Analysis of diurnal and seasonal dynamics of water quality parameters and their implications on metabolism and physiology of nile tilapia (oreochromis niloticus) cultured in pond.
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Date
2024
Authors
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Journal ISSN
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Publisher
Sokoine University of Agriculture
Abstract
Aquaculture production of Nile tilapia (Oreochromis niloticus) has
attracted considerable attention around the world because of their
fastest growth rate, good quality meat, and fascinating
physiological characteristic that allow them to adopt and strive in
a broad range of production systems. Because of
these
characteristics, Nile tilapia has been adopted as the key
aquaculture species in Tanzania. Nile tilapia production in
Tanzania takes place primarily in earthen ponds, mostly under a
semi-intensive system without the use of water treatment
technology. In Tanzania more than 80% of the aquaculture
produced fish comes from Nile tilapia raised ponds. The
production of Nile tilapia in ponds has for long time been
unsuccessful due to poor growth performance. This is because,
unlike in other production systems such as Recirculating
Aquaculture Systems (RAS), in small water bodies such as
earthen ponds water quality parameters vary considerably on a
daily basis, depending on the equilibrium reached with the regular
physical and chemical characteristics of the surrounding
environment. These variation are likely to considerablly effect
physiology and welfare of the cultured fish. Unfortunatly, the
extent of diurnal, and seasonal variation in water quality
parameters in the production ponds across the country are not
yet known. This makes it difficult for estimating the influence of
changes in fish ponds water quality influences on physiology and
performance of cultured Nile tilapia.
This study was carried out to determine the current status and
dynamics of the diurnal, and seasonal fishpond water quality
parameters in Tanzania. It was also carried out to assess the
effects of the observed water quality variation on the physiology,
metabolism and performance of the pond-cultured Nile tilapia. To
achieve the study objectives, a field survey of fish onds and three
laboratory experiments were performed. The diurnal and
seasonal trends in pond water quality parameters were
documented and the effect of changes of some selected water
quality parameters on the Nile tilapia physiological characteristics
and performances were reported.
Study 1. A field survey was conducted between June 2019 and
April 2021 in Tanzania. Four agroecological zones (Easter,
Southern Highland, Northern, and Lake Zones) were selected as
study sites. A total of 120 fish farmers were interviewed using a
well structured questionnaire to collect information on the current
general farm characteristics and farming practices. In addition, a
fish farm with identical ponds stocked with Nile tilapia was
selected in each agro-ecological zone, and an in-situ
measurement of the water parameters was carried out for one
year, covering two seasons (the cold: June to September and the
warm season November to January). Results revealed
significantly difference in mean pond water temperature between
the study zones (p <0.001) with the highest in the Eastern and
the lowest in the Southern Highland zones. Results show that the
temperature in the Northern zone and Southern highlands during
the cold season (June to September) was below the level
required for Nile tilapia growth. Other water quality parameters
(pH, DO, turbidity, and TDS) varied between seasons within
zones, but, were within the range suitable for Nile tilapia growth.
The annual fish yield was higher in the Lake zone (629 ±124
kg/ha/yr) suggesting a positive relationship between temperature
and fish yield. High net primary production during the warm
season in all zones indicates the need for good water quality
management to prevent algae blooms, which may result in acute
and prolonged stress from hypoxia and hypercapnia, which are
the primary physiological stressors for fish. The available data
from the survey study was used as a baseline in a series of
experiments conducted in the lab at Denmark Technical
University (DTU-aqua) to critically assess the effects of the
variation in water parameters on the physiology and performance
of the cultured Nile tilapia.
Experiment 1. High level s of dissolved carbon dioxide (CO 2 )
occurs nightly in earthen ponds characterized by high respiration
rates. Exposure to high CO 2 conditions (hypercapnia) leads to
acidosis in fish, which can be compensated by an accumulation
of HCO 3- to recover intra - and extracellular pH levels, with a
capacity that appears to be species-specific. For Nile tilapia, a
freshwater tropical teleost traditionally produced in earthen
ponds, little information is available on the tolerance to dissolved
levels of CO 2 and associated acid-base disturbances. Here, we
investigated first the effects of acute and progressively increasing
CO 2 , from normocapnic conditions to 60 mg CO 2 L -1 , on oxygen
uptake rates (MO 2 ). This was followed by exposure to three
concentrations of CO 2 ; 10, 30, and 60 mg L -1 (equivalent to pCO 2
of 5.4, 16.2, and 32.4 mmHg) against a normocapnic control
(pCO 2 0.3 mmHg), to investigate acute (1h) or prolonged (24h)
effects on standard (SMR) and maximum metabolic rate s
(MMR), haematology, and extra - and intracellular acid-base
status in adult Nile tilapia (mean BM 435 ± 16 g ±SE). Acute
exposure to hypercapnia led to concentration-dependent
decreases in both SMR and MMR. Fish were able to fully or
partially recover MMR and metabolic scope (MS) after 24h, while
depression of SMR persisted at all CO 2 levels. Acute exposure to
CO 2 caused intra-and extracellular pH levels to decrease by up to
0.5 units in a concentration-dependent manner. Only the lowest
hypercapnic treatment (pCO 2 5.4 mmHg) was able to fully
recover within 24h. Changes in haematological variables
appeared minor, being restricted to increasing haematocrit,
haemoglobin concentration, and mean cell volume in the highest
CO 2 treatments after 24h exposure. Although the Nile tilapia is
generally considered a species able to tolerate poor water quality,
the modest or slow acid-base regulation following hypercapnic
exposure suggests sensitivity to hypercapnia.
Experiment 2. Dissolved gases (carbon dioxide and oxygen) in
earthen ponds exert strong opposing effects due to
photosynthesis and respiration. This leads to daily conditions that
fluctuate between being hyperoxic and hypocapnic during the day
and becoming severely hypoxic and hypercapnic at night. The
interacting effects of hypoxia and hypercapnia and fluctuating
conditions on pond-farmed Nile tilapia have not received any
attention. This study evaluated the effects of daily variations in
dissolved O 2 and CO 2 on the appetite and feed utilization in Nile
tilapia using groups reared under normoxic - normocapnic
conditions (control, C), diurnal hypoxia (HO), diurnal hypercapnia
(HC), or combined diurnal hypoxia and hypercapnia (HO × HC) in
a digestibility system. Results show that hypoxia and hypercapnia
exerted strong individual effects on appetite, specific growth rate,
and feed conversion and that simultaneous hypoxia and
hypercapnia amplified these effects. Appetite depression induced
by nocturnal hypoxia or hypercapnia was restored at mid-day, but
simultaneous exposure resulted in a day-long loss of appetite.
Hypoxia and hypercapnia reduced specific growth rates by up to
40% and increased feed conversion ratio by up to 80%, while
combined exposure to hypoxia and hypercapnia reduced specific
growth rates by >60%. Surprisingly, the digestibility of dry matter,
protein, and lipid was improved in groups exposed to single or
combined diurnal variation in dissolved oxygen and carbon
dioxide. This study concludes that nocturnal exposure to hypoxia
or hypercapnia has adverse effects on the growth performance of
Nile tilapia driven by a combination of reductions in appetite and
poor feed utilization. The results emphasize that there is a need
for careful management of dissolved gases in pond culture.
Application of aeration technology, combined with management
of the nutrient status of the pond and sludge accumulation to
control oxygen consuming and CO 2 liberating processes, is likely
able to greatly improve production performance.
Experiment 3. Temperature plays a significant role in controlling
oxygen solubility in water and its availability to aquatic breathing
species including fish. In fish ponds, the water temperature can
increase gradually during the day resulting in elevated
temperature and an extended period of low dissolved oxygen. In
this study, the response of Nile tilapia (Oreochromis niloticus)
interms of thermal tolerance limit (CTmax), metabolic rate (SMR),
Cardiac performance (ƒ H ), and ventilation frequency (ƒv) to the
acute increase in temperature upon changing dissolved oxygenv
saturation was investigated. Nile tilapia were experimentally
exposed to an increased temperature of 0.1 °C min -1 at 100% DO
saturation (Normoxia), 70% (hypoxia), and 130% (hyperoxia)
from 26 °C (Optimum) until the onset of loss of equilibrium (LOE).
Oxygen consumption rate (MO 2 ), heartbeat, and opercula activity
were averaged at each temperature increase for three DO
saturation levels. Results show that high DO saturation (130%)
increased Nile tilapia’s Critical Thermal Maximum (CTmax) by
10%. The Nile tilapia metabolic rate (SMR) varied significantly
with the changing DO saturation at acute increased temperature
levels (One-way ANOVA; F(2,44) = 4.596, P = 0.016).
Temperature-DO-limited
metabolic
relationship
affected
cardiorespiratory activities and maximum thermal tolerance
(CTmax) in Nile tilapia (one-way ANOVA; F (2,44) = 0.34, P =
0.713). Different levels of DO saturation were found to
significantly affect Nile tilapia’s respiration ability (Opercula beats
min -1 ) (P < 0.001).
This study is concluding that diurnal and seasonal pond water
quality varies significantly between zones and between seasons
within the investigated zones and affectsthe production
performance of the pond-cultured Nile tilapia. Nile tilapia might
only be stocked in the warm season in the Northern and southern
highland zone if farmers are to achieve the best growth of Nile
tilapia. Although the Nile tilapia is generally considered a species
able to adapt to and tolerate poor water quality, the modest or
slow acid-base regulation following hypercapnic exposure
suggests that it is quite sensitive to CO 2 exposure. Although Nile
tilapia is considered resilient to environmental stress, feeding and
feed utilization, oxygen uptake, and Critical thermal maximum are
strongly influenced by daily fluctuations in dissolved CO 2 , O 2 and
Temperature.
Description
PhD Thesis
Keywords
diurnal, dynamics of water, water quality, parameters, nile tilapia