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 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 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 Crassostrea 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, oysters 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).