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Environmental Monitoring and Assessment (2006) 122: 275–287
DOI: 10.1007/s10661-005-9180-7 c
Springer 2006
INTEGRATING REMOTE SENSING APPROACH WITH POLLUTION
MONITORING TOOLS FOR AQUATIC ECOSYSTEM RISK
ASSESSMENT AND MANAGMENT: A CASE STUDY OF LAKE
VICTORIA(UGANDA)
SILVIA FOCARDI1,∗, ILARIA CORSI2, STEFANIA MAZZUOLI1,
LEONARDO VIGNOLI3, STEVEN A. LOISELLE1and SILVANO FOCARDI2
1Dipartimento di Scienze e Tecnologie Chimiche e dei Biosistemi. Universit`
a di Siena, Via Aldo
Moro 1, 53100, Siena, Italy; 2Dipartimento di Scienze Ambientali” G.Sarfatti”, Universit`
adi
Siena, Via Mattioli 4, 53100, Siena, Italy; 3Dipartimento di Biologia, Universit`
a degli Studi Roma
Tre, Viale G. Marconi 446, 00146, Roma, Italy
(∗author for correspondence, e-mail: focardis@unisi.it)
(Received 19 May 2005; accepted 28 December 2005)
Abstract. Aquatic ecosystems around the world, lake, estuaries and coastal areas are increasingly
impacted by anthropogenic pollutants through different sources such as agricultural, industrial and
urban discharges, atmospheric deposition and terrestrial drainage. Lake Victoria is the second largest
lake in the world and the largest tropical lake. Bordered by Tanzania, Uganda, and Kenya, it provides
a livelihood for millions of persons in the region. However, the lake is under threat from eutrophica-
tion, a huge decline in the number of native fish species caused by several factors including loss of
biodiversity, over fishing and pollution has been recently documented. Increasing usage of pesticides
and insecticides in the adjacent agricultural areas as well as mercury contamination from processing
of gold ore on the southern shores are currently considered among the most emergent phenomena
of chemical contamination in the lake. By the application of globally consistent and comprehensive
geospatial data-sets based on remote sensing integrated with information on heavy metals accumula-
tion and insecticides exposure in native and alien fish populations, the present study aims at assessing
the environmental risk associated to the contamination of the Lake Victoria water body on fish health,
biodiversity from agricultural activities which occur in the cachment surrounding the lake. By the
elaboration of Landsat 7 TM data of November 2002 and Landsat 7 TM 1986 we have calculated the
agriculture area which borders the Lake Victoria bay, which is an upland plain.
The resulting enhanced nutrient loading to the soil is subsequently transported to the lake by
rain or as dry fall. The data has been inserted in a Geographical information System (ARCGIS) to
be upgraded and consulted. Heavy metals in fish fillets showed concentrations rather low except for
mercury concentrations being higher than others as already described in previous investigations. In the
same tissue, cholinesterases activity (ChE) as an indicator of insecticides exposure showed significant
differences among fish species in both activity and sensitivity of selected inhibitor insecticides. This
integrated approach aims at identifying and quantifying selected aquatic environmental issues which
integrated with monitoring techniques such as contaminant concentrations and biological responses to
insecticides exposure in fish populations provide a basis for aquatic management and assist in policy
makers at the national and international levels.
Keywords: remote sensing, ARCGIS, Lake Victoria, heavy metals, insecticide exposure, fish
276 S.FOCARDI ET AL.
Introduction
Lake Victoria, the World’s second largest fresh water lake is shared by East African
Community Partner States of Kenya, Tanzania and Uganda and is of great socio-
economic development to the East African people. The Lake basins endowed with
a rich and unique terrestrial and aquatic biodiversity ranging from forests, wildlife
and fisheries and abounds in minerals. The Lake catchment supports one third of
East African Community population of approximately 30 million people. The ri-
parian states rely highly on the Lake for fish, hydropower generation, transport
and communication, water supply for domestic, agricultural and industrial use,
recreation and biodiversity conservation. Lake wide fish production is estimated
at 500,000 metric tons per annum valued at USD 400 million. The Lake is of
great scientific interest globally as it harbours over 350 endemic fish species. The
rate of population increase of the countries is about 3.4% per annum (Hecky and
Bugenyi, 1992; Hecky 1993) and is increasing (Alabaster,1981). Due to increased
multiple human activities in the last 40 years, the Lake basin has experienced pat-
terns in unsustainable practices and exploitation of its resources. These trends have
caused serious socio-economic and environmental problems including deteriora-
tion of water quality, loss of biodiversity, water hyacinth infestation, deforestation,
severe soil erosion, wetlands destruction and poverty among the Lake basin com-
munities. By the application of globally consistent and comprehensive geospatial
data-sets based on remote sensing integrated with information on heavy metals ac-
cumulation and insecticides exposure in native fish populations, the present study
aims at assessing the environmental risk associated to the contamination of the
Lake Victoria water body on fish health, land cover distribution, biodiversity and
the agricultural area surrounding the lake. By the elaboration of Landsat 7 TM
data of November 2002 and the Landsat 7 TM 1986, we calculated the agricul-
tural area which surrounding the bay, and the potential increase of nutrient load-
ing to the soil, which is subsequently transported to the lake by rain or as dry
fall.
Remote sensing is one of the emerging technologies that has the potential to
extend measurements over a range of spatial scales ranging. Remote sensing offers
tools to formulate and test ecological hypotheses at larger scales.
Present satellite systems provide information on the physical measurements of
the absorptance, reflectance, and emittance properties of landscapes, obtained over
a spectral interval that goes from ultraviolet to thermal infrared and microwave.
Optical Remote sensing measurements convert photons received by a sensor from
pixels (smallest resolvable surface areas) arrayed in a spatial context into voltages
that are digitised. Information about the surface is derived from the spectral char-
acteristics and their spatial patterns. These data can be used to explore ecological
properties and processes after conversion of the raw data to radiances and then to
reflectances, which can be considered a property of the surface observed. This data
INTEGRATING REMOTE SENSING APPROACH WITH POLLUTION MONITORING TOOLS 277
when confronted on a spatial or temporal basis can be used to predict states and
processes.
A fundamental property of matter is that it absorbs and emits energy at specific
wavelengths; the absorption spectrum is determined by the chemical composition
and structure of the compounds present. Many plant compounds now are identi-
fied routinely using spectroscopic assays n the laboratory (Weyer, 1985; Marten
et al., 1989), suggesting that spectral characteristics could be used to measure
biogeochemical properties at other scales. Vegetation spectra are most varied at
the level of biochemical constituents and cell structure. As spatial scales increase,
spectral variation decreases nonlinearly, with a trajectory that is not completely
understood. The changing variance results largely from the averaging of some
components, including the vegetation cover, vegetation structure and shape, soils,
and other extraneous effects, for example, atmospheric conditions, as spatial scales
increase.
Regarding biological effects related to the exposure to agricultural insecticides
such as organophosphates (OPs) and carbamates (CBs) in native and alien fish
populations of Lake Victoria, cholinesterases (ChEs) activity was investigated in
muscle tissue of two alien species Nile tilapia (Oreochronis niloticus) and the
redbelly tilapia (Tilapia zilli) and one native species the ngege (swahili) (Orechronis
variabilis).
Cholinesterases (ChEs) are a class of serine hydrolases ubiquitous in the animal
kingdom. They are widely classified in vertebrates in two homologous groups:
acetylcholinesterase (AChE, EC 3.1.1.7) and butyrylcholinesterase (BChE, EC
3.1.1.8) also called pseudocholinesterase or non-specific cholinesterase. AChE,
mainly present in brain, plays the physiological role of degradation of the transmit-
ter acetylcholine (ACh) in the cholinergic synapses and neuromuscular junctions
while BChE, mainly found in serum, contribute to ChE activity in muscle and nerve
tissue (Massouli`e et al., 1993). ChEs are strongly inhibited by exposure to neuro-
toxic compounds including organophosphate (OP) and carbamate (CB) insecticides
therefore they represent a well established and specific biomarker of insecticides
exposure in non-target terrestrial and aquatic organisms (Weiss, 1964; Fultonet al.,
2001). More recently, its field application in biomonitoring programmes on fish
species has been reported to be less specific, thus it is now emerges as a more
general marker of exposure to neurotoxic contaminants including heavy metals
and organochlorines (Gill et al., 1990a,b; Sturm et al., 1999). The sensitivities of
ChEs to insecticides exposure concentrations may differ from species to species
and may also display variations due to peculiar conditions such as place of ori-
gin. In addiction, since more than one ChE seems to be present in muscle tissue
of marine fish, a biochemical characterization of ChEs and of their sensitivity to
selective inhibitors and insecticides are needed in order to validate their use as a
biomarker.
278 S.FOCARDI ET AL.
Materials and Methods
STUDY SITE
Lake Victoria is the world’s most extensive tropical lake (ca. 69,000 km2) but is rel-
atively shallow (ca. 80 m max. depth; Crul, 1995). Fishes champion was collectedin
Jinja Bay Uganda (Figure 1; 0◦18N, 33◦20E). The bay floor dips northward to a
depth of 12 m and papyrus swamps fringe the shoreline. More than 90% of Lake
Figure 1. Site map (A) Jinja Bay. (B) Lake Victoria watershed. 1 =Pilkington Bay 2 =Damba
Channel. (C) Africa with Lake Victoria watershed dotted.
INTEGRATING REMOTE SENSING APPROACH WITH POLLUTION MONITORING TOOLS 279
Victoria’s water enters and leaves via the atmosphere with most rain falling around
March–May (long rains) and October–December (short rains) with the arrival of
the Intertropical Convergence Zone (ITCZ). Between rainy seasons, dry winds as-
sociated with the Afro-Asian monsoon system contribute to the breakdown of ther-
mal stratification (Talling, 1966). The past 50 years have brought major changes to
Lake Victoria, including eutrophication and persistent thermal stratification (Hecky,
1993). Native fishes have declined and the phytoplankton, formerly dominated by
green algae and diatoms (Aulacoseira spp.) that require strong mixing, are now
dominated by cyanobacteria and diatoms (Nitzschia spp.) that are typical of stable
water columns (Ochumba and Kibaara, 1989; Hecky, 1993).
Until the 1960s, Lake Victoria could boast a rich, well-balanced plant and animal
species complex (Greenwood, 1956). Overfishing, pollution from industrial and
agricultural sources, noxious water weeds and predatory introduced fish species
have all threatened the sustainability of Lake Victoria’s resources and, consequently,
the economies and well-being of the surrounding governments and people. Many
of the threats to Lake Victoria have been the result of increases in the surrounding
human populations (Food and Agriculture Organisation (FAO), 1990, 1996; LVFO,
1996).
In the early 1960s, the Nile perch (Lates niloticus) was introduced into Lake
Victoria in an effort to improve the declining fishery of the popular native species,
partially for sport fishing purposes and partially to boost the fishing economy. At
the time, there were heated disputes about the possible costs and benefits to such an
introduction (Fryer, 1960; Anderson, 1961). Nile perch introduction resulted in the
disappearance of many native species that were abundant in the lake. At present,
the huge haplochromines cichlid flock has been driven to near extinction and only
pockets of some species may be seen in protected bays, rocky shores and inlets
acting as refugia (Ogari and Dadzie, 1988). Scientists estimate that 200 cichlid taxa
have been lost (Barel et al., 1991). The native tilapia (Oreochromis esculentus), pre-
viously a fish of great commercial importance, has fallen to insignificant numbers in
Lake Victoria (Goldsmidt and Witte, 1992; Witte et al., 1992). It is likely that many
other biota such as aquatic insect, crustacean and plant species have been affected
by the radically altered trophic structures in the lake. At present, two exotic species
(Nile perch and Nile tilapia) and a native sardine dominate the commercial fishery.
The lake fishery that was once multi-species is now dominated by three species.
The fishery resources of Lake Victoria had high historical value as a source of
protein and employment opportunities, especially for the lakeside communities.
Nile perch is a vital foreign exchange earner for the riparian states, cherished in-
ternationally, while the other fishery resources had ready regional markets. This
demand for fish and the environmental degradation highlighted above placed con-
siderable pressure on the fisheries resources of Lake Victoria.
The traditional fish fauna and fishery of Lake Victoria was dominated by Cich-
lids. Two Tilapia species (O. esculentus and Oreochromis variabilis) plus a few other
fishes (Bagrus docmac, and Labeo victorianus) were the main stay of the fishery,
280 S.FOCARDI ET AL.
while, haplochromines, which evolved in the lake into about 300 species through ex-
plosive speciation, dominated the itchthiofauna. Many of the haplochromine species
were endemic to Lake Victoria. There were only about 50 non-cichlid fish species
in the lake, Lowe-McConnel but the traditional commercial fishery depended on
relatively few taxa. Four alien tilapiines (Oreochromis niloticus, Monochromes leu-
costictus, Oreochromis melanopleura and Tilapia zilli) and alien Nile perch were
introduced into Lake Victoria in the 1950s and early 1960s, respectively.
Some information has been collected by local and national authorities on the
scale and location of polluting industries, and there are a number of basic industries
that are common to most of the major urban areas, for example, breweries, tanning,
fish processing, agroprocessing (sugar, coffee) and abattoirs. Some of these have
implemented pollution management measures but in general the level of industrial
pollution control is low. Small scale gold mining is increasing, in Tanzania in
particular, and this is leading to some contamination of the local waterways by
mercury which is used to amalgamate and recover the gold. Some traces of other
heavy metals, such as chromium and lead, are also found in the lake, although the
problem has not yet reached major proportions.
The fish were collected along the shoreline within the ecotone area between
papyrus stand and open water using different methodologies: gill nets for the tilapia
sensu lato (Oreochromis spp. and Tilapia spp.) and electrofishing equipment for the
haplochromines species. Each specimens collected was then preserved in ethanol
solution (70%).
Remote Sensing Analysis
In order to characterise the of Jinja bay ecosystem, satellite based measurements of
reflected and emitted electromagnetic radiation in the visible (400–700 nm), near
infrared (700–1100 nm) and thermal infrared (10000–13000 nm) wavebands were
used to examine spatial and temporal trends in the environment. The availabil-
ity of a historic series of data permits the analysis of environmental changes due
to natural or induced factors. As satellite data is obtained nearly instantaneously
over a wide area, the study of the spatial distribution of specific territorial char-
acteristics is facilitated. The availability of information on the emission in several
wavebands from the surface of a territory gives valuable information about the
conditions and cover of these areas. The use of high or medium resolution data
permits the creation of specific indices that can be used to make intra-territorial
comparisons and decisions. Landsat Thematic Mapper (TM) images were used
to map the cover of Jinja area. TM data have advantages over the other sensors
that TM records an additional infrared channel at 1.55–1.75μm. This is impor-
tant for discriminating different vegetation types (Fuller et al., 1989; Townshend,
1992).
Image interpretation: Vegetation is the dominant and important component in
most ecosystems and useful indicator environmental conditions. Many remote
sensing mechanisms operate in the green, red and near infrared regions of the
INTEGRATING REMOTE SENSING APPROACH WITH POLLUTION MONITORING TOOLS 281
Figure 2. In this figure a comparison between the band 3 and the band 4 of the Landsat 7 ETM image
of November 2002. Black - and - white images represent light intensity variations for a single band.
Band 3 ( red) is strongly absorbed by active vegetation, whereas band 4 (near infrared) is strongly
reflected. Because of this vegetated areas are bright in the band 4 image, and dark in the band 3 image
(Aber, 2000). On left side in the band 3 image the light area is the modified area and the wetland are
light too. Permanent and temporary waterbodies are black. Light areas refersto population centers
and roadways. The band 4 image shows waterbodies in black, wetlands in dark grey and agricultural
areas in ligh and dark grey, depending upon irrigate regime.
electromagnetic spectrum. They can discriminate radiation absorption and re-
flectance of vegetation. Changes in vegetation are useful for recognizing changes
in other environmental factors. Identifying vegetation in remote sensing images
depends on plant characteristics: leaf shape and size, overall plant shape, water
content, and associated background (e.g. soil types and spacing of the plants).For
interpretation is used single band images (bands 3, and 4) and bands compositions
are particulary useful (Figure 2).
The studied area is the Jinja Bay, in the northwest Ugandan part of the Lake
(Figure 1). Around the bay there is extensive area with agricultural production.
From this area there is a series of streams which cross the crops and flow into
the Lake.
We elaborated a technique of false color composition and unsupervised
(Figure 2) classification the Landsat 7 TM image of November 2002 and November
1986. These techniques have shown that from 1986 to 2002 the land cover around
the studied bay has changed.
Biological Effect of Insecticides Exposure: ChE In Vitro Test
ChEs were extracted from crude homogenates of portion of dorsal muscle accord-
ing the colorimetric method of Ellman et al. (1961). Dried tissue was homog-
enized in 0.1 mg/ml of 20 mM Tris-HCl, 5 mM MgCl2, 0,1 mg/ml Bacitracin,
8×10−3TIU/ml Aprotinin, 1% Triton X−100 and centrifuged at 9,000g for
20 min recovering the supernatant. ChE activity was assayed at 30◦C using
282 S.FOCARDI ET AL.
TABLE I
IC50 calculate to the inhibition curve of ChE activity (nmol min−1mg
protein−1) versus ASCh (1 M) in in vitro exposure to insecticides at
range of 10−9to 10−3M
Nile tilapia Redbelly tilapia Ngege
OP insecticides (O. niloticus)(Tilapia zilli)(O. variabilis)
DFP 1,E-05 1,E-05 1,E-06
Fenitrothion 1,E-03 1,E-03 1,E-05
Chlorpyriphos 1,E-04 1,E-04 1,E-04
1 mM acetylthiocholine iodide (ASCh) and butyrilthiocholine iodide (BSCh) as
substrates. Selective ChE inhibitors [tetra(monoisopropyl)pyrophosphor-tetramide
(iso-OMPA) for BChE and 1,5-bis (4-allyldimethylammoniumphenyl)-penthan-3-
one dibromide (BW284c51) for AChE] were tested at a fixed concentration of 3 mM
while three OP insecticides [diisopropyl fluorophosphate (DFP), fenitrothion and
chlorpyriphos], were tested at 10−9to 10−3M and the resulting IC50 calculated.
Both inhibitors and insecticides were tested by incubating 15 min crude muscle
homogenates with them and then adding substrate ASCh (1 mM) and DTNB. Re-
action was read in 5 min (linearity) at 30 ◦C in a 550 Model microplate reader and
ChE versus ASCh activity expressed as nmol minutes−1mg proteins−1. Total pro-
tein content of crude homogenates was measured by the method of Bradford et al.
(1976) in a Shimadzu UV-Visible recording spectrophotometer λ595 nm, Biorad
Protein and bovine serum albumin (BSA), as reference standard, were used. In the
Table I are expressed the results of the CHe in vitro test.
Heavy Metals Analysis
Muscle and liver tissue of fishes were freeze dried (lyophilisator LIO 5 PASCAL
220 V with Vacuum Pump RV8) before analysis. Samples of about 0.15 g of both
materials were digested with 2.5 ml nitric acid in PTFE digestion bombs at 120 ◦C
for 9 h (Jackwerth and Wurfels, 1997). Before mineralization, known amounts of the
elements analysed was added to 0.15 g of each specimen type to obtain an internal
standard from which a standard calibration curve is derived. The digested material
was analysed for Cd and Pb by electrothermal atomic absorption spectrometry with
Zeeman background correction (Perkin Elmer ZETAAS), for Zn, Cu and P by in-
ductively coupled plasma emission spectrometry (Perkin Elmer ICP-AES) and for
Hg by flow injection mercury system (Perkin Elmer FIMS 400). In order to prevent
contamination, during every digestion cycle more that one blank analysis was per-
formed by introducing only the reagents in the Teflon vessels. The accuracy of the
results was verified by analysing Standard Reference Material DORM-2 (dogfish
muscle), obtained from the National Research Council Canada, Institute for Envi-
ronmental Chemistry, Ottawa, Canada. Uncertainty related to sample homogeneity,
digestion and analysis was assessed by replicate determination of samples and was
INTEGRATING REMOTE SENSING APPROACH WITH POLLUTION MONITORING TOOLS 283
TABLE II
Trace element values are expressed as μg/g dw
Sample Hg Cd Pb Zn P Cu
O. variabilis Fillet <0.005 <0.01 <0.05 24.648 8373 <1
O. variabilis Fillet 0.0126 <0.01 <0.05 31.637 4587 1.0619
Nile tilapia (O. niloticus) Fillet 0.0335 <0.01 <0.1846 48.173 86190 1.6244
Nile tilapia (O. niloticus) Fillet 0.0526 <0.01 0.0962 40.794 11958 1.4286
Redbelly tilapia (Tilapia zilli) 0.1204 <0.01 0.2031 78.603 17123 1.8436
found to be below 10%. Data are expressed as μgg
−1dry weight. In the Table II
are expressed the concentration of the metals in the issiue of the fishes.
Results and Discussion
From the remote sensing analysis we can see that the land use around Lake Victoria
has changed in the last 10–15 years. Many of the natural shrub areas presents in
1986 have been replaced by intensive agricultural area (Figure 3). Wetlands also
appears to have decreased.
The agriculture area is located at higher elevation with respect to the Lake.
Therefore channels crossing the bring sediments and nutrients into Jinja Bay carring
along with pesticides and heavy metals.
Figure 3. False colour composition. In this figure the two Landsat images respectively in the band
4,3,2 are shown. In the picture A, the around Jinja Bay in 1986 are shown where the uniform red color
shows natural areas while the green colour shows agriculture crops. In the figure B, the same area is
shown in 2002, showing an increase in agricultural areas.
284 S.FOCARDI ET AL.
Both satellite images were elaborated using a unsupervised classification pro-
cedure to extract the land cover information. The principals crops grown in the
watershed include maize, cotton, sisal, tobacco, beans, sugarcane, and coffee. Most
of these are intensive crops that require significant uses of herbicides and agricul-
tural chemicals.As there is a significant pressure to increase export related products,
the continue expansion of crops is likely.
Regarding the biological effects on insecticide exposure on fish populations, ChE
activities were investigated and the following results were found. Crude muscle ho-
mogenates of the three species showed similar ChE activities versus ASCh as sub-
strate and comparable higher selectivity to ASCh as substrate compared to BSCh.
An apparent lower selectivity was observed for the native species O. variabilis com-
pared to both alien Nile tilapia and redbelly tilapia. Both tilapia species showed
comparable hydrolysis of ASCh and no substrate inhibition at concentration >of
1 mM.
Regarding sensitivity to specific ChE inhibitors, significant inhibition with
both Iso-OMPA (against BChE) and BW248 c51 (against AChE) was observed
in all three species (from 40 to 70%). These findings are consistent with the well
established views that there are only two types of ChE in vertebrates including fish
(AChE and BChE) in addition to which an atypical ChE cleaving BChE substrates
and also sensitive to BW284c51 seems to be present in fish muscle tissue (Sturm
et al., 1999). Similar data resulted in agreement with previous observations reported
by Rodriguez-Fuentes et al. (2004) in muscle tissue of the Nile tilapia (O. niloticus)
from Mexico and with previous investigation in other fish species (Var`oet al., 2003).
No substrate inhibition was in fact reported supporting the hypothesis that the ChE
present in muscle tissue (as well as in liver) might have properties that resemble a
butyrylcholinesterase (BChE, EC 3.1.1.8) due to the high affinity to BSCh.
The in vitro exposure to OP insecticides DFP, fenitrothion and chlorpyriphos
showed a slight different sensitivity in the native species compared to tilapias
(Table I). IC50 values suggest that ChE activities (versus ASCh) of both tilapia
species (Nile tilapia and redbelly tilapia) might be more resistant to OPs than ChE
activities of native O. variabilis which resulted more inhibited even at the lowest
concentrations (10−7–10−6M) (Table I).
Conclusion
Environmental impacts to the lake Victoria ecosystem includes enhanced siltation,
nutrient enrichment and pollution loading due to destruction of the buffering capac-
ity of the wetlands; loss of biodiversity, and other lake resources; and degradation
of fish habitats and fish stocks.
The studied areas of the rivers feeding the lake and the shoreline is particu-
larly polluted by municipal and industrial discharges. Some information has been
collected by local and national authorities on the scale and location of polluting
INTEGRATING REMOTE SENSING APPROACH WITH POLLUTION MONITORING TOOLS 285
industries, and there are a number of basic industries that are common to most of
the major urban areas, for example, breweries, tanning, fish processing and agro-
processing (sugar, coffee). Some of these have implemented pollution management
measures but in general the level of industrial pollution control is low. Small scale
gold mining is increasing, in Tanzania in particular, and this is leading to contamina-
tion of the local waterways by mercury which is used to amalgamate and recover the
gold. Some traces of other heavy metals, such as chromium and lead, are also found
in the lake, although the problem has not yet reached major proportions. The use
of fertilizers and animal manure has increased tremendously, causing accelerated
eutrophication of Lake Victoria waters.
Lake Victoria has undergone a dramatic change associated with excessive eu-
trophication causing its bottom waters, which were previously oxygenated by wind-
inducing mixing, to remain stratified and anoxic year-round (Lowe-McConnell,
1997). The use of agrochemicals is gaining momentum in the lake basin even
among the small-scale producers, particularly in Kenya and Tanzania where there
are large-scale farms of coffee, tea, cotton, rice and maize.
ChE activities in term of sensitivity to substrate and inhibitors resulted in agree-
ment with previous investigation on Nile tilapia and in general with other fish
species of both Atlantic and Mediterranean ecosystems. Sensitivity to insecticides
exposure reveal a similar response to that observed in other fish species even the
high sensitivity of the native species ngege (O. variabialis) compared to the alien
tilapia suggest the need for further investigation for conservation of the native
species biodiversity in the lake.
Further studies on target insecticides tissue such as brain and in vivo exposure to
pesticides readily used in the agricultural area are needed a toxicological risk for fish
populations can be determined in relation to the increasing impact of agriculture in
the area surrounding the lake. Moreover, the lack of accurate data on fertilizer and
pesticide usage continues to hinder decision-making about appropriate measures
of control. New environmental impact assessment measures being put in place by
the national environment authorities of the three countries bordering the lake may
help clarify many of the issues related to data and monitoring. In conclusion ,the
integrated approach based on ecotoxicology analysis and remote sensing analysis
applied in the present study clearly revealed that conversion and unsustainable
management of catchment areas of Lake Victoria can have a negative impact on
resource quality and in particular fish.
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