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.

PhD Thesis

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.

Danish International Development Agency (DANIDA)