Hydrobiologia, vol. 40, 1, pag. 101--119, 1972

Causes of Mortality in the endemic Tilapia of Lake Chilwa (Malawi)

by

PETER R. MORGAN

University of Malawi

INTRODUCTION

Lake Chilwa is considerably important because of its potentially high, but somewhat inconsistent yields of fish. It lies 25.7 km east of Zomba, within a saucer shaped closed drainage basin 620 m above sea level. Most maps of the area indicate an open water area of about 750 km 2, and a surrounding swampy area about 1000 km 2. The area of open water fluctuates however, and the lake dried up in 1968, but was refilled in 1969. The lake has a muddy bottom, and consequently the water is turbid.

The high productivity of this tropical lake is partly due to its shallowness and warmth, but these features also lead to its insta- bility. The maximum open water depth was 2.5 m in 1969 and 2.55 m in 1970. The annual loss of water by evaporation is high and may amount to 1.53 m (Moss & Moss, 1969). Consequently the depth varies considerably from year to year, depending on rainfall, and there have been net upward and downward trends in water level for many years (MORGAN & KALK, 1970). Changes in water level are directly associated with changes in the chemical composition of the water (MoaOAN & KALK, 1970), the flora (Moss & Moss, 1969), and the benthos (McLACHLAN & MCLACHLAN, 1969). There is every reason to suspect that these changes may also affect the fishes.

Three of the thirteen species of fishes recorded in L. Chilwa are of commercial value. They are Barbus paludinosus (PETERS), Tilapia shirana chilwae (TREwAVAS) and Clarias mossambicus (PETERS). This paper draws particular attention to the endemic Tilapia.

Received April 27, 1971.

101

During the years 1965 to 1968 the lake was transformed from a large body of water into a dry lake basin, and these drastic changes were paralleled by an appreciable fall in the fish landings. Records for the second half of 1964 and the first three months of 1965 indicate an annual fish production of about 9.06 x 106 kg, over one third being Tilapia (MzuMARA, 1967a). Fish exports for the first half of 1966 (no information is available for the second halves of 1965 and 1966) were still high (7.39 x 106 kg), although the proportion by weight of Tilapia had dropped to less than a quarter (KIRK, 1970). This proportion by weight had fallen to less than 2% by the first half of 1967, and the fish exports for the whole of 1967 amounted to ,n ly 3.32 x 106 kg. (RENSEN, 1969). The gill net fishery for Tilapia was virtually abandoned after June 1967, when fishing became both difficult and unprofitable. At this time most of the fishing effort was diverted to the streams and lagoons in the Chilwa basin. In 1968 an estimate for the total catch amounted to only 0.09 x 106 kg, which consisted almost entirely of Clarias.

The most critical period in the natural decline of Tilapia took place during the second half of 1966 and the first half of 1967, more than 1½ years before the lake dried up. A mass mortality was re- corded after two days of very strong wind on 17th and 18th October, 1966, and more prolonged and drastic mortalities were recorded during December, 1966 and January 1967 in calm hot weather, when the lake reached its lowest level for 17 years (KIRK, 1970). Large numbers of Barbus were also killed during these periods, but Clarias was unaffected. During the period June 1966 to June 1967 the chemical composition of the water in L. Chilwa was analysed at fortnightly intervals (MORGAN, 1968; MORaAN & KAr.K, 1970), and these made it possible to build up a picture of the chemical environment in which Tilapia lived during its most critical period. This paper attempts to clarify what the most likely causes of mor- tality might have been.

MATERIALS AND METHODS

Since the catastrophic conditions of 1966/67 could not be simulated on the new lake in 1969/70 when the experiments were carried out, it became necessary to analyse causes of mortality under experi- mental conditions in the laboratory. In certain experiments, com- parisons were made between the endemic Tilapia and Tilapia mela- nopleura (DuMERIL), a fish of more widespread distribution (JuBB, 1967), but one that also inhabits L. Chilwa in relatively small numbers. In all experiments fingerling Tilapia shirana chilwae and

102

Tilapia rnelanopleura measuring between 70 and 100 mm were used. These were obtained from a Government fish farm, and transferred to holding tanks near the laboratory; they were fed "madea" , a local maize product.

Effect o f re leased m u d During the periods of high winds and rough weather in October,

1966, considerable amounts of mud were released into the water from the bottom. Attempts were therefore made to determine the effect of released mud on fish and on the quality of lake water.

Two alloy tubes, 2 m long and 26 cms wide were constructed, and a neatly fitting chicken wire basket was placed into each of them. The baskets were open at the top and closed at the bottom with a bag of fine nylon netting, through which fish could be ex- tracted. The tubes were placed in the lake so that they stood up- right in the mud, with a clearance of 15 cms above the water. 20 T. s. chilwae were placed into each of the baskets, one acting as a control and left alone. The basket in the experimental tube was removed and the mud at its base was stirred into suspension. Water samples were taken to test for suspended solid contant. The basket of fish was then replaced into the tube, and removed at regular intervals for inspection of dead fish. Water samples from top and bottom were also collected at intervals and tested for dissolved oxygen by WINKLERS method (MAcKERETH, 1963) -- see below.

The oxygen uptake of Chilwa mud was determined by preparing a sample of liquid homogenous mud and adding different quanti- ties of this to a series of 500 ml plastic bottles containing tap water. The bottles were sealed to exclude air, shaken, and the mud was allowed to settle for 15 minutes before a sample of water was ex- tracted for the WlNKLER determination of oxygen. This settling procedure was found to be necessary for the determination of oxygen in the presence of reducing mud. In pilot experiments, water containing mud in suspension yielded lower values for oxygen than samples taken from water above settled mud in sealed bottles. Under these conditions values for dissolved oxygen did not rise after 15 minutes of mud settlement (Table IV).

The influence of high concentrations of aerated mud particles on fish was tested by placing 10 T. s. chilwae and 6 7-. melanopleura into each of two 40 litre glass tanks filled with aerated and me- chanically stirred tap water. Quantities of washed and aerated mud were added to one of the tanks on three successive occasions one week apart so that the suspended solid content was raised by 4,000 ppm on each occasion. The fish were regularly inspected, and samples were taken at weekly intervals, their gills being fixed in

103

Bouins f lu id , a n d e x a m i n e d h i s t o log i c a l l y . F i sh f r o m the c o n t r o l t a n k w e r e t r e a t e d in a s i m i l a r w a y .

Oxygen L e t h a l levels o f o x y g e n w e r e d e t e r m i n e d a t f irst b y e n c l o s i n g 20

7-. s. chilwae w i t h i n a 2.5 1 s e a l e d glass vessel f i l led w i t h t a p w a t e r . W a t e r s a m p l e s w e r e d r a w n of f a t i n t e r v a l s a n d t e s ted for o x y g e n (MAcKERETH, 1963). N i t r o g e n gas was used to d i s p l a c e w a t e r . F i s h b e h a v i o u r a n d d e a t h t imes w e r e r e c o r d e d . A c o n f i r m a t o r y tes t was c o n d u c t e d b y e n c l o s i n g 10 fish ( t o t a l w e i g h t = 60 g) in e a c h o f 8 s e a l e d 1.25 1 p l a s t i c c o n t a i n e r s . T h e e n c l o s u r e t i m e was v a r i e d b e t w e e n 15 a n d 105 m i n u t e s , a n d tests for d i s so lved o x y g e n a n d n u m b e r o f s u r v i v i n g fish we re m a d e w h e n e a c h c o n t a i n e r was re- o p e n e d .

Concentration of salts I n o r d e r to tes t w h e t h e r w a t e r s o f h i g h s a l i n i t y a n d a l k a l i n i t y

a d v e r s e l y a f fec t Tilapia in L. C h i l w a i t b e c a m e n e c e s s a r y to p r e p a r e l a k e w a t e r w i t h h i g h c o n c e n t r a t i o n . S i n c e l a r g e v o l u m e s w e r e n e e d e d c o n c e n t r a t e d l a k e w a t e r was p r e p a r e d a r t i f i c i a l ly . I n L. C h i l w a t h e Na+ , CI- , C O z - - a n d H C O a - ions a c c o u n t for a b o u t 9 7 . 4 % o f t he t o t a l ions ( M o s s & M o s s , 1969), a n d i t was poss ib l e

TABLE I

Chemical records for L. Chilwa during critical periodfor Tilapia. (Data of A. MORGAN (1968), with monthly rain.fall figures &fish deaths.

Con- duct. Oxy-

/~mho/ Alk. Temp. gen Depth Rain Tilapia Date cm meq/1 °C mg/1 pH m cms deaths

Sept. 5. 1966 5050 27.4 25.0 8.3 - - 1.07 - - - - Sept. 19. 1966 5800 28.6 25.2 7.0 8.8 0.97 - - - - Oct. 3. 1966 6000 32.0 22,0 6.4 8.6 - - 0,63 - - Oct. 17/18. 1966 - - - - - - (high winds) + + + + Oct. 28. 1966 5600 34.2 26.4 5.7 8.6 - - - - - - Nov. 14. 1966 7000 42.8 29.0 9.0 8.8 - - 2,64 - - Nov. 26, 1966 7100 44.0 23.0 7.4 8.8 0.63 - - - - Dec. 10. 1966 12000 61.6 28.7 3.7 9.3 0.50 11,76 ÷ + + + Dec. 30. 1966 6000 34.0 26.0 10.2 9.5 0.43 - - - + - + + + Jan. 11. 1967 5550 27.0 35.2 16 .1 9.6 0.46 15.49 + + + + Feb. 1. 1967 6800 41.6 37.4 15.7 9.6 0,52 11.68 ÷ + + Feb. 24. 1967 6480 35.6 27.0 4.9 9.5 0.48 - - -t- + March. 11. 1967 1630 9.8 30.0 - - 8.4 0.51 29.92 + March. 29. 1967 276 1.3 25.4 7.7 8.0 0.84 - - ÷ April. 19. 1967 2630 15.4 33.4 8.0 8.4 1.01 3.30 + May. 2. 1967 4090 22.8 30.0 - - 8.4 1.07 - - +

104

to make up artificial water by adding equal parts by weight of NaC1 and Na2CO~ to natural lake water to make the required concentration. This artificial water was analysed for conductivity, alkalinity and pH over a wide range of concentrations, and com- pared favourably with natural water (Fig. 1). The proportion of

2 5 0 -

I - l § O -

..J

._I

I 0 0 -

SO- P

I i I I I

Io, o o o 2c~ooo 3o, ooo 40,000 Sc~:x:)o

co.oucr,,,,Tv tJ,..Q/c,.. Fig. 1. Relationship between alkalinity and conductivity in artificial lake water. Points indicate experimental results, circles readings from the lake itself.

minor ions was obviously less however. Lethal concentrations of this water were tested, after two siting runs, by preparing four 40 1 aquaria filled with dilute Chilwa water, 14 T. s. chilwae and 14 T. melanopleura. Each day equal amounts (5 g) of NaC1 and Na2CO~ were added to three of the tanks, the fourth being a control. Daily measurements of conductivity, alkalinity and pH were taken to- gether with the number of live fish. In order to test whether the concentration of NaC1 alone might have caused mortalities, 10 T. s. chilwae were placed into each of four 8 1 buckets, and daily additions of NaC1 (10 g) were made to three of them, the fourth

105

being a control. The concentration at which the 1st, 5th and 10th fish died in each tank was recorded.

Temperature The mean point of heat coma and the upper tolerance limit of

temperature were tested by placing Tilapia into tanks in which water temperatures were raised with aquarium heaters. 12 fish, previously acclimated to 24°C were added to a 40 1 tank, and the water temperature was slowly raised with constant stirring until all the fish were dead. The behaviour and death temperature for each fish was recorded. Duplicate experiments were carried out for T. s. chilwae and T. melanopleura. In a 2nd experiment, 10 7-. s. chilwae were placed into each of 12 ten 1 buckets filled with aerated tap water. The fish had previously been acclimated to a diurnal temperature range of 24°C to 31 °C. Each bucket was in turn raised to a particular temperature ranging from 40°C to 42.75°C with a small water heater, and then allowed to cool. The rate of rise and fall of temperature was recorded together with the number of fish surviving each temperature limit. Finally , 16 T. s. chilwae and 16 T. melanopleura previously acclimated to 24°C were placed in a well aerated vessel containing 30 1 of tap water, and the temperature was slowly raised from 23.5°C to 35.5°C over 11.5 hours (0.016°C/m). The temperature was held at 35.5°C overnight and raised again the following day from 35.5°C to 40.0°C over 7 hours (0.01°C/m) and held between 40.0°C and 40.3°C for 9 hours. During the final period the time of death was recorded for each fish.

RESULTS

Effect of released mud Mud released into the water by stirling within experimental

tubes caused a sudden depletion of oxygen, and subsequently the death of all fish. The suspended solid content of stirred water was raised from 1,400 ppm to 18,000 ppm within a few minutes, and oxygen levels fell to zero within two minutes of stirring (Table II). The relatively low values for oxygen at the beginning of the ex- periment were probably caused by a small release of mud at the base of each tube at it was introduced. Normally dissolved oxygen levels in L. Chilwa range from 5.4 mg/1 to 8.2 mg/1. Fish came to the surface gulping for air after only one minute, and continued this for 15 minutes. After this they progressively became weakened and were unable to gulp, and fell t o the bottom; all were dead within 50 minutes. Conditions were favourable in the control tube,

106

TABLE II Oxygen levels and.fish mortality within alloy tubes in lake. Temperature = 28°C; Depth

= 1.52 m.

Control tube Experimental tube Minutes Oxygen Oxygen Fish + Oxygen Oxygen Fish

after start top bottom alive top bottom alive

0 3.3 rag/1 2.2 mg/l 20 3.0 mg/1 1.6 rag/1 20 Stirred

2 - - 20 0.0 rag/1 0.0 rag/1 20 15 - - - - 20 . . . . 20 25 2.6 mg/1 - - 20 . . . . 7 45 - - - - 20 . . . . 1 50 2.0 rag/1 - - 20 0

a n d fish d id no t g u l p a n d were in exce l len t c o n d i t i o n at the t e rmi -

n a t i o n of the e x p e r i m e n t . L a b o r a t o r y e x p e r i m e n t s c o n f i r m t h a t s u s p e n d e d m u d has a h igh

aff ini ty for oxygen , a n d t h a t it is c a p a b l e of d e o x y g e n a t i n g 16 x its

o w n v o l u m e of a e r a t e d t ap water . Resul t s in T a b l e I I I show t h a t

TABLE III Effect of Lake Chilwa mud on dissolved oxygen in tap water at 23°C.

ml mud added to suspended oxygen mg/1 no. 500 ml water solids after 15 mine.

1 0.0 ml - - 6.0 (6.1) 2 0.0 ml - - 6.2 3 10.0 ml 4,280 ppm 3.4 (3.45) 4 10.0 ml 3.5 5 15.0 ml 6,420 ppm 2.1 (2.1 ) 6 15.0 ml 2.1 7 20.0 ml 8,560 ppm 0.8 (0.7 ) 8 20.0 ml 0.6 9 25.0 ml 10,720 ppm 0.2 (0.225)

10 25.0 ml 0.25 11 30.0 ml 12,860 ppm 0.0 (0.0 ) 12 30.0 ml ,, 0.0

TABLE I V Effect of L. Chilwa mud on dissolved oxygen in tap water at 23°C. Samples of water were withdrawn at intervals Jrom a sealed 1.25 l plastic bottle filled with water and a known amount of mud. The let sample was withdrawn with mud in suspension, and subsequent samples were taken atfer 10, 30 and 120 minutes. Withdrawn water was replaced with

nitrogen.

Initial level after after after of oxygen 10 min. 30 min. 120 min.

0.1 mg/1 0.3 mg/l 0.3 rag/1 0.3 mg/l 0.2 mg/1 0.5 mg/1 0.5 mg/1 0.5 mg/1 0.3 mg/1 0.9 mg/1 0.9 mg/1 0.95 mg/1 0.4 mg/1 1.0 mg/1 1.0 mg/1 1.0 mg/1 0.45 mg/1 1.1 mg/1 1.1 mg/1 1.2 mg/l 0.5 mg/1 1.3 mg/1 1.3 mg/1 1.05 rag/1

107

total deoxygenation had occurred when sufficient mud had been added to the tap water to raise its suspended solid content to 12,860 ppm. The tests also confirm that suspended mud had a marked effect on the WINKLER test, reducing the detectable oxygen levels by a factor of about 3 x (Table IV). During the experiments very little change occurred in detectable oxygen levels after 10 minutes had been allowed for mud settlement, and figures tabulated in Table III represent the maximum values for oxygen that could have been determined from the mud/water sample. Referring to the field experiment, mud released sufficient to raise the suspended matter to 18,000 ppm would have been more than sufficient to deoxygenate the water.

No deaths occurred in the laboratory where fish were subjected to high levels of fully aerated mud. Histological examination of the gill epithelium did reveal hyperplasia however, presumably the result of mechanical irritation caused by mud particles.

Oxygen 7-. s. chilwae appears to be able to tolerate levels of oxygen as

low as 0.6 mg/l for brief periods, but levels between 0.3 mg/1 and 0.4 mg/1 were found to be lethal. The fish became oxykinetic be- tween 0.6 mg/l and 0.8 rag/1.

TABLE V Behaviour of 7-. s. chilwae and oxygen levels wzthin enclosed chambers (28°C) . Experiment 1. Fish no. = 20; weight = 112 g ; volume oJ chamber ~ 2.5 1. Experiment 2. Fish no. = 18; weight = 210 g ; volume of chamber = 5.0 1.

Experiment 1 Experiment 2

Minutes after Behaviour Fish oxygen Behaviour Fish oxygen start of fish dead rag/1 of fish dead mg/1

0 - - - - 6.2 - - - - 7.3 Fish inserted:

5 N 0 4.2 N 0 5.7 I0 N 0 2.4 N 0 4.7 15 N 0 1.4 N 0 3.3 20 G 0 0.9 N 0 1.9 25 H 0 0.6 N 0 1.4 30 Q 0 0.4 H 0 0.8 35 Q 0 0.4 H 0 0.8 40 U 0 0.4 U 0 0.5 45 U 0 0.3 U 4 0.4 50 U 3 0.3 U 5 0.4 55 U 18 0.3 U 6 0.3 60 U 20 0.3 U 10 0.3 65 U U 12 0.3 70 U 18 0.3

Behaviour: N = Normal; G = Gulping; Q = Quiescent; U = Upturning

108

TABLE VI Survival of T. s. chilwae after enclosure in sealed chambers (23°C) Fish no. = 10 (60

gms) in each chamber; chamber volume = 1,250 ml.

Oxygen Duration of Final oxygen No. fish No. start mg/1 enclosure mg/1 dead

1 7.2 15 mine. 3.8 0 2 7.2 25 ,, 2.2 0 3 7.2 35 ,, 1.0 0 4 7.2 45 ,, 0.8 0 5 7.2 60 ,, 0.8 0 6 7.2 75 ,, 0.6 0 7 7.2 90 ,, 0.5 1 8 7.2 105 ,, 0.4 8

Concentrat ion of sa l ts Both species of Tilapia are clearly euryhaline, but the endemic

Tilapia was consistently more resistant to high ionic stress. Median lethal alkalinities were found to depend on the rate of acclimation, and the 3rd experiment in which the rate of increase of conductivity of 340 #mho/cm/day was similar to the rate of increase during the critical phase of the lakes history (26th Nov. to 10th Dec., 1966, 350 #mho/cm/day) was the most reliable. Median lethal conduc- tivities under these conditions were 12,800 #mho/cm for 7-. s. chilwae and 10,800 #mho/cm for T. melanopleura. Median lethal alkalinities were 61.6 meq/1 (pH 9.4) and 53.6 mgq/1 (pH 9.4) respectively. Maximum lethal conductivities were 15,000 #mho/cm for 7". s. chilwae and 12,000 #mho/cm for T. melanopleura (Table VII) . A

TABLE VII Tolerance of 2-. s. chilwae and 7-. melanopleura to artificial lake water. Duration of experiment = 43 days; mean dazly increase in conductivity = 340 i~mho/cm/day ; temp. =

22.5°C; 3 tanks + control.

Fish type 7. melanopleura 7". s. chilwae Number 14 + 14 + 14 14 + 14 + 14

Conduct. (Alk.) (pH) at let mortality:

Conduct. (Alk.) (pH) at 50% mortality:

Conduct. (Alk.) (pH) at 100% mortality:

9000 (44.8) (9.43 5200 (26.7) (9.2) 8475 (44.4) (9.4) 2550 (13.8) (9.0)

10500 (50.0) (9.4) 4800 (24.5) (9.2)

11000 (55.0) (9.4) 12600 (63.0) (9.4) 10500 (50.0) (9.4) 12000 (61.0) (9.4) 11000 (56.0) (9.4) 12000 (61.0) (9.4)

11500 (58.0) (9.4) 15000 (75.0) (9.4) 11000 (56.0) (9.4) 14000 (71.0) (9.4) 12000 (61.0) (9.4) 15000 (71.0) (9.4)

Conductivity in/~mho/cm; Alkalinity in meq/1.

109

TABLE VIII

Tolerance of 7-. s. chilwae to NaCl solutions. Duration of experiment = 29 days; mean daily increase in conductivity = 1,790 #mho/cm/day ; temp. = 22°C.

Conductivity #mho/cm and g/1. NaC1 in solution. 1st mortality 50% mortality 100% mortality

Tank I 25,000 (12.5) 38,500 (20.2) 51,000 (27.2) Tank 2 27,000 (13.7) 37,500 (19.8) 52,000 (27.8) Tank 3 38,000 (20.0) 44,000 (23.0) 47,000 (25.0)

certain proportion of deaths resulting from finrot occurred in the control tank amongst 7-. s. chilwae (35%), although this disease did not occur in other tanks. It is likely that elevated salinity stalled the onset of finrot, although it caused opaqueness in the corneas of the eyes.

When 7". s. chilwae was subjected to NaC1 solutions without the inclusion of Na,CO8 it withstood much higher ionic concentra- tions. The median lethal conductivity of NaC1 solutions was 40,000 #mho/cm (alkalinity = 2.5 meq/1; NaC1 = 21.0 g/l). In a pilot experiment 4 fish withstood a solution containing 16 g/1 NaCl (31,000 #mho/cm), although a further addition of Na2COs suffi- cient to raise the conductivity to 33,750 #mho/cm caused all the fish to die.

Temperature Median lethal temperatures for T. s. chilwae acclimated to 24°C

were 42.00°C and 42.05°C in duplicate experiments; values for T. melanopleura were 41.4°C and 40.95°C. The variation in resistance to heat stress was considerable, ranging from 40.7°C to 42.35°C in 7". s. chilwae, and 40.65°C to 41.95°C in T. melanopleura (Table IX). T. s. chilwae acclimated to a daily range of temperature were slightly more resistant. All but one recovered healthily from 41.75°C, but subsequent mortalities were noted in excess of this value, the maximum limit being 42.7°C. When subjected to tem-

TABLE IX Response of T. s. chilwae and 7-. melanopleura to high temperature

Experiment no. 1 2 3 4 Rate of rise of temperature: 30---40 °C 0.070°C/m 0.15 °C/m 0.09°C/m 0.09°C/m Fish type: 7". s. ch. 7". s. ch. 7-'. mel. 7". mel. Number 12 12 12 12 Median lethal temperature 4 2 . 0 0 ° C 42.05°C 41.40°C 40.95°C Minimum lethal temperature 41.50°C 40.70°C 40.70°C 40.65°C Maximum lethal temperature 42.25°C 42.35°C 41.90°C 41.40°C

110

TABLE X Recovery of 7-. s. chilwae from high temperatures

n o .

Highest Rate of Rate of Temp. at temp. rise fall % start °C reached ° C / m i n . ° C / m i n . recovery

1 27 40.00 0.225 0.250 100 2 27 40.25 0.237 o. 150 100 3 27 40.50 0.225 0.140 100 4 27 40.75 0.212 0.140 100 5 27 41.00 0.200 0.160 100 6 27 41.25 0.160 0.200 90 7 27 41.50 0.212 0.150 100 8 27 41.75 0.187 0.175 100 9 27 42.00 0.175 0.150 40

10 27 47.25 o. 175 0.200 50 11 27 42.50 0.175 0.175 30 12 27 42.75 0.162 0.175 0

peratures over 40°C for extended periods, T. s. chilwae was again more resistant than T. rnelanopleura. Between 40.0°C and 40.3°C the mean time of survival for T. s. chilwae was 420 minutes (range 170--540 m), whereas it was only 166 minutes (range 125--215 m) for 7-. melanopleura.

DISCUSSION

During October 1966 no prolonged chemical changes occurred in the lake (Table I), and the mass mortality resulted from the effect of high winds ("mwera") on the lake bed. A layer of liquid mud 0.3 m deep persisted in about 1.0 m of water, 2 days after the winds had subsided (MzuMARA, pets. comm.). Experimental results indicate that 12,860 ppm of released mud would cause oxygen depletion and a death of fish. Although the proportion of fish killed in this catastrophe is not known, many survived. Most deaths occurred in the north and west, where the lake was shallowest and the fetch of the S.E. winds was the greatest. The likelihood of toxic gases causing mortality is minimal, for although methane is pro- duced, it has little effect on T. s. chilwae, and hydrogen sulphide, which is very lethal, is almost undetectable in the lake. The possible toxic effects of other gases and substances released from the mud cannot be excluded however, for these may have enhanced lethal effects caused by lack of oxygen.

It is noteworthy that similar mortalities of Tilapia took place in 1955 and 1960 when the lake was low, the latter, and probably the former after heavy winds. Gauge heights for the low levels of 1955, 1960 and 1966 were 1.06 m, 0.83 m and 0.43 m respectively. It is possible that a gauge height close to 1.0 m may represent a critical

111

point below which bottom erosion is likely to occur from wave action caused by winds.

Mud with a high oxygen demand is often found in highly pro- ductive eutrophic water, where a considerable amount of organic matter falls to the lake bed. Tilapia mortalities attributed to the overturn of mud or deoxygenation have been reported from several shallow lakes in tropical Africa. These include L. George in 1957 (HICKLING, 1961); the Nampongwe River (Zambia) in 1964 (TAIT, 1965); L. Victoria (FISH, 1955) and Mweru-wa-Ntipa (Zambia), (BowMAKER, 1965). In some cases the flushing out of stagnant mud by excessive inflows of rain water can cause deoxygenation, but whether this happens in L. Chitwa is not known.

Little published information is available for the tolerance of Tilapia to low oxygen regimes, although FISH (1955) has estab- lished that 7-. esculenta GRAHAM is more sensitive to anoxia than Mormyrus kannume FORSK and Bagrus docmac FORSK in L. Victoria, and is more likely to succumb as a result of overturns causing deoxygenation. It is known that the tolerance of low oxygen decreases in more saline water, and FARMER & BEAMISH (1969) have presented evidence to show that in T. nilotica (L.) approxi- mately 29% of the total oxygen consumption is required for osmoregulation in 30% S.W.

In common with many other fish, 7-. s. chilwae becomes oxy- kinetic at low levels of oxygen (Table V), and this behaviour enables them to escape into more oxygenated water if it is close by. THOMPSON (1925) noted that fish have the ability to avoid water deficient in oxygen, but are able to recover quickly from anoxia if they are forced into deoxygenated water. Similar reac- tions have been observed in 7-. s. chilwae.

Respiratory distress in fish occurs more readily at higher levels of temperature, pH and alkalinity (ERICHSEN-JONES, 1964), and it is likely that the dense blooms of Anaebena during January and February 1967, which caused supersaturation of oxygen during the day, may have caused low and lethal levels of oxygen at night. The nocturnal respiration of algae is known to cause deoxygenation (OLsEN, 1932 ; HUTCHINSON, 1936), and the decomposition of algae during the day can cause catastrophic effects on fish (MOORE, 1942). The toxic effects of high concentrations of algae have also been reported by SHILO (1951). COE (1966) reported extensive mortalities of 7-. grahami in L. Magadi in 1960 following periods of excessive rainfall when the water was filled with blue-green algae (Arthrospira). CoE reports similar events in L. Natron in 1962 and ascribes both events to deoxygenation of the water following algal flushes.

112

The incidence of gill hyperplasia noted in these experiments is of interest, a l though the condition is unlikely to occur in the lake itself, where the suspended solid content rarely exceeds 1,500 ppm. The conditions found are similar to those described by HERBERT & MERKINS (1961), and T. s. chilwae, in common with many fish is able to tolerate very high levels of suspended matter for brief periods. In one experiment 9 out of 10 fish survived over 36 hours in water containing fully washed and aerated mud at 140,000 ppm.

The widespread occurrence and extended duration of the December/January mortality of Tilapia suggests that ionic con- centrations had reached lethal levels, and experimental results indicate that the alkalinity recorded in December 1966 (61.6 meq/1) may have been sufficient to cause a 50% mortali ty of Tila- pia. Although lethal alkalinities were not maintained during January (Table I), fish deaths continued (KIRK, pers. comm.), and probably resulted from anoxia at night (see above). The incidence of corneal opaqueness in alkali stressed fish, both in the laboratory and in natural conditions is indicative of similar physiological responses to stress in the two environments, and may represent deposition of CaCO3.

Although 7-. s. chilwae is moderately tolerant of high ionic con- centrations, it is much less resistant than T. grahami (BouLENOER) of L. Magadi where alkalinities rise above 1,000 meq/1 and T. alca- lica (HILGEND) of L. Natron where alkalinities exceed 2,000 meq/1 (data of TALLING & TALLING, 1965). T. s. chilwae is also less tolerant of NaC1 solutions than other species. The conditions im- posed by POTTS et al. (1967) and DHARMAMBA • NISHIOKA (1968) led to 100% acclimation in T. rnossambica (PETERS), al though identical treatments caused 100}~ mortality in T. s. chilwae. T. mossarnbica survives 200% seawater for over one week (POTTS et al., 1967), but T. s. chilwae can just withstand 100°/~ sea water. The tolerance of T. mossambica, T. galilaea (ARTEDI), (CHERVlNSm, 1961), and 7-. nilotica (LINNE), (LOTAN, 1960), for sea water can be attr ibuted to their association in rivers and estuaries to the sea. The freshwater 7-. melanopleura has a low tolerance of seawater (MORTIMER, 1962). T. graharni and T. alcalica may have become secondarily adapted to high ionic conditions in the soda lakes of East Africa. It is evident that the tolerances of individual species of Tilapia are closely allied to their histories of physiological stress.

The lack of blood analysis makes the possible effects of high concentrations of CO3-- and HCO3- difficult to assess. Chloride is exchanged for bicarbonate in the gills during salt uptake at low salinities, and if too much bicarbonate is present the fish may become chloride deficient. Absent chloride could be replaced by

113

bicarbonate, but this might diffuse out too readily as CO2, and the fish could die, either because the blood was too alkaline, or simply because the osmotic pressure of the blood was too low (PoTTS, pers. comm.). It is thought, however, that at higher con- centrations the bicarbonate/chloride mechanism no longer operates because the ionic fluxes are so large that bicarbonate and ammo- nium ions produced metabolically are no longer adequate, and regulation takes place by sodium/sodium and sodium/potassium exchanges (POTTS pers. comm.). Since fish drink more readily in saline water it is possible that the intestinal fluid may become alkaline if large amounts of HCO% and CO-s are present, and this could markedly change the rate of entry of sodium and chloride (CoNTE, 1969). COE (1966) considers that the hind gut of T. grahami which is thickened may be the main organ of salt balance in this resistant fish.

T. s. chilwae is remarkably tolerant of high temperature (Table IX and X), and this factor alone would not have caused natural mortalities. The highest temperatures on record are 40°C (December, 1968) and 39.5°C (March 1969), both made in Kachulu Bay in calm hot weather. Periods of calm hot weather are rare on L. Chilwa, and a light breeze has a considerable cooling effect on the water. T. grahami does not appear to be more resistant to heat stress than T. s. chilwae. CoE (1966) reports that T. grahami browses on algae in waters below 40°C, but if it passes up the naturally occurring temperature gradient beyond 40°C it is likely to succumb. It lives quite normally between 38°C and 39°C how- ever, and lethal temperatures lie in the range 41°C to 42°C (CoE, 1966). Median lethal temperatures for T. mossambica lie in the range 36.94°C to 38.25°C (ALLANSON • NOBLE, 1964), and are well below those for T. s. chilwae, possibly because it lives in waters having mean monthly summer temperatures below 28°C. Summer temperatures for L. Chilwa are above this. The differences in thermal tolerances of T. mossambica, T, s. chilwae and T. melano- pleura may be ascribed to their thermal histories.

Although heat coma may not have caused Tilapia deaths in L. Chilwa, elevated temperatures may have decreased the fishes re- sistance to other stress factors. Temperature, like salinity has a considerable influence on the oxygen requirements of fish (EVANS et al., 1962), and increases in temperature are invariably associated with increased branchial irrigation and blood circulation. These responses prompt increases in endosmosis and salt efflux and lead to reductions in the CO2 tension (HOUSTON et al., 1968). Conse- quently the ionic balance of fish may be disturbed, and acute tem- peratures may impair regulatory ability altogether. Increased irri-

114

gation promotes the passage of water across the gills (MACKAY & BEATTY, 1968), and HOUSTON et al. (1968) have established a well defined inverse relationship between water content and tempera- ture in trout; tissue water declines with warm acclimation. Thus elevated temperatures during January and February 1967, may, have placed a considerable stress on both respiratory and osmotic mechanisms in Tilapia and caused mortalities at alkalinities below the lethal limits set for lower temperatures. Similarly osmotic stress caused by high alkalinity during December 1966 may have promoted the onset of thermal stress and anoxia in January.

Although little direct evidence is available, it is likely that L. Chilwa rarely enters the class I I I type of TALLING • TALLING (1965), with alkalinities in excess of 60 meq/1. The exceptionally low levels of 1914/15 and 1967/68 when the lake became dry (CHIPETA, 1968), and possibly the low of 1931/33, may represent the only occasions when the lake entered the class III type this century. Taking evidence from local reports (CHIPETA, 1968), the recessions of 1900, 1923, 1943, 1949, 1953/55, 1960/61 and 1966, do not appear to have been low enough to cause lethal concentra- tions of salts, although the figure for December 1966 places the lake in class III . Records of reduced fish catches or fish mortalities are available for 1914/15 (CHIPETA, 1968), 1931/33, 1955, 1960/61 and 1966/67 (MzuMARA, 1967b). The catastrophes of 1914/15 and 1966/68 caused famine and hardship.

A study of L. Chilwa leads one to suppose that it is in the process of receding. Indeed as the lake shrinks the wasteage of water from the affluent streams increased, and the likelihood of low water levels leading to deoxygenation and increased salt concentration of the water progressively becomes more frequent. In the soda lakes of E. Africa Tilapia survives considerable levels of stress, and it is thought that these fish evolved, together with their lakes from the last pluvial period (BEADLE, 1969). It is probable that fish like 7". grahami have progressively become physiologically adapted to L. Magadi, and thus able to withstand the considerable ionic stress. Future limiting factors in L. Chilwa may not be solely associated with increased ionic concentrations however, but also with regular deoxygenation of the water. If this is the case, the salt resistant and air breathing Clarias mossambicus may become dominant in the future.

The repeated mortalities of Tilapia in L. Chilwa are the result of natural catastrophes which man might have no power to control. At times they seriously affect the fishery, and the overall effect may be one of reducing the capacity of Tilapia to fully exploit the re- sources of the lake. Stabilising the Tilapia fishery effectively means

115

stabilising the lake itself by maintaining water levels artificially, but the high cost and technical difficulties involved make such schemes prohibitive. However, with further investigations it may become possible to propose a management program for the lake so as to maintain a more consistent output of fish.

ACKNOWLEDGEMENTS

Thanks are due to the Lake Chilwa Co-ordinated Research Project of the University of Malawi, and the Leverhulme Trust for financial support. I am indebted to Mr. J. S. LIKONGWE, for his unfailing assistance and to Mr. C. M. CHISALA, for a constant supply of fish. Sincere thanks are also due to Mr. A. COCKSON and members of the research project for their assistance. I should like to acknowledge the Department of Fisheries for access to their reports.

SUMMARY

During 1966 and 1967 extensive mortalities of Tilapia shirana chilwae took place in Lake Chilwa, Malawi. By collating field observations and physico-chemical data from the lake with experimental evi- dence from the laboratory, it has been possible to predict what the most likely causes of mortality of Tilapia might have been.

High winds caused bottom erosion in October 1966, and the released mud deoxygenated the water. Total deoxygenation of the water results if the level of suspended matter is raised to 12,860 ppm. Tilapia are able to survive levels of oxygen as low as 0.6 mg/1, but become distressed below this figure.

The median lethal alkalinity for T. s. chilwae in artificial lake water is 61.6 meq/1. This concentration was reached in the lake in December 1966, and probably caused further mortalities.

T. s. chilwae is remarkably tolerant of high temperature, and this factor alone is unlikely to have caused natural mortalities. Elevated temperatures can cause decreased resistance to low oxygen and high ionic concentrations, however, and the dense blooms of Anaebena may have caused sufficient deoxygenation of the water at night to cause further mortalities.

T. s. chilwae is more resistant to high alkalinity and high tem- perature than T. melanopleura. This undoubtedly reflects the long history of stress in the endemic Tilapia. No doubt the heightened resistance of T. s. chilwae has enabled it to successfully populate L. Chilwa, despite its eharaeteristie instability.

116

ZUSAMMENFASSUNG

Betrachtliche Mortalit~iten des Fisches Tilapia shirana chilwae erfolgte im Chilwa See (Malawi) in 1966 und 1967. Bei der Zusam- menfiigung von den Beobachtungen am See und den physisch- chemischen Daten des Sees mit experimentellen Daten aus dem Laboratorium, war es m6glich die wahrscheinlichen Ursachen des Mortalit~its festzustellen.

Starke Winde verursachten die Erosion des Bodens im Oktober 1966 und der aufgel6ste Schtamm entfernte die Sauerstoffe des Wassers. Totale Entfernung des Sauerstoffes folgte als das Niveau des aufgel6sten Materials einen Wert yon 12,800 t.p.m, erreichte. Obgleich Tilapia einen so niedrigen Sauerstoffniveau yon 0.6 rag/1 fiberleben k6nnen ffihlen sie sich unheimlich unter diesem Wert.

Die mediane letale Alkalinit~it ftir T. s. chilwae im ktinstlichen Seewasser ist 61.6 meq/1. Diese Konzentration wurde im Dezember 1966 im See erreicht und verursachte wahrscheinlich eine weitere Mortalit~it.

T. s. chilwae ist aufl'allig tolerant hohen Temperaturen gegen- fiber und es ist unwahrscheinlich dasz diese Tatsache allein natfir- liche Mortalit~iten verursacht hat. Dennoch k6nnen erh6hte Tem- peraturen eine abnehmende Resistenz gegen niedrigen Sauerstoff und hohe Ionen-Konzentration verursachen und die dicke Bltite yon Anaebena k6nnte eine genfigende Desoxygenation des Wassers in der Nacht veranlasst haben um weitere Mortalit~iten zu verur- sachen.

T. s. chilwae zeigt st~rkere Resistenz gegen~ber hoher Alkalinit~it und hoher Temperaturen als T. melanopleura. Dieses reflektiert, ohne Zweifel die lange Geschichte von Stress in die endemische Tilapia. Die erh6hte Resistenz des T. s. chilwae, dasz es ftir diese Fische m6glich war den Chilwa See zu bev61kern, trotz ihrer charakte- ristischen Instabilit~it.

117

REFERENCES

ALLANSON, B. R. & NOBLE, R. G. - 1964 - T h e tolerance of Tilapia mossambica (PETERS) to high tempera ture . Trans. Amer. Fish Soc., 93, 4: 323--332.

BEADLE, L . C . - 1969 - Osmotic regula t ion and the adap ta t ion of f reshwater animals to in land saline waters. Verb. Internat. Verein. Limnol. 1 7 : 4 2 1 - - 429.

BOWMAKER, A. P. - 1965 - T h e fisheries of Zambia , Mweru-wa-Nt ipa . Na tu ra l Resources Handbook , Minis t ry of Lands and Na tu ra l Resources, Zambia . 58--60.

BROWN, M. E. - 1957 - T he physiology of fishes. Vol. 1. Academic Press Inc., New York.

CHERVINSKI, J . - 1961 - Study of the growth of Tilapia galilaea (ARTEDI) in various saline concentrat ions. Bamidgeh, 13, 3/4: 71--74.

CmPETA, W. - 1968 - Cyclical and secular changes in Lake Chi lwa wate r level and its fisheries. Cyclostyled repor t No. 1. Univers i ty of Malawi.

CoE, M. J . - 1966 - T he biology of Tilapia grahami in Lake Magadi , Kenya. Acta Trop~ca, 23: 146--177.

CONTE, F. P. - 19C9 - Salt secretion, in Fish physiology, Vol. 1. Eds. HOAR, W. S. & RANDALL, D. J . Academic Press, London.

DHARMAMBA, M. & NIsmor~ , R. S. - 1968 - Responses of "prolactin-secreting" cells of Tilapia mossambica to env i ronmenta l salinity. Gen. Comp. Endo- crinol. 10, 3: 409--420.

ERmHSEN JONES, J . R. - 1964 - Fish and r iver pollution. But te rwor th & Co. Ltd., London.

EVANS, R. C., PURDm, F. C. & HXCKMAN, C. P. - 1962 - The effect of t empera tu re and photoper iod on the respira tory metabol ism of r a inbow trout, Salmo gairdneri. Can. J. Zool., 40: 107--118.

FARMER, G. J . & B ~ I S H , F. W. H. - 1969 - Oxygen consumpt ion of Tilapia nilotica in re la t ion to swimming speed and salinity. J. Fish, Res. Bd. Canada 2 6 : 2 8 0 7 - - 2 8 2 1 .

FISH, G. R. - 1955 - Some aspects of the respirat ion of six species of fish f rom Uganda . J. Exp. Biol. 33, 1 : 186--195.

I-IERBERT, D. W. M. & MF.RKINS, J . C. - 1961 - T h e effect of suspended minera l solids on the survival of trout. Int. J. Air Wat. Poll. 5, 1 : 46--55.

HmKLXNO, C. F. - 1961 - Tropical in l and fisheries. Longmans. London. HOAR, W. S. & RANDALL, D. J . - 1969 - Fish Physiology. Vol. 1. Academic

Press, London. HOUSTON, A. H., REAVES, R. S., MADDEN, J . A. & DE WILDE, M. A. - 1968 -

Env i ronmen ta l t empera tu re and the body fluid system of the f reshwater teleost - 1. Ionic regula t ion in thermal ly accl imated ra inbow trout , Salmo gairdneri. Comp. Biochem. Physiol. 2 5 : 5 6 3 - - 5 8 1 .

HUTCHINSON, G. E. - 1936 - Alkali deficiency and fish mortal i ty. &ience 84: 18. JUBB, R. A. - 1967 - T he freshwater fishes of Sou the rn Africa. Balkema, Cape-

town/Amste rdam. KINK, R. G. - 1967a - T h e fishes of Lake Chilwa. Soc. Malawi J. 20: 35---48. KIRK, R. G. - 1967b - Cyclostyled Repor t , D e p a r t m e n t of Fisheries, Malawi. KIRK, R. G. - 1970 - A study of Tilapia shirana chilwae TREWAVAS in Lake Chilwa,

Malawi. Ph.D. Thesis, Univers i ty of London. LOTAN, R. - 1960 - Adaptab i l i ty of Tilapia nilotica to various saline concent ra-

tions. Bamidgeh, 12, 4: 96--100. MACKAY, W. C. & BEATTY, D. D. - 1968 - T h e effect of t e m p e r a t u r e on renal

functions in the whi te sucker fish, Catostomus commersonii. Comp. Physiol. Biochem. 26: 235--245.

118

MACKERETH, F. J . H . - 1963 - Some methods of water analysis for limnologists. Sci. Publ. Fresh. Biol. Ass. 21 : 1--71.

MeLACHLAN, A . J . & MeLAeHLAN, S. M. - 1969 - T h e bo t tom fauna and sedi- ments in a d ry ing phase of a saline Afr ican Lake (L. Chilwa, Malawi) , Hydrobiologia, 34: 401--413.

MOORE, W . G . - 1942 - Field studies of the oxygen requi rements of cer tain freshwater fishes. Ecology, 23: 319--329.

MORGAN, A. - 1968 - A physico-chemical study of Lake Chilwa. M. Sc. Thesis. Univers i ty of Malawi .

MORGAN, A. & KALK, M. - 1970 - Seasonal changes in the waters of Lake Chi lwa (Malawi) in a d ry ing phase, 1966--68. Hydrobiologia, 36: 81--103.

MORTIMER, M . A . E . - 1962 - Notes on the salinity tolerances of cer tain Tilapia species (Cichlidae: Pisces) in Nor the rn Rhodesia. J.F.R.O. Ann. Rep. 9 : 63- -64 .

Moss, B. & Moss, J . - 1969 - Aspects of the l imnology of an endorheic Afr ican lake (L. Chilwa, Malawi) . Ecology, 50 (1): 109--118.

MZUMARA, A. J . P. - 1967a - T h e Lake Chilwa fisheries. Soc. Malawi. J. 20: 58--68.

MZUMARA, A. J . P. - 1967b - Cyclostyled report , D e p a r t m e n t of Fisheries, Malawi . Governmen t .

OESEN, T. A. - 1932 - Some observations on the interrela t ionships of sunlight, aquat ic p lan t life and fishes. Trans. Amer. Fish Soc. 62: 278--289.

POTTS, W. T. W., FOSTER, M. A., RubY, P. P., & PARRY HOWELLS, G. - 1967 - Sodium and water ba lance in the Cichlid Teleost Tilapia mossambica. J. Exp. Biol. 47: 4 6 1 4 7 0 .

RENSEN, H. L. F. - 1969 - Fish Market ing . Repor t of Minis t ry of Agricul ture Malawi Governmen t .

Smco , M. - 1951 - Toxic blue-green algae in fish ponds in Israel. Bamidgeh, 3: 146--153.

TAIT, C. - 1965 - Mass fish mortalit ies. Fisheries Res. Bull. Dept. G a m e and Fisheries, Zambia. 3: 28--30.

TALLING, J . F. & TALLING, I. B. - 1965 - Chemical composit ion of African lake waters. Int. Revue ges. Hydrobiol. 50: 421--463.

THOMPSON, D. H. - 1925 - Some observations on the oxygen requi rements of fish in the Illinois River. Bull. Ill. Lab. Nat. Hist. 15: 423--437.

119