Nile tilapia (Oreochromis niloticus) show high tolerance to acute ammonia exposure but lose metabolic scope during prolonged exposure at low concentration.

Ammonia is a respiratory gas that is produced during the process of protein deamination. In the unionised form (NH3), it readily crosses biological membranes and is highly toxic to fish. In the present study we examined the effects of unionized ammonia (UIA), on the resting oxygen consumption (MO2), ventilation frequency (fV), heart rate (HR) and heart rate variability (HRV) in Nile tilapia (Oreochromis niloticus). Fish were either exposed to progressively increasing UIA concentrations, up to 97 µM over a 5 h period, or to a constant UIA level of 7 µM over a 24 h period. For both treatment groups resting MO2, HR and fV were recorded as physiological variables. Relative to the control group, the fish groups exposed to the incremental UIA levels did not exhibit significant changes in their MO2, HR and fV at UIA concentrations of 4, 10, 35, or 61 µM compared to control fish. Exposure to 97 µM UIA, however, elicited abrupt and significant downregulations (p < 0.05) in all three responses, as MO2, HR and fv decreased by 25, 54 and 76 % respectively, compared to control measurements. Heart rate became increasingly irregular with increasing UIA concentrations, and heart rate variability was significantly increased at 61 and 97 µM UIA. Prolonged exposure elicited significant changes at exposure 7 µM UIA. Standard (SMR) and maximum metabolic rate (MMR) were significantly reduced, as was the corresponding fV and HR. It is evident from this study that Nile tilapia is tolerant to short term exposure to UIA up to 61 µM but experience a significant metabolic change under conditions of prolonged UIA exposures even at low concentrations.

Relatedness of hypoxia and hyperthermia tolerances in the Nile tilapia (Oreochromis niloticus) and their relationships with cardiac and gill traits.

In fish, thermal and hypoxia tolerances may be functionally related, as suggested by the oxygen- and capacity-limited thermal tolerance (OCLTT) concept, which explains performance failure at high temperatures due to limitations in oxygen delivery. In this study the interrelatedness of hyperthermia and hypoxia tolerances in the Nile tilapia (Oreochromis niloticus), and their links to cardiorespiratory traits were examined. Different groups of O. niloticus (n = 51) were subjected to hypoxia and hyperthermia challenges and the O2 tension for aquatic surface respiration (ASR pO2) and critical thermal maximum (CTmax) were assessed as measurement endpoints. Gill filament length, total filament number, ventricle mass, length and width were also measured. Tolerance to hypoxia, as evidenced by ASR pO2 thresholds of the individual fish, was highly variable and varied between 0.26 and 3.39 kPa. ASR events increased more profoundly as O2 tensions decreased below 2 kPa. The CTmax values recorded for the O. niloticus individuals ranged from 43.1 to 44.8 °C (Mean: 44.2 ± 0.4 °C). Remarkably, there was a highly significant correlation between ASR pO2 and CTmax in O. niloticus (r = -0.76, p < 0.0001) with ASR pO2 increasing linearly with decreasing CTmax. There were, however, no discernible relationships between the measured cardiorespiratory properties and hypoxia or hyperthermia tolerances. The strong relationship between hypoxia and hyperthermia tolerances in this study may be related to the ability of the cardiorespiratory system to provide oxygen to respiring tissues under thermal stress, and thus provides some support for the OCLTT concept in this species, at least at the level of the entire organism.

Acute hyperthermia and hypoxia tolerance of two improved strains of nile tilapia (Oreochromis niloticus).

Tilapia production in Ghana has been hit with episodes of stress and pathogen-induced mass fish kills which have anecdotally been linked to the culture of illegally imported Genetically Improved Farmed Tilapia (GIFT) strains of Nile tilapia, Oreochromis niloticus. This study was thus set up to comprehensively assess the stress tolerance of the GIFT strain and a native strain of Nile tilapia (the Akosombo strain) following exposures to hyperthermic and hypoxic stressors. In a series of experiments, oxygen consumption (MO2), aquatic surface respiration (ASR), thermal limits and hypoxia tolerance were assessed. The effects of these stressors on haematological parameters were also assessed. The GIFT strain was less tolerant of hypoxia and performed ASR at higher O2 levels than the Akosombo strain. Under progressive hypoxia, the GIFT strain exhibited higher gill ventilations frequencies (fV) than the Akosombo strain. The thermal tolerance trial indicated that the Akosombo strain of O. niloticus has higher thermotolerance than the GIFT strain and this was reflective in the higher LT50 (45.1℃) and LTmax (48℃), compared to LT50 and LTmax of 41.5℃ and 46℃ respectively. These results imply that it is crucial to consider how the GIFT strain performs under various environmental conditions and changes during culture. Particularly, raising the GIFT strain of Nile tilapia in earthen ponds rich in phytoplankton and subject to protracted episodes of extreme hypoxia may have a detrimental physiological impact on its growth and welfare.

Air-breathing behavior in Heterotis niloticus fingerlings: Response to changes in oxygen, temperature, and exercise regimes.

Air-breathing in fish is believed to have arisen as an adaptation to aquatic hypoxia. Although air-breathing has been widely studied in numerous fish species, little is known about the obligate air-breathing African bonytongue, Heterotis niloticus. We evaluated if abiotic factors and physical activity affect air-breathing patterns in fingerlings. The air-breathing frequency (fAB ) and behavioral responses of H. niloticus fingerlings were assessed in response to environmental oxygen, temperature, and exhaustion and activity in a series of experiments. The air-breathing behavior of H. niloticus fingerlings under optimum water conditions was characterized by swift excursions lasting less than 1 s to the air-water interface to gulp air. The intervals between air-breaths were highly variable, ranging from 3 to 259 s. Body size only slightly affected fAB , while hypoxia, hyperthermia, and exercise stress significantly increased fAB . Progressive hypoxia from 17.69 to 2.17 kPa caused a ~2.5-fold increase in fAB . Increasing temperatures to 27 and 32°C, from a baseline temperature of 22°C, significantly increased fAB from 0.4 ± 0.2 to 1.3 ± 0.5 and 1.6 ± 0.4 breaths min-1 , respectively. Lastly, following exhaustive exercise, fAB increased up to 3-fold. These observations suggest that H. niloticus fingerlings are very reliant on aerial oxygen, and their air-breathing behavior is sensitive to environmental changes and activity levels.

Influence of the chemical content of sawdust on the levels of important macronutrients and ash composition in Pearl oyster mushroom (Pleurotus ostreatus).

Influence of chemical composition of sawdust on the nutritional profile of oyster mushrooms (Pleurotus ostreatus) has yet to receive significant research attention. This information will help mushroom growers to select specific sawdust for the production of mushroom with desired dietary preferences. This study assessed the influence of the chemical composition of sawdust on the macronutrients and ash content of the pearl oyster mushrooms. The American Standard for Testing Materials and other widely accepted protocols were used to determine the C-N ratio, pH, lignin, hemicellulose and cellulose contents of mixed sawdust from tropical wood species. The study evaluated the fat, crude fibre, crude protein, carbohydrate, and ash content of the oyster mushroom cultivated on the sawdust. Cellulose constituted the largest component of the sawdust (47.82%), followed by lignin (33.29%). The yield of the mushroom (on 0.05 kg of sawdust) ranged from 490.1 to 540.9 g (biological efficiency: 44-50%); the average carbohydrates constituent in the mushroom was 56.28%. pH of the sawdust influenced the crude protein, carbohydrate, fat and ash content of oyster mushrooms (p<0.05) most significantly. The hemicelluloses also had a significant effect (p<0.05) on the mushroom's minerals, fat and crude fiber content. The study revealed that the mushroom producers would likely obtain high protein content using sawdust with low pH (slightly acidic to slightly basic) in the oyster mushroom. Mushrooms grown on substrates, rich in hemicelluloses, had low fat and high crude fiber content.

Growth, metabolism and respiration in Nile tilapia (Oreochromis niloticus) exposed to chronic or periodic hypoxia.

Tropical earthen ponds for extensive aquaculture are characterised by daily fluctuations in the availability of dissolved oxygen in the water. Primary production during the daytime ensures excess oxygen availability with oxygen partial pressures (pO2) exceeding 220 mmHg, while nocturnal respiration of fish, plankton and bacteria leads to nightly episodes of severe hypoxia (pO2 < 20 mmHg), often persisting for several hours. To investigate how oxygen availability affects feeding, growth, digestive performance, metabolism and behaviour in Nile tilapia (Oreochromis niloticus), a series of experiments were conducted under different oxygen regimes. To assess growth performance, triplicate groups of fish were held either under constant normoxia (pO2 17.4 ± 0.4 kPa), constant hypoxia (pO2 8.1 ± 0.6 kPa), or diel-cycling between normoxia (pO2 17.1 ± 0.6 kPa from 6 a.m. to 11 p.m.) and severe nocturnal hypoxia (0.4 ± 1.0 kPa from 11 p.m. to 6 a.m.). Chronic hypoxia led to significant affected feed intake and FCR, compared to the normoxic group, whereas nocturnal hypoxia was associated with a compensatory increase in appetite later in the day. Overall, this resulted in a significant increased feed intake compared to the normoxic group. Interestingly, exposure of fish to 6-h nocturnal hypoxia (diel-cycling hypoxia) for 9 weeks resulted in the best growth performance indicators among the treatment groups. Respirometry showed that tilapia respond to nocturnal hypoxia by metabolic depression, allowing them to return to normoxia with a modest oxygen debt. Behavioural observations revealed that aquatic surface respiration is employed when pO2 approaches 2.1 kPa.