Aquaculture 184 Ž2000. 349–361
www.elsevier.nlrlocateraqua-online
Blood oxygen transport, rheology and
haematological responses to confinement stress in
diploid and triploid Atlantic salmon, Salmo salar
Joanne Sadler
b
a,)
, Rufus M.G. Wells b, Patricia M. Pankhurst a ,
Ned W. Pankhurst a
a
School of Aquaculture, UniÕersity of Tasmania, Launceston, Tasmania 7250, Australia
School of Biological Sciences, The UniÕersity of Auckland, PriÕate Bag 92019, Auckland, New Zealand
Accepted 20 September 1999
Abstract
Diploid and triploid all-female Atlantic salmon Ž Salmo salar . smolts were challenged with 2.5
h of confinement stress in aerated seawater. Subsequent stress responses were quantified by
plasma cortisol, glucose, and lactate, and by the haematological parameters haematocrit ŽHct., red
blood cell count ŽRBCC., mean cell volume ŽMCV., blood haemoglobin concentration ŽHb., mean
cell haemoglobin ŽMCH., mean cell haemoglobin concentration ŽMCHC., total protein and
erythrocyte adenosine triphosphate ŽATP.. The magnitude of stress responses was similar between
diploid and triploid smolts. Triploid smolts displayed a higher MCH, but lower Hb than diploid
smolts. The blood oxygen affinity of diploid and triploid fish was similar at 158C over the pH
range 6.76–7.99 and the isohaemoglobin components were identical. The enlarged triploid
erythrocytes showed reduced shear-dependence on blood viscosity at constant Hct and are
therefore unlikely to contribute to greater peripheral vascular resistance. The results show that
despite having fewer, larger erythrocytes, triploids have very similar oxygen carrying capacity and
haematological response to stress as diploids. This suggests that the higher farm mortality reported
for triploids in response to stress is not generated by their failure to show respiratory homeostasis
in the face of stress. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Atlantic salmon; Haematology; Stress; Triploidy; Oxygen transport; Viscosity
)
Corresponding author. Tel.: q61-363-2438-62; fax: q61-363-2438-04; e-mail: [email protected]
0044-8486r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 4 - 8 4 8 6 Ž 9 9 . 0 0 3 2 1 - X
350
J. Sadler et al.r Aquaculture 184 (2000) 349–361
1. Introduction
The induction of triploidy in salmonid fish has become a valuable tool in the
Tasmanian aquaculture industry. In Canada, the production of triploid salmonids has
been proposed to address concerns regarding the environmental impact of domestic
escapees ŽBenfey, 1999.. All-female triploid Atlantic salmon Ž Salmo salar . remain
sexually immature, subsequently energy investment in somatic growth is not hindered by
metabolic costs of sexual maturation ŽBenfey, 1999.. As a result, somatic growth
continues in triploids and flesh quality is maintained over the period when diploid fish
are sexually maturing and undergoing associated reduction in flesh quality. In Tasmania,
all-female triploid Atlantic salmon are routinely used to extend the annual harvest period
and thereby improve commercial returns ŽJungawalla, 1991..
In order to accommodate the extra genetic material, triploid fish are composed of a
smaller number of larger cells, though outwardly the fish cannot be distinguished from
natural diploids. Several researchers have called attention to the theoretical problem of
carrying out basic metabolic processes in enlarged cells Žreviewed by Benfey, 1999..
Diffusion processes and receptor binding chemistry, for example, may be affected by the
reduced surface area to volume ratio of enlarged cells.
Fish erythrocytes are nucleated and contain the oxygen-binding protein haemoglobin,
which is critical for the delivery of oxygen to metabolising tissues. Triploid salmonids
possess fewer and larger erythrocytes ŽBenfey and Sutterlin, 1984; Benfey et al., 1984.,
and thus erythrocyte count and haematocrit ŽHct. measurements are a less useful
measure of oxygen carrying capacity than is haemoglobin concentration. Atlantic salmon
triploids have lower blood haemoglobin concentration ŽBenfey and Sutterlin, 1984.,
although this is not always the case with triploids of other species, including some
salmonids ŽBenfey, 1999..
Additional factors affecting blood oxygen transport which might be influenced by
erythrocyte dimensions are the rheological flow characteristics of blood Žincluding
viscosity and erythrocyte deformability., and blood oxygen affinity Žnotably regulation
by pH and erythrocyte ATP.. These aspects are poorly understood in triploid fish, yet
they are likely to be important in regulating oxygen delivery during the stresses imposed
by handling and exercise, confinement, and temperature and oxygen perturbations.
It appears that triploid salmonids may be more sensitive to the stresses imposed by
handling and seawater transfer during routine farm management, and suffer higher
mortality rates under sub-optimal environmental conditions ŽJohnson et al., 1986; Quillet
and Gaignon, 1990; Jungawalla, 1991; Yamamoto and Iida, 1994a; Ojolick et al., 1995..
Therefore, we examined the above-aspects of the blood oxygen transport system with a
view to discerning whether differences in the respiratory component of the stress
response might account for higher mortality in triploids. Accordingly, we have subjected
diploid and triploid Atlantic salmon to handling and confinement stress Žsee Pickering,
1992., and compared haematological variables, and the plasma metabolites glucose and
lactate before and after the stress regime. Further, the response of blood oxygen affinity
to pH, and the effect of shear rate on blood viscosity, were assessed to evaluate
functional differences, if any, between the two groups.
J. Sadler et al.r Aquaculture 184 (2000) 349–361
351
2. Materials and methods
2.1. Fish production and maintenance
All-female diploid Ž2N. and triploid Ž3N. smolts were produced using standard
commercial techniques ŽJohnstone et al., 1991. carried out at the SALTAS Wayatinah
Hatchery in Tasmania. At 14 months post-fertilisation, smolts were transferred to the
School of Aquaculture, University of Tasmania, where 2N and 3N populations were
each maintained in a 2000-l tank with a closed recirculating seawater system at
12.5 " 0.58C and a stocking density of 15 kg my3 . Water quality and nutritional
regimen followed commercial protocols. Fish were acclimated to seawater conditions for
4 weeks prior to experimentation. Mean body mass Ž"SE, n s 20. for 2N and 3N fish
was 78.1 " 11.1 and 76.4 " 15.6 g, respectively.
2.2. Stress induction
All fish were left undisturbed and fasting for 24 h prior to manipulations Žs‘rested’
fish.. Ten ‘rested’ 2N and 10 ‘rested’ 3N fish were randomly sampled by scoop net
from each tank. A further 10 2N and 10 3N fish were exposed to a stress protocol
known to elevate plasma cortisol values ŽPickering, 1992.. These fish were separated
according to ploidy status, subjected to handling and confined for 2.5 h, in one of two
20-l containers of aerated seawater, prior to blood sampling. Approximately 500 ml of
blood was sampled immediately from the caudal vein and placed on ice in paediatric
vials containing K 2 EDTA anticoagulant, and analysed as described below. Ploidy status
of each fish sampled was confirmed by determining mean erythrocyte nucleus length
ŽBenfey et al., 1984..
2.3. Plasma steroid measurement
Plasma cortisol was measured by radioimmunoassay after extraction with ethyl
acetate as described by Pankhurst et al. Ž1992.. Extraction efficiency, determined as the
recovery of 3 H-labelled steroid extracted with plasma, was 94% and assay values were
corrected accordingly. The assay detection limit was 2.4 ng mly1 plasma and interassay
variability Ž%CV., using a pooled internal standard, was 5.4% Ž n s 3..
2.4. Haematology and plasma chemistry
Haematocrit ŽHct., haemoglobin ŽHb., and red blood cell counts ŽRBCC. were
determined using standard haematological techniques ŽDacie and Lewis, 1984. with the
added step of centrifugation to remove cell debris for Hb determination. Mean cell
haemoglobin concentration ŽMCHC. was calculated from wHbxrŽHctr100., mean cell
volume ŽMCV. from HctrRBCC, and mean cell haemoglobin ŽMCH. from wHbxrRBCC.
Plasma glucose was estimated using a standard enzymatic test kit based on the
hexokinase reaction Ž15-u.v., Sigma, St. Louis, USA., and plasma lactate using the
enzymatic test kit no. 826-u.v. ŽSigma.. Plasma protein was determined using the biuret
reaction ŽSigma test kit no. 541.1. with a certified albuminrglobulin standard.
352
J. Sadler et al.r Aquaculture 184 (2000) 349–361
Total cellular RNA was isolated from 50 ml blood samples using Trizol reagent ŽLife
Technologies, USA.. The single-step RNA isolation method is based on a phenol–
guanidine thiocyanate reagent ŽChomczynski, 1993.. Purified RNA was quantified from
the relationship A 260 nm of 1.0 s 40 mg RNA mly1 . All samples showed A 260:280 ) 1.7
and were therefore judged to be free from protein contamination. Total cellular RNA for
each blood sample was divided by RBCC to obtain red cell RNA. The contribution of
white cell RNA to total cellular RNA was thought to be negligible.
2.5. Haemoglobin oxygen transport
Functional evaluation of the blood oxygen transport system was carried out by
determination of oxygen affinity using pooled blood from five individuals from diploid
and triploid populations. Blood was taken from undisturbed fish by rapid caudal
venepuncture into a heparinised syringe. Oxygen equilibrium curves were measured
using a series of 50 mmol ly1 Hepes buffers at constant Ž125 mmol ly1 . chloride
concentration ŽWeber, 1992., and decreasing pH Ž6.76–7.99., in a modified tonometric
system ŽWells and Weber, 1989.. Oxygen saturation was determined for each pH level
at 158C with a 5-nm bandpass Novaspec II spectrophotometer for at least six points of
equilibration, and the affinity coefficient, P50 , and cooperativity coefficient, n 50 , were
determined by interpolation and slope of Hill plots according to Weber and Wells
Ž1989.. The effect of pH on oxygen transport at various oxygen partial pressures was
evaluated from the Bohr factor, Ø, s Dlog P50 rDpH. The Root effect was estimated
from the change in saturation Ž P100 . with decreasing pH using the method of Pelster and
Weber Ž1990..
Total nucleoside triphosphates were extracted from red blood cells using cold 12%
trichloroacetic acid and the extract was analysed using an enzymatic NTP test kit
ŽSigma, no. 366-u.v... The method does not distinguish ATP from other nucleoside
triphosphates, but ATP is essentially the only NTP in salmonid erythrocytes ŽWells and
Weber, 1990..
Isoelectric focusing of haemoglobin components from 2N and 3N populations was
carried out on lysates prepared for the PhastGele ŽPharmacia. system in the pH range
3–9. Gels were unstained in order to eliminate non-haemoglobin proteins from analysis
and immediately scanned for analysis.
2.6. Blood Õiscosity
Viscosity was measured in 500 ml aliquots of erythrocyte suspensions from 2N and
3N fish using a cone-plate viscometer with a cone angle of 88 Žmodel LVTD CPr11,
Brookfield Engineering Laboratories, USA., capable of shear rates from 2.3–450 sy1 .
The temperature range of the sample cup was regulated to 15.0 " 0.28C using a
circulating water bath. Calibration of the viscometer was checked with Brookfield
standards, and found to be within specification. The technical problem of erythrocyte
aggregation at low shear rates arises from the bridging effects of large plasma protein
molecules such as fibrinogen and globulins ŽFletcher and Haedrich, 1987. and was
avoided by resuspending erythrocytes in Cortland’s solution ŽWolf, 1963., a physio-
J. Sadler et al.r Aquaculture 184 (2000) 349–361
353
logical saline. The viscosity of blood samples with adjusted Hct values was also
measured. Erythrocytes pooled from five fish in each ploidy group were separated from
plasma by light centrifugation Ž1200 = g . and resuspended in Cortland’s solution to
provide a range of Hct values.
2.7. Statistical analysis
Data from blood haematology experiments were tested for normality and equal
variances within treatments using Bartlet’s test prior to analysis. A two-way ANOVA
analysis Ž P s 0.05. was used to determine the effects of ploidy and confinement stress
on respective parameters. We acknowledge the experimental design did not preclude
possible tank effects, however, precluding fluctuation of parameters due to photoperiod,
feeding status and disturbance was of greater concern. A Welch ANOVA was also used
for data with unequal variances ŽJMP 3.1 Software.. Student’s t-test Ž P s 0.05. was
used to compare blood viscosity and corresponding Hct values between diploids and
triploids ŽExcel Software.. A three-way ANOVA was used to compare the effects of
ploidy, Hct and shear rate on viscosity.
3. Results
3.1. Plasma steroid measurement
Plasma cortisol levels of all fish exposed to 2.5 h of confinement stress were
increased approximately two-fold above ‘rested’ values Ž P - 0.05, Table 1.. Thus, both
diploid and triploid smolts displayed a significant primary endocrine response to stress,
but there was no difference in the response between 2N and 3N fish Ž P ) 0.05.. These
results indicate it is unlikely that there was any tank effect within treatments.
Table 1
Haematology and plasma metabolites in diploid and triploid Atlantic salmon subjected to confinement stress.
Data are expressed as means"SD, ns10
All-female diploids
Hct Ž%.
Hb Žg ly1 .
RBCC Ž10 6 ly1 .
MCHC Žg ly1 .
MCV Žfl.
MCH Žpg.
Glucose ŽmM.
Lactate ŽmM.
Protein Žg%.
Cortisol Žng mly1 .
a
b
All-female triploids
‘Rested’
Stressed
‘Rested’
Stressed
32.5"3.5
83.7"17.3
0.94"0.15
259.9"57.8
35.4"6.7
90.8"23.2
3.3"0.7
0.89"0.13
2.37"0.92
24.6"6.1
35.7"6.9
79.7"9.6
0.97"0.12
227.5"29.7
36.9"6.4
82.8"10.0
4.7"1.2 b
3.67"0.79 b
1.83"0.72
75.5"11.6 b
35.4"6.3
68.8"7.4 a
0.69"0.13 a
201.8"49.4
47.8"6.1a
102.6"24.9 a
4.0"1.1
0.79"0.14
1.93"0.62
32.6"5.0
33.5"4.9
71.9"8.7 a
0.66"0.09 a
214.3"15.2
51.1"4.0 a
111.0"12.9 a
5.3"1.5 b
3.73"1.1b
2.40"0.53
74.3"15.7 b
Triploids significantly different from diploids Ž P - 0.05..
Stressed significantly different from ‘rested’ Ž P - 0.05..
354
J. Sadler et al.r Aquaculture 184 (2000) 349–361
3.2. Haematology and plasma chemistry
Comparisons of haematological measurements, plasma glucose, lactate, and protein
for ‘rested’ and stressed 2N and 3N fish are summarised in Table 1. Triploid fish had
lower RBCC, greater MCV, lower wHbx and higher MCH than did diploid fish
Ž P - 0.05.. Confinement stress did not result in any changes to the haematological
profile within ploidy groups Ž P ) 0.05.. Plasma glucose and lactate increased as a result
of confinement stress Ž P - 0.05., but no differences were detected between ploidy
groups Ž P ) 0.05.. Total plasma proteins remained constant in all groups. Red blood
cell RNA values were similar for both diploid and triploid ‘rested’ fish Žmean " SD s
36.8 " 5.9 and 36.1 " 2.0 pg RBCy1 , respectively..
3.3. Haemoglobin oxygen transport
Functional studies failed to reveal differences between 2N and 3N fish. The effect of
pH on haemoglobin–oxygen affinity Ž P50 ., Hill’s cooperativity coefficient Ž n 50 ., and
Fig. 1. Blood haemoglobin–oxygen binding data at 158C for diploid and triploid S. salar smolts showing Ža.
the pH-dependence of oxygen affinity Ž P50 ., Žb. cooperativity coefficient Ž n 50 ., and Žc. Root effect. Data for
Ža. and Žb. are from blood pooled from five fish in each group and each point is the average of duplicate
determinations. Data from Žc. are mean values"SD for ns 5.
J. Sadler et al.r Aquaculture 184 (2000) 349–361
355
the Root effect Ž P100 . are shown in Fig. 1, where the Bohr factors Ž Dlog P50rDpH.
were y0.48 Ž2N. and y0.40 Ž3N., and n 50 max was 2.3 Ž2N. and 2.4 Ž3N.. In addition,
whole blood ATP, the principal allosteric regulator of haemoglobin–oxygen affinity in
salmonids ŽWells and Weber, 1990., was similar for both 2N and 3N fish Žmean " SD
s 1.78 " 0.08 and 1.58 " 0.37 mmol ly1 , respectively..
Isoelectric focusing revealed four equivalent bands for both 2N and 3N fish and thus
the expression of isohaemoglobin components was identical.
3.4. Blood Õiscosity
Viscosity was strongly dependent on Hct with 3N salmon erythrocytes showing lower
viscosity at high Hcts ŽFig. 2.. The effect was most marked at low shear rates. Viscosity
readings could not be obtained at high shear rates for the highest Hct samples.
Fig. 2. Dependence of viscosity on shear rate and Hct for erythrocyte suspensions at 158C from diploid and
triploid S. salar smolts. Blood sample for each ploidy group pooled from five fish.
356
J. Sadler et al.r Aquaculture 184 (2000) 349–361
4. Discussion
4.1. Haematology
Atlantic salmon subjected to confinement stress showed elevated levels of plasma
cortisol, and the magnitude of the increase was similar in both diploid and triploid fish.
Our results are in close agreement with those of Biron and Benfey Ž1994. obtained
following acute handling stress in diploid and triploid brook trout Ž SalÕelinus fontinalis.,
and confirm that the primary endocrine response to stress in the triploid fish follows the
typical salmonid response ŽPickering, 1992; Mazur and Iwama, 1993; Pankhurst and
Van Der Kraak, 1997..
The haematological profile of fewer, and larger, erythrocytes in the triploid salmon is
also consistent with findings for other triploid species ŽBenfey and Sutterlin, 1984;
Graham et al., 1985; Small and Randall, 1989; Biron and Benfey, 1994; Yamamoto and
Iida, 1994a,b.. The effect of triploidy on haemoglobin concentration, a direct measure of
oxygen carrying capacity of the blood, appears equivocal and varies between species
Žsee Benfey, 1999.. Nonetheless, our finding of reduced wHbx in triploid salmon is
consistent with earlier observations from S. salar ŽBenfey and Sutterlin, 1984; Graham
et al., 1985. and other salmonids ŽSmall and Randall, 1989; Yamamoto and Iida,
1994b.. Given the similarity in cortisol responses, but important differences in erythrocytes between the ploidy groups, we were surprised to find no significant haematological
changes following confinement stress. In the only other study of biochemical and
physiological responses to confinement stress in triploid fish, Biron and Benfey Ž1994.
found no change in Hct, although other haematological characteristics were not reported.
By contrast, Virtanen et al. Ž1990. reported a markedly greater increase in Hct, and
reduction in MCHC, from triploid rainbow trout forced to swim in a flume. These
responses to exercise are well-described in salmonids and are adrenergically mediated,
resulting in splenic contraction, and activation of an erythrocyte surface proton exchange
causing the cells to swell ŽWells and Weber, 1990, 1991; Randall and Perry, 1992.. The
relative contributions of the initial and rapid adrenergic flush, and the slower, more
persistent cortisol response are complex and depend on the nature and duration of stress
imposed. The responses are qualitatively and quantitatively different in confined fish and
fish swimming freely ŽLowe and Wells, 1996.. We would therefore anticipate that
adrenergic responses are fully manifested in ‘rested’ fish acutely sampled, and that
cortisol effects will persist beyond the restoration of baseline catecholamine levels
ŽGamperl et al., 1994.. Further, there occur complex interactions between the two
endocrine responses at the level of the erythrocyte which appear to regulate oxygen
transport in response to either acute or chronic stress ŽPerry and Reid, 1993.. Accordingly, it is most likely the corticosteroid rather than the adrenergic stress response which
has been evaluated in our study and is relevant to fish husbandry protocols.
4.2. Plasma metabolites
The increase in plasma glucose and lactate with handling and confinement stress of
both diploid and triploid fish indicated a typical salmonid response, and confirms the
J. Sadler et al.r Aquaculture 184 (2000) 349–361
357
rise in plasma glucose for triploid brook trout under confinement stress ŽBiron and
Benfey, 1994.. The persistence of altered states of glucose, lactate, and protein, and
recovery rates following stress are well-defined indicators of the robustness of the stress
response ŽBarton, 1997; Morgan and Iwama, 1997.. This response is an important
metabolic indicator of available energy reserves, and healthy rainbow trout may show
elevated plasma lactate for up to 24 h post-stress ŽPankhurst and Dedual, 1994..
Disturbances to plasma protein, however, occur only under nutritional stress or extreme
physical disturbance ŽWood et al., 1983; Wells et al., 1986., and were not noted for
either diploid or triploid salmon in the present study.
Given the higher DNA content of triploid cells Žreviewed by Benfey, 1999., we were
interested to see whether RNA content indicated a significant post-translational activity
in the triploid erythrocytes of the Atlantic salmon. Total RNA content in the epaxial
muscle of S. salar appears to be a useful index of somatic growth rate ŽArndt et al.,
1994. and recent measurements confirm increased production of RNA in muscle cells of
triploid rainbow trout, Oncorhynchus mykiss ŽSuresh and Sheehan, 1998.. The similarity
of RNA content in 2N and 3N erythrocytes was therefore unexpected. However, protein
turnover in erythrocytes is unlikely to match that of muscle cells during phases of rapid
growth.
4.3. Blood oxygen transport
Multiple haemoglobin components are functionally heterogeneous in salmonids and
their oxygen binding is regulated by the intracellular concentration of ATP ŽWeber and
Wells, 1989.. Analysis of the haemoglobin components and erythrocyte ATP in Atlantic
salmon revealed no differences between diploid and triploid populations. The oxygen
transport characteristics of blood from diploid and triploid salmon were similar with
respect to the pH-dependence of the oxygen affinity coefficient, P50 , and cooperativity
coefficient, n 50 ŽFig. 1.. These data indicate that in response to oxygen demand, tissue
oxygen unloading is effectively regulated in both groups by the Bohr effect in response
to carbon dioxide and lactate loading. Moreover, both groups showed similar dependence of oxygen saturation Ž P100 . on pH ŽRoot effect. allowing for oxygen secretion to
the eye ŽIngermann, 1982.. An earlier observation reporting similarity in P50 for diploid
and triploid S. salar was determined at a single pH value above the physiological range
ŽGraham et al., 1985..
Accordingly, the responses of the oxygen transport system in triploids to stresses
induced by exercise, anaesthesia, acute or chronic hypoxia, and hypercapnia can be
compensated by allosteric phosphate regulation, and the Bohr effect, as is the case in
diploid salmonids ŽSoivio et al., 1980; Bushnell et al., 1984; Milligan and Wood, 1987;
Weber and Wells, 1989..
Triploid erythrocytes are greater in length and width than those of diploids but not in
depth ŽBenfey, 1999., hence, it is likely that oxygen diffusion across the erythrocyte
surface is compromised neither in the gills nor the tissues. Nevertheless, in a different
experiment, a high incidence of branchial abnormalities was observed in the triploid
population sampled in the present study ŽSadler, J., Pankhurst, P. and King, H.,
unpublished data.. The occurrence of jaw deformities in older triploid fish ŽSutterlin et
358
J. Sadler et al.r Aquaculture 184 (2000) 349–361
al., 1987; King and Lee, 1993; McGeachy et al., 1996; O’Flynn et al., 1997. and
branchial abnormalities might be expected to result in compensation in haematological
and oxygen transport characteristics.
4.4. Blood Õiscosity
Blood viscosity contributes to vascular resistance and hence is a determinant of the
cardiac output required to generate adequate peripheral blood circulation and oxygen
delivery to tissues. Fish erythrocytes in suspension show complex, non-Newtonian
behaviour thus, viscosity decreases with increasing shear rate and reducing Hct ŽGraham
and Fletcher, 1985; Fletcher and Haedrich, 1987.. Moreover, fish display a remarkable
interspecies diversity in erythrocyte dimensions which is reflected in their rheological
behaviour such that smaller erythrocytes tend to show less shear-dependence on
viscosity ŽWells and Forster, 1989; Wells and Baldwin, 1990; Baldwin and Wells,
1990.. However, erythrocyte size is not constant in salmonids, and adrenergically
stress-mediated increases in MCV are thought to modulate haemoglobin–oxygen affinity
and lower erythrocyte viscosity ŽWells and Weber, 1991; Wells et al., 1991.. Further,
erythrocytic deformability in rainbow trout increases with adrenergic swelling during
exercise and hypoxia ŽHughes and Kikuchi, 1984.. Confinement stress was shown to
reduce blood viscosity in a marine teleost ŽPankhurst et al., 1992.. The larger erythrocytes in triploid Atlantic salmon appear to parallel these effects and the most marked
effects of triploid cell viscosity occurred at low shear rates Žequivalent to blood flow in
vivo., and higher Hct. Aside from shear dependence, the lower viscosity of triploid
compared to diploid cells at constant Hct is in part the consequence of the colligative
nature Ždependence on particle number. of viscosity. Given that blood vessel diameters
do not appear affected by ploidy status ŽBenfey, 1999., the improved shear dependence
of larger triploid erythrocytes should not result in increased vascular resistance in
capillary beds or in the branchial lamellae.
4.5. Conclusion
Despite having fewer and enlarged erythrocytes, all-female triploid Atlantic salmon
showed only slightly reduced oxygen carrying capacity and haematological responses to
a confinement stress challenge were similar to diploids. The stress response, as indicated
by plasma cortisol, glucose, and lactate concentrations, was equivalent for both groups.
Further, haemoglobin–oxygen affinity, Bohr and Root effects, and the isohaemoglobin
pattern were similar for both diploids and triploids. The larger triploid erythrocytes,
however, showed lower shear dependence on blood viscosity, and thus oxygen transport
is unlikely to be compromised. The similarity of the haematological response to stress in
diploid and triploid fish suggests that the higher mortality reported for triploids reared
under farm conditions is not generated by their failure to show respiratory homeostasis
in the face of stress. The greater heterozygosity displayed by triploid salmonids
ŽAllendorf and Leary, 1984. may result in different responses to environmental perturbations, although there is little evidence for this ŽOliva-Teles and Kaushik, 1987, 1990a,b;
Yamamoto and Iida, 1994a; McCarthy et al., 1996; Stillwell and Benfey, 1996a,b,
1997..
J. Sadler et al.r Aquaculture 184 (2000) 349–361
359
Acknowledgements
This research was funded by an APAIR grant awarded to P.M. Pankhurst, ARC
infrastructure funding allocated to N.W. Pankhurst and was supported by Salmon
Enterprises of Tasmania ŽSALTAS., Wayatinah, Tasmania. We thank Polly Hilder for
conducting RIAs and for technical assistance in fish husbandry, Mark Hilder for
technical assistance in fish husbandry, and the staff of SALTAS for their assistance and
provision of the smolts.
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