Sangaréah Bay is one of the coastal ecosystems closest to Conakry, the capital city of the Republic of Guinea. The bay is located to the north-west of the city of Conakry between 9˚11'N and 10˚5'N in latitude and between 13˚29'W and 13˚45'W in longitude, it covers an area of 38,000 ha and was home to 49,318 inhabitants from 51 villages, including 6 mangrove villages, in 1999 [12] . The study area is bounded to the east by Mount Kakoulima and part of the coastal chain, to the west by the Konkouré river, to the north by the Kabitaye classified forest and the sub-prefectures of Wassou and Tanènè, and to the south by the Atlantic Ocean and the Kaloum peninsula. It is fed by the Konkouré River, the Soumba River and the Sonfonia River. Sangaréah Bay is characterised by alluvial hydro morphic soils of marine or continental origin [13] . It is a typical estuarine zone used by fish as a breeding ground, nursery, refuge or feeding ground. This ecosystem is also a transit area for juvenile fish before they migrate to deeper waters [14] ( Figure 1 ).

Taking into account the habitat heterogeneity and the variety of activities in the Bay (wood cutting, salt farming, fish smoking, fisheries, domestic and agricultural wastewater), eight sampling point, distributed along the estuaries and mangroves in the bay, have been selected following a downstream-upstream gradient as in Table 1 .

Ichthyoplankton in the Sangareah Bay was sampled monthly during two years from January 2022 to December 2023. Samples were taken in the first week of each month following the same methodology as Tamoïkine [6] . After sampling, all fish larvae were sorted, identified and counted. The taxonomic determination was made based on the guide in [6] [15] - [21] .

Standard length and total length of larvae were measured under a microscope using an eyepiece micrometer. Weight of the larvae was also measured.

In total 5938 ichthyoplankton specimen were sampled in this study. These specimens belong to 30 species divided into 20 families, dominated by the Carangidae, Clupeidae and Sciaenidae, which each account for 10%, and the Ariidae, Cynoglossidae, Mugilidae and Polynemidae, which represent 7% ( Figure 2 ). Table 2 shows the list of inventoried species.

The dominant species were as follows: Ethmalosa fimbriata 9%, Sardinella aurita 8%, Sardinella maderensis 8% ( Figure 3 ).

The qualitative analysis shows that the study areas are rich in ichthyoplankton organisms.

Species of the Clupeidae family are widely represented Sangaréah Bay, accounting for 26%, followed by 6% and Trichiurus lepturus with 2% ( Figure 3 ).

For the quantitative study of the ichthyoplankton population, the density, biomass and egg count were determined in time and space.

Evaluation of the variation in density and mean biomass ( Figure 4 ) of the ichthyoplankton population reveals peaks during the months of June to September, corresponding to the rainy season in the study area. In June, which totals 8.01 ind/m³, 1.57% of which is represented by the clupeid family, which is the most diverse and abundant along the upstream-downstream gradient. Within this family, the genera Sardinella and Ethmalosa predominate, with biomasses of 58.29 and 65.56 g/m³ respectively. In July, with a density close to 13.63 ind/m³, 26% of which belonged to the class Clupeiformes, the genera Sardinella and Ethmalosa also predominated, with a biomass of 12.72 g/m³ and a density of 13.63 ind/m³ respectively. August, with an average biomass of 10.9 g/m³ and a density close to 9.72 ind/m³, of which 8.5% were clupeiformes; the genus Sardinella also recorded the highest density. Although less pronounced, in September, with 4.81 ind/m³ of which 5% are represented by the family Clupeidae, the genera Sardinella and Ethmalosa predominate with densities of 5.8 and 2.9 ind/m³ respectively and a biomass of 65.26 g/m³.

The monthly variation in the number of eggs ( Figure 4 ) shows abundance during the months of June to August, with a total average of 1134 eggs, compared with an annual average of 156 eggs. At the height of the rainy season (July-August), this average rises to 1501 eggs.

Looking at the different values obtained ( Figure 4 ), we can see that July has the highest concentration of ichthyoplankton in terms of density, biomass and number of eggs. These first two months justify the choice of the period for the introduction of biological rest in Guinea.

The high concentration of ichthyoplankton organisms is observed in the Sonfonia, Soumba, Arabanty and Samatran areas, where the density varies between 156.1 and 198.3 ind/100m³ ( Figure 5 ).

These areas are characterised by a massive presence of poison eggs. This suggests the hypothesis of ontogenetic development.

In short, the constant presence of eggs and larvae of ontogenetic stages make these estuary and mangrove zones environments of advection, migration, ontogenetic development and feeding.

Furthermore, the quantitative study of ichthyoplankton shows significant variations depending on the location of the estuary and mangrove areas in relation to communication with the sea. Remote areas, which receive little marine input, have very low and unstable abundances or specific richness.

It is worth noting the presence of various larval stages of meroplankton: molluscs, cephalopods (cuttlefish, squid), crustaceans, decapods (prawns, spiny lobsters, crabs, squid, etc.) and phytoplankton, which demonstrates the diversity of the planktonic ichthyo net in hydrobiont sampling.

In order to get an idea of the organisation and structure of the various ichthyoplankton populations inventoried, we calculated the frequency of occurrence of the various families inventoried and the similarity between the structures of the ichthyoplankton populations.

The frequency of occurrence or constancy C in % provides information on the characteristic species of the zones. A total of 10 families (i.e. 72.16%) are considered constant, with a cosmopolitan distribution and a broad ecological spectrum, and are found in all the study areas. Also, 3 families (20.10%) are considered accidental and 7 families are accessory. In terms of frequency of occurrence, there is a relative numerical similarity between the groups of constant and accidental genera. The majority of species reported as constant tolerate the various abiotic parameters characterising their habitat.

The presence of 72% of ichthyoplankton families with a “constant” frequency of occurrence (C > 50%) indicates that these species have a high ecological value and can withstand the conditions of different habitats ( Figure 6 ). Accessory species, on the other hand, may develop in a particular type of biotope or at fairly limited periods of the year.

Based on the presence or absence of species in the areas studied, we compared the composition of ichthyoplankton populations taken in pairs, using Sorensen’s similarity index. This index is obtained from the common genera, taken in pairs ( Table 3 ).

The similarity index calculated is greater than 50% in most cases of comparisons between two zones taken in pairs. The highest rate of similarity was noted between the Sonfonia, Samatran, Bouramaya, Bonèya and Arabanty zones, where the similarity coefficient was 63.79%. The similarity between the zones taken in pairs can be explained by the fact that they generally offer the same ecological conditions to the ichthyoplankton population. It is certainly not accidental, as it reflects the action of situational ecological conditions, in particular the action of the physico-chemical properties of the water on the development of these ichthyoplankton populations.

The structure of the ichthyoplankton population is similar in the different areas studied, demonstrating the similarity of the environmental conditions, particularly the physico-chemical properties of the water.

The composition of the ichthyoplankton population in the Sonfonia area is closest to that in the Soumba and Samatran areas, where the values recorded are 63.79% and 62.65% respectively. This similarity represents the greatest resemblance between the estuary and mangrove zones studied and the physicochemical parameters of the environment that characterise them.

The grouping of the sampling zones by means of a dendrogram ( Figure 7 ) using the simple link and for a minimum level of similarity makes it possible to distinguish 2 distinct homogeneous groups which are:

Furthermore, grouping the variables using the simple link gives a similarity rate of (63.79%), which is greater than 50%.

Our results show that there are significant differences between the 8 zones studied, which may be linked to seasonal differences. In addition, the differences depend on the diet of the ichthyoplankton population in each zone sampled, the physiological state of the individuals at the time of sampling and even the sampling period.