ISSN : 0976-9927
EISSN : 0976-9935
OFFEM B.O.1*, AYOTUNDE E.O.2, ADA F.B.3
1Department of Fisheries and Aquatic Sciences, Cross River University of Technology Obubra Campus, P.M.B 102 Obubra, Nigeria.
2Department of Fisheries and Aquatic Sciences, Cross River University of Technology Obubra Campus, P.M.B 102 Obubra, Nigeria.
3Department of Fisheries and Aquatic Sciences, Cross River University of Technology Obubra Campus, P.M.B 102 Obubra, Nigeria.
* Corresponding Author : benbeff06@yahoo.com
Received : 16-03-2013 Accepted : 31-05-2013 Published : 10-06-2013
Volume : 4 Issue : 1 Pages : 67 - 74
J Fish Aquaculture 4.1 (2013):67-74
DOI : http://dx.doi.org/10.9735/0976-9927.4.1.67-74
To improve on management strategy of mochokid catfish, (Synodontis clarias) in the Cross River, biological and biometric data including; stomach content, standard length and total weight of 500 fish were collected monthly, from January, 2009 to December, 2011 and used to investigate the trophic ecology and population parameters. Stomach content showed the fish to be euryphagus and benthic feeder, with inter-seasonal difference in food item. Length-weight relationship was described as W = -3.97 + 0.98 L (male), W = -19.7 + 1.00L (female) and W = -6.73 + 0.99L (combined sex). Condition factor ranged from 0.32±0.7 minimum for males (midriver) to 2.25±0.6 maximum also for males (downriver) with monthly variation for both sexes peaked between June and July, and November and January, with better conditions for males than females. Growth parameters were L∞ = 72.12, 79.23 and 76.34; k = 0.22 year-1 0.34 and 0.31 for males, females and combined sex, respectively. Index of growth performance (Φ’) = 3.11 (males), 2.65 (females) and 3.78 (combined sex). Longevity potential (tmax) was 30.34, 33.56 and 32.33 years, for male female combined sex, respectively. Z and M were 0.45 year-1, 0.67 year-1, 0.55 and 0.24, 0.38 and 0.29 year -1 while F, E and Emax values were 0.18, 0.34 and 0.28 years-1 , 0.66, 0.86, and 0.78) and 0.78, 0.87 and 0.98 for males, females and combined sex, respectively, indicate that overfishing of S. clarias is not occurring in the Cross River (i.e., E < Emax). However, E exceeded E0.5 (0.29, 0.34 and 0.39) and was not close to E0.1 (0.54, 67 and 0.66) for males, females and combined sexes, indicating that the present exploitation rate must be maintained for males, females and combined sexes to achieve the optimum yield per recruit.
Cross River, Length-weight relationship, Condition factor, Growth performance index, Diet habit, Mortality rates
Among the Cross River fishes, the family Mochokidae, known as squeaker or upside-down catfish [1] is very common. Local riverine people regard it as palatable with high protein content and in great demand. Among the three species in Cross River, Synodontis clarias is dominant species [2] . It is of great economic importance as food fish with good aquaculture attributes and well prized ornamental qualities. Their aquaculture attributes includes; ability to withstand handling stress, disease resistance, palatability and high potential yield. Thus culture fishery resource distributed in the Nigerian inland water mass, offer a promising potential in the Nigerian fishery scenario which can be further improved by a thorough knowledge of the exploitable biomass of the river system.
The species are common and widely distributed in tropical and sub-tropical water systems, experience frequent growth fluctuations due to changes in food composition and environmental variables among others. S. clarias is fatty with considerable amount of fat in the abdominal cavity [3] and adipose dorsal fin is present [1] . They have formidable defences against predation with strong and serrated spines at the front of dorsal and pectoral fins [3] . It is sluggish, slow during the day but active at night [4] .
The essential component in understanding the dynamic changes in the fishery of any water-body lie in the determination of age and growth of the fish species. Age and growth determination are essential for the estimates of production, stock size, recruitment and mortality [5] and helps to estimate the potential yield per recruit in the study of fish population dynamics [6] . Understanding the relationship between body structures and fish diet could be important for predicting the diet habit. Studies on stomach composition could provide useful information in positioning of the fishes in a food web in their environment and in formulating management strategy options in multi species fishery [7] , is useful in defining predator-prey relationships, estimation of trophic level [8] , in the creation of trophic models as a tool to understanding complex ecosystems [9] and the use to reduce intra and inter specific competition for ecological niche [10] . Research on the growth parameters; age trophic ecology of different species in various water bodies have been quite intense [12-15] . No research was available on these parameters in Synodontis clarias in Cross River.
Therefore objective of this paper was to characterize the population dynamics of S. clarias in the Cross River, using parameters of age, size, growth of the population and feeding. We also analyzed parameters related to mortality (total, natural, and fishing) and the current status of the stock of this species using Beverton and Holt’s model of analysis of yield per recruit.
Study Site
The study site is the Cross River, a floodplain river located at the south eastern part of Nigeria on Latitude 4°, 25´- 7°.00´ N, Longitude 7°.15´-9°.30´ E [Fig-1] . It is bounded in the south by the Atlantic Ocean, east by the Republic of Cameroun, the Nigerian states of Benue in the North, Ebonyi and Abia in the west and Akwa Ibom in the south-west. Climate of the study area is characterized by a dry season from November-March and wet season from April-October. Highest precipitation (3050 ± 230mm) occurs in August, and lowest (300 ± 23mm) in March. The mean annual temperature ranges from 15.5 ± 7.6°C in wet period to 32.6 ± 5.4°C in dry period. Three sampling sites were selected randomly, one each upriver, middle river and downriver. Upriver site is located 3km from the river source with rocky, gravel and sandy substratum. The shoreline is covered with savanna grassland, and wood and paper industries are located close to the source. The middle river site is at 100km distance from river source; it has a rocky substratum and shoreline and is sparsely shaded by forest and savanna grassland. Downriver has a muddy substratum and opens into the Cross River estuary, with shoreline thickly shaded with rainforest.
Physico-chemical parameters of water were determined monthly for two years (January 2006 -December 2007). Standard methods for the examination of water and waste water [16] were used for all measurements. Monthly rainfall data for the study area were obtained from meteorological stations, located in each of the three reaches. Habitat variables, water level and river width were measured in three places in each reach and averaged.
Fish samples for all assessments in each site were randomly collected twice every month, in the day (2400- 0600 hours and night (0830-1200 hours) and during wet and dry seasons, from randomly selected artisanal fishermen whose fishing gears were mainly seine net (10-34mm stretched mesh size) and gill net (22-76mm stretched mesh size). Fish samples, 300 males and 200 females were preserved in 10% formalin prior to laboratory examination. In the laboratory, data obtained from each fish included; date of capture, length, weight, sex and food records. Standard length (SL) and total length (TL) were measured to the nearest 0.1cm and weighed (wt) to the nearest 0.1g. Samples were identified using FAO Species Identification Sheet [17] and sexed by visual observation.
Specimens for diet studies were dissected and their guts removed immediately after capture and stored in formaldehyde solution (4%) until the contents were analyzed. Gut analysis was later carried out, food items identified to the lowest possible taxon and analyzed quantitatively for percentage composition by number (N) and frequency of occurrence (FO). Diet breath estimates the diet spectrum and was calculated using the diversity index of Shannon- Wiener (H); H = -∑pi ln pi where, pi is the proportion by the number of food type i, Food richness; expressed using Margalef’s index: d= (S-1)/logN) where d= Margalef’s index, S is the number of species and N is the number of individuals and Gut Repletion Index (GRI) which is the percentage of non-empty stomach was estimated for samples from each reach.
Length-Weight Relationship (LWR)
Length-weight relationship of fish was estimated from the equation
W = aLb [18] .
W = weight of fish in grams, L = total length of fish in centimeters, “a†is proportionality constant and “b†is the allometric coefficient both estimated by method of least squares [19] using logarithmically transformed expression;
log W = lag a + b log L.
To investigate whether the species followed cube law the values of the exponent “b†was tested against ‘3’ applying Student’s t test. An ANOVA was used to determine if there were significant differences in the LWR between the sexes. Fulton’s condition factor (CF) was determined using the expression by [20] as
K= condition factor, W = total weight (g) and L = total length (cm).
Age determination and growth were analyzed for the species from length frequency modes. This based on the integrated approach of [18] illustrated in [22] where the length frequency distribution in the respective years of sample were placed sequentially. A smooth curve interconnecting the peaks, assumed to be new year-class entering recruitment, intercepts the succeeding horizontal lines, read as length- at- age of the species. Based on the monthly distributions of standard length frequency, the following growth parameters were obtained: L∞ = asymptotic standard length (cm) and k = growth coefficient (years). These parameters were evaluated using the ELEFAN I routine within the computer program FiSAT II [23] , which is based on the Von Bertalanffy equation:
Lt = 72.12(1-exp (-0.22 (t-t0)) for males
Lt = 79.23(1-exp (-0.34 (t-t0)) for females
Lt = 76.34(1-exp (-0.31 (t-t0)) for combined sex
The length-age structure was evaluated using the Von Bertalanffy equation. We use the parameters L∞ and K calculated in this paper and replaced t with ages ranged from 0 to 4.5 years. The parameter t0 was considered to be o because biologically speaking it is not significant [25] .
The natural mortality rate was evaluated using [18] empirical model:
L∞ and k are the growth parameters obtained from the Von Bertalanffy equation and T = mean water temperature (°C), which was 27.1°C. Total mortality rate (Z) was estimated using Beverton and Holt’s model
Lc = mean length of first capture (i.e., the time at which the fish are recruited for the fishery) and Lm = mean length starting from Lc. For the purpose of this study Lc was considered to be 26.67, 30.45, 28.55 for male, female and combined sex. The mortality rate due to fishing (F) was calculated as the difference between the total mortality rate (Z) and the natural mortality rate (M) (i.e., F = Z-M).
To compare the overall performance of the fish species, growth performance (Ï•) [26] has been used as follows
ϕ = Log 10 K + 2 Log 10 L∞ for length
ϕ = Log10 K + 2/3 Log 10 W∞ for weight.
Yield per recruit model (Y’/R) was calculated using Beverton and Holt’s method and the Knife Edges option in the program FiSAT II [22] according to the following model:
Y’/R = EUM/K {1-(3U/1 + m) + (3U2/1 + 2m)-U3/1 + 3m)}
where m = (1-E)/(M/k) = k/Z; U = 1-(Lc/L∞) and E = F/Z (exploitation rate). We also calculated Emax (exploitation rate of maximum sustainable yield), E0.1 (exploitation rate at which the marginal increment of Y’/R is 10% of its virgin stock), and E0.5 (exploitation rate that will result in a 50% reduction of the non-exploited biomass). The relative biomass per recruit (B’/R) was estimated as
B’/R = (Y’/R)/F.
For these estimates we used the routine ELEFAN I in the program FiSAT II [22] .
The χ2 test was used to test the null hypothesis that the male to female ratio of the population was 1:1. Before applying parametric tests, the data were transformed (log + 1) and their normality and homogeneity were tested using Kolmogorov-Smirnov’s and Bartlett’s tests, respectively. A one-way ANOVA was applied to test the null hypothesis of equal RGS values for different periods of the year and Tukey test was accomplished to detect statistical differences. The t test was applied to test whether the value of b of the length-weight relationship was equal to zero and whether b = three (the condition of isometric growth). Because the data for the abiotic factors did not meet the assumption of variance normality and homogeneity, Spearman’s nonparametric correlation test was applied to examine the possible relationships between abiotic factors and reproductive period through values of RGS. For all statistical analyses p < 0.05.
Water depth, water discharge, water velocity and dissolved oxygen vary significantly between reaches (p>0.05) [Table-1] .
There was no spatial variation in the dietary items found in stomach of S. clarias but there was diel difference with Gut Repletion Index of 100% [Table-2] . During the dry season, predominant food items in the stomach of S. clarias were planktons; cyanophytes (87%), dinoflagellates (64.2%), copepods (54.2%), cladocerans (64.4%) and fish species (62.2%) while insects (55.5%), gastropods (75.5%), trichocerca (66.4%) and trichotria (70.4%) and mollusk were more frequent in the wet [Fig-2] . Also occurred in the diet but in lesser quantity were ostracods, diatoms and detritus constituting only 24.4% of the diet. The prominence of detritus in the diet indicated that the fish is a bottom or benthic feeder and feed more actively at night. The wide variability in food supply enables S. clarias to maintain its overwhelming prominence in Cross River, and its euryphagus habit makes it suited for pond culture.
The standard length varied from 10.2-50.4, 9.8-48.6 and 9.7-49.2cm with mean of 25.6 ± 4.9, 28.65 ± 5.3 and 26.4 ± 3.2cm, whereas the total weight varied from 30.5-1223.5, 29.4-1406.2 and 28.7-1387.4g with mean of 426.4 ± 13.2, 506.4 ± 23.2 and 489.6 ± 18.9 for males, females and sex combined of S. clarias, respectively.
Length-weight relationship and condition factor analysis of S. clarias comprising 300 individuals showed significant regression (P<0.005) of weight on length with r values of 0.990, 0.997 and 0.997 for Lower river, mid-river and upriver samples respectively [Table-3] .
The length-weight relationship can be described as
Wt = -3.97 + 0.98 Lt (Upriver)
Wt = -19.7 + 1.00Lt (Midriver) and
Wt = -6.73 + 0.99Lt (downriver).
The value of the mean annual allometric coefficient ‘b’ was 0.99 ± 0.134 and the overall LWR significantly deviated from the cube value (b=3) (P>0.05). The populations of S. clarias in the study sites can therefore be considered as having heterogenous groups with body weight varying differently from the cube of total length. The high values of the correlation coefficient (r) perform a good measure for the strength of these equations and closeness of the observed and calculated of the fish weight.
Mean condition factor ranged from 0.32±0.7 minimum for males (midriver) to 2.25±0.6 maximmum also for males (downriver) with significant (P < 0.05) difference between reaches. Also, there was significant monthly variation in the condition factor with both sexes recording highest between June and July, and between November and January. Least values were obtained from March to April and September to October [Fig-3] . The values of the condition factors revealed better conditions for males than females.
The growth parameters were estimated to be L∞ = 72.12, 79.23 and 76.34cm and k = 0.22 year-1, 0.34 and 0.31 for males, females combined sex respectively [Table-4] . Accordingly, the growth of S clarias was described by the von Bertalanfy’s growth equation:
Lt = 72.12{1-exp (-0.22 (t-t0))} for males
Lt = 79.23( 1-exp (-0.34 (t-t0)) for females
Lt = 76.34( 1-exp (-0.31 (t-t0)) for combined sex
The equations of the theoretical growth in weight were obtained by applying the length-weight relationship to the growth in length equations as follows
Wt = 3456.23 {1-exp (-0.12 (t-t0))}2.27
Wt = 3893.34 {1-exp (-0.11 (t-t0))}2.29
Wt = 3145.22 {1-exp (-0.22 (t-t0))}2.28 for male, female and combined sex respectively.
Age Composition
The data revealed that age group (IV) is the least and contributed about 1.01 and 2.28% for males and combined sexes, respectively, while for the females it was found that age group zero and contributed about 2.49%. the frequency of fishes of age group I and II were dominant in the catch and constitute about 41.23 and 28.98, 38.26, and 27.23, 39.34 and 29 12% for S. clarias males, females and combined sexes, respectively [Fig-4] .
The longevity potential was estimated to be tmax = 30.34, 33.56 and 32.33 years for males, females and combined sexes. The length-age structure analysis showed that most of the S. clarias caught in the fishery were 10.5-15.5 years old (94.9%), of which 60.2% were 10.5 years old [Table-5] .
Z and M were estimated to be 0.45 year-1, 0.67 year-1, 0.55 and 0.24, 0.38 and 0.29 year-1 for males, females and combined sexes, respectively. Thus, F was estimated to be 0.18, 0.34 and 0.28 years-1 for males and females and combined sexes, respectively. Mortality due to fishing was less important than the other causes of mortality (F < M).
The mean selection length (Lc) of S. clarias, the size at which 50% of the fish are retained in the gear was found to be 26.67, 30.45, 28.55cm for males, females and combined sexes, respectively. The corresponding mean selection age tc was estimated to be 3.57, 4.34 and 4.78 years, where as the corresponding mean selection weight 288.34, 411.34 and 401.34g for S. clarias males, females and combined sexes, respectively.
The values of Lr of S. clarias were 13.50, 21.87, 17.94cm and those of tc were 0.56, 1.89 and 0.79 years, whereas the corresponding Wr were 54.67, 201.45 and 189.56 years for S. clarias for males, females and combined sexes, respectively.
Growth performance index (Φ) and the maximum age (tmax) were found to be 3.11, 2.65 and 3.78 of growth performance in length (ΦL) and 1.22, 2.87 and 3.02 of growth performance in weight (Φw), respectively, whereas the values of maximum age (tmax) were 30.34, 33.56 and 32.23 years for S. clarias males, females and combined sexes, respectively.
Y/R and B/R were determined as functions of L50 / L∞ and M / K, respectively. It means that the calculated yield per recruit (Y/R) was 54.44, 176.34 and 1.8,12g, whereas, biomass per recruit B/R was 304.55, 278.23 and 288.45g for S. clarias male, female and combined sex, respectively. The results show that the yield per recruit (Y/R) and Biomass per recruit (B/R) of S. clarias irrespective of sex, as a function of fishing mortality, by F-values. The estimated yield per recruit increases continuously with the increase in the fishing mortality reaching its climate at Maximum Sustainable Yield (MSY) then it remained constant or decrease.
The relative yield per recruit (Y/R)’ increase with the increase in exploitation rate. The values of E (0.66, 0.86, and 0.78) and Emax (0.78, 0.87 and 0.98) for males, females and combined sex, respectively, indicate that overfishing of S. clarias is not occurring in the Cross River (i.e., E < Emax). However, the value of E exceeded the value of E0.5 (0.29, 0.34 and 0.39) and was not close to E0.1 (0.54, 67 and 0.66) for males, females and combined sexes hence exploitation rate must be maintained for males, females and combined sexes to achieve the optimum yield per recruit.
Diet Studies
There was no spatial variation in the dietary habit of S. clarias but there was seasonal difference. The feeding habit of S. clarias has been found to be overlapping [28] . They utilize various materials found in the environment, thus can live as herbivores, predators, detrivores, Omnivores, as well as plantivores [29] . A number of factors are attributable to changes in the feeding habits of fish species. The size of the fish, sex, season, water temperature, habitat and competition as some of these factors [31,32] . Morphological changes in the feeding apparatus of the fish as a result of age may also lead to a change in the feeding habits. The feeding habits depend on the availability of feed in water [32] .
Phytoplankton dominate food items in the stomach during the dry season while crustaceans and mollusk were more abundant during the wet. This findings agree with some other authors [32,33] who mentioned that synodontis lives at the bottom of the river feeding on mollusk, crustaceans, mud sand and algae. Further confirmation about Synodontis sp. being omnivorous had been made [33] .
Seasonality in the diet of Siluriformes has been reported by Offem et al [34] and Wooton [35] and both agreed that is caused by variations in the availability of food in the environment, and these variations can occur for many reasons, including life cycle changes of prey, changes in the predators preying on the food item, and changes in the foraging habitat [36] . In fresh waters in Nigeriaa, the diet of silurids varied from phytoplankton to zooplankton to detritus depending on the availability of the items in the environment [36] . In addition to phytoplankton and zooplankton, insects, vegetation, and fish were part of the diet in Agbokim waterfalls [37] .
The length-weight relationship showed high correlation (r) and values of the exponent (b) showed that the ponderal growth was allometric in males, females and combined sexes. This is in agreement with other findings [38,39,33] for Synodontis schall in Lake Nasser, River Nile at El-kanater and River Nile at Assuit, respectively. The value of exponent was not affected by sex, position and season, which disagree with the findings by [40-42] , that the geographic location and the associated environmental condition like season, disease and parasites may affect the value of “bâ€.
The condition coefficient of the fish which measures the physiological wellbeing of the fish [43,44] is considered as a measure of “fitness†of the fish in a population. It may also be considered as a rough measure of the state of the fish, weather heakthy or unhealthy, starved or well-fed, spawning or spent [34] . The condition coefficient in this study showed that the value of “K†increased with decreased length of fish, which agrees with findings by Hassan [33] who observed decrease in K with increased length. The interspatial difference of the condition factor with lower reaches having higher K value may be due to different ecological and feeding conditions of the habitats [44] . The seasonal difference can be as a result of seasonal changes in food availability and spawning activity [45] .
The growth parameters for S. clarias estimated were biologically good because the growth performance rate of ΦL = 3.11 for males, 2.65 for females and 3.78 for combined sex was within the range estimated for other Synodontis populations [41,47,33] . The growth parameters estimated for the species male, female and combined sex (L∞ = 72.12, 79.23, 76.34; k = 0.22, 0.34 and 0.31 and longevity 30.34, 33.36, and 32.23 years) respectively, indicated rapid growth and relatively higher longevity as against 6, 5 and 7.6 years recorded by Tharwat [48] , Hassan [33] and Authman et al [42] in River Nile respectively. According to some authors [49,50] , species at a low trophic level, such as S. clarias, tend to have rapid growth, short longevity, and early sexual maturation, which did not fit our results. When we compared the growth parameters (L∞ and k) and the mortality rates (Z, M, and F) estimated for Synodontis species in our study with those estimated in other studies, several differences were apparent. Various endogenous and exogenous factors influence fish growth [51] , and these might be responsible for the observed differences. For instance, growth and mortality rates can be affected by the level of fishing effort [25] , and exploitation rate of different fishery resources as well as overfishing throughout the past and present years [6] . It is likely that fishing effort will affect the growth and mortality rates of S. clarias in the Cross River.
The results also indicated that the growth in weight of fish downriver is higher than those of other sites. The variation of weights at different lengths among different regions of the river are mainly attributed to water temperature which can affect fish growth directly by affecting the physiology of the fish [52,53] and food availability in these areas [54,55] . Also this may be due to interplay between a complex of genotype, body size, physiological conditions of the fish and environmental condition [55] .
The main causes of mortality in fish can be either fishing or natural mortality [26] Natural mortality coefficient of S. clarias from Cross River is the higher than the average given by [18] for 175 fish stocks (0.2 and 0.3). The high survival rate indicate lower fishing ie this further shown by the exploitation ratio (E) and fishing mortality (F) in this study as 0.78 and 0.28, respectively. The high survival rate in this study is indicating that the gears used is not effective in catching S. clarias due to the fish behavior peculiarities which enables them to escape fishing recruitment. There was selective mortality among different age groups for the combined sex and the selection is in favor of smaller fish sizes due to their vulnerability to the fishing gears as they outnumbered matured fish. Also the presence of age zero-group for S. clarias from Cross River indicate that the fish are subject to accidental fishing early in life. This may be attributed to the fact fishing in African rivers and the tributaries is mainly concentrated on tilapias which are relatively smaller in size [46] .
The population parameters of S. clarias from the different regions of the Cross River. The patterns of growth (Ï•), L Íš and K values of von Bertalanffy model, fishing, natural and total mortality of the different population in the various regions were different. The findings refer to the variability of fishery characteristics of S. clarias populations of different localities [46] . Also, the variation in population parameters of S. clarias in different areas can be due to different prevailing conditions of food and temperature in these areas [55] .
The value of Y/R in this study indicates that the current exploitation level (E) of the S. clarias in the Cross River is significantly higher than that associated with E0.5, E0.1 and Y/R’, for male, female and combined sex and very close to Emax. The stock can therefore be considered as over- fished. Also, F > M, which suggest that the current fishing effort is higher than the maximum sustainable level. S. clarias is therefore not well established in the Cross River and the fish stock currently is being over fished because the level of exploitation by the artisanal fisheries is higher than maximum sustainable level, and this implies that measures to prevent an increase in fishing effort in the region are necessary.
The assessment of the present state of the S. clarias resource indicates that the current exploitation rate is higher than the estimated maximum exploitation level. It is also higher than the exploitation rate which will maintain 50% of the unexploited stock biomass, and this implies that measures to prevent an increase in fishing effort in the region are necessary. Besides there is selective mortality towards smaller fish sizes. This implies that juvenile individual are the target of the fishery and the stock dynamics of the species in this area of study will be seriously affected. The high vulnerability of juvenile fish to capture by gears will result to the reduction of future yield of this species. Thus, the protection of immature fish is probably the key factor to preserve the spawning stock, as a priority in the sustainability of the resource and management of S. clarias fishery and thus will protect both biodiversity in the area of study and the fish as a food source.
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 | Fig. 1- Map of Cross River State showing study site |
 | Fig. 2- Food item frequency distribution of S. clarias in Cross River sampled per season from artisanal fishery |
 | Fig. 3- Length-Weight Relationship of the sexes of S. clarias from Cross River |
 | Fig. 4- Monthly mean variation of condition factor of male and female S. clarias combined from Cross River by reach and sex |
 | Table 1- Mean variation and F-values of the analysis of variance (ANOVA) of physico-chemical parameters of water measured at three sampling sites. I: Upriver, II: Mid-river, III: Downriver |
 | Table 2- Diel variations in the food habits of S. clarias in the Cross River inland wetlands |
 | Table 3- Population parameters of S. clarias from Cross River, Nigeria |
 | Table 4- Comparison of population parameters of S. clarias (irre-spective of sex) from Cross River Upriver, Mid-river and Downriver |
 | Table 5- Age- mean standard length structure of S. clarias, sampled from artisanal fishery at Cross River |