Stomach pH,feeding rhythm and ingestion rate in Oreochromis niloticus L. (Pisces: Cichlidae) in Lake Awasa,Ethiopia

Fish were caught in February and October, 1985 with gill and trawl nets at different time intervals. The length and weight of the fish, the stomach pH and the weight of the stomach contents were determined. The feeding pattern of O. niloticus was observed to have a diel rhythm. The stomach became more acidic towards the afternoon and most of the evening. The pH of the stomach had a significant regression on stomach fullness. The feeding rate of O. niloticus in Lake Awasa was estimated from daily feeding periodicity and rate of gastric evacuation. The fish ingests 11.5% of their wet body weight daily at an average water temperature of 21.5 °C. It is believed that the fish ingests and assimilates adequate supply of energy (9.1 KJ/day) for growth, maintenance and other energy requiring activities of the fish.     

The food habits of an herbivorous fish (Oreochromis niloticus Linn.) in Lake Awasa,Ethiopia

The natural food ofOreochromis niloticus in Lake Awasa was studied from the stomach contents of 10 fish measuring 18–32 cm (standard length) monthly from August 1984 to April 1986.Chroococcus, Oscillatoria andBotryococcus were found to be important food items in the diet. Animal foods were observed on rare occasions and these were mainly Rotifera. Blue-green algae as a group contribute about 28.1% of the ash free dry weight (AFDW) whereas diatoms and green algae (excludingBotryococcus) make 2.31% and 2.39% of AFDW, respectively.Botryococcus, detritus and other algae comprised 67.2% of AFDW and it was suspected that more than half of this was contributed byBotryococcus. Blue-green algae, which were especially abundant in the diet during the dry season, November–February, are nutritionally most important. Seasonal variation in algal species composition could influence the quality of food ingested by the fish.     

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.     

Changes in condition and maturation of the Oreochromis niloticus L. population of Ferguson's Gulf, Lake Turkana, Kenya

The condition and maturation of the Oreochromis niloticus population of Ferguson's Gulf, Lake Turkana was assessed in light of the declining lake level and compared with similar data taken at the high lake level period 10 years ago. Results show a dramatically lower length/weight relationship and an earlier length at maturity for the 1980s population. A defined breeding period was seen for the 1980s population, unlike in the 1970s, perhaps in response to stressful conditions. The reduction in area of the Ferguson's Gulf breeding/nursery grounds is suggested as the major factor contributing to stress in the O. niloticus population, and it is suggested that measures be taken to protect what is left of these breeding/nursery areas.     

Species composition,distribution and seasonal dynamics of Rotifera in a Rift Valley lake in Ethiopia (Lake Awasa)

The species composition of rotifers in Lake Awasa was studied and 40 species recorded. Seven species appeared in large numbers in the plankton seasonally. Brachionus and Keratella species made up more than 50% of the rotifer community by numbers during the observation period (1983–1987). The standing stock numbers of rotifer species are low with a maximum of 50 individuals L^–1, and some possible reasons for this observation are discussed. Most rotifer species are distributed randomly in the lake and show 3-fold fluctuations between consecutive days.The seasonal dynamics of most rotifer species are correlated with mixing periods in the lake, and the amplitude of seasonal fluctuation may be as high as 50-fold. Clear-cut seasonal succession of rotifer species was observed during the study period, but no consistent seasonal pattern for individual species was obvious. Also, observations indicate that rotifer biomass is partly sustained by availability of carbon through the bacterial pathway and that competitive exclusion for food by nauplii and ciliates probably keeps rotifer abundance low.     

A study on an herbivorous fish, Oreochromis niloticus L., diet and its quality in two Ethiopian Rift Valley lakes, Awasa and Zwai

The natural food of Oreochromis niloticus from two Ethiopian Rift Valley lakes, Awasa and Zwai, was studied from the stomach contents. The food was analyzed to determine its chemical composition. The major consumed nutrient by the Awasa population was carbohydrate whereas in the Zwai population it was lipid. Microscopic identification revealed that Microcystis sp. and Botryococcus braunii are the dominant algae present in the food of the fishes from Lakes Zwai and Awasa, respectively. Assimilation efficiency of total organic matter is somewhat low, 28.3% for Awasa and 14.7% for Zwai populations. Both populations assimilated protein to a greater extent than other nutrients. In both lakes the quality of the food, expressed as a ratio of digestible protein to digestible energy, is adequate (>4 mg kJ ⁻¹) for growth, but the Awasa population appears to have higher growth and better condition.     

Shifts in the food of Nile tilapia, Oreochromis niloticus (L.) in Lake Victoria, Kenya

Studies of the food of introduced Nile tilapia, Oreochromis niloticus (L.) with respect to size, habitat and season were conducted between November 1998 and October 2000 in Kenyan waters of Lake Victoria. Stomach contents of 1980 specimens collected by demersal trawl and seining were analysed. Nile tilapia originally known to be herbivorous, feeding mostly on algae has diversified its diet to include insects, fish, algae and plant materials. The major diet of fish <5 cm total length was zooplankton whereas bigger fish included a wider range of food items in their diet. There was spatial variation in diet with insects and algae dominating in the gulf and open water habitats respectively. There was no seasonal variation in the food items ingested and diel feeding regime indicated that O. niloticus is a diurnal feeder. The shift in diet could be due to ecological and environmental changes in Lake Victoria, which have been associated with changes in composition and diversity of fish and invertebrate fauna, emergence and dominance of different flora including water hyacinth Eichhornia crassipes (Mart.) Solms‐Laub., and algae communities. The feeding habit of O. niloticus is discussed in the context of changes occurring in the lake.     

The benthic and weed-bed faunas of Lake Awasa (Rift Valley,Ethiopia)

The community structures of the benthic and weed-bed faunas of Lake Awasa, Ethiopia Rift Valley, are described and related to the various types of substratum present. L. Awasa is non-saline (conductivity 860 µS cm^-1), and lies in a shallow basin with maximum depths of 21–22 m. It supports an important commercial fishery mainly based on the tilapia Oreochromis niloticus.The large weed beds of the lake extend out to the 4 m. isobath in most places; they consist mainly of the grass Paspalidium geminatum. Below the 6–8 m isobaths the bottom is covered with a deep layer of flocculant mud, with organic matter constituting about one third of the dry weight. No benthic animals are able to colonize this mud. In shallower regions the bottom consists of hard sand covered with a thin layer of organic detritus, except for a narrow region around the 6 m isobath where the sand is covered with a layer of ostracod shells up to 25 cm deep. All this firmer bottom is colonized by a benthic community consisting mainly of cyclopoids, ostracods, Limnodrilus and chironomids; there are no prosobranch or bivalve molluscs. The weed-bed community consists mainly of Cladocera, ostracods, Ephemeroptera, Heteroptera, chironomids and pulmonate molluscs. These communities are compared with those of other Afro-tropical lakes, and are shown to have much in common with that of Lake Chad (Dejoux, 1983), specially the chironomids (Dejoux, 1976) but L. Awasa has fewer species, the most noticeable absentees being the decapod Crustacea.     

The breeding season of tilapia, Oreochromis niloticus L. in Lake Awassa (Ethiopian rift valley)

The breeding season of Oreochromis niloticus in Lake Awassa was studied from fish caught monthly from December 1987 to November 1988 in relation to a number of environmental factors. Fish at various stages of gonad development and spawning were caught in almost all months, but breeding fish were more frequent twice a year: main peak occurred during January–March and a secondary one occurred during July–September. Gonadosomatic index (GSI) values peaked twice in the study year, a major peak being during January–March and a less pronounced one during July–September. Thus, it was concluded that O. niloticus in Lake Awassa breeds intensively in the former and less intensively during the latter period, and that some breeding occurs at any time of the year. Intensive breeding activity during January–March appeared associated with increase in solar radiation and sunshine hours d^–1 whereas that during July–September appeared associated with heavy rainfall. However, intensive breeding in both periods coincided with increase in phytoplankton biomass. Previously, phytoplankton biomass in this lake is shown to increase following increase in nutrients due to mixing- and rainfall-associated changes in the hydrography and hydrology of the lake. Thus, although further studies are required, increase in phytoplankton may be one of the environmental cues to stimulate spawning in O. niloticus in Lake Awassa whereas other factors such as sunshine and rainfall may have indirect roles through their effects on the hydrology and hydrography of the lake ultimately resulting in changes in phytoplankton biomass.     

Comparative feeding efficiency on indigenous and introduced phytoplanktivores from Lake Victoria: Experimental studies on Oreochromis esculentus and Oreochromis niloticus

The ngege, Oreochromis esculentus Graham (Perciformes;Cichlidae), was a numerically important phytoplanktivore inLakeVictoria, East Africa prior to its extirpation and subsequentreplacement following the introduction of its sister species,O. niloticus Boulenger (Cichlidae). The disappearance of O. esculentus has been attributed to predation, competition,andintrogression by introduced species and to overfishing. Anoverlooked factor is a shift in algal community dominance inLakeVictoria, from large filamentous diatoms (a preferred fooditem ofO. esculentus) to smaller colonial bluegreen and greenalgae.We performed a series of experiments to test the hypothesisthatfeeding performance of the two species differs sufficientlythatalgal shifts could have contributed to the speciesreplacement.Adults of the two tilapiine species were fed individually on amonospecific culture of the small green alga Scenedesmusacuminatus, which grows in a range of colony sizes. Clearingrateswere compared at 4 algal food densities and at the 4 colonysizes.The results indicate that O. niloticus has the abilitytofeed on either smaller or less bulky particles than O. esculentus such as prevail in the postdisturbance LakeVictoria. O. niloticus is also a more efficient filterfeederoverall, with a differential that is inversely proportional toparticle size. Additional data would be required to show thatfeeding performance and limnological shifts were primary tothereplacement of O. esculentus by O. niloticus.Researchers should however, be cautious about invokingcompetitionas the proximal cause of exotic impacts when simpler,extrinsic mechanisms are equally plausible.     

Seasonal variation in phytoplankton primary production in relation to light and nutrients in Lake Awasa,Ethiopia

The biomass and primary production of phytoplankton in Lake Awasa, Ethiopia was measured over a 14 month period, November 1983 to March 1985. The lake had a mean phytoplankton biomass of 34 mg chl a m^–3 (n = 14). The seasonal variation in phytoplankton biomass of the euphotic zone (mg chl a m^–2 h^–1) was muted with a CV (standard deviation/mean) of 31%. The vertical distribution of photosynthetic activity was of a typical pattern for phytoplankton with light inhibition on all but overcast days. The maximum specific rates of photosynthesis or photosynthetic capacity (Ø_max) for the lake approached 19 mg O₂ (mg chl a)^–1 h^–1, with high values during periods of low phytoplankton biomass. Areal rates of photosynthesis ranged between 0.30 to 0.73 g O₂ m^–2 h^–1 and 3.3 to 7.8 g O₂ m^–2 d^–1. The efficiency of utilisation of PhAR incident on the lake surface varied from 2.4 to 4.1 mmol E^–1 with the highest efficiency observed corresponding to the lowest surface radiation. Calculated on a caloric basis, the efficiency ranged between 1.7 and 2.9%. The temporal pattern of primary production by phytoplankton showed limited variability (CV = 21 %).     

Recruitment control and production of market-size Oreochromis niloticus with the predator Lates niloticus L. in the Sudan

Experiments were conducted in earth ponds (242–1260m²) to evaluate the Nile perch (Lates niloticus L.) as a predator for recruitment control and the production of marketable Oreochromis niloticus L. in the Sudan. Supply of the predatory fish was maintained by induced spawning of L. niloticus brooders by raising the water level in a 1–0 ha earth pond. Lates niloticus fingerlings (7–5 cm total length) were then stocked with same size O. niloticus at the ratios 1:5, 1:10 and 1:15, Lates:Oreochromis , respectively. Over a period of 7 months Lates niloticus reduced young 50g) Oreochromis population and enhanced the production of preferred-size ( 200 g) Oreochromis. The ratio 1:5 Lates:Oreochromis was established to be the most desirable for Lates -with a total production of 0.2428 kg/m² of O. niloticus with 55.7% (by weight) at the target size averaging 287.5g. The ratios 1:10 and 1:15 produced 0.2106 and 0.2153 kg m² of O. niloticus , with 49.4% and 16.9% averaging 235.0 and 210.0 g, respectively.     

Mortality rates of Lates niloticus,L. in Lake Nasser

Landings of Lates niloticus from Lake Nasser belonged to age groups I–VII, with II–III predominent. Mortality rate, as computed by two different methods, ranges around 75%. Turbid flood water did not reach Lake Nasser in 1972, and this fact may be responsible for the decline in the commercial catch from 1972 to 1973.     

Seasonality and abundance of some dominant crustacean zooplankton in Lake Awasa,a tropical rift valley lake in Ethiopia

The zooplankton of a Rift Valley lake in Ethiopia, Awasa, was sampled at 3 stations for 2 years (1986 and 1987) concurrently with various meteorological and limnological measurements. The spatial and temporal variation in abundance of some numerically dominant crustaceans, Mesocyclops aequatorialis similis (Copepoda), Thermocyclops consimilis (Copepoda) and Diaphanosoma excisum (Cladocera) is discussed. Temporal (months, sampling dates) rather than spatial (station) variability accounts for more than 50% of the total variance in zooplankton abundance but horizontal patchiness exists during periods of high zooplankton density. Sampling errors were generally low, except for counts of cyclopoid nauplii (subsampling) and Diaphanosoma (inter-replicate variance). Zooplankton showed distinct seasonality associated with the mixing cycle of the lake. Total numbers increased to more than 200 000 m⁻³ during the unstratified period (July to September). Low numbers were evident during stratification (February to May) when zooplankton numbers did not exceed 15 000 m⁻³. Individual zooplankton species and age classes showed variable seasonal amplitudes, ranging from 6.4 (nauplius 3) to 44.8 (copepodite 3 of Mesocyclops). We discuss some possible causes for zooplankton seasonality in Lake Awasa, and also review zooplankton seasonal cycles in other tropical lakes, especially African ones.     

Some biological aspects and life history strategies of Nile tilapia Oreochromis niloticus (L.) in Lake Victoria, Kenya

The life history characteristics of introduced Nile tilapia (Oreochromis niloticus L.) in Lake Victoria, including, sex ratio, fecundity, reproduction, weight‐length relationship and body condition were studied and compared with those of other populations. Samples were collected by trawling and seining in the Kenyan sector of Lake Victoria between June 1998 and December 2000. Males predominated over females (sex ratio 1.42 : 1 : 00). O. niloticus spawned throughout the year but with a peak between December and June. Length at first maturity was (mean ± SD) 30.81 ± 0.09 for females and 34.5 ± 6 0.48 for males. There was little seasonal variation in relative condition, which ranged from 0.92 to 1.05 in males and 0.94 to 1.07 in females. Gonadosomatic index (GSI) was low during the postspawning period (July to October) and high during the protracted breeding period (December and June). Fecundity ranged from 905 to 7619 oocytes for fish of 28 to 51 cm total length (TL) respectively. The relationships between fecundity (F) and total length (L), weight (W) and ovary weight (OW) were: F = 8.159L^1.53, F = 96.269W^0.4504, F = 1806 + 39.4OW. The slope b of the weight‐length relationship was 3.08–3.32 for males and 3.07–3.22 for females. Growth was allometric in both cases and was significantly different from the expected value of 3. The life history strategy of O. niloticus is discussed in context of environmental changes occurring in the lake.     

Triploidy induction in Nile tilapia,Oreochromis niloticus L. using pressure,heat and cold shocks

Summary The results of a study aimed at the identification of treatment optima for triploidy induction in recently fertilised Oreochromis niloticus L. eggs by altering the intensity, duration and timing of application of pressure, heat and cold shocks are reported. Preliminary, but not directly comparable, trials suggested the following treatments to be close to the individual agent optima. Pressure: 8,000 psi 2-min duration applied 9 min after fertilisation (a.f.); heat: 41 °C, 3.5-min duration applied 5 min a.f., cold: 9°C, 30-min duration applied 7 min a.f. In a directly comparable trial in which the eggs of eight different females were separately exposed to the optimum shocks listed above, individual triploid yields were more variable following cold shocks and mean triploid yields were, therefore, higher following pressure and heat shock. These and other results obtained are presented and the light they shed on the timing of the second meiotic division in this species is discussed.     

Population dynamics and life-history styles of Nile tilapia,Oreochromis niloticus,in Ferguson's Gulf,Lake Turkana,Kenya

Synopsis Three observed dynamic aspects of the Nile tilapia population around Ferguson's Gulf at Lake Turkana, Kenya are evaluated and discussed: the seasonality in catch rates, the enormous inter-annual abundance variations, and the large changes in median size at first maturity. A clear understanding of the regulating mechanisms behind these features has never been achieved, although seasonal changes in the hydrology of shallow sheltered refuges seems to play an important role. This paper suggests a further holistic approach taking the impacts and interrelationships of both the primary productivity and the various predators into account. A synthesizing ecological hypothesis is elaborated, which concludes that most observations on the tilapia dynamics can be explained from changes in the oxygen concentrations and size-specific mortality pressures. Variations in these two proximate factors can ultimately be explained by the floodplain-type fluctuations in the Ferguson's Gulf environment.     

Growth parameters and total mortality in Oreochromis niloticus (Linnaeus) from Nyanza Gulf,Lake Victoria

Length-frequency data collected from fish landings in the Kenya waters of Lake Victoria were used to estimate the growth parameters, total mortality rate and growth performance index in Oreochromis niloticus. The asymptotic length, (L _∞) and the ratio of the total mortality rate (Z) to the growth constant (K), were estimated to be 64.6 cm and 3.219 respectively. K was 0.254 y^-1, Z was 0.818 y^-1 and the growth performance index θ′ = Log₁₀ K + 2 log₁₀ L∞ = 3.025, which is rather high as compared to other tilapia populations in natural waters.