Influence of two organochloride pesticides, Thiodan and Lindane on survival of fingerlings of Oreochromis niloticus (Linnaeus) and Tilapia zillii (Gervais).
Richard Musanhi Gurure
AFRICAN REGIONAL AQUACULTURE CENTRE, PORT HARCOURT, NIGERIA
CENTRE REGIONAL AFRICAN D'AQUACULTURE, PORT HARCOURT, NIGERIA
UNITED NATIONS DEVELOPMENT PROGRAMME
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
NIGERIAN INSTITUTE FOR OCEANOGRAPHY AND MARINE RESEARCH
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RICHARD MUSANHI GURURE**
Toxic effects of two organochloride pesticides, Thiodan and Lindane on two tilapia species, Oreochromis (= Sarotherodon) niloticus and Tilapia zillii, were investigated in fresh water at 25–30°C under laboratory conditions. In the case of Thiodan LT 50 (median lethal time) values increased from 78 minutes at 1 ppm to 6.5 days at 0.001 ppm for T. zillii. As for Lindane the LT 50 values increased from 60 minutes at 2 ppm to 4.7 days at 0.125 ppm for O. niloticus while the corresponding LT 50 values for T. zillii were 46 minutes and 2.7 days. No fish died within one week exposure at 0.0625 ppm Lindane for both O. niloticus and T. zillii.
Median lethal concentrations (LC 50), obtained from regressions of pesticide concentrations and LT 50, indicated that the 96 hour LC 50's for Thiodan and Lindane were 0.00142 and 0.11590 ppm for O. niloticus, while the corresponding values were 0.00083 and 0.00597 ppm respectively for T. zillii.
The safe concentration of Thiodan for O. niloticus was estimated as 0.000588 ppm while that for T. zillii was 0.000321 ppm, and for Lindane the safe concentration was 0.0232 ppm for O. niloticus and 0.000762 ppm for T. zillii. The results showed that Thiodan was about 50 times more toxic than Lindane to O. niloticus and about 6 times more toxic than Lindane to T. zillii.
*This study formed a part of a project report/thesis submitted for Postgraduate Diploma/M. Tech. (Aquaculture) degree of the African Regional Aquaculture Centre/Rivers State University of Science and Technology, Port Harcourt, Nigeria.
**Present address: Branch of Aquatic Ecology, Department of National Parks and Wildlife Management, P. O. Box 8365, Causeway, Harare, Zimbabwe.
Pesticide pollution is a subject of global concern. Pesticides are increasingly being used in Africa. Stout et al (1979) stated that pesticide usage in Africa was 20% of world's total in 1974 and could be 36% (almost 846,000 tons) in 1985. In Ivory Coast 600 tons of Lindane are used in cocoa plantations annually (FAO, 1981). George et al (1978) also stated that 4 kg/ha/year of pesticides are applied in irrigation channels of the Gesira in Sudan, and fish in the channels periodically suffer mortalities. In Kenya approximately 500 tons of pesticides are used in cotton and maize crops annually (Mbote, 1979). In Zimbabwe about 237g of active ingredient of DDT is sprayed per hectare per year to control tsetse fly (Matthiessen, 1982). DDT is also still widely used in malaria control in Zimbabwe with about 77g DDT a.i. being sprayed per hectare per year, Matthiessen (1982).
In aquaculture, the control of predator fish and weedfish is sometimes done by the use of pesticides. Pesticides are also commonly used in the storage of fish especially the dry product. Pesticides especially non-biodegradable organochlorides bioaccummulate in food fishes and ultimately in man. Wassermann et al (1974) found large amount (up to 0.4 ppm) of pesticides in adipose tissue and milk of Ugandans. Griechus et al (1977), detected pesticides residues in soil and water in three African reservoirs. Apart from their accummulation pesticides exert acute and chronic toxic effects on different non-target organisms in the ecosystem. In the present study acute toxic effects of two pesticides Lindane and Thiodan on two tilapia species are investigated. It is also necessary to find information on physiological effects of the pesticides, other than just survival.
The objective of these investigations are to study the comparative tolerance of two commercially cultured tilapia to two organochloride pesticides, Thiodan and Lindane. This information might be of value to the ecologist, agriculturist, public health worker and the aquaculturist who are concerned with the effect of the pesticides on the ecosystem. More information regarding pesticide effects on our fish with specific reference to our local conditions, is needed.
MATERIALS AND METHODS
The pesticides for this study were obtained from the stocks available at the International Institute of Tropical Agriculture (ITTA), Onne, Rivers State, Nigeria.
Thiodan used was a product of Hoechst Limited, HOE 2671 (U.S.A.) The pesticide is marketed as ‘Thiodan’, ‘Endosulfan’ or “Thiosulfan” and its chemical formula is 6, 7, 8, 9, 10, 120. Hexachloro - 1, 5, 5a, 6, 9a hexahydro - 6,9 methano - 2, 4, 3 - benzodioxathiepin - 3 oxide. Thiodan is chemically composed of two isomers with melting points of 109.2 C and 213.3 C. It is marketed as wettable powders (WP) of 35% and 50%, emulsifiable concentrates (Ec), granules or dusts; the product used in this study was wettable powder (WP 50%).
Lindane which was used is a product of I.C.I. Plant Protection (Great Britain). Chemically Lindane is a gammaisomer of 1, 3, 4, 5, 6 - hexachloro cyclohexane also known as Benzene Heachloride (BHC). Lindane that was used was the emulsifiable concentrate (Ec) from. It is however also available as wettable powders (WP), oil-base sprays, granules, dusts, aerosols or smoke generators.
The fish used were collected from ponds at the African Regional Aquaculture Centre, Aluu, Port Harcourt, Nigeria. The fish used were ingerlings of O. niloticus of size range 3.2 – 11.0 g in weight and 5.0–9.0cm total length, and fingerlings of T. zillii of size range 3.5 – 9.2g in weight and 5.0 – 9.0cm total length. The fish were initially collected and maintained in large circular plastic pools for two months prior to commencement of the experiments. Subsequently the fish were acclimated for 7 days in glass aquaria provided with compressed air supply in fresh water (ambient temperature 24.8 – 29.5 C) prior to tests. The fish were fed once daily with pelleted feed (NIOMR pellets - 35% protein). Feeding was stopped 24 hours before commencement of the experiments.
The experimental set-up consisted of 12 litre glass aquaria (20cm × 20cm × 30cm) with various concentrations of pesticides in dechlorinated tap water which was aerated by an aquarium aerator. Pesticide concentrations were obtained by diluting stock solution prepared weekly. Th pH of the ambient test water was measured before and after the experiment and ranged from 6.5 to 7.0. A Fisher Accumet portable field pH Meter Model 150 was used for the pH measurements. The temperature was also measured before and after the experiment and it ranged from 24.8 to 29.5 C. The average value for hardness was 40mg CaCO3/litre.
Screening tests were carried out in one-litre conical flasks with single fish. Concentrations of pesticides which caused fish death within 30 minutes were omitted from test. In each case eight fingerlings of O. niloticus and eight fingerlings of T. zillii were introduced into each aquarium with known concentration of pesticide. The test media were changed every 24 hours, replacing the old media with fresh solution in tests longer than 24 hours. Observations for loss of equilibrium and death were done in each test. The fish were considered dead when respiratory movements of the gills (opercular movements) stopped. Death was also confirmed when fish did not react to touch stimuli. The time to death of the fish, and length and weight measurements were taken. Observations on each test were continued until all the fish were dead or upto a maximum of 10,000 minutes. The experiments were done following procedures described by Alabaster and Lloyd (1982), Wuhrmann and Worker (1949), Fry et al (1972), Kutty et al (1977), Kassim (1978) and other workers.
The basic data on times to death of individual fish (O. niloticus and T. zillii) exposed to Thiodan and Lindane is summarised in Appendix I, where statistical analysis of the data are presented for each test. The percent mortality plotted against time to death on arithmetic scale gave the characteristic sigmoid curves as obtained for similar data on fish mortality by earlier workers (Fry, 1971 and Kutty et al 1977). An example of such a plot is Fig. 1 showing curves for O. niloticus exposed to various concentrations of Thiodan. The percent mortality on probability scale was also plotted against logarithm of time to death. These plots transformed the sigmoid curves to straight lines as shown in Fig. 2.
The median lethal times (LT 50's) were obtained by calculation, these were compared to LT 50 values obtained from arithmetic and probit plots. The values were found to be quite close as shown in Table I. Plots of logarithm of pesticide concentration on logarithm of median lethal time for the two pesticides and the two species of fish are shown in Fig. 3. The regression lines were fitted using the regression equations shown in Table IV. The regression equations that were used were obtained from median lethal times (LT 50's) and pesticide concentrations shown in Table II. The pesticide concentrations which kill half of the fish (median lethal concentration, LC 50) in 24, 48 and 96 hours were obtained by calculation from the regression equations (see Table III). The LC 50 values obtained are presented in Table IV. The pesticide concentrations which are presumed harmless (taken here as safe concentrations) were calculated for both the species exposed to Thiodan and Lindane. The calculations were based on the 24 and 48 hour LC 50 values using the method described by Hart et al. (1945) and Henderson et al (1959). According to Hart et al. (1945), the safe concentration can be obtained from the following formula:
C = 48 hours LC 50 × Application factor (A) × S2
where: C = Safe concentration
A = application factor i.e. 0.3 according to Henderson et al (1959)
The safe concentrations calculated from the above equations for the two fish exposed to the two pesticides are shown in Table V. Safe concentration is often taken as a fraction of 96 hr LC 50 (i.e. × 0.01) (EIFAC, 1983). The present estimations approximate values obtained similarly.
Comparison of median lethal times, LT 50 (mins) of O. niloticus exposed to the pesticides, Thiodan and Lindane, as obtained from arithmetic plots, probit plots and from geometric mean calculations
LT 50's for fish exposed to Thiodan
|O. niloticus||T. zillii|
|CONC. (ppm)||ARITH PLOTS||PROBIT PLOTS||GEOMETRIC MEAN||ARITH PLOTS||PROBIT PLOTS||GEOMETRIC MEAN|
LT 50's FOR FISH EXPOSED TO LINDANE
|O. niloticus||T. zillii|
* No death in one week
Median lethal times, LT 50 (mins) of O. niloticus and T. zillii exposed to various concentrations of Thiodan and Lindane (ND indicates no death in one week)
|O. n.||T. z.||O. n.||T. z.|
Regression equations and correlation coefficient (r) of log pesticide concentrations in ppm (y) against log median lethal times (LT 50) in minutes (X) for O. niloticus and T. zillii.
|O. niloticus||THIODAN||Y = 2.5261 – 1.4287×||0.938|
|LINDANE||Y = 1.2655 – 0.5854×||0.9871|
|T. zillii||THIODAN||Y = 2.0453 – 1.3637×||0.9981|
|LINDANE||Y = 2.1458 - 1.2338×||0.9794|
24, 48 and 96 hour median lethal concentrations (LC 50) (ppm) of Thiodan and Lindane for the fingerlings of O. niloticus and T. zillii.
Safe concentrations (ppm) of Thiodan and Lindane for fingerlings of O. niloticus and T. zillii (values in parenthesis obtained according to the method described in EIFAC, 1983)
|O. niloticus||0.0005888 (0.00042)||0.0232 (0.01159)|
|T. zillii||0.000321 (0.000083)||0.000762 (0.000597)|
The series of experiments on survival of O. niloticus and T. zillii exposed to various concentrations of the pesticides Thiodan and Lindane reveal that survival times vary with pesticides concentrations. The survival times increased with decrease in pesticides concentrations. The median lethal times (LT 50) are shown in Table I. It is evident from Table IV and Fig. 3., that there is difference in the effect of the pesticides themselves and in relation with the two pesticides. Thiodan caused fish mortalities at a concentration of 0.001 ppm, while Lindane killed fish at 0.125 ppm within 10,000 minutes. The LT 50 for O. niloticus and T. zillii exposed to 0.5 ppm Thiodan were 108 and 53 minutes respectively. At a concentration of 0.001 ppm LT 50's were 9419 and 5069 minutes for the two tilapias respectively. When exposed to 2 ppm Lindane LT 50's for O. niloticus and T. zillii were 6808 and 3910 minutes respectively.
It is evident from Tables II and IV that Thiodan is 50 times more toxic than Lindane. The median lethal concentration (LT 50's) for 96 hours period for O. niloticus and T. zillii were 0.00142 and 0.000826 ppm Thiodan respectively. For O. niloticus and T. zillii exposed to the 96 hour LC 50's were 0.1159 and 0.00597 ppm respectively. This shows that O. niloticus is 1.7 times more resistant to Thiodan than T. zillii. O. niloticus was observed to be 19.4 times more resistant to Lindane than T. zillii. The safe concentrations (presumably harmless) are 0.000588 and 0.000321 ppm Thiodan for O. niloticus and T. zillii respectively. When exposed to Lindane the safe concentrations for O. niloticus and T. zillii were 0.0232 and 0.000762 ppm respectively. The observations are similar to those found by Yoshihiro et al, (1958) and Hardjumalia et al, (1972) who found that Thiodan was more biodegradable but more toxic than Lindane. They also found Thiodan was biodegradable and with 24 hour LC 50 of 5–8 ppb, whereas 8-BHC (Lindane) was persistent but less toxic with 48 hour, LC 50 of 22–53 ppb. Hardjumalia et al, (1972) also found that Endrin and Thiodan caused fish kills within 11 and 18 days respectively. Kutty et al (1977) found that Endosulfan (Thiodan) killed common carp (Cyprinus carpio) and Oreochromis mossambicus at 0.00056 and 0.00001 ppm respectively in 48 hours. These findings are comparable with present observations. Suegue et al (1983) found Lindane more toxic than Carbaryl and Fenthion. Lakota et al (1983) in studies on Toxaphene, Lindane and DDT found DDT exerted toxic effects on carp (Cyprinus carpio) fry at concentrations ranging between 1–10 ppm when exposed for 24 hours.
Sublethal effects of pesticides have been investigated by several workers. Matthiessen, (1981) investigated the sublethal effect of Endosulfan on haemotological changes in tilapias. The total haemoglobin did not change, haemoglobin count per erythrocyte decreased from 159 to 39 pg and leucocyte count and plasma protein concentration were not altered. Kulshretha et al, (1984) found that Endosulfan and Carbaryl reduced number of oocytes, their size and caused deformation to oocytes and also damaged yolk vesicles.
The present study deals with only the survival of the tilapias on exposure to two major pesticides used in agriculture. In view of the potential damages pesticides could do to organisms in the ecosystem it is important to study the sublethal effects of the chemicals on aspects such as growth, reproduction etc. of the fish.
It is however hoped that the information now obtained on the tolerance limit, and safe concentration of the two pesticides on tilapias in the tropical environment would be of some value as examples of acute pesticide effects.
I am grateful for the supervision, suggestion and advice of Dr. M. N. Kutty (ARAC) in the course of the study. I also thank Dr. T. G. Pillai and Mr. L. Risch for their valuable help, critism and suggestions in the course of study. The (IITA) International Institute of Tropical Agriculture unit at Onne, Rivers State, Nigeria was very generous with supplies of pesticides used in the study. The Rivers State University of Science and Technology's Pollution Institute was most helpful in supply of reference material. My gratitude is also extended to Dr. Magadza and Mr. Mabaye for advice and help on the write-up. My sincere thanks to the Zimbabwe National Parks and Wildlife Department for allowing me to attend the 1983/84 ARAC/FAO course which enabled me to undertake the study and to CFTC, London for awarding me a fellowship.
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Statistical analysis of individual tests, for O. niloticus and T. zillii exposed to the two pesticides, Thiodan and Lindane.
|O. niloticus and Thiodan|
|T. zillii and Thiodan|
|O. niloticus and Lindane|
|T. zillii and Lindane|
Time mortality curves of Oreochromis niloticus fingerlings for various concentrations of Thiodan. The numbers next to the curves indicate persticide concentrations in ppm. Crosses indicate median lethal times (LT 50) in minutes.
Plots of percent mortality in probit and logarithm of time to death for Tilapia zillii fingerlings exposed to various concentrations of Thiodan. The crosses indicate median lethal times (LT 50). The numbers next to the curves indicate the pesticide concentration.
Plots of logarithm of pesticide concentrations (ppm) and logarithm of median lethal times (min) for fingerlings of O. niloticus and T. zillii exposed to Thiodan and Lindane. The broken lines indicate regression lines for Lindane and the continuous lines are for Thiodan.