MATERIALS AND METHODS

Fingerlings of Oreochromis niloticus, C.gariepinus and hybrid (mean total length 3.0 – 7.9cm; mean weight, 0.6 – 2.3g) were acclimated to laboratory conditions in glass tanks for a period of two weeks.  The fish were fed compound diet (30% protein) at 3% body weight ad libitum. The tanks were cleaned daily and water exchange was done every 48 hours. Mortality during acclimation period was less than one percent. The industrial ammoniacal effluent was obtained in 50l plastic containers each from the National Fertilizer Company of Nigeria, Onne near Port Harcourt on three occasions. These represent different ranges of the discharge at the outfall. The physico-chemical characteristics of the wastewater were determined according to APHA (1980) and shown in Table 1. Twelve 50l glass aquaria were used for the exposure of the three species, respectively. Test solutions were prepared by serial dilution from each effluent sample. Serial dilutions were prepared on a volume to volume (v/v) ratio so that the percentage (%) concentration in each test solution is obtained by using the formula below (FAO, 1984):

Where, V_E = Volume of effluent, V_DW = Volume of dilution water. 

The determined volume of effluent was added to the desired quantity of dilution water and stirred vigorously to disperse the effluent. Screening test was done in 1l conical flask with single fish. Concentrations of the effluent   that caused death within 30 minutes were omitted from the definitive test (Gurure, 1987). Each aquarium held 20l of test medium. Test organisms were randomedly introduced into each aquarium with each holding 20 fingerlings within an hour after preparation of test solution. The fish were not fed 24 hours before and during the biossay. The test media were analyzed for physico-chemical parameters (APHA, 1980, Tables 2) at the Environmental Laboratory of the Nigerian National Petroleum Corporation, Port Harcourt, Nigeria.

Each fish species was exposed to four levels of the effluent with a control. The test media were analyzed for their characteristics (Tables 2). Every treatment was replicated thrice. The mean TBFm^-1 and OBF min^-1 were record at 6, 24, 48, 76 and 96 hours for all species and treatments, respectively. The relationship between TBF min^-1 and OBFmin^-1 and the various physico-chemical parameters of effluent were determine with the linear regression model of Statistical Analysis System, SAS, (1985). Mean and standard error (S.E.) were calculated for the TBF and OBF of the various species exposed to the different effluent concentrations. Root mean square error (R.M.S.E) and coefficient of determination (R²) were used to establish the fitness of the model chosen.

RESULTS AND DISCUSSION

Table 1 shows that the values of the physico-chemical parameters of all treated effluents fall within the acceptable limits, except the values for ammonia in the first and second samples, dissolved oxygen, urea, and phosphate in all the samples (FEPA, 1991).  The physico-chemical characteristics of various concentrations of effluents employed in the bioassay are shown in Table 2.  The values of pH, temperature and conductivity are within the FEPA (1991) guidelines/standards for those parameters.  But the free ammonia, urea, phosphate were 118.2,4.1 and 30 times, respectively higher than the limits; and the DO values 0.35 times below the 5mg/l limit set by FEPA (1991).  There was a progressive increase in ammonia, percent un-ionized ammonia (UIA), pH, phosphate, urea and conductivity values with increase in effluent concentration in the bioassay for O. niloticus and hybrid.  But this was not true for ammonia and un-ionized ammonia for C. gariepinus bioassay.  The above may imply an increased toxicity with the  raised values of the physicochemical parameters (Table 2). The mean OBF and TBF and their standard errors for O. niloticus, C. gariepinus and hybrid exposed to various percent concentrations of effluent are shown in fig. 1. Table 3 shows the relationships between OBF and TBF of the various species and the physico-chemical parameters of the effluence

Powell and Fielder (1982) stated that toxicity of pollutants to fish usually increased with temperature which may be due to an increased uptake of toxin to an added environmental stress e.g. reduced oxygen solubility (Ananthakrishnan and Kutty, 1974).  The recorded values of water temperature were within tolerable range for these tropical species and may not have affected the toxicity of the effluent and hence the OBF and TBF of the exposed fishes.

It has been observed by Thomas and Rice (1975) that increased opercula rate may be caused by decreased efficiency in oxygen uptake, transport or increase metabolic rate.  The dissolved oxygen levels in the bioassay solutions were generally low (1.54 – 24 ppm) and may be accounted for by respiration of fish species, fast decomposition of organic matters due to static condition and reduction of other gases e.g. CO₂ among others.  The low dissolved oxygen value only had significant influence on TBFmin^-1 of C. gariepinus contrary to the raised ventilation volume in sockeye salmon exposed to bleached kraft pulp mill effluent (Davis, 1973).  The observed response may be due to the   ability of the fishes to tolerate very low oxygen levels (Kutty, 1984).

Although phosphate levels did not significantly affect OBF and TBF in all fish species, yet high levels in aquatic environment may indicate pollution (Sehgal, 1980).  Such levels are known to have adverse effects on the embryonic development and survival of common carp (Toor et al. 1983).  Hence we may safely suspect that aquatic organisms exposed to the effluent may suffer similar fate as reported for carp.  The high level of urea in the stock and test solutions may be due its usage in the production of fertilizer (Tables 2).  The urea molecule is dipole in aqueous solution, and is less toxic than ammonia (Forster and Goldstein, 1970).  Very high conductivity values reflect the presence of a large number of dissolved ions such are characteristic of industrial wastewaters ( Chukwu, 1993).  But all the values recorded in this study were far below that reported by other authors, which may be why conductivity caused significantly raised TBF in the hybrid only.

Ammonia had significant effect on the TBF and OBF of O. niloticus and OBF of the hybrid respectively, with no ammonia effect on C. gariepinus (Tables 3) due to the absence of ammonia in the stock solution (Table.1).

The response pattern of the three species to un-ionized ammonia and pH are similar, and the most sensitive parameter was OBF. Nonetheless the degree of responses (TBF and OBF) differ as indicated by the slope function values in the regression equation of un-ionised ammonia and pH against TBF and OBF. (Table 3) Ammonia toxicity has been shown to be dependent on its concentration in the un-ionised form; and pH and temperature are major determinates of the proportion of ammonia present in the un-ionized form (EIFAC, 1973). Kuma and Krishnamoorthi (1983), implicated ammonia as a major pollutant in fertilizer wastewater as noted in this study.  Un-ionised ammonia was thought to be lipophilic which easily diffuses across respiratory membranes (Kormakik and Cameron, 1981), accounting for their more toxic effects.  Also it was shown to produce impairment of cerebral energy in fish (Walker and Schenker, 1970). Many of the characteristics associated with acute ammonia toxicity were noted in the O. niloticus and hybrid

Figure 1 shows that the fish species had variable OBF and TBF responses to the various grades of the effluent. The responses and their levels have some relationship to the toxicity to the stock solutions from which the test solutions are prepared. The OBF response of C gariepinus in the control is abnormally high when compared to the control of other species. Moreover, the responses of O niloticus and hybrid are more variable than that of C gariepinus are shown in the S.E. values. It was observed that the TBF and OBF in O. niloticus and hybrid rose proportionally with increment in effluent concentration, but declined with time as reported in O. niloticus  exposed to acetellic (Omorogie and Ufodile, 1991). ) The fish increased its activity to cope with rise in the level of stress (concentration), but the hyperactivity waned with the depletion of fish energy (Davis, 1973). However, the OBF responses of C gariepinus and hybrid declined with increase in the concentration of the test solutions. (Table 3).

Generally, the results of this study showed that OBF is a more sensitive physiological tool than TBF for monitoring environmental stress in fish as a result of  exposure to NAFCON effluent as noted in fish exposed to sublethal levels of pollutants   (Drummond et al, 1973).  Besides, these effluents have variable compositions whose values may sometimes far exceed the allowable limits thereby posing great danger to the aquatic biota.  There is therefore the need for the authorities concerned to ensure that treated effluent discharges comply with acceptable standard to save our environment from destruction.

REFERENCES 

Copyright 2001 - Journal of Applied Sciences & Environmental Management