Research Article
Length-weight Relationship and Diet of African Carp Labeo ogunensis (Boulenger, 1910) in Asejire Lake Southwestern Nigeria
Department of Zoology, Hydrobiology and Fisheries Unit, University of Ibadan, Ibadan, Nigeria
Fishes of the genus Labeo are widely distributed throughout Africa and consists of at least 80 species which comprises 16.40% of the African cyprinid ichthyofauna (Reid, 1985). Labeo species are found to be food fishes and support fishing industry in Africa (Corbet, 1961; Siddiqui and Naseem, 1978; Weyl and Booth, 1994).
The fish fauna of Nigeria has been classified into five ichtyo faunal zones and L. ogunensis (Boulenger, 1910) is found only in the Ogun Oueme zone. It occurs in Oshun River, Ogun River, River Oueme and other rivers in South Western Nigeria (Syndenham, 1977). Other species of Labeo occur in other parts of Nigeria and Africa as reported by Burchard (1967) and Munro (1967).
A sound knowledge of the food and feeding habit will highlight the dietary features of the species and position in the food chain. Length-Weight Relationship (LWR) of fishes are important in fisheries biology because they allow the estimation of the average weight of the fish of a given length group by establishing a mathematical relation between the two (Beyer, 1987). They are also useful for assessing the relative well being of the fish population. LWR has a number of important applications in fish stock assessment. Various studies have been done on the length-weight relationship and food habits of fish species (Mortuza and Rahman, 2006; Ayoade and Ikulala, 2007; Ayoade et al., 2008; Hosseini et al., 2009; Jud et al., 2010; Lawson, 2011).
In Nigeria, studies on diet of L. ogunensis have been reported as part of general surveys of the food ad feeding habits of several species (Adebisi, 1981).
This work reports the length weight relationship and diet of this little studied fish, L. ogunensis from Asejire Lake and this will enhance effective management of this species.
Study area: Asejire Lake is a man-made lake constructed on River Oshun at about 30 km east of Ibadan, Oyo State, Nigeria (04° 07E and 07° 21N at an altitude of 137 m above sea level) . The entire length of the lake is 19.5 km and is enclosed between 07° 21N and 07° 33N .
The catchment area above the dam is about 7,800 km2 and the impoundment area is 2,342 ha. The dam was constructed in 1972 with a normal pool elevation of 150 m and a maximum flood elevation of 152.4 m. The fish fauna of the lake comprises of 23 species belonging to 13 families. Eleven families including Cyprinidae were represented by one species.
Sampling and Laboratory procedure: A total of 321 L. ogunensis were collected from fishermen at the landing site in Asejire Lake from May to August 2009 and gill nets of mesh sizes 50-55 mm were the fishing gear used. The fish were examined fresh or kept in a deep freezer for subsequent examination. The total and standard lengths were taken with measuring board to the nearest 0.1 cm. Body weight of individual fish was measured to the nearest 0.1 g with an electric balance after removing the adhered water and other particles from the surface of body.
The relationship between length and weight is described by the equation:
where, W is total weight of fish (g), L is total length of fish (cm), a is constant and b is growth exponent. A logarithmic transformation was used to make the relationship linear.
The condition factor (k) was calculated using the formula:
where, K is condition factor, W is total body weight (g) and L is total length (cm).
The slopes of length weight regressions were compared to 3 using students t-test (Sokal and Rohlf, 1987) to determine whether species grew isometrically.
The specimens were cut open on the ventral side and paired gonads from each specimen were removed. The sex of individuals with ripe gonads was easily recognized while immature gonads were classified by observing under the microscope.
The alimentary tracts were removed and kept in 4% formalin. Then the gutted weight (IF) (weight of the fish without intestine) was taken. An index of fullness (total mass of gut contents as percentage of total mass of each specimen was estimated (Arnauld and Hureau, 1966). Each intestine was slit open and the content emptied into a petri-dish. Random samples of the gut contents were taken with dropping pipette and dropped on slides. These were observed and identified under a light microscope. The stomach contents were analysed using the frequency of
occurrence, numerical, points and volumetric methods as described by Hyslop (1980) and Bagenal (1978). Since each of these methods emphasizes the importance of different categories of food items, the relative importance index of each food item based on Absolute Importance index (AI) was calculated (George and Hadley, 1979; Hyslop, 1980).
The specimens examined were divided into three size groups in order to establish if there were changes in Length-Weight relationship in relation to size.
Statistical analyses applied to the data are mean, standard deviation and ANOVA.
Length Weight Relationship (LWR): The results of the length weight analyses are presented in Table 1. All LWRs were highly significant (p<0.05) with r values greater than 0.82. The slopes of the LWR (b) ranged from 2.34 (14-19.9 cm) to 3.15 (26-31.9 cm) for the size groups; 2.58 (male) to 2.85 (Female) for sex and 2.78 for combined. The regression coefficient (b) of the smallest size group (14-19.9 cm) is significantly different from 3.
Condition factor: The mean condition factor of L. ogunensis during study period was 1.108±0.233. It varied between 1.09±0.2041 (26-31.9 cm) and 1.15±0.248 (14-19.9 cm) for the size groups, thus higher K was recorded for smaller specimens (Table 2). The mean condition factor recorded for the male (1.13±0.1993) and female (1.13±0.124) were the same. Monthly variation in mean K is shown in Table 2. Highest mean K was recorded in June, followed by gradual decrease in condition factor.
Fullness of GutOut of 321 specimens of L. ogunensis examined, 210 (65.4%) had food in their gut. Female specimens recorded higher mean fullness index and monthly variation occurred in the Fullness Index (IF) with decrease from May to July and highest IF was recorded in the smallest size group. There was no significant difference in IF between sexes, months except between size groups (Table 3).
Qualitative assessment of Gut content: Desmids were more important (100% by occurrence, 10.98% by volume, 13.4% by points and 46.8% by number) than other phytoplankton. A variety of diatoms were included in the diet. Dominant among these were Melosira, Navicula, Diatomata, while Fragilaria and Frustulia occurred sparingly. Diatoms occurred in 95.7% of guts examined and made up 9.5% by volume, 7.1% by points and 26.4% by number (Table 4). Green algae encountered in its diet included Microspora, Cladophora and Spirogyra.
Table 1: | Length-weight relationships of Labeo ogunensis in Asejire Lake |
n: Sample size, a and b: Parameters of the relationship, SE (b) is the standard error of the slope b. r = Correlation co-efficient. *: Significantly different 3 (p>0.05) |
Table 2: | Variation in Condition Factor of Labeo ogunensis in Asejire Lake |
K: Condition factor, SD: Standard deviation |
Table 3: | Variation in Fullness Index (IF) of gut with sex, month and sizes groups of Labeo ogunensis in Asejire Lake |
SD: Standard deviation, * Significantly different p>0.05 |
Table 4: | Summary of the stomach contents of Labeo ogunensis in Asejire Reservoir |
NA: Not applicable |
Zooplanktonic organisms present in diet of African carp included rotifer and crustacean appendages (21.48% by occurrence, 13% by volume, 1.11% by points and 2.8% by number). Detritus occurred in all the guts examined and accounted for 68% by volume and 71% by points.
Sand grains were recorded in many guts (67.1%). Relative importance index recorded for group of food items in order of importance were phytoplankton (40.54%), Detritus (38.58%), Sand grain (11.56%), Zooplankton (4.52%) and unidentified organism (4.79%) (Fig. 1).
Fig. 1: | Relative Importance Index (IRI) of food items in Labeo ogunensis in Asejire Lake |
Length-weight relationship provides information on growth patterns and growth rates of animals. During their development, fish are known to pass through stages in their life history which are defined by different length-weight relationship.
Statistical analyses of the LWR showed that the smallest size group exhibited negative allometric growth, that is they favour increase in length than in mass, since b (b<3) value is significantly different from 3 while remaining size groups were observed to show isometric growth. Thus different growth pattern is demonstrated by the size groups. Generally, L. ogunensis demonstrated isometric growth in Asejire Lake. This agreed with Konan et al. (2007) who reported isometric growth for L. coubie in coastal rivers in Laleye (2006) who reported L. parvus in Oueme River exhibited positive allometric growth. Variation in b could be due to the fact that the studied species is obtained from freshwater body while the work of Konan et al., is from coastal rivers. Several other factors could also be the cause of variation in b values such as period of year and stage of maturity (Weatherley and Gill, 1987); water quality and food availability (Mommsen, 1998); sexual dimorphism (Artiques et al., 2003); sample size and length range (Ecoutin and Albaret, 2003). The mean K for the species is greater than one and this show that this fish species is above average condition within the lake (Wade, 1992). Also, the higher K for the smallest size group could be due to this group feeding more than others and indicated by the higher fullness index.
The lower percentage of empty stomach indicates availability of food for the species in the lake. The higher stomach fullness index recorded for smaller size group could be associated to the increased energetic needs of small specimens displaying highest growth rates during first year of their life (Vassilopulou and Ondrias, 1999).
The results of the stomach content analyses revealed presence of detritus and sand which showed L. ogunensis is a bottom deposit feeder.
The diet of L. ogunensis consisted mainly of algae, detritus and sand. This agrees with Imevbore and Bakare (1970) who reported that L. senegalensis fed on bottom ooze of dead and decaying matter together with diatoms. Montchowui et al. (2009), also indicated that Labeo species are specialized feeders on algae, aufwuchs and detritus from the substratum.
The high K-factor obtained for L. ogunensis in this study indicated that this fish was able to procure sufficient nutrients from the bottom deposit it fed on.