Evaluation of the Distribution of Shrimp Species Found in Southern Benin Through the Lake Nokoué-ocean Complex

The export of shrimp to the European Union was one of the mainstays of the Beninese economy. It is an income-generating activity for the populations living along lake Nokoué. The lack of a sheries management strategy has caused a drastic decline in shrimp production in lake Nokoué since 2003. To remedy this problem, it is necessary to hypothesize on their spatio-temporal dynamics in the lake Nokoué-Cotonou channel complex. This is investigated by combining a literature review on their life cycles and habitat suitability with spatio-temporal evolution of salinity obtained from in-situ observations. The literature review recognizes the presence in southern Benin of species of the genus Macrobrachium, Atya and Desmocaris (seven, two and one respectively) and ve species of marine shrimps belonging to three genera: the genus Penaeus, the genus Holthuispenaeopsis and the genus Parapenaeus. From information on the ecology and physical environment it is shown that adults of the species of the genus Macrobrachium and genus Atya have a favorable environment in lake Nokoué from August to November, their larvae must remain in the lake from December to June to ensure their survival. The species of the genus Desmocaris, which carry out their entire life cycle in fresh water, can stay in lake Nokoué from August to November. The species of the genus Penaeus can nd a favorable environment in lake Nokoué from December to June but their larvae can only survive in the sea. Finally, the species of the genus Holthuispenaeopsis and Parapenaeus spend their entire life cycle at sea. This work allows understanding contrasted life cycles of shrimp’s genera Macrobrachium and Atya compared to Penaeus in a complex characterized by strong saline variation. This research allows to predict the or of shrimp’s in a system, on the basis of for a sustainable we we raw data of the rainfall of Cotonou are obtained at the direction METEO-Benin against payment. Information related to shrimp species life cycles, breeding migrations and anities the with the combination of the following Penaeidae, Atyidae, Desmocaris, Atya, classication, fresh, water, salt, cycle, annual, West, coast, Benin.

gives raise to signi cant variations in certain parameters, in particular the salinity of the lake [17,25]. The seasonal variation in the salinity of lake Nokoué alternately confers a favourable environment for Penaeus (saltwater shrimp) and certain Caridea (freshwater shrimp) [26]. The main scienti c question we are trying to solve in this study is: What is the potential spatio-temporal distribution of these different shrimp species in lake Nokoué and the Cotonou channel? To answer this question, we rst present a review of the literature on the shrimp species found in southern Benin and their life cycle in the natural environment. Secondly, we evaluate their potential spatio-temporal distribution in the lake Nokoué-Cotonou channel complex, by comparing the evolution of the salinity of this complex with their a nity to live in a more or less saline environment. The presented work provides an update of current knowledge on the distribution of shrimp in lake Nokoué through the Cotonou channel. This work aims to complete the information related to the distribution of shrimp found in southern Benin, in order to propose avenues for their conservation for the development of the lake Nokoué-Ocean system.

Palaemonidae
Belonging to the superfamily Palaemonoidea, the Palaemonidae Ra nesque, 1815 represent one of the few groups of decapods that, in addition to the oceans, have succeeded in colonizing estuaries and rivers in subtropical and tropical regions (Fig. 1, right) thanks to a catadromous migration [26]. Furthermore, the genus Macrobrachium is the most diverse of the family Palaemonidae with currently 256 species described worldwide [16,64,70]. In West Africa, eleven species have been described [12,27,31,43,53] [5,31].

Atyidae
Belonging to the superfamily Atyoidea, the Atyidae are characterized by a life cycle similar to that of the Palaemonidae (Fig.  1 right). There are thirteen described species belonging to the genus Atya [70], four of which inhabit West African fresh waters: A. africana; A. gabonensis; A. intermedia and A. scabra [43,49]. Two of them have been reported in Beninese rivers: A. africana, A. gabonensis [31].

Desmocarididae
Belonging to the superfamily Palaemonoidea, the family Desmocarididae is characterized by a life cycle exclusively in freshwater, i.e. in an environment favourable to the plant Eichhornia crassipes. According to [16], the Desmocarididae are found in freshwater. This family contains only the genus Desmocaris with currently two species including one found in Benin, which is D. trispinosa [31].
Life cycle of each shrimp species found in southern Benin Table 1 presents a general summary of the life cycle of shrimp species found in southern Benin. This review was made on the basis of available information along the West African coast from Sierra Leone to Angola. Atya africana Lake, lagoon, river or estuary; with a peak during the rainy season [15,23,51] Brackish water [15] Fresh water [23,51] Fresh water [15,51] _ _ _ Desmocarididae Desmocaris trispinosa River, lake or other freshwater. [15,51,52] Fresh water [15,51] Fresh water [15,51] Fresh water [15,51] _ _ _ Relationship between the life cycle of shrimps and the geochemical characteristics of lake Nokoué.

Bathymetry of lake Nokoué -Cotonou channel
The depth data allowed us to produce a bathymetric map of lake Nokoué which showed that the relative elevation of the water level varies between 2 m and 3.2 m on average in the centre of the lake. The depth of the lake varies between 1 m and 2 m to the east and west of the lake. In the north, the depth is between 1 m and 1.6 m, whereas it is close to 3 m in the centre of the lake and in the south of lake Nokoué near the Cotonou channel. The depth in the Cotonou channel varies between 4 m and 6 m on average (Fig. 2). In general, lake Nokoué has a fairly at bottom with a very shallow depth. Shrimps are benthic already from the juvenile stages [23,44,50,54]. Therefore, the bathymetric characteristics of lake Nokoué are suitable for a homogeneous distribution of shrimps because the relatively at bottom should not favour a strong accumulation of organic debris in a deeper zone.

Seasonal evolution of salinity in lake Nokoué-Cotonou channel
The interpolated data allowed to generate maps of the spatio-temporal distribution of salinity in the basin of the lake Nokoué-Cotonou channel complex (Fig. 3, 4). These maps showed in December the beginning of saline intrusion from the Atlantic Ocean via the Cotonou channel, on both surface (Fig. 3) and bottom (Fig. 4). This entry of sea water into lake Nokoué is progressively increasing from the southern part (entrance to the channel) and progressively extending to the northern side of the basin. This saline intrusion continues to reach the western side rst, then progressively the northern and eastern sides of the basin, during the month of January, due to the ow of the Sô river and the Ouémé river. The highest salinity level of the lake is observed in April. During this period, the salinity values of the lake are almost oceanic on the surface as well as on the bottom (salinity > 30 PSU). Lake Nokoué is then more subject to tidal currents than to the low ow of the lake's tributaries, notably the Sô and Ouémé rivers (Figs. 3,4). During the rainy season mid-April to mid-August (Fig. 6), there is a signi cant increase in the ow of the rivers owing into lake Nokoué, and the surface salinity of the lake and the channel begins to fall from May to reach very low values between July and August. This desalination of the lake occurs more rapidly on the northeast side than on the southwest side of the basin where a portion of the water on the west side and in the channel remains slightly salty in July. Maps of spatial distributions of salinity show that, overall, the bottom of the lake is proportionally saltier than the surface (Fig. 4). During the December-January period, the salinity of the lake increases with the exception of the areas located at the mouth of the Sô and Ouémé rivers which continue to have a low ow into the lake, despite the end of the rains (Fig. 6).
Evaluation of the spatio-temporal distribution of different shrimp species according to the bottom salinity in the complex composed of lake Nokoué and the Cotonou channel The spatio-temporal distribution of salinity is a determining factor in the spatio-temporal distribution of species in the lake- gabonensis) that are entering their reproductive period to lake Nokoué (Figs. 1 right, 4, and 5). Adults of the genera Macrobrachium and Atya have an a nity with the fresh water of lake Nokoué during the months of August to November and begin their migration from lake Nokoué to the Ouémé River in December (Fig. 1 right, 4, and 5). The larvae of the Macrobrachium and Atya species are forced to remain in the brackish waters of lake Nokoué from December to June, until they reach the post-larval stage to have a chance to survive (Figs. 1 right and 4). As for the Desmocaris species (D. trispinosa), which carries out its entire life cycle in the freshwater tributaries of lake Nokoué and can only pass through lake Nokoué during the period from August to November when the environment is favourable for the development of the Eichhornia crassipes plant (freshwater). Tables 1 and 2 and Fig. 5 summarises the annual migration of shrimp species through the tributaries, lake Nokoué and the Cotonou channel in Benin.

Discussion
The bathymetry of lake Nokoué is rather shallow and doesn't show a lot of variation. These results are consistent with those of [39,69]. The depth of the lake is not expected to in uence the distribution of shrimps because, according to the work of [46], it is the nature of the area substrate (dead wood and leaves, invertebrates, and other organic matter) that attracts shrimp and not the depth, even though in deep area's, more debris is present. The shallow depth gives lake Nokoué the characteristics of a polymictic lake. Thus, lake Nokoué should regularly undergo mixing. This would prevent any kind of temperature or salinity strati cation. However, maps of spatial distributions of salinity in lake Nokoué and the Cotonou channel showed that the bottom of the lake is proportionally more saline over time than the surface of the lake (Figs. 3 and   4). These results could be explained by the tidal pressure on lake Nokoué at the time of saline intrusion. This tidal pressure affects the bottom water compartment before rising to the surface with time.  57,59]. These results are con rmed by the results of four months of sampling of Penaeidae conducted by [27] on lake Nokoué. Juvenile Penaeus are, moreover, abundantly caught in lake Nokoué from December onwards, as soon as the period of saline intrusion starts [27]. As for the hatching of the eggs of these Penaeus (reproduction), it must therefore take place at sea a few weeks before. On this basis, we estimated that the period of peak reproduction of Penaeus would extend from August to November. Comparing these results with other studies [41,58,61], it is concluded that the same species of the family Penaeidae could have different reproduction periods in different geographical areas. [9], who studied the biology, reproduction and population dynamics of the deep water shrimp Parapenaeus longirostris at the level of the Algerian west coast (port of Oran and Arzew), showed that the period of strong reproduction observed in Parapenaeus longirostris extends from May to June. This same period has been widely described for P. longirostris in the western Mediterranean, suggesting a minimal reduction mechanism of intra-speci c competition [1,45,58]. The reproductive period observed for P. longirostris in the waters of the Algerian western coast coincides with other studies carried out in the western Mediterranean [41] as well as in Italy and Portugal [8,58]. In contrast, in Senegal, the egg-laying period in P. longirostris is spread over the whole year with two distinct peaks: the most important in winter (February-March) and the second in autumn, October-November [61]. For [10], oviposition in P. longirostris occurs between December and January, with the months of June to August corresponding to the sexual rest period with a resumption of ovarian maturation in September. These differences in the observation of the reproductive period of the same species could be explained by the inequality of climate observed between the West African side and the Algerian west coast. Thus the high temperature highlighted as a factor in uencing reproduction by [14], indicates that spawning takes place in P. longirostris in the cold season and in the warm season. In the context of this study, as the lake Nokoué temperature varies only little (27°C -29°C), the impact of temperature on the seasonal dynamics of shrimp reproduction has not been taken into account. In the framework of this work, the salinity maps of the basin indicate that freshwater shrimp of the genus Macrobrachium, Atya and Desmocaris have an a nity for lake Nokoué during the months of August to November. This period of ooding of lake Nokoué (August to November) marks the end of the rainy seasons in Benin. Since the arrival of freshwater shrimp in lake Nokoué coincides with their reproduction, it is likely that the rainy season in uences or contributes to initiate the gonad maturation period in freshwater shrimp. These results are consistent with those obtained by several researchers [42,66,67,68], who have located the period of egg laying and reproduction during these months based on the study of the ovarian cycle in some freshwater shrimps. According to these authors, the adults Macrobrachium live in the fresh waters of the rivers where the fertilization takes place, especially during the rainy season. Vitellogenesis, according to these authors, is linked to the rainy season and their migration to brackish waters. Rainfall also seems to have a determining role in the beginning and end of saline intrusion through the tributary -lake Nokoué -ocean complex (Fig. 6). Indeed, the beginning of the saline intrusion indicates, according to Météo-Benin data, the end of the short rainy season in Cotonou (December 2017) (Figs. 3, 4 and 6). The salinity of lake Nokoué reaches its peak in April, (period corresponding to the beginning of the long rainy season in Cotonou). We note that during the long rainy season (May 2018), the salinity of lake Nokoué begins to decrease progressively and practically drops to zero during the short dry season (August 2018).

Conclusion
By combining information on the life cycle of shrimps found in southern Benin, from the literature, with the spatio-temporal distribution of salinity in the lake Nokoué-Cotonou channel complex, the evolution of the potential distribution of shrimp species in this complex could be established. The present work has shown that the salinity of lake Nokoué increases from December to reach a peak in April. This period (December-June) of high salinity in the lake indicates a favourable environment for Penaeus and freshwater shrimp larvae such as Macrobrachium and Atya. Furthermore, the salinity of the water of lake Nokoué remains very low during the months of August to November, which is the period favourable for the presence of adult freshwater shrimps. Our results have allowed us to assess the spatio-temporal distribution of shrimp species found in southern Benin, but also to evaluate their life cycle and seasonal migration through lake Nokoué and the ocean via the Cotonou Channel in Benin. This information, in conjunction with pluriannual eld data that have started in 2020, will contribute to a better regulation of shing in the lake Nokoué-Cotonou channel complex for a sustainable management of the shrimp stock. Complementary studies are underway to evaluate in situ the presence and relative abundance of these species in the lake Nokoué-Cotonou channel complex and to deepen their spatio-temporal distribution based on the physico-chemical parameters of the lagoon system. Future research will compare the current faunal richness of the lake Nokoué-Cotonou channel complex with that indicated by the literature in southern Benin in general years ago.

Study area
The study setting in this work is the entire lake Nokoué-Ocean system (Fig. 7). Located in south-eastern Benin between 6° 22′ N and 6° 30′ N and 2° 20′ E to 2° 35′ E, the lake Nokoué covers an area of 150 km 2 . This lake is located in a sub-equatorial climate characterized by a long rainy season concentrated between mid-March and mid-July, a short dry season observed between mid-July and mid-September, a short rainy season between mid-September and mid-November, and long dry season between mid-November and mid-March [2,3]. The average annual water temperature is 27-29 °C and the average annual rainfall is 900-1100 mm [7,40]. Lake Nokoué is mainly fed with freshwater by tributaries (Ouémé river and Sô river) and it is connected to the brackish ecosystem of the Porto-Novo lagoon via the "Totchè" canal to the Atlantic Ocean, via the arti cial channel that is the Cotonou channel [22]. The Cotonou channel contributes mainly to the hydrological and environmental uctuations of the lake. The main tributaries of lake Nokoué are: -The Ouémé, with a catchment area of 46,500 km 2 and a length of 523 km, crosses the country from north to south. In terms of fresh water supply, it is largely in uenced by the rainfall of its upper basin (Upper Ouémé); -The Sô, with a catchment area of 1,000 km 2 and a length of 70 km, is connected to the Ouémé River in high water and maintains a good level of ow in the dry season; -The Cotonou channel is 4.5 km long, 300 m wide and between 5 and 10 m deep. It is the sea water tributary of lake Nokoué.

Review of the literature
The objectives of the literature review were: -To list the shrimp species that are present in the southern Benin region and in lake Nokoué.
-To synthesize the available knowledge on the life cycle of these different species.
-To evaluate the in uence of chemical and physical parameters such as salinity, hydrology and bathymetry on the distribution of shrimp throughout the lake Nokoué -Ocean complex.
-The raw data of the rainfall of Cotonou are obtained at the direction METEO-Benin against payment.
Information related to shrimp species life cycles, breeding seasons, migrations and a nities were obtained from the databases https://scholar.google.com/; http://www.ask.com; http://www.freefullpdf.com/; https://www.aquaportail.com/ with the combination of the following keywords: Penaeidae, Palaemonoidea, Atyidae, Macrobrachium, Desmocaris, Atya, Penaeus, migration, reproduction, classi cation, distribution, ecology, shrimp, fresh, water, salt, cycle, annual, salinity, biology, West, African, coast, Benin. In addition, reports, dissertations and theses were also consulted in the libraries of the Ministry of Agriculture of the Universities and the Directorate of Fisheries of Benin and, the World Register of Marine Species Database [70] was used to update the data related to the classi cation of shrimp species. A total of 105 theses, 375 scienti c articles and 80 technical reports were consulted, whose only 12, 41 and 17 respectively were used. The choice to use them or not was guided by the relevance of the documents that address the topic.
In-situ data and analysis Salinity and bathymetry data of the lake Nokoué -Cotonou channel complex were obtained during monthly campaigns carried out at 54 sampling stations (Fig. 7) by the IRHOB in collaboration with the IRD between November 2017 and August 2018 (http://nodc-benin.odinafrica.org/nous-joindre.html). Vertical pro les were conducted each month to determine depth and salinity using a CTD probe at each of the 54 stations using a motorized boat. A GPS was used to acquire the geographic coordinates of all sampling stations. For the analysis, monthly surface and bottom salinity data have been interpolated over a 1 km x 1 km grid. Monthly depth data were averaged to obtain the average depth of each station, and then interpolated on the same grid. The Matlab software was used to produce the salinity and bathymetry maps.