Saloum Estuary Research Articles

Subfossil diatoms from Bandiala (southern branch of the Saloum estuary, Senegal): Evidence of a wet period in the upper part of the Néma Ba core.

Subfossil diatoms from Bandiala (southern branch of the Saloum estuary, Senegal): Evidence of a wet period in the upper part of the Néma Ba core.

Sub-fossil diatoms from the Saloum Estuary, Senegal, West Africa: Floristic inventory and paleo-productivity evolution along the Ndangonkha core

As part of our program to reconstruct the estuaries and lagoons history in Senegal and the diatom microflora inventory in Sene-Gambia, a 400 cm long core was taken at Ndangonkha in the Saloum Estuary, near Foundiougne. The dating obtained showed that sedimentation began during the Nouakchottian age, around 6000 years BP, with an average rate of sedimentation higher at the base than at the top where the high hydrodynamics of the environment would have caused sediment reworking. The comparative analysis of the lithology and the microflora showed a perfect correlation between the lutite fraction, the absolute abundance of diatoms and their specific richness. The study involved an inventory of 110 species and varieties belonging to 62 genera. The most represented genera being Nitzschia (10 species) and Cyclotella and Diploneis (5 species each). Five species are cited for the first time in Sene-Gambia and 61 species are also reported from Ndangane Babou. The microflora is dominated by euryhaline forms headed by Cyclotella striata. This dominance could be linked to the frequent variations in salinity between the rainy and dry seasons, while the abundance of marine species could be related to the dominant influence of marine waters. The rare freshwater species encountered could be brought by wind, insects or birds. The analysis of the microfloristic and lithological data showed that the primary paleo-productivity and the specific richness are weak at the base but showed an increase upwards. They are high in zone A, maximum in zone C whereas in zone D they are decreasing before disappearing in the last 10 cm, in relation to the hydrodynamics of the environment.

A review of the ecological knowledge on the species Batrachoides liberiensis in estuarine and lagoon environments of West Africa

AbstractScientific fishing programmes were carried out between 1979 and 2013 in several West African estuarine, coastal and lagoon ecosystems. Batrachoides liberiensis represented only 5.2% of the occurrences, of which the majority were recorded in the Sine Saloum estuary. This originality makes it possible to use this population to characterise certain features of the biology and ecology of this little described species. B. liberiensis was recorded in waters with a salinity of 25–46, transparency of 1–3.5 m, at depths of 2–10 m. It was particularly abundant in areas at a distance of 15–35 km from the mouth. The water temperature did not appear to be a determining factor for this species. The sex ratio of the population, whose total length ranged from 10 to 371 mm, was in favour of females (87%) with an estimated size at first maturity of 189 mm (total length). The parameters of the length‐weight relationship were estimated at K = 1.0009*10−5 and b = 3.097. The diet of B. liberiensis consisted mainly of molluscs and crabs, present in respectively, 53.6% and 50.7% of the stomach contents analysed.

CONTRIBUTION TO THE ASSESSMENT OF PERSISTENT ORGANIC POLLUTANTS (POPS) IN MARINE ORGANISMS: THE CASE OF SAROTHERODON MELANOTHERON (RUPPELL, 1852) AND MUGIL BANANENSIS (PELLEGRIN, 1928) IN THE SALOUM DELTA

It is known that the consumption of seafood has several health benefits for humans. However, the accumulation of organic pollutants, such as polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs), in their flesh could pose public health problems. Thus, the concentrations of PCBs and OCPs in edible fish species (Mugil bananensis, Sarotherodon melanotheron) from the Sine Saloum Estuary located in central-western Senegal are investigated in order to assess the potential risks of these contaminants to human health. After analysis, PCB52 and chlordane show the highest concentrations in the edible flesh of these two species. The concentration of PCB52 is almost identical in these two species, while that of chlordane is higher in S. melanotheron. Regarding the distribution of PCBs and OCPs in M. bananensis, the highest concentration is found in Fimela for PCBs and in Sokone for OCPs. For S. melanotheron, the highest concentration is found in Toubacouta for PCBs and in Felir for OCPs. The cancer risk values obtained after calculation do not reveal any serious risk because they do not exceed the threshold value established by the U.S. EPA (2005) which is 1×10-4. However, it is recommended to the populations a moderate consumption because of the phenomenon of bioaccumulation.

Length–weight relationships of 5 fish species from the Sine Saloum estuary, Senegal, West Africa

Length-weight relationships (W = aLb) were estimated for 5 fish species, the pearl stingray Fontitrygon margaritella (Compagno & Roberts, 1984), the West African spadefish Chaetodipterus lippei (Steindachner, 1895), the East Atlantic African spadefish Ephippus goreensis (Cuvier, 1831), the law croaker Pseudotolithus senegallus (Cuvier, 1830) and the Guinean sole Dagetichthys cadenati (Chabanaud, 1948). Fishes were collected during several scientific fishing surveys using purse seine (length 250 m, height 20 m, 14 mm mesh size) and beach seine (180 m length, 9 m height and 25 mm mesh net) conducted from April 1990 to October 2012 in the Sine Saloum estuary, Senegal. They were weighed to the nearest g and measured to the nearest mm (disc width for Fontitrygon margaritella and total length for the 4 other species). The values of parameter b in the LWR equations varied from 3.048 (Chaetodipterus lippei) to 3.274 (Dagetichthys cadenati) and the coefficient of determination (r²) varied between 0.969 (Fontitrygon margaritella) and 0.998 (Chaetodipterus lippei). In addition, new maximum lengths are proposed for Fontitrygon margaritella, Chaetodipterus lippei and Dagetichthys cadenati.

Length-weight relationships of 20 fish species from the Sine Saloum estuary, Senegal, West Africa

Length-weight relationships (W=aLb) were estimated for 20 fish species collected during scientific fishing surveys conducted from April 1990 to October 2012 in the Sine Saloum estuary, Senegal, using purse seine, beach seine and fyke-net. The number of individuals varied from 10 (Hemicaranx bicolor) to 777 (Carlarius latiscutatus). Fish length ranged from 4.5 cm total length (Sphoeroides marmoratus) to 130.0 cm total length (Sphyraena afra). The values of parameter b in the LWR equations varied from 2.687 (Hemicaranx bicolor) to 3.334 (Citharichthys stampflii). The coefficient of determination (r²) varied between 0.953 (Sphoeroides marmoratus) and 0.996 (Sphyraena afra). The length–weight relationship has never been studied before in Sine Saloum estuary for 13 fish species (Carlarius parkii, Batrachoides liberiensis, Caranx senegallus, Hemicaranx bicolor, Trachinotus teraia, Elops senegalensis, Lutjanus goreensis, Citharichthys stampflii, Psettodes belcheri, Dagetichthys lusitanicus, Sphyraena afra, Sphyraena guachancho and Sphoeroides marmoratus). This study provided LWRs that are not available in FishBase (Froese and Pauly, 2020) for Carlarius latiscutatus, Carlarius parkii, Batrachoides liberiensis, Caranx senegallus, Hemicaranx bicolor, Cynoglossus senegalensis, Fontitrygon margarita, Plectorhinchus macrolepis, Pomadasys perotaei, Lutjanus goreensis, Citharichthys stampflii, Psettodes belcheri, Dagetichthys lusitanicus and Sphoeroides marmoratus. It also provided size ranges that are not specified for LWRs in FishBase for Ephippion guttifer, Elops senegalensis, Pseudotolithus senegalensis, Pseudotolithus senegallus and Sphyraena afra. In addition, new maximum lengths are presented for Plectorhinchus macrolepis and Citharichthys stampflii. The parameters obtained from this study will be useful for proper fish population management in West African fisheries.

Evolution of coastal erosion in Palmarin (Senegal)

Coastal erosion is a major concern in Senegal, which is increasing over time. In the Palmarin peninsula, in the Saloum estuary, previous studies showed shoreline retreat rates of 1.20 m/yr in 1982 and 2.20 m in 1954–2002; our results show an increase to 2.45 m/yr in 2005–2010, 2.60 m/yr in 2010–2014 and 3.05 m/yr in 2014–2018. The main cause seems to be sea level rise, as there are not significant human factors. Short-term estimates show probable damages in two villages (Ngallou and Djiffer), where buildings are already being destroyed, tourist camps near the beaches and on the road that crosses the peninsula. There is also a significant risk that coastal erosion would connect the sea and the intertidal mud flat areas of the Saloum estuary, fragmenting the peninsula in several islands, changing the evolution of erosive processes and producing a decline of the mangroves, and with them of fishing. It is urgent to develop an adaptation plan for this area, to address the possible consequences of coastal erosion on the assets and activities of the population.

Évolution des terres salées dans le nord de l’estuaire du Saloum (Sénégal)

Le milieu d’etude, au Senegal, est caracterise par une forte salinisation des terres. Il est compris entre la lagune de Joal-Fadiouth et la limite de la rive droite de la ria du Saloum entre les latitudes 13°97’ et 14°32’ Nord et les longitudes 16°39’ et 16°20’ Ouest. Ces terres salees existent depuis le Quaternaire recent, mais leur evolution actuelle est amplifiee par la variabilite climatique au cours de ces dernieres decennies et l’action de l’homme. L’objectif de cette recherche est d’analyser la dynamique des terres salees dans le nord de l’estuaire du Saloum. La demarche methodologique adoptee est basee sur le traitement des images satellitaires Landsat MSS de 1973, TM de 1984, 1992, ETM+ de 2006 et OLI et TIRS de 2014. La classification supervisee par maximum de vraisemblance validee par des verifications de terrain, par determination des matrices de confusion et du calcul des indices de Kappa, a permis de caracteriser la dynamique des terres salees. Les resultats obtenus par la classification ont montre une forte avancee des terres salees durant ces deux dernieres decennies. De 1994 a 2014, les terres salees ont augmente de 21 000 ha. Les terres salees gagnent chaque annee 1 000 ha en moyenne depuis 1994.

Data on spawning energetics and otolith microchemistry in female Ethmalosa fimbriata sampled in Senegal (Atlantic coast and Sine Saloum estuary)

Data on spawning energetics and otolith microchemistry in female Ethmalosa fimbriata sampled in Senegal (Atlantic coast and Sine Saloum estuary)

Seasonal growth differences of larval Hyporhamphus picarti (Hemiramphidae) in the Sine Saloum estuary, Senegal

Summary The African halfbeak Hyporhamphus picarti (Hemiramphidae) is one of the most abundant species within the ichthyoplankton community of the Sine Saloum estuary (Senegal). A year-round occurrence of larvae suggests that the Sine Saloum is an important spawning habitat for this species. Annual fluctuations in water temperature, however, can have severe impacts on the survival probabilities of marine fish larvae. To determine whether temperature has an effect on the growth of H. picarti during its larval development, larval age at length and somatic growth rates were investigated for two contrasting spawning seasons in 2014: February (cold season, 20.8°C) and June (warm season, 26.4°C). In both months H. picarti larvae were sampled at the mouth of the Saloum River using neuston nets. Sagittal otoliths’ increments were counted to estimate the larva age at a given standard length (SL). The age of larvae ranged between 2 and 22 days, with SL of 3.86–21.68 mm, respectively. In order to describe larval age at length during the contrasting spawning seasons, two distinctive Gompertz functions were applied. Accordingly, specimens sampled in June (0.94 ± 0.17 mm per day) exhibited significantly higher somatic growth rates than those sampled in February (0.60 ± 0.06 mm per day). These findings suggest that water temperature is an important factor influencing larval growth in H. picarti. Information concerning the early life stages of H. picarti are scarce and the results of the present study may contribute to a better understanding of the species’ biology and ecology.

Ethmalosa fimbriata (Bowdich 1825), a Clupeid Fish That Exhibits Elevated Batch Fecundity in Hypersaline Waters

Little is known about the concerted influence of temperature and salinity on the fecundity of clupeid fishes. Due to a globally changing climate, both physical parameters might act as stressors, severely affecting the reproductive potential of clupeid fish populations inhabiting tropical estuaries. Differences in relative batch fecundities, the gonado-somatic index, and the condition index of bonga shad (Ethmalosa fimbriata) were analysed in individual females sampled at the Senegalese coast and inside the inverse Sine Saloum estuary, where salinity increases upstream in all seasons. Multiple linear regression models on fecundity and gonadal energy storage show that clupeids can adapt towards increasing their reproductive investment at temperatures (26–30 °C) and salinities (42–51), which by far exceed marine conditions, in an effort to maximize recruitment success. This reproductive strategy, however, is accompanied by a trade-off between reproductive effort and somatic growth, which ultimately limits the species’ reproductive potential inside the estuary. The observed high variability in batch fecundities might be a viable mechanism to adjust to fluctuating and rather extreme environmental conditions. Understanding the spawning biology of exploited clupeid fishes in drastically changing environments is crucial for evaluating the reproductive potential of stocks at the outer reach of their physiological performance curve.

Composition and structure of the larval fish community related to environmental parameters in a tropical estuary impacted by climate change

Mangrove ecosystems have long been considered essential habitats and are commonly viewed and referred to as “nursery areas”. They are highly sensitive to climate change, and environmental transformations in these ecosystems are expected. The Sine Saloum estuary is a case of a system affected by global climate change where reduced precipitation and temperature increase have resulted in an inversion of the salinity gradient. Within the estuary, the composition and structure of the larval fish community related to environmental parameters were investigated using neuston and ring trawl nets. Larval fishes were sampled at 16 stations distributed along a salinity and distance-to-the-sea gradient during four field campaigns (November 2013, February, June, and August 2014) covering an annual cycle. This is the first study documenting the spatial and temporal assemblages of fish larvae in an inverse estuary. The total of 41 taxa representing 24 families and 34 genus identified in this study was lower than that of other tropical estuaries. Clupeidae spp. was the dominant taxon, accounting for 28.9% of the total number of fish larvae caught, followed by Gerreidae spp. (21.1%), Hyporamphus picarti (18.8%), Diplodus bellottii (8.9%), Hypleurochilus langi (4.8%), Mugilidae spp. (4.4%), and Gobiidae sp.1 (3.5%). A total of 20 taxa were recorded within the upper estuary region, whereas 29 and 37 taxa were observed in the middle and lower reaches, respectively. While larval fish were captured at all sites and during all seasons, abundances and richness decreased with increasing salinity. Larval fish assemblages also showed a clear vertical structure corresponding to three distinct water strata. Salinity, water temperature, and dissolved oxygen were the variables that best explained the spatial and temporal differences in larval fish assemblages. It is difficult to forecast the future situation for this system but so far, compared to other mangrove estuarine systems, we have observed the loss of freshwater species in favour of species of marine origin. The information provided in the present study is a contribution to the knowledge of tropical biodiversity and modifications of the ichthyoplankton communities in the context of climate change and future green fund action.

Temporal changes in groundwater quality of the Saloum coastal aquifer

Abstract Study region Groundwater in the southern part of the Saloum Basin in Senegal. Study focus The Saloum estuary is a hypersaline and ‘inverse’ estuary where the salinity of river water increases in the upstream direction. This region is problematic in that due to the underlain superficial Continental Terminal aquifer bordered by the hypersaline estuary constitutes the unique fresh groundwater reservoir for water supply for its estimated 466,000 residents living in 18 rural districts (belonging to the regions of Fatick, Kaolack and Kaffrine). This is of high value given that the deep Maastrichtian aquifer (200–300 m depth) is saline. This study aims to describe and understand temporal changes in the chemical and isotopic compositions of groundwater, the geochemical processes and especially the groundwater salinization. New hydrological insights for the region The analytical data were discriminated into 3 groups on the basis of the water types. Na-Cl, Ca-Cl and Ca-SO4 rich waters derived from saline water intrusion at the vicinity of the Saloum River accompanied by ion exchange reactions and pollution dominate the first group. The second group located mainly in the centre and eastern parts of the region is featured fresh groundwater of Ca-HCO3 derived from calcite dissolution reactions. The third group of Na-HCO3 type and less mineralized indicates freshening processes by recently infiltrating rainwaters. Slight seasonal chemical variations are observed due to new infiltrating water reaching the water table. High variation in rainfall between the 2 reference years (2003 and 2012) also changes chemical patterns in the groundwater. Chemical evolution of the groundwater is geographically observed and is due to a combination of dilution by recharge, anthropic contamination and seawater intrusion. The results of environmental isotopes (δ18O, δ2H) compared with the local meteoric line indicate that the groundwater has been affected by evaporation processes before and during infiltration. The results also clearly indicate mixing with saltwater and an evolution towards relative freshening between 2003 and 2012 in some wells near the Saloum River.

Effect of a marine protected area on tropical estuarine fish assemblages: Comparison between protected and unprotected sites in Senegal

A comparative study of fish assemblages of a marine protected area and an exploited area was carried out in the Sine Saloum estuary (Senegal, West Africa). Data were collected at these two sites between 2008 and 2011. The two areas are morphologically similar, and comparison of the physicochemical parameters (salinity, transparency, temperature and percentage of saturation of dissolved oxygen) showed that the water masses are similar. The bio-ecological indicators and ecological, trophic and size structures were also compared. Given the environmental similarity of the two sites, their differences in bio-ecological characteristics and fish assemblage organization can be explained by the protection status of the MPA. The species richness and fish abundance per sampling unit were higher in the exploited site, but the mean trophic level and the observed maximum size of individuals were higher in the MPA. Although slightly higher for the MPA, the differences observed for biomass and average size between the two sites were not significant. The beta diversity in the MPA was higher than in the exploited site. The fish assemblage was dominated in terms of abundance by two pelagic herbivores Ethmalosa fimbriata (80%) and Sardinella maderensis (12.3%) and in terms of biomass by E. fimbriata (37.4%) and a benthic predator, Arius latiscutatus (23.7%). The assemblage of the MPA had a marine affinity characterized by the presence of piscivorous or generalist predators; large individuals were not uncommon. In the exploited site, the fish assemblage was relatively stable, dominated in abundance by E. fimbriata (33.4%) and S. maderensis (32.8%). In terms of biomass, a species of mullet, Liza dumerili, ranked first (26.2%), followed by E. fimbriata (22.4%) and S. maderensis (14.9%). The assemblage in the exploited site had a more estuarine affinity, dominated by detritivorous or phytophagous herbivores. Individuals observed there were mostly of small or medium size. This study confirms that the establishment of MPA is an effective tool for restoring marine biodiversity and trophic structure of fish assemblages.

Ecological field experiment of short-term effects of fishing ban on fish assemblages in a tropical estuarine MPA

Marine Protected Areas (MPAs), in addition to their important role in the conservation of critical habitats and vulnerable species, are now also used in a context of ecosystem-based fisheries management. The aim of this study was to assess the observed changes in the fish population structure in the years following a fishing ban. The experiment took place in the Bamboung bolong, a small bay in the Sine Saloum estuary in Senegal, West Africa. The protection of this zone was undertaken with local fishermen support, and a formal MPA was created in 2004. The survey of the fish assemblages was initiated in 2003, the year before the fishing ban, and went on until 2007, with 12 sampling sites being monitored three times per year.In this paper, we describe and analyze the evolution of the fish assemblage during this five years' experimental period. Some changes have been highlighted on which the effect of the fishing ban was shown. After the fishing ban, an increase of total biomass and of maximal fish length has been registered. In the same time, the number of species has increased and the percentage of large and/or iconic species targeted by fishing has also increased. The community structure has been modified, with more small fish, more big fish (new large species and more large individuals in the original species) and fewer medium sized fish. The contribution of marine affinity species has increased in depend of the estuarine part of the assemblage. Finally, the trophic structure has been modified with an overall increase of the mean trophic level, resulting from an increase of the percentage of generalist or piscivorous predators and a sharp decrease of herbivorous and detritivorous low trophic level species. Marine predators which numbers and size were reduced by fisheries are again important components of the system. We argue that despite the intrinsic variability of the tropical estuarine environments, the introduction of a total ban on fishing in such estuarine zones can substantially improve the health status of the fish assemblage.

Parasitic Burden and Pathologic Effects of Anisakis Sp. (Nematoda : Anisakinae, Anisakidae) and Contracaecum Sp. Larvae (Nematoda : Anisakinae, Anisakidae) on Mugilids from Senegalese Estuaries

Consumption of mugilids is widespread in Senegal, especially in the estuaries, where there production is of economic importance. The prevalence, mean intensity and pathologic effects of Anisakis sp. and Contracaecum sp. larvae which infected mugilids from Senegalese estuaries were investigated. In 2009, individuals of mugilids belonging to 6 and 5 species were trapped from the estuaries of Saloum, Senegal and Casamance rivers respectively. The prevalence and mean intensity of Anisakis sp. and Contracaecum sp. are higher in Senegal and Saloum estuaries respectively. The increase in the load of Contracaecum sp., related to the season in L. falcipinnis, the season, the size, the weight and/or the sex in M. cephalus from the estuaries of Saloum and Senegal and M. curema from Saloum estuary were due to an accumulation of parasites over time. Liver, kidney, body cavity and mesentery and ovaries are the respective sites of infection of Anisakis sp. and Contracaecum sp. larvae. The immune response observed is the formation of a fibrous capsule surrounding the parasites with melanin granules all around the capsule. The implementation of the capsule and the presence of melanin granules suggest the involvement of lymphocytes, monocytes, neutrophils and macrophages. The lesions caused by Anisakis sp. and Contracaecum sp. larval settlement in target organs may cause the death of the host and/or a reduction in fertility.

A New Species ofCucullanus(Nematoda: Cucullaninae, Cucullanidae) fromMugil curema(Mugilidae) in Senegal (West Africa)

ABSTRACT: The presence of Cucullanid nematodes was investigated in 10 individuals of Mugil bananensis, 115 Mugil cephalus, 109 Mugil curema, 89 Liza dumerili, 37 Liza falcipinnis, and 40 Liza grandisquamis, all collected from the Saloum estuary in Senegal, West Africa. Cucullanids were found only in intestines removed from M. curema and were identified using light microscopy. The results showed that 13 individuals of Mugil curema were found to be parasitized by 22 adult nematodes of a species new to science, Cucullanus djilorensis n. sp. (prevalence 11.9%, intensity 1.7). This species differs from all other species of the genus Cucullanus by a number of features including the form of the esophagus, the number and distribution of caudal papillae, the position of the excretory pore, and the length of the spicules. Cucullanus djilorensis n. sp. is the first species of the genus described from an estuarine fish in West Africa.

Seasonal and Spatial Distribution of Nematode Larvae of the Genera Anisakis and Contracaecum (Anisakidae) in Two Populations of Mugil Cephalus (Mugilidae) from Saloum and Senegal Rivers

Anisakis sp. and Contracaecum sp. are nematodes belonging to the family of Anisakidae. They are worldwide distributed and use a great number of fish species as intermediate, paratenic or final host. Despite their effects on fish and human beings, they play ecological role as biological tags to identify fish population or bioindicators of marine pollution. From January to December 2009, the helminthological examinations of 129 and 167 specimens of Mugil cephalus originating from Saloum and Senegal estuaries respectively revealed the presence of Anisakis sp. and Contracaecum sp. larvae. Generally, whatever the season or locality, the kidney lodges more larvae of Anisakis sp. than the liver or the general cavity . A ll Contracaecum sp. larvae were found in the mesentery of Mugil cephalus whatever the locality and season. In each locality, the prevalence of Anisakis sp. larvae was higher in rainy season than in dry season whereas the larvae of Contracaecum sp. were more frequent in dry season than in wet season. Mean intensity of the Anisakidae larvae showed significant differences according to the season in the estuary of Saloum and non significant variation in the estuary of Senegal. The mean abundance of these larvae varied significantly depending on the locality, the season or the site of infection. These results are explained by factors related to the host (fat content in infested organs), factors in relation to the environment (temperature, salinity) and the presence of birds, final hosts. In the Saloum estuary, Anisakis sp. was dominant in rainy season, while Contracaecum sp. dominates in dry season. However, in the estuary of Senegal, Anisakis sp. was dominant all the year. So we can say that Anisakis sp. is characteristic of the estuary of Senegal while Contracaecum sp. is characteristic of the Saloum estuary. Distribution of anisakidae larvae is more related to the locality, the season and the site of infection rather than the fish gender.

The impact of environmental degradation on reproduction of the black-chinned tilapia Sarotherodon melanotheron from various coastal marine, estuarine and freshwater habitats

There is growing evidence that climate change has greatly altered environmental conditions in many aquatic ecosystems over the last decades, leading to changes in fish distribution and life history traits. Recent works conducted in Senegalese and Gambian coastal marine, estuarine and freshwater ecosystems have shown important changes in the intensity, frequency and breeding timing of wild population of Sarotherodon melanotheron in response to changes in salinity regimes. In addition to salinity, this study investigates the potential influences of other environmental factors that have received less attention on the reproduction of S. melanotheron from three different aquatic ecosystems in Senegal. The results demonstrate that day-length and temperature affect sexual maturity in both males and females from Guiers Lake and Hann Bay, but no such effects were found in upstream of the Saloum Estuary, where the spawning activity seems to be under the synergetic control of rainfall and salinity that apparently predominate over all the other ambient factors. This study demonstrated for the first time that aside from photoperiod and temperature, rainfall also influences fish reproduction in Hann Bay probably through its effects on water quality. Furthermore, our results confirm previous findings that changes in salinity regimes resulting from seasonal variations in precipitation and evaporation are the ultimate causes of temporal changes in the reproductive activity of the species in this estuary.

Genetic diversity and adaptability of Sarotherodon melanotheron (Cichlidae) in coastal ecosystem

We analysed the genetic diversity of nine populations of two subspecies of Sarotherodon melanotheron using allozymes in relation to the existence of environmental pressures. The genetic variability of Sarotherodon melanotheron melanotheron in the Ivorian ecosystem (A = 1.23, He = 0.04 and P(95%) = 11.11) was lower than that of Sarotherodon melanotheron heudelotii of the estuaries in Gambia and Saloum (A = 1.68, He = 0.08 and P(95%) = 22.23). The analysis of allelic diversity revealed that the individuals of S. m. melanotheron subspecies from a much polluted environment (lagoon of Grand-Lahou or Ebrié) presented a relatively higher frequency of allele GPI-2*80 than those of a less polluted environment (Aby lagoon and Ayamé lake). The allele IDH-2*90 was expressed only in the specimens of S. m. heudelotii found in the Saloum estuary, a very saline environment compared to the estuary of Gambia. A deficit of heterozygosis was observed with the tilapia Sarotherodon melanotheron. This study confirmed the possibility of using allozymes as biomarkers and bioindicators of environmental disturbances.