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International Journal of Aquaculture, 2014, Vol.4, No.24 1
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8
http://ija.biopublisher.ca
3
Visual assessment of mass mortality
The first incidence was reported by the local
fishermen from Lake Babogaya on the 10
th
of April
2013, whilst on 17
th
of April 2013 similar tragic
incident was reported in the adjacent lakes
Hora-Arsedi and a week later in L. Kuriftu. It was
observed that the extent of fish mortalities in these
lakes fulfills the criterion for mass fish kill according
to the existing standard definitions (NCDWQ, 2000;
La and Cooke, 2011).
Field sampling and analyses
The investigation included some limnological parameters,
clinical, histopathological and bacteriological
examinations of live but affected fish samples. Data
on physico–chemical variables such as water
temperature, dissolved oxygen (DO), conductivity
(corrected to 25
o
C) and pH were measured
in
–
situ
using a multi-probe (Model HQ40D, HACH
Instruments). Water for nutrient analysis was collected
from depth profiles (surface, 1, 2, 5, 7, 10, 15, 20, 30,
40, 50, 60, 62 m depth) using a 5 L Schindler sampler.
Samples were transferred to 2 L acid–prewashed
plastic bottles and transported to the laboratory, stored
in dark cooling boxes. The samples were subsequently
filtered within 4 h of sampling through glass fibre
filters (Whatman GF/C) paper and the filtrate was
used for nutrient analyses, which were done within 48
h of sample collection following the standard methods
(APHA, 1998). Samples for estimation of the relative
abundance of phyto- and zoo-plankton were collected
with a plankton net (40 µm mesh size). Phytoplankton
taxa were enumerated and identified under a
microscope using keys of Whitford and Schumacher
(1973), Popovsky and Pfiester (1990), and Komarek
and Anagnostidis (2005), while Zooplankton was
enumerated and identified down to species level under
a WILD stereoscope microscope (magnification 40x)
using keys of Defaye (1988), Korovchinsky (1992)
and Fernando (2002). Fish samples were collected
according to the safety and aseptic handling
procedures for diseased fish (Meyer and Barclay,
1990). Post-mortem and live fish histopathological
and bacteriological examinations were carried out on
four tissues samples derived from the heart, brain,
liver and kidney at the National Animal Health
Diagnostic and Investigation Center (NAHDIC). Live
fish were also examined for clinical symptoms. Aseptic
techniques have been employed for bacteriological and
parasitological examination using appropriate bacterial
culture and isolation media. Inoculums were taken from
live fish demonstrating clinical symptoms in the form of
skin lesion and injured gills.
Results
Physicochemical parameters
Both lakes were characterised by medium to higher
water transparencies, with secchi depths in
Hora-Arsedi never exceeding 1.35 m and in Babogaya
3.05 m, respectively. The mean water temperatures
generally exceeded 20 °C. Vertical gradients in water
temperature revealed that lakes Babogaya and
Hora-Arsedi were stratified during the mortality
incident with well-defined thermocline between 6 and
10 m depth in Babogaya and between 3 and 6 m in L.
Hora-Arsedi (Figure 2). The mean DO concentrations
were 4.14 ±1.09 mg L
−1
(SE) and 3.44 ±0.98 mg L
−1
(SE) for Babogaya and Hora-Arsedi, respectively. The
surface water was well oxygenated in both lakes with
DO concentrations up to 9.4 mg L
-1
and 9.0 mg L
-1
in
Babogaya and Hora–Arsedi, respectively (Figure 2),
however the hypolimnion was anoxic in both lakes.
DO levels of 8.9 and 7.1 mg L
-1
was measured early in
the morning at 06:00 A.M in Babogaya and
Hora–Arsedi, respectively (Table 1). The pH value
ranged between 7.3 and 8.1 in both lakes. The specific
conductivity ranged from 768 to 951 μS cm
−1
in
Babogaya and 2420 to 2310 μS cm
−1
in Lake
Hora-Arsedi (Figure 2). Mean unionized ammonia
which was derived from the total ammonium content
(TAN) were 2.1 and 0.1 mg L
-1
in Babogaya and
Hora-Arsedi, respectively (Figure 2). The mean TAN
values were 6.50 ±1.64 mg L
-1
(SE) and 0.039 ±0.01
mg L
-1
(SE) in Babogaya and Hora-Arsedi, respectively.
Table 1 Some physico-chemical measurements (mean ±SE) taken early in the morning at near shore and open water sites of Lakes
Babogaya and Hora-Arsedi.
Note
: sulfide values are only for bottom-sediment samples
Site
Lakes
Mean DO (mg L
-1
) Mean pH
Conductivity (µS cm
-1
) Total sulfide (mg L
-1
)
Near shore
Babogaya
9.47±0.29
8.00+0.01
815±1.4
Hora-Arsedi
6.98±0.10
7.05+0.00
2350±2.9
Open water Babogaya
7.23±0.02
8.13±0.02
817±14
8.29
Hora-Arsedi
7.09±0.11
7.11±0.08
2368±12