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INTRODUCTION
Aquaculture is the science dedicated to the production of aquatic organisms. According to the species being cultured, it has a specific name that serves as a factor indicating the type of culture being carried out such as algaculture, malacoculture, carciniculture, fish farming or pisciculture. The pisciculture is the part of aquaculture that focuses on the cultivation of fish. This activity has been developed around the world for over 2000 years B.C. (Tidwell, 2012) and every day has proven to be one of the best options for the supply of food needs and economic growth of developing countries in the African region.
In Angola, fish farming has been developed since 2008 and the most produced species is tilapia of the genus Oreochromis niloticus (Manjarrez et al., 2016; Aguilar et al., 2016). Currently it takes place in 16 of the 18 provinces that make up the nation. The study was developed with the application of various production systems and different kinds of cultivation where production in concrete tanks, in net pens and in earthen ponds stand out. The tilapia Oreochromis niloticus is known to be a specie that is not very sensitive to environmental changes and resists very well the adverse conditions that are not adequate for its production when compared to other productive species. The physical and chemical conditions considerate ideal technical and scientifically for tilapia production are presented in Table 1.
TABLE 1 Range of values considered ideal for physical (temperature and transparency) and chemical (pH, dissolved Oxygen and ammonia) parameters for tilapia Niloticus
| Productive parameter | Ideal productive interval | Reference |
|---|---|---|
| Temperature (℃) | 25 - 30 | Pereira & Silva (2009) |
| Transparency (cm) in pound | 20 - 30 | Da Silva (2015) |
| Hydrogen potential (pH) | 6,5 - 8,0 | Borges (2009) |
| Dissolved Oxygen (DO mg/L) | 4,0 - 8,0 | Kubitza (2017) |
| Total ammonia (NH4+ and NH3 mg/L) | Up to 0,15 | Kubitza (2017) |
The success of any fish production depends largely on the water quality in the production facilities. According to Leira et al. (2017) the water used in fish farming can be classified in three different ways:
(i) The source water, that comes from a source and that will supply the production system;
(ii) the water of use, which is the water that is retained in the spot of production in direct contact with the specie in production;
(iii) the discharge water, which is the water that leaves the spot of production to the wastewater discharge site.
Poor quality water in use leads to reduced fish growth and prolongs the production cycle, promotes the reduction of resistance to diseases which can lead to death, poor quality of the product obtained, low production and low economic yield making production projects unviable. Water quality for aquaculture has to do with physical, chemical and biological parameters that demonstrate the water's ability to maintain the health and well-being of cultured organisms. Therefore, having all physical, chemical and biological water conditions in recommended proportions within tolerance limits is a sine qua non condition for optimal productivity. Among all the physical and chemical water quality parameters for tilapia production the most relevant are transparency, temperature, pH, dissolved Oxygen and ammonia (Pereira y Silva, 2012; Oliveira, 2019; SENAR, 2019). This article aimed to evaluate the range of variation of the main physicochemical parameters in water quality to use in the tilapia production ponds of the Tchissola II farm thus determining its suitability for the development of the fish farming activity.
MATERIALS & METHODS
The study area
The samples were collected in the farm Tchissola II located at 23 km north of the municipality of Huambo. Samplings and evaluations were done in two ponds with 20 m wide, 40 m long and 2 m deep which were stocked with water from a retention basin built next to a natural water source (spring), during the first three months of production (December, January and February 2020 to 2021) and fingerlings of the genus Oreochromis niloticus. The depth of water sample collection was 60 cm. The evaluations were done with the frequency that each parameter required. For temperature, transparency and dissolved Oxygen the evaluation occurred during one week in a row. For Hydrogen potential and ammonia, the evaluation was done once a month for December and January and twice in the month of February (at the beginning and end of the month), because a considerable increase in ammonia levels was recorded and dilution measures were adopted for them. The frequency of evaluation of the last two parameters was the minimum recommended and possible because of the need to preserve the reagents for a longer time since they are expensive and imported from Brazil.
Physical Parameters
Temperature evaluation (T°C)
The temperature was evaluated in situ using the thermometer that comes coupled with the Alfakit Oximeter which is part of the Alfakit combo kit for water quality analysis in aquaculture (Alfakit, 2020). The evaluation was done monthly in the months of December 2020 to February 2021, during 7 days with the frequency of 5 times a day at different times (4, 8, 12, 16 and 20 h) at a depth of up to 20 cm from the bottom of the nursery. The methodology consisted on the calibration of the equipment keeping the probe in the air holding by the handle, since the heat of the hand on the metal part may interfere with the temperature. After calibration, was performed the procedure of the probe immersion in the incubator so that the stainless steel body was completely immersed waiting until the temperature sensor reached thermal equilibrium with the sample, approximately 1 minute (instructions in AT 150/155/160/170 Oximeter Operations Manual).
Transparency Assessment
The transparency was assessed in situ using the Secchi disk from the Alfakit combo kit for aquaculture water quality analysis. The evaluation was done monthly in the months of December 2020 to February 2021 for 7 days with the frequency twice a day at different times (8 and 14 h). The methodology consisted of immersing the Secchi disk in the scallop water until the equipment could not be seen, at this point the depth at which the disk could no longer be seen was marked on the measuring tape according to the instructions in Alfakit Instruction Manual (Filizola et al., 2006; Da Silva, 2015).
Chemical Parameters
Evaluation of the Hydrogen potential (pH)
For the pH the sample was collected at 60 cm depth at 10 o'clock in the morning trying to avoid the beginning and end of the day because the conditions at these times can interfere with the real pH values of the ponds as indicated by Oliveira (2017). The parameter was evaluated using the pH meter of the Alfakit combo kit for water quality analysis for aquaculture. The evaluation was done monthly in the months of December 2020 to February 2021, once per month. The methodology consisted of connect the pH electrode (AT315) to the equipment to proceed with equipment calibration. Once calibration was complete, the reading was carry out according to the Instruction Manual AT 315 code 5382/6424/5791 version 2.0.
Dissolved Oxygen evaluation (DO mg/L)
The dissolved Oxygen (DO) was evaluated in situ using the Alfakit combo kit oximeter for aquaculture water quality analysis. The evaluation was done monthly in the months of December 2020 to February 2021, for 7 days with the frequency of 5 times a day at different times (4, 8, 12, 16 and 20 h) at a depth of up to 20 cm from the bottom of the nursery. The methodology consisted on calibration of the equipment (Oximeter) and the immersion of the probe in the nursery so that the stainless-steel body was fully immersed, and was shacked the probe in circular motions according to the instructions in the AT 150/155/160/170 Oximeter Operations Manual.
Ammonia Evaluation (NH4+ mg L-1 N-NH3)
The water sample for the ammonia (NH4+) evaluation was collected at a depth of 65 cm and its evaluation was done using the ammonia evaluation kit that comes attached to the Alfakit combo kit for aquaculture water quality analysis. The evaluation was done monthly in the months of December 2020 to February 2021, once per month with the frequency of once a day at 10 am. The methodology consisted of collecting 50 mL of water sample from the ponds with the syringe sampler reaching a depth of 1 m. Following the sample was transfer into a cuvette up to the 5 mL mark. After this operation was added three drops of Reagent 1 - 3613, three drops of Reagent 2 - 3614 and three drops of Reagent 3 - 3615, then closed and shacked. After waited for 10 min, was opened the cuvette and done the color comparison according to the Instruction Manual ACQUAcombo kit freshwater producer, code 5355/5356.
Processing of the collected data
The data obtained for the variables evaluated in the two ponds were summed and divided by the number of observations made in each parameter finding the monthly average for each ponds. Those results gave the average variation of the water quality parameter in production in the month being evaluated. These final values were analyzed based on the simple descriptive statistics methodology that allows the interpretation of the average results obtained and subsequently compared to the normal values recommended by fish farming scholars (Table 1) and by the Water Quality Manual for Aquaculture of the Alfakit Company according to the methodology applied by Leira et al. (2017) and Mata (2018).
RESULTS
The evaluation results of the physical and chemical parameters estimated in the tilapia production ponds at the Tchissola II farm varied as presented in Table 2.
TABLE 2 Ranges in variation of physical and chemical parameters evaluated in the tilapia production ponds at Fazenda Tchissola II between the months of December 2020 - February 2021
| Parameters | Months | ||||||
|---|---|---|---|---|---|---|---|
| Dec-2020 | Jan-2021 | Feb-2021 | Unit | Max. | Min. | Average | |
| Transparency | 48 | 44 | 31 | Cm | 60 Dec. | 20 Feb. | 40 |
| Temperature | 25,5 | 26,7 | 27,4 | °C | 29,7 Jan. | 21,8 Dec. | 25,75 |
| Dissolved Oxygen | 7,3 | 3,78 | 6,75 | mg/L | 11,5 Feb. | 2 Feb. | 6,75 |
| Hydrogen potential | 6,5 | 6,2 | 8,82 | mg/L | 8,80 Feb. | 6,2 Jan. | 7,5 |
| Total ammonia | 0,11 | 0,73 | 1,35 | mg/L | 3,0 Feb. | 0,10 Dec. | 1,55 |
DISCUSSION
The transparency, which is the capacity of the water to pass the sun's rays (Alfakit) in the two ponds evaluated, had an average variation of 40 cm between December and February, reaching its highest expression of 60 cm in the month of December. These results, at the beginning of the productive process in the farm, demonstrate that at this time the ponds were poor in planktonic population due to the reduced quantity of organic matter. This fact lead to a high transparency rate considered inadequate for the productive process in this type of facility, also signaling the need for fertilization of the ponds in order to stimulate plankton production.
The lowest transparency level recorded was 20 cm in February, demonstrating that the plankton level increased considerably during the production cycle, as organic matter was introduced into the ponds through feeding, and through the waste expelled by the fish such as urine and feces. The addition of these elements lead to increase the levels of organic matter, which consequently lead to considerable plankton growth, resulting in a reduction in the level of transparency from 60 to 20 cm 60 days after the production start. Such levels are within those considered adequate for the production of tilapia Oreochromis niloticus according to Leira et al. (2017).
When the transparency range is between 20 - 40 cm means that there is an adequate amount of plankton in the ponds that serve as a nutritional source for fish larvae. Nevertheless, this range is relative since there is no consensus among the specialized literature as to the ideal transparency range. The results are also closer to those found by Leira et al. (2017) who obtained a range of transparency between 29.80 - 31.07 cm. Results reported by Martins (2007) on water quality in tilapia Oreochromis niloticus ponds found that diurnal characterization of physical, chemical and biological variables reached a water transparency between 25 - 35 cm presenting an average of 30 cm. In contrast, the results obtained in this study, present a considerable difference compared to the results offered by Mata (2018) who during the study obtained a maximum transparency of 23.5 cm.
The temperature variation ranged from a low of 21.8°C in December to a high of 29.7°C in January, with the average recorded for both ponds for the same period being 25.75°C. The data showed that the lowest temperature was recorded just at the beginning of the production and as with transparency, as the production cycle unfolded, the temperature increased. Some authors state that as the organic matter increases so does the water temperature in the ponds, because the feeding activity of the fish induces an increase in temperature Leira et al. (2017).
The results obtained, are within the temperature range considered ideal for tilapia culture. According to Borges (2009); Pereira & Silva (2012); Da Silva (2015) e Kubitza (2017), temperatures between 25-30ºC correspond to the ideal temperature range for tilapia culture where its manifest the maximum of their growth potential, greater immunological stability that results in greater resistance to diseases and adequate feed conversion. Similar results were reported by Mata (2018) who described that in the evaluation of the temperature, the acceptable for the development of the fish remained within an average of 23.3ºC with maximum around 26.5 and minimum 21ºC.
The dissolved Oxygen during the evaluated period varied between 2 mg/L and 11.5 mg/L for the minimum and maximum levels respectively, both in February, reaching an average of 6.75 mg/L. The Oxygen concentration in production ponds is dependent on factors such as temperature, in which colder places permit the higher dissolved Oxygen concentration. Also the salinity, in which the higher values offer the lower Oxygen level. Respiration of living organisms present in the water and the photosynthesis process there are factors with influence on maximum and minimum oxygenation levels. The values reached in February reveals that the content of plankton in the ponds was increased considerably throughout production causing Oxygen consumption in a period when there was low solar radiation. As phytoplankton reduce the photosynthesis process and consequently reduce the releasing of Oxygen, thus decrease the Oxygen available in the ponds.
Likewise, the reverse process also happens in periods of higher solar radiation. Due to the high level of plankton available in the nurseries, in periods of adequate solar radiation, the photosynthetic activity is high resulting in higher Oxygen availability in the nurseries, hence both maximum and minimum levels of Oxygen availability are recorded at the same period. Such levels, are within those considered adequate for the survival of tilapia according to Borges (2009); Pereira & Silva (2012); Da Silva (2015) e Kubitza (2017). Even though the lowest level (2 mg/L) was within the range considered as alert, it has no considerable negative influence because the period that fish was exposed to this value was short, mostly during the transition period between evening and morning. Mata (2018) reported similar results on limnology studies and its correlation with the productivity of tilapia Oreochromis niloticus when evaluating the variation of Oxygen levels. This author found that the data obtained on dissolved Oxygen showed a quality plateau with results between 6.92 - 9.63 mg/L. Authors such as Mercante et al. (2006); Barbosa et al. (2009); Dantas & Apolinário (2014) found values between 5, 0 and 12.0 mg/L.
The Hydrogen potential levels showed fluctuation between 8.80 to 6.2 mg/L having reached the maximum level of 8.80 mg/L in the month of February and the minimum level of 6.2 mg/L in the month of January being the recorded average in 7.5. These data inform that the pH levels increased throughout the production process, however, the numbers recorded for pH are within the range considered adequate for the cultivation of tilapia Oreochromis niloticus (Borges, 2009; Pereira & Silva, 2012; Da Silva, 2015, Kubitza, 2017) with tolerance to the variations presented in the fishponds evaluated. These results are in accordance with the results presented by Leira et al. (2017), which on the evaluation of water quality for tilapia raised in ponds at the Institute Federal Fluminense (IFF) CAMPUS-CAMBUCI identified that the pH values considered normal in those conditions were around 7,93 - 7,96.
The last variable evaluated was total ammonia. Ammonia levels in the tilapia production ponds at Tchissola II farm ranged from 0.10 mg/L minimum recorded in December to 3.0 mg/L in February leading to an average of 1.55 mg/L. It is visibly noticeable that ammonia levels increased proportionally with the intensification of production. At first the level of 0.10 mg/L recorded in the month of January was considered within the ideal standards for tilapia (Borges, 2009; Pereira & Silva, 2012; Da Silva, 2015; Kubitza, 2017).
On the other hand, the same authors consider the levels recorded in January and February as critical and lethal respectively, being unsuitable for production because can be toxic to the specie generating intoxication mortality problems. Similar results are reported by Mata (2018) who described an average of 2.07 mg/L and classified it as optimal for tilapia production in ponds. These studies emphasizes that continuous exposure of tilapia to irregular ammonia concentrations can lead to high fish mortality rates or make them more susceptible to disease presentation.
CONCLUSIONS
From the evaluation of water quality parameters in tilapia production ponds at the Tchissola II farm in Chipipa commune, Huambo municipality, it was found that physical (temperature and transparency) and chemical (dissolved Oxygen and Hydrogen potential) parameters are within normal levels considered tolerable in tilapia production, although they tend to increase with the course of production.
Ammonia levels reached inadequate proportions for tilapia production indicating that the management of the culture needs to be done with more caution minimizing the increase of the parameters with the progress of the production process.
