This research was conducted in Majang Zone, Gambela Regional State, Ethiopia. The Lake ‘Bishan Waka’ is an alternative local name for Lake Kabo, a kind of natural lake created by volcano phenomenon. The name, Bishan Waka means the gift of God. The maximum depth  of  the lake was 20.5m during the study period. It is located about 651 km from Addis Ababa and is situated very far from the government. Local people benefit from the lake through traditional fishing activity. We encountered dead fish floating on  the  lake  during  preliminary  survey  and were interested in finding the reason for their sudden death. Water quality is the totality of physical, biological and chemical parameters that affect the growth and welfare of organisms. Quality of water is an indispensable factor to be considered when planning to produce fish (Mallya, 2007).

The chemistry of natural surface waters is complex, and is based on the equilibrium reached with the normal physical, chemical and biological characteristics of the surrounding (Bronmark and Hansson, 2005). The life and growth of fish depend on pH of the water.  pH value of the water will rise during the day as the amount of carbonic acid is reduced; on the other hand dissolved oxygen concentrations rise during the day. At night, the level of carbon dioxide rises, leading to reduction in pH, and the level of dissolved oxygen also falls (Svobodová et al., 1993). Algae require a pH of about 7, and a slightly lower (alkaline) pH of 6.5 favors good zooplankton (tiny animals in the pond water on which fishes feed) and fish growth.

Fish have a limited ability to adapt to changes in these factors, if they occur sufficiently slowly; rapid changes can be harmful. Water pH also has a significant influence on the toxic action of a number of other substances (e.g. ammonia, hydrogen sulphide, cyanides, and heavy metals) on fish. The suitable water temperature for carp culture, for example, is between 24 and 30°C. In their natural environment, fish can easily tolerate the seasonal changes in temperature, e.g. a decrease to 0°C in winter and increase to 20-30°C (depending on species). Increase in temperature can cause stress and expose them to disease. Fish kill can be caused by a variety of factors including dissolved oxygen depletion, extreme water temperatures, fish diseases or introduction of pollutants.  Most fish kills are natural events.

Secchi disc is an all white or a black and white metal disc which can easily be made by hand. The disc is attached to a cord that is marked every 5 cm along its length (Assiah et al., 2004). Turbidity is the decreased ability of water to pass on light caused by suspended particulate matter and phytoplankton. Secchi disk transparency from 30 to 40 cm shows optimum productivity of lake and 50 cm is high. Algae,  microscopic  aquatic animals,  water  color,  and  suspended  sediment  interfere with light  penetration  and lessen  water  clarity. Hence, secchi transparency is considered an indirect measurement of how much algae and sediment is in the water (Tushar, 2012).

Cyprinids types of fish species have less demand than other fishes; they can succeed in water containing 6-8 mg/L DO and show signs of suffocation only when the oxygen concentration falls to 1.5-2.0 mg/L. Increase in water temperature from 10 to 20°C doubles the oxygen demand; a higher total weight of fish per unit volume of water can lead to increased activity and increased respiration as a result of overcrowding (Svobodová et al., 1993). Tilapia fish prefer >5 mg/l and tolerate 3-4 mg/L  of DO level (Lloyd, 1992). Alkalinity is the total sum of ions reacting to neutralize hydrogen ions when an acid is added to water. The ideal value for fish is 50-300 mg/L (Santhosh and Singh, 2007). With low levels of oxygen in the water, lower concentrations non-dissociated ammonia can kill fish.

Fish populations are highly dependent on the variations of physicochemical characteristics of their aquatic habitat which supports their biological functions (Mushahida-Al-Noor and Kamruzzaman, 2013; Blaber, 2000; Furhan et al., 2004; Koloanda and Oladimeji, 2004; Ojutiku and Kolo, 2011). The availability of good water quality is important for all fish farming systems but water quantity is even of greater importance for fish farming systems. Right through the century’s fish has been vital element of humans’ food. The need to improve fish yield by farming became a critical matter (Carballo et al, 2008). Phytoplankton produces carbohydrate using sunlight and releases oxygen. This is the major source of energy and oxygen in the ecosystem. Zooplanktons living on the phytoplankton form major source of food for fishes (Santhosh and Singh, 2007).

In winter, fish are commonly killed by suffocation in polluted storage ponds and in summer this often happens in polluted water courses with high temperatures and low flow rates. Oxygen deficiency causes asphyxiation and fish will die, depending on the oxygen requirements of the species and to a lesser extent on their rate of adaptation. Fish exposed to oxygen deficient water do not take food, collect near the water surface, gasp for air (cyprinids), gather at the inflow to ponds where the oxygen levels are higher, become torpid, fail to react to irritation, lose their ability to escape capture and ultimately die (Svobodová et al., 1993). The dissolved oxygen (DO) is the most important source of the aquatic atmosphere and photosynthetic process for the green plants and also a determining factor of the water quality of an aquatic ecosystem (Rajeev and Ajay, 2010).

Generally, phytoplankton development in waters is determined by the same physical and chemical parameters like temperature, light availability, nutrients. Chlorophyll-a concentration is also determined as a measure of phytoplankton biomass (Hötzel and Croome, 1999). A Secchi disk is used to estimate phytoplankton density and the fertility of lake. This study aims to study the physicochemical characteristics of the Lake and provides reason for sudden fish death.

Physicochemical feature of water has direct effect on survival, growth, reproduction and distribution of fishes. Any improper environmental characters threaten the life of fish. Quality of water is, therefore, an indispensable factor to be considered when planning the production of fish (Mallya, 2007). The chemistry of natural surface waters is complex and is based on the equilibrium reached with the normal physical, chemical and biological characteristics of the surrounding (Bronmark and Hansson,   2005).   Therefore,  the   present   study   was conducted to find out the information about the Physico-chemical parameters of Kabo Lake water for the first time and find out possible reasons for sudden fish death. So, the result of this study may provide primary data about the lake for further scientific study, help to extend sustainable lake management, monitoring of water quality, indicate the problem of the lake identified and recommend accordingly and point out possible solutions.

Description of the study area

Lake Kabo is a small, circular and natural lake located in Ethiopia in Gambela Regional State, Majang zone, Godere worada, Gubeti kebele (Ethiopia). It is located about 651 km from Addis Ababa and 40 km far from Teppi town. It lies at Latitude: 7°18’10.36’’N and Longitude: 35°16’5.42’’ E, at an elevation of 1397 m above mean sea level.

Sampling technique and sample collection methods

Four sampling stations on the lake were taken to identify and analyze the problem of the Lake. These selected sites were according to the iterance of water to the lake, the exit site of water out of the lake and area mostly preferred by fish or traditional fishing activity performed. Samples were taken from all four selected stations every month starting from October 2017 to August 2018 in the morning by direct immersing of polyethylene vessels (PVC) just below the surface of the lake at the depth of 50 cm. Sites on the lake’s body coded SS1 for entering of small river into the lake, SS2 (exit site of the lake), SS3 (at the center of the lake) and SS4 (sites of fishery activities performed by local society traditionally).

Experimental methods and procedures

One liter of water sample from each sampling station was taken and placed in a polyethylene bottle, coded and transferred cold to the laboratory for analysis as per the standard methods suggested by APHA (1992). The samples were analysed in triplicate. Samples for pH, temperature (T^°), transparency, TDS (total dissolved solid and electrical conductivity (EC) were analyzed in situ using a portable standard laboratory pH meter (pH 600, range 0.0/14 pH, accuracy + 0.1); electrical conductivity (EC) was determined by portable EC meter (conductivity meter cc–101, Elmron I P67 model). Water samples for DO and total alkalinity were analyzed in the Teppi National Soil Laboratory. All field meters and equipment were checked and calibrated according to the manufacturer’s specification. Total alkalinity was estimated titrimetrically using 0.02N, HCl with Bromocresol Green and DO was measured titrimetrically based on Winkler method/procedure. The transparency of the lake was determined with the aid of the Secchi disc. The disc was lowered into the water and the depth at which it disappeared was observed and recorded. It was there after gradually withdrawn from the water and the depth at which it became visible was noted and recorded. The transparency of the water body was calculated as the mean of the two readings.

The Lake water was analyzed for its minerals following AOAC (1990). Sample (0.5 L) was digested by using HNO₃ and perchloric acid at ratio of 7:3 on hot plate until solution turned colorless. Digested sample was diluted for mineral analysis. Sodium, potassium and calcium were calculated on  Flame  Photometer-410 (Sherwood Scientific Ltd., Cambridge); on the other hand, copper, iron, magnesium and phosphorus were determined through Atomic Absorption Spectrophotometer (Varian AA240, Australia).

Statistical analysis

Comparison of physicochemical characteristics of the lake water was made with WHO standards (1993), BIS standards (1991) and other related documents. The data were exposed to descriptive statistical analysis. Statistical package for social sciences (SPSS version 19) was used to govern the mean, standard deviation and range values. Pearson correlation was performed using simple Pearson correlation method. The results of water parameters were also compared with water quality standards to check whether the lake water is within the acceptable range to fish or not.

Monthly physicochemical analysis was made in different parameters. The maximum pH was registered in March (8.7), TDS (251.3 mg/L) in June, TA (195 mg/L) in August and February, EC (191 mg/L) in August, DO (4.7 mg/L) in June and the maximum temperature was 24.6°C in the month of April (Table 1). The annual mean temp., pH, TDS, total alkalinity, EC and DO of the lake were 23°C, 7.65, 205.64, 173.3 and 4.34 mg/L respectively (Table 2). The annual mean DO value of the Lake Kabo was 3.033±0.18 to 4.2 ± 0.24 mg/L. The least value was 2.8 mg/L while 4.7 mg/L was the maximum DO value (Table 2). The annual mean values of PO₄^-3, Chlorides and Ca⁺⁺ were 0.1, 1.5 and 7.6 mg/L respectively (Table 3).

In present study, the lowest value of water temperature was found in the month of August (19°C) and highest in May (25.1°C); while the annual average temperature was 23 ± 1.6°C (Table 4). Sampling stations SS1 and SS4 scored maximum annual mean value (23.16°C) than the rest two sampling sites. The result was found below permissible limit for aquatic life as recommended by BIS (1991) (30-35°C) and Dennis et al. (2009) for tilapia fish (25-32°C) but it agreed with the report presented by Assiah et al. (2004) for fish (20- 30°C). However, the variations in water temperature may be due to different timings of collection, influence of the season and the effect of atmospheric temperature (Rajeev and Ajay, 2010).

Electrical conductivity estimates the amount of total dissolved salts (TDS), or the total amount of dissolved ions in the water. The conductivity is proportional to the amount of salts dissolved in water. In the present investigation, the lowest value of EC was obtained in October (98 μs/cm) and highest values (235 μs/cm) registered in July (Table 2), with the annual mean value of 150.59 μs/cm (Table 5). The maximum average result was registered in August (206 μs/cm and the lowest in February (108 μs/cm). The result of electric conductivity and total dissolved solid positively correlated with TDS (r=0.569). The increase in electric conductivity was completely associated with the amount of dissolved solid in the analyzed sample waters. The EC level annual mean of SS4 was significantly greater than the rest three sampling sites (158.60 ± 43.653). The result obtained during the study period was low when compared with the value specified by BIS (1991) permissible limit for aquatic life. The lower value of total alkalinity and TDS directly correlated with Na⁺ and Ca⁺⁺ concentration of the lake.

From the result obtained during the study period, major conclusions were drawn. The Physicochemical water quality of Lake Kabo at the study time revealed that water dissolved oxygen (mg/l), temperature and pH level fluctuation were found as the reasons for the sudden death of the fish.

The critical problem of the lake was fluctuation of DO and pH level. So, it is recommended that the level of phytoplankton in the lake should be enhanced through wise use of minerals for their growth and farther production of oxygen. Forest canopy that prevents the penetration of sun light should be avoided.

The author has not declared any conflict of interests.

Author appreciate and thank Mizan Teppi University, Research and community service Directorate for his Financial and material supports for the success of our work.