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West African Ilisha was the third most abundant species of Clupeidae off Benin coastal waters after
*Sardinella spp* and
*Ethmalosa frimbriata*. The fishing effort of these fisheries increased with a dominance of small-sized specimens in the catches. This paper allowed updating some demographic parameters and the exploitation rate of
*Ilisha africana* collected between July 2013 and June 2014 from the coastal waters of Benin for management and conservation of these fisheries. The growth pattern showed a negative allometric growth with an abundance of small-sized specimens. The von Bertalanffy growth function (VBGF) estimations were:
*L*
_{∞}= 21.31 cm standard length;
*K* = 1.20 year
^{-1}; and
*t*
_{0} = -0.138 year. The total mortality rate (
*Z*), natural mortality rate (
*M*) and fishing mortality rate (
*F*) were 4.040 year
^{-1}, 2.27 year
^{-1} and 1.77 year
^{-1} respectively. The
*Z*
*/*
*K* ratio was 3.667 and the exploitation rate (
*E* =
*F/Z*) was 0.44 showing an under exploitation of this species. The estimated potential longevity (
*t*
_{max}) was 2.5 years. In addition, the fisheries management should be devolved from the state to the local level to compel fishermen to take greater responsibility for the sustainability and conservation of the fisheries such as size-limit regulation by gradually increasing fishing gears mesh size.

The direct consequence of maritime congestion caused by the presence of an important number of local and foreign fleets remains overfishing, one of the big threats on marine fauna and flora in West-Africa (Diouf, 1996 [

The small pelagic species of commercial importance for artisanal marine fishing in Benin are Sardinella aurita, Sardinella maderensis and Ilisha africana (FAO, 1991 [

Due to the high demand for these species, it is necessary to evaluate their population parameters to ensure proper management of this fishery.

Studies on the fish community are useful for a durable management of fishing activities (Aliko et al., 2010 [_{∞}) and growth coefficient (k), mortality rates and exploitation level were studied with the major objective of rational management and resource con- servation (Tah et al., 2010 [

Growth information provides a lot of tools that are used in fishery management. The data on the age of a fish can provide tools in fishery management such as the general background information needed for management decisions. It aids in the diagnosis of management needs such as the recognition of overcrowding and stunting (Carlendar, 1955 [

Mortality can be defined as the death of an organism. It is a very important aspect of population biology since it provides information about changes in the population. Mortality can be caused by fishing activity (fishing mortality) or by natural action (natural mortality) (Abowei and Hart, 2009 [

The present work is the first attempt to investigate growth rates, mortality coefficients and the exploitation rate of this species in the coastal waters of Benin to ensure the proper management of this fishery.

Specimens of I. africana were sampled from catches off Benin’s coastal waters (West Africa) landing at the artisanal fishing port of Cotonou (

The analysis of the length-frequency distribution was used to determine the size modal distribution. Histograms were obtained from the distribution.

The sex ratio calculated as

al., 2013 [

The length-weight relationships were expressed as: W = aL^{b} (Ricker, 1975 [^{2}) between length and weight was computed from the

regression curve. The value of b is a good indicator of the type of growth: the growth is isometric if b = 3 (Wootton, 1990 [

The asymptotic length (L_{∞}) is defined as the length that the species would reach if it lived indefinitely and the growth coefficient (K) is a measure of the rate at which the maximum size is attained (King, 1995 [_{∞} and K (We the rall, 1987 [_{0}) was estimated using Pauly’s empirical equation (Pauly, 1979 [_{10}(−t_{0}) = −0.392 − 0.275 Log_{10}L_{∞} − 1.038Log_{10}K.

To compare the growth of Ilisha africana from the study area with those from other studies, the growth performance index (φ') was calculated. The estimates of L_{∞} and K were used to compute the φ' (in terms of length) of the species (Munro and Pauly, 1983 [_{10}K + 2Log_{10}L_{∞}.

The annual instantaneous rate of total mortality, Z, was estimated from the linearized length-converted catch curves (Pauly and David, 1981 [_{10}M = −0.0066 − 0.279 Log_{10}L_{∞} + 0.6543Log_{10}K + 0.463Log_{10}T (Pauly, 1980 [

The instantaneous fishing mortality rate, F, was computed as Z − M, and the exploitation rate (E) was expressed as E = F/Z (Pauly, 1980 [

The value of the mean coefficient of growth has been used to generate longevity as shown by the formula t_{max} = 3/K (Anato, 1999 [

The probability of capture provides a clear indication of the estimated real size of fish in the fishing area that are vulnerable to a specific gear. This parameter is an important tool for fishery managers who, by regulating the minimum mesh size of a fishing fleet, can mostly determine what should be the minimum size of the target species of a fishery. The probability of capture was computed from the length-converted catch curve. A selectivity curve was generated using linear regression fitted which was used to estimate the final value of L_{25}, L_{50} and L_{75} (i.e., lengths at which 25%, 50% and 75% of the fish will be vulnerable to the fishing gear, respectively). Estimates of at-first-capture length (L_{50}) were derived from the probabilities of capture generated from the catch curve analysis output by FiSAT.

The exploitation rate (E) was generated by FiSAT. 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 (Guland, 1983 [

A total of 1,296 sampled specimens were sexed comprising 683 males and 613 females (

N (S − R) | Mean SL (cm) (range) | Mean W (g) (range) | |
---|---|---|---|

Males | 683 | 12.68 ± 0.05 (9.4.5 - 15.0) | 17.33 ± 0.01 (8.7 - 27.1) |

Females | 613 | 12.85 ± 0.03 (10.5 - 16.0) | 17.92 ± 0.02 (9.2 - 33.5) |

Males + Females | 1,296 (111.41%) | 12.77 ± 0.01 (11.5 - 16.0) | 17.63 ± 0.01 (8.7 - 33.5) |

The results of the L − W relationship of I. africana by sex are consigned in

females (p > 0.05). The respective L − W regressions for males, females and pooled samples are presented in Figures 5-7. The value of the Person correlation coefficient is close to 1; that means a positive correlation between the Total Weight (W) of the fish and its Total Length (TL). As well in males (b = 2.355) as in females (b = 2.327), b is significantly different from 3 (p < 0.05). The I. Africana exhibited a negative allometric growth (

Sex | Number | TL (cm) | W_{t} (g) | Equation | r^{2 } |
---|---|---|---|---|---|

Males | 683 | 11.3 - 18.5 | 8.7 - 27.1 | W_{t} = 0.028TL^{2.355}^{ } | 0.907 |

Females | 613 | 11.6 - 20.2 | 9.2 - 33.5 | W_{t} = 0.030TL^{2.327}^{ } | 0.886 |

Males + Females | 1296 | 11.3 - 20.2 | 8.7 - 33.5 | W_{t} = 0.026TL^{2.369}^{ } | 0.856 |

The asymptotic length output by FISAT L_{∞} (cm), the Growth coefficient K (year^{−1}), the Hypothetical age (t_{0}) and the Growth performance index φ' of I. africana in the coastal waters of Benin obtained are reported in

The Von Bertalanffy growth equation is as follows:

Authors | Countries | Length-weight relationship |
---|---|---|

Current study | Benin | W_{t} = 0.0684TL^{2.369 } |

Fiogbe et al. (2003) [ | Benin | W_{t} = 0.0057TL^{3.6 } |

King (1996) [ | Nigeria | W_{t} = 0.0078TL^{2.99 } |

Anyangwa (1991) [ | Sierra Leone | W_{t} = 0.0038TL^{3.35 } |

Stokholmand Isebor (1993) [ | Benin and Nigeria | W_{t} = 0.09287TL^{2.92 } |

Valiky and Cham (2003) [ | Sierra Leone | W_{t} = 0.008TL^{2.94 } |

Parameters | Values |
---|---|

Asymptotic length (L_{∞}) | 21.31 cm |

Growth coefficient(K) | 1.200 yr^{−}^{1} |

Hypothetical age (t_{0}) | −0.138 yr |

Growth performance index φ' | 2.736 |

The estimated values of asymptotic length (TL_{∞}) and growth coefficient (K) of the Von Bertalanffy growth equation were 21.31 (cm) and 1.200 (year^{−1}) respectively. _{0}) was t_{0} = −0.138.

The length-converted catch curve (

The longevity of I. africana in the Benin coastal waters is (t_{max}) = 2.50 yrs.

The estimated length at first capture was 12.92 cm (

The exploitation rate for I africana in the coastal waters of Benin was 0.44 (_{opt} = 0.5 criterion (Pauly and Munro, 1984 [

The asymptotic length for I. africana (TL_{∞} = 21.31 cm) suggests that the I. africana

Authors | Regions | M (yr^{−}^{1}) | F (yr^{−}^{1}) | Z (yr^{−}^{1}) |
---|---|---|---|---|

Villanueva (2004) [ | Ivory Coast | 1.421 | 0.138 | 1.559 |

Villanueva (2004) [ | Gambia | 0.991 | 0.518 | 1.509 |

Valiky and Cham (2003) [ | Sierra Leone | 1.8 | - | 5.1 |

StokholmandIsebor (1993) [ | Benin et Nigeria | - | - | 3.66 |

stock being exploited consists of relatively smaller-sized individuals. Larger specimens reaching TL = 34.54 cm have been reported in Ivory Coast where the modal length was 13.12 cm (range 13 - 15 cm) (Villanueva, 2004 [

The value of the Person correlation coefficient between the length and the weight was positive (r = 0.856, N = 1296,

Fish grows throughout its life. Fish growth is the result of the actions of specific endogen factors (genetic luggage) and exogenous factors that consist of abiotic characteristics (temperature, dissolved oxygen) and biotic characteristics (availability of food resources, feeding, intra- or interspecific competition) (Ezenwaji and Ikusemiju, 1981 [

According to literature (_{∞} = 21.31 cm) is close to what was found in Gambian estuary (Villanueva, 2004 [_{0} = −0.138. Few studies inform on the value of φ' of I. africana. In this study, the value of φ' was 2.736. Growth performance indexes of I. africana in other countries are reported in _{∞} = 21.31 cm) to a rate of K growth of 1.200 year^{−1} is coherent and close to those gotten in Gambian estuary (Villanueva, 2004 [_{∞} value results in the decrease of K value.

Mortality and growth parameters are antagonistic factors in the dynamic of populations (Ahouansou Montcho, 2011 [

Authors | Regions | TL_{∞} (cm) | K (an^{−}^{1}) | φ' |
---|---|---|---|---|

This study | Benin | 21.31 | 1.2 | 2.736 |

Villanueva (2004) [ | Ivory Coast | 34.54 | 0.75 | - |

Villanueva (2004) [ | Gambia | 22.75 | 0.38 | - |

Valiky et Cham (2003) [ | Sierra Leone | 28.2 | 1 | - |

Stockholm and Isebor (1993) [ | East of Niger delta | 22.0 | 2.33 | 2.90 |

King (1997) [ | Nigeria (marine) | 29.6 | 0.80 | 2.85 |

ratio < 1, the population is marked by the predomination of growth over mortality; if Z/K ratio > 1, then mortality predominates over growth; if Z/K ratio = 1, then the population is in an equilibrium where mortality balances growth (Barry and Tegner, 1989 [^{−1}) obtained in this study is higher than the fishing mortality (F = 1.77 yr^{−1}). These results suggested a weak exploitation of the I. africana stock in the study area.

The instantaneous total mortality Z reported in this study is close to what was found (Z = 5.1 yr^{−1}) in Sierra Leone (Valiky and Cham, 2003 [^{−1}) [^{−1} found (Villanueva, 2004 [_{max} = 2.5 yrs and the natural mortality M is 2.27 yr^{−1}. Based on these results and what was reported in Ivory Coast (t_{max} = 4 yrs, M = 1.421 yr^{−1}) (Villanueva, 2004 [

Fifty percent (50%) of fish specimens measuring at least 12.92 cm are vulnerable to the fishing gear. Comparing these results with the length-at-first-sexual-ma- turation (L_{50}) reported in Sierra Leone (Valiky and Cham, 2003 [

The exploitation rate E = 0.44 yr^{−1} in this study is inferior to the value of the optimal exploitation rate E_{opt} = 0.5 (Pauly and Munro, 1984 [_{opt} = 0.5 indicates that the stock of I. africana is slightly underexploitated in the coastal waters of Benin. The fact that samples were dominated by small-sized specimens implies management measures such as size-limit regulation by gradually increasing fishing gears mesh size (Sossoukpe et al., 2013 [

Edmond, S., Wilfrid, A.S.H. and Didier, F.E. (2017) Growth, Mortality Parameters and Exploitation Rate of West African Ilisha (Ilisha africana Bloch, 1795, Clupeidae) off Benin Coastal Waters (West Africa): Implications for Management and Conservation. Open Journal of Marine Science, 7, 327-342. https://doi.org/10.4236/ojms.2017.73024