Length-weight Relationships of 30 Fish Species in Aby Lagoon , Southeastern Côte d ’ Ivoire

Most West African lagoons are very productive ecosystems that support important fishery activities. However, increases in human populations put heavy pressure on their resources. This raised the importance of sustainable management tools for these ecosystems. Length-Weight Relationships (LWRs) which is an important fish stock assessment tool was investigated in the Aby lagoon, southeastern Côte d’Ivoire. Fishes were caught monthly from February 2012 through March 2013 using gillnets. Fishes captured were identified, weighted to the nearest gram and standard length was measured to the nearest millimeter. The 30 fish species selected for this study belong to 18 families. The highest species number (n = 7) was recorded for Cichlidae family. All length-weight regressions were significant with a coefficient of determination (r2) varying from 0.462 (Parailia pellucida) to 0.984 (Elops lacerta). Growth coefficient, b of the LWR ranged from 2.229 (P. pellucida) to 3.811 (Eleotris vittata). Nineteen species exhibited negative allometric growth pattern while 11 had positive allometric growth. These results are useful references for West African coastal lagoon management and particularly for that of the Aby lagoon system.


INTRODUCTION
Worldwide, lagoon systems represent 13% of the coastline (Knoppers, 1994) and together with other coastal ecosystems contribute a large part of the ecological richness of the biosphere (Costanza et al., 1997).Due to their location between the continent and the sea and their shallow depths, lagoons are among the most productive ecosystems (Knoppers, 1994) but also very sensitive to both climatic and human impacts (Dumay et al., 2004).Lagoons can be high in productivity and serve as important nursery habitats that provide food and shelter for juvenile stages of many economically valuable fishes (Parrish, 1989).
In West Africa, estuaries and lagoons are sites of an important fishing industry for mollusks, crustaceans and fishes (Albaret, 1994;N'Goran, 1998).In this region the estuarine and coastal environments are also subject to heavy human pressure, with an annual population growth rate of 4-7 and 60% of industries located in coastal cities (Charles-Dominique, 1994;UNEP, 1999;Baran, 2000).Studies performed in Ebrie lagoon (Côte d'Ivoire) showed that fishing pressure could induced important changes on the fish community (Albaret and Laë, 2003): the main changes included lowering of fish diversity in catches, fish biomass, average catch length and trophic level of catches.These data raised the importance of management tools for a sustainable use of fisheries resources in these lagoons.Moreover, there is an urgent need to manage and regulate the small-scale coastal fishery in the region (Albaret and Laë, 2003;Njifonjou et al., 2006) and this requires basic population dynamics information for the target species.
Data on the functional Length-Weight Relationship (LWR) is important for fish stock assessment (Muto et al., 2000).Length and weight data are essential for estimating growth rates, age structure (Kohler et al., 1995), calculate the standing stocks biomass (Martin-Smith, 1996), condition indices (Safran, 1992) and several other aspects of fish population dynamics (Morato et al., 2001).
The present study describes the length-weight relationships for 30 coastal fish species in the Aby lagoon system which supports important fishery activities in south east Côte d'Ivoire.
Data collection and analysis: Fish samples were collected monthly from February 2012 through March 2013 using two batteries of gillnet (06 to 60 mm stretched mesh).Fishes captured were identified following Paugy et al. (2003a, b) and Decru et al. (2012).
Data on Standard Length (SL) in mm and total weight (W) in g were recorded for each fish.Lengthweight relationships of 30 species were estimated by fitting an exponential curve (Le Cren, 1951): Parameters a and b were then estimated by the linear regression analysis from log-transformed data: where, a = The intercept b = The slope (Tesh, 1971) For each species, the data collected were validated by the analysis of the graph corresponding to length-weight relationships (Andrade and Campos, 2002;Ecoutin and Albaret, 2003).The degree of association between the variables was assessed by the determination coefficient (r²).In order to verify if calculated b was significantly different from 3, the Students t-test was employed (p<0.05)(Sokal and Rohlf, 1987).

RESULTS
Summary of length-weight relationships for 30 fish species was presented in Table 1.
A total of 4466 individuals were collected and samples size ranged from 11 individuals for Clarias buettikoferi to 621 for Pellonula leonensis.Fish size was as small as 28.9 mm SL (28.6 g body weight) in Chrysichthys nigrodigitatus and as large as 400 mm (1145 g body weight) in C. nigrodigitatus.

DISCUSSION
Fish species studied in the Aby lagoon varied in size between 28.9 mm SL (28.6 g BW) in C. nigrodigitatus and 400 mm SL (1145 g BW) in C. nigrodigitatus.Typical classical estuaries are distinguished by a higher proportion of juvenile sizes which utilize this habitat as nursery and feeding grounds.Therefore the absence of fish below 28.9 mm SL may be associated with fishing gear selectivity rather than implying the absence of small sized individuals.
Except for P. pellucida the correlation coefficient (r²) for the length-weight relationship of the fishes (ranging from 0.725 in P. leonensis to 0.984 for E. lacerta) is high which indicate increase in length with increase in weight.This agreed with earlier studies involving fish species from different water bodies (Fagade, 1983;Lalèyé, 2006;Ayoada and Ikulala, 2007).
Several authors have reported both isometric and allometric growth for different species from various water bodies (King, 1991;Oribhabor et al., 2009).Parameters of length weight relationships are affected by several factors such as season, sample size, habitat, gonad maturity, sex, diet and stomach fullness, health, fish activities, seasonal growth rates and preservation techniques (Bagenal and Tesch, 1978;Mizuno and Furtado, 1982;Lowe-McConnell, 1987).Other factors such as temperature, trophic level and food availability in the community are also important (Mizuno and Furtado, 1982;Lowe-McConnell, 1987).All of these effects were not considered in the present study.Our results could be considered as mean annual values for the species since the fish samples were collected during different seasons throughout the sampling period and the data do not represent a particular season or time of the year.The regional differences of estimated b values may present spatial variations resulting from the influence of water quality or food availability on fish growth (Sparre et al., 1989;Mommsen, 1998).
In the current study maximum fishes showed deviations from the ideal value (b = 3).Similar observations were made for other West African fish population: in two reservoirs (Tah et al., 2012) and in some small coastal rivers of Côte d'Ivoire (Konan et al., 2007).Overall, it is recognized that only a fair number of species seems to approach the ideal growth pattern with b value equal to 3 (Anbalagan et al., 2009;Oribhabor et al., 2011).
This study provided the basic information on the length-weight relationships of 30 fish species from the Aby lagoon system that will be useful for the management of fishery resources.

ACKNOWLEDGMENT
This study is part of the research project entitled "Evaluation of refuge and nursery roles of the Tanoe-Ehy swamp forest for fishes of adjacent lagoon and marine ecosystems" and was funded by the Program

Fig. 2 :
Fig. 2: Distribution of the value b of length-weight relationships for 30 fish species caught in Aby lagoon (Côte d'Ivoire)

Table 1 :
Number of specimens (n), standard length (minimum and maximum), total weight (minimum and maximum) and parameters of the length-weight relationship of fish species caught in Aby lagoon system, southeastern Côte d'Ivoire