Morphometry and diet of Chrysichthys nigrodigitatus ( Lacépède ) in Epe Lagoon , Nigeria

The morphometry and diet of Chrysichthys nigrodigitatus in Epe lagoon were investigated between January and July 2008. The major fishing methods employed for collecting the 600 specimens were cast netting and set netting. The total lengths ranged from 11.00 to 30.20 cm (mean size 19.67 ± 4.56 cm) while the standard lengths ranged from 8.30 to 24.50 cm (mean size 15.09 ± 3.39 cm).The length-weight relationship was described by the equation: Log W = 1.5389 + 2.8168 Log L. The value of the coefficient of regression ‘b’ for this species was nearly 3 thus, indicating isometric growth. The mean condition factors ranged from 1.68 to 1.76. The food organisms consisted of phytoplankton, crustaceans, molluscs, plant materials and fish parts. The population of males was significantly (p < 0.05) higher than females in the lagoon.


INTRODUCTION
The catfish, Chrysichthys nigrodigitatus (Lacépède), a siluroid fish of the family Bagridae is widely distributed in fresh and brackish waters in West Africa (FAO, 1969;Holden and Reed, 1991).In Nigeria, C. nigrodigitatus is highly valued food-fish and is among the dominant fishes of commercial catches as well as culturable fish species from the wild (Ezenwa et al., 1986(Ezenwa et al., , 1990)).Available information on C. nigrodigitatus is predominantly taxonomic (Reed et al., 1967), comparative racial study in Lagos and Lekki lagoons (Kusemiju, 1975), food and feeding habits in Lagos lagoon (Kusemiju and Olaniyan, 1989), age and growth determination (Ezenwa and Kusemiju, 1981) and Ezenwa et al (1986) on the comparative studies of catfish in three isolated geographical areas in Nigeria for breeding purposes.
Fisheries management parameters such as lengthweight relationship, condition factor and sex composition are essentially used to predict the potential yield and determination of size at capture for obtaining optimum yield (Bagenal, 1978;Offem et al., 2008).Food studies reveal the status of foraging, rate of growth and seasonal *Correspondence author.E-mail: lawdeen2003@yahoo.com.Tel: 08023825115.life history changes in fish species which are useful for rational exploitation of the species (Tudorancea et al., 1988;Ugwumba and Ugwumba, 2007).
Most of the information provided on the biology of the species in Nigeria has been on the Lagos (brackish) and Lekki (fresh water) Lagoons.To date, gaps in current knowledge on the biology of this species, which is of commercial value, is still apparent in Epe lagoon.Therefore, this paper examines the diet, growth pattern, condition factor and sex composition of C. nigrodigitatus in Epe (fresh water) lagoon, for proper management and conservation of the fish species in the study environment.

Study area
Epe lagoon is located in Lagos State, South-West, Nigeria.It lies between longitudes N 06° 33.710 E 004° 03 .710and latitudes N 06° 31.893E 003° 31.912(Figure 1).It has a surface area of about 243 km 2 with a maximum depth of about 2.8 m (Edokpayi et al., 2008).The lagoon is sandwiched between two lagoons, Lekki lagoon (freshwater) in the east and Lagos lagoon (brackish) in the south with the Oshun River being the main river discharging to the lagoon.Epe lagoon connects to the sea via Lagos harbour and the vegetation around the lagoon is characterized by stilt rooted trees with dense undergrowths of shrubs and herbs; Raphia sudanica, Elaeis guineensis and Cocos nucifera (Edokpayi et al., 2008).

Collection of specimens
The 600 specimens of C. nigrodigitatus used for this study were purchased from local fisher-folks on landing at Epe jetty.The specimens were fished with cast nets and set nets in the lagoon.The specimens were preserved in an ice-chest containing ice cubes in the field and later transferred into a deep freezer (temperature -20°C) in the laboratory prior to further examination.

Laboratory procedures
The preserved specimens were thawed and wiped dried before laboratory analysis.The standard and total lengths (in centimeters) were measured on a measuring board while the weights (in grammes) were determined using a sensitive Sartorious balance (Model 1106).The condition factor was calculated for C. nigrodigitatus and examined in relation to size and sex.Each stomach content was emptied into a petri dish and examined under a binocular microscope.The state of fullness of each stomach was recorded and expressed as empty ( 0 /4), one-quarter full ( 1 /4), half-full ( 2 /4), three-quarters full ( 3 /4) and full ( 4 /4).The food habits were studied using the numerical and occurrence methods (Hyslop, 1980).

Statistical analysis
The length-weight relationship was expressed by the equation:

Length-frequency distribution
The total lengths of C. nigrodigitatus from Epe lagoon ranged from 11.00 to 30.20 cm (mean size 19.67 ± 4.56 cm) while the standard lengths ranged from 8.30 to 24.50 cm (mean size 15.09 ± 3.39 cm).The length-frequency distribution of C. nigrodigitatus is shown in Figure 2.

Length-weight relationship
The weight of C. nigrodigitatus ranged from 11.40 to 201.73 g. Figure 3 illustrates the log length-log weight relationship of the species.
The length-weight relationship reflected a common general increase in weight with increasing length.The least squares common fit of the transformed data gave the following linear equation:

Food habits
A summary of food items that constituted the diet of C. nigrodigitatus specimens from the Epe lagoon is given in Table 2 and Figure

Sex ratio
The monthly sex ratio is shown in Table 3.The overall result shows that, of the 600 specimens examined 393 (65.50%) were males and 207 (34.50%) were females giving a sex ratio of 1: 0.50.The population of males was significantly higher than females in the lagoon.A chisquare analysis of the result shows that there is a significant difference between the preponderance of the male C. nigrodigitatus and that of the female ( ² = 57.66,p < 0.05).

DISCUSSION
The length-frequency distribution showed a triple mode and this depicted that the species were made of three age groups during the study period.The length-weight relationship of C. nigrodigitatus reflected an increase in weight with a corresponding increase in length.The value of the regression coefficient (b) is approximately 3 which show that the species exhibit isometric growth in Epe Lagoon (Bagenal and Tesch, 1978).The correlation coefficient (r) of 0.8683 showed a positive correlation between length and weight.Isometric growth pattern exhibited by this species was an indication that this species have homogenous groups in their populations with body weights varying indifferently with the total lengths.Offem et al. (2008)    There was a general decrease in condition factor with increasing length of the specimens.This is due to the landing of mainly sexually immature and juvenile fish which are in active growth stage of their lifecycles.However, the mean condition factor of 1.80 obtained for the population of C. nigrodigitatus in Epe lagoon is higher than 0.9673, 0.9159 and 0.7859 reported for Badagry Lagoon, Warri River and Imo River respectively (Ezenwa et al.,1986) and 0.996 reported for Cross River (Offem et al., 2008).This is an indication of the good condition of the specimens (Bannister, 1976), therefore C. nigrodigitatus population from Epe lagoon would provide excellent brood stocks for aquaculture elsewhere.
C. nigrodigitatus in the lagoon feed on variety of food items of which phytoplankton (mainly algae and diatom), crustaceans and molluscs are of primary importance.Other food items that probably serve as supplements are plant materials and fish parts indicating that C. nigrodigitatus is an omnivorous feeder.Similar results have been obtained for C. nigrodigitatus from Abidjan (Oteme, 1987) and from Lekki lagoon (Ugwumba and Kusemiju, 1994).The wide food spectrum of C. nigrodigitatus is an indication of flexibility in trophic level which gives the fish ecological advantage to feed effectively on different categories of diet based on the availability of the food items (Warren, 1993;Offem et al., 2008).
The overall sex ratio values showed that population of males was significantly higher than the females.Similar observation was reported in population of C. nigrodigitatus in some Nigerian water bodies by Fagade and Adebisi (1979) and Offem et al. (2008).Large number of the male specimens over the female specimens recorded in this study could be due to the fact that fishing gears were not set close to the breeding ground.In African water bodies it is common, that in the population of fish the males dominate because, they generally present more growth than females without this representing a risk situation for the fishery (Fagade and Adebisi, 1979).The sex ratio varies monthly and the variation observed might be a reproductive strategy to ensure successful spawning over certain environmental conditions.

Figure 1 .
Figure 1.Map showing Lagos State and Epe Lagoon .
Log weight = Log a + b Log length Where a and b are regression constants.The condition factor was calculated using the formular: K = [100 W] / L 3 Where K = condition factor, L = standard length (cm) and W = weight (g).

Figure 4 .
Figure 4. Distribution of food items in the stomachs of C. nigrodigitatus from Epe lagoon.

Table 1 .
Condition factor of C. nigrodigitatus from Epe lagoon.

Table 2 .
Summary of stomach contents of C. nigrodigitatus from Epe lagoon.

Table 3 .
Monthly variation in sex ratio of C. nigrodigitatus.