Dec 12, 1981 - Interactive effects of starvation and low pH on body composition were examined in .... h and then weighed a final time. Elemental composition ( ...
Interaction of Low pH and Starvation on Body Weight and Compositio of Young-of-Year Sma mouth Bass Mimpterus dolomieui G. L. Cunningham Department of Zoology, University of Toronto, Torontp, Ont. M5S 7A 1
and B. 1. Shuter Ontario Ministry of Natural Resources, Research Section, Fisheries Branch, Box 50, Maple, Ont. LO) IEO
Cunningham, G. L., and B. J. Shuter. 1986. Interaction of low pH and starvation on body weight and composition of young-of-year srnallmsuth bass (Micropterers dolcsrnieui). Can. 1. Fish. Aquat. Sci. 43: 869 -876. Effects sf overwinter starvation sn the body composition of young-sf-year smal lmouth bass ( Micropterus ds%omieui) were examined in laboratory and field studies. Interactive effects of starvation and low pH on body composition were examined in the laboratory. Changes in body composition associated with starvation were similar in laboratory and field studies: on a dry weight basis, the amount of water, ash, Ca, K, and %\laincreased progressively in both laboratory and wild fish. The amount of CI per unit dry weight showed a moderate increase in the laboratory fish and a large decline in the wild fish. Phe effects on starving fish of 6-wk chronic exposures to nonlethal pH levels of 4.8-7.0 were examined in the laboratory. N o pH effect was noted for levels greater than 5.0. Losses sf water, ash, Ca, K, 61, and Na increased as the pH declined. There was no consistent effect of pH on rate sf loss of ash-free organic matter. The results suggest that starvation may reduce the tolerance sf young smallmouth bass to low pH through progressive weakening of the osmoregusatcary system. Les effets du manque de nourriture en hiver sur la composition chimique du corps de l'aehigan a petite bouche (Microptesus dolomieui) dans sa premiere ansske ont kt6 examines en laboratoire et sur Je terrain. Les effets interactifs du manque de nssdrritsdre et d'uss faible p H sur la composition chimique ont kt6 examin4s en laboratoire Les changements dans la compssiticsn chimique associ6s au manque de nourriture etaient semblables au laboratsire et sur le terrain : en masse sPche, les quantites d'eau, de cessdre, de Ca, de K et de Na augrnesstaient progressivement dans le poisson tant en laboratoire qu'en Biberte. La quantite de 61 par unit6 de rnasse sGche prksentait une augmentation rnsdkrke dans le poisson en iabcaratsire et une forte diminution dans le poisson en liberte. On a examin4 aussi en laboratsire les effets, sur le poisson privk de nourriture, d'une exposition constante de 6 sem a des niveaux non Ietaux de pH de 4,0 3 7,O. Aucun effet dO au pH n'a kt6 not6 pour des valeurs sup4rieures A 5,8. Les p r t e s d'eau, de cendre, de Ca, de K, de CI et de Na ont augrnente au fur et mesure que te pH dirninuait. Le pH n'a pas eu d'effet unifsrme sur I'importance des pertes de matiere srganique sans cendre. Les r6sultats indiquent que le manque de nsurriture peut reduire la tolkranee du jeune achigan a petite bouche aux Bai bles pH par affaibl issernent progressif des systerne osrnorkgeslatoire.
Received ApaiB 30, 1985 Accepted Ianeraay 7, 1986 (.la227)
he smdlmouth bass (Micropterus do&omieui)is a w water sports fish of economic importance to the recreation and tourism industry of Ontario. Data on the disappaance of wild populations indicate that the species is sensitive to acidification through recruitment failure ( H m e y 1980). Baynes (1981) and Beamish (1976) suggested that reproductive failure becomes likely when pH falls within the range from 5 -5 to 6.0. However, the phases sf the life cycle vulnerable to reduced pH have yet to be identified. Shuter et al. (1980) argued that much of the considerable year-to-year variation in cohort strength, which is characteristic of northem populations sf this species, can be accounted f r by variations in the winter survival of young-of-yearf..Feeding at winter temperatures is significantly reduced, and metabolic
T
' Contribution 85-02 of the Ontario Ministry of Natural Resources, Research Section, Fisheries Braweh, Box 50, Maple, Ont. LOJ 1EO. Can. J . Fish. Aqwr. Sci., V s l . 43, 1986
requirements must be largely supplied by energy reserves accumulated during the first growing season. Under annual $emprature cycles typical sf north-central Ontario ( i s . at the northern limit of the species9range), the first growing season is sufficiently short and the winter sufficiently long that significant mortality from stmation is likely. For such populations, an increase in metabolic costs arising from exposure to reduced pH levels could lead to a significant decrease in the winter survival of young-of-year and, consequently, a greater likelihood sf recruitment failure. In this study, we examine the effects of low pH on the metabolic costs of young-of-year smallmouth bass held under simulated winter conditions, and we document the interactive effects of stmation and low pH on whole-body elemental composition. We also compare changes in body composition sbsewed under laboratory conditions with changes observed in -82. three wild populations over the winter of 198% $69
EXPERIMENTAL TANKS ( 61
PH METER 1
AGNETIC VALVE -RE&CIBIFIE&) WATER HEADBOX FROM
RG.1. pH control system. Water from the headbox is fed to six experimental tanks, each maintained at a different pH.
TABLE1. Summaq of conditi~nsfor three experiments successftd%glcompleted. In each experiment, the six pH levels tested were checked at least once every 2 dl; for 97% of these checks, the observed pH was within 0.2 unit of the desired level. Temperatures were checked daily and were within 1°C sf the reported mean for 96% of these checks.
Source of fish
Experiment 1
OMNR hatchery
2
Northeastern Biologists
3
Northeastern Biologists
Duration
pH levels tested
Dec. 12, 1981 - 3.5, 4.2, 4.9, Jan.26,1982 5.6,6.3,7.0 Sept. 15, 1982 - 4.0, 4.3, 4.6, O ~ t . 2 4 ~ 1 9 8 2 4.9,5.6,6.5 Nov. 1. 1982 4.0, 4.3, 4.6, Dec. 12, 1982 4.9: 5.6, 6.5
Materials and Methds Young-of-year smallmouth bass for laboratory experiments were obtained from the Ontario Ministry of Natural Resources (OMNR) hatchery on Mmitoulin Island in 1981 and from Northeastern Biologists in Rhinebeck, New York, in 1982. In the late summer of each y e a , approximately 1300 fish were transferred to a large (3000 %) flow-through holding tank at the University of Toronto. Initially, photoperiod, and temperature conditions in the holding tank were set to match conditions exgehienced by the fish in the hatchery. Day length and temperature were then decreased gradually to simulate an accelerated change to winter conditions. On reaching 18.25 h light: 13.75 h dark and 6"C, no further adjustments were made to photoperiod or temperature. The holding and test tanks were maintained under these conditions throughout the experiments. Temperature was maintained using Minicooll refrigeration units and themoregulator-controlled glass rod heaters. Opaque fiberglass covers were placed over both the holding and test tanks to minimize the amount of light reaching the fish, thus
Mean fork length (cram>
Fork length range (cm)
Mean temperature
6.25
4.9-7.6
5.86
6.7
5.3-9.6
7.8%
6.4
5.3-8.0
6.85
b"cb
simulating the effects of ice and snow cover. In both holding and test tanks, one piece of PVC tubing (2.5 em diameter, 12 cm long) was provided for each fish to simulate the rocky shelters based by wild young-of-year during the winter. Four experiments were started: two in the winter sf 1981-82 and two more in the fall of 1982. Of these, only three were completed successfully (Table 1). Each experiment lasted six weeks. In each experiment, six test tanks were maintained at different pH levels, 1 tank for each pH level (Table 1). Because the available water (dechlorinated tap water) was quite h a d (120- 140 mg CaC03*L-' similar to levels observed at both hatcheries), direct acidification would liberate potentially toxic Thus, the water was pretreated before reachamounts of @02. ing the test tanks by mixing it with 1 A4 &SO4 in the first section of a headbsx divided into eight sections by perforated Plexiglas partitions (Fig. I). The pH in the first section was maintained at 5.5 by a Radiometer BHM84 pH meter and TFT80 titrator, in conjunction with a GK2401C combination electrode and an MNV2 magnetic valve. Each section in the headbox was vigorously aerated to drive off evolved COz. As
a consequence of C02 loss, the pH in the final section fluctuated fmm 6.5 to 7.0. Water llgswed by gravity from the last section of the headbox to the six 90-E test tanks. The flow rate to each tank was maintained at approximately 0.075 L min-' . A six-channel switch (Bach-Simpson SAS-2) was used to permit a single pH meter (gPHM84) and titrator (TTTSO) to control the pH in all six test tanks. The switch caused the pH meter and titrator to successively monitor (via a GK2401C electrode) and adjust (using 0.1 M H2S04delivered via an MNV2 magnetic valve) the pH of each tank for 2 min out of every 10. Each tank was well aerated to insure adequate mixing of the acid, acceptable O2 levels (12 - 11 3 mg t-' ), and to drive off any further C 0 2 produced by acidification. At the start of each 6-wk experiment, 300 fish were taken fmm the holding tank and equally distributed among the six test tanks. An additional 563 fish were then taken from the holding tank and killed. This latter sample was used to obtain data on the body composition of the fish at the start of each experiment. Each test tank was maintained at a different pH for a period of 6 wk. Temperatures were checked and adjusted on a daily basis. A complete pH check, and calibration of the pH control system, was carried out at least every second day. Tanks were checked twice daily for dead fish. Periodically, the water in the headbox and test tanks was analyzed for metal (AH, Cu, Fe, Mn, Ni, Zn) levels. These were always well below the minimum toxic levels reported by Spry et al. 1981). At the end of 6 wk, all fish in the test tanks were killed. Fish that lived through an experiment, those that died during an experiment, and those sacrificed from the holding tank at the beginning of an experiment were a11 processed in the same manner. Live fish were killed with an overdose of MS-222. Fish were blotted dry, weighed to the nearest milligram, measured (fork and total length to the nearest millimetre), wrapped in aluminum foil, and frozen. Frozen fish were reweighed and measured, oven dried at 1110 + 5°C for 24 h, placed in a desiccator for 8 h, and then re-weighed to determine dry weight. Dried fish were placed in a muffle furnace at 550 2 10°C for 24 h. Remaining ash was placed in a desiccator for 8 h and then weighed a final time. Elemental composition (levels of Na, K,C1, Ca in micrograms per gram) of the whole-body ash was determined by neutron activation analysis using a Slowpoke-2 nuclear reactor. Samples were irradiated at a neus-' for t min. After a tron flux of 2 x 106 neutrons l-min delay, the radioactivity of each sample was determined using a gamma ray spectrophotometer. The concentration (micrograms per gram) of each element in the sample was detemined by comparing its activity with the activity of a standard of known concentration. Data on elemental composition of fish are available for part of experiment 1 and all of experiments 3 and 4. The fish held at pH 3.5 in experiment I all died within 2 wk of the start of the experiment. The pH 3.5 fish in experiment 2 all died within 2 d of the start. In experiment 2, a fungus infection broke out in five of the test tanks part way through the experiment, and many fish died before the end of the 6 wk. Therefore, the data from pH 3.5 in experiment 1 and all of the data from experiment 2 were excluded from further analysis. The effects of starvation on the test animals were evaluated by compkng the following measures of body composition in the samples taken from the holding tank at the beginning of each experiment with similar measures taken on fish in the highest pH tank at the end of 6 wk: wet weight, ash weight,
-
Can. 9. Fish. Aquat. Sci., Vod. 43, 1986
ash-free dry weight (i.e. dry weight minus ash weight), and the Na, K, Cl, and Ca concentrations. The interactive effects of stmation and reduced pH were evaluated by comparing the same measures of body composition across the samples collected from the six test tanks at the end of each 6-wk experiment. Comp&sons of wet weight, ash weight, and ash-free dry weight across samples were done using analysis of covariance on log-transformed data with log fork length as the covariate. Results are presented in the form of estimates of mean weight at a standardized fork length of 6.4 cm. Comparisons of Na, K, C1, and Ca across samples were done using one-way analysis of variance on untransfomed data. Length was not used as a covariate here, because it was not significantly correlated with any of the elemental values. The statistical significance of each c o m p ~ s o nwas evaluated using a two-tailed t-test. Tests on independent replicates of the same cornpaison were pooled using a x2 procedure (Sokal and Rohlf 1981, p. 779) to obtain an overall test of significance. Samples of young-of-year smallmouth bass were obtained from wild populations resident in three small lakes located in south-central Ontario (Table 2). Samples were collected from the littoral zone of each lake, in the fall of 1981 and the spring of 1982, using electrsfishhg gear and dip nets. Fish were treated in the same manner as laboratory fish, except stomach contents were removed before weighing and freezing. Changes in body composition over winter were evaluated using the same set of statistical procedures used in evaluating changes observed in the laboratory. Water chemistry data for the lakes were obtained from samples analyzed by the Water Chemistry Laboratory of the Ontario Ministry of the Environment (OMOE), according to methods outlined in OMOE (1981).
Results Starvation Effects Comparisons between fish killed at the start of each expeiiment and fish killed after 6 wk of starvation revealed significant ( p < 8.01) decreases in the mean wet weight and mean ashfree dry weight per individual (Table 3). In addition, significant (p < 0.01) decreases in the mean K concentration of the ash and increases in the mean Na and Ca concentrations of the ash were observed. No significant 4p > 0.05) changes in the mean fork length per individual, mean ash weight per individual, or mean Cl concentration of the ash were noted with stmation. Significant (p < 0-01) changes observed over winter in the three wild populations are consistent across lakes and largely parallel the changes observed in the laboratory starvation experiments: mean wet weight, mean ash-free dry weight, and mean K concentration declined; mean Ca concentration increased (Table 3); mean length and mean ash weight remained reaatively constant. The changes in the Na and C1 concentrations observed in the field differed from those observed in the laboratory. In the field, mean Na and C1 concentrations declined significantly ( p < 0.01). In the laboratory, the mean Na concentration increased, while the response of the mean Cl concentration was neither consistent nor statistically significant ( p > 0.05). Interactive Effects of Starvation and pH on Body Composition There was no consistent trend linking low pH with greater losses of ash-free dry weight during starvation. Only in expri-
TABLE2. Location, physical characteristics, water chemistry data, and sampling infomation for the t h e e lakes supporting the wild smallmouth bass populations studied. For each lake, water chemistry qata are the means sf two independent samples: the first taken in Aug. 1981 and the second taken in Nov. 1982. Lake Kelly
Johnson
Lochlin
Oct. 19 May 87-28
act. 20 May 20-29
03. 27 May 20-29
Descriptor Location Area (ha) Mean depth (m)
PH Gonductivity (pRecm-I at 25°C) Alkalinity (TIP, mg CaCo3 L - ' 1 Na (mg .L-') C1 (mg L-') K (mgn~-~) Ca (mg*L--') Dates fish sampled 1981 1982 n 1981 1982 Mean length (cm) 198 1 1982 Length range (cm) 198 1 1982
TABLE3. Statistically significant ( p < 0.01) effects on h d y composition induced by stmation alone and additional significant effects induced by exposing starving fish to reduced pH levels. Wet weight, ash-free dry weight, and ash weight refer to absolute values for a fish of standard size (fork length = 6.4 cm); elemental concentrations refer to concentrations in ash derived from whole fish.
Component
Observed change
Lab
Wet weight Ash-free dry weight Na concentration K concentrdtiosl Ca concentration
Decrease Decrease Increase Decrease Increase
Starvation
Field
< 4.6 < 4.6 < 4.9 < 4.6 < 4.6
Lab Lab Lab Lab Lab
Wet weight Ash-free dry weight Na concentration K concentration @a concentration C1 concentration Wet weight Ash weight Na concentration K concentration C1 concentration
Decrease Decrease Decrease Decrease Increase Decrease Decrease Decrease Decrease Decrease Decrease
Effect
Lab OF field data
Stmation
pH pH pH pH pH
ment 3 did the fish held at low pH levels have significantly ( p < 0.05) lower ash-free dry weights than those fish held at higher pH levels. In contrast, there were consistent and significant (Table 3) trends in the data linking low pH with Iower values for mean wet weight, mean ash weight, and the mean Na, e l , and K concentrations in the ash. The data also suggested that exposure to reduced pH led to an increase in the Ca 872
concentration in the ash. All of these effects became evident a1 about pH 4.5 and appeared to intensify at lower pH levels. Figure 2 provides a quantitative picture of the changes taking place in the components of a fish of standardized length (6.4 cm), forced to exist on the energy stored in the ash-free dry weight fraction of its body. It also illustrates how these changes are influenced by exposure to low pH. Under moderate Can. .I Fish. . Aquat. Sci., Vol. 43, 8886
0.3
8.4
8.5
0.6
0.7
0.8
ASH - FREE DRY WEIGHT ( g )
0.3
04
8.5
0.6
0.7
ASH -FREE DRY WEIGHT
g g1
FIG.2. Log-log relations between (A) moisture, (B) ash, (C) Na, (D) Cl, (E) K, (F) Ca, and ash-free cfhy weight. Points represent estimates of the absolute amount of each component fim a fish of fork length 6.4 cm. Declines in ash-free cfhy weight are indicative sf increasing stmation. Ordinates all fdlbw the same logarithmic scale; therefox, the heights of the graphs indicate the overall variation observed in each eeampnent. Bmken lines in Fig. 2A, 2E, and 2F were derived from least squares regression; p i n t s used in each Egression are circled. For laboratory fish, closed symbols (A, experiment B ; experiments 3 and 4) are for pH > 5.5 and open symbols are far pH < 5.5; for pH < 4.5. the actual value is given within the symbol. For wild fish, letters designate the lake (L = Lwhlin, K = Kelly, I = Johnson).
+,
Can. J . Fish. Aquar. Sci., V01. 43. 1984
6.8
pH conditions (pH > 5.5) in both the field and the laboratshy, a decline in ash-free dry weight is accompanied by an increase in @aand declines in b t h moisture and K.Stmation-induced decreases in ash-free dry weight were not accompanied by consistent changes in the other three components studied: ash, @l, and Na. All of the moisture data, from both the wild fish and the laboratory fish held at moderate (> 5.5) pH levels, closely ( r = 0.98. fa = 16, p < 0.01) folowed the regression equation where M is moisture (grams per fish) and AFBW is ash-free dry weight (grams per fish) (Fig. 2Pa). The exponent in the equation is significantly less than 1.0, indicating that the amount of water per unit ash-free dry weight increased progressively with increasing starvation. Four of the five pints derived from fish exposed to pH levels less than 4.5 fall below the regression line. suggesting that exposure to low pH may result in accelerated moisture loss in stming fish (Fig. 2A). Hn contrast with the results for moisture at moderate (15.5) pH levels, no consistent relationship was observed between changes in ash weight and changes in ash-free dry weight (Fig. 2B). However, there is evidence that exposure to low pH leads to an abnomal loss of ash: values from laboratory fish exposed to low (< 4.5) pH levels lie well below all of the values from ldmratory fish expsed to pH 4.6 or above (Fig. 2B). There was little consistency in the response of Na content to reductions in ash-free dry weight (Fig. 2C). There was also considerable variation in mean Na levels across data sets: fish from one lake (Eochlin Lake) contained much higher Na levels than fish from the other two lakes, and fish from experiment 1 contained more Na than fish from experiments 3 and 4. At lower pH (below 5.0), an abnomal loss of Na was observed. The extent of this loss increased progressively with decreasing pH (Fig. 2C). There was a general tendency for Cl content to decline during stmation at moderate (> 5.5) pH levels. However, both the rate of decline and the average level of Cl in the body varied considerably from data set to data set (Fig. 2D). At lower pH (below 5.0), there was a progressive increase in the normal rate of Cl loss with stmatiion (Fig. 2B). At moderate pH levels (> 5.5), K content declined progressively as ash-free dry weight declined. Data from the wild fish and all data from laboratory fish held at moderate (> 5.5) pH levels closely ( r = 0.974, PI = 14, p < 0.01) followed the regression equation where K is potassium (rnillimoles per fish) (Fig. 2E). The exponent is significantly less than 1.0, indicating that the amount of K per unit ash-free dry weight increased progressively with increasing stmatism. As pH fell below 5.0, there was a progressive increase in the normal rate sf K loss during stmation (Fig. 2E). At moderate pH levels, Ca content increased as stmation progressed (Fig. 2F). Data from two wild ppulations and all data from laboratory fish held at moderate (> 5.5) pH levels closely ( r = -0.961, n = 12, p < 0.01) followed the regression equation where Ca is calcium (millimoles per fish) (Fig. 2F). The aver-
age Ca content of the fish from one lake (Lockdin Lake) was much higher than that found in fish from other locations but, here toe, Ca content increased as ash-free dry weight declined. All of the data points from fish exposed to low (< 4.5) pH levels lie well below the Ca/ash-f~edry weight regression line, suggesting that exposure to low pH leads to a reduction in the normal rate of @aaccumulation during starvation (Fig. 2F).
Many studies of the effects of stmation on the body composition of fish have been published in the last 38 yr (see Love 1970, 1980 for reviews of studies published prior to 1978). A number of general pattms are evident in this work. Losses of lipid and other carbohydrate energy stores are common along with cepncornitant increases in the water content of various tissues (Johnston and Goldspink 1973; Stirling 1976; Jobling 1980; Love 1980; Toneys and Coble 1980; Amnachalam and Reddy 1981; Mustafa 1983; Reinitz 1983). The latter is usually interpreted as reflecting an increase in the relative size of the extracellular space due to lipid loss. Muscle protein, particularly protein associated with the contractile fibres of white muscle, is often mobilized as an additional energy source (Creach and Sedaty 1974; Moon and Johnston 1980; Johnston and Goldspink 1973; Stirling 1976; Moon 1983a, 1983b; Beardall and Johnston 1983; Love 1980). Concomitant changes in muscle structure include a decline in the cross-sectional area and volume of contractile fibres (Beardall and Johnston 1983; Moon 1983b; Creach and Serfaty 1974). a decline in the total mitoehondrial volume of muscle cells (Beardall and Johnston 2983; Moon 1983b), a decline in the vascaalarization of muscle tissue (Beardall and Johnston 19831, and shrinkage of muscle cells with a consequent increase in the extracellular space between cells (Creach and Serfaty 1974; Love 1986)). The changes listed above lead to an increase in the relative size of the extracellular space in muscle tissue and an increase in the water content of muscle cells. These changes, in turn, lead to a considerable increase in the water content of muscle tissue. Changes in the relative sizes of the extracellular and intracellular spaces have been invoked to explain observed changes in the Na, C1, and K content of stming fish (Love 1970, 1980). Since Na and Cl are primarily confined to the extracellular space and K is primarily confined to the intracellular space (Holmes and Donaldson 1969; Love 19701, one would expect the Na and Cl concentrations in the tissues of starving fish to increase and the K concentration to decrease. Such changes have been reported in a number of studies of muscle tissue (Moon 1983b; Kanevskii 1981 ; Soletter and Huggins 1977; Sutton 1968). Our observations on stmation-induced changes in the water and elemental content of whole fish were qualitatively similar; the amount of water per fish declined less rapidly than dry weight so that water content, on a dry weight basis, increased progressively; Na per fish increased (labratory) or remained relatively constant (field) so that Na csncentmtion, on a wet or dry weight basis, increased; K per fish declined progressively so that K concentration, on a wet weight basis, either decreased (laboratory) or remained relatively unchanged (field). K concentration on a dry weight basis increased in both the laboratory and the field, but this probably reflects the tendency for the significant amounts sf K associated with skeletal material to mask shifts in the K concentration of soft body tissue. The fact that the amount. of Na per fish actually increased Can. J . Fish. Aquar. Sci., Vol. 43, 1986
(Iaboratov) while the total moisture content per fish declined suggests a greater accumulation of Na than that necessary to maintain isotonicity in an enlarged extracellular space. Similar obsewations were made by Meyer et al. (1955) and Creach and Sedaty (19'74) who both suggested that some of this extra Na partially replaces K lost from the intracellular space. In experiments 3 and 4, the rate of decline in K (Fig. 2E) is essentially identical to the rate of increase in Na (Fig. 2C). This suggests that, under the conditions existing during these experiments, some exchange of Na for K may have occuned. In our work on whole-body composition, we did not find the paallelism between Cl and Na behavior reported by authors working on muscle alone (Sutton 1968;Love 1980). Cl concentration, on a wet or dry weight basis, declined in the wild ppulations, while in the laboratory experiments there was a moderate increase. Our data, therefore, are not consistent with the hypothesis that starvation-induced shifts in these elements can be explained on the basis of changes in the relative sizes of extracellular and intracellular spaces. This discrepancy could arise if the dynamics of Na and Cl in muscle are masked by other processes, operating at the level of the whole fish. However, Houston and Meaow (19'79) have shown that, for most of the major tissues in fish, the Cl space provides a much more accurate measure of the extracellular space than the Na space. Therefore, it may be more appropriate to regard shifts in Cl concentration as indicative of shifts in the relative sizes of celular spaces, while shifts in Na concentration are seen as indicative of the combined effect of changes in both the size of cellular spaces and the distribution of Na between those spaces. A few repofis in the literature parallel our observation of a systematic increase in Ca absolute amount and/or coneentration (Reufs and Weinland 1913, cited in Meyer et al. 1955; Moon 1983b). Moon (198%) interpreted his observation that Ca and Na concentrations increased in muscle and declined in ro~trata)during stmation as indicaplasma of eels (Angui&&a tive of a progressive decline in the effectiveness of osmoregulation during stmation. This interpretation is supported by observations in fish (Jurss et al. 1983) and other organisms (rats, Machado et al. 19'71) that Na-K ATPase activity progressively declines as stamation prwedes and by the observation, in mammals, that stmation is accompanied by electrolyte imbalance (Creach and Serfaty 19'74). This interpretation of the changes in elemental composition induced by stmation is of particular interest when we consider the impact of exposure to low pH. Our results do not support the hypothesis that chronic ewp s u r e to low pH conditions leads to a significant increase in metabolic costs. However, they do demonstrate that some of the effects of low pH exposure on elemental composition are, in essence, an accentuation of the changes that accompany stmation: moisture and K losses are accelerated. Hn addition, outright reversals of starvation effects occur: abnormal losses in e l , Na, and Ca are induced. Such changes are the result of impaired osmoregulatoy function (Wood and McDonald 1982). Low-pH toxicity has itself k e n explained in terns of a cardiovascular collapse induced by osmoregaalatory failure. If we accept this explanation of pH toxicity and the hypothesis that stmation is accompanied by a progressive weakening of the osrnoregulatory system, it follows that stmation should be accompmied by a reduction in tolerance to low pH conditions. Evidence for this effect has recently been presented by Kwain et al. (1984) who showed that the pH LCs0 (96 h) of youngof-year smallmouth bass increased progressively as starvation time increased. Can. .J0Fish. Aqua. Sci., Vo1. 43, 1986
We thank the following for their help with this work: J. A. M a c k a n , H. A. Regier, G . Beggs, J . M. Casselmaw, J. Fraser, R. Hancock, H. H. H w e y , P. Hhssen, F. Hicks, D. Jeffrey, and D. Wales. Financial support was provided by the Ontario Ministry of Natural Resources and the University of Toronto.
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Can, J . Fish. Aquat. Sci.,
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