Journal of Experimental Marine Biology and Ecology 348 (2007) 183 – 190
www.elsevier.com/locate/jembe
Influence of salinity on survival, growth, plasma osmolality and gill
Na + –K + –ATPase activity in the rabbitfish Siganus rivulatus
I. Patrick Saoud ⁎, Sawsan Kreydiyyeh, Antoine Chalfoun, Mazen Fakih
Department of Biology, American University of Beirut, Bliss Street, Beirut, Lebanon
Received 7 October 2006; received in revised form 30 April 2007; accepted 3 May 2007
Abstract
Rabbitfish (Siganidae) are an Indo-Pacific family of teleosts considered very suitable for aquaculture. The rabbitfish Siganus
rivulatus has established populations in the Eastern Mediterranean and in now part of the commercial fishery in some
Mediterranean countries. In the present work, we studied the salinity tolerance of S. rivulatus and the effect of various salinities on
plasma osmolality and gill Na+–K+–ATPase activity (NKA). Three salinity experiments were performed. In the first experiment,
fish were maintained at nine salinities between 10 ppt and 50 ppt for 3 weeks and survival and plasma osmolality evaluated. In the
second experiment fish were maintained at salinities of 25, 30, 35 and 40 ppt for 6 weeks to study survival, growth, blood
osmolality and gill NKA. In the third experiment, fish were maintained at salinities of 10, 15, 20 and 25 ppt for 8 weeks and again
survival, growth, blood osmolality and gill NKA were estimated. Results of the first experiment show that S. rivulatus can survive
for 3 weeks at salinities ranging from 10 ppt to 50 ppt and that they can maintain a relatively stable blood osmolality (between 398
and 435 mmol kg− 1). In the second experiment we observed no differences in survival and growth of S. rivulatus in all treatments.
However, gill NKA was lowest at 35 ppt and increased at salinities above and below 35 ppt. In the third experiment, survival was
similar at all salinity treatments but growth decreased slightly at 10 ppt. Gill NKA increased as salinity decreased. Results show
that S. rivulatus is a highly euryhaline fish and a very strong osmoregulator. Furthermore, although iso-osmolality is at 14.6 ppt,
NKA activity is lowest at 35 ppt, which is the salinity in the Indo-Pacific where the fish evolved but not in the Mediterranean where
juveniles for the present experiment were collected. Thus, we conclude that 35 ppt is the optimum salinity for S. rivulatus.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Na+–K+–ATPase; Osmolality; Rabbitfish; Salinity
1. Introduction
Siganids (rabbitfish) are a relatively small family of
herbivorous fishes widely distributed in the Indo-West
Pacific Region (Woodland, 1983). They are economically important and relatively easy to rear and are thus
considered suitable for aquaculture (Juario et al., 1985;
⁎ Corresponding author. Tel.: +961 1 350 000x3919.
E-mail address: [email protected] (I.P. Saoud).
0022-0981/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jembe.2007.05.005
Hara et al., 1986). The opening of the Suez Canal in
1869 linked the Red Sea to the Mediterranean and
resulted in the invasion of the eastern Mediterranean Sea
by various siganid species (Galil, 2000; Quignard and
Tomasini, 2000). Siganus rivulatus Forsskål, 1775 was
first recorded in the Levant basin in 1927 (Tortonese,
1970). The species has since established large populations in its new environment and can be considered
amongst the most successful of Lessepsian fishes (BenTuvia, 1985; Papaconstantinou, 1990). Although the
life history of S. rivulatus is well studied (see Bariche,
184
I.P. Saoud et al. / Journal of Experimental Marine Biology and Ecology 348 (2007) 183–190
2005), its salinity preferences are not documented. The
species is well established in the Indian Ocean where
salinity is approximately 35 ppt, in the eastern Mediterranean where salinity is between 37 and 39 ppt, and in
the red sea where salinity is between 39 and 41 ppt.
Environmental factors such as salinity strongly
influence fish growth (Morgan and Iwama, 1991;
Kuwaye et al., 1993; Gaumet et al., 1995; Bœuf and
Payan, 2001). Osmoregulation is energy consuming. In
fish it is carried out mainly via branchial chloride cells
that utilize the Na+ gradient established by the Na+–K+–
ATPase (NKA) on their basolateral side to drive Cl− and
Na+ extrusion from saltwater regulating fish (Marshall
and Bryson, 1998). A reduction in fish growth when
reared in non-optimal salinity waters could be due to an
increase in NKA activity and concomitant energy expenditure (Bœuf and Payan, 2001; Sampaio and Bianchini,
2002). The present study had two goals. One was to
determine optimal salinity for S. rivulatus, and the second was to study the effect of salinity on blood osmolality and branchial NKA activity.
2. Materials and methods
2.1. Preliminary trial
The present work was performed at the marine biology laboratory at the American University of Beirut,
Lebanon. A preliminary salinity tolerance experiment
was performed in which six rabbitfish were placed in
each of nine aquaria filled with seawater at 35 ppt. Each
aquarium was fitted with a submerged biological filter
and an air diffuser. Salinity in the aquaria was reduced at
a rate of 5 ppt per 12 h every day and fish allowed to
acclimate to the new salinity for 12 h until endpoint
salinities of 10, 15, 20, 25, 30, 35, 40, 45 and 50 ppt
were attained. The fish were offered a 35% protein feed
(Zeigler Bros., Inc. Gardners, PA) at 3% body weight
daily divided into two equal morning and evening rations. They were maintained in the aquaria for 3 weeks
and 50% of the water in each aquarium was replaced
with seawater of the same salinity every 4 days. At the
end of the 3 weeks, fish were harvested and anaesthetized with MS-222. Blood from each fish was taken
using heparinised capillary tubes after caudal severance.
The osmolality of the blood and water from each aquarium were estimated using a Wescor 5520 vapor pressure
osmometer.
2.1.1. First fish growth experiment
Fish were reared in a system composed of four batteries of four 180 L square tanks (60 × 60 × 50 cm;
L × W × H). Each battery was connected to a common
biofilter and settling tank equipped with a 300 W submersible heater that maintained water temperature
at 28 °C. Water was recirculated between the filter and
the tanks using a submersible pump and aeration was
provided via a regenerative blower and submersed air
diffusers. Oxygen concentration, salinity and temperature were measured twice daily and ammonia and
nitrite concentration twice weekly using the Solorzano
(1969) method and Parsons et al. (1985) method,
respectively.
Juvenile rabbitfish, S. rivulatus, were captured in
traps in Batroun Bay and transported to the lab where
they were size sorted by hand, divided randomly into
four equal groups and then each group divided among
the four tanks of a battery at 35 ppt. Salinity in two of the
recirculating systems were reduced to 30 ppt and 25 ppt
over 3 days using dechlorinated tap water and the salinity in the fourth system was allowed to increase to
40 ppt through evaporation. Fish in each system were
then pooled and small and large fish removed. Sorted
fish 12.9 g ± 0.1 g (mean ± SE) were randomly divided
into 4 groups of 12 fish. Each group was weighed and
stocked into one of the 4 square fiberglass tanks from
which the fish had just been removed. Twenty of the fish
remaining in each salinity treatment were individually
weighed and total length measured for condition index
estimation. Stocked fish were offered a 35% protein feed
(Zeigler Bros., Inc. Gardners, PA) at 3% body weight
daily divided into two equal mornings and evenings
rations. Fish in every tank were captured and group
weighed weekly and ration size increased accordingly.
Six weeks after the start of the experiment, fish were
harvested, group weighed, individually weighed and
individual total length recorded. Four fish from every
tank were then randomly selected for osmolality and
NKA activity measurements. Remaining fish were returned to their respective tanks.
2.1.2. Second fish growth experiment
The same battery of tanks and experimental procedures described above were used for this experiment.
However, salinities evaluated were 10, 15, 20 and
25 ppt. Rabbitfish were captured in the same area as in
the first experiment and treated similarly. The fish at
each salinity were sorted and 13 fish (7.4 g ± 0.3; mean ±
SE) stocked into each tank. Fish were offered a 50%
protein, 20% lipid trout feed at 2.5% daily ration divided
into two feedings. The experiment was terminated after
8 weeks. Fish were harvested, group weighed, individually weighed and individual total length recorded. Four
fish from every tank were then randomly selected for
I.P. Saoud et al. / Journal of Experimental Marine Biology and Ecology 348 (2007) 183–190
osmolality and NKA activity measurements. Remaining
fish were returned to their respective tanks.
2.1.3. Blood and gill sampling
Fish were anaesthetized using MS222 and blood was
taken using heparinised capillary tubes after caudal
severance. The fish were then killed by spinal transaction and gills removed. The gill filaments were dissected
from the gill arches using a scalpel and the cartilage free
tissue was homogenized in ice-cold Tris buffer (200 mM
NaCl; 5 mM MgCl2 · 6H2O; 2 mM EGTA;
5 mM KCl; 200 mM Tris-HCl, pH 7.4) using a Polytron
homogenizer at 26,000 rpm for 4 min. Protease inhibitor
was added to each sample and it was placed on ice and
allowed to settle for 15 min. Aliquots were then removed for protein quantification and to assay for NKA
activity.
2.1.4. Plasma osmolality measurement
Capillary tubes with blood samples were centrifuged
to separate cells from plasma. Plasma osmolality was
then determined using a Wescor 5520 vapor pressure
osmometer and reported as mOsmol kg− 1. Water samples from each recirculating system were taken using the
heparinised capillary tubes centrifuged similarly to the
blood samples and their osmolality determined.
2.1.5. Na+–K+–ATPase activity measurement
Samples of crude gill homogenate were diluted with
Tris buffer to a concentration of 0.5 mg protein ml− 1 and
assayed for the NKA activity. Protein quantification was
performed using the Bio-Rad protein assay (Bio-Rad
Laboratories, 2000 Alfred Nobel Drive, Hercules, CA
94547, USA) and bovine albumin was used as a
standard. After a 30 min incubation period with saponin
Table 1
Water osmolality (mmol kg− 1), blood osmolality and standard error of
mean of blood osmolality of rabbitfish maintained for 3 weeks at
various salinities
185
Table 2
Initial weight (Wti), initial length , initial Fulton condition index,
weight at harvest, length at harvest, and condition at harvest, of
Siganus rivulatus reared at four different salinities
Wti (g)
Li (cm)
CIi
Wtf (g)
Lf (cm)
CIf
A
25 ppt
30 ppt
35 ppt
40 ppt
PSE
9.1
9.1
9.1
9.1
9.5
9.5
9.5
9.5
1.05
1.05
1.05
1.05
23.3
22.7
24.0
24.1
0.36
12.5
12.4
12.2
12.3
0.09
1.2a
1.2a
1.3b
1.3b
0.02
B
10 ppt
15 ppt
20 ppt
25 ppt
PSE
7.4
7.4
7.4
7.4
9.0
9.0
9.0
9.0
1.01
1.01
1.01
1.01
29.10a
29.98a,b
30.70b
31.38b
0.404
12.30b
13.00a
13.13a
13.15a
0.169
1.33a
1.30a
1.33a
1.30a
0.018
Values with different letters within a column are significantly different
from each other (P b 0.05).
PSE = Pooled standard error.
(0.02% final concentration) at room temperature, aliquots from each sample were pre-incubated at 37 °C for
10 min in presence or absence of ouabain, and for an
additional 1 h in presence of ATP (12.5 mM). The
reaction was terminated by addition of an equal volume
of trichloroacetic acid (11.5%) and samples were centrifuged at 3000 ×g for 5 min. The amount of inorganic
phosphate liberated in the supernatant was measured
colorimetrically according to Taussky and Shorr (1953).
Since ouabain is a specific inhibitor of the NKA, enzyme activity was assayed by measuring the amount of
ouabain-inhibitable inorganic phosphate liberated. Preliminary results indicated that NKA activity at salinities
of 15 and 20 ppt were similar so NKA activity measurements were repeated using two extra fish from every
tank at the two salinities and results added to the data
set.
2.2. Statistical analysis
Salinity (ppt) Water osmolality Blood osmolality Standard error
10
15
20
25
30
35
45
50
PSE
261
412
532
656
794
926
1185
1333
–
398a
394a
395a
408a
404a
401a
416a
435b
5.46
7.16
4.00
5.66
4.21
3.33
6.07
3.14
10.04
Values in the same column with similar superscript are not
significantly different from each other.
PSE = pooled standard error.
All statistical analyses were performed using SPSS
statistical software and a level of significance of 0.05.
The Fulton condition index (CI) of the fish was calculated as: CI = 100 W/L3, where W = fish weight (g) and
L = total length (cm). Survival, growth, CI, blood
osmolality and Na+–K+–ATPase activity were analyzed
using a one-way ANOVA and Student Newman–Keuls
multiple-range test to determine significant differences
(P b 0.05) among treatment means. Osmolality and
NKA activity at 25 ppt was compared among the two
growth experiments using ANOVA and were similar.
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I.P. Saoud et al. / Journal of Experimental Marine Biology and Ecology 348 (2007) 183–190
Table 3
Tank water osmolality (mmol kg− 1) and blood osmolality (mmol kg− 1)
and standard error of mean of blood osmolality of rabbitfish Siganus
rivulatus reared at four salinities
Salinity
Water osmolality
Blood osmolality
Standard error
A
25 ppt
30 ppt
35 ppt
40 ppt
PSE
716
870
1028
1180
383.6a
387.0a
393.6a,b
408.3b
5.56
5.04
6.08
6.00
5.51
B
10 ppt
15 ppt
20 ppt
25 ppt
PSE
241
382
527
693
378.6a
373.4a
416.8b
419.8b
6.02
6.38
3.31
3.65
8.94
Values with different letters within a column are significantly different
from each other (P b 0.05).
PSE = Pooled standard error.
Accordingly, data for the 25 ppt treatment from the two
experiments were pooled.
3. Results
3.1. Preliminary trial
One fish in the tank with 40 ppt salinity died on day
18 of the experiment. Water quality in the tank deteriorated and the fish were excluded from the data set.
Blood osmolality of fish at salinity 50 ppt was 435 mmol
kg− 1, significantly larger than blood osmolality of fish
at all other salinities (P b 0.05). Blood osmolalities of
fish in all other treatments were not significantly different from each other although there was a slight increasing trend of plasma osmolalities with an increase in
salinity (Table 1).
3.2. Fish growth experiments
There were no differences in survival (99%–100%)
among fish reared at 25, 30, 35 and 40 ppt. There were
also no statistical differences in length (P N 0.05) or
weight (P N 0.05) among treatments (Table 2A). However, the CI of the fish reared at 35 ppt and at 40 ppt
were significantly greater (P b 0.05) than CI of fish
reared at 25 ppt and at 30 ppt (Table 2A). Plasma
osmolality of fish reared at 40 ppt was significantly
greater than that of fish reared at 25 ppt and at 30 ppt
(Table 3A). However, the range of osmolalities
remained within that observed in the preliminary trial.
Survival was 100% in all tanks when fish were reared
at 10, 15, 20 and 25 ppt. The final weight of fish was
significantly greater at 20 and 25 ppt than at 10 ppt
(P b 0.05) (Table 2B). Also, final length of fish was
significantly less at 10 ppt than at all other treatments
but was not different among fish reared at 15, 20
and 25 ppt. There were no differences in CI among
treatments. Plasma osmolality at 20 and 25 ppt was
significantly greater than at 10 and 15 ppt (P b 0.05)
(Table 3B).
Fig. 1. Plasma osmolality of Siganus rivulatus maintained at salinities from 10 ppt to 50 ppt (values are pooled from the three experiments).
Osmolality at 50 ppt is significantly greater than at all other salinities. No differences were observed among plasma osmolalities at salinities between
10 ppt and 45 ppt.
I.P. Saoud et al. / Journal of Experimental Marine Biology and Ecology 348 (2007) 183–190
Table 4
Na+–K+–ATPase activity (μg Pi mg protein− 1 h− 1) in gill tissue of
rabbitfish Siganus rivulatus reared at various salinities
Salinity (ppt)
10
15
20
25
30
35
40
PSE
Na+–K+–ATPase activity
a
283.16
200.76b
191.88b
132.14c
95.86d
69.63e
212.52b
6.49
Standard error
14.36
10.64
8.14
3.09
4.36
5.07
6.64
Values with different letters within a column are significantly different
from each other (P b 0.05).
PSE = Pooled standard error.
3.3. Osmolality and Na+–K+–ATPase activity
Blood osmolality of fish reared at 40 ppt (408.3 mmol
kg− 1) was significantly greater than that of fish reared
at 25 and 30 ppt (383.6 and 387.0 mmol kg− 1, respectively). Also, blood osmolality of fish reared at 20
and 25 ppt (416.8 and 419.8 mmol kg− 1, respectively)
was significantly greater that blood osmolality of fish
reared at 10 and 15 ppt (378.6 and 373.4 mmol kg− 1,
respectively). The data from the three experiments were
pooled and plotted against salinity (Fig. 1). The slope
and intercept of the graphs for water and blood osmolality were determined and used to calculate a salinity
of 14.6 ppt (388.2 mmol kg− 1) as point of isosmolality
for S. rivulatus.
Na+–K+–ATPase activities in gill tissue of fish
maintained in various water salinities were significantly
187
different from each other (P b 0.05) (Table 4; Fig. 2).
Gill NKA activity was greatest in fish reared at 10 ppt
(283.16 μg Pi mg protein− 1 h− 1), decreased with increasing salinity until 35 ppt (69.63 μg Pi mg protein− 1
h− 1), then increased again in gills of fish reared at
40 ppt. There was no significant difference in NKA
activity among fish reared at 15 ppt and fish reared at
20 ppt. If NKA results at 15 ppt are removed from the
data set and a line is plotted through means of NKA
activity between 10 ppt and 35 ppt, the resultant is a
straight line with a regression coefficient R2 = 0.989.
4. Discussion
Effects of salinity on fish growth vary greatly among
fish and among salinity ranges tested (see review by
Bœuf and Payan, 2001). In the present study, salinities
between 25 ppt and 40 ppt did not affect the growth rate
of S. rivulatus. This fact might be attributed to the
eurihalinity of S. rivulatus or to a relatively short experimental period. Kelly et al. (1999) found that salinity did
not affect growth of black sea bream after 42 days of
culture but did have an effect on growth after 84 days of
culture. Furthermore, most researchers compared effects
of salinities below, close to and above isosmotic levels
on fish osmoregulation and/or growth (Kelly et al.,
1999; Rodríguez et al., 2002; Lin et al., 2003; LaizCarrión et al., 2005). In the present study, all salinities
evaluated but one were above the salinity of isosmolality (14.6 ppt) and that might partly explain the lack of
growth differential in a relatively short experimental
period. Fish maintained at 10 ppt did grow slower than
fish at other salinities.
Fig. 2. Na+–K+–ATPase activity (μg Pi.mg protein− 1 h− 1) of gill tissue of S. rivulatus reared at salinities between 10 ppt and 40 ppt.
188
I.P. Saoud et al. / Journal of Experimental Marine Biology and Ecology 348 (2007) 183–190
Fish growth can be expressed as change in length or
change in weight. Both factors were only affected by a
salinity below the point of isosmolality. However, fish
condition as expressed by a weight to length relationship
(Fulton's condition index) was significantly affected by
salinity. Fulton's condition index was significantly greater
at 40 ppt and 35 ppt than it was at 25 and 30 ppt, thus
suggesting a preference of the fish for high salinity. In
contrast, Laiz-Carrión et al. (2005) found that salinity
affected growth of gilthead sea bream, Sparus aurata, but
not condition index. These results hint that comparison of
effects of salinity on teleostan performance under culture
conditions is not feasible since various species of fish
respond differently to salinity changes even if they inhabit
similar natural environments. Moreover, various researchers conduct their experiments at different salinity regimes,
making comparisons of results even more complicated.
For example, Bœuf and Payan (2001) list various species
of fish and optimal salinity for growth ranges from 5 ppt
for the Atlantic croaker (Peterson et al., 1999) to 30 ppt for
the Amarillo snapper, Lutjanus argentiventris (SerranoPinto and Caravea-Patiño, 1999), both marine fishes.
Further complications are seen when comparing contradicting results of experiments performed on the same fish
species. Swanson (1998) reports an optimal salinity for
the growth of milkfish Chanos chanos as 55 ppt while
Alava (1998) reports 0 ppt as an optimal salinity for
growth of the same fish. Due to such discrepancies, and
the fact that this is the first reported work on salinity
tolerance of S. rivulatus, we will refrain from further
comparisons of salinity effects on growth between present
work and that of other researchers.
Blood osmolality in teleost fish is ≈ 280–360 mmol
kg− 1, and is tightly regulated in a species-dependent
range of salinities (Varsamos et al., 2005). Fish blood
osmolality in the present study ranged between
398 mmol kg− 1 and 435 mmol kg− 1, in fish reared at
salinities from 10 ppt to 50 ppt, respectively. Blood
osmolality was significantly greater at 50 ppt than at
other salinities, indicative of the strong osmoregulatory
capacity of the fish. Lin et al. (2003) found no effect of
salinities ranging from 0 ppt to 35 ppt on blood osmolality of milkfish and Hwang et al. (1989) found no
difference in blood osmolality of Oreochromis mossambicus reared in freshwater or in saltwater. Laiz-Carrión
et al. (2005) found a small but significant difference in
blood osmolality of gilthead sea bream reared at 12 ppt
and 6 ppt but no difference between blood osmolalities of
fish reared at 12 ppt and 38 ppt. Results of the present
experiment show that in some fish, fish have to be reared
in extreme salinities before plasma osmolality variations
are observed.
The present study strengthens the argument that
teleostan fish tend to be strong osmoregulators. The
slope of the curve depicting blood osmolality increase
with salinity is much smaller than that depicting the
increase in water osmolality (0.95 vs. 26.4; Fig 1). Similar
results are reported by Sampaio and Bianchini (2002)
working with the flounder Paralichthys orbignyanus.
They state that when salinity only slightly affects plasma
osmolality, a species is probably adapted to face the
salinities it osmoregulated in. Therefore, our results suggest that S. rivulatus is adapted to survive in low salinity
estuaries in India as well as in the high salinity environments of the Red Sea and Eastern Mediterranean. Along
the coast of Lebanon, S. rivulatus lives near shore where it
can graze on algae growing on shallow rocks. These areas
range in salinity from 20 ppt near mouths of seasonalrivers to 38 ppt during the dry summer months (unpublished data). Fish that live in such areas need to be
adapted to flourish in a range of salinities. However, rivers
in Lebanon flow only during the cold winter months and
salinity tolerance might be affected by temperature.
Imsland et al. (2003) reported a significant effect of
temperature on osmoregulation of turbot Scophthalmus
maximus. They found that when reared at 10 °C, 14 °C
and 22 °C, salinity had a significant effect on osmoregulation, but at 18 °C salinity did not affect blood osmolality. In the present experiment temperature was
maintained at 28 °C but coastal temperature ranges from
12 °C to 30 °C. Results presented herein might not be
applicable during cold winter months. However, our
results suggest that S. rivulatus tolerate salinities between
10 ppt and 40 ppt when reared in closed aquaculture
systems where temperature is maintained at it’s upper
limits to encourage growth.
Gaumet et al. (1995) suggested that NKA activity is
generally lowest in fish living in a medium whose
salinity is equivalent to that of the blood. However, here
again the reports in the literature cannot be compared to
each other. Some papers report no effect of salinity on
NKA activity (Yoshikawa et al., 1993; Kelly et al.,
1999) while others report a strong effect of medium
salinity on gill NKA activity (Piermarini and Evans,
2000; Rodríguez et al., 2002; Imsland et al., 2003).
Further confusion arises in that some researchers report
a positive correlation between environmental salinity
and NKA activity (Kültz et al., 1992;Uchida et al.,
1997) while others find a negative correlation between
water salinity and NKA activity (Marshall et al., 1999;
Lin et al., 2004). In the present study, NKA activity
in gill tissue was significantly affected by water salinity. Activity was lowest in fish reared at 35 ppt and
higher in fish reared at salinities lower and higher than
I.P. Saoud et al. / Journal of Experimental Marine Biology and Ecology 348 (2007) 183–190
35 ppt. Similar U-shaped curves were also reported by
(Yoshikawa et al., 1993; Lin et al., 2004; Laiz-Carrión
et al., 2005). However, these researchers reported lowest
NKA activity at salinities close to isosmotic levels for
the fish while in the present experiment, gill NKA
activity increased as the environmental salinity decreased from 35. An ecologically sound theory would
state that fish would be adapted to spend the least
amount of osmoregulatory energy in environmental
salinities they evolved to live in. However, physiologically we would expect the energy consuming NKA
activity to be minimal at environmental salinities isosmotic to blood. The review by Bœuf and Payan (2001)
cites conflicting reports in the literature regarding effects
of isotonic salinities on metabolic rate of fish. Further
research on the topic is warranted if we are to understand
the physiological processes involved with osmoregulation when comparing environmental salinities to isosmotic salinities.
Although S. rivulatus growth was similar at all salinities tested in the present experiment, results suggest
that the fish perform better at 35 ppt than at other
salinities. Condition index at 35 ppt was greater than at
lower salinities and NKA activity associated with energetic loss due to osmoregulation was lowest at 35 ppt.
As suggested by Kültz et al. (1992), 34 ppt appears to
represent a threshold at which NKA activity starts increasing rapidly with an increase in salinity. Therefore,
we propose using a salinity of 35 ppt as a control for
future research into salinity tolerances of S. rivulatus
and as salinity of choice in experiments to study other
environmental tolerances such as temperature, culture
density, diet etc.
Acknowledgements
We wish to thank the Lebanese National Council for
Scientific Research and the American University of
Beirut Research board for funding the present project.
We would also like to thank Dr. John Grizzle and Dr.
Allen Davis for their advise and comments.[SS]
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