The study aimed to estimate population parameters of the mangrove oyster, Cassostrea gasar Dautzenberg (1891), such as asymptotic length (L∞), growth coefficient (K), and recruitment pattern and their relationship to environmental factors. 420 samples were measured for standard length and analyzed using FISAT II. Frequency histograms showed the existence of two recruitments per year with a single spawning event occurring at the study sites in May-June at the start of the rainy season when the salinity levels ranged between 10 and 18 ‰. Best growth performances were observed at Lake Zowla with the asymptotic length and growth coefficient reaching 85.10 mm and 10.86g yr-1, respectively. Growth model showed negative allometric growth (b <3), with an asymptotic weight (W∞) of approximately 10.86 g. Oyster reaches an average length of 8.17 cm after 8 months. Results also reveals that the presence of C. gasar in the Zalivé channel and in Lake Zowla is seasonal; indeed, by the end of the little rainy season (end of November), all oyster settlements at both sampling stations were eliminated, and only a few scattered individuals remained. The cycle begins again in December-January the following year with the recruitment of larvae from nearby Aného Lagoon.

1. Aspects of Population Dynamics of the Mangrove Oyster, Cassostrea gasar Dautzenherg (1891) (Ostreida: Ostreidae) from the Lake Zowla-Aného Lagoon system in Togo
JFAR
Aspects of Population Dynamics of the Mangrove Oyster,
Cassostrea gasar Dautzenherg (1891) (Ostreida: Ostreidae)
from the Lake Zowla-Aného Lagoon system in Togo
Hodabalo Dheoulaba SOLITOKE1 ⃰, Komlan Mawuli AFIADEMANYO2, Kamilou OURO-SAMA1,
Gnon TANOUAYI1, Tchaa Esso-Essinam BADASSAN1, Kissao GNANDI1
1
Laboratoire de Gestion, Traitement et valorisation des déchets, Faculté des Sciences ; Université de Lomé : BP. 1515,
Lomé -TOGO.
2
Laboratoire d’Ecologie Animale et d’Ecotoxicologie, Département de Zoologie et Biologie Animale. Faculté des
Sciences, Université de Lomé, BP.1515, Lomé-Togo
The study aimed to estimate population parameters of the mangrove oyster, Cassostrea gasar
Dautzenberg (1891), such as asymptotic length (L∞), growth coefficient (K), and recruitment
pattern and their relationship to environmental factors. 420 samples were measured for standard
length and analyzed using FISAT II. Frequency histograms showed the existence of two
recruitments per year with a single spawning event occurring at the study sites in May-June at
the start of the rainy season when the salinity levels ranged between 10 and 18 ‰. Best growth
performances were observed at Lake Zowla with the asymptotic length and growth coefficient
reaching 85.10 mm and 10.86g yr-1
, respectively. Growth model showed negative allometric
growth (b <3), with an asymptotic weight (W∞) of approximately 10.86 g. Oyster reaches an
average length of 8.17 cm after 8 months. Results also reveals that the presence of C. gasar in the
Zalivé channel and in Lake Zowla is seasonal; indeed, by the end of the little rainy season (end of
November), all oyster settlements at both sampling stations were eliminated, and only a few
scattered individuals remained. The cycle begins again in December-January the following year
with the recruitment of larvae from nearby Aného Lagoon.
Keywords: Cassostrea gasar, population dynamics, growth performance indices, recruitment, Lake, Zowla, Togo
INTRODUCTION
Oysters are keystone species in most estuaries and
lagoons along the Atlantic and Gulf coast worldwide. They
maintain a healthy ecosystem through filter feeding and
several of them are considered valuable marine organisms
for environmental monitoring (Grabowski et al. 2012).
Apart from their great ecological value, oysters are
commercially important molluscs (Jouzier, 1998). In most
tropical and subtropical countries, Crassostrea-type
oysters are a major source of much needed protein for
rural communities and are not considered luxury food
items as in the temperate zones (Agadjihouede et al.,
2017). They are rich in vitamins (A and D) and essential
minerals (iodine, selenium and calcium), low in fat and a
good source of omega-3 fatty acids and other well
established health benefits (Schug et al., 2009).
Several species occur around the coasts of Africa. The
most widely distributed species is the mangrove oyster
Crassostrea gasar Dautzenherg (1891). It occurs naturally
from Senegal to the south of Angola and on the Isle of
Principe (Diadhiou, 1995); it is now present on both shores
of the Atlantic Ocean (probably introduced by humans to
South America) (Lazoski et al., 2011; Lapegue et al.,
2002). The species’ ability to adapt to a wide range of
environmental conditions (e.g. tolerance for low dissolved
oxygen and wide ranges in salinity and temperature)
makes it resilient (Marche-Marchad, 1969). It can be found
in shallow saltwater bays, lagoons and estuaries, in water
2.5 to 10 m deep (Sandison and Hill, 1966). In Togo, C.
gasar is found isolated and / or grouped on the roots and
lowest branches of the mangroves trees bordering lakes
Research Article
Vol. 5(1), pp. 93-106, September, 2020. © www.premierpublishers.org, ISSN: 9901-8810
Journal of Fisheries and Aquaculture Research

2. Aspects of Population Dynamics of the Mangrove Oyster, Cassostrea gasar Dautzenherg (1891) (Ostreida: Ostreidae) from the Lake Zowla-Aného Lagoon system in Togo
Islam et al. 94
and the network of channels of the Lake Zowla-Aného
lagoon complex (Figure 1). They are also found in the
Mono estuary and in the Aného Pass where they are fixed
to hard substrates or on the shells which line the sandy or
muddy beds (Solitoke, 2012).
The Cassostrea gasar fishery is an important source of
livelihood for rural communities in a number of coastal
West African countries. Several authors (Asare et al.,
2019; Anyinla et al., 20011; Ansa and Bashir, 2007;
Yankson, 2004; Afinowi, 1985)) have commented on the
regular consumption of the mangrove oyster in coastal
communities where they occur in the West African sub-
region. A number of studies have dealt with ecological
factors determining the nature of oyster communities and
evaluated its potential for aquaculture, considering
biological characteristics, as well as economic and
marketing aspects, which may be relevant to the future
development of oyster farming in the sub region. These
include Agadjihouede et al. (2017), Adite et al. (2013)
(Benin); Otchere (2003), Obodai (1991), Asare et
al.(2019), (Ghana), Afinowi (1975), Ajana (1978), Adisa-
Bolanta et al. (2013), Sule and Sotolu (2016), (Nigeria),
Diadhiou and Le Pennec (2000); Diadhiou and Ndour
(2017) (Senegal); Hunter (1969) and Kamara (1982);
(Sierra Leone).
In Togo, oysters and clams are harvested from wild stocks
for food for centuries by coastal villagers in the south east
of the country (PNAE, 2002). In this part of the country,
molluscs’ meat is consumed dried or smoked (locally in
part) or sold to passengers travelling on the Lomé-
Cotonou international highway. Despite its commercial
importance for people inhabiting the coastal areas, local
conditions for growth and reproduction of C. gasar relative
to environmental factors and its situation in the hydro
system have not been assessed. This information is
necessary for formulating management and conservation
policies as well as the further development of the fishery
for this species in the country. From this perspective, this
study assessed the growth factors of C. gasar taken from
the lake Zowla- Aného lagoon complex, based on oyster
size frequency histograms in Zowla and Zalivé sampling
stations while it also determined the recruitment pattern
and identify environmental conditions that influence
reproduction and mortality of C. gasar in the study area.
MATERIALS AND METHODS
Study sites
The Lake Zowla-Aného Lagoon Complex is located on the
Togolese littoral and consists of Lake Zowla (6.55 km2),
and the Aného Lagoon and its network of narrow channels
in the Southeast (4 to 11 m). This Complex belongs to the
Togolese littoral zone between latitudes 6° 17' 37'' and 6°
14' 38'' North and longitudes 1° 23' 33'' and 1° 37' 38'' East
(Figure 1). The System communicates downstream with
the sea through the Aného Pass, which has remained
continuously open since 1989 (MERF, 2007; Millet, 1986).
The hydrological regime of the lagoon system is mainly
dependent on the regimes of the Zio, Haho, Boco and
Mono Rivers (Atanle et al., 2012; MERF, 2007;
ONUDI/TGO, 2007; Millet, 1986). Figure 1 shows the
study area and sample sites.
Figure 1: Map showing the oyster sample sites in the lake Zowla-Aného lagoon hydro system

3. Aspects of Population Dynamics of the Mangrove Oyster, Cassostrea gasar Dautzenherg (1891) (Ostreida: Ostreidae) from the Lake Zowla-Aného Lagoon system in Togo
J. Fish. Aquacul. Res. 95
Sampling and laboratory procedure
Quantitative sample taking of C. gasar in the hydro system
was carried out at monthly intervals from January 2017 to
December 2017 at two sites (Zowla and Zalivé). The
sampling sites were chosen considering previous studies
in the area (Solitoke, 2012; Ouro-sama et al.,2014), the
resource stock, oyster harvesting activities and position of
sample sites concerning the sea. All conspicuous (visible)
faunal elements were identified and particular attention
was paid to real and potential enemies of oysters. On each
sampling occasion, hydro graphic parameters in particular,
pH, temperature and salinity were measured in situ at the
surface.
To obtain monthly length frequency distributions, shell
length of oysters collected was measured to the nearest
0.1 mm using a digital vernier caliper. Shell length was a
measurement of the furthest dorsal to ventral distance
from the umbo to shell periphery (Ben Messaoud, 1987;
Kourradi, 2007). Individual oysters were then allocated to
one of the fifteen size classes of 0,5 cm amplitude after the
initial 1 cm entries. They are: C1 ([1,1;1,6[), C2 ([1,6; 2,1[),
C3 ([2,1; 2,6[), C4 ([2,6; 3,1[), C5 ([3,1 ;3,6[), C6 ([3,6; 4,1[),
C7 ([4,1; 4,6[), C8 ([4,6; 5,1[), C9 ([5,1; 5,6[), C10 ([5,6; 6,1[),
C11 [6,1; 6,6[, SC12 ([6,6; 7,1[), C13 [7,1; 7,6[, C14 [7,6; 8,1[
and C15 (Lt >8,1).
The class size relative frequencies are calculated by
dividing the number of individuals in each class by the total
number of individuals in all classes and multiplying by 100.
Class size frequency was obtained using the FISAT II
software (FAO, 2005).
Growth parameters
The growth model developed by von Bertalanffy (1938)
has been found to be suitable for the observed growth of
most marine species. This model expressed length as a
function of age of the animal. The integration of the
generalized von Bertalanffy growth function (VBGF) is well
documented and has been discussed in detail in Pauly
(1979). The VBGF equation, in terms of length, is:
Lt = L∞ [1 – e–K(t–t
0
)]
In this equation, Lt is the predicted mean length at age t.
L∞ is the theoretical maximum (or asymptotic) length that
the species would reach if it lived indefinitely, K is a growth
coefficient which is a measure of the rate at which
maximum size is reached and t0 the initial condition
parameter.
Estimation of growth parameter to
There are three ways to express the age of a mollusc,
considered individually, or the average age of a cohort
(Belhoucine, 2012; Sidibé, 2003). In this study, given the
difficulties in determining the average date of birth of
cohorts or recruitment, we will use a conventional age.
Consequently, the initial condition parameter (t0) is
determined from the average size of the first mode
observed in December-January (oysters of 1 month) using
the following relation:
t0= 1+ [ln (1-L1/ L∞)]/K
The growth performance 𝜑′ of C. gasar population in terms
of length growth was computed using the index of Murno
and Pauly (1983) i.e.,
𝜑′=2 × log10 𝐿∞+ log10 𝐾
To assess Daily Growth Rate, the following formula was
used:
DGR= (Xt+1 – Xt) / dt
In which X t+1 is the mean height (mm) or the total weight
(g) in the current month; Xt is the mean length (mm) or the
total weight (g) in the previous month; and dt the time (in
days) between t and t+1. This formula used by Lopes et al.
(2013) makes it possible to follow the daily growth of
oysters and to relate growth to fluctuations in
environmental parameters. The asymptotic length (L∞)
and growth constant (K) were estimated using the FISAT
II (version 1.2.2) software. Growth (length increment)
between sample sites was compared using the Mann-
Whitney U-test, and between oyster groups by means of
the Kruskal-Wallis test.
Length - Weight Relationships
All the wet flesh from a sample of 210 individuals was
weighed to the nearest 0.01g using a precision balance.
The length-weight relationship was established according
to the formula:
W = a.Lb (Huxley and Teissier 1936)
The values of a and b are obtained after a logarithmic
transformation of the exponential function to a linear
function (Melouah et al., 2014)
log W = loga +blogL
Where W is the body weight in grams and L is the standard
length (SL) in mm, a is a constant determined empirically,
b is an exponent. If the shellfish is growing isometrically
then the length exponent is 3, in which case weight
increases as the cube of the length. A value significantly
larger or smaller than 3 indicates allometric growth). The
adjustment line will be calculated using the right axis
method (Teissier 1948) using the STATISTICA software
version 6.1.
The weight growth equation results from the combination
of Von Bertalanffy's (1938) linear growth equation and the
height-weight relationship.
Wt= W∞ [1 – e–K(t–t
0)]b
Where, Wt is the total weight of the bivalve at time t; W∞
is the weight corresponding to L∞. Parameters K and t0
are the same as those used in the linear absolute growth
equation (Mezedjri et al., 2008; Belhoucine, 2012).

4. Aspects of Population Dynamics of the Mangrove Oyster, Cassostrea gasar Dautzenherg (1891) (Ostreida: Ostreidae) from the Lake Zowla-Aného Lagoon system in Togo
Islam et al. 96
Condition index (C.I)
The Condition index is a traditional way of assessing the
relative weight status of individuals studied. C.I may serve
as a measure of the life-history changes in the animal’s
body for e.g. during the reproductive season (Blackwell et
al., 2000). In this study, we used the condition index to
determine whether or not there are seasonal cycles in the
life-history, if so, whether seasonal cycles vary with
geographic location within the System. Different
methodologies are used to estimate the condition index
(Rainer and Mann, 1992). In this study, the monthly mean
condition index (K) was obtained from Quayle and Newkirk
(1989):
BW/L3*100.
Where BW is the body weight of the oyster in grams and L
is the length of the oyster in centimetres.
Condition Index data were tested for homogeneity of
variance using Levene’s test and were found to be
“normal” (F1,46 = 0.9, P > 0.05) and analyzed using a one-
way ANOVA against location.
RESULTS
Fluctuations in hydro graphic factors
Seasonal variations in hydro graphic factors in the two
water bodies are illustrated in Figure 2. On the whole,
changes in pH were gradual and steady with no definite
pattern in both study areas. However, pH values were
slightly lower in the rainy season without being significantly
different. In lake Zowla, it ranged from 6.9 (August) to 7.9
(April) averaging 7.33 while in the Zalivé channel the range
was from 6.9 to 8.1 averaging 7.39. There was also a
noticeable increase in temperature during the dry season
(November to March) in both study areas. Temperatures
ranged from 27.4°C to 31.2°C at Zalivé and from 27.2°C to
31°C at Zowla sample station.
On the contrary, there was a significant monthly variation
in salinity, with the highest value recorded at the end of the
dry season (Kruskal-Wallis tests, p>0.05). At Zalivé salinity
dropped quickly from its maximum value of 18.8 ‰ in April
to 0.1‰ in November then went up to 15 ‰ March. The
same pattern was observed at Zowla where salinity values
ranged from 0.1 to 16.6‰.
a. b c.
Figure 2: Seasonal variations in pH (a), temperature (b) and salinity (c) in Lake Zowla and Zalivé channel in 2017.
Length - Frequency Distribution
Figures 3 and Figure 4 Showed the length frequency
distribution of C. gasar collected at Zalivé and Zowla from
January 2017 to July 2017. During the study period, the
smallest size class of C. gasar encountered is 1-1.5cm
obtained in January, June and July at the Zalivé and Zowla
stations with a minimum size of 1.1 cm, whereas, the
largest size class encountered is 8.1-8.6 cm found in April
at Zowla with a maximum size of 8.3cm. Analysis of the
size frequency histograms, revealed that in January, a
batch of juveniles belonged to the smallest size class (1-
1.5 cm). The latter disappeared in the following months
(February to May) and does not reappear until June-July.
In January, the most represented sizes belonged to
classes C4 to C6 with more than 50% of the individuals.
From February through Mai 2017, there was an increase
in oyster numbers for size classes C4, C5, C6, C7, C8 and
C9 in both study areas. From May, we observed a decrease
in sizes, the C6 class represented a little more than 25%
of the total. From the population structure indicated by the
length frequency histograms (Figure 4 and Figure 5), the
incorporation of recruits can be inferred from the
appearance of the smallest size classes or their increase
in frequency. Two recruitment periods can be inferred at
the two locations: the main one from January to March,
and the minor period from May to August.
J F M A M J Jt At S O N D
6,0
6,2
6,4
6,6
6,8
7,0
7,2
7,4
7,6
7,8
8,0
8,2
pH
Collection period
Zalivé
Zowla
J F M A M J Jt At S O N D
27,0
27,5
28,0
28,5
29,0
29,5
30,0
30,5
31,0
31,5
Température(°C)
Collection period
Zalivé
Zowla
J F M A M J Jt At S O N D
0
5
10
15
20
salinité(g/l)
Collection period
Zalivé
Zowla

5. Aspects of Population Dynamics of the Mangrove Oyster, Cassostrea gasar Dautzenherg (1891) (Ostreida: Ostreidae) from the Lake Zowla-Aného Lagoon system in Togo
J. Fish. Aquacul. Res. 97
Figure 3: Monthly distribution of size structure in C. gasar oysters in the Aného lagoon (January 2017-July 2017) (N:
numbers).
Figure 4: Monthly distribution size structure (mm) in C. gasar oysters in lake Zowla (January 2017-July 2017) (N:
numbers).
Monthly variations in length
Results of the growth increment study are shown in Table
1. At Zowla, oysters grew at a mean rate of 6 mm month-1
from January to April with a mean increase of 18 mm in
this period. At Zalivé, oysters grew at a mean rate of 3,4
mm month-1 during the same period with a mean increase
of 10 mm. The monthly increment then gradually
decreases. Growth was very fast during the first two
months of the year. Overall, oysters in the Lake grew faster
than those in the channel.
Table 1: Average monthly length, monthly length
increment and daily length increment in C. gasar
individuals at Zalivé and Zowla in 2017.
Sampling Periods
21/01/
17
25/02/
17
27/03/
17
29/04/
17
27/05/
17
24/06/
17
22/07/
17
Zalivé
Lm 3.49 4.0 4.33 4.49 3.74 3.52 3.06
Im - 0.51 0.33 0.16 -0.75 -0.22 -0.46
Id 0.02 0.01 0.005 -0.027 -0.008 -0.02
Zowla
Lm 3.14 3.97 4.52 4.94 4.01 3.67 3.35
Im - 0.83 0.55 0.42 -0.93 -0.34 -0.32
Id 0.03 0.018 0.013 -0.033 -0.012 -0.01
Lm= mean shell length; Im =monthly increment; Id= daily
increment.

6. Aspects of Population Dynamics of the Mangrove Oyster, Cassostrea gasar Dautzenherg (1891) (Ostreida: Ostreidae) from the Lake Zowla-Aného Lagoon system in Togo
Islam et al. 98
The measurements of total length and total weight of 420
specimens of C. gasar were used to estimate the length-
weight relationship (Figure 5). The broad range in size of
oyster collected in lake Zowla and in the channel varied
respectively between 11 and 83 mm and 11 and 6.9 mm,
while total weight varied respectively from 0.5 to 8.1 g and
from 0.4 to 5.2 (Table 2). The length-weight relationship of
C. gasar was described by the equation:
W = 0.5633 x SL r = 0.89 (Lake Zowla)
W = 0.6939 x SL r = 0.84 (Zalivé Channel).
The relationship between size and weight of oysters in the
Lake showed proportionality in linear weight growth,
expressed by a high correlation coefficient. On the other
hand, the value of the exponent "b" was <3, in the two
study areas thus reflecting a negative allometric growth, in
which size grows faster than weight. Figure 5: Curves representative of the growth of oysters
as a function of age during the year 2017.
Table 2: Length-weight relationship parameters of C. gasar in the Lake Zowla-Zalivé channel
a b Length/weight relationship R N Growth type
Zalivé 0.5633 1.2565 W=0.5633L1.2565 0.84 210 b < 3
Zowla 0.6939 1.3251 W=0.6939L1.3251 0.89 210 b < 3
a. …. b
Figure 6: Linear regression relationship between the length (L) and the fresh weight (W) in the oysters of Lake Zowla (a)
and from Zalivé channel (b) during the year 2017: scatter plot and regression line.
Growth parameter estimates
From the length frequency data, growth parameters L∞, K
and t0 for the oyster from the lake and the lagoons were
computed and compiled in Table 3. Table 4 shows the final
estimates of growth parameters of the oysters. The
maximum observed length (Lmax) of C. gasar living in the
lake was 83 mm and the asymptotic length (L∞) was 85.1
mm whereas in Zalivé, these same parameters only
reached 65 mm and 45 mm respectively. The best oyster
growth performance was registered at Zowla (𝜑′ =2.67)
even though the Zalivé site presented the best growth
coefficient (k=0.84 year-1vs 0.65 year-1)
Table 3: Performance indexes and von Bertalanffy growth parameters of C. gasar from Zowla and Zalivé
Parameters Equation
L∞ K t0 𝜑′ Rn Lt = L∞ [1 – e–K(t–to)] Lmin Lmax
Zalivé 6.4 0.84 -0.05 2.54 0.357 Lt = 6.4[1 – e-0.84(t+0.05)] 1.1 6.3
Zowla 8.51 0.65 0.48 2.67 0.386 Lt = 8.51[1 – e-0.65(t-0.48)] 1.1 8.3
L∞= asymptotic shell height; K=von Bertalanffy growth constant; t0= von Bertalanffy growth parameter; 𝜑′= growth
performance index. Lmin= minimum length; Lmax= maximum length
0 1 2 3 4 5 6 7 8 9 10 11 12 13
0
1
2
3
4
5
6
7
8
9
Length(Cm)
Age (month)
Zalivé
Zowla
0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9
Log(L)
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8
Log(W)
Log(W) = -0,5739 + 1,2565Log (L)
R=0,84
0,0792
0,2041
0,2788
0,3617
0,4314
0,5051
0,5798
0,6532
0,7243
0,7924
0,8633
Log(L)
-0,2218
-0,0458
0,0792
0,1761
0,2788
0,3802
0,4771
0,5798
0,6812
0,7993
0,8976
Log(W)
Log(W) = -0,3654+ 1,3151Log(L)
R=0,89

7. Aspects of Population Dynamics of the Mangrove Oyster, Cassostrea gasar Dautzenherg (1891) (Ostreida: Ostreidae) from the Lake Zowla-Aného Lagoon system in Togo
J. Fish. Aquacul. Res. 99
Table 4: Results of variations in weight and size of
samples of C. gasar in Zalivé and Zowla used for relative
growth.
Fresh weight (g) Height (cm)
Zalivé Zowla Zalivé Zowla
Mean 1.52 2.97 3.85 4.13
Median 1.30 2.70 3.80 4.10
S.D. 0.72 1.61 1.17 1.47
Min 0.4 0.5 1.1 1.1
Max 5.2 8.1 6.9 8.3
N 210 210 210 210
Table 5: Performance indexes and von Bertalanffy growth
parameters for C. gasar in the Lake Zowla-Aného lagoon
complex in 2017.
Parameters Equation
b W∞ t0 k Wt.= W∞ [1 – e–K(t–to
)] b
Zalivé 1.2565 5.80 -0.05 0.84 Wt=
5.8[1 – e–0.84(t+0.05
)] 1.2565
Zowla 1.3151 10.86 0.48 0.65 Wt=
10.86[1 – e–0.65(t–0.48
)] 1.3151
Regarding oyster total weight, significant differences (p
<0,05) were observed between the two sample areas. At
the end of the field experiment, the maximum weight
reached was higher (p <0,05) in the lake (10.86g) than in
the channel (5.80g)
Figure 7: Curves of weight growth of oysters as a function
of age in 2017.
The monthly mean condition index (CI) of adult oysters
from the two habitats are shown in Figure 8. The CI of C.
gasar ranged from 3.6 to 6.08 with a mean of 4.3 in Lake
Zowla and 1.85 to 3.7 with a mean of 2.55 in the Zalivé
channel. In both study areas, CI were low in January and
declined slightly until April before increasing significantly
until June. The same pattern was observed at both
stations. However, it is apparent from Fig.8 that the CI was
generally higher for the oysters in Lake Zowla than those
in the Zalivé channel with values in the former being
statistically higher throughout the study period (t= 1.35,
p<0.05).
Figure 8: Condition index in C. gasar collected from Lake
Zowla and the Zalivé channel in 2017.
DISCUSSION
The monthly evolution of salinity follows the hydrological
regime of continental and marine waters which condition
the functioning of the Lake Togo-Aného Lagoon hydro
system (Ouro-Sama et al., 2018; Atanle et al., 2012).
Recruitment has been described as a continuous
phenomenon of tropical marine and estuarine species
because of the stable and elevated water temperatures
allowing year round breeding (Pauly, 1979). However, for
a given Crassostrea species, a variety of reproductive
strategies have been observed between populations
throughout the range of the species and accordingly,
oyster recruitment has been shown to vary spatially and
temporally (Borsa and Millet, 1992). In Brazilian mangrove
estuaries, studies on C. gasar and C. rhizophora indicated
continuous recruitment throughout the year, suggesting
that the species reproduce during all months, although, the
higher densities obtained were registered in the dry period
(Funo et al.; 2019; Christo and Absher, 2006; Nascimento
and Pereira, 1980) or late spring and early autumn
(Nalesso et al. 2008). In West Africa, year-round
recruitment pattern in C. gasar with a single peak pulse
was observed in the Niger delta (Afinowi, 1985) and Sierra
Leone (Kamara, 1982). In contrast, Obodai et al. (1996)
observed up to three seasonal pulses in the recruitment
pattern of C. gasar in Benya lagoon from February 1998 to
January 1999. In our study area, the temporal length
frequency distributions showed the presence of two
important modes corresponding to two essential periods of
recruitment: the major recruitment extended from the end
of December to February while the minor period of
recruitment extended from June to July. The position of the
recruitment peaks was inferred to be the months of
January and June which coincided with the preponderance
or peaks of juveniles of the smaller size class, indicating a
juvenile recruitment pattern into the shellfishery.
0 2 4 6 8 10 12
0
2
4
6
8
10
12
Weight(g)
Age(Month)
Zalivé
Zowla
J F M A M J Jt
2
3
4
5
6
ConditionIndex
Collection period (Month)
Zalivé
Zowla

8. Aspects of Population Dynamics of the Mangrove Oyster, Cassostrea gasar Dautzenherg (1891) (Ostreida: Ostreidae) from the Lake Zowla-Aného Lagoon system in Togo
Islam et al. 100
One of the characteristics of the relationship between
oyster dynamics and physical factors is that oysters are
sensitive to changes in the thermal cycle (Thompson et al.
1996). Related to the reproduction, it is generally accepted
that temperature plays a role on three levels: action on the
speed of gametogenesis, an action on the triggering of
laying and an indirect action which, through the
development of food, can also play on the importance of
gametogenesis (Wang et al., 2017; La Peyre et al. 2016;
Yankson, 1990). Observations and histological data on
eastern oyster populations (C. gasar and C. gigas) showed
that oysters at the early growth stage were first observed
during the months when the water temperature was near
18°C. As the temperature continued to rise from November
through April, oysters reached the mature and spawning
stages (Cham et al., 2014; Chávez-Villalba et al., 2008).
Cáceres et al (2007), reported that C. corteziensis only
spawns at temperatures above 25.5°C. Nevertheless,
Lenz and Boeh (2011) studied the reproduction biology of
C. rhizophorae, in the Bay of Camamu and admitted that
the reproductive cycle of oysters in this region was less
subject to the thermal cycle.
In Western tropical lakes and lagoons, seasonal
temperature fluctuations are less pronounced than in
temperate and boreal regions. Accordingly, it is likely that
the reproductive cycle of C. gasar populations in these
waters is less subject to seasonal thermal influences than
it would on the Pacific coast, and that oysters would then
be more sensitive to other environmental events. Yankson
(1990) and Obodai et al; (1996) observed that in the Pra
estuary the period of low breeding activity coincided with
that of low salinity and low water transparency and vice
versa. They, therefore suggest that the more rapid sexual
differentiation, maturation and spawning of lagoon oysters
may be attributed to the higher salinity of this habitat.
Hunter (1969) and Blanc (1962) have come to the same
conclusion when they investigated the feasibility of
culturing C. gasar in Sierra Leone and Senegal. Some
other studies on the reproduction of C. gasar, suggest that
spawning periods correspond to a change in season (dry
to wet season or vice versa) (Hunter, 1969; Gills, 1992;
Diadhiou and Le Pennec, 2000). It was therefore
impossible to extract the main variable responsible, since
several elements of climate vary at the same time. That
could probably be the case in this study. Indeed, the
population structure of the length frequency histograms
corroborated by data generated using the condition index
(CI) method suggested the occurrence of two reproductive
events per year in the Aného hydro system. The first
spawning event occurs in October-November
corresponding to the transition between small rainy
season and main dry season and the second event took
place in May-June (transition months from the major dry
season to the major rainy season in the coastal region of
Togo). During these periods, the average temperature was
around 29°C±1°C and salinity was low (5-10‰). The
decrease in salinity during these periods is explained by
the entry of flood water, via the Voukpo, Hato, Haho and
Mono rivers and direct precipitation. This causes dilution
of pre-existing lagoon waters strongly influenced by the
ocean. These results are consistent with the finding of
Diadhiou and Le Pennec (2000) who also observed that
the main spawning period in Southern Senegal occurs
during the periods of high Casamance River flooding at the
end of the rainy season. However, the salinity values
measured (35‰) in Casamance during spawning periods
(Diadhiou and Le Pennec (2000), was much higher than
those encountered in Zowla and Zalivé. Thus, as Obodai
et al. (1996) pointed out, hydrographic factors may interact
intricately in directing the biological processes in tropical
lagoons.
Conspecific oysters generally display a considerably inter-
specific range of growth rates (Quayle and Newkirk, 1989).
For the Cassostrea genus the lowest growth performance
(𝜑′) has been obtained for C. tulipa in Brazil (Legat et al.
2017) and C. madarensis in Bangladesh (Amin et al.,
2008), while the highest would appear to have been
obtained for C. gasar in Côte d’Ivoire (Yapi et al., 2017a
and b). In our study, the 𝜑′ values of 2.54 and 2.67
obtained for C. gasar at Zowla and Zalivé, respectively, are
among the lowest recorded for mangrove oyster
populations in estuaries and lagoons throughout West
Africa (Table 6). Furthermore, the calculated growth
performance index was outside the (2.65-3.32) range
designated for fish and shellfish species with fast growing
performance, indicating that C. gasar has a slow
performance in the Aného lagoon complex. They are
however higher than the values reported by Legat et al.
(2017) and by Lopez et al. (2013). Likewise, it was
observed that, the calculated asymptomatic length (L∞) of
C. gasar at Zalivé (64 mm) and Zowla (85.10 mm) were
also lower than those recorded in wild populations in the
Ebrié and Aby (135.5 mm) Lagoons (Yapi et al. 2017 a)
but were higher than those obtained for the same species
farmed in Brazilian estuaries (Lopez et al. 2013; Legat et
al. 2017).
Intra specific variation in oyster growth rates between sites
is common and was confirmed in this study, with significant
slower growth of C. gasar in Zalivé than in Zowla. In
Canada Brown and Hartwick (1988) evaluated the growth
of C. gigas in 10 areas with different environmental
characteristics. Based on the performance of cultured
oysters, these authors classified the areas as low, medium
and high growth sites. Differences in hydrological
conditions such as turbidity, salinity and temperature,
make the growth of C. gasar highly variable, from season
to season, from year to year and from site to site even
when colonies are sited together (Quayle and Newkirk,
1989). The assumption is that the growth of oysters is
promoted when these parameters are within their
tolerance ranges or when the animals are not exposed
long enough to extreme changes in the parameters.
Yankson (1990) show that combined temperature and
salinity ranges of 25-30°C and 10-30‰, respectively

9. Aspects of Population Dynamics of the Mangrove Oyster, Cassostrea gasar Dautzenherg (1891) (Ostreida: Ostreidae) from the Lake Zowla-Aného Lagoon system in Togo
J. Fish. Aquacul. Res. 101
Table 6: Estimates of growth parameters from this study and values from previous studies
Location Species Statute L∞(mm) K(year-1
) Φ' Source
Araioses TT (Brazil) C. tulipa* Cultured 55.75 0.020 1.79 Legat et al. 2017
Zalivé channel (Togo) C. gasar Wild 64.0 0.84 2.54 Present study
Banjul (Gambia) C. gasar Wild 70.4 1.9 3.97 Vakily (1992)
São Francisco do (Brazil) C. tulipa* Cultured 72.16 0.021 2.05 Lopez et al (2013)
Lake Zowla (Togo) C. gasar Wild 85.1 0.65 2.63 Present study
Lagoon Ebrié (CI) C. gasar Wild 135.45 0.58 4.03 Yapi et al. 2017b
Lagoon Aby (CI) C. gasar Wild 135.45 0.88 4.52 Yapi et al. 2017b
Cananéia, SP (Brasil) C. brasiliana Cultured 68.36 0.69 3.51 Pereira et al. (2001)
Bahía Guásimas (Mexico) C. corteziensis Cultured 98.17 1.69 4.21 Chávez-Villalba et al (2008)
India C. madrasensis Cultured 119.0 0.77 4.04 Vakily (1992)
Bangladesh C. madrasensis Wild 208.8 0.35 2.18 Amin et al. (2008)
Venezuela C. rhizophorae Cultured 76.0 3.96 4.34 Angell (1986)
Colombia C. rhizophorae Cultured 149.0 0.90 4.30 Mancera and Mendo (1996)
* C. tulipa synonymized with C. gasar
supported satisfactory C. gasar larval development, while
the ranges found during the study period did not restrict
the growth rate of oysters. Furthermore, according to
Calabrese and Davis (1969), the pH range for normal
oyster growth is 6.75 to 8.75, values that fall within the
range found during the study period. (6.9-8.1). Thus,
temperature and salinity would not constitute a significant
limiting factor to optimal growth at both sites. This is not
the case for food availability, known to influence bivalve
development, affecting the energy reserves of the
spawners, the duration of maturation, quality and quantity
of eggs, and larval development (Sara and Mazzola, 1997;
Lopez et al. 2013). Accordingly, the relatively low growth
rate and performance recorded in this study could be
attributed to low primary productivity in the Aného Lagoon
complex (plankton biomass level) as a result of the
negative impacts of damming the Mono River. It has been
shown that dams not only interrupt the flow of sediment but
also the flow of nutrients with consequences for the
productivity in the river downstream, and, in the case of
large rivers, the productivity of coastal areas (Rossi, 1996;
Ferarreze et al. 2015). Moreover, barnacles, bryozoans,
tube-dwelling polychaetes and other colonial organisms
observed at sites, may compete with oysters spat for
space and/or food (Alvarenga and Nalesso, 2006; Gilles,
1992; Afinowi, 1984; Dabo 1979).
Nevertheless, satisfactory final lengths and weights were
reached after 8-9 months of C. gasar development in Lake
Zowla (85.1 mm) when compared with results reported
elsewhere. Indeed, Adisa-Bolonta et al., (2013) reported
maximum sizes of 50.3cm and 52.3cm with weights of
20.8g and 18.8g respectively for oysters grown in the Niger
Delta. On average it requires about 7-8 months for the
mangrove oyster to attain the local market size of 35-69
mm (Asare et al. 2019; Afinowi, 1985; Kamara, 1976). On
the other side of the Atlantic forecast for cultivation in
equatorial waters are that C. gasar reaches commercial
size (80 mm) at 10-11 months and according to Pereira et
al. (2001), who studied the growth of Cassostrea sp.,
oysters attached to mangrove roots, commercial size (≥50
mm) was obtained after 19.5 months. It should also be
noted that there is a good correlation between the size and
weight of the mangrove oyster of the Lac Zowla- Aného
Lagoon hydrosystem (R = 0.84 for Zalivé and R = 0.89 for
Zowla). The results of the height-weight relationship gave
values of the allometric parameter both less than 3 (b =
1.25 in Zalivé; b = 1.31 in Zowla) characterizing a lowering
allometry. This shows that the oyster in the Lake Zowla-
Aného Lagoon hydrosystem grows more in length than in
weight.
Analysis of the size frequency distribution shows
significant fluctuations in numbers within the size classes
of C. gasar. During February, March April and May 2017,
there was an increase in enrollment at Zalivé and Zowla
for classes C4, C5, C6, C7, C8 and C9. For each of these
classes, the increase in enrollment for the upper classes
could be explained by the integration of new individuals
previously having a smaller size. From June through
August, there was a gradual depletion of larger individuals.
The majority of individuals are small and this could be
attributed to fishing mortality due to selected fishing
pressure on larger individuals. Indeed, results of informal
interviews with harvesters (mostly women) and sample
site visits indicated that oyster fishing peaks in June-July
when most of the bivalves reach the optimal local market
size (≥60 mm) and catch differentially impacts upper
classes.
By the end of the little rainy season (end of November), all
oyster settlements at both sampling stations were virtually
eliminated by predators or lower salinity, and only a few
scattered individuals remained. This means that neither
the first generation issue from dry season larval
settlement, nor their offspring survived to suggest that the
presence of C. gasar in the Zalivé channel and Lake Zowla
is seasonal. This seasonal presence of C. gasar had been
reported by authors studying West Africa lagoons and
deltas. For example, a study of the distribution of molluscs
in the Ébrié lagoon in Côte d'Ivoire revealed that, each
year C. gasar colonizes the eastern part of the lagoon
during the long dry season and it is destroyed by a
reduction in salinity at the start of the wet season Binder

10. Aspects of Population Dynamics of the Mangrove Oyster, Cassostrea gasar Dautzenherg (1891) (Ostreida: Ostreidae) from the Lake Zowla-Aného Lagoon system in Togo
Islam et al. 102
(1968). Similarly, Sandison (1966) and Ajani (1980) found
that C. tulipa populations in the Niger Delta and Lagos
harbor were not permanent during the year; they suffer
almost 100% mortality during the rainy season (July-
October) when the salinity of the water becomes as low as
0.5‰.
The heavy post wet monsoon oyster mortality highlighted
in this study could be attributed to the hydrographic
conditions prevailing in the water bodies during that period.
Indeed, from October onwards, the lagoon environment at
both sample sites becomes ß mixo-oligohaline (salinity
between 0.5 and 3‰), because of heavy rainfall and
entries of floodwater into the hydro system. Furthermore,
during the same period, temperature, hitherto relatively
low, started rising sharply to over 30°C, thus exacerbating
the harmful effects of low salinity. It would therefore appear
that the synergistic effect of prolonged exposure to low
salinity and high temperature probably leads to the
observed high oyster mortality encountered in Zalivé and
Zowla. Although oysters are well-known for their broad
tolerance to salinity, laboratory experiments have
demonstrated that, bivalve mortality generally increases
as salinity decreases and temperature increases.
Supporting evidence was provided by Kennedy (1996) and
Sutton et al. (2012) who state that if short periods of low
salinity exposure during rainfall happen, oysters could still
survive, as it could continue to feed but at a slower rate.
However, if rainfall persists for many months, the death of
oyster due to starvation or hypoxia from prolonged valve
closure could happen. However, if one believes the
assumption of some authors (Sandison, 1966; Hunter
1969; Kamara 1982), it seems that the almost total
disappearance of C. gasar from estuaries and lagoons is
due, mainly to the very high mineral load suspension
created by strong currents, accompanied by swirls, which
inhibit the feeding mechanisms of the bivalve, rather than
a drop in salinity.
To the action of causal factors mentioned above, would be
added those of many competitors and predators found at
the sample sites. Indeed, the high mortality observed in
oysters must have been at least partly due to death caused
by competition from gregarious animals in a crowded
situation (Afinowi, 1985). Predation probably came mostly
from oyster drills (Thais haemastoma and T. nodosa) and
crabs (Callinectes amnicola, Uca tangeri). Gastropods
were very abundant during the dry period and the
beginning of the wet season.
CONCLUSION
The study revealed that the West African mangrove oyster
(C. gasar) exhibits an annual distribution pattern in Zalivé
channel and Zowla lagoon. The cycle starts with
recruitments of larvae from the Aného lagoon to replace
depleted populations. Growth rates and performances
were medium probably as a result of restricted food
availability. The only spawning events in the study sites
occur in Mai – June, at the onset of the wet season when
salinity levels are between 10 and 18‰. However, further
investigations must focus on histological analysis for a
better understanding of the reproduction cycle of C. gasar
and its relationship with the hydro-system environment
and to better situate the maturation period of parent
breeders whose larvae recolonize lake Zowla and Zalivé
channel. Furthermore, a study on the diversity and
distribution of phytoplankton in the two aquatic
environments will allow a better understanding of the
observed medium growth and performances of the
oysters.
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*CORRESPONDING AUTHOR: Hodabalo Dheoulaba SOLITOKE; Email: dominiquesolitoke@yahoo.fr
CO-AUTHORS: Komlan Mawuli AFIADEMANYO, Email: kafiade@gmail.com
Kamilou OURO-SAMA, Email: ouro_kamilou@yahoo.fr
Gnon TANOUAYI, Email : tanouayit@yahoo.fr
Tchaa Esso-Essinam BADASSAN, Email : badassan13@gmail.com
Kissao GNANDI, Email: kgnandi@yahoo.fr
Accepted 24 August 2020
Citation: Solitoke HD, Afiademanyo KM, Ouro-Sama K, Tanouayi G, Badassan TE, Gnandi K (2020). Aspects of
Population Dynamics of the Mangrove Oyster, Cassostrea gasar Dautzenherg (1891) (Ostreida: Ostreidae) from the Lake
Zowla-Aného Lagoon system in Togo. Journal of Fisheries and Aquaculture Research, 5(1): 093-106.

14. Aspects of Population Dynamics of the Mangrove Oyster, Cassostrea gasar Dautzenherg (1891) (Ostreida: Ostreidae) from the Lake Zowla-Aného Lagoon system in Togo
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