Identification and characterization of critical sites for small pelagic fish in the coastal marine area of Senegal, West Africa

Better management of coastal marine resources generally requires good knowledge of habitats and resources. This need led to the performance of an identification and bioecological characterization study of critical sites for small pelagics in the Senegalese coastal marine zone. Biological and ecological data were collected using beach seines (12 to 28 mm mesh) and surface drifting gillnets (28 to 36 mm mesh) at sites identified by fishermen on the basis of surveys. The results of the study highlight the bioecological characteristics of these sites: (i) the Port-Baie Hann site a nursery and spawning site with a high diversity for the target species; it is under marine pollution threat; (ii) the Mbodiène-Ngazobil site, with high diversity for target species and the Kafountine-Cap site are nursery and spawning sites under high fishing pressure, (iii) the Diakhanor-Sangomar site a spawning site under the potential threat of exploration of offshore Sangomar oil in addition to the high fishing pressure. This work is a first flagship contribution to a process that should lead to the development and control of new tools for the effective and sustainable management of small pelagics at national and regional scales.


Introduction
In Senegal, small pelagic fishery provide the basis for a multitude of activities (fishing, fish processing and selling) and jobs. It supplies of cheap protein. So, 75% of the proteins consumed come from fishing, especially small pelagics (Dè me et al., 2012). Sardinella catches represent 70% of the total landings of artisanal fisheries in Senegal (Thiaw et al., 2017). The Sardinella fishery contributes mainly to the supply of local and regional markets (Dè me et al., 2012;Ba et al., 2017). It employs more than 25% of artisanal fishermen (16,000 fishermen) and provides about 70% of Senegal's fish consumption (Ba et al., 2017). In addition to these socio-economic benefits (Dè me, 2012), small coastal pelagic species (PPC) are also of ecological interest because of their position in the trophic food web (Cury et al., 2000). ISSN 2157-6076 2020 Despite these socio-economic and ecological benefits, these resources are under high fishing pressure due to increased effort and fishing capacity. In addition, the installation of fishmeal factories since 2010 along the coastline, closest to the landing sites, increases the fishing pressure on small pelagics (Mbaye, 2018). The development of illegal fish processing sites, mainly Sardinella and ethmalose, also encourages the exploitation of juveniles of small pelagics in Senegal. In such a context, it is necessary to strengthen the management of the small pelagic fishery by setting up appropriate management and conservation tools for the stocks and their habitats. This is the objective of this study, which aims to contribute to improving the management of small pelagics, mainly sardinella (Sardinella maderensis and S. aurita), ethmalose (Ethmalosa fimbriata), yellow horse mackerel (Caranx rhonchus), black horse mackerel (Trachurus trecae), mackerel (Scomber colias), anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus), through the bio-ecological characterization of critical sites for small pelagics in Senegalese waters.

Journal of Biology and Life Science
The concept of "critical site" is defined by the SRFC-RAMPAO Working Group of October 2018 as being: "a site of bioecological importance for small pelagics with a high socio-economic or cultural interest, exposed to threats likely to disturb environment and to compromise its ecosystem goods and services ". By this definition, the characterization of critical sites for small pelagics is of great interest for better management of this resource for the benefit of local communities.

Study Area
The Senegalese coast is divided into three large zones: the Grande Côte which extends from St. Louis (16 ° 04'N) to the Cape Verde Peninsula (14 ° 36'N), the Petite Côte that goes from Dakar (14 ° 36'N) to the North Sea border of The Gambia (13̊ 36'N) and the Casamance between the latitudes13̊ 04 'and 12̊ 20'N (Roy, 1989;Dème-Gning et al., 1990). Identified by previous work as the main nursery and spawning grounds for small pelagics, Cape Verde, Petite Côte and Casamance were chosen for this study (Figure 1). Sites previously identified by local fishermen and located in these coastal areas were all sampled. These were the sites of Soumbé dioune-Ouakam, Port-Baie de Hann, Yenne-Bargny, (Cape Verde), Cape de Naze, Mbodiè ne-Ngazobil, Diakhanor-Sangomar (Petite Côte), and Kafountine -Cap (Casamance) (Figure 1). Figure. 1. Location of critical sites for small pelagics sampled along the coastal marine area of Senegal

Surveys
In order to identify the critical sites of small pelagics (nursery and spawning grounds), surveys were carried out among 459 people composed mainly of fishermen. According to the socio-demographic profiles of the people interviewed, the Lebou (52%), guet-ndarien (17%) and niominka (16%) communities were the most represented. Considering residency status, indigenous were the majority (92%) in our surveys. The majority of the fishermen interviewed (30%) had fishing experience range of 20-30 years. In terms of fishing specialty, fishermen using mixed gear (targeting pelagic and demersal species) and those specializing in pelagic species were the most represented in our surveys, 52% and 39% respectively.

Experimental Fishing
Experimental fishings were conducted using a beach seine (SP) of 12 to 28 mm mesh and a surface drift gillnet (FMDS) of 28 to 36 mm mesh. The lengths of fishing gear ranged from 100 to 200 m. The choice of these fishing gear was explained by their complementarity for good longitudinal, bathymetric and species size coverage. The FMDS targets relatively large pelagic and semi-pelagic species, while SP catches the same species in smaller sizes. In addition, the fishing areas of the beach seine were more coastal than those of the FMDS. In each site, two fishing trips were made with each gear, to ensure the representativeness of the sample in terms of spatial coverage and number of individuals captured.
For the FMDS, the sampling was done at night: laying the net was done the day before at 7 pm and it was removed at 7 am the next day. As for the beach seine, the samplings were performed during the day between 7 am and 1 pm. After each fishing trip, fish were sorted by species. The individuals were measured, sexed, and sexual maturity stages determined according to the Albaret & Legendre (1985) scale. In addition, total catches as well as catches by species were noted.

Data Processing
Characterization of critical sites was performed on the basis of bio-ecological indicators such as size spectrum, sexual maturity stages and condition factor.
The analysis of the size spectrum makes it possible to verify the presence of juveniles or adults of the targeted species in the site. Total length (LT) was measured in fish to the nearest centimeter. Size classes of 0.5 cm were established.
The stage of sexual maturity informs about the occurrence of immature (stage ˂ 3) or mature (stage ≥ 3) fish, especially spawners in the laying phase (stages 4 and 5), according to the Albaret & Legendre (1985) scale.
The Fulton's condition factor K (Ricker, 1975) makes it possible to analyze the relationship between the species and its environment. Fish that live in unfavorable conditions generally have low weights relative to their length, i.e a low K, and vice versa. The K factor is a parameter that provides information on the physical shape of the fish. It was calculated according to the standard formula below: Where W is the total weight of the fresh fish (g) and TL is the total length of the fish (cm).
The sizes of first sexual maturity (L 50 ) of the target species obtained in previous work carried out especially in Senegal or in general in the sub-region were used to separate the immature individuals from those which were mature. Otherwise, the L 50 of the targeted species were collected from Fishbase. The relative abundance of the species was calculated from the percentage of the number of individuals.

Statistical Analysis
Multivariate analysis techniques such as factorial and automatic classification analysis methods were applied to resume the complex structure of data used in this study. The Hierarchical Classification Analysis (HCA) was used to group sites according to their affinity or similarity in terms of abundance and of diversity. Factorial Correspondence Analysis (FCA) was also carried out using fish abundance to investigate the pattern of small pelagic assemblage between sites. The dendrograms were performed using the Euclidean distance and the Ward minimum variance clustering method. Statistical analyses were carried out using R software and ade4 package.

Identification of Critical Sites
The results of surveys on critical sites for small pelagic fish showed, by zone, the main nursery and spawning sites (Figures 2A, B, C). In the Cap-Vert area, the sites of Soumbé dioune-Ouakam (37.7% and 27.2%), Port-Baie de Hann (20.2% and 20.7%) and Yenne-Bargny (21.9% and 17.9%) were identified, by fishermen, as the most important critical sites ( Figure 2A). The sites of Diakhanor-Sangomar (34.1% and 39.6%) and Mbodiè ne-Ngazobil (28.8% and 25.4%) were considered by the fishermen as being the main nursery and spawning sites in the Petite Côte area ( Figure 2B). In the Casamance zone ( Figure 2C), the main critical site identified by fishermen was Kafountine-Cap (70% and 69.8%). Petite Côte and C) Casamance

Fish Composition and Relative Abundance
A high diversity of species was noted in the sites of Port-Baie de Hann (8 species), Mbodiè ne-Ngazobil and Diakhanor-Sangomar (5 species each). Although the number of species was lower, the latter two sites had the highest relative abundances with respectively 51.6% and 26.2% of the catches. The species C. rhonchus (38.6%), E. fimbriata (27.9%), S. maderensis (17.4%) and S. aurita (8.5%) were the most abundant in all sites (Table 1). However, the highest catches were recorded at the sites of Diakhanor-Sangomar (54.2%) and Port-Baie de Hann (33.1%) ( Table 2). Scomber colias -   ISSN 2157-6076 2020 individuals. In the Mbodiè ne-Ngazobil site, a modal size of 9 cm, represented by 85% of individuals, was noted. Two modal sizes, one of 7 cm (18%) and the other of 15 cm (50%) characterized the species in the Soumbé dioune-Ouakam site (Figure 2A).
For Sardinella maderensis, a presence of only juveniles was noted in the Soumbé dioune-Ouakam site with a modal size of 9 cm (55%). In contrast, in the Bargny-Yenne site, only adults were observed, with a modal size of 25 cm (70%). At the Kafountine-Cap and Diakhanor-Sangomar sites, both juveniles and adults were observed. However, high dominance of juveniles (TM: 15 cm (70%)) and adults (TM: 20 cm (35%)) of S. maderensis were noted in these sites, respectively. As for the Mbodiè ne-Sangomar site, it was characterized by an important presence of both juveniles and adults of the species with modal sizes of 15 cm (30%) and 23 cm (20%), respectively ( Figure 2C).
For S. aurita, only juveniles were found in the Kafountine-Cap site (TM: 13 cm (65%)). In contrast, in the Mbodiè ne-Ngazobil site, both juveniles and adults were observed ( Figure 2D). As for Engraulis encrasicolus, although the presence of some adults was noted, most of the individuals caught were juveniles (TM: 8 cm (70%)). In the Port-Baie de Hann site, all individuals of Scomber colias and Trachurus trecae had sizes larger than their L 50 ( Figure  2D).

Sexual Maturity Stages
With the exception of the Port-Baie de Hann site where adults (stage ≥ 3) at the spawning and post-laying stage were observed, immatures (stage ˂ 3) of C. rhonchus were encountered in the others sites where the species was captured ( Figure 3A).
For E. fimbriata, at the sites where the species was encountered, significant proportions (57% at Cape de Naze) of adults from stages 3 to 6 were observed. Immatures were also captured at these sites, but at relatively low proportions (30% Mbodiene-Ngazobil) compared to adults ( Figure 3B).
The individuals of S. aurita and E. encrasicolus captured in the Mbodiè ne-Ngazobil site were Journal of Biology and Life Science ISSN 2157-6076 2020, Vol. 11, No. 1 28 sexually undifferentiated (89%) and of stage 3 (70%), respectively. The species Scomber colias and Trachurus trecae captured only in the Port-Baie de Hann site were represented by mature individuals (stages 3 to 6), ( Figure 3D). The highest K values were recorded at the Cap de Naze, Mbodiè ne-Ngazobil and Diakhanor-Sangomar sites (1.05 to 1.14). In contrast, the lowest K values were noted in the sites of Port-Baie de Hann and Soumbé dioune-Ouakam (0.89). Intermediate values were noted in the Yenne-Bargny and Kafountine-Cap sites (0.73 to 0.87). However, for these last two sites, high K values were noted respectively for C. rhonchus (0.88) and E. fimbriata (0.89), (Figure 4). ISSN 2157-6076 2020 Figure 4. Evolution of the condition factor for target species according to sites along the south marine coastal area of Senegal

Multivariate Analysis
The Hierarchical Classification Analysis (HCA) method allowed to classify sites in four groups relative to their species abundance ( Figure 5A). A projection of these groups was done using the Factorial Correspondence Analysis (FCA). The FCA, carried out on the species abundance ( Figure 5B), showed that the 1-2 factorial plane explained 81.7% of the total inertia (45.5% for axis 1 and 36.2% for axis 2). Therefore, results obtained from the first two axes were plotted. The factorial plan 1-2 showed that these groups were clearly distinct ( Figure 5B).  ISSN 2157-6076 2020 Figure 5. erarchical Classification Analysis (HCA) performed using species abundance from the different critical sites and Factorial Correspondence Analysis (FCA) relating associations of critical sites to species abundance

Discussion
Lebous, guet-ndariens and niominkas are the main traditional fishing communities in Senegal holding empirical knowledge of fishing activity and the dynamics of fisheries resources. As for the natives, they are the ones who have better control over the realities of the evolution of the fishery resources in their localities. The predominance of fishermen with experience between 20 and 30 years in the survey justifies the importance of fishing experience and especially the identification of critical sites for small pelagics. The predominance of specialized fishermen in pelagic fisheries or in the use of mixed fishing gear was also an asset in the validity of the information provided by fishermen in view of their knowledge of the target species.

Size Spectrum
The analysis of the size spectrum of small pelagic species provided a clear overview of the bio-ecological characteristics of the sites identified along Senegal's coastal zone. The Mbodiè ne-Ngazobil, Port-Bay of Hann, Kafountine-Abé né and Yenne-Bargny sites were characterized by high concentrations of both juveniles and potential small pelagic spawners. These results are consistent with previous observations, which showed that small individuals are encountered in the southern zone from July to December (Freon, 1988;Brochier et al., 2018).
In addition, the sites of Mbodiè ne-Ngazobil and Port-Baie de Hann had a high target species diversity (S. aurita, S. maderensis, Engraulis encrasicolus, Ethmalosa fimbriata and Caranx rhonchus), for the vulnerable stages. T. trecae and Scomber colias were exclusively found in the site of Port-bay of Hann. Kafountine-Abé né represented a nursery for both Sardinella and a concentration site for spawners of Ethmalosa. For the Yenne-Bargny site, juveniles of Caranx rhonchus and both juveniles and adults of Sardinella maderensis were observed. The site of Diakhanor-Sangomar was marked by the presence of high concentrations of mature adults of S. maderensis and E. fimbriata. In contast, that of Soumbé dioune-Ouakam was mainly home to concentrations of juveniles of C. rhonchus and S. maderensis.
These results are a refinement of earlier works conducted along the Senegalese continental shelf. Those works had identified the Petite Côte area (southern Cape Verde) and Casamance as nursery areas (Tiedemann and Brehmer, 2017;Brochier et al., 2018), and also an eggs and larvae retention area for small pelagics (Mbaye et al., 2015;Ndour et al., 2018). The effect of topographic features, such as capes and headlands modulates dramatically the onshore movement of upwelled water and favors the formation of stationary coastal upwelling centers (Faye et al., 2015;Capet et al., 2017). The retention of near-shore surface waters plays a major ecological role in the breeding of small pelagic fish (Ndoye et al., 2018).

Sexual Maturity Stages
The encountered sexual stages of the targeted species explained that the sites, where they were found, were nurseries and spawning grounds for small pelagics. However, there was a trend to spawning or nursery according to the sites, and a specificity of certain sites to some species. In fact, the Port-Baie de Hann and Mbodiè ne-Ngazobil sites were nursery and spawning sites that had the highest biological diversity in terms of target species. In addition, the Port-Baie de Hann site was characterized by the presence of species such as T. trecae and S. colias that had not been found in the other sites. In addition to these sites, there was Kafountine-Abé né , which was both a spawning site for ethmalose and nursery for both Sardinella species. The sites of Cap de Naze, Yenne Bargny and Soumbé dioune-Ouakam had a strong trend for the nursery and that for a certain number of species according to the site (C. rhonchus, S. aurita, S. maderensis, and E. fimbriata). The Diakhanor-Sangomar site had a strong spawning trend for the species S. aurita, E. fimbrata and S. maderensis. Like the work of Ba et al. (2019), our results did not show a clear link between marine protected areas and critical sites for small pelagics identified in the study. The main common characteristic of the sites described was that they were generally located outside the MPAs and were located most often at the outlet of the mouths of rivers.
These results are consistent with those of Sarré et al. (2018), which showed that areas of high concentrations of adults of Sardinella were located south of Cape Verde and off Casamance. They also confirm the results obtained on the retention zones (Mbaye et al., 2015), of nursery , spawning grounds (Tiedemann and Brehmer, 2017), and of high concentration of pelagic fish larvae (Ndour et al., 2018) along the coastal marine area of Senegal.

Vulnerability of the Sites
High K values for most target species at the Cap de Naze, Mbodiè ne-Ngazobil, and Diakhanor-Sangomar sites could be explained by the advection of upwelled waters arriving in these areas (Capet et al., 2017;Ndoye et al., 2018). To this, could also be added the quality and availability of food in the Petite Côte, an area recognized as one of the richest in primary and secondary production (Diadhiou et al., 2017;Ndour et al., 2018), and very favorable to small pelagics (Diankha et al., 2018). The low values of K noted in the sites of Port-Baie de Hann and Soumbedioune-Ouakam could be the result of an environment unfavorable to the target species under the effect of marine pollution in the coastal area of Dakar, and Hann Bay in particular (Bouvy et al., 2008;Net et al., 2015;Diop and Amara, 2016).
The clustering of sites, based on multivariate analyzes, demonstrates their different characteristics in terms of relative abundance and species diversity. The irregularity of the continental shelf structure along the southern coastal zone, which could affect environmental conditions, could explain the observed situation. Ba et al. (2016) showed that Sardinella are not very sensitive to long-term climatic fluctuations. However, the work of Diankha et al. (2018) have shown that the variability of Sardinella recruitment is related to fluctuations in surface temperature. These results are also consistent with those of Baldé et al. (2018), which explain the variability of recruitment in Ethmalosa fimbriata, by the effect of local climate variability.
In addition to the bioecological characteristics, there were threats that increase the vulnerability of the sites. According to the fishermen, the main threat identified in the Mbodiè ne-Ngazobil site was the high fishing pressure and especially the large number of beach seines in the area. As for the Diakhanor-Sangomar site, besides the threat of high fishing pressure (including fishing boats), exploration of oil off Sangomar could be a risk factor. For the Yenne-Bargny site, it was threatened by the installation of illegal sites for juveniles of small pelagics processing, which increased the fishing pressure on the resource. In addition, marine pollution affects fish habitats. A potential threat that could be related to oil exploration off Rufisque was also identified by the fishermen. The sites of Port-Baie de Hann and Soumbedioune-Ouakam were threatened by a very marked coastal marine pollution. The ongoing construction of the port of Ndayanne (Petite Côte) was a significant potential threat to the Cap de Naze site according to fishermen. Regarding the Kafountine-Abé né site, it was affected by high fishing pressure, because of the transfer of the fishing effort to the south part due to the scarcity of the resource north part of Senegal.

Conclusion
This study provides a better understanding of the structuring of sites of bioecological interest for small pelagic species along the coastal marine zone of Senegal. This result was obtained through the identification and characterization of sites of bio-ecological interest within the framework of the new management concept of small pelagic resources called "critical site", developed by the Sub-Regional Fisheries Commission (SRFC). This work is a flagship contribution to the improvement of the management of small pelagic resources, which until now has been the weak link in the sub-regional sustainable management system for small pelagics.