The composition and nutritional status of the diet of Oreochromis niloticus in Lake Chamo, Ethiopia

The stomach contents of Oreochromis niloticus from L. Chamo were microscopically examined and chemically analysed in order to study the relationship between food quality, growth and condition. More than 20 genera of phytoplankton plus Copepoda, Rotifera and nauplii were present in the stomach contents. The diatom, Melosira , was the most dominant alga in the diet, contributing about 30% of the organic matter. The composition of the food in terms of organic nutrients was dominated by carbohydrate (20.3% dry weight). Both protein and fat made low contributions to the diet; 10% and 5.8% dry weight, respectively. The overall performance of the fish in assimilating food was high: 43.3% for ash free dry weight, 61.2% for protein, 57.2% for lipid, 29% for carbohydrate and 55.7% for energy. The digestible protein-energy ratio was 15.7 mg kJ⁻¹, suggesting that the food was qualitatively good both for fast growth and for maintaining good condition. It was concluded that the small amount of good quality food ingested, the low fish population density in the lake and the high temperature of the water are the major factors responsible for making O. niloticus a fast-growing fish in Lake Chamo.     

Sources of fish in the ephemeral western iishana region of the Cuvelai–Etosha Basin in Angola and Namibia

The triangle between the Kavango and Kunene rivers is drained by the Cuvelai, an ephemeral and deltaic drainage system covering more than 100 000 km². In good rainfall years, the area becomes populated by fish communities dominated by five species migrating southward towards the endorheic Etosha Pan, the basin’s terminal sump. When water dries up, fish subsequently die-off and their sudden appearance in rainy years has captivated scientists for decades. The current study was prompted by hitherto untapped indigenous knowledge through narratives of opportunistic fish harvesting of migrating fish at temporary connections between the Kunene River and the Cuvelai- Etosha Basin. A reconnaissance fish survey in 2017 was complemented by digital satellite images and elevation data analyses. Results support the presence of at least three major ephemeral fish migration routes. The dominant fish genera migrating upstream in Kunene tributaries comprise Enteromius, Oreochromis and Clarias, all eurytopic and known to undertake upstream, lateral and downstream migrations on floodplains. Although other notable fish refugia in the Cuvelai–Etosha Basin are yet to be identified, there is a necessity for the protection and management of these migration routes in tandem with studies on the nature and extent of this inter-basin fish migration under climate change and variability.     

A place preference test in the fish Nile tilapia

The Nile tilapia fish (Oreochromis niloticus) has a high potential to be used as a model in neuroscience studies. In the present study, the preference of the Nile tilapia between a gravel-enriched (GEE), a shelter-enriched (SEE) or a non-enriched (NEE) environment was determined, for developing a place preference model. Nile tilapia had an initial preference for GEE, but after 1 day of observation, the fish stabilized their frequency of visits among compartments. Hence, any stimulus motivating tilapia increase in compartment visiting indicates a positively reinforcing effect. This feature is very useful for the development of new behavioural paradigms for fish in tests using environmental discrimination, such as the conditioning place preference test.     

The effects of temperature acclimation on the oxygen consumption and enzyme activity of red and white muscle fibres isolated from the tropical freshwater fish Oreochromis niloticus

The standard oxygen consumption rate and the activities of muscle citrate synthase, creatine phosphokinase and lactate dehydrogenase in the tropical fish Oreochromis niloticus acclimated to either 20.5 ± 0.3° C or 26.5 ± 0 ± 5 ± C for at least 3 months were investigated. The standard oxygen consumption rate of individual fish from the two acclimation temperatures was determined at 20, 25 and 30 ± C. At all experimental temperatures, the standard oxygen consumption rate of fish acclimated to 20.5 ± 0.3° C was significantly higher than that of fish kept at 26.5 ± 0.5 ± C. In both groups smaller individuals had a higher oxygen consumption rate than large ones.
Analyses of the activity levels of citrate synthase (CS), creatine phosphokinase (CPK) and lactate dehydrogenase (LDH) in both red and white muscles isolated from fish kept under the two temperature regimes were performed at 26 ± C. The activity of CS in both red and white muscles isolated from the 20.5 ± 0.3° C acclimated fish was significantly higher than that of muscles isolated from the 26.5 ± 0.5 ± C acclimation group. Similarly, the CPK activity in white muscles isolated from fish acclimated to 20.5 ± 0.3 ± C was higher than that of muscles obtained from the 26.5 ± 0.5 ± C acclimation group. However, the CPK activity in red muscles isolated from the two fish groups was not significantly different. The opposite results were obtained for LDH activity. For example, the LDH activity of white muscles isolated from fish acclimated to 26.5 ± 0.5 ± C was significantly higher than that of the same muscles but from the 20.5 ± 0.3 ± C acclimated fish. No differences were observed in the LDH activity of red muscles isolated from the two fish groups.