I I
. .
em
,
+
t
MEAN RED FIBRE
t
--t-
+-t~t--+
--t---t+--
AREA (~1
1
:
I
. .
.
. .
I
225
325
L.25
5.25
6.25 FISH
LENGTH
7.25
8.25
925
10.25
11.25
km)
Fig. 3. (a) Changes in red fibre number with fish length. (b) Changes in mean red fibre size with fish length. n - unimodal fry; 0 -lower mode; l - upper mode.
42
+/I=‘Y!=‘/“‘JJ
J’UP”J’l”~/‘!‘~u)
1 NOIldWnSN03
N30AXO
601
NOIldWnSNO3
NJOAXO
00-1
~‘4/I=“IUD/oY)
J’4/I“‘l”=‘/“w)
NOILdWIlSN03
NOIldWflSN03
N3ShXO
N30AXO
001
“‘7
43 TABLE I Oxygen consumption at 7.5%. Equations describing the line of best fit 0, consumption (mge animal-‘. h-’ ) = a fish weight (g) b For comparison of regressions, see text l
Smolts Non-smolts Upper mode Lower mode
a
b
Obs.
P
0.0129 0.0433 0.0268 0.0477
1.2641 0.8982 1.1168 0.7554
14 54 29 25
<.OOl <.OOl <.OOl <.OOl
Gill weight Highly significant (P
Smolts Non-smolts Upper mode Lower mode
a
b
Obs.
P
0.0382 0.0334 0.0348 0.0371
0.8340 0.9316 0.8942 0.8713
19 180 67 133
<.OOl <.OOl <.OOl <.OOl
Heart weight Highly significant (P
44
(0) lHDI3R 1lID 0O-l
. . . . . .- .
I
\
. .
..
. .
\-
(0) lHOI3H 1lID 601
(0) LHOI3fl1110 e0-l
(6)
lHOI3fl
lW3H
601
(6)
lHOI3fi
lW3H
EOl
(6)
lH013M
lW3H
601
46 TABLE III Equations describing the line of best fit of heart weights Heart weight (g) = a - fish weight (g) b For comparison of regressions, see text
Smolts Non-smolts Upper mode Lower mode
a
b
Obs.
P
0.00135 0.00139 0.00149 0.00140
1.0538 1.0558 1.0337 1.0416
19 180 67 133
<.OOl <.OOl <.OOl <.OOl
exception that upper mode cept than lower mode fish.
fish had a significantly
greater
(P
Feeding Food in take On each of the five (monthly) sampling occasions the upper mode fish took a significantly (KO.01) larger meal (dry weight of food as percentage of dry body weight) than the lower mode fish (Fig. 7 and Table IV).
Oct.
Nov
Fig. 7. Food intake of upper and lower mode fish, sampled from October 1983 to March 1984. Intake estimated as dry weight of food as a percentage of dry fish weight. **differences significant at FYO.01; ***difference significant at P
47 TABLE IV 24-h food intake in feeding experiments Meal (%) = (dry weight food * dry weight fish-‘) * 100 N = number of observations; P = level of significance of differences between modes at that date Date
Temp. (“0
Mode
Length (-1
Meal (%I
S.E.
N
P
13 Oct.
7.00
upper lower upper lower upper lower upper lower
7.83 6.52 8.05 6.46 8.22 6.54 8.38 6.57 8.52 6.61 8.77 6.69
2.95
0.216 0.342
15 6
0.136 0.125
13 8
< 0.001
0.131 0.116
12 8
0.253 0.111
13 7
0.074 0.030
15 5
0.148 0.162
14 6
10 Nov.
5.38
08 Dec.
4.50
06 Jan.
3.50
08 Feb.
0.88
15 Mar.
4.00
upper lower upper lower
1.76 1.51 0.63 1.72 0.90 1.52 0.30 0.67 0.15 1.76 0.50
DISCUSSION
Growth Specific growth rate remained higher in the upper modal group throughout the study period and mirrored the time-divergence in length shown by other experimental populations (Bailey et al., 1980; Thorpe et al., 1980). Although this high growth rate was maintained through the autumn and winter, a substantial increase was not evident until March (Fig. 1). This coincided with a rapid increase in fibre number in the upper mode fish (Figs. 2a, 3a) which Weatherley et al. (1979,198O) have established as a characteristic of rapid growth in rainbow trout. This should be expected as the surface area/volume ratio of large cells (above a certain size) limits their metabolic efficiency (Von Bertalanffy, 1960). Although no measurements of swimming activity were made in the present study, a tendency towards increasing fibre number is unlikely to be the result of elevated activity levels in the upper mode fish as, according to Johnston (1981), exercise leads to fibre hypertrophy. Seasonal changes in ambient temperature and photoperiod are the main environmental factors which might be expected to influence growth and smoltification (Wedemeyer et al., 1980). Although artificial elevation of water temperature appears to accelerate growth and produce smolts in a shorter time (Wagner, 1974; Saunders, 1976), most evidence points to photoperiod being the primary influence. This is thought to operate by controlling neuroendocrine activity and metabolism and thereby growth rate
(Hoar, 1965, 1976; Saunders and Henderson, 1970) and smoltification (Saunders and Henderson, 1970; Wagner, 1974). That changing photoperiod should be the more reliable influence seems reasonable as it is not prone to the “noise” often recorded in temperature seasonality. Despite an unusually cold winter and spring, both modes of the microtagged population showed a rapid increase in specific growth rate from January onwards which seems to be associated with the seasonal change in photoperiod. The actual daylength does not seem as critical as the direction of the trend (increasing or decreasing) in photoperiod and this is in accord with the findings of Saunders and Henderson (1970), Knutsson and Grav (1976) and Komourdjian et al. (1976). A similar response has also been recorded for brown trout by Wingfield (1940) and Swift (1961), but was not found by Elliott (1975a). As both modes demonstrated the same form of growth response (but not to the same degree) they were probably responding to the same stimuli at this stage of development. Although smolts have a higher growth rate than non-smolts (Saunders and Henderson, 1970; Komourdjian et al., 1976) the initiation of this difference does not appear to be linked to spring photoperiod changes, and probably occurs at the initiation of bimodality which Thorpe et al. (1982) and Villarreal(l983) put at midsummer of the first year of growth.
Respiration Von Bertalanffy (1951) and Pauly (1981) have proposed that anabolism in fish is dependent on respiration rate, which is in turn limited by the extent of a respiratory surface (e.g. gills). Pauly discounts food supply as the main factor limiting growth as fish can to some extent store food, but this is not the case with oxygen. By drawing on the literature available on growth, oxygen consumption and gill surface area for a wide range of fish species, he puts forward a convincing argument that the “limiting respiratory surface” for growth is the gill surface area. He also argues (per-s. comm., 1982) that although the full capacity of the gills is only used during exercise, there must be a constant factor relating the area used at rest with the total surface area available. As a general trend, in the present study the fish which demonstrate the highest growth rates in winter and spring (smolts) have the highest weight-specific rates of oxygen consumption and associated gill weights. This lends broad general support to the proposition of Von Bertalanffy and Pauly. However, as oxygen consumption was relatively higher in the upper mode compared to the lower mode, we might have expected larger gills, whereas no differences were found. It may therefore be that gill size is not a factor limiting the growth of the lower mode fish and that some other process is responsible for their reduced growth rate (e.g. photoperiod acting on the neuroendocrine system as suggested earlier). More simply, gross changes in gill weight may not be a sensitive enough estimator of gill surface area. Some consideration should also be given to changes in the ionic regulation capability of the gills, as for example, Langdon and Thorpe (1985) have
49
shown that there are intermodal differences in the pattern of development of ionoregulation and that the previously observed changes in e.g. Na/K-ATPase (Saunders and Henderson, 1978) are not solely a function of smoltification. Changes of this type might be expected to have some impact on the respiratory function of the gills. The only difference between the heart weights of the groups was that upper mode fish had significantly larger hearts as a function of body weight than the lower mode fish. This points to a difference in respiratory capacity between the modes but it is perhaps surprising that this was not also found for smolts. Poupa et al. (1974) found an increase in relative heart weight after migration and throughout the marine phase. They noted no relative increase in gross heart weight in smolts but did find an increase in the thickening of the ventricular shell and compact ventricular volume in relation to the ventricle volume. The sample size of smolts for their investigation was small (6) and they did not have the opportunity to compare upper and lower modal groups. The results of the two studies may therefore complement each other, and the larger hearts found in the upper mode fish may represent a greater facility to carry out its role in respiration. Feeding Brett and Groves (1979) point out that in almost all species investigated “the higher the ration, the greater the growth”. The fish used in the feeding experiments were offered excess food and so their situation is comparable with the brown trout fed on maximum rations by Elliott (1975a). Under these conditions, Elliott found that over a wide size and temperature range the smaller the fish the higher its specific growth rate, and the higher its food intake as a function of body weight. He also demonstrated that the weightspecific maintenance ration decreased as a function of increasing fish size (Elliott, 1975b). Thus, according to Elliott’s evidence, if the modal differences are simply a function of size itself, we should expect the higher weightspecific food intake and growth rate to be found in the lower mode fish. In fact, the converse is the case (Figs. 1,6). These results can be compared with the generalised ration/temperature relationship for sockeye salmon (Oncorhynchus nerka) of an equivalent size range (Brett et al., 1969, fig. 8). The daily meal taken by the upper mode fish is approximately equivalent to the optimum ration size for a young sockeye salmon at the same temperature, though the lower mode food intake is generally in the region of or just above maintenance. One important difference, however, is that our fish were subject to a winter photoperiod whilst Brett’s sockeye experiments were conducted between spring and autumn. Feeding in juvenile Atlantic salmon is limited in periods of darkness (Talbot and Higgins, 1983; Higgins and Talbot, 1985), and thus a winter photoperiod would represent a reduced opportunity to feed. The upper mode fish therefore seem to have fed optimally despite the shorter daylength, whereas the lower mode fed sub-optimally.
50
Clearly, the higher growth rate maintained by the upper mode through the winter, which appears to be a prerequisite of smolting, is supported by the optimum size of ration under the prevailing environmental conditions. Commentary The changes in respiration recorded in the present experiments are clearly not simply associated with smoltification as they also relate to upper mode fish. Evidence will be presented elsewhere to show that modal differences in metabolism exist throughout the period of bimodal development (Higgins, in prep.). Furthermore, elevated respiration rate is not simply a function of changes in body composition recorded by Komourdjian et al. (1976), Woo et al. (1978) and others (see review by Wedemeyer et al., 1980). In separate body composition analyses of samples of fish throughout the first year of growth (Higgins and Talbot, 1985), upper mode fish maintained higher reserves of lipid and a lower proportion of body weight as water than the lower mode fish. Therefore, the elevated respiration rate in upper mode fish and smolts cannot be due solely to catabolism of lipid stores. As the results of oxygen consumption measurements presented here are generally lower than those measured by Baraduc and Fontaine (1956) and Power (1959), it seems unlikely that the results for the upper mode and smolts (around 40 to 45 mg * kg-’ - h-l) represent a state of elevated metabolism. Rather, as was argued for the results of food intake and growth, the lower mode fish may be considered as demonstrating a “suppressed” metabolic response relative to the upper mode fish. These bimodal differences, and in particular any suppression of growth and metabolism in the lower mode fish, might, as was suggested by Simpson and Thorpe (1976), be initiated and regulated by environmental influences (e.g. photoperiod as emphasised here) acting through the endocrine system. ACKNOWLEDGEMENTS
This work was carried out under an award from the Science and Engineering Research Council, held at the University of Aberdeen and the Freshwater Fisheries Laboratory, Pitlochry. I am grateful to friends and colleagues for their help and encouragement throughout the study period, and in particular to Dr. John Thorpe for assistance in the preparation of the manuscript.
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