Length-weight relationships for 36 freshwater fish species from two tropical reservoirs : Ayamé I and Buyo , Côte d ’ Ivoire

Nowadays, the successful management of small scale fisheries requires the use of biometric data collected in the field, in order to transform them into suitable indicators. The present study describes the lengthweight relationships for 36 freshwater fish species from two tropical reservoirs Ayame I and Buyo, in Côte d’Ivoire. The main objective of the study was to provide a length weight key for a wide range of freshwater fish species from these tropical reservoirs exploited by the inland fisheries. The samplings were carried out at Buyo from July 1997 to August 1998, and from August 2004 to July 2005 in Ayame I. Fish specimens were collected from catches of artisanal fisheries using gill-nets, cast-nets, beach seines and bamboo traps. After landings, samples were identified, total weight for each specimen was recorded to the nearest gram and standard length was measured to the nearest millimetre. A total of 12 724 individuals belonging to 15 families and 24 genera were obtained in this study. The results indicated that the family with the highest number of species was Cichlidae with eight species. Six families were recorded with only one species per family. The value of the exponent b in the length weight relationships (W=aLb) ranged from 2.173 for Marcusenius furcidens to 3.472 for Polypterus endlicheri and the median of b was 2.756. The modal value of the exponent b equal to 2.70 indicates that most of the fish species in Ayame I and Buyo Reservoirs have negative allometric growth. The length weight parameters of the three species, Lates niloticus, Synodontis koensis and S. punctifer are described for the first time in these regions. The present length-weight key for 36 freshwater fish species could be used as a valuable tool for fishery managers, in order to improve the inland fisheries statistics largely based on hydropower reservoirs in Côte d’Ivoire. Rev. Biol. Trop. 60 (4): 1847-1856. Epub 2012 December 01.

inland fisheries in Côte d'ivoire are exclusively artisanal and fishers operate various The impoundment of hydropower reservoirs in Côte d'ivoire has allowed the development of inland fisheries, mostly in the large reservoirs of Ayame i, Kossou and Buyo (Da Costa et al. 2002).Officially, inland fisheries catches estimated at 18 000 tons are composed by Cichlidae (Oreochromis niloticus, Sarotherodon spp, Tilapia spp.), Osteoglossidae (Heterotis niloticus), Claroteidae (Chrysichthys spp.) which represent more than 88% of the gears depending on the season, the investment level, the fishing areas and the species targeted.These gears are gillnets, cast nets, long-lines, beach seines and various kinds of traps (Reizer 1968).Besides, reservoirs fisheries are characterized by the cohabitation of autochthonous population and non indigenous fishermen coming from the neighboring country of Mali.As of 1998, fishing activities in Ayame i were operated by the indigenous inhabitants of the reservoir district, after the eviction of the non indigenous fishermen (Tah et al. 2009).Regarding Buyo Reservoir, since September 2002 after the civil war event, a large part of this area has resulted inaccessible by fisheries managers.Consequently, no reliable data on fisheries statistics are available since.
According to Ecoutin & Albaret (2003), the management of small scale fisheries requires the use of biometric relationships.Among them, the length-weight relationships (LWR) are widely presented by authors as useful tools with several applications in domains of fisheries sciences, population dynamics, ecology and stocks assessments (Petrakis & Stergiou 1995, Santos et al. 2002).Knowledge of LWR allows, in a given geographic zone, the estimation of the average weight at given length (Ferreira et al. 2008).Moreover, this tool may provide important information concerning morphometric comparison between species and populations (King 1996, Gonçalves et al. 1997) and life history comparisons between regions (Weatherley & Gill 1987).
Studies on fish population biology have been carried out in these two reservoirs since their impoundment.Most of these studies focused mainly on fish fauna, biological and population parameters of some commercially important fish species (Traore 1996, Kone & Teugels 1999, Tah et al. 2010).However, none actually considered the length-weight parameters of fish species exploited by inland fisheries from the man-made reservoirs.Therefore, the aim of the present study is to avoid this deficiency and provide a length-weight key for a wide range of freshwater fish species.

Study areas:
Two reservoirs were surveyed in this study.Ayamé i is located in the South-East (05°30' -06°00' N and 03°00' -03°05' W) and Buyo in South-West (6°20' -07°03' N and 06°50' -07°4' W) (Fig. 1).For both reservoir areas, the climate characterized by an equatorial transition zone, with two rainy seasons separated by a short dry period from August to September, and a more pronounced one from December to March.The oldest hydroelectric reservoir is Ayamé i; it was built in 1959 on the Bia River, and has an average surface of 135km 2 (Laë 1997); the artificial reservoir is 80km long and 27km wide (at the maximum water level).The second hydroelectric reservoir of Buyo is the most recent dam and was built on the river Sassandra in 1980; it has a main channel total length of 100km and a wide varying from 200m to 10km, with an average surface of 920km 2 (Traore 1996).
Data collection and analysis: Samplings were carried out in a monthly basis in four landings sites at Buyo Reservoir during July, August, September and December 1997, and March and June 1998; and at three landing sites in Ayame i, between August 2004 and July 2005 (Fig. 1).Fish species were collected directly from the artisanal fishermen catch, that use gill nets of various mesh sizes (15 to 60mm stretched mesh), beach seines, bamboo traps and cast nets.The most frequently encountered fishing gears on both reservoirs were gillnets that are used throughout the year.Gillnets are usually set during the afternoon at about 16:00 h (GMT) and lifted the following morning at about 07:00 h (GMT).Large and medium-sized mesh gillnets mainly targeted high-valued large size species of Cichlid.Beach seines with 14mm mesh size and a length of 300m on average are activated mostly during the early part of the rainy seasons, when the water level rises.Bamboo traps and cast nets targeting Chrysichthys spp.are operated during the greater rainy season (March-July).
At Buyo Reservoir between 1997 and 1998, like at Ayame i Reservoir between 2004 and 2005, measurements were made in situ by the research team.Standard length for each fish was measured to the nearest millimetre using a measuring board ichtyometer (STAN-LEY Model CE-M 10-0594).Body weight was recorded with a precision balance to the nearest gram (OHAUS Model CT 6000).Species identification was based on Lévêque et al. (1990Lévêque et al. ( , 1992) ) and Paugy et al. (2003) keys.For each reservoir, data from all landing sites were pooled together.As mentioned by Lalèye (2006) and Konan et al. (2007), only species presenting a sample size higher than 10 individuals were taken into account.Taxonomic orders of fish species in this work is based in Nelson (2006) classification.
The LWR of fish was estimated by using the equation: W=a L b , where W=weight in grams (g), L=standard length in centimetres (cm), a is a scaling constant and b is the allometric growth coefficient.After logarithmic transformation of this relation (log10 W=log10 a + b log10 L), parameters (a) and (b) were determined via least squares linear regression (Zar 1999).The 95% confidence limits for b (CL 95%) were computed using the equation:  The value of b gives information on the kind of growth of fish: The growth is isometric if b=3 and the growth is allometric if b≠3 (negative allometric if b<3 and positive allometric if b>3).All the statistical analyses were considered at significance level of 5% (p<0.05).

RESULTS
The mean water level and temperature monthly fluctuations for the period of 1997-1998for Buyo, and of 2004-2005 for Ayame i, are illustrated in figure 2; these data were provided by the operator of the hydroelectric energy (Compagnie ivoirienne d'Electricité).The maximum water level reached 200.15m at Buyo in November 1997, and 90.5m at Ayame i in December 2004; while the minimum water levels were recorded at 192.6m in July 1997, and 85.32m in August 2004, respectively.The   A total of 49 data sets (24 from Ayamé i and 25 from Buyo) were obtained and the LWR could be estimated.The 36 freshwater fish species found belong to 15 families and 24 genera (Tables 1 and 2).Cichlidae resulted the family with the highest number of species with eight species: Chromidotilapia guentheri, Hemichromis bimaculatus H. fasciatus, Oreochromis niloticus, Sarotherodon galileaus, S. melanotheron, Tilapia busumana and T. zillii.Only one species per family was recorded for Polypteridae, Osteoglossidae, Hepsetidae, Distichodontidae, Malapteruridea Channidae and Centropomidae.And the other families had two or four species each.
Table 1 shows the 13 fish species collected in both reservoirs; and table 2 shows the 23 species collected either in Buyo or Ayame i reservoirs.A total of 12 724 individuals were used in the present study (Tables 1 and 2).The samples ranged from 14 individuals for Alestes baremoze in Buyo Reservoir to 2 016 individuals for Sarotherodon melanotheron in Ayamé i Reservoir.The sizes of the collected fish specimens ranged from 4cm (Oreochromis niloticus) to 122.5cm (Heterobranchus isopterus), and the intervals of size classes varied from 2.2cm (Hemichromis bimaculatus at Ayame i) to 114.7cm (H.isopterus at Buyo).
The estimates of b ranged from 2.173 for Marcusenius furcidens to 3.472 for Polypterus endlicheri with a mean value of 2.771 (SD=0.256).The median and mode values of b were 2.756 and 2.70 (Fig. 3).
The kind of growth, determined by Student's t-test, revealed that six species, Distichodus rostratus, Labeo coubie, Chrysichthys nigrodigitatus, Synodontis schall at Buyo, and Clarias anguillaris and Synodontis schall at Ayame i, showed isometric growth (b=3).For the other species, b was significantly (Student t-test: p<0.05) different from 3. Seven species of this category (four from Buyo and three from Ayamé i) showed positive allometric growth (b>3) and the 36 last ones a negative allometric growth (b<3).it should be pointed out that four species, Hepsetus odoe, Chrysichthys nigrodigitatus, Parachanna obscura and Hemichromis bimaculatus studied in the both reservoirs showed different growth type according to the reservoir (Table 2).

DISCUSSION
The range of b values (2.173-3.472) in our study is similar to the values (2.458-3.473)recorded by Ecoutin & Albaret (2003), which studied the length-weight relationships of 52 species of West African lagoons and estuaries.it is also similar to the b values (2.213-3.729)obtained in Konan et al. (2007), which studied the length-weight relationships of 57 fish species in the coastal rivers of South-Eastern  Côte d'ivoire.According to Bagenal & Tesch (1978), the range of the parameter b usually encountered in fishes is within the expected range of (2-4).This is in accordance with the range of b for the 49 LWRs from the present study.Moreover, the modal value of b equal to 2.70, can suggest, that the «cube law» (Froese 2006) cannot be applied to most of the fish species in the Ayame i and Buyo reservoirs.On the 30 species appearing both in this study and those obtained by Konan et al. (2007), only 10 species (Heterotis niloticus, Mormyrops anguilloides, Brycinus imberi, Labeo coubie, L. parvus, Chrysichthys maurus, Clarias anguillaris, Synodontis bastiani, Oreochromis niloticus and Sarotherodon melanotheron) have similar b values.For the other 20 species, several factors such as sampling procedure (sample size and length range) (Ecoutin & Albaret 2003) sexual dimorphism (Artigues et al. 2003), water quality or food availability on fish growth (Henderson 2005) could explain this variation in b values.
No values of b for the six species of Alestes baremoze Distichodus rostratus, Sarotherodon galileaus, Lates niloticus, Synodontis koensis and S. punctifer were reported by Konan et al. (2007).However, LWRs data on three species were published in Fishbase (Froese & Pauly, 2010) for this region: Alestes baremoze (Daget & iltis 1965), Distichodus rostratus and Sarotherodon galileaus (Bauchot & Bauchot 1978).Concerning the last three fish species of Lates niloticus, Synodontis koensis and S. punctifer no previous data on length-weight relationships were available in FishBase (Froese & Pauly 2010) for this region and our results provide the first references on LWRs for them.
inland fishery statistics in Côte d'ivoire mostly consist of rough estimates of total catch for all species, exploitation indices (e.g., number of canoes, fishermen) and, at best, average length or weight for some species of commercial importance.Therefore, the present lengthweight key for 36 freshwater fish species could be used as an useful tool for more effective management of these fisheries.Certainly, this key can facilitate fisheries biologists to derive weight estimates for un-weighed but measured fish, and assist them in estimating the biomass of captured fish species.Besides, new data on length-weight relationships were recorded for three fish species in this region.According to that, this study represents an additional contribution to the available LWRs for the region.

ACKNOWLEDGMENTS
The present work was fully supported by Laboratory of Hydrobiology (University of Cocody, Abidjan) and Ministry of Animal Resources and the Halieutic Production of Côte d'ivoire.
CL= b ±(1.96 x SE) where SE is the standard error of b. in order to check if the value of b was significantly different from 3, the Student's t-test was conducted as expressed by the equation according toSokal & Rohlf (1987): ts=(b-3)/SE, where ts is the t-test value, b the slope and SE the standard error of the slope b.

Fig. 3 .
Fig. 3. Frequency distribution of the allometric growth coefficients for 36 freshwater fish species from two tropical man-made lakes, Ayame I and Buyo (Côte d Ivoire; West Africa).

TABLE 1
Descriptive statistics and LWR parameters for 13 selected freshwater fish species collected in both reservoirs Buyo and Ayame i

TABLE 2
Descriptive statistics and LWR parameters for the 23 freshwater fish species collected either in Buyo or Ayame I reservoirs