Journal of Fisheries and Aquaculture Research JFAR Vol. 5(1), pp. 93-106, September, 2020. © www.premierpublishers.org, ISSN: 9901-8810

Research Article

Aspects of Population Dynamics of the Mangrove , Cassostrea gasar Dautzenherg (1891) (: ) 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 1Laboratoire de Gestion, Traitement et valorisation des déchets, Faculté des Sciences ; Université de Lomé : BP. 1515, Lomé -TOGO. 2Laboratoire 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 Several species occur around the coasts of Africa. The lagoons along the Atlantic and Gulf coast worldwide. They most widely distributed species is the mangrove oyster maintain a healthy ecosystem through filter feeding and gasar Dautzenherg (1891). It occurs naturally several of them are considered valuable marine organisms from Senegal to the south of Angola and on the Isle of for environmental monitoring (Grabowski et al. 2012). Principe (Diadhiou, 1995); it is now present on both shores Apart from their great ecological value, are of the Atlantic Ocean (probably introduced by humans to commercially important molluscs (Jouzier, 1998). In most South America) (Lazoski et al., 2011; Lapegue et al., tropical and subtropical countries, Crassostrea-type 2002). The species’ ability to adapt to a wide range of oysters are a major source of much needed protein for environmental conditions (e.g. tolerance for low dissolved rural communities and are not considered luxury food oxygen and wide ranges in salinity and temperature) items as in the temperate zones (Agadjihouede et al., makes it resilient (Marche-Marchad, 1969). It can be found 2017). They are rich in vitamins (A and D) and essential in shallow saltwater bays, lagoons and estuaries, in water minerals (iodine, selenium and calcium), low in fat and a 2.5 to 10 m deep (Sandison and Hill, 1966). In Togo, C. good source of omega-3 fatty acids and other well gasar is found isolated and / or grouped on the roots and established health benefits (Schug et al., 2009). lowest branches of the mangroves trees bordering lakes

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

Figure 1: Map showing the oyster sample sites in the lake Zowla-Aného lagoon hydro system

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 Consequently, the initial condition parameter (t0) is determined from the average size of the first mode Quantitative sample taking of C. gasar in the hydro system observed in December-January (oysters of 1 month) using was carried out at monthly intervals from January 2017 to the following relation: December 2017 at two sites (Zowla and Zalivé). The t0= 1+ [ln (1-L1/ L∞)]/K sampling sites were chosen considering previous studies in the area (Solitoke, 2012; Ouro-sama et al.,2014), the The growth performance 휑′ of C. gasar population in terms resource stock, oyster harvesting activities and position of of length growth was computed using the index of Murno sample sites concerning the sea. All conspicuous (visible) and Pauly (1983) i.e., faunal elements were identified and particular attention 휑′=2 × log10퐿∞+ log10퐾 was paid to real and potential enemies of oysters. On each sampling occasion, hydro graphic parameters in particular, To assess Daily Growth Rate, the following formula was pH, temperature and salinity were measured in situ at the used: surface. DGR= (Xt+1 – Xt) / dt

To obtain monthly length frequency distributions, shell In which X t+1 is the mean height (mm) or the total weight length of oysters collected was measured to the nearest (g) in the current month; Xt is the mean length (mm) or the 0.1 mm using a digital vernier caliper. Shell length was a total weight (g) in the previous month; and dt the time (in measurement of the furthest dorsal to ventral distance days) between t and t+1. This formula used by Lopes et al. from the umbo to shell periphery (Ben Messaoud, 1987; (2013) makes it possible to follow the daily growth of Kourradi, 2007). Individual oysters were then allocated to oysters and to relate growth to fluctuations in one of the fifteen size classes of 0,5 cm amplitude after the environmental parameters. The asymptotic length (L∞) initial 1 cm entries. They are: C1 ([1,1;1,6[), C2 ([1,6; 2,1[), and growth constant (K) were estimated using the FISAT C3 ([2,1; 2,6[), C4 ([2,6; 3,1[), C5 ([3,1 ;3,6[), C6 ([3,6; 4,1[), II (version 1.2.2) software. Growth (length increment) C7 ([4,1; 4,6[), C8 ([4,6; 5,1[), C9 ([5,1; 5,6[), C10 ([5,6; 6,1[), between sample sites was compared using the Mann- C11 [6,1; 6,6[, SC12 ([6,6; 7,1[), C13 [7,1; 7,6[, C14 [7,6; 8,1[ Whitney U-test, and between oyster groups by means of and C15 (Lt >8,1). the Kruskal-Wallis test.

The class size relative frequencies are calculated by Length - Weight Relationships dividing the number of individuals in each class by the total number of individuals in all classes and multiplying by 100. All the wet flesh from a sample of 210 individuals was Class size frequency was obtained using the FISAT II weighed to the nearest 0.01g using a precision balance. software (FAO, 2005). The length-weight relationship was established according to the formula: Growth parameters W = a.Lb (Huxley and Teissier 1936) The growth model developed by von Bertalanffy (1938) The values of a and b are obtained after a logarithmic has been found to be suitable for the observed growth of transformation of the exponential function to a linear most marine species. This model expressed length as a function (Melouah et al., 2014) function of age of the . The integration of the log W = loga +blogL generalized von Bertalanffy growth function (VBGF) is well Where W is the body weight in grams and L is the standard documented and has been discussed in detail in Pauly length (SL) in mm, a is a constant determined empirically, (1979). The VBGF equation, in terms of length, is: –K(t–t ) b is an exponent. If the shellfish is growing isometrically Lt = L∞ [1 – e 0 ] then the length exponent is 3, in which case weight

increases as the cube of the length. A value significantly In this equation, Lt is the predicted mean length at age t. larger or smaller than 3 indicates allometric growth). The L∞ is the theoretical maximum (or asymptotic) length that adjustment line will be calculated using the right axis the species would reach if it lived indefinitely, K is a growth method (Teissier 1948) using the STATISTICA software coefficient which is a measure of the rate at which version 6.1. maximum size is reached and t0 the initial condition parameter. The weight growth equation results from the combination of Von Bertalanffy's (1938) linear growth equation and the Estimation of growth parameter to height-weight relationship. –K(t–t b Wt= W∞ [1 – e 0)] There are three ways to express the age of a mollusc, considered individually, or the average age of a cohort Where, Wt is the total weight of the bivalve at time t; W∞ (Belhoucine, 2012; Sidibé, 2003). In this study, given the is the weight corresponding to L∞. Parameters K and t0 difficulties in determining the average date of birth of are the same as those used in the linear absolute growth cohorts or recruitment, we will use a conventional age. equation (Mezedjri et al., 2008; Belhoucine, 2012).

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) RESULTS

The Condition index is a traditional way of assessing the Fluctuations in hydro graphic factors relative weight status of individuals studied. C.I may serve as a measure of the life-history changes in the animal’s Seasonal variations in hydro graphic factors in the two body for e.g. during the reproductive season (Blackwell et water bodies are illustrated in Figure 2. On the whole, al., 2000). In this study, we used the condition index to changes in pH were gradual and steady with no definite determine whether or not there are seasonal cycles in the pattern in both study areas. However, pH values were life-history, if so, whether seasonal cycles vary with slightly lower in the rainy season without being significantly geographic location within the System. Different different. In lake Zowla, it ranged from 6.9 (August) to 7.9 methodologies are used to estimate the condition index (April) averaging 7.33 while in the Zalivé channel the range (Rainer and Mann, 1992). In this study, the monthly mean was from 6.9 to 8.1 averaging 7.39. There was also a condition index (K) was obtained from Quayle and Newkirk noticeable increase in temperature during the dry season (1989): (November to March) in both study areas. Temperatures BW/L3*100. ranged from 27.4°C to 31.2°C at Zalivé and from 27.2°C to 31°C at Zowla sample station. Where BW is the body weight of the oyster in grams and L is the length of the oyster in centimetres. On the contrary, there was a significant monthly variation in salinity, with the highest value recorded at the end of the Condition Index data were tested for homogeneity of dry season (Kruskal-Wallis tests, p>0.05). At Zalivé salinity variance using Levene’s test and were found to be dropped quickly from its maximum value of 18.8 ‰ in April “normal” (F1,46 = 0.9, P > 0.05) and analyzed using a one- to 0.1‰ in November then went up to 15 ‰ March. The way ANOVA against location. same pattern was observed at Zowla where salinity values ranged from 0.1 to 16.6‰.

Zalivé 8,2 Zalivé Zalivé 8,0 20 Zowla Zowla 31,5 Zowla 7,8 31,0 7,6 30,5 15 7,4 30,0 7,2 29,5 7,0 10 29,0 pH 6,8 28,5 6,6 salinité (g/l) 5 6,4 28,0

6,2 Température (°C) 27,5 6,0 27,0 0 J F M A M J Jt At S O N D J F M A M J Jt At S O N D J F M A M J Jt At S O N D Collection period Collection period Collection period 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 In January, the most represented sizes belonged to classes C4 to C6 with more than 50% of the individuals. Figures 3 and Figure 4 Showed the length frequency From February through Mai 2017, there was an increase distribution of C. gasar collected at Zalivé and Zowla from in oyster numbers for size classes C4, C5, C6, C7, C8 and January 2017 to July 2017. During the study period, the C9 in both study areas. From May, we observed a decrease smallest size class of C. gasar encountered is 1-1.5cm in sizes, the C6 class represented a little more than 25% obtained in January, June and July at the Zalivé and Zowla of the total. From the population structure indicated by the stations with a minimum size of 1.1 cm, whereas, the length frequency histograms (Figure 4 and Figure 5), the largest size class encountered is 8.1-8.6 cm found in April incorporation of recruits can be inferred from the at Zowla with a maximum size of 8.3cm. Analysis of the appearance of the smallest size classes or their increase size frequency histograms, revealed that in January, a in frequency. Two recruitment periods can be inferred at batch of juveniles belonged to the smallest size class (1- the two locations: the main one from January to March, 1.5 cm). The latter disappeared in the following months and the minor period from May to August. (February to May) and does not reappear until June-July.

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). Table 1: Average monthly length, monthly length Monthly variations in length increment and daily length increment in C. gasar individuals at Zalivé and Zowla in 2017. Results of the growth increment study are shown in Table Sampling Periods 1. At Zowla, oysters grew at a mean rate of 6 mm month-1 21/01/ 25/02/ 27/03/ 29/04/ 27/05/ 24/06/ 22/07/ from January to April with a mean increase of 18 mm in 17 17 17 17 17 17 17

this period. At Zalivé, oysters grew at a mean rate of 3,4 Lm 3.49 4.0 4.33 4.49 3.74 3.52 3.06 mm month-1 during the same period with a mean increase Im - 0.51 0.33 0.16 -0.75 -0.22 -0.46 of 10 mm. The monthly increment then gradually Id 0.02 0.01 0.005 -0.027 -0.008 -0.02 decreases. Growth was very fast during the first two Zalivé Lm 3.14 3.97 4.52 4.94 4.01 3.67 3.35 months of the year. Overall, oysters in the Lake grew faster Im - 0.83 0.55 0.42 -0.93 -0.34 -0.32 than those in the channel. Id 0.03 0.018 0.013 -0.033 -0.012 -0.01

Zowla Lm= mean shell length; Im =monthly increment; Id= daily increment.

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- 9 weight relationship (Figure 5). The broad range in size of 8 oyster collected in lake Zowla and in the channel varied respectively between 11 and 83 mm and 11 and 6.9 mm, 7 while total weight varied respectively from 0.5 to 8.1 g and 6 from 0.4 to 5.2 (Table 2). The length-weight relationship of 5 C. gasar was described by the equation: 4 W = 0.5633 x SL r = 0.89 (Lake Zowla) W = 0.6939 x SL r = 0.84 (Zalivé Channel). 3 Zalivé Length(Cm) 2 Zowla The relationship between size and weight of oysters in the 1 Lake showed proportionality in linear weight growth, 0 expressed by a high correlation coefficient. On the other 0 1 2 3 4 5 6 7 8 9 10 11 12 13 hand, the value of the exponent "b" was <3, in the two study areas thus reflecting a negative allometric growth, in Age (month) 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

0,8 Log(W) = -0,5739 + 1,2565Log (L) 0,8976 Log(W) = -0,3654+ 1,3151Log(L) R=0,89 0,6 R=0,84 0,7993 0,6812 0,4 0,5798 0,4771 0,3802 0,2 0,2788 0,1761 0,0 Log (W) 0,0792

Log (W) -0,0458 -0,2 -0,2218 -0,4

-0,6 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9

0,0792 0,2041 0,2788 0,3617 0,4314 0,5051 0,5798 0,6532 0,7243 0,7924 0,8633 Log(L) Log(L)

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 lake was 83 mm and the asymptotic length (L∞) was 85.1 mm whereas in Zalivé, these same parameters only From the length frequency data, growth parameters L∞, K reached 65 mm and 45 mm respectively. The best oyster and t0 for the oyster from the lake and the lagoons were growth performance was registered at Zowla (휑′ =2.67) computed and compiled in Table 3. Table 4 shows the final even though the Zalivé site presented the best growth estimates of growth parameters of the oysters. The coefficient (k=0.84 year-1vs 0.65 year-1) maximum observed length (Lmax) of C. gasar living in the

Table 3: Performance indexes and von Bertalanffy growth parameters of C. gasar from Zowla and Zalivé Parameters Equation –K(t–to L∞ K t0 휑′ Rn Lt = L∞ [1 – e )] Lmin Lmax -0.84(t+0.05 Zalivé 6.4 0.84 -0.05 2.54 0.357 Lt = 6.4[1 – e )] 1.1 6.3 -0.65(t-0.48 Zowla 8.51 0.65 0.48 2.67 0.386 Lt = 8.51[1 – e )] 1.1 8.3 L∞= asymptotic shell height; K=von Bertalanffy growth constant; t0= von Bertalanffy growth parameter; 휑′= growth performance index. Lmin= minimum length; Lmax= maximum length

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

Zalivé Table 4: Results of variations in weight and size of Zowla samples of C. gasar in Zalivé and Zowla used for relative 6 growth. Fresh weight (g) Height (cm) 5 Zalivé Zowla Zalivé Zowla Mean 1.52 2.97 3.85 4.13 4 Median 1.30 2.70 3.80 4.10 3

S.D. 0.72 1.61 1.17 1.47 Condition Index Min 0.4 0.5 1.1 1.1 2 Max 5.2 8.1 6.9 8.3 N 210 210 210 210 J F M A M J Jt Collection period (Month)

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

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

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 noted that there is a good correlation between the size and the ranges found during the study period did not restrict weight of the mangrove oyster of the Lac Zowla- Aného the growth rate of oysters. Furthermore, according to Lagoon hydrosystem (R = 0.84 for Zalivé and R = 0.89 for Calabrese and Davis (1969), the pH range for normal Zowla). The results of the height-weight relationship gave oyster growth is 6.75 to 8.75, values that fall within the values of the allometric parameter both less than 3 (b = range found during the study period. (6.9-8.1). Thus, 1.25 in Zalivé; b = 1.31 in Zowla) characterizing a lowering temperature and salinity would not constitute a significant allometry. This shows that the oyster in the Lake Zowla- limiting factor to optimal growth at both sites. This is not Aného Lagoon hydrosystem grows more in length than in the case for food availability, known to influence bivalve weight. development, affecting the energy reserves of the spawners, the duration of maturation, quality and quantity Analysis of the size frequency distribution shows of eggs, and larval development (Sara and Mazzola, 1997; significant fluctuations in numbers within the size classes Lopez et al. 2013). Accordingly, the relatively low growth of C. gasar. During February, March April and May 2017, rate and performance recorded in this study could be there was an increase in enrollment at Zalivé and Zowla attributed to low primary productivity in the Aného Lagoon for classes C4, C5, C6, C7, C8 and C9. For each of these complex (plankton biomass level) as a result of the classes, the increase in enrollment for the upper classes negative impacts of damming the Mono River. It has been could be explained by the integration of new individuals shown that dams not only interrupt the flow of sediment but previously having a smaller size. From June through also the flow of nutrients with consequences for the August, there was a gradual depletion of larger individuals. productivity in the river downstream, and, in the case of The majority of individuals are small and this could be large rivers, the productivity of coastal areas (Rossi, 1996; attributed to fishing mortality due to selected fishing Ferarreze et al. 2015). Moreover, barnacles, bryozoans, pressure on larger individuals. Indeed, results of informal tube-dwelling polychaetes and other colonial organisms interviews with harvesters (mostly women) and sample observed at sites, may compete with oysters spat for site visits indicated that oyster fishing peaks in June-July space and/or food (Alvarenga and Nalesso, 2006; Gilles, when most of the bivalves reach the optimal local market 1992; Afinowi, 1984; Dabo 1979). size (≥60 mm) and catch differentially impacts upper classes. Nevertheless, satisfactory final lengths and weights were reached after 8-9 months of C. gasar development in Lake By the end of the little rainy season (end of November), all Zowla (85.1 mm) when compared with results reported oyster settlements at both sampling stations were virtually elsewhere. Indeed, Adisa-Bolonta et al., (2013) reported eliminated by predators or lower salinity, and only a few maximum sizes of 50.3cm and 52.3cm with weights of scattered individuals remained. This means that neither 20.8g and 18.8g respectively for oysters grown in the Niger the first generation issue from dry season larval Delta. On average it requires about 7-8 months for the settlement, nor their offspring survived to suggest that the mangrove oyster to attain the local market size of 35-69 presence of C. gasar in the Zalivé channel and Lake Zowla mm (Asare et al. 2019; Afinowi, 1985; Kamara, 1976). On is seasonal. This seasonal presence of C. gasar had been the other side of the Atlantic forecast for cultivation in reported by authors studying West Africa lagoons and equatorial waters are that C. gasar reaches commercial deltas. For example, a study of the distribution of molluscs size (80 mm) at 10-11 months and according to Pereira et in the Ébrié lagoon in Côte d'Ivoire revealed that, each al. (2001), who studied the growth of Cassostrea sp., year C. gasar colonizes the eastern part of the lagoon oysters attached to mangrove roots, commercial size (≥50 during the long dry season and it is destroyed by a mm) was obtained after 19.5 months. It should also be reduction in salinity at the start of the wet season Binder

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 availability. The only spawning events in the study sites that C. tulipa populations in the Niger Delta and Lagos occur in Mai – June, at the onset of the wet season when harbor were not permanent during the year; they suffer salinity levels are between 10 and 18‰. However, further almost 100% mortality during the rainy season (July- investigations must focus on histological analysis for a October) when the salinity of the water becomes as low as better understanding of the reproduction cycle of C. gasar 0.5‰. and its relationship with the hydro-system environment and to better situate the maturation period of parent The heavy post wet monsoon oyster mortality highlighted breeders whose larvae recolonize lake Zowla and Zalivé in this study could be attributed to the hydrographic channel. Furthermore, a study on the diversity and conditions prevailing in the water bodies during that period. distribution of phytoplankton in the two aquatic Indeed, from October onwards, the lagoon environment at environments will allow a better understanding of the both sample sites becomes ß mixo-oligohaline (salinity observed medium growth and performances of the between 0.5 and 3‰), because of heavy rainfall and oysters. 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 BIBLIOGRAPHIE the harmful effects of low salinity. It would therefore appear that the synergistic effect of prolonged exposure to low Adisa-Bolanta A S, Sikoki F D, Ansa E J, (2013). 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*CORRESPONDING AUTHOR: Hodabalo Dheoulaba SOLITOKE; Email: [email protected] CO-AUTHORS: Komlan Mawuli AFIADEMANYO, Email: [email protected] Kamilou OURO-SAMA, Email: [email protected] Gnon TANOUAYI, Email : [email protected] Tchaa Esso-Essinam BADASSAN, Email : [email protected] Kissao GNANDI, Email: [email protected]

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.

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

Copyright: © 2020 Islam et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.

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