1 Introduction

Fish are totally dependent on water to breathe, feed, grow, excrete wastes, maintain salt balance and reproduce. Water is always a limiting factor in commercial fish production and its quality determines not only how well fish will grow in an aquaculture operation, but whether or not they will survive. Fish influence water quality through processes such as nitrogen metabolism and respiration (Bhatnagar and Devi, 2013). Water quality includes; all physical, chemical and biological factors that influence the beneficial use of water. There are many water quality variables in pond fish culture but only a few of these normally play important roles. Water used for growth of fish will not give maximum production if the condition is not optimal for the fish. Therefore, it is very important to maintain the quality of the water for a successful aquaculture operation. Water quality is determined by variables like temperature, transparency, turbidity, water colour, carbon dioxide, pH, alkalinity, hardness, unionised ammonia, nitrite, nitrate, primary productivity, dissolved oxygen, plankton population and others (Bhatnagar and Devi, 2013). Clarias gariepinus has a wide tolerance to temperature as well as low dissolved oxygen and high salinity (David et al., 2010). Some physic-chemical parameters such as pH, temperature, dissolved oxygen, total dissolved solids, nitrate, nitrite and ammonia were evaluated. FAO (2013) reported that catfish species had long been regarded as one of the most suitable species for culture in Africa because of their hardy qualities. Therefore, the objectives of this study are to study the effect of culture systems and seasonal change on the physic-chemical properties of water used in culturing Clarias gariepinus in Ibadan.

2 Materials and Methods

Six commercial farms (OOF, SFL and KFF operating earthen pond system and VFF, EKF and ASF operating concrete tank systems) were used in this study (Figure 1). Fingerlings and juveniles of average weight 2.7 ± 1.34 g and 10 ± 5.0 g – 20 ± 5.0 g, respectively were stocked in each pond and tank. The fish farms were monitored for the period of two wet seasons (June - September) and two dry seasons (November - February). On the farm assessment was done for some parameters (pH, nitrites, nitrates, ammonia and temperature) using NT LABS Pondlab 200 Test Kit. Temperature was measured using mercury in glass thermometer. Dissolved oxygen and total dissolved solids (TDS) were analyzed in the laboratory using standard methods for examination of water and waste water (APHA, 1998). The farms were visited fortnightly for water sampling for four months and replicated twice. Data were analyzed using ANOVA and the means were separated using Duncan Multiple Range test.

3 Results and Discussion

The results of selected water physico-chemical parameters from different culture systems and seasons are presented in Table 1, Table 2, Table 3, and Table 4.

pH: Mean values of pH recorded in the culture systems were 7.23 ± 0.03 and 7.36 ± 0.07 in earthen pond and concrete tank, respectively. Bhatnagar and Devi (2013) stated that pH generally between 7.0 and 8.5 is more optimum and conducive to fish life. According to Santhosh and Singh (2007), the suitable pH range for fish culture is between 6.7 and 9.5 and ideal pH level is between 7.5 and 8.5, above or below this is stressful to the fishes. If pH readings are outside this range, fish growth is reduced. At values below 4.5 or above 10, mortalities occur. There was no significant difference (p > 0.05) in the pH between culture systems and also between seasons (p > 0.05). The pH values in water samples analyzed showed that water from the culture systems are conducive to survival of catfish.

Temperature: Temperature is defined as the degree of hotness or coldness in the body of a living organism either in water or on land. It is the most important physical variable affecting the metabolic rate of fish and is therefore one of the most important water quality attributes in aquaculture (IEPA, 2001). Each species has an optimum temperature range of where it grows best. At temperatures above or below optimum, fish growth is reduced. Mortalities may occur at extreme temperatures above or below optimum temperatures (Joseph et al., 1993). The mean value obtained in this study was between 29.1 ± 0.04^°C and 28.2 ± 0.06^°C for EPs and CTs, respectively. Delince (1992) stated that, temperature between 30-35^°C is tolerable to fish. Bhatnagar et al. (2004) suggested the levels of temperature 28-32^°C as good for tropical major carps; < 12^°C as lethal but good for cold water species; < 20^°C is sub lethal for growth and survival of fishes and > 35^°C as lethal to maximum number of fish species. Temperature was significantly different among the culture systems (Table 5).

Dissolved Oxygen (DO): DO is an important environmental parameter for the survival of aquatic life. D.O. values measured in the culture systems ranged between 4.0 mg/L and 15.4 mg/L with a mean of 7.5 ± 0.05 mg/L and 4.06 mg/L and 8.5 mg/L with a mean of 6.1 ± 0.01 mg/L for EPs and CTs, respectively. There was no significant difference (p > 0.05) between culture systems and also between seasons (p > 0.05). Dissolved oxygen affects the growth, survival, distribution, behaviour and physiology of aquatic organisms. The amount of oxygen consumed by the fish is a function of its size, feeding rate, activity level and temperature. DO in a culture system must be maintained above levels considered stressful to fish. Fish require oxygen for respiration. Oxygen depletion in water leads to poor feeding of fish, starvation, reduced growth and more fish mortality, either directly or indirectly (Bhatnagar and Garg, 2000). The results from this study indicated that there was no significant difference (p > 0.05) in DO between culture systems and also between season (p > 0.05). According to Santhosh and Singh (2007), catfishes and other air breathing fishes can survive in low oxygen concentration of 4 mg/L. Ekubo and Abowei (2011) recommended that fish can die if exposed to less than 0.3 mg/L of DO for a long period of time, minimum concentration of 1.0 mg/L D.O is essential to sustain fish for long period and 5.0 mg/L are adequate in fish ponds. Prolonged exposure to low DO will cause the fish to stop feeding, reduce their ability to convert ingested food into fish flesh and make them more susceptible to disease (Joseph et al., 1993).

Ammonia: Ammonia is the by-product from protein metabolism excreted by fish and bacterial decomposition of organic matter such as wasted food, faeces, dead planktons, sewage and others. The unionized form of ammonia (NH₃) is extremely toxic while the ionized form (NH₄⁺) is not and both the forms are grouped together as “total ammonia” (Bhatnagar and Devi, 2013). Ammonia levels depend on the temperature of the pond’s water and its pH (Joseph et al., 1993). Mean value of ammonia obtained in this study was between 0.4 ± 0.08 mg/L and 0.6 ± 0.03 mg/L for EPs and CTs, respectively. Ammonia in the range > 0.1 mg/L tends to cause gill damage, destroy mucous producing membranes, “sub- lethal” effects like reduced growth, poor feed conversion, reduced disease resistance at concentrations that are lower than lethal concentrations, osmoregulatory imbalance, and kidney failure. Stone and Thomforde (2004) stated the desirable range as Total NH₃-N (0.2 mg/L) and Un-ionized NH₃-N (0 mg/L) and acceptable range as Total NH₃-N (< 4 mg/L) and Un-ionized NH₃-N (< 0.4 mg/L). Bhatnagar and Singh (2010) recommended the level of ammonia (<0.2 mg/l) suitable for pond fishery. Ammonia was significantly different among the culture systems (values were higher in CTs than EPs).

Total Dissolved Solids (TDS): TDS refers to any matter either suspended or dissolved in water. Dissolved solids include; bicarbonate, sodium, organic ions and other ions in sustaining aquatic life. TDS in culture systems ranged from 200 mg/L to 402 mg/L. The values were higher during the dry season than wet season (p < 0.05) but there was no significant difference (p > 0.05) between culture systems.

Nitrate: Nitrate is a form of nitrogen and a vital nutrient for growth, reproduction and the survival of organisms. Santhosh and Singh (2007) described the favourable range of 0.1 mg/L to 4.0 mg/L in fish culture water. The results indicated high level of nitrate during the dry season. There was significant difference (p < 0.05) between seasons (values were higher during the dry season than the wet seasons) and nitrate level was not significantly different (p > 0.05) between culture systems.

Nitrite: Nitrite can be termed as an invisible killer of fish because it oxidizes haemoglobin to methemoglobin in the blood, turning the blood and gills brown and hindering respiration. It also damages the nervous system, liver, spleen and kidneys of the fish (Bhatnagar and Devi, 2013). The ideal and normal measurement of nitrite is zero in any aquatic system. Stone and Thomforde suggested that the desirable range is 0 – 1 mg/L NO₂ and acceptable range less than 4 mg/L NO₂. Nitrite content recorded in this study was between 0.01 mg/L and 0.5 mg/L and 0.04 mg/L and 0.16 mg/L for EPs and CTs, respectively.

Linear correlations between water quality variables revealed that due to inter-correlation of variables, their linear combinations were constructed by applying principal component analysis. Ammonia and nitrate had positive correlation and significant at p < 0.05. A unit increase in the value of Nitrate will result in 0.55 increases in the value of ammonia. pH had positive correlation with temperature, TDS and nitrite but they were not significant (Table 6). Comparison of mean of water quality in different farms revealed that nitrite values obtained in OOF and KFF farms during the wet seasons were significant at p < 0.05 (Table 5). During the dry season, water quality among the farms operating earthen pond systems were not significantly different (p > 0.05). There was no significant difference (p > 0.05) in water quality parameters in farms operating concrete tank systems during the wet and dry seasons (Table 5).

4 Conclusions

The study on the effect of season of the year on the physico-chemical properties of pond water revealed that; pH, DO, TDS, nitrate and nitrite were not affected by culture systems and season of the year. Temperature, ammonia and TDS were significantly different among culture systems and season of the year. The values of water quality parameters obtained in this study were within the recommended water quality (Table 7) parameters for warm water fish (Ajani et al., 2011).

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