Biological aspects of Schilbe intermedius (Ruppell, 1832) in the Nyanza Gulf of Lake Victoria,Kenya

Silver butter catfish (Schilbe intermedius) is an indigenous fish species in Lake Victoria. It has a good economic value because of the quality and freshness of its flesh, increasing its demand for consumption. It is potamodromous, with its stock in the lake declining because of overexploitation by the use of illegal fishing gears at the river mouths on their way to spawning areas. Fish samples (321) were obtained from commercial gill net (1.5–3 in.) fishers within Nyanza gulf. The fish samples were weighed (g) and measured (cm), dissected and sex determined by visual inspection of the gonads. Ovaries for mature female fish specimens were preserved for analysis of fecundity. Fecundity was determined by the gravimetric technique. The mean (±SE) length and weight for all fish were 18.2 ± 0.2 cm TL and 51.9 ± 1.9 g, respectively. The female fish (20.0 ± 0.2 cm) were significantly (F = 199.80, p < .05) larger than the males (15.9 ± 0.1 cm). Similarly, the females (68.8 ± 2.6 g) were significantly (F = 152.61, p < .05) heavier than the males (30.93 ± 1.02 g). The overall sex ratio was 1.23:1.0 (female:male). The length frequency exhibited a unimodal distribution, with the modal class between 16 and 18 cm for either sex. The regression slope (b) of the length–weight relationship was 3.2, 3.1 and 3.0 for all fish, females and males. The absolute fecundity ranged from 6,510 to 59,818 eggs, with a mean of 34,473 eggs. The length at 50% maturity was estimated to be 16.0 cm (female) and 18.0 cm (male) fish. The mean length and size at maturity of S. intermedius have declined, likely attributable to fishing effects, changes in food resources, competition and predation. Thus, the present study provided relevant biological data of S. intermedius, which is currently limited for this region.     

Growth,mortality and recruitment of silver cyprinid (Rastrineobola argentea) in the open waters of Lake Victoria,Kenya

This study investigated the growth, mortality and recruitment of Rastrineobola argentea in Lake Victoria on the basis of length–frequency data collected during the period 2014–2015. The asymptotic length (L _∞ ) had a mean (±SE ) value of 53.50 ± 0.50 mm SL , growth curvature (K ) of 0.81 ± 0.06 year^?1, total mortality (Z ) of 2.96 ± 0.12 year^?1, a natural mortality (M ) of 1.23 ± 0.06 year^?1, a fishing mortality (F ) of 1.74 ± 0.07 year^?1, an exploitation rate (E ) of 0.59 ± 0.01, a growth performance index (?′ ) of 3.36 ± 0.02 and a length at 50% capture (L ₅₀) of 28.25 ± 0.43 mm SL . The fish exhibited a peak breeding during the months of May and August. The Beverton and Holt relative yield‐per‐recruit model indicated mean (±SE ) indices as 0.37 ± 0.01 for optimum sustainable yield (E _0.5), 0.76 ± 0.01 for maximum sustainable yield (E _max) and 0.66 ± 0.01 for economic yield (E _0.1). Compared with previous studies, there is a great decline in the sizes of R. argentea stocks in Lake Victoria. Thus, management measures should include restriction on illegal seine nets <10 mm mesh size and re‐enforcement of a fishing ban or a closed season.     

Ontogenic Changes in Prey Ingested by Nile perch (Lates niloticus) Caught in Nyanza Gulf of Lake Victoria,Kenya

A total of 289 fish were analysed in this study, with Caradina nilotica (44%) being the dominant food item in their diet, while tilapia (8%) contributed the least. Haplochromines, unidentified fish prey and juvenile Nile perch also were observed. An ontogenic shift was observed, with C. nilotica contributing highest (61.1%) of the diet of fingerlings, but decreasing to 21% in the diet of adults. Nile perch contributed 35.7% of the diet in the adults and 8.1% in the fingerlings. This study indicates Nile perch is a predatory fish, feeding mainly on C. nilotica, haplochromines, tilapia, Nile perch and other fish materials.     

Interannual variability of water quality conditions in the Nyanza Gulf of Lake Victoria,Kenya

The Nyanza Gulf of Lake Victoria is considered eutrophic with degrading water quality, as shown by harmful algal blooms formed by cyanobacteria of the genus Microcystis. The dilution of gulf water occurs mainly through the wide Rusinga Channel, whereas the much narrower and shallower Mbita Channel was reopened in May 2017. Specific conductivity and phytoplankton biovolume composition were monitored monthly from July 2017 to July 2018 at three stations and compared with records during 2008 – 2009, when higher eutrophication was observed, in the west gulf (WG, closely located to the Mbita Channel), the mid gulf (central part of the gulf), and the east gulf (EG, close to Kisumu City). As previously observed, conductivity showed a distinct increase from the WG to the EG but compared with records 2008–2009 it had decreased. Water level fluctuations, resulting from wind possibly causing water exchange with the main basin, were related to the decrease in conductivity at the EG. When all three stations were compared, the environmental variable transparency was positively related to most phytoplankton genera, whereas the genera Dolichospermum, Microcystis, and Aulacoseira were related to eutrophication. Compared with the period 2008–2009, the proportion of Microcystis decreased, while the proportion of Aulacoseira increased. Compared with the period 2008–2009, the observed dilution at the EG was primarily related to the overall increased water level in the main basin. The observed dilutional influence might have led to a decline in nuisance phytoplankton and overall improved water quality conditions.     

Spatial variations in sedimentary organic matter in surficial lake sediments of Nyanza Gulf (Lake Victoria,Kenya) after invasion of water hyacinth

No detailed studies have been conducted on the spatial distribution of sediment characteristics within Nyanza Gulf of Lake Victoria, especially after the invasion of water hyacinth. Accordingly, a rapid, inexpensive method was used to determine the spatial sediment organic matter variations, expressed as loss on ignition (LOI). Surficial sediments were collected during different survey periods from about thirty‐two sampling sites between 1994 and 2012, in order to characterize the sediments. Although the deeper (>40 m) depositional areas are located in the open lake, visual observations indicate that most of the offshore sediments consist of muddy deposits, plus significant shell remains, with a more sandy type of sediments located in some areas off the inflowing river mouths. The surficial sediments were characterized by a relatively high water content, with over 89% of the samples containing water contents greater than 75%. The sediment organic matter contents from the 2012 survey were more variable, ranging from 1.90% to 33.47%. The sediment organic carbon contents varied spatially, although there were no significant variations between the different sampling periods. Evaluating the SOM at different heating durations at a constant temperature (550 °C) and a very high temperatures (950 °C) did not identify any significant differences. High primary production and greater settling in the deeper areas contribute to the nature of sediments deposited, also being reflected in the relatively high OC contents. Spatially, it is thought that detritus from water hyacinth and terrestrial external loads are important sources of organic matter deposited on the lake bottom within the gulf and may support its growth because of enrichment of nutrient elements. The importance of sediments in adsorption of contaminant substances and influencing geochemical processes within the lake is also emphasized.     

Population Biology of Nile tilapia (Oreochromis niloticus) in Lake Naivasha,Kenya

The Nile tilapia (Oreochromis niloticus) was introduced into Lake Naivasha to provide exploitable fisheries, being one of the commercially important fish providing livelihoods to communities around the lake. Its stocks in the lake have been declining, however, because of intense fishing, ecological changes and effects of other exotic species. The present study was undertaken to investigate the population and biological parameters of Oreochromis niloticus. Fish samples (1,021) were caught monthly from January to December 2017, collecting a total of 1,021 O. niloticus, with an overall sex ratio of 2.21:1.0 (male: female). The length of the fish ranged from 9.0 to 40.0 cmTL and weighed between 20 and 1,220 g. The mean length and weight for all fish was 22.6 cm and 272.6 g. The slope b of length–weight relationship was 2.86 for all fish, indicating negative allometry. The mean condition factors were 0.99, 1.04 and 1.01 for male, female and all fish, respectively. The length at first maturity (Lm₅₀) was estimated to be 28.0 cmTL for all sexes, indicating the fish matures at smaller sizes. The length at first capture (Lc₅₀) was calculated to be 19.38 cm, a value much lower than its Lm₅₀, reflecting the capture of juvenile fish. The asymptotic length (L∞) was 42 cm, the growth coefficient (K) was 0.21/year, and the growth performance index (Ф) was 2.57. The natural (M), fishing (F) and total mortality (Z) coefficients were estimated to be 0.55, 0.26 and 0.80/year, respectively. The optimum sustainable yield (E_0.5), maximum sustainable yield (E_max) and maximum economic yield (E_0.1) indices of 0.23, 0.35 and 0.26, respectively, imply an optimal exploitation of O. niloticus. The results of the present study provided some important insights into the biology and management needs of the O. niloticus fishery in Lake Naivasha, as well as revealing a gap for further research on its reproductive biology.     

Use of the fish endoparasite Ligula intestinalis (L., 1758) in an intermediate cyprinid host (Rastreneobola argentea) for biomonitoring heavy metal contamination in Lake Victoria, Kenya

Use of some fish parasites as bioindicators of heavy metal pollution has been demonstrated as a promising approach because of their capacity to bioconcentrate such elements. This study evaluated the effects of a cestode parasite, Ligula intestinalis , on the accumulation of lead (Pb), cadmium (Cd), chromium (Cr) and copper (Cu) in the cyprinid fish, Rastreneobola argentea , in Lake Victoria, Kenya. This L. intestinalis / R. argentea model also was assessed as a bioindicator system for heavy metal contamination in the lake. Samples of 125 fish, 63 parasites, water and sediments were collected at four sites in the Kenya portion of the Lake Victoria basin characterized by variable heavy metal concentrations, for ICP-OES element analysis. The concentration of all four heavy metals in the fish and parasite samples exhibited site-specific variations relative to the metal concentrations in the water. The Pb, Cr and Cd concentrations in the L. intestinalis were higher than in the fish samples by a factor 11, 18 and 14 respectively, whereas the Cu concentration in L. intestinalis was increased by a factor of 2.5, relative to the Cu concentration in fish. The Pb, Cd and Cr concentrations in the parasite body increased, relative to their concentrations in fish samples, suggesting in the bioaccumulation of these metals by the parasite. The Cu concentration in the fish parasite decreased, relative to increased Cu levels in the fish. This finding was interpreted as being a competition for these elements between the parasite and its fish host. Moreover, the increased Cd and Cr levels in the fish were significantly influenced by the increased abundance of the parasites in the abdominal cavity of the fish samples. Based on the results of this study, the L. intestinalis / R. argentea system is proposed as a promising bioindicator model for evaluating environmental Pb, Cd and Cr concentrations where these species occur.     

Biogeochemical characterization,phosphorus sources and intrinsic drivers to its speciation within the Nyanza Gulf of Lake Victoria

Lake Victoria, like many African great lakes, was formed through tectonic activity that formed East African Rift Valley. The lake's ecology has undergone dramatic reorganization dating back to the 1920s, before agricultural mechanization and high urban populations were observed, transforming from it from a desirable to less desirable state. The present study was conducted to better understand Nyanza Gulf's biogeochemical characteristics, phosphorus sources and the driving forces to their speciation. Five littoral‐limnetic‐littoral transects were sampled for water‐associated phosphorus (soluble reactive phosphorus, SRP; total phosphorus, TP_W); sediment‐associated phosphorus (non‐apatite inorganic phosphorus. NAIP); apatite phosphorus, AP; inorganic phosphorus, IP; organic phosphorus, OP; total phosphorus, TP_S); elemental compositions (calcium, Ca; iron, Fe; aluminium, Al; manganese, Mn); organic matter (OM); and organic carbon (OC) contents in the sediment. The SRP and TP_W concentrations ranged from 22.9 to 142.9 μg/L, and 57.1 to 277.1 μg/L, respectively. The littoral sampling sites exhibited relatively higher TP_W concentrations than the limnetic sites. Sheltered bays and sites located off sewerage discharge points had higher NAIP concentrations exceeding 400 mg/kg, compared with strong current areas. The AP and TP_S concentrations ranged from 136.7 to 1,511.3 mg/kg, and 512.5 to 2,254.4 mg/kg, respectively. The AP and TP_S concentrations were generally higher (>500 mg/kg) within the littoral zones compared to the limnetic zones; with the littoral sites located close to documented carbonatite rock substrates manifesting exceptionally high concentrations. Nyanza Gulf's eutrophic/hypertrophic status is derived highly from shoreline erosions of phosphorus‐enriched carbonatite rocks associated with rift valleys and from municipal sewerage discharges. Re‐afforestation, fringing wetland restorations and tertiary treatment of municipal waste waters are vital for its ecological restoration.     

Population biology of common carp,Cyprinus carpio (Linnaeus, 1758), in Lake Naivasha,Kenya

The common carp (Cyprinus carpio) is an exotic fish, being one of the commercially important species in Lake Naivasha. The present study investigated its growth, mortality, maturity, condition and exploitation. Sampling was done monthly from January to December 2017 with gillnets of 2–5 inches mesh size. Each individual fish was measured (cm) and weighed (g) in the field. The specimens were preserved in ice and transported to the laboratory for analysis of maturity. The fish lengths ranged from 12.0 to 69.0 cm TL, and weighed between 32 and 6,840 g. The overall population sex ratio was 1:1.66 (female:male). The length at first maturity (L_m50) was calculated to be 54.0 and 49.0 cm for females and males, respectively. The length at first capture (23.9 cm) was lower than its length at first maturity, suggesting the capture of immature fish. The mean condition factor for all fish was 1.05. The peak condition values were observed during April (1.35), September (1.59) and October (1.80). The highest (1.377) mean condition was observed for size class 56–60 and the lowest (0.933) in class 26–30. The asymptotic length (L∞) was 73.5 cm, and the growth curvature (K) was 1.05 year^‐1, with a growth performance index (Ф) of 3.72. The total (Z), natural (M) and fishing (F) mortality coefficients were 2.85, 1.34 and 1.78 year^‐1, respectively. The exploitation rate (E) of 0.57 year^‐1 was higher than the optimum sustainable yield (E_0.5) of 0.32, but was not different from the maximum sustainable yields (E_max) 0.54 and economic yield (E_0.1) 0.46. The results of the present study provide parameters of fish biology and population that should prove useful in managing the fishery of Lake Naivasha.     

Bioavailability of particle‐associated nutrients as affected by internal regeneration processes in the Nyanza Gulf region of Lake Victoria

Land‐use changes have been implicated a lot in the eutrophication of many lakes while forgetting the role of internal loading as influenced by the hydrological regimes of a given lake. The phosphorus loading of Nyanza Gulf is influenced both by internal and external loading with the internal loading playing a greater role in its eutrophication. The shear on the Apatite Phosphorus (AP) rich residual rock in the western end of the gulf through strong currents across the Rusinga Channel erodes the rock into non‐apatite inorganic phosphorus (NAIP) which is readily available for primary productivity. The current suspends the phosphorus rich apatite sediment together with the reserved phosphorus within sediments to the water column. The NAIP concentration on the western end of the gulf is exceptionally high, >1500 mg kg^?1, and together with the hydrological forcing; is believed to be the driving force of Nyanza Gulf eutrophication. External loading through rivers and municipal discharges exacerbates the problem. The external loading mainly influences the inner gulf on the eastern shore while the internal loading affects mainly the western end of the gulf. Nyanza Gulf eutrophication can be managed by adopting the following measures: (i) the Mbita Causeway needs to be opened and a bridge erected in its place in order to reduce the strong current through the Rusinga Channel and the residence time within the gulf, by increasing the flushing rates; (ii) the farming communities within the basin need to be sensitized on the controlled use of fertilizers; (iii) the municipal wastes should be treated to tertiary level before discharge into the lake; and (iv) reduce erosion within the basin through re/afforestation.     

Some aspects of the biology of Nile perch,Lates niloticus,in the open waters of Lake Victoria,Kenya

Samples (6404) of Nile perch were collected monthly from commercial catches between June 2014 and June 2015. The total length (cm), weight (g) and sex of fish were all determined in the field, with 3771 (59%) being male and 2059 (32%) being female fish, for an overall sex ratio of 1.83:1.00 (male: female). The average length and weight for all fish were 55.38 ± 0.14 cm TL and 2355.10 ± 22.30 g, respectively. The size of male fish ranged from 15.5 to 128.5 cm TL and weighed between 190 and 25 000 g, whereas that of female ranged from 21.0 to 130.0 cm TL and weighed between 124 and 25 800 g. The value of the regression slope b of the length–weight relationship was equal to 3.04. The mean (±SD) condition factor for all fish was 1.23 ± 0.13. The condition factor was different in the length groups, with the highest (1.32 ± 0.13) and lowest (1.12 ± 0.20) values recorded in length groups 100–110 and <30 cm TL, respectively. The highest and lowest condition factors were recorded between January and March, respectively. The length–weight relationship and condition of Nile perch in Lake Victoria have deteriorated greatly, this finding being linked to the reduction of its prey species in the lake.     

Growth,mortality and recruitment of Nile perch (Lates niloticus) in Lake Victoria,Kenya

This study investigated the growth, mortality and recruitment of Lates niloticus in Lake Victoria basis on length–frequency data collected during the period 2014‐2015. The asymptotic length (L _∞ ) had a value of 124 cm TL , growth curvature (K ) of 0.22 year^?1, total mortality (Z ) of 0.96 year^?1, a natural mortality (M ) of 0.42 year^?1, a fishing mortality (F ) of 0.54 year^?1, an exploitation rate (E ) of 0.57 and a growth performance index () of 3.53. Logistic selection model showed that 50% of fish of 46.09 cm TL encountering the gear are retained. There were two peak recruitment periods, a minor one in March and a major one in July, accounting for 12.04% and 22.04%, respectively, of the total fish catch. The Beverton and Holt's relative yield‐per‐recruit model indicated the indices for sustainable yields are 0.32 for optimum sustainable yield (E _0.5), 0.60 for maximum sustainable yield (E _max) and 0.51 for economic yield (E _0.1). Compared to previous findings, there is a great decline in the sizes of Nile perch stocks in Lake Victoria. Thus, managing the fishery requires strict adherence to the slot size of 50–85 cm TL , and restrictions on illegal gear and methods, by the devolved governments through monitoring, control and surveillance in liaison with the Beach Management Units (BMU s).     

Growth and population parameters of Nile tilapia,Oreochromis niloticus (L.) in the open waters of Lake Victoria,Kenya

The Nile tilapia (Oreochromis niloticus) was introduced into Lake Victoria in the early 1950s and 1960s and has since become the dominant tilapiine in the lake. This study investigated the growth and population parameters of O. niloticus in Lake Victoria on the basis of length–frequency data collected during the period June 2014 and June 2015. The asymptotic length (L_∞) had a mean (±SE) value of 46.24 ± 0.04 cm TL, growth curvature (K) of 0.69 ± 0.25 year^?1, total mortality (Z) of 2.18 ± 0.80 year^?1, a natural mortality (M) of 1.14 ± 0.28 year^?1, a fishing mortality (F) of 1.05 ± 0.53 year^?1, an exploitation rate (E) of 0.46 ± 0.08, a growth performance index (?) of 3.14 ± 0.17 and a length at first capture (L_C50) of 20.31 ± 0.40 cm TL. Comparing the results of this study with previous studies indicates the parameters K, Z and M have increased, whereas ?, F, E and L_C50 have decreased. Changes in these parameters could be attributed to the existing high fishing capacity, and changing lake conditions. Thus, management measures should include continued restriction on illegal fishing methods and gears, such as the use of undersized gillnets (<5 in. mesh size) and beach seines. More attention also should be directed to the implementation of measures to control pollution of the lake from its various sources.     

Aspects of the biology of Labeo cylindricus (Pisces: cyprinidae) in Lake Baringo,Kenya

Aspects of the biology of Labeo cylindricus from Lake Baringo were investigated, based on experimental beach seining and gillnetting between August and October 2007. The length–weight relationship indicated the species exhibited positive allometric growth (b = 3.7083), with a condition factor (K) of 0.84 ± 0.0298 S.D. Males dominated fish catches, with an adult sex ratio of males/females of 1:1.7. The fish length‐at‐50% maturity (Lm50) was 127.7 and 126.1 mm total length (TL) for males and females, respectively. Fish age and growth were determined from sagittal otoliths considered to be from the young‐of‐the‐year. Clear circuli observed in the otoliths were used to determine fish age in days. Validation by oxytetracycline marking indicated that a single growth increment (1.1 ± 0.1 SE) is formed daily on fish otoliths. Fish length (TL mm) was correlated significantly with age (in days), TL = 1.398 Age + 26.523 (r² = 0.91). Fish growth was subsequently estimated to be 1.398 mm day^?1. The weights and ages (in days) of fish exhibited a significant (P < 0.05) power relationship defined by the equation: W = 0.0003 Age^2.5804 (r² = 0.88). The results of this study provide parameters that, if appropriately monitored, can be used to predict responses of fish populations in Lake Baringo and elsewhere to human interventions (exploitation) and natural environmental change.     

Trace metal concentration in water and sediments of satellite lakes within Lake Victoria (Kenya) basin

Lakes Kanyaboli, Sare and Namboyo are three important freshwater satellite lakes in the northern region of the Lake Victoria (Kenya) basin. Lake Simbi, a small alkaline -saline crater-lake is located near the southern shoreline of the Nyanza Gulf of Lake Victoria. The three freshwater lakes provide unique aquatic habitats for the important indigenous fish species and other aquatic biodiversity, as well as serving as water resources for the surrounding communities. Surface and sediment samples were collected and examined to characterize these lake systems. Metal partitioning among the various defined geochemical phases also was determined. Based on the measured concentrations, the satellite lake waters are relatively uncontaminated with regard to Al (aluminium), manganese (Mn), zinc (Zn), copper (Cu) and chromium (Cr). Levels of most elements were below detection limits, as well as being well below the desirable drinking water guideline values proposed by the World Health Organization. The exception was the elevated iron (Fe) contents (>300 μg L⁻¹). The sediments exhibited a relatively low level of contamination with regard to heavy metals. The mean values of the measured metals in the sediments were compared to levels in adjacent Lake Victoria. The study results from these poorly-studied, and relatively unperturbed ecosystems, compared to the main body of lake Victoria, highlight their importance as conservation areas, despite the small-scale fishery activity, potential threats from human activities and their sensitivity to fluctuating environmental conditions.     

Development of a fish‐based index of biotic integrity (FIBI) for monitoring riverine ecosystems in the Lake Victoria drainage Basin,Kenya

The long term monitoring of aquatic ecosystems based on chemical analysis is expensive resulting in a lack of time series data for management purposes thus necessitating the need for new cost effective methodologies. This study developed a fish‐based index of biotic integrity (FIBI) for monitoring environmental conditions in riverine ecosystems within the Lake Victoria Basin, Kenya. The fish fauna in the basin is poorly studied, less diverse and ecologically specialized than the temperate fauna that provided the origins of the Index of Biotic Integrity. Fish samples were collected by electrofishing from 24 sites during baseflow periods in February, March and July 2004. Validation data were collected from 7 sites with varying levels of degradation. Fish samples were identified, counted and grouped into different trophic groups, relative tolerance to pollution, habitat guild and whether exotic or native to riverine environment. Thirty‐three candidate metrics were evaluated for responsiveness to habitat quality and twelve were selected for inclusion in the final index. The index classified 6 of the 7 validation sites according to their levels of degradation. As a bioassessment tool, the index was useful in laying the basis for long‐term monitoring of rivers in the Lake Victoria drainage basin. Copyright © 2010 John Wiley & Sons, Ltd.     

Spatio‐temporal trends of nutrients and physico‐chemical parameters on lake ecosystem and fisheries prior to onset of cage farming and re‐opening of the Mbita passage in the Nyanza Gulf of Lake Victoria

Many large lake ecosystems are experiencing increasing eutrophication and persistent cyanobacteria‐dominated algal blooms affecting their water quality and ecosystem productivity because of widespread non‐point and point nutrient sources. Accordingly, the present study utilized data of July 2003 and January–February 2004, as well as previous measurements of nutrients and physico‐chemical variables (electrical conductivity, dissolved oxygen, temperature, pH, turbidity and chlorophyll‐a), to characterize the spatial and temporal trends, as a means of better understanding the factors influencing lake environmental conditions, as support tools for long‐term ecosystem management and for better understanding the long‐term trends and effects. Inshore gulf areas were found to represent zones of maximum nutrient concentrations, compared to the deep main lake zones, with significant inter‐parameter correlations. Phosphorus, silicon and chlorophyll‐a concentrations were significantly correlated. Water electrical conductivity was also significantly and positively correlated with soluble reactive silicon (SRSi), alkalinity hardness DO, while exhibiting a negative association with water transparency. Water turbidity and transparency, electrical conductivity, and SRSi concentrations clearly describe a gradient from the gulf into the main lake. For such a shallow gulf, these findings suggest primary productivity is influenced mainly by the availability of nutrients, light transparency and the extent of availability of resuspended nutrients. The increasing eutrophic state of Lake Victoria is a serious concern since it contributes to an increased potential of more frequent occurrences of cyanobacterial blooms, a potential public health risk to both humans and wildlife. Improved understanding of influences from previous fish species introductions and concomitant changes in indigenous fish species, increased lake basin population and anthropogenic activities, water hyacinth resurgences, sustainability of biodiversity, and current interests in cage farming, are among the major concerns and challenges facing the contemporary Lake Victoria. The trends regarding nutrients and physico‐chemical characteristics are intended to support better monitoring efforts and data to promote the lake's ecosystem services and the sustainable management of the lake ecosystem.     

Assessment of catfish (Clarias gariepinus) stock in Lake Baringo,Kenya

The management measures used for sustainable utilization of Clarias gariepinus in Lake Baringo do not have a stock assessment reference, attributable mainly to a lack of information on biological limits and target reference points. Assessment of Clarias gariepinus stock in Lake Baringo was carried out between August 2013 and July 2014. A total of 2772 fish were sampled from 25 boats (40%) for 5 days each week for length and weight measurements. Fish Stock Assessment Tools and yield model were used to estimate population parameters, exploitation rate and optimal fishing scenarios. Annual C. gariepinus standing biomass was estimated at 21 383 kg, L_∞ = 114.30, K = 0.37 year^?1, W = 0.0147L^2.81, Z = 1.14 year^?1, M = 0.61 year^?1, F = 0.53 year^?1, and exploitation rate = 0.46 year^?1. The relative yield‐per‐recruit (Y′/R) and biomass‐per‐recruit (B′/R) resulted in E_max = 0.44 and F_MSY = 0.50 year^?1. The yield‐per‐recruit ratio at maximum sustainable yield was 29.12%, and the SSB_MSY per recruit ratio = 56.10%. The steady‐state biomass, exploitation rate and optimal fishing scenario indicated a pristine fishery for the lake, suggesting the current fishing efforts should not be exceeded to enable sustainable economic utilization of C. gariepinus.     

Shifts in the food of Nile perch (Lates niloticus) in Lake Victoria

The study investigated diet of Nile perch in Lake Victoria and compared results with past data from the same lake in order to analyse diet shifts over time. Fish samples (2020) were collected by seining and trawling from 2012 to 2016. In overall, Caridina nilotica (59%) was the dominant food item in the diet, while Rastrineobola argentea (5%) contributed the least. The diet of Nile perch of 1 cm was copepods, cladocerans and rotifers. The diet changed to only copepods and cladocerans at 2 cm. The proportion of the relatively large calanoids increased with Nile perch size in 1988/89, comprising between 35 and 80% of the diet of fishes of 3–4 cm. Nile perch of 6–25 cm TL fed more on C. nilotica in 2012/2016, compared to 1988/1989 and 2006/08. Haplochromines were fed on more in 2006/2008 than in 2012/2016 by Nile perch of 6–30 cm TL as fish size increased. The frequencies of occurrence of R. argentea in the diet were highest in 1988/1989, and lowest in 2006/2008, for fish size of up to 30 cm TL. The proportion of Odonata in the diet of size class 16–20 and 21–30 cm TL were highest in 2012/2016 and 1988/1989, respectively. Thus, shifts in Nile perch diet was observed from zooplankton to C. nilotica, then to haplochromines and finally to fish prey as size increased. Nile perch preferred haplochromines with C. nilotica as the substitute food. Cannibalism was observed to have decreased, only being evidenced in Nile perch above 35 cm TL, which could actually have a positive impact on the Nile perch fishery. The information on shifts in diet of Nile perch in Lake Victoria is of considerable ecological importance.     

Spatial-temporal dynamics of water hyacinth, Eichhornia crassipes (Mart.) and other macrophytes and their impact on fisheries in Lake Victoria,Kenya

Invasive aquatic macrophytes in Lake Victoria including water hyacinth (Eichhornia crassipes) exhibit periodic cyclical patterns of decline and proliferation with attendant ecological and economic impacts. This study aimed to monitor the extent of macrophytes and other invasive weeds on Lake Victoria to establish their impact on fisheries. The study employed a combination of remote sensing and geographical information system (GIS) techniques to estimate the coverage of water hyacinth and other invasive macrophytes. Data on fish landings and their respective market values were acquired from the Electronic Fish Market Information Service (EFMIS) database, which is hosted by the Kenya Marine and Fisheries Research Institute (KMFRI). Analysis of consistent temporal satellite data showed that the weeds frequently cover sheltered bays and river mouths. These areas of Winam Gulf had higher coverage (average 5000?ha) of macrophytes than the open waters (<200?ha). The proliferation of the invasive weeds showed fluctuations over the study period reaching the highest peak between September and November 2016. Other aquatic plants that have invaded the littoral areas of Lake Victoria include Egeria densa, Ceratophyllum demersum and Potamogeton spp. Increased coverage of macrophytes was found to be correlated inversely with the commercially important tilapia Oreochromis niloticus, but not with other species. The study concludes that there is a need for sustained monitoring of the invasive macrophytes alongside ecosystem modelling studies using the available time series data to clearly identify the ecological factors that drive water hyacinth dynamics and predict more precisely its impact on the fishery.