Open Journal of Marine Science, 2012, 2, 167-176
http://dx.doi.org/10.4236/ojms.2012.24020 Published Online October 2012 (http://www.SciRP.org/journal/ojms)
Effect of Salinity on Growth of Mussels, Mytilus edulis,
with Special Reference to Great Belt (Denmark)
Hans Ulrik Riisgård*, Line Bøttiger, Daniel Pleissner
Marine Biological Research Centre, University of Southern Denmark, Kerteminde, Denmark
Received August 14, 2012; revised September 14, 2012; accepted September 28, 2012
ABSTRACT
The effects of salinities between 10 and 30 psu on the growth of blue mussels, Mytilus edulis, were studied in laboratory
feeding experiments and compared to the growth of mussels suspended in net-bags in the brackish water Great Belt,
Denmark. In the laboratory, 3 series of growth experiments were conducted: in Series #1, groups of mussels were ex-
posed to 10, 15, 25 and 30 psu, in Series #2, two groups of mussels were exposed to 10 and 30 psu, respectively, for 15
days (first period) where upon the mussels were exposed to the reversed salinities for another 15 days (second period).
In Series #3, two groups of mussels were initially exposed to 15 and 25 psu for 22 days whereupon the mussel groups
were exposed to the reversed salinities for another 17 days. In the laboratory experiments there was a tendency towards
reduced growth with decreasing salinity, reflected as reduced shell growth rate and decreasing weight specific growth
rate with falling salinity. The shell growth rate was relatively low in the first feeding period compared to the second
period, and mussels that were initially exposed to 10 psu, where the growth was low, exhibited fast growth when subse-
quently exposed to 30 psu, and reversed when 30 psu mussels were exposed to 10 psu. The study showed that mussels
are able to adjust growth at changing salinities, and the observed effect of salinity could partly be explained by a tempo-
rary shell valve closure after a sudden change in salinity. The specific growth rate of mussels measured in laboratory
experiments at salinities between 15 to 25 psu (4.2% to 4.8% d–1) were comparable to the growth of mussels in the field
experiment (3.2% to 4.0% d–1) where the salinity varied between 24 and 13 psu during the growth period.
Keywords: Mussels; Mytilus edulis; Salinity Effects; Growth Rates; Condition Index; Doubling Time
1. Introduction
The blue mussel, Mytilus edulis, is a common member of
the filter-feeding zoobenthos of coastal and estuarine
intertidal areas where it may often encounter different
salinities. Salinity is a key environmental factor that may
directly influence the feeding behaviour and growth of
mussels. Thus, constantly low salinities, frequency and
amplitude of salinity changes, as well as the changing-
rate of salinity will influence the behavioural response
(siphon and shell valve closure), the filtration activity,
actual growth rate, the maximal size, and the early deve-
lopment and survival of mussels [1-9].
Due to low salinities in the inner Baltic Sea (6 to 8 psu
in the northern Baltic proper) Mytilus edulis is dwarfed in
this area [10-12], and in the northern Baltic Sea along the
southern coast of Finland where the salinity decreases
from 6.5 psu in the west to become only 3 psu in the east,
M. edulis lives at the margin of its salinity tolerance, and
here a marked decline in mussel size can be observed
along the decreasing salinity gradient [8]. Davenport [3]
studied the response of both sudden and slowly falling
seawater salinities on M. edulis’ exhalant-siphon and
shell valve closure. The behavioural response of the
mussel was found to be dependent upon the rate of
changes in salinity rather than being triggered by a fall to
a critical salinity. At a changing rate of 16.75 psu·h–1
(which may be compared to 10 to 12 psu·h–1 in the Con-
way estuary, UK, [13]), Davenport [3,4] found it likely
that oxygen depletion in the mantle water may be de-
layed “as a result of the mussels’ partial isolation re-
sponse to falling salinity”. The effect of fluctuating salin-
ity with a rate of 5.33 psu·h–1 (sinusoidal variation be-
tween 0 and 32 psu during 6 h) on shell growth of small
M. edulis has been studied by Almada-Villela [5] who
found that the growth was “markedly depressed”. Further,
Almada-Villela [5] studied the long term (38 d) effect on
shell growth of different lowered salinities, and found
that growth was significantly depressed at 6.4 and 16 psu,
but not at 22.4 psu. Likewise, Hiebenthal et al. [14] re-
cently found that M. edulis performed best at 25 psu and
further that temperature effects may interact with salinity
effects. However, acclimation to reduced salinities may
take place [3,4,9,15-17], but due to shell valve closure
*Corresponding author.
C
opyright © 2012 SciRes. OJMS

H. U. RIISGÅRD ET AL. 168
and reduced filtration rate in the acclimation period, the
resulting temporarily reduced growth may blur the actual
ability of mussels to grow more or less unrestrained at
low salinities, possibly down to 10 psu as studied in the
present work.
Great Belt is one of the Danish Straits that form the
transition between the tidal North Sea and the non-tidal
Baltic Sea. The water exchange between the Baltic Sea
and the open sea is driven both by the river run-off and
by the meteorological conditions over the North Sea-
Baltic Sea area [18-20]. In Great Belt the salinity varies
according to changing flow situations; outflow of water
from the Baltic Sea gives salinities down to less than 10
psu whereas inflow to the Baltic Sea gives salinities up to
27 psu in the upper layer [20-22]. Frequencies and am-
plitudes of salinity changes above the permanent halo-
cline in the northern Great Belt appear from Table 1
which shows data for the 10 year period 2000 to 2010.
The mean salinity is about 17 psu, major salinity changes
of about 8.5 psu take place with an annual frequency of
about 10, a mean duration time of 21 d, and with a mean
salinity changing rate of 0.4 psu·d–1. Therefore, in order
to evaluate the potential for future line-mussel cultivation
in the Great Belt it has been of interest to know the possi-
ble effect of the frequently changing salinities on the
growth rate of mussels, which nevertheless seem to be
able to reach the full size of marine mussels (10 cm shell
length of 5 years old line-mussels; Mads Anker van
Deurs, Nordshell, personal communication).
In the present work, we studied the effect of different
salinities (between 10 and 30 psu) on Mytilus edulis in
laboratory feeding and growth experiments, and the ob-
tained growth rates have been compared to the actual
growth of mussels suspended in net-bags in Great Belt.
2. Materials and Methods
2.1. Laboratory Feeding and Growth
Experiments
Blue mussels, Mytilus edulis, were collected in Kerte-
minde Bugt in the autumn of 2009 and transported to the
nearby Marine Biological Research Centre (SDU) in
Kerteminde, Denmark, where the feeding and growth
experiments were conducted on groups of 20 or 30 mus-
sels (mean shell length about 20 mm) in 16 l aquaria with
seawater of different salinity (10, 15, 25 or 30 psu) ad-
justed by means of distilled water or salt (Read Sea Ma-
rine Salt) added to natural sea water, measured by means
of a salinity meter (YSI 30). A dosing pump supplied the
growth aquaria holding the experimental mussels with
suspension of pure algae (the about 6 µm diameter flag-
ellate Rhodomonas salina from a batch culture) which
were kept homogeneous by strong aeration with 4 air
stones. The growth experiments were carried out as 3
series, each performed at a well-defined algal concentra-
tion, adjusted by means of the dosing pump within the
concentration interval where the mussels exploit their
filtration capacity (i.e. above lower trigger concentration
leading to closure of the valves, and below the satiation
concentration where the mussels shut down [23-26] thus
ensuring a steady-state between the addition and removal
(feeding by mussels, wash out with through flow) of al-
gal cells [27]. The algal concentration was measured by
means of an electronic particle counter (Elzone 5380)
several times a day during the experimental period to
ensure that the concentration of Rhodomonas salina in the
stock solution flask could be adjusted to be near constant.
Three series of growth experiments were conducted. In
Series #1, 4 groups of mussels were exposed to 10, 15,
Table 1. Annual mean (Smean), maximum (Smax) and minimum (Smin) salinities, frequency (fS) of major (>4 psu) salinity
changes, amplitude of major salinity changes (ΔS), duration time of major salinity changes (Δt), and salinity changing rates
(ΔSr = ΔS/Δt) in the period 2000 to 2010 measured above the permanent halocline in the northern Great Belt (STB 53, Fyn
County 2001, Figure 8.4 therein) by the Danish Nature Agency, Danish Ministry of the Environment. Standard deviations (±)
are indicated.
Year Smea n (psu) Smax (psu) Smin (psu) fS (year−1) ΔS (psu) Δt (d) ΔSr (psu·d−1)
2000 17.2 ± 4.5 27.1 10.7 7 10.0 ± 3.4 15 ± 9 0.7
2001 16.3 ± 3.3 22.6 10.6 15 8.5 ± 1.2 22 ± 9 0.4
2002 16.1 ± 4.4 24.8 9.6 12 7.7 ± 2.3 17 ± 11 0.5
2003 17.2 ± 4.2 25.6 9.9 12 9.7 ± 2.1 21 ± 10 0.5
2004 17.4 ± 3.8 24.2 10.7 10 8.7 ± 2.1 29 ± 17 0.3
2005 16.0 ± 3.5 25.3 10.5 11 7.7 ± 2.7 18 ± 10 0.4
2006 16.7 ± 3.5 24.1 10.1 10 8.9 ± 2.1 20 ± 9 0.4
2007 16.4 ± 3.4 23.2 10.8 9 7.5 ± 2.3 21 ± 10 0.4
2008 16.8 ± 4.6 25.2 9.4 8 9.6 ± 2.1 28 ± 15 0.3
2009 17.4 ± 3.9 24.4 10.5 11 8.0 ± 2.1 23 ± 14 0.3
2010 15.4 ± 3.7 22.4 10.2 10 7.7 ± 1.8 16 ± 5 0.5
Mean 16.6 ± 0.6 24.4 ± 1.4 10.3 ± 0.5 10 ± 2 8.5 ± 0.9 21 ± 5 0.4 ± 0.1

H. U. RIISGÅRD ET AL. 169
25 and 30 psu, respectively, for 14 days to allow the
mussels to adjust, and subsequently during the following
17 days a mean steady-state algal concentration was es-
tablished in order to compare the growth rate of the
mussels. The shell length and the dry weight of soft parts
were determined at the beginning and at the end of the
experiment. In Series #2, two groups of mussels were
exposed to 10 and 30 psu, respectively, for 15 days
whereupon the 2 mussel groups were exposed to the re-
versed salinities for another 15 days. At regular intervals
during the experiment, the shell length was measured,
and 5 mussels were taken out on Day 15 for determina-
tion of body dry weight. In Series #3, 2 groups of 30
mussels were initially exposed to 15 and 25 psu for 22
days whereupon the 2 mussel groups were momentarily
exposed to the reversed salinities for another 17 days. At
regular intervals during the experiment, the shell length
was measured, and 5 mussels were taken out on Day 10,
15, 22, 30, and 32 for determination of body dry weight.
All experiments were conducted at 12˚C to 15˚C. No
mortalities of experimental mussels were observed.
2.2. Field Growth Experiment
In the present study, the growth rate of Mytilus edulis
sampled in Great Belt was compared with the growth of
mussels collected in Limfjorden where the salinity is
generally higher (about 25 to 28 psu in the central parts,
e.g. [28,29]) than in the Great Belt surface water (about
17 to 21 psu during summer, [20], Table 1) in which the
field growth experiment was performed. Small mussels
(18 to 23 mm) Mytilus edulis were obtained from line-
mussel farms in the Great Belt (near Svendborg) and in
Limfjorden (Hvalpsund) two days before the onset of a
field growth experiment conducted in the period 9 Sep-
tember to 30 October 2009 with 3 size groups of mussels
in net bags at Stavreshoved (Great Belt, 55˚28.36'N,
10˚44.60'E, depth 8.5 m). The mussels were measured
with a vernier gauge, sorted into size groups (Great Belt:
shell length 18.6 ± 0.3 mm and 23.1 ± 0.3 mm; Limfjor-
den: 20.1 ± 0.3 mm) and put into net bags (Go Deep In-
ternational Inc.) before they were transferred to the field
location and hung up in a buoy system 1 m below the
water surface, and weights were tight to the net bags to
stabilize them in a vertical position. The net bags (widths
of masks = 10 × 10 mm) were made of polypropylene
fibres and cotton strings that disintegrate after about 1
week which result in an increase of mask width. The net
bags were 50 cm long and placed approximately 1 m
apart to avoid entanglement. Samples were subsequently
collected with about 14 days interval from the buoy sys-
tems and transported to the Marine Biological Research
Centre, Kerteminde, for analysis. Dry weight of shells
and soft parts were measured by removing the soft parts
from the shells, measure the wet weight and then drying
the soft parts on pieces of tin foil in an oven for 24 h at
90˚C; shell lengths were measured with a vernier gauge.
2.3. Equations
The condition index (CI, mg·cm–3) of the mussels was
calculated from the dry weight of soft parts (W, mg) and
the shell length (L, cm) according to the formula:
3
W
CI L  (1)
The growth of an exponentially growing organism can
be expressed by the equation: Wt = Woeµt where Wo and
Wt is the body dry weight on Day 0 and Day t, respec-
tively, and thus the specific growth rate (μ, d–1) can be
calculated according to the equation:
1
ln t
o
W
µ
W




t
(2)
or obtained from slope of regression line for semi-ln plot
of W versus time.
The time (t2, d) for doubling the dry weight of soft
parts of any given size of mussel is given by the equa-
tion:
2
ln 2
t

 (3)
2.4. Statistical Analysis
Statistical analyses of impact of salinity on specific
growth rate, doubling time and shell growth rate (Tables
2-4) were carried out by “One way repeated measures
analysis of variance (ANOVA)” in Sigma Plot.
3. Results
3.1. Laboratory Studies
The measured increase in shell length, weight specific
growth rate, and doubling time of mussels grown at dif-
ferent salinities in steady-state feeding experiments are
shown in Tables 2-4. The experimental data from Series
#2 are depicted in Figures 1 and 2. From Figure 1 it
appears that the mussels closed their shells when the sa-
linities were changed from 10 to 30 psu and from 30 to
10 psu on Day 15 and that they remained more or less
closed (as easily observed by eye) during the following
week. On Day 22 the mussels were again fully open and
filtering thus allowing the steady-state feeding condition
to be re-established. It appears from the first growth pe-
riod that the weight specific growth rate was low at 10
psu, µ =1.6% d–1, compared to µ = 5.6% d–1 at 30 psu,
and further, after transfer of mussels from high to low and
vice versa, this resulted in increased and reduced growth
at the high and low salinity, respectively (Table 3). Fig-

H. U. RIISGÅRD ET AL. 170
Table 2. Mytilus edulis. Growth of mussels (Series #1) constantly exposed to 10, 15, 25 or 30 psu at maintained algal concen-
tration (C) during the whole experimental time (t) inparallel steady-state experiments. Lo and Lt = initial and final shell length,
respectively; Wo and Wt = initial and final body dry weight, respectively; ΔL/t = daily increase in shell length; µ [= ln(Wt/Wo)t–1
× 100] = weight specific growth rate; t2 (=ln2/µ) = doubling time. Mean ± S.D., n = 20.
Series S (psu) t (d) C (cells·ml–1) Lo (mm) Lt (mm) ΔL/t (mm·d–1) Wo (mg) Wt (mg) µ (% d–1)t2 (d)
#1A 10 17 2761 ± 910 20.0 ± 0.421.3 ± 0.50.071 22.9 ± 3.751.1 ± 8.1 4.7 14.8
#1B 15 17 2614 ± 1257 19.6 ± 0.321.3 ± 0.50.100 26.7 ± 2.840.7 ± 5.8 2.5 27.7
#1C 25 17 2353 ± 1212 19.5 ± 0.322.0 ± 0.70.147 26.7 ± 2.843.2 ± 5.4 2.8 24.8
#1D 30 17 2852 ± 722 20.0 ± 0.422.7 ± 0.70.159 22.9 ± 3.772.0 ± 7.7 6.7 10.4
Table 3. Mytilus edulis. Growth of mussels (Series #2) exposed to initially 10 and 30 psu whereupon the salinities were inter-
changed during the following 15 d at maintained algal concentration (C) during experimental time (t) in steady-state experi-
ments. Lo and Lt = initial and final shell length, respectively; Wo and Wt = initial and final body dry weight, respectively; ΔL/t
= daily increase in shell length; µ = weight specific growth rate (from slopes in Figure 2); t2 = doubling time. Mean ± S.D., n =
20.
Series S (psu) t (d) C (cells·ml–1) Lo (mm) Lt (mm) ΔL/t (mm·d–1)Wo (mg) Wt (mg) µ (% d–1)t2 (d)
#2A 10 15 2978 ± 1684 19.5 ± 0.319.6 ± 0.30.01 16.8 ± 2.821.5 ± 4.2 1.6 43.3
#2B 30 15 1934 ± 874 19.5 ± 0.320.4 ± 0.40.06 16.8 ± 2.839.5 ± 6.3 5.6 12.4
#2C 10 to 30 15 3212 ± 1000 19.5 ± 0.321.3 ± 0.20.12 21.5 ± 4.166.3 ± 10.3 6.2 11.2
#2D 30 to 10 15 2578 ± 720 20.3 ± 0.421.3 ± 0.40.06 39.5 ± 6.369.8 ± 8.6 3.1 22.4
Table 4. Mytilus edulis. Growth of mussels (Series #3) exposed to initially 15 and 25 psu during 22 d whereupon the salinities
were interchanged during the following 11 d at maintained algal concentration (C) during experimental time (t) in
steady-state experiments. Lo and Lt = initial and final shell length, respectively; Wo and Wt = initial and final body dry weight,
respectively; ΔL/t = daily increase in shell length; µ = weight specific growth rate (from slopes in Figure 5); t2 = doubling time.
Mean ± S.D., n = 20.
Series S (psu) t (d) C (cells·ml–1) Lo (mm) Lt (mm) ΔL/t (mm·d–1)Wo (mg) Wt (mg) µ (% d–1)t2 (d)
#3A 15 22 3351 ± 1407 19.6 ± 0.320.0 ± 0.50.02 12.5 ± 2.233.4 ± 8.1 4.6 15.1
#3B 25 22 3185 ± 1371 19.5 ± 0.320.3 ± 0.60.04 12.5 ± 2.234.8 ± 6.6 4.8 14.4
#3C 15 to 25 11 2959 ± 961 20.0 ± 0.521.5 ± 0.90.14 33.4 ± 8.154.0 ± 9.9 4.0 16.5
#3D 25 to 15 11 3533 ± 1697 20.3 ± 0.621.6 ± 0.80.12 34.8 ± 6.652.8 ± 10.1 4.2 17.3
L= 0.004t+ 19.45
R² = 0.993
L= 0.157t+ 16.08
R² = 0.995
L= 0.060t+ 19.45
R² = 0.981
L= 0.079t+ 18.62
R² = 1
18
19
20
21
22
23
0510 15 20 25 30 35
Shell length, L( mm)
Time , t(d)
10 30 30 10
Figure 1. Mytilus edulis. Mean (± SD) increase in shell length
(L) of mussels in Series #2 (Table 3) where the mussels in 2
parallel growth experiments were exposed to 10 and 30 psu,
respectively, during the first 15 days, whereupon the salini-
ties were changed from 10 to 30, and from 30 to 10 psu,
respectively. The data are split into 2 growth periods: Pe-
riod I with closed symbols for 10 and 30 psu, Period II with
open symbols for mussels transferred from 10 to 30 psu,
and from 30 to 10 psu, respectively. Linear regression lines
and their equations are shown. The slopes of the lines indi-
cate the daily increase in shell length (mm·d–1). During the
time period indicated with dotted lines the mussels were
closed or not fully open.
lnW= 0.016t+ 2.823
lnW= 0.062t+ 2.129
lnW= 0.056t+ 2.823
lnW= 0.031t+ 3.200
1
2
3
4
5
0510 15 20 25 30 35
ln W
Time , t(d)
10 30
30 10
Figure 2. Mytilus edulis. The natural logarithm (ln) of mean
dry weight of soft parts (W, mg) of mussels in Series #2 (Ta-
ble 3). The data are split into 2 growth periods: Period I
with closed symbols, Period II with open symbols. Linear
regression lines and their equations are shown. The slopes
of the lines indicate the weight specific growth rate (µ, d–1).
ure 3 shows that the condition index increased during the
growth period, from 2 to about 7 mg·cm–3. The growth
data from Series #3 are depicted in Figures 4 and 5, and
it is notable that the weight specific growth rate was near
constant, between 4.0% and 4.8% d–1, during the whole

H. U. RIISGÅRD ET AL. 171
0
1
2
3
4
5
6
7
8
9
10→30 30→10
CI (mg cm
-3
)
Day 0Day 15Day 33
CI (mg·cm
-3
)
Figure 3. Mytilus edulis. Condition index (CI) for mussels in
two parallel growth experiments, Series #2 (Table 4) where
the salinity was changed from 10 to 30 psu, and decreased
from 30 to 10 psu, respectively. Mean + S.D., n = 20.
15, L= 0.023t+ 19.44
R² = 0,864
25, L= 0.109t+ 17.85
R² = 0.956
25, L= 0.037t+ 19.37
R² = 0.880
15, L= 0.122t+ 17.50
R² = 0.860
18
19
20
21
22
23
0510 15 20 25 30 35
Shell length, L(mm)
Time, t(d)
15 25 25 15
R
2
= 0.864
Figure 4. Mytilus edulis. Mean (±S.D.) increase in shell
length (L) of mussels in Series #3 (Table 4). The data are
split into 2 growth periods: Period I with closed symbols,
Period II with open symbols. Linear regression lines and
their equations are shown. The slopes of the lines indicate
the daily increase in shell length (mm·d–1).
15, ln W= 0.046t+ 2.495
R² = 0.997
25, ln W= 0.040t+ 2.651
R² = 0.995
25, ln W= 0.048t+ 2.475
R² = 115, ln W= 0.042t+ 2.552
R² = 0.984
1
2
3
4
5
0510 15 20 25 30 35
ln W
Time , t(d)
15 25 25 15
Figure 5. Mytilusedulis. The natural logarithm (ln) of mean
dry weight of soft parts (W, mg) of mussels in Series #3 (Ta-
ble 4). The data are split into 2 growth periods: Period I
with closed symbols, Period II with open symbols. Linear
regression lines and their equations are shown. The slopes
of the lines indicate the weight specific growth rate (µ, d–1).
growth period irrespective of the salinity, whereas the
increase in shell length was relatively low, 0.02 to 0.04
mm·d–1, during the first period, where the condition in-
dex increased increased from less than 2 to about 6
mg· c m–3 (Figure 6). The doubling times of body dry
weight for mussels grown at different salinities in all the
experiments are shown in Figure 7. Although there was
a tendency of longer doubling times at the lower salini-
ties, it is remarkable that the most pronounced effect of
salinity wasa temporary closure of the shells which lasted
for several days (cf. Figure 1) when the salinity was sud-
denly changed.
Figure 8 shows the growth rate of shells of Mytilus
edulis exposed to different salinities. It is seen that the
growth rate of shell length increased with increasing
salinity when the salinity was kept constant, whereas the
growth was lower in the first period of growth experi-
ments in which the salinity was either increased or de-
creased in the middle of the experiment. The weight spe-
0
1
2
3
4
5
6
7
15→25 25→15
CI (mg cm
-1
)
Day 0Day 10Day 22Day 26Day 33
CI (mg·cm
-1
)
Figure 6. Mytilus edulis. Condition index (CI) for mussels in
two parallel growth experiments, Series #3 (Table 4) where
the salinity was changed from 15 to 25 psu, and from 25 to
15 psu, respectively. Mean + S.D.
Figure 7. Mytilus edulis. Doubling time of body dry weight
for mussels grown at different salinities during 11 to 22
days on Rhodomonas salina cells. Regression line, its equa-
tion, and the 95% confidence interval (dashed lines) are in-
dicated. Data from Tables 2-4.

H. U. RIISGÅRD ET AL. 172
0
0.04
0.08
0.12
0.16
0.2
0 10203040
Shell growth (mm d
-1
)
Salinity (psu)
S1
S2 P1 -10
S2 P1 -30
S2 P2 -10
S2 P2 -30
S3 P1 -15
S3 P1 -25
S3 P2 -15
S3 P2 -25
Shell growth (mm·d
-1
)
Figure 8. Mytilus edulis. Increase rate of shell length of
mussels fed with algal cells and exposed to different salini-
ties. Symbols: S1 = Series #1; S2P1-10 = Series #2, Period I,
10 psu, etc.
cific growth rates of mussels exposed to different sali-
nities in the various growth experiments are shown in
Figure 9 which indicate that there was a tendency of
reduced growth with decreasing salinity, and it is striking
that mussels exposed to 10 psu in the first period were
growing slowly (1.6% d–1) but when the water was
changed to 30 psu, the mussels obtained a fast growth
rate (6.2% d–1). Likewise, mussels that were initially
exposed to 30 psu had a fast growth rate of 5.6% d–1, but
when the salinity was reduced to 10 psu, the growth rate
decreased to 3.1% d–1.
One way repeated measures ANOVA revealed that the
specific growth rate (P = 0.034) and shell growth rate (P
= 0.028) are statistically significant different and the hy-
pothesis of reduced specific growth rate and shell growth
rate with decreasing salinity (Tables 2 and 3, at 10 and
30 psu, and change from 30 to 10 psu and reverse) can be
accepted. Doubling times, however, are not statistically
significant different (P = 0.190). Further, one way re-
peated measures ANOVA revealed that the weight spe-
cific growth rates (P = 0.580), shell growth rates (P =
0.084), and doubling times (P = 0.178) are not statistic-
cally significant different, and likewise not between 15
and 25 psu (Tables 2 and 4, at 15 and 25 psu, and for
change from 25 to 15 psu and reverse). Thus, the statis-
tical analysis indicate significant decrease of weight spe-
cific growth rate and shell growth rate when salinity was
changed from 30 to 10 psu and reverse, but not when
changed from 25 to 15 psu and reverse.
3.2. Field Studies
The growth rates of mussels in net bags in the field are
shown on Figures 10 and 11. The increase in shell length
is expressed by the slope of regression lines (Figure 10),
and for Great Belt mussels the growth rate was 0.177 and
0.128 mm·d–1 for the initially 18.6 and 23.1 mm mussels,
respectively, while the shell growth rate of the 20.1 mm
0
1
2
3
4
5
6
7
8
0 10203040
µ (% d
-1
)
Salinity (psu)
S1
S2P1 -10
S2P1 -30
S2P2 -10
S2P2 -30
S3P1 -15
S3P1 -25
S3P2 -15
S3P2 -25
Figure 9. Mytilus edulis. Weight specific growth rate (µ) of
mussels in Series #1 (S1) at different constant salinities, and
in Series #2 & #3 with high/low salinities during first period
(PI), and second period (PII) with reversed salinities, see
Figures 2 & 5. Symbols: S2P1-10 = Series #2, Period I, 10
psu, etc.
L= 0.128t+ 22.96; R
2
= 0.94
L = 0.177t+ 17.85; R
2
= 0.97
L= 0.221t+ 19.65; R
2
= 0.95
0
5
10
15
20
25
30
35
0 20406
Shell length , L(mm)
Time, t (d)
0
GB 23.1 mm
GB 18.6 mm
L 20.1 mm
Figure 10. Mytilus edulis. Increase in mean (±S.D.) shell
length as a function of time of 3 size classes of mussels
transferred from Limfjorden (L) and Great Belt (GB) and
hung up in a buoy system at Stavreshoved (Great Belt). The
initial shell length of the 3 size classes are indicated.
ln W = 0.037t + 4.02; R2= 0.92
ln W = 0.045t+ 3.20; R2= 0.90
ln W= 0.048t+ 3.48; R2= 0.91
0
1
2
3
4
5
6
7
0 20406
ln W
Time, t(d)
0
GB 23.1 mm
GB 18.6 mm
L 20.1 mm
Figure 11. Mytilus edulis. Natural logarithm (ln) to dry
weight of soft parts (W, mg) as a function of time of 3 size
classes of mussels transferred from Limfjorden (L) and
Great Belt (GB) and hung up in a buoy system at Stavre-
shoved (Great Belt). The initial shell length of the 3 size
classes are indicated.

H. U. RIISGÅRD ET AL. 173
mussels from Limfjorden was 0.221 mm·d–1. Likewise,
the weight specific growth rates are expressed by the
slope of regression lines (Figure 11). Thus, the weight
specific growth rate of 20.1 mm mussels from Limfjor-
den was 4.8% d–1 which may be compared with 4.5 and
3.7% d–1 for 18.6 and 23.1 mm Great Belt mussels, re-
spectively. From Figure 12 it appears that the initial
condition index was low, <3 mg·cm–3, but rapidly in-
creased during especially the first 30 days to become
high, about 9 to 12 mg·cm–3. The mussels from Limfjor-
den had the shortest body-dry weight doubling time of t2
= (ln2/0.048 =) 14.4 d, compared to 15.4 and 18.7 d for
the 18.6 and 23.1 mm Great Belt mussels, respectively.
Clearly, the mussels from Limfjorden were not impeded
by being transferred to the lower saline Great Belt.
4. Discussion
In all the laboratory mussel-growth experiments there
was a tendency towards reduced growth with decreasing
salinity, reflected as increasing doubling time of the body
dry weight (Figure 7), reduced shell growth (Figure 8),
and as decreasing weight specific growth rate with fal-
ling salinity (Figure 9). However, it is notable that the
shell growth rate was relatively low in the first feeding
period compared to the second period (Figure 8). This
phenomenon is likely to be due physiological and be-
havioural adaptation correlated with shell valve opening-
closing [1,3,4,16], possibly combined with an increasing
condition index during the experiments (Figures 3 and 6).
It is also striking that mussels initially exposed to 10 psu
where the growth was low (1.6% d–1) exhibited a faster
growth when exposed to 30 psu, and reversed, when 30
psu mussels were exposed to 10 psu (Figure 2, Table
3).
The cumulative shell length increase of small (15 mm)
0
2
4
6
8
10
12
14
16
0816 30 41 51
CI (mg cm
-3
)
Time (d)
L 20.1 mmGB 18.6 mm
GB 23.1 mm
C
I
(mg·cm
-3
)
Figure 12. Mytilus edulis. Condition index (CI, mean + S.D.)
as a function of time of 3 size classes of mussels transferred
from Limfjorden (L) and Great Belt (GB) and hung up in a
buoy system at Stavreshoved (Great Belt). The initial shell
lengths of the 3 size classes are indicated.
Mytilus edulis at different constant lowered salinities, be-
tween 12.8 and 32 psu, was studied in a long-term ex-
periment by Almada-Villela [5] using a laser technique.
It was found that the growth was initially delayed at 12.8
and 16 psu but recovered eventually, and mussels in the
range 19.2 to 32 psu grew at similar rates. The delay of
growth was overcome rapidly for mussels in 16 psu
whereas those in 12.8 psu “suffered severely from low
salinity” (i.e. shell closure for 8 to 9 days, only 50% of
the mussels survived more than 12 days). It was sug-
gested by Almada-Villela [5] that acclimation of shell
growth does take place and that the threshold for shell
growth is close to 12.8 psu. Further, Hiebenthal et al. [14]
found that effects of salinity on both growth and mortal-
ity may be influenced by temperature. In the present
work no mortalities were observed at 10 psu where a
considerable shell growth was observed in Series #1 and
Series #S2P2-10 (Figure 8). However, the present study
confirms that mussels are able to adjust shell growth at
reduced salinities, and the observed tolerance of the
brackish water Great Belt mussels to lower salinities than
experienced by Almada-Villela [5] may be explained by
the high saline (32 psu) mussels used by the last-men-
tioned [7,30,31]. The growth rate and maximum size of
M. edulis is much lower in the Baltic Sea (7 psu) than in
the North Sea (28 psu), and reciprocally transplanted
mussels were found by Kautsky et al. [7] to grow at rates
similar to those of native mussels at each site, and there-
fore the authors suggested that the observed variation
should be explained by “physiological differences due to
environmental salinity”. The mortality was very high
(>90%) among North Sea mussels transferred to the Bal-
tic Sea, whereas the survival of Baltic mussels trans-
ferred to the North Sea was very high, and therefore
Kautsky et al. [7] suggested that the different survival
rates may have “genetic causes”, and differences be-
tween Baltic and North Sea mussels may thus be due to
“differential selection” as suggested by Tedengren et al.
[30] and Johannesson et al. [31].
The weight specific growth rates measured in the la-
boratory growth studies on Mytilus edulis exposed to sa-
linities between 15 to 25 psu (Figure 5) are comparable
to the growth rates obtained in the field growth experi-
ment (Figure 11). Thus, the doubling times of body dry
weight in the laboratory experiments varied between 10.4
and 24.8 d (Tables 2-4), which may be compared with
14.4 to 18.7 d for mussels in the field, and this inspires
confidence to both relevance and credibility of the labo-
ratory growth experiments.
The chlorophyll a concentration in Great Belt is rou-
tinely monitored by the Danish Nature Agency, Danish
Ministry of the Environment. The mean (±S.D.) concen-
tration measured during the field-growth period in the
northern Great Belt (55˚30.46'N, 10˚51.72'E, 12 km NE

H. U. RIISGÅRD ET AL. 174
of Stavreshoved) was 3.0 ± 1.1 µg·chla·l–1 which is equi-
valent to (3.0/1.251 × 103 =) 2400 Rhodomonas salina
cells·ml–1 [32]. This value is fairly close to the mean al-
gal concentration used in the laboratory feeding experi-
ments, and this indicates that the observed effects of sa-
linity on growth may also apply for mussels in the field.
Here, it should be mentioned that available monitoring
data on the biomass of autotrophic and heterotrophic
plankton in the Great Belt has recently been examined by
Riisgård et al. [33] who found that the heterotrophic
biomass generally accounts for less than 10% of the total
pelagic biomass, and that it therefore can be concluded
that the autrotrophic plankton, which can be quantified
by measurement of the chla concentration, generally
dominates the pelagic microplankton in the Great Belt. In
the actual field-growth period for mussels, the salinity
varied between 24.4 and 13.0 psu (Figure 13), and the
salinity decreased from 23.3 to 13.3 psu in the period 13
to 26 October, thus resulting in a decrease of ([23.3 –
13.3]/13 =) 0.8 psu·d–1. This salinity changing rate may
be compared with the values listed in Table 1 from
which it appears that the typical salinity changing
0
3
6
9
12
15
18
21
24
27
30
Temperature / Salinity / Chl a
S T C
0.0
3.0
6.0
9.0
12.0
15.0
18.0
21.0
24.0
27.0
30.0
Temperature / Salinity / Chl a
S T C
A
B
Figure 13. Salinity (S, psu), temperature (T, ˚C), and chlo-
rophyll a (C, µg·l–1) in the northern Great Belt (STB53,
close to the experimental net-bag mussel growth site) (A) in
2009, and (B) during the mussel growth period (marked by
broken vertical lines). Based on data provided by the Dan-
ish Nature Agency, Danish Ministry of the Environment.
rate in the Great Belt is about 0.4 psu·d–1 and that major
salinity changes take place about10 times per year with at
duration time of about 21 d.
The shell-closure effect observed in the laboratory
when the salinity was suddenly changed and which lasted
for several days (Figure 1) does not represent the normal
environmental conditions in the Great Belt (Table 1,
Figure 13) where the salinity-changing rate is relatively
low and where Mytilus edulis may therefore be able to
adjust (acclimate) to the occasionally high (up to about
25 psu) and low salinities (down to about 10 psu) in such
a way that the growth rate remains rather unaffected.
More studies on the effect of Great Belt-relevant salinity
changing rates on feeding behaviour as well as growth
experiments with mussels that are well-adjusted to low
salinities (i.e. fully open valves and exhalant siphon, cf.
[3]) are needed to separate the acute shell-closing effect
on food uptake and growth from the possible physio-
logical effect of low salinities on the metabolism and
excretion of nitrogen [11,34,35] resulting in reduced
growth.
5. Acknowledgements
This work formed part of the MarBioShell project sup-
ported by the Danish Agency for Science, Technology
and Innovation for the period January 2008 to December
2012. Thanks are due to the Danish Nature Agency,
Danish Ministry of the Environment, for providing moni-
toring data from the Great Belt, and to Kim Lundgreen
for comments on the manuscript.
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