DFO
Lb ay MPO Bibjio heque
1 Î 1 1 1 111
12021472
Seasonal Changes In Meat Yield
of Cultivated Blue Mussels
(Mytilus edulis L.)
G. Robert
Department of Fisheries & Oceans Canada
Resource Branch
Invertebrates and Marine Plants Division
Halifax, Nova Scotia B3J 2S7 r7es & Oceans
-•■■••■■■■■•
LIBRARY
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ret
April, 1981
10T"ÈQUE
Canadian Manuscript Report of
Fisheries and Aquatic Sciences
1609
Sfri
cUP3
fee
it.
lee?
lip Gove rn ment of Canada Gouvernement du Canada
Pêches et Oceans
Fisheries and Oceans
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Canadian Manuscript Report of
Fisheries and Aquatic Sciences 1609
^pri1 1981
SEASONAL CHANGES IN MEAT YIELD OF CULTIVATED
BLUE MUSSELS (MYTILUS EDULIS L.)
by
Ginette Robert
Fisheries & Oceans Canada
Resource Branch
Invertebrates & Marine Plants Division
P.O. Box 550
Halifax, Nova Scotia
B3J 2S7
ji
0
Minister of Supply and Services Canada 1981
Cat. No. Fs 97-4/1609
ISSN 0706-6473
Correct citation for this publication:
Robert, G.
1981.
Seasonal changes in meat yield of cultivated blue mussels (Mytilus edulis L.). Can. MS Rep.
Fish. Aquat. Sci. 1609: iv + 8 p.
iii-
CONTENTS
Preface
ii
Abstract/Resume
iv
Introduction
1
Methods
1
Results
3
Live meat weight / total drained weight
3
Dried meat weight on shell length
3
Dried meat weight for a fixed shell length
5
Discussion
6
Market acceptability
6
Changes in meat yield
6
Acknowledgements
7
References
7
iv
ABSTRACT
Robert, G. 1981. Seasonal changes in meat yield of cultivated
blue mussels (Mytilus edulis L.). Can. MS Rep. Fish.
Aqua. Sci. 1609: iv 4 8 p.
Seasonal changes in meat yield were studied in cultivated blue mussels (Mytilus edulis) from a suspended culture system in a shallow (5 m) location in Malpeque Bay, P.E.I. Changes were observed through different types of condition indices
Maximum values, nearly 2 times greater than the minimum values
were reached just prior to spawning. After a rapid decrease
during spawning, condition gradually improved during summer
and fall before falling to low 'winter' levels. Seasonal
changes in meat yield are important factors to two aspects of
mUssel cultivation, spat collection and harvest optimisation.
In 1980 there was one spawning period at the experimental site,
during June. Condition was highest during the latter part of
the summer and early fall.
RESUME
Robert, G. 1981. Seasonal changes in meat yield of cultivated
blue mussels (Mytilus edulis L.). Can. MS Rep. Fish.
8 p.
Aqua. Sci. 1609 -: iv
On a étudié les changements saisonniers de rendement en
chair de moules bleues (Mytilus edulis) cultivées en suspension
à un site peu profond (5 m) dans la baie de Malpêque, I.-P.-E.
On a observé ces changements au moyen de différents indices de
la condition de la qualité de la chair. Des valeurs maximales,
presque le double des valeurs minimales furent atteintes tout
juste avant la ponte. Après un déclin rapide de la condition
durant la ponte, la qualité s'améliora graduellement au cours
de l'été et l'automne avant de diminuer aux bas niveaux hivernais. Les changements saisonniers de rendement en chair sont
importants pour deux aspects de la mytiliculture, la capture
du naissain et l'optimisation de la récolte. En 1980, il ne
se produisit qu'une seule période de ponte au site expérimental,
au début de l'été. La qualité de la chair était la plus élevée
de l'année pendant la deuxième partie de l'été et au début de
l'automne.
INTRODUCTION
Mussel cultivation has been in practice for quite some
In - North America, natural blue mussel beds
time in Europe.
(Mytilus edulis) provided an inexpensive source of protein
during World War II (in U.S.: 4,320 MT/year) but landings
subsequently declined (Scattergood and Taylor, 1949) to low
levels, supplying only the demand of ethnic groups. As a
larger consumer market appreciated this seafood, interest increased in mussel cultivation. This phenomenon has also reached Atlantic Canada. In Atlantic Canada average annual landings
of 20 MT between 1969 and 1979 increased to 200 MT in 1980.
A quarter of the 1980 production was from cultivated stocks.
Mussel cultivation is in its early stages and establishing high
product quality for market acceptability is important to the
mussel grower.
Significant seasonal changes in meat yield take place
in both natural (Dare, 1976) and cultivated (Mason, 1972)
stocks. These changes are known to reflect the complex interactions of food availability and temperature with growth, gonad developemnt and spawning. The study determines the seasonal changes in meat yield of cultivated mussels and their
effect on production of a mussel culture operation. Results
based on the 1980 data are presented because of immediate needs
of the industry. Annual variability could be the subject of
another investigation.
METHODS
Analyses were performed on cultivated mussels age 1
from an experimental suspended culture in a shallow (5 m) location in Malpeque Bay, P.E.I. (Fig. 1). Sampling was at weekly
intervals from mid-May through October and once each of November and December. Frequent sampling allowed close observation
of the spawning period.and of the relatively rapid growth.
The study was limited to the ice-free season as previous studies
under similar climatic conditions (Freeman, 1974; Haamer, 1977)
have shown that mussel growth was slow during the winter.
Samples of 100 mussels were collected and divided into
at least 3 length-classes (40-44 mm; 45-49 mm; 50-54 mm).
In
May few mussels were over 50 mm while by December 40 mm individuals were rare. Shells were cleaned of epibionts and measured
to the nearest mm on their longest axis. Meats (soft tissues)
were quickly dipped in boiling water to release the adductor
muscles, extracted, drained, weighed (±0.001 g) (live meat
weight), dried at 65 ° for 24 hours, then weighed again (dried
meat weight). Shells were air-dried before weighing.
Total
drained weight equals live meat weight + shell weight. All
animals over 40 mm were mature.
- 2 -
Measures of changes in meat weight in relation to another variable such as total weight, shell weight, etc. or condition index are often used in studies of reproduction and biomass production.
Shell length, a convenient size index, lacks
information on the shell content (.meat weight) which usually
shows temporal and spatial variability. Many types of condition
indices have been applied to mussel data: meat-volume to cavityvolume ratio (Baird, 1958), meat weight to total weight ratio
(Andreu, 1968), meat weight to shell weight ratio (Freeman, 1974)
However, such indices are not free from distortions brought
about by the size composition of the samples. Following Freeman (1974) I used a,condition index that could relate changes
in meat weight to a shell linear dimension through a log-log
linear relationship:
log (meat weight) = log a + b log (shell length)
and was independent of the size composition of the samples.
Such an index is valid only if there is no significant difference in the growth of the shell (shape). Allometry was verified from log plots of shell weight over shell length of all
2,500 mussels collected (rz=0.79, P<0.05) (Table 1).
Table 1. Parameters of least squares regressions of log shell
weight (g) on log shell length (mm). (n=100 per analysis)
Date
D/M/Y
210580
290580
050680
120680
190680
270680
040780
110780
180780
280780
040880
110880
190880
260880
020980
090980
170980
240980
011080
081.080
161080
231080
301080
141180
011280
intercept-95% conf.
-3.43 ± 0.53
-4.23
0.42
-3.40
0.72
-3.29
0.54
-3.88
0.50
-3.44
0.40
-4.35
0.46
-4.14
0.56
-3.47
0.65
-3.99
0.49
-3.94
0.43
-3.91
0.40
-4.20
0.46
-3.82
0.49
-3.83
0.47
-3.86
0.54
-4.21
0.36
-4.43
0.39
-4.03
0.27
-4.06
0.36
-4.45
0.32
-4.54
0.39
-4.48
0.31
-4.03
0.43
-4.15
0.37
2
slope±95% conf.
2.26 ± 0.33
2.77
0.26
2.27
0.43
2.20
0.33
2.58
0.30
2.30
0.24
2.86
0.27
2.77
0.34
2.37
0.39
2.69
0.30
2.67
0.26
2.64
0.25
2.80
0.28
2.60
0.30
2.62
0.28
2.63
0.32
2.84
0.22
2.95
0.21
2.71
0.16
2.72
0.22
2.93
0.19
3.00
0.23
2.95
0..19
2.71
0.25
2.77
0.22
r
.656
.818
.524
.640
.751
.783
.815
.732
.601
.773
.810
.827
.803
.755
.782
.734
.877
.868
.921
.865
.907
.872
.912
.822
.868
3
As a first approximation of condition, the ratio of
live meat weight to total drained weight was established for
.the three length-classes. The log-log relationship of meat
weight (g) and shell length (mm) was computed for each sampling
period.
Dried meat weight is preferred to live meat weight
to reduce variability caused by the possibility of liquids
"liquor" being trapped in the mantle cavity. On average, 94 %
of the variability encountered in live meat weight was due to
dried meat weight (P<0.05).
RESULTS
Live meat weight / total drained weight
The ratio of live meat weight over total drained weight
expressed as a percentage illustrates in a coarse way the variations in condition in the three length-classes (Fig. 2).
Maximum percentages of about 60 % were reached in the latter
part of May to rapidly decrease through June to levels between
38 and 45 % for the rest of the sampling period. Weekly changes in this index were 5 percentage points or less except in
late spring, prior to spawning.
Dried meat weight on shell length
Parameters for regressions of log dried meat weight on
log shell length are presented in Table 2 with 95 % confidence
intervals of a (intercept) and b(slope). The coefficient of
determination r2 where r2 x 100 would give the percentage of
variability in meat weight due to variation in shell length.
From May to early September, the main growing season, the model would account for approximately 50 % of the variability
with a downward trend during June (Fig. 3). The second downward trend, from September through December is more uniform
in the mussel population as r2 averages 79 $(Table 2).
The intercept may reflect the condition under changing
environments provided that the slope remains constant. However, important differences exist between the slopes (Table 2).
When tested by t-test those differences were not found to be
significant d.f.:23, P<0.05. Since we are dealing with a single population (genetic strain) and all animals were exposed
to the same environmental conditions, food availability, etc.,
changes in both slope and intercept could only result from high
physiological variability likely caused by animals at different
stages of gonad maturity (Lubet, 1959). Mussels from the same
sample were heterogeneous in 'fullness!, particularly in June.
4
Table 2. Parameters of least squares regressions of log dried
meat weight (g) on log shell length (mm). (n=100 per analysis)
2
Date
D/M/Y
intercept ± 95% conf.
210580
290580
050680
120680
190680
270680
040780
110780
180780
280780
040880
110880
190880
260880
020980
090980
170980
240980
011080
081080
161080
231080
301080
141180
011280
-3.61
-3.88
-4.00
-3.44
-5.07
-4.15
-5.37
-3.89
-4.47
-3.93
-4.12
-3.80
-4.72
-4.20
-4.46
-4.41
-5.29
-6.08
-5.65
-5.93
-6.41
-6.48
-6.46
-6.09
-6.80
•
0.75
0.71
1.08
1.03
1.02
0.95
0.90
1.20
0.86
0.78
0.67
0.86
1.00
0.73
0.84
0.96
0.65
0.79
0.53
0.63
0.55
0.57
0.54
0.57
0.60
slope ± 95% conf.
1.99
2.21
2.20
1.80
2.79
2.23
2.92
2.10
2.44
2.12
2.26
2.08
2.66
2.35
2.53
2.49
3.03
3.45
3.23
3.34
3.64
3.65
3.67
3.45
3.86
0.47
0.44
0.66
0.63
0.62
0.58
0.54
0.73
0.52
0.47
0.41
0.53
0.60
0.44
0.50
0.58
0.39
0.48
0.32
0.38
0.33
0.34
0.32
0.40
0.35
.419
.506
.314
.248
.452
.373
.542
.251
.470
.447
.557
.387
.443
.534
.507
.431
.709
.682
.810
.760
.835
.824
.842
.751
.830
Since maturing gonads contribute significantly at
spawning time to the meat weight of intertidal molluscs such
as the soft-shell clam and the blue mussel, changes in meat
weight may be used as a spawning index. Gametes release produces a loss in meat weight. Because of little synchronisation
of gonad maturation one would expect poor or no significant
correlation between meat weight and shell length during the
spawning period. Correlation analyses of meat weight-shell
length (Table 3) identified this period as late May (2905)
to early July (0407), the month of June approximately. High
levels of significance were obtained when comparing the different slopes through time (Table 4).
(A plankton study of
mussel larvae and results from a spat collection experiment
support the time period of the spawning event. This investigation will be reported elsewhere.)
-5
Table 3. Correlation analyses of log dried meat weight - log
shell length. 100 cases per sampling date. For P<0.05, significant r 2 =.197.
Sampling dates included in analysis (day,month) n
(2905), (0506), (1206), (1906), (2706), (0407) 600
300
(2905), (0506), (1206)
400
(2905), (0506), (1206), (0407)
r2
.160
.155
.099
Table 4. Analyses of variance of slopes of regressions log
Data grouped by
dried meat weight on log shell length. n=25.
month (May to December) or data grouped by stage. Pre-spawn:
May; spawning: June and early July; post-spawn: July, August,
early September; 'winter': from September on.
variance d.f.
f ratio
f prob.
Data grouped by month
between
within
1.0928
.1135
7
17
9.630
.0001
Data grouped by stage
between
within
2.3280
.1242
3
21
18.747
.0000
Dried meat weight for a fixed shell length
Since there may be much heterogeneity in the slope of
the regression of log meat weight on log shell length, the intercept by itself is not a satisfactory condition index. As
suggested by Freeman (1974) the two parameters of the regression
line were used to calculate an estimated condition (dried meat
weight) for a mussel of fixed shell length (Fig. 4). These
results are in agreement with ratios of live meat weight / total
drained weight (Fig. 2). Peak values, nearly 2 times greater
than the minimum values occurred just prior to spawning in late
May. Largest weight changes took place at, or just after spawning. After spawning, the condition improved during the summer
and early fall when it gradually fell to low 'winter' levels.
6
Significant correlation (r 2 =.72, P<0.05) between log
meat weight and log shell length, especially during the sampling
interval from mid-September to November does not support the
possibility of a fall spawning at this location in 1980. The
apparent weight surge, early October sample, is not significant
in the downward trend taking place with the onset of au4umn.
DISCUSSION
Market acceptability
Andreu (1968) reported that condition indices from
the ratio meat weight / total weight varying between 35 and
40 % are considered acceptable in Spanish culture systems.
Excluding peak values just prior to spawning, my indices compare well with these acceptable levels. Giguêre and Poirier
(1978) reported similar condition indices for cultivated mussels from Magdalen Islands' lagoons.
Changes in meat yield
Seasonal changes in meat yield result from the storage
and utilisation of food reserves in relation to the complex
interactions of food availability and temperature with growth
and spawning processes (Dare and Edwards, 1975). It is further
postulated that high values of dried meat weight in summer corresponds to maximal levels of protein and carbohydrate. These
levels decrease through the 'winter' to a post-spawning minimum.
The weight loss in 'winter' results from a rapid utilisation of
carbohydrate as glycogen reserves (De Zwaan and Zandee, 1972;
Gabbott and Bayne, 1973) and a depletion of both protein and
lipid content.
For a mussel culture operation certain activities like
the collection of spat after spawning and the harvest at the
time of optimal condition are dictated by seasonal changs in
meat yield. The mussel grower may use the substantial meat
losses at spawning time as an indicator to set out spat collectors. After a minimum stay of 3 weeks in the plankton, larvae
are ready to set. Collecting spat which may have originated
from the cultivated stocks is one way for the grower to recuperate meat losses. Of course, this step is unnecessary if the
operation collects wild spat to grow to market size.
Seasonal changes in meat yield have important implications for a culture operation. An optimal exploitation of the
stocks should concentrate the harvesting season at the peak
periods of condition in summer and early fall (Fig. 4). Although condition would be lower in 'winter', texture and keeping
quality of the meats are still good (Slabyj et al., 1978).
-7--.
Condition is very high just prior to spawning but, since spawning may then be induced by handling the resulting product quality would be poor and the shelf life considerably shortened
•
(Incze and Lutz, 1980).
Seasonal changes in meat yield will probably differ
somewhat with geographical location since differences in the
spawning period(s) of mussels are apparent in different areas
of the East coast. Spawning is known to occur more than once
a year in some areas: Magdalen Islands (Gigue and Poirier,
1978), Nova Scotia (MacLeod, 1975). Only a long term study
will show whether the experimental site with one spawning
event in 1980 was deviating from normality or whether this
section of Malpeque Bay is similar to areas such as the south
shore of Nova Scotia (Freeman, 1974) and Maine (Incze et al.,
1978) with one annual spawning event.
ACKNOWLEDGEMENTS
Thanks are expressed to Mr. A.H. Bryan, Ms M. Harris
and E. Lyons for their technical assistance. Review by Mr.
T. Amaratunga and Dr. R. Miller is gratefully acknowledged.
REFERENCES
Andreu, B. 1968. Fishery and culture of mussels and oysters
in Spain. in: Proc. of the Symp. on Moll. Part III.
Cochin, • Jan. 1968: 835-846.
Baird, R.H. 1958. Measurement of condition in mussels and
oysters. J. Cons. int. Explor. mer 23(2): 249-257.
Dare, P.J. and D.B. Edwards 1975. Seasonal changes in flesh
weight and biochemical composition of mussels (Mytilus
edulis) in the Conwy estuary, North Wales. J. exp.
mar. Biol. Ecol. 18: 89-97.
Dare, P.J. 1976. Settlement, growth and production of the
mussel, Mytilus edulis L., in Morecambe Bay, England.
Fish. Invest., Lond., Ser. 2, 28(1): 1-25.
Freeman, K.R. 1974. Growth, mortality, and seasonal cycle of
Mytilus edulis in two Nova Scotia embayments. Fish.
Mar. Serv., Tech. Rep. 500, 112p.
Gabbott, P.A. and B.L. Bayne 1973. Biochemical effects of
temperature and nutritive stress on Mytilus edulis L.
J. mar. biol. Ass. U.K. 53: 269-286.
8
Giguère, M. and L. Poirier 1978. Croissance et engraissement
de la moule bleue (M tilus edulis 14.), placée en élevage aux Iles-de-la-Ma e eine. Québec, MIC, Dir. gén.
Pêches marit., Dir. Rech., Cah. Info. 84, 47p. ,
Haamer, J. 1977. Mussellodling.
Forum, 144p.
Stockholm: Bokforlaget
Incze, L.S., B. Porter and R.A. Lutz 1978. Experimental culture of Mytilus edulis L. in a northern estuarine gradient: growth, siii=i1 and recruitment. Proc. World
Maricult. Soc. 9: 523-541.
Incze, L.S. and R.A. Lutz 1980. Mussel culture: an east coast
perspective. in: R.A. Lutz ed. Mussel culture and
harvest: a north-american perspective. Elsevier, New
York, 350p.
Lubet, P. 1959. Recherches sur le cycle sexuel et l'émission
des gamètes chez les Mytilidés et les Pectinidés. Rev.
Tray. Inst. Pêches marit. 23: 387-548.
•
MacLeod, L.L. . 1975. Experimental blue mussel (Mytilus edulis)
culture in Nova Scotia waters. Interim report I.
Fish. & Mar. Serv., Pictou, Nova Scotia, 38p.
Mason, J. 1972. The cultivation of the European mussel, Mytilus edulis L. Oceanogr. Mar. Biol. Ann. Rev. 10: 437460.
Scattergood, L.W. and C.C. Taylor 1949. The mussel resources
of the North Atlantic region. Part III. Development
of the fishery and the possible need for conservation
measures. Commer. Fish. Rev. 11(11): 3-13.
Slabyj, B.M., D.L. Creamer and R.H. True 1978. Seasonal effect
on yield, proximate composition, and quality of blue
mussels, Mytilus edulis, meats obtained from cultivated
and natural stocks. Mar. Fish. Rev. 40 (8): 18-23.
Zwaan, A. de and D.I. Zandee 1972. Body distribution and seasonal changes in the glycogen content of the common sea
mussel, Mytilus edulis. Comp. Biochem. Physiol. 43A:
53-58.
lo
Northumberland Strait
Figure 1. Malpeque Bay, Prince Edward Island. The arrow
indicates the location of the experimental suspended culture
system.
;o 1
7
6/
0
?°.
2.
50_
r
E
e ilVe
\KA\
40
Ô
.- - - .
,fc
\
0
\C
c„.......,.0 B
s—.
0...................A
•
May
o
June
1
July
I
Aug.
1
I
Sept.
Oct.
1
Nov.
r
Dec.
1
Figure 2. 'Condition' represented by the percentage ratio
live meat weight over total drained weight in three length
classes, A: 40-44 mm, B: 45-49 mm, C: 50-55 mm.
km
Figure 3. 'Condition' represented h1^ the constant a (intercept)
of the regression log dried meat weight (g) on log shell length
(mm) (with 95 % confidence interval).
Figure 4. 'Condition' in dried meat weight
of A: 45 mm, B: 50 mm, C: 55 mm.
(g)
for a mussel