Sitoki L.,Kenya Marine and Fisheries Research Institute |
Sitoki L.,University of Innsbruck |
Kurmayer R.,Austrian Academy of Sciences |
Rott E.,University of Innsbruck
Hydrobiologia | Year: 2012
The Nyanza Gulf is a large shallow bay of Lake Victoria suffering from eutrophication by human activities. In order to characterize the harmful algal bloom formation as a consequence of eutrophication, both spatially and seasonally, environmental conditions, phytoplankton community composition, and microcystin (MC) concentrations were investigated monthly from Kisumu Bay, and bimonthly from the center of the gulf, as well as quarterly from the Rusinga Channel and the main basin of Lake Victoria between July 2008 and September 2009. The sites located in Kisumu Bay and the central gulf were most strongly affected by eutrophication, including increased nutrient concentrations and phytoplankton growth. More than 90% of the samples obtained from the gulf were dominated by cyanobacteria, whereas diatoms only dominated in the samples obtained from Rusinga Channel and the main lake. In general, Microcystis accounted for the largest part (>50-90%) of cyanobacterial biovolume. MCs were found in 35 (54%) out of 65 samples and were detected throughout the study period in the gulf, but only in two out of eight samples from the Rusinga Channel and the main lake. A significant linear relationship between Microcystis biovolume and MC concentration was observed (n = 65, R 2 = 0.88, P < 0.001). The highest MC concentrations were recorded in Kisumu Bay between November and March (max. 81 μg l -1) when Microcystis showed max. biovolume (18 mm 3 l -1 in November 2008). The results suggest that seasonal variability did not outweigh the spatial differences in phytoplankton composition and MC production, which is seasonally persistent in Kisumu Bay. © 2012 The Author(s).
Feikin D.R.,Centers for Disease Control and Prevention |
Tabu C.W.,Centers for Disease Control and Prevention |
Gichuki J.,Kenya Marine and Fisheries Research Institute
American Journal of Tropical Medicine and Hygiene | Year: 2010
Cholera outbreaks continue to occur regularly in Africa. Cholera has been associated with proximity to lakes in East Africa, and Vibrio cholerae has been found experimentally to concentrate on the floating aquatic plant, water hyacinth, which is periodically widespread in East African lakes since the late 1980s. From 1994 to 2008, Nyanza Province, which is the Kenyan province bordering Lake Victoria, accounted for a larger proportion of cholera cases than expected by its population size (38.7% of cholera cases versus 15.3% of national population). Yearly water-hyacinth coverage on the Kenyan section of Lake Victoria was positively associated with the number of cholera cases reported in Nyanza Province (r = 0.83; P = 0.0010). Water hyacinth on freshwater lakes might play a role in initiating cholera outbreaks and causing sporadic disease in East Africa. Copyright © 2010 by The American Society of Tropical Medicine and Hygiene.
Mwamburi J.,Kenya Marine and Fisheries Research Institute
Lakes and Reservoirs: Research and Management | Year: 2016
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. © 2016 John Wiley & Sons Australia, Ltd
Mugo R.,Hokkaido University |
Mugo R.,Kenya Marine and Fisheries Research Institute |
Saitoh S.-I.,Hokkaido University |
Nihira A.,Ibaraki Prefecture Fisheries Research Station |
Kuroyama T.,Ibaraki Prefecture Fisheries Research Station
Fisheries Oceanography | Year: 2010
Skipjack tuna habitat in the western North Pacific was studied from satellite remotely sensed environment and catch data, using generalized additive models and geographic information systems. Weekly resolved remotely sensed sea surface temperature, surface chlorophyll, sea surface height anomalies and eddy kinetic energy data were used for the year 2004. Fifteen generalized additive models were constructed with skipjack catch per unit effort as a response variable, and sea surface temperature, sea surface height anomalies and eddy kinetic energy as model covariates to assess the effect of environment on catch per unit effort (skipjack tuna abundance). Model selection was based on significance of model terms, reduction in Akaike's Information Criterion, and increase in cumulative deviance explained. The model selected was used to predict skipjack tuna catch per unit effort using monthly resolved environmental data for assessing model performance and to visualize the basin scale distribution of skipjack tuna habitat. Predicted values were validated using a linear model. Based on the four-parameter model, skipjack tuna habitat selection was significantly (P < 0.01) influenced by sea surface temperatures ranging from 20.5 to 26°C, relatively oligotrophic waters (surface chlorophyll 0.08-0.18, 0.22-0.27 and 0.3-0.37 mg m-3), zero to positive anomalies (surface height anomalies 0-50 cm), and low to moderate eddy kinetic energy (0-200 and 700-2500 cm2 s-2). Predicted catch per unit effort showed a trend consistent with the north-south migration of skipjack tuna. Validation of predicted catch per unit effort with that observed, pooled monthly, was significant (P < 0.01, r2 = 0.64). Sea surface temperature explained the highest deviance in generalized additive models and was therefore considered the best habitat predictor. © 2010 Blackwell Publishing Ltd.
Mbaru E.K.,Coral Reef Conservation Project |
Mbaru E.K.,Kenya Marine and Fisheries Research Institute |
McClanahan T.R.,Wildlife Conservation Society
Fisheries Research | Year: 2013
There is increasing effort to develop fishing methods that increase sustainability of the fishery and reduce bycatch without sacrificing the incomes of fishers. Consequently, we explored the use of modified African basket traps (experimental traps) retrofitted with 4. cm. ×. 30. cm escape gaps and compared their catches with those from unmodified traps lacking these gaps (controls). Studies were undertaken in a heavily fished Kenyan coral reef lagoon dominated by sand, seagrass, and coral reef. Of the 1202 fish captured, we distinguished 64 species from 23 families with significant differences in catch composition between the two trap types. Among the bycatch, numbers of butterflyfish and other low value species were reduced in the experimental traps. Overall, at the trap level, there were no significant differences in terms of mean length, weight and value of the target species. Nevertheless, fish captured in experimental traps were 31% longer and 55% heavier and a decline in the capture of low value species accounted for the lack of difference at the whole trap level. Due to a strong size-price relationship in this fishery, there was a 25% increase in the economic value of the gated compared to control traps. © 2013 Elsevier B.V.