International Journal of Salt Lake Research 8: 253-266, 1999. �9 1999 Kluwer Academic Publishers. Printed in the Netherlands.

Organochlorine pesticide and metal residues in a cichlid fish, Tilapia, Sarotherodon (=Tilapia) alcalicus grahami Boulenger from Lake Nakuru, Kenya

J.K. KAIRU Department of Wildlife Management, Moi University, P.O. Box 1125, Eldoret, Kenya

Abstract . 29 specimens of a cichlid fish Sarotherodon (=Tilapia) alcalicus grahami were collected from Lake Nakuru between September and October 1990 and samples of liver, kidney, muscle, brain and fat were removed for analysis of organochlorine pesticide and metal residues. Fat was extracted and the concentration of three lindane (BHC/HCH) isomers (alpha, beta and gamma), aldrin, heptachlor, heptachlor-epoxide, endrin, dieldrin, DDD, DDE and DDT was determined. Fish muscle samples were digested and the concentration of mercury, arsenic and cadmium was also determined.

No residues of o,pl-DDD, p,pl-DDD, aldrin, endrin and dieldrin were detected. The highest residue concentration detected was 0.062 mg kg -1 of p,pl_DDT. The mean pesti- cide residue concentration levels were generally low. The [p,pl-DDT]/[p,pl-DDE] ratio of 1.22 indicated that the residues of the parent DDT compound exist in the Lake Nakuru ecosystem. There was a negative fish length: DDE concentration relationship. Similarly, there was a very weak negative relationship between arsenic concentration and length of fish. The concentrations of metal residues were considerably low. The median arsenic and cadmium concentrations were 0.03 mg kg -1 and < 0.1 mg kg -1 , respectively. The concentrations of mercury, with a median level of < 0.01 mg kg -1 , were particulary low and did not approach a level that would give rise to public health concern. Metal and pesticide residue concentration in fish in 1970 and 1990 does not show a significant increase. From these results, Lake Nakuru is presently not exposed to heavy pesticide and metal contamination, but there is a gradual build-up of these residues in the biota.

Key words: cichlid fish, metals, pesticides, Lake Nakuru, Tilapia

Introduction

Increasing expansion of agricultural and industrial activities without proper environmental safeguards can have a particularly severe impact on the envi- ronment. The increase in industrial and agricultural activities has hastened the mobilization of many metals into the environment (UNEE 1980). Chemicals are among the chief pollutants of the environment and unless their effects are carefully monitored and their use carefully controlled, they can result

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in serious environmental and adverse health effects (World Commission on Environment and Development (WCED), 1987). Short-term research on the effect of chemicals is mainly undertaken after a catastrophe, and ironically without plans for long-term monitoring work.

Lake Nakuru is a shallow, endorheic and hypereutrophic lake. The lack of an outlet endangers the lake from a buildup of industrial effluent, agro- chemical residues and sewage. Prior to 1961, there was no fish in Lake Nakuru. In 1961, a cichlid fish, Sarotherodon (=Tilapia) alcalicus graharni, was introduced into Lake Nakuru from Lake Magadi (another saline-alkaline lake, where it was endemic), The tilapia is a small-sized mouthbreeding fish. It has an average size of 10 cm and a maximum of 19 cm. During favourable conditions, the fish is very prolific and attains a very high density in the lake, which supports more than 30 species of fish-eating birds. The density of the fish in Lake Nakuru decreases as one moves offshore while the individual size increases (Vareschi, 1979).

Tilapia is a filter-feeder, feeding principally on insect larvae and phyto- plankton (Vareschi, 1979). However, benthic invertebrates contribute signi- ficantly to its diet. These food sources bioconcentrate pollutants from the aquatic medium, sediments and detritus. Lake Nakuru is relatively shal- low, with a mean depth of < 1 m (Koeman et al., 1972), and the highly mobile tilapia is in frequent contact with the suspended food material (e.g. algae), bottom sediments and detritusl Lake Nakuru usually exhibits a daily rhythm of stratification and circulation (Vareschi and Jacobs, 1985), a char- acteristic that certainly fosters the recycling of pesticides and metals from sediments. Higher pesticide residue concentration has been reported on windy days, perhaps due to mixing, in shallow water-bodies (Mauck et al., 1976). Organochlorine pesticides have a tendency to be associated with sediments (Arrunda et al., 1988), especially DDT, since it is less water soluble (Locker- bie and Clair, 1988). Suspended material is known to be a good scavenger for heavy metals (Sims and Presley, 1976). Blue-green algae (Spirulina platensis Nordst.), one of the food sources of the tilapia in Lake Nakuru, is a highly prolific primary producer (Vareschi and Jacobs, 1985) and this fast growth may lead to quick uptake of heavy metals and pesticides from the lake water.

The unexplained mass death of fish in Lake Nakuru in 1971 (Vareschi, 1979) and in 1991 (unpublished observations) has created concern amongst wildlife conservationists and park visitors. It was estimated from fish washed onto the lake's shoreline that a half million fish died in 1991. Analysis of dead fish in 1991 showed elevated levels of malathion and lead in the fish tissues (unpublished data). The possibility of these occurrences being related to pollution has been alleged given to increasing agricultural activities in the

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catchment and the lake's close proximity to a large and fast growing industrial town of Nakuru. This alleged pollution of the lake waters and of the organ- isms that feed on the lake was first investigated by Koeman et al. (1972). However, results from investigations by Greichus et al. (1978) in samples collected in 1975 did not show any significant increase in pesticide and metal residue concentrations in the lake. The present study was done to deter- mine the present levels of pesticide and metal residues in tilapia and to test whether there have been changes since Koeman et al. (1972) reported their findings.

Study area

Lake Nakuru lies at 0~ S, 36005 ' E, and at an altitude of 1759m above sea level (Vareschi, 1978). The mean temperature in the area is 18 ~ while the mean rainfall is 876 mm yr -1 (Nimira, 1976). The lake's geographical posi- tion in Kenya, land-use in its catchment and the major physical and chemical parameters are reported in an earlier publication (Kairu, 1994). The lake has an exceptionally high productivity and it supports one of the highest bird populations in the world. The catchment basin of the lake has a high agricul- tural potential and is subjected to varying applications of biocides (Thampy, 1995). A survey showed that 56% of small-scale farmers had intentions of making greater use of agro-chemicals in the future (Wachira, 1991). Industrial effluent entering the lake from the nearby town of Nakuru is on the increase and is only partially treated (Ndetei, 1995).

Materials and methods

Sample collection

Between September and October 1990, 29 tilapia fish specimens were caught by gill nets from three different points of Lake Nakuru. The fish were imme- diately placed in cooler boxes prior to their delivery to the laboratory. Using a digital balance, the fresh weight of the fish specimens was taken not later than four hours after trapping. Lateral muscle samples from all fish, and liver, fat and brain samples from some fish were removed. Using a scalpel and pincers, fat samples were carefully taken from a thin layer of fat tissue covering the intestines. All the tissues were collected in duplicate, one set for metal analysis and the other for pesticide analysis. Samples for metal and pesticide analysis were collected in plastic and universal (glass) bottles, respectively. Immediately after dissection, all the samples were kept in well

256

sealed and carefully labelled bottles and preserved in a deep freezer at - 20 ~ The samples were later transported to the College of Agriculture and Veterinary Medicine, Kabete, Kenya, and stored in a deep freezer at -20 ~ until analysis.

Samples for metal analysis were sent to the Norwegian College of Veterinary Medicine, Norway, in February 1991. To avoid putrefaction on transit, the samples were packed in dry ice at -79 ~ They were transferred immediately on arrival in Norway to a deep freezer at -20 ~

Samples for analysis were removed from the freezer and allowed to stand for one hour under room temperature to thaw. The samples were weighed and this was taken to represent the wet weight of sample.

Residue analysis and extraction

(i) Pesticide residues Only 24 samples were analyzed for organochlorine pesticide residues: 19 muscle samples, 3 pooled liver samples, 1 pooled fat sample and 1 pooled brain sample. Analysis of samples for organochlorine pesticide residues was done following Kairu (1994), in which each sample was subjected to the same treatment of taking fresh weight of samples, extraction, clean-up, injection into a gas liquid chromatograph (GLC) with an electron capture detector (ECD) and quantification.

(ii) Metal residues A total of 61 muscle samples were analyzed for metal content: 20 for cadmium, 20 for arsenic and 21 for mercury. All muscle samples were digested in a mixture of nitric and perchloric acid (Norheim, 1989). Cadmium concentrations were determined by flame atomic absorption spectroscopy. The detection limit for cadmium was 0.1 mg kg -1. Mercury and arsenic concentrations were determined by hydride generator atomic absorption spec- troscopy (Haugen et al., 1985). The detection limits for these two elements was 0.10 mg kg -1.

Calculations

(i) Sum-DDT was calculated as p,pl-DDT + 1.1 I(p,pl-DDE + p,pl-DDD). The factor 1.11 is used to correct for the lower molecular weight (Skaare et al., 1985).

(ii) Linear regression and correlation relationships were used to determine the variation of residues with the weight and length of fish.

Table 1. Mean pesticide residue levels (mg kg-1; wet weight) in tissues of Tilapia Sarotherodon (=Tilapia) alcalicus grahami from Lake Nakuru, Kenya (N = total samples analyzed; n = number of samples positive for the pesticide; nc = range not calculable, either only one sample was analyzed or none of the samples was positive for the pesticide)

257

Pesticide Parameter Muscle Fat Brain Liver

residue

alpha-BHC mean 0.010 0.002 0.055 0.046

range 0-0.017 nc nc 0-0.046

N 19 1 1 3

n 16 1 1 1

gamma-BHC mean 0.010 0.008 0 0

range 0--0.015 nc nc nc

N 19 1 1 3

n 2 1 0 0

beta-BHC mean 0.002 0 0 0

range 0-0.003 nc nc nc

N 19 1 1 3

n 3 0 0 0

heptachlor mean 0.007 0 0 0

range 0-0.014 nc nc nc

N 19 1 1 3

n 9 0 0 0

heptachlor- mean 0.004 0.051 0 0

epoxide range 0-0.004 nc nc nc

N 19 1 1 3

n 1 1 0 0

p,pl-DDE mean 0.002 0.051 0.001 0.011

range 0-0.004 nc nc 0.005-0.022

N 19 1 1 3

n 16 1 1 3

o,pl-DDT mean 0.027 0 0 0

range 04).027 nc nc nc

N 19 1 1 3

n 1 0 0 0

p,pl-DDT mean 0 0.062 0 0

range nc nc nc nc

N 19 1 1 3

n 0 1 0 0

sum-DDT mean 0.004 0.119 0.001 0.0121

range 0-0.029 nc nc 0.006-0.024

258

I0') v .

0') g 0

e -

t - O

L) uJ n a

0.0045 "

0.0030 - " ~ * - * . . . . . �9 �9 (x2)

0.0015 -

�9 �9 �9 ~'"'-~ �9 (x3)

0.0000 ', ', '. : ~ m, ,, ~ 8.0 8.8 9.6 10.4 11.2 12.0

L e n g t h ( c m )

Figure 1. The relationship between the concentration of DDE (mg kg-1) in tilapia muscle and length (cm) of tilapia; (r = -0.675, N = 19, P = 0.002, the slope is significantly different from zero).

(iii) To determine whether the slopes of the regression lines were significantly different from zero, the null hypothesis Ho:B1 = 0 was tested using a t-test (Johnson and Bhattacharyya 1987).

R e s u l t s

In September-October 1990, most of the fish trapped had little or no fat. Consequently only one pooled fat sample from three fish was analyzed for organochlorine pesticide residues.

Eight organochlorine pesticide residues were detected (Table 1). The most prevalent compounds were p,pl-DDE and alpha-BHC, each found in 79% (19 out of 24) of the samples. The other pesticide residues were detected in fewer than 15% of samples except heptachlor (38%; Table 1). No residues of o,p 1-DDD, p,pl-DDD, aldrin, endrin and dieldrin were detected. A composite fat sample had the highest sum-DDT level of 0.119 mg kg -1 .

There was a high positive correlation (r = 0.913) between length and weight of fish. The length (r = -0.675, P = 0.002) and weight (r = -0.609, P = 0.006) of tilapia had a significant negative correlation with concentration of p,pl_DDE. Older fish had lower levels of p,pI_DDE. The regression equa- tion of length of tilapia against concentration of p,pl-DDE is: Y = 0.011 -

259

1:3)

E v

0.048"

E Q 0.032" 0 0

E

0.016 <

e(x2) �9

" " ' = = ' = ' = ' - - = . . . . = . . . . . . = . . . = . .

�9 �9 �9 . . . . . . . . . . . . . . . . . 4k., �9 (x2)

�9 �9 �9 �9 (x2) �9

I I I I I I 9.0 9.6 10.2 10.8 11.4 12.0

Length (crn)

Figure 2. The relationship between the concentration of arsenic (mg k g - 1) in tilapia muscle and length (cm) of tilapia; (r = -0.16, N = 20; P = 0.502, the slope is not significantly different from zero).

0.00085X1 (where Y = concentration of DDE; X] = length of tilapia). The relationship between concentration of DDE and length of tilapia is shown in Figure 1.

The ratio of [p,pl-DDT]/[p,p]-DDE] was 1.22 (see Table 1). The ratio is calculated from the DDT and DDE residues in fat only as there were no residues of p,pI_DDT detected in muscle, liver and brain samples.

The concentrations of three metal residues in fish muscle are presented in Table 2. The concentration of cadmium in all 20 samples analyzed was below the detection limit which varied between 0.1 mg kg -a and 0.2 mg kg -1. The range for the concentration of mercury and arsenic was < 0.01 - 0.01 mg kg -1 and 0.01 - 0.06 mg kg -1, respectively. Only 4 samples out of the 20 analyzed for mercury had concentrations at the detection limit of 0.01 mg kg -1. The median concentrations of arsenic, mercury and cadmium were 0.03 mg kg -1, < 0.01 mg kg -1 and < 0.1 mg kg -1 , respectively.

A negative correlation, which was not significant (P = 0.502), was found between fish length (r = -0.16) and arsenic concentration. The regression equation of length of tilapia against concentration of arsenic is: Y = 0.057 - 0.0024X1 (where Y = concentration of arsenic; X1 = length of tilapia). The relationship between concentration of arsenic and the length of tilapia is shown in Figure 2.

A comparison of pesticide and metal residue concentrations in fish in 1970 and this study is shown in Table 3. There was a slight increase in concentra-

260

Table 2. Size of fish and concentrations (rag k g - 1 ; wet weight) of

metal residues in muscles of tilapia f rom Lake Nakuru, Kenya

Specimen Length Weight Cadmium Arsenic Mercury

number (cm) (g)

1 9.50 18 < 0.1 0.02 < 0.01

2 9.75 16 na a 0.04 na a

3 8.75 15 na a na a < 0.01

4 10.50 18 < 0.2 na a na a

5 10.00 17 na a 0.06 0.01

6 8.00 14 < 0.1 na a na a

7 9.50 15 < 0.2 na a < 0.01

8 10.50 19 na a 0.04 na a

9 12.00 23 < 0.2 na a < 0.01

10 10.00 18 na a 0.06 na a

11 10.00 15 < 0.2 na a < 0.01

12 12.00 25 na a 0.03 na a

13 12.00 25 na a na a < 0.01

14 12.00 27 na a 0.02 < 0.01

15 11.50 25 < 0.1 0.02 0.01

16 10.50 20 < 0.1 0.06 < 0.01

17 10.00 15 < 0.1 0.03 < 0.01

18 11.00 25 < 0.1 0.02 < 0.01

19 11.50 20 < 0.1 0.02 0.01

20 10.00 16 < 0.1 0.01 < 0.01

21 9.00 18 < 0.1 0.03 < 0.01

22 11.50 22 < 0.1 0.04 < 0.01

23 11.00 20 < 0.1 0.02 0.01

24 9.00 15 < 0.1 0.02 < 0.01

25 9.75 17 na a 0.03 < 0.01

26 11.00 21 < 0.1 na a na a

27 10.00 18 < 0.1 0.04 < 0,01

28 9.75 20 < 0.1 na a na a

29 12.00 18 < 0.1 0.03 < 0.01

aNot analyzed.

Table3. Comparison of mean pesticide and median metal residue (mg k g - 1 ; wet weight) levels in 1970 a and 1990 b in tilapia from Lake Nakuru, Kenya

261

1970 a 1990 b

Pesticide/metal residue Total body Muscle

alpha-BHC < 0.002 0.0095

gamma-BHC 0.0016 0.0095

beta-BHC < 0.007 0.0023

dieldrin < 0.0018 0.0 c

aldrin < 0.002 0.0 c

p,pl_DDE 0.0016 0.0021

mercury 0.016 < 0.01

cadmium 0.0 c < 0.001

arsenic 0.086 0.030

aKoeman et al. (1972). bpresent data. CNon-detectable.

tions of alpha-BHC, gamma-BHC, beta-BHC and p,pl-DDE pesticides in 1990 as compared to 1970, while aldrin and dieldrin which were detected in 1970 were not detectable in 1990. Mercury and arsenic concentrations showed a decrease in 1990 and cadmium, which was not detected in 1970, was detectable in 1990 at very low levels.

Discussion

Only alpha-BHC and p,pl-DDE were detected in all the four tissues analyzed. However, the detection of eight organochlorine pesticide residues is evidence that these pesticides are possibly in use in the catchment of Lake Nakuru. The detection of the parent DDT compound is noteworthy in view of the fact that DDT has been banned for agricultural use in the country, except in mosquito breeding grounds (Pest Control Products Board, 1986). If in reality the restriction and/or ban on the use of DDT in the area is effective, the pres- ence of DDT and DDE indicates the high persistence of these compounds in the environment. The persistence of DDT in Lake Nakuru area has, however, been shown to be relatively short, with a half life of 120-200 days (Wandiga, 1986). The higher occurrence of DDE instead of DDT is not surprising, as its incorporation into fish tissues usually involves altering the form of the residue (Brooks, 1974). The highest mean pesticide residue concentration of 0.062 mg kg -1 of p,p~-DDT, found in a fat sample, was generally low. It was

262

lower than a mean level of 0.15 mg kg-1 of p,pl_DDT found in fish from Lake Victoria (Mitema and Gitau, 1990). The mean residue concentration levels of sum-DDT (0.004 mg kg -1) and heptachlor (0.007 mg kg -1) were far below the United States Food and Drug Administration (FDA) (1982) action limits of 5.0 mg kg -1 and 0.03 mg kg -1, respectively.

The non-detection of aldrin, endrin and dieldrin was not unexpected since these residues were also detected very infrequently in piscivorous secondary consumers, white pelican Pelecanus onocrotalus and white-necked cormorant Phalacrocorax carbo, collected from the same lake (Kairu, 1994). It is possible that either the piscivorus birds pick up these residues elsewhere during their migration to other lakes or they could have bioaccummulated gradually with time. The non-detection of o,p I-DDD was unexpected as the parent DDT was detected in a fat sample. The detection of the parent p,pl_ DDT only in a fat sample, and the detection of p,pI_DDE in muscle, fat, brain and liver samples may possibly confirm that the latter compound is more stable and persistent than the former.

The negative correlation between length/weight (and therefore age) of fish and DDE concentration probably means that young tilapia may be susceptible to pesticide stress but with time develop physiological strategies to ease it. As reported by Murty (1986), it has been found experimentally that the young of some fish species may bioaccumulate more contaminants than the adult. The negative correlation is also in agreement with results of Lockerbie and Clair (1988) who found no correlation between fish size (and thus age), sex or weight versus contaminant levels. In contrast, however, a study by Reinert (1970) found that the concentration of DDT and dieldrin increased with the age of lake trout and walleye in the Great Lakes.

Organochlorine pesticide residues are lipophilic and the higher content of sum-DDT in fat tissue relative to other tissues was not unexpected. Except in a fat sample, sum-DDT in the other tissues was comprised only of DDE. This correlation suggests that where funds for analysis are really limited, DDE should be the compound mainly considered for analysis in the DDT group of organochlorine pesticides.

For purposes of comparison, the pesticide residues in the fish collected in 1970 and this study are given in Table 3. It is usually difficult to make comparison due to differences in methods of analysis, presentation of results, sample sizes and tissue samples analyzed. Whereas the data may not be strictly comparable, the 1970 levels and those of the present study were both based on the wet weight of samples, thereby making comparison reasonably valid. It is assumed, therefore, that the comparison give an indication of the difference or similarity in the residue levels.

263

Koeman et al. (1972) observed nearly similar level of mean p,pl-DDE residues to that reported in this this study (0.0016 against 0.002 mg kg-1). Koeman et al. (1972) analyzed the total body of 5-10 fish as opposed to the lateral muscle samples from individual fish analyzed in this study. Their results on total body DDE concentrations were lower than of the lateral muscle from results of this study. Kent and Johnson (1979) reported that residues in the whole body of fish are usually greater than the concentration in the edible part. The greater DDE concentration in the fish muscle in this study, therefore, suggests that the fish were more exposed to higher DDE concentrations in the lake in 1990 than in 1970. The increases in the pesticide levels were not substantial, suggesting that their increase is rather gradual. Although low levels of DDD, dieldrin and aldrin were detected in 1970, none of these was detected in 1990. This may mean that their use in the catchment has diminished.

None of the fish samples analyzed had mercury levels exceeding the widely accepted tolerance limit of 1.0 mg kg -1 . The median mercury concen- tration of < 0.01 mg kg -1 was particulary low and did not approach a level which would give rise to public health concern.

Arsenic concentrations in 1990 and 1970 are comparable, with the 1990 levels slightly lower (0.03 mg kg -1 against 0.086 mg kg-1). In 1990, the concentration of mercury was much lower than in 1970 (< 0.01 mg kg -1 against 0.016 mg kg-1). Since the concentrations of mercury in 1970 came from analysis of the total body of 5-10 fish, the higher levels of arsenic and mercury are not surprising, and possibly lower levels would have been detected if muscles from single fish specimens had been analyzed. The concentration of cadmium was below detection limit in 1970 and in 1990, showing no appreciable increase in cadmium levels in tilapia over two decades. The general lack of appreciable change in metal concentration for two decades possibly indicates that the concentrations in the organisms studied represent natural background levels.

The lack of relationship between the concentration of arsenic and the length and weight of fish is contrary to the expectation that persistent metal concentrations increase with increasing age. It is well established that, in many species of fish, mercury concentration increases with increasing body weight (Koirtyohann et al., 1974), length (Friberg and Vostal, 1972) or age (Bache et al., 1971). Cadmium is also known to increase with age (Norheim, 1987). Arsenic, unlike mercury and cadmium, has a weak tendency to bioac- cumulate with age and does so only in some species. The low correlation values relating arsenic levels in fish muscle to the weight and length of fish may be attributed to the weak tendency of arsenic to bioaccumulate with

264

age, small sample and size range, as well as individual variation in the fish examined in this study.

Metal concentrations (wet weight) in tilapia muscle from Lake Nakuru are comparatively lower than in muscle from fish caught in the North Sea coast, United Kingdom and Swedish waters in 1985 (UNEP, 1989). The median cadmium concentration in tilapia muscle (< 0.1 mg kg -1) was much lower than in plaice (0.06 -0 .14 mg kg-1), cod (0.06 - 0.14 mg kg -1) and flounder (0.36 - 0.89 mg kg -1) from the North Sea coast. The median concentration of mercury in tilapia (< 0.01 mg kg -1) was comparatively much lower than in cod (0.07 - 0.27 mg kg-1), plaice (0.04 - 0.15 mg kg -1) and flounder (0.08 - 0.20 mg kg -1) from the North Sea coast and the United Kingdom. The concentration of mercury in herring from Swedish waters ranged from 0.017 - 0.037 mg kg -1 .

It can be concluded that the pesticide and metal residue levels in the fish studied are low and considerably lower than the levels reported to cause chronic effects in fish. There was no evidence to show that the residues had any deleterious effects on the health of the fish. The presence and possible build-up of alpha-BHC and sum-DDT does not, however, augur well for the future. Studies (e.g. Pratap, 1989) have shown that sublethal stress of different types of pollutants resulted in transient disturbances in osmo-ionic regula- tion, significantly elevated plasma cortisols and glucose levels and alteration of gill structure in African freshwater tilapia Oreochromis (Sarotherodon) mossambicus. The toxicological effects of the residue levels recorded in this study are difficult to assess without controlled experimentation studies on physiological performance of the species.

Acknowledgements

This study was carried out with the financial assistance from NORAD. I wish to thank the Director, Kenya Wildlife Service, for permission to collect fish for this work. I also thank the late Dr Carl Berg for his invaluable advice during the initial stages of this project. My sincere thanks to Dr J.U. Skaare and Dr Tore Sivertsen, Veterinary Institute, Norwegian College of Veterinary Medicine, for without their able and dedicated guidance this work would have been impossible. Special thanks to Ame Godal, Gunhild Sand and Hanne Line Daae, of Veterinary Institute, Norwegian College of Veterinary Medicine, and EK, Gitan, College of Agriculture and Veterinary Medicine, Kabete, Kenya, for assisting in the laboratory analysis of the samples. My field work benefitted greatly from assistance given by Dr L.W. Kanja, College of Agriculture and Veterinary Medicine, Kabete. D. W. Mugwe, Provincial

265

Wild l i fe Officer, and A. M. Kisee, Senior Warden, Lake Nakuru Nat iona l Park.

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