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Demographic Structure and Level of Exploitation of the Small Pelagic Pellonula leonensis Boulenger, 1916 (Pisces; Clupeidae) Fishery in the Aghien lagoon, West Africa

Demographic structure and level of exploitation of Pellonula leonensis in the Aghien lagoon were examined by applying length-weight relationship (LWR), Condition factor, Von Bertalanffy model, Mortality parameters, Exploitation rate, Recruitment pattern and Beverton and Holt analysis. Fish population were sampled monthly during one year between June 2014 and May 2015 from artisanal and experimental captures in the Aghien Lagoon. Except LWR and the condition factor, studied parameters were provided by the FiSAT II package. The negative allometric growth (b = 2.61) was reported for Pellonula leonensis. Mean values of the condition factor (CF) vary significantly from one month to another (Anova, p ˂ 0.05). Concerning growth parameters, results indicated that the asymptotic length (L∞) has been estimated at 126.84 mm SL, growth coefficient (K) was 0.67 year -1 and growth performance index (Φ') was 2.03. Growth modelization revealed 4 cohorts for Pellonula leonensis. The estimates of the total (Z), natural (M) and fishing (F) mortalities were 1.87, 0.92 and 0.95 year -1 respectively. The recruitment pattern was continuous throughout the year with two Gauss curve. The exploitation rate obtained (E = 0.51) was close to E0.1 (E0.1 = 0.55), thus indicating that the P. leonensis stock is in an optimum state of exploitation. Journal of Biology and Life Science ISSN 2157-6076 2019, Vol. 10, No. 1 40


Introduction
In Côte d'Ivoire, fish represents 70% of the animal proitein consumed and its consumption level is between 11 and 14 kg per inhabitant per year (FAO 2008). National fish consumption is between 250000 and 300000 T per year for an average local production of 80000 T (FAO, 2014). Of the fishery resources landed, particularly, fish called "small pelagic" and "semi-pelagic" have a proportion of 80 to 90% (FAO, 2008). They play a role as a "food safety net" (Gaoussou, 2011). According Yao (2008), the species Pellonula leonensis has a considerable contribution in these groups of fish called "small pelagic" and "semi-pelagic". P. leonensis is the subject of special fisheries in lagoons and main Côte d'Ivoire lakes (Yao et al., 2015). For these authors, the production of this fish is quite important with average monthly catches estimated at 11 T in Lake Taabo. Koné et al. (2011), note that this species of fish is of considerable economic interest in the locality of Kossou where it is marketed in dry form. This small pelagic belonging to the family Clupeidae is a fish known in Côte d'Ivoire under the vernacular name of "mimi la go". It is remarkable by its first pre-pelvin shield of identical size to the following eccussions without ascending arms and located after the origin of the pectoral (Froese and Pauly, 2017). In the vast majority of continental and estuarine environments in West Africa, P. leonensis is responsible for the most targeted fish species in commercial fisheries (Uneke et al., 2010). For better stock management and conservation of the species, a thorough knowledge of population dynamics is needed. Indeed, knowledge of the parameters of growth and exploitation is indispensable for the study of population dynamics. Research has been conducted on the species P. leonensis. These studies relate in particular to the length-weight relationship and population parameters of the species in Lake Taabo , The reproduction in the Bandama River  and the diet in the Buyo Dam (Kouamé et al., 2006). This study aims to analyze the demographic structure and estimate the exploitation level of P. leonensis in the Aghien lagoon, West Africa.

Study Area
Aghien Lagoon is situated on the Ivorian coast of Atlantic Ocean, northern of Ebrie Lagoon, between latitudes 5°22'N and 5°26'N and longitudes 3°49'W and 3°55'W ( Figure 1). This lagoon is separated by from the Atlantique Sea by the Potou Lagoon and the Ebrie Lagoon. The Aghien and Potou Lagoons communicate through a natural channel (Koffi et al., 2014). The Aghien Lagoon could reach 11 m deep (Guiral and Ferhi, 1989). This lagoon has a surface area of 20 km² for a perimeter of 40.72 Km. Three rivers Me, Djibi and Bé té are effluents of Aghien Lagoon and is almost exclusively continental all year long. This gives to the hydrosystem a fluvial character (Koffi et al., 2014). Located in an estuarine zone, the ichthyological diversity of this lagoon is strongly influenced by species of marine and continental origin. The result is a very diverse fish community with intense fishing activity (Bé dia et al., 2009;Traoré et al., 2014).

Mortality Parameters and Exploitation Rate
The total mortality coefficient (Z), was estimated using the linear length-converted catch curve method incorporated in FiSAT, using the final estimates of L∞, K and the length distribution data for the specie (Gayanilo and Pauly, 1997). Assuming constant recruitment and constant mortality the length converted catch curves takes the form: Where N is the number of fish in length class i, dt i is the time needed for the fish to grow through length class i, t i age of the mid-length of length class i corresponding to the mid length of class i, and where Z, with sign changed, is an estimated total mortality. The von Bertalanffy growth parameters derived as described in the previous section were used as input data for the estimation.
The temperature used for this study was measured in-situ monthly during the period from June 2014 to May 2015.
Fishing mortality (F) was derived as the difference between of total mortality coefficient (Z) and natural mortality (M): (Abowei et al., 2010).
Following the estimations of Z, M and F, the exploitation ratio €, was obtained from: (Pauly, 1985).
The exploitation rate indicates whether the stock is lightly (E < 0.5) or strongly (E > 0.5) exploited, based on the assumption that the fish are optimally exploited when F = M or E = 0.5 (Gulland, 1971).

Size at First Capture
The catch-curve analysis was extended to an estimation of probabilities of capture by backward projection of the number (N) that would be expected if no selectivity had taken place (Sparre, 1987). From the analysis, the size at which 50% of a fish population is likely to be caught by fishing gear (L 50 or Lc) was estimated. By analogy, L 25 and L 75 were estimated.

Recruitment Pattern
The recruitment pattern was obtained from the estimated growth parameters by backward projection of length-frequency data, as done in ELEFAN, onto the time axis (Moreau and

Journal of Biology and Life Science
ISSN 2157-6076 2019, Vol. 10, No. 1 Cuende, 1991). This type of back-calculation allows identification of the number of seasonal pulses of recruitment that have been generated by the population represented in the length frequency data (Gayanilo et al., 2002). Input parameters were L∞, K and t 0 . Recruitment pattern was presented in terms of the percentage of recruitment versus time (months).

Estimation of Relative Yield (Y'/R) and Biomass Per Recruit (B'/R)
The relative yield per recruit (Y'/R) and biomass per recruit (B'/R) were estimated according to Beverton and Holt (1966) using the Knife-edge selection. Beverton and Holt (1966) method as modified by Pauly and Soriano (1986) were used to predict the relative yield per recruit (Y'/R) and relative biomass per recruit (B'/R) of the species to the fisheries. Y'/R was computed following the formula below: Where U = 1-(Lc / L∞) is the fraction of growth to be completed by the fish after entry into the exploitation phase; m = (1-E) / (M / K) = (K / Z) and E = F / Z is the fraction of mortality of the fish caused by the fishermen.
The predicted values were obtained by substituting the input parameters of Lc/L∞ and M/K in the FiSAT II package.
The relative biomass per recruit (B'/R) was estimated from the relationship:

Estimation of the Biological Reference Points
Yield per-recruit (Y'/R) and biomass per-recruit (B'/R) analysis to obtain reference points and evaluate the exploitation status of the specie was conducted. From the analysis, the maximum allowable limit of exploitation (E max ) giving maximum relative yield-per-recruit was estimated. The exploitation rate (E 0.1 ) at which the marginal increase in relative yield-per-recruit is 10% as well as the exploitation rate corresponding (E 0.5 ) to 50% of the unexploited relative biomass per-recruit were estimated.
Except the Length-weight relationship and the condition factor, all the parameters evaluated during this study are made using the FiSAT II package. This software is the most frequently used to estimate fish population parameters (Al- Barwani et al., 2007), because of relatively simple application, requiring only length frequency data. In addition, the only necessary and sufficient condition for the use of the FiSAT II package, for a species, is to have data of frequency distributions of lengths of at least 100 fish distributed over 10 to 20 length classes (Ahouanssou, 2011).

Variation of the size of Pellonula Leonensis
In this study, a total of 884 Pellonula leonensis were sampled in the Aghien lagoon for length frequency data. The standard lengths were measured of P. leonensis captured varying from 38 to 118 mm and of 78.04 ± 15.41mm average (Figure 2). The average size of P. leonensis varied significantly from one month to another (Anova, p ˂ 0.05). However, mean sizes of P. leonensis varied only slightly from December (SL = 77.68 mm) to Mars (SL = 74.25 mm) (Anova, p ˃ 0.05). The smallest average size is obtained in July (SL = 71.20 mm) and the largest in October (SL = 80.40 mm), June (SL = 80.92 mm), April (SL = 83.63 mm) and November (SL = 89.17 mm).

Length-Weight Relationship
The value of Log (a) = -4.25 and b = 2.61 were better fitter at r 2 = 0.71 ( Figure 3; Table 1). The allometric coefficient (b = 2.61) was significantly lower to 3 (p ˃ 0.05), indicated that growth was negative allometric for P. leonensis.

Condition Factor
Mean values of the condition factor of P. leonensis vary significantly from one month to another (Anova, p ˂ 0.05) (Figure 4).

Growth Parameters
Growth curves indicated that captured fish belonged to 4 cohorts for Pellonula leonensis ( Figure 5). The value of the adjustment index Rn is 0.20.The asymptotic length of P. leonensis (L∞) was 126.84 mm SL while the performance growth index (Ф') was 2.03. The growth coefficient (K) and the potential longevity (t max ) were respectively 0.67 year -1 and 4.47 years. The age at zero length was estimated as -0.45 year, which gave the von Bertalanffy growth equation for this species as: e -0.67 (t + 0.45) ).

Mortality Parameters and Exploitation Rate
The estimated Z value and linearized regression parameters obtained from the length curve are reported in Table 2.
Total mortality coefficient (Z) obtained by the length converted catch curve was 1.87 year -1 for 1.33 -3.16 years of age ( Figure 6; Table 2). The natural mortality (M) at 28.39°C was 0.92 year -1 while the fishing mortality (F) was 0.95 year -1 . Exploitation rate (E) for Pellonula leonensis in the Aghien Lagoon was 0.51.

Size at First Capture
The length at first capture (L 50 or Lc) i.e. the length at which 50% of the Pellonula leonensis species are vulnerable to capture was estimated at 57 mm SL (Figure 7). The logistics model selection also showed that 25% of fish at 51.94 mm SL and 75% of fish at 62.06 mm SL were caught. Figure 7. Probability of capture of P. leonensis from Aghien Lagoon

Recruitment Pattern
The recruitment pattern showed that Pellonula leonensis was recruited in the fishery continuously throughout the year with two Gauss curve ( Figure 8; Table 3). Monthly values of recruitment percentages for P. leonensis are shown in Table 3. The two periods of intense recruitment of the specie to the Aghien lagoon are from April to May and October to November.

Yield and Biomass Per Recruit and Biological Reference Points
The relative Y'/R and B'/R analysis were estimated using Lc/L∞ = 0.44 and M/K = 1.37 as input for knife-edge selection procedure (Figure 9). The different colors correspond to relative yields (Figure 9 a) and relative biomasses (Figure 9 b) per recruit. The red zones delineate the optimal values of relative yields and relative biomasses per recruit. The values of relative yield per recruit and relative biomass per recruit based on exploitation rates are shown in Table 4.
The relative yield per recruit (Y'/B) increases to a maximum then decreases while the curve of the relative biomass gradually decreases with the increase of the level of exploitation (Figure 9 c). The maximum allowable limit of exploitation level (E max ) that gives the maximum relative Y'/R (E max = 0.63) was greater than the exploitation rate (E = 0.51).
The exploitation level (E 0.1 ) at which the marginal increase in relative yield per recruit is 10% of its value at E = 0 was 0.55 whereas the exploitation level (E 0.5 ) which corresponds to 50% of the relative B'/R of an unexploited stock was 0.34. The current exploitation rate (E = 0.51) is almost equal to E 0.1 , but it is greater than E 0.5 .

Journal of Biology and Life Science
ISSN 2157-6076 2019, Vol. 10, No. 1 50 Figure 9. Relative yield-per-recruit (Y'/R) and biomass-per-recruit (B'/R) curves for P. leonensis from Aghien lagoon using knife-edge procedure Table 4. Values of relative yield per recruit and relative biomass per recruit based on exploitation rates for P. leonensis from the Aghien Lagoon

Structure in Size
The population structure of Pellonula leonensis in the Aghien lagoon indicates that the average monthly size of fish varies significantly. The higher average sizes obtained in October, June, April and November could be explained by the fact that during these months, the food resources are more available to the growth of P. leonensis at the Aghien lagoon. Indeed, Lé vê que (1999) states that a fish species is maintained in a long-term environment to the extent that it finds a set of conditions that allow it among others to grow and reproduce. In addition, these months correspond to the rain and flood season at the Aghien lagoon (Ettien, 2010). During this period trophic conditions become more favorable for growth because of the better availability of food resources. The average size obtained at the Aghien lagoon (mean = 71.2 mm LS) is higher than that recorded by Koné et al. (2014) in Lake Taabo (mean = 32.59 mm LS).

Length-Weight Relationship
The negative allometric growth (b = 2.61) obtained for Pellonula leonensis in the Aghien lagoon corroborates the results of Ecoutin and Albaret (2003) which obtained the same type of growth for the species in the Ebrié lagoon. However, the type of growth observed in this study differs from that of Koné et al. (2014) indicate positive allometric growth, with a value of b = 3.24 for P. leonensis in Lake Taabo. This difference between the values of the allometric coefficient b could be explained by differences in the environmental factors, the physiological conditions of the fish at the time of sampling (gonad development and nutrient conditions), food competition in the ecosystems and to the different size ranges observed as underlined by Froese (2006) ; Kalhoro et al. (2014).

Condition Factor
The average values of the condition factor of Pellonula leonensis at the Aghien lagoon vary significantly from one month to another. Analysis of this parameter revealed that low average KC values are recorded in the months of September and October. According to Kingdon and Allison (2009), September and October correspond to the reproductive period of P. leonensis in the Niger Delta. In addition, Edoukou et al. (2017) ; Djadji et al. (2013) note that during the breeding season, fish feed less and begin to tap into their reserves to meet energy requirements. This could explain the low condition factor values of P. leonensis obtained in September and October at the Aghien lagoon.

Growth Parameters
Concerning the growth parameters according to the von Bertalanffy model, the determination was based on the electronic analysis of the size frequencies. The adjustment of the von Bertalanffy equation to the available size frequencies is expressed by an index (Rn) which can vary between 0 and 1 according to Pauly (1987). For the population of Pellonula leonensis exploited at the Aghien lagoon, the value of Rn obtained (Rn = 0.20) is included in this range. This indicates that the current method used best fits and that the estimated Journal of Biology and Life Science ISSN 2157-6076 2019 parameters are reliable. Growth coefficient analysis (K = 0.67) reveals that P. leonensis has a tendency to grow more rapidly towards asymptotic length (Branstetter, 1987). In addition, based on the K value, Jutagate and De Silva (2003) note that this fish species is a long-lived species. The growth performance index (Ф' = 2.03) recorded during the present study is close to those of Bé dia (2015) obtained for another Clupeidae Ethmalosa fimbriata in the Aghien lagoons (Ф' = 2.71) and Potou (Ф '= 2.64). In addition, Pauly and Munro (1984); Pé rez-Bote and Roso (2012) indicated that species within the same family are expected to have similar values of growth performance index.

Mortality Parameters and Exploitation Rate
This study shows that, in Pellonula leonensis at the Aghien lagoon, the natural mortality rate (M = 0.92 year -1 ) is equal to the fishing mortality rate (F = 0.95 year -1 ). This result indicates an optimum exploitation status for P. leonensis stock (Gulland, 1971). This result is confirmed while considering the analysis of Beverton and Holt's model (1966). Indeed, this analysis revealed a value of E 0.1 of 0.55 identical to the value of the exploitation rate E (E = 0.51). Otherwise, in Lake Taabo (Bandama Basin, Ivory Coast), Koné et al. (2014) reported in the same species a natural mortality (M = 2.55year -1 ) greater than the fishing mortality (F = 0.93 year -1 ). They also pointed out that the P. leonensis stock is under exploited in Lake Taabo with an exploitation rate of 0.27. This difference in the stock status of the two populations in the two environments could be justified by the mathematical expression of the exploitation rate. In fact, the natural mortality of P. leonensis in Lake Taabo is much higher than that of the species at the Aghien lagoon. The exploitation level of the species was higher than E 0.5 (E 0.5 = 0.34) for which the stock is reduced by 50% of its unexploited biomass. This result indicates that the current exploitation of P. leonensis in the Aghien lagoon may not ensure the durability of the biomass of the species.

Recruitment Pattern and Probabilities of Capture
Recruitment of Pellonula leonensis shows two Gauss curves. This observation is consistent with Pauly's (1982) assertion that tropical fish exhibit double annual recruitment. This result translates two breeding periods per year for P. leonensis in the Aghien lagoon. Recruitment is continuous throughout the year but is marked by significant peaks. The periods of intense recruitment coincide with the rainy seasons when the trophic conditions become favorable for the growth of young individuals of the studied species. In fact, the strategy for tropical fish would be to match the larval emergence period to a time when the environmental factors are more likely to survive the larvae. For example, the reproductive biology of P. leonensis (Koné et al., 2011) reveals that the monthly variation of the gonad index peaks in April in Lake Kossou (Côte d'Ivoire) and another in October in the Niger Delta (Kingdom and Allison, 2009). The diagnosis of Pellonula leonensis fishing at the Aghien lagoon through the Lc / L∞ ratio (Lc / L∞ = 0.44) reveals that catches are dominated by small individuals (Moreau and Cuende, 1991).

Conclusion
This study suggests that for optimal exploitation of Pellonula leonensis at the Aghien lagoon, it would be necessary to increase the size of the first capture and maintain the current exploitation rate (Figure 10).

Figure10. Schematization of the intervention on
Lc and E for optimal exploitation of P. leonensis at the Aghien lagoon. Lc: The length at first capture; L∞: The asymptotic length