Reproductive biology of Tilapia zillii (Gervais) in Lake Naivasha,Kenya

Synopsis Tilapia zillii breeds all year round in the equatorial Lake Naivasha with slightly high breeding intensity during wet months (March, May and July–August). A sex ratio of 1 : 1.28 was found between males and females. The smallest mature male and female were 9.0 and 11.0 cm respectively. The fecundity ranged from 2990 to 12344 eggs, with a mean of 6606 eggs. Fecundity showed a curvilinear relationship with total length (log F = 1.7034 + 1.6370 log L) and a linear relationship with body weight (F = 2197 + 28.76 W)     

Changes in fish species composition in Lake Naivasha,Kenya

The fish species composition in Lake Naivasha has been modified during recent years. Out of seven introduced and one indigenous species reported in 1962, only three introduced ones are found today. The elimination of species has been brought about by predation and changing ecological conditions. The most common fish species today is Sarotherodon leucosticuus. The hybridization found by previous workers between S. leucostictus x S. spilurus niger has stopped, as S. s. niger is no longer found in the lake. Most of the hybrid characters have been lost by the few surviving hybrids due to backcrossing between hybrids and between hybrids and S. leucostictus. The hybrids now mostly resemble S. leucostictus.     

Reproductive biology, length-weight relationship and relative condition of Tilapia leucosticta (Trewavas) in Lake Naivasha, Kenya

In Lake Naivasha Tilapia leucosticta (Trewavas) spawns throughout the year. Males attain maturity at 180 mm and females at 160 mm total length. Males predominate over females in the catches (sex ratio 1-00 : 0-51). Fecundity ranged from 320 to 1328 eggs with an average of 751. The relationship between fecundity (F) and total length (L), body weight (W) and gonad weight (GW) is:
The length-weight regression coefficient is near to the cube. There is little seasonal fluctuation in relative condition of the fishes.     

The status and future of the Lake Naivasha fishery,Kenya

I describe a laboratory system for investigating the role of light as a proximate cue for diel changes in locomotor activity and vertical location on the substrate of stream macro-invertebrates. The system consisted of computer-controlled halogen lamps positioned over a laboratory stream in which video-recordings were made of Stenonema modestum mayfly nymphs located on the undersides of unglazed tile substrates. Locomotor activity of study organisms in response to light changes were quantified during computer-programmed and reproducible light/dark (LD) cycles. The system provided the flexibility to simulate a variety of light environments so that the separate influences of light intensity and light change on diel activities of individuals and populations could be examined, which is difficult under natural light conditions. As a group, nymphs responded similarly to simulated twilight (light decrease from 7.9 × 10² to 6.9 × 10^–2 W cm^–2 at a constant –1.9 × 10^–3 s^–1 rate of relative light change) and to natural twilight, suggesting that proposed mechanisms of light control of diel activities in nature can be adequately tested in the simulated environment. However, locomotor activity and vertical movements among individual mayflies were highly variable under controlled conditions, suggesting that physiological differences influence their responses to environmental conditions.     

Diel vertical distribution of zooplankton in Lake Naivasha,Kenya

Diurnal and diel vertical distribution of limnetic zooplankton species in relation to temperature and dissolved oxygen profiles was examined at a central station in Lake Naivasha. During calm days thermal stratification developed gradually from late morning to reach maximum formation at mid-day. Dissolved oxygen concentrations showed similar vertical profiles to temperature. These stratifications were, however, short lived and were broken up in late afternoons by the wind induced poly-holomictic nature of the lake. During the day most zooplankters aggregate at the top 3–4 metre zone of the water column coincident with maximum photosynthetic activity. The pattern of diel vertical distribution of zooplankton in Lake Naivasha is undefinedly even. The absence of significant diel changes in the distribution of the limnetic zooplankton may be related to the absence of permanent physico-chemical boundaries and lack of predation pressure in the open water.     

Toxicity of silver nanoparticles on the brain of Oreochromis niloticus and Tilapia zillii

BackgroundSilver nanoparticles (Ag-NPs) are widely used nowadays in a variety of commercial applications including medical, health care, textiles and household supplies.ObjectivesThe current study was designed to determine the median lethal dose (LC₅₀) of Ag-NPs on Oreochromis niloticus and Tilapia zillii.MethodsAcute and sub-acute toxicity study of the Ag-NPs on brain tissues was carried out using different concentrations of the NPs at 2 mg L and 4 mg L. These concentrations were dispersed in deionized water with the exception of the control groups in the experiments. Biochemical and molecular analysis were conducted on tissue homogenates in order to evaluate the potential effects of NPs on the antioxidant system.ResultsThe Ag-NP acute toxicity (96 h LC₅₀) values of 19.5 ± 2 and 20 ± 2.4 mg/L were reported for O. niloticus and T. zillii respectively. Fish exposed to 2 mg/L Ag-NPs did not show any significant change in the levels of reduced glutathione (GSH), total glutathione (tGSH) levels, superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR), glutathione peroxidase (GPx) and glutathione-S-transferase (GST) activity or genes expression and malondialdehyde (MDA) level. In contrary, a dose of 4 mg/L showed a significant reduction in the levels all the above-mentioned parameters except in MDA level where it was significantly induced.ConclusionResults indicate that exposure of O. niloticus and T. zillii to Ag-NPs (4 mg/L) has deleterious effects on brain antioxidant system, whereas a dose of 2 mg/L has no effects.     

Nitrogenase activity in the papyrus swamps of Lake Naivasha,Kenya

Nitrogenase activity (acetylene reduction activity) was found to occur universally in the Cyperus papyrus swamp in Lake Naivasha. Low rates of acetylene reduction activity (0.9–104.9 nmol C₂H₄ g d.wt. roots^-1 h^-1) were associated with excised roots of C. papyrus but higher rates of activity (89.0–280.4 nmol C₂H₄ g d.wt. roots^-1 h^-1) were associated with intact root systems of the plant. It was estimated that nitrogen fixation associated with young roots alone could supply about 26% of the nitrogen requirements of growing papyrus plants. Acetylene reduction activity in the lake bottom sediments was generally low and associated with adjacent papyrus stands. Plate counts of putative aerobic and facultatively anaerobic N₂-fixing bacteria associated with papyrus roots showed the presence of high numbers of diazotrophs (5.4 × 10⁶ CFU g d.wt. roots^-1). Fewer numbers of N₂-fixing bacteria were detected in the sediments (1.9 × 10³-3.2 × 10⁴ CFU g d.wt. sediment^-1).     

Nutrient control of phytoplankton production in Lake Naivasha,Kenya

Lake Naivasha, a shallow tropical lake in Kenya's Rift Valley, has an unstable water column and is moderately eutrophic. Nutrient (bottom-up) control of primary production is more important than grazing (top-down) control. Experimental nutrient enrichment was used to investigate bottom-up control in more detail. Minor nutrients were not found to be limiting, whilst nitrogen was more limiting than phosphorus with an algal preference for ammonium over nitrate. Sediments form a phosphorus sink but there is hypolimnetic release from the one area showing regular temporary stratification. This indicates that the rate of primary production in the water column could double if conditions change to allow lake-wide nutrient release from sediments. Both external and recycled nutrient regeneration are important.     

Recent introgressive hybridization revealed by exclusive mtDNA transfer from Oreochromis leucostictus (Trewavas, 1933) to Oreochromis niloticus (Linnaeus, 1758) in Lake Baringo, Kenya

Nuclear DNA and mtDNA polymorphisms were surveyed in various species of East African Oreochromis . In Lake Baringo, where only Oreochromis niloticus baringoensis is present, alien mtDNA haplotypes were observed, apparently the result of introgressive hybridization with Oreochromis leucostictus . This introgression is not accompanied by any substantial or recorded transfer of nuclear genes into O. n. baringoensis .     

Evaluation of Hydrodictyon reticulatum as protein source in feeds for Oreochromis (Tilapia) niloticus and Tilapia zillii

The green filamentous alga Hydrodictyon reticulatum was evaluated as a protein source in fish-meal substituted diets for Oreochromis niloticus and Tilapia zillii fingerlings. The fingerlings were fed in duplicate groups each of six different diets for 50 days. Five of the diets contained 30% crude protein supplied by varying proportions of fish meal and H. reticulatum meal. The five diets were formulated to supply fishmeal protein: H. reticulatum meal protein ratios of 30:0 (diet 1), 15:5 (diet 2), 20:10 (diet 3); 15:15 (diet 4), 10:20 (diet 5) respectively. A sixth diet containing only 25% crude protein supplied entirely by H. reticulatum meal was also fed. The best growth and protein utilization was obtained at lower levels of H. reticulatum substitution for both species of fish. Carcass analysis revealed a decrease in lipid contents of the fishes with increasing levels of the alga in the diet.     

Assessing the potential health risk of cyanobacteria and cyanotoxins in Lake Naivasha,Kenya

Hydrobiologia - This study discerned the causes of cyanobacteria blooms in Lake Naivasha (Kenya). We hypothesized that phytoplankton and cyanobacteria biomass respond to hydrologic cycles, peaking...     

Organochlorine residues in fish from Lake Victoria, Kenya

Eighty-two samples of either nile perch fish fat or muscle were collected from the Kenyan region of Lake Victoria for detection of organochlorine residues. Nine organochlorine residues were detected in the following percentages: α-BHCIHCB-40%; P-BHC/HCB-40%; y-BHC/HCB/lindane-4%; aldrin-9%; dieldrin-1%; p, p'-DDE-73%; p, p'-DDD-9%; o, p'-DDT-170; and p, p'-DDT-11%. All levels of organochlorine residues were below the Extraneous Residue Limit (ERL), apart from just one sample of fish fat which had 4.51 ppm of sum DDT above ERL. There was a higher level of organochlorines from Mbita than from Luanda fishing point.

Résumé

Eighty-two échantillons de graisse ou de muscle de perche du Nil ont été récoltés dans la région kényane du lac Victoria pour rechercher des résidus organochlorés. On a détecté 9 résidus organochlorés, avec les pourcentages suivants: α-BHC/HCB-40%; β-BHC/HCB-40%;γ-BHC/HCB/lindane-4%;aldrine-9%; dieldrine-1%; p, p'-DDE-73%; p, p'-DDD-9%; o, p'-DDT-1%; and p, p'-DDT-11%. Tous les taux de résidus organochlorés ktaient infkrieurs a la 'Extraneous Residue Limit' (ERL) sauf pour un seul échantillon de graisse de poisson qui contenait 4,51 ppm de DDT total au-dessus de 1'ERL. I1 y avait un taux plus élevé d'organochlorés a Mbita qu'au point de pêche de Luanda.     

Gonadal and reproductive patterns in Tilapia leucosticta (Teleostei: Cichlidae) in an equatorial lake, Lake Naivasha (Kenya)

The cichlid fish T. leucosticta (Trewavas) was studied by fortnightly seining an area of an equatorial lake (Lake Naivasha) in Kenya to study their reproductive and gonadal patterns. The study lasted 16 months during which period temperature, rainfall and (for the last 11 months) sunlight patterns were obtained and analysed. Testicular and ovarian development was lowest in the months of July-August. No evidence of breeding was detected during this period. This period is also marked by the lowest rainfall, the lowest temperatures and least sunlight. Slight but fluctuating breeding activity and gonadal development is resumed in October following increased sunlight and temperatures from September. The period December to February is marked by sustained high temperatures, high sustained maximal sunlight and low rainfall. It is also the period of rising gonadal development in most mature males and females reaching a peak in both sexes in February. This is followed shortly after by a sharp peak in reproductive activity. It is concluded that sunlight and temperature are the most important factors influencing gonadal development and subsequent breeding in Tilapia even under equatorial conditions. When preceded by conditions of sustained high temperatures and good sunlight which are conducive to the maximal development of the gonads of both sexes, the onset of rainfall seems to stimulate the initiation of the peak breeding activity. Sustained heavy rainfall on the other hand seems to check breeding.     

Phosphorus inputs to Lake Naivasha,Kenya, from its catchment and the trophic state of the lake

The main river supplying Lake Naivasha, Kenya, the Malewa, drains a catchment given over to largely subsistence cultivation and animal husbandry. The lake itself is the focus for an intensive horticultural industry based upon irrigation from the lake. The Malewa, however, is relatively independent of the impact of industry, and so its contribution to eutrophication of the lake was evaluated. Two periods of study, a very wet-dry and a `normal' wet-dry season showed that the river contribution of phosphorus led to a total phosphorus loading of 1.4 g m<sup>–2</sup> lake surface ann<sup>–1</sup> in the very wet period compared to 0.2 in the `normal'. Chlorophyll `a' in the open water of the lake was significantly related to soluble reactive phosphorus. The lake is now eutrophic by normal limnological criteria.     

The diet of largemouth bass, Micropterus salmoides, in Lake Naivasha, Kenya

Lake Naivasha is a freshwater lake situated in the eastern rift valley of Kenya. Only five species of fish are present, all of which have been introduced. Of these, Oreochromis leucostictus, Tilapia zillii and Micropterus salmoides (largemouth black bass) support an important gillnet fishery with bass also being taken for sport. Until bass reached 260 mm f.l. they depended upon invertebrate food organisms. Thereafter crayfish, fish and frogs became increasingly important the larger the size of the bass. The most important invertebrate prey species was the water boatman, Micronecta scutellaris , followed by chironomid and culicid pupae. Zooplank-ton was consumed but only in large quantity by fish smaller than 80 mm. For bass over 260 mm the crayfish, Procambarus darkii , was the principal food. The largemouth bass in Lake Naivasha are generalized macro-predators, feeding principally on free-living animals of a kind most likely to be found in the littoral zones.     

Magnetic susceptibilities of lake sediment and soils on the shoreline of Lake Naivasha,Kenya

On steep, unvegetated slopes, sands (particle sizes 0.063 to 2 mm) and gravels (2–64 mm) erode from the shoreline of Lake Naivasha (Kenya) and enter the lake basin. This occurred freely where fringing papyrus (Cyperus papyrus) swamp had been cleared in favour of landing jetties or agriculture. Gravel-sized particles have been recovered up to 60 m offshore and sands to 80 m. In an area where papyrus was undisturbed and the swamp margin was 52 m wide, gravel did not enter the lake and sands penetrated to around 35 m. Large particles are much easier to trace to source and to manage than the finer silts and clays (<0.63 m) that form the bulk of Lake Naivasha's sediment. The pattern of mass specific magnetic susceptibilities for the <63 m fraction of lake sediment around the southern shoreline of the lake suggests that fine particles enter open water directly from the shoreline and are not transported from the lake's only perennial inflow, the River Malewa in the north. Such particles originate from a hinterland that supports high intensity horticulture and are therefore a potential source of contamination. Mean ± SD susceptibility (_If) immediately offshore papyrus fringe was 0.49 ± 0.08 × 10^-6 m³ kg^-1 compared with higher values of 1.33 ± 0.14 × 10^-6 m³ kg^-1 where there was no papyrus barrier (P<0.0001). The value for five sites in the middle parts of the lake was 0.45 ± 0.02 × 10^-6 m³ kg^-1 with 1.38 ± 0.10 × 10^-6 m³ kg^-1 near the mouth of the River Malewa. The results of this study are evidence, therefore, that conservation of a continuous papyrus margin of about 50 m width is a priority for intercepting particulate material.     

The aquatic plant communities of the Lake Naivasha wetland,Kenya: pattern,dynamics and conservation

The spatial and temporal patterns of the wetland plant communities at Lake Naivasha over the past decade, and the past five years in particular, are discussed in relation to the major controlling factors. The four communities are:- the emergent swamp, dominated by Cyperus species; the floating raft, dominated by the aliens Salvinia molesta and Eichhornia crassipes; the floating-leaved plants, represented only by Nymphaea caerulea; and the submerged angiosperms, consisting of three species of Potamogetons — P. pectinatus, P. schweinfurthii, P. octandrus — together with Najas pectinata. The major factors affecting their dynamics are:- water level changes which influence agricultural clearance, introduced crayfish Procambarus clarkii, and interactions between communities such as the physical effects of mobile floating rafts. The value of the different communities to the ecological and economic value of the lake is highlighted, and the possible damage from future activities discussed.     

Breeding biology of the Louisiana red swamp crayfish Procambarus clarkii Girard in Lake Naivasha,Kenya

This paper describes the breeding biology of the Louisiana red swamp crayfish (Procambarus clarkii Girard) that has been introduced in a tropical lake, Naivasha. Notable differences in crayfish biology are found between the place of origin in Louisiana and lake Naivasha.In Naivasha sexual maturity in females is reached at a larger size (42 mm carapace length (CL), 76 mm full length (FL)) than in males (40 mm CL, 71 mm FL). The corresponding figures for Chalmette area of Louisiana are 31–32 mm CL for both sexes and at Ben Hur Research Centre, Louisiana, 33 mm CL. The females weigh between 15 and 20 g at the onset of egg deposition. The corresponding Louisiana values are from 5 to 10 g. Crayfish in Naivasha as in Louisiana breed both inside burrows which are 10–70 cm deep, and on the sediment in shallow water at a depth of between 0.5 and 4 m. These observations suggest that reproduction is very much a lake edge activity. The number of breeding individuals in Naivasha varies with the trend of the water surface level. Sexually active females (form I) as judged from secondary sexual characters (appearance of basal hooks at the base of 3rd and 4th walking legs) persist throughout the year at no less than 60% of the adult male population. This is confirmed by the appearance of recently hatched juveniles (3.4 to 6.5 mm CL) in the water throughout the year. Similarly berried females in Naivasha are found throughout the year, though with peaks at irregular intervals. In Louisiana breeding takes place only from August through November with a single peak in October. The fecundity of crayfish in Naivasha is 433 (SE ± 21, N = 133) and there is a positive correlation between body length and egg number.     

Community structure of helminth parasites of the tuna, Euthynnus affinis, from the Visakhapatnam coast, Bay of Bengal

An analysis is made of the community structure of the helminth parasites of the tuna Euthynnus affinis collected off the coast of Visakhapatnam, Bay of Bengal. The helminth fauna comprised 23 species, consisting of two monogeneans, 18 digeneans, one larval cestode, one nematode and one acanthocephalan. Didymozoids, represented by 14 species, were the dominant members of the parasite spectrum. The parasite assemblage is characterized by species richness as well as a high diversity. Most of the parasites found, the didymozoids in particular, were host specialists, exhibiting a high degree of host specificity. The fauna comprised three core species, four satellite species and many secondary species, with no potential for interaction among them but the degree of predictability is high in that each infracommunity comprised two or three core species superimposed by a few secondary species. Many host factors such as the varied diet, high vagility, long life span and endothermy appear to have contributed to the development of the species rich and diverse parasite communities in the tuna. There was evidence for a decrease in parasite density and an increase in diversity with increase in host size, indicating that host size has a profound influence on the component community structure.