RECENT STATUS OF THE POPULATIONOF INDUS DOLPHINTOSHIO KASUYA AND MASAHARU NISHIWAKI
Ocean Research Institute, University of Tokyo, Tokyo

ABSTRACTA survey of the population of Indus dolphin Platanista gangetica indi
(Blyth, 1859) was conducted in December 1974. The range of the significant
abundance of this subspecies is restricted to the section between Sukkur andGuddu Barrages, and to the section between Guddu Barrage and Taunsa andPanjnad Barrages. The size of the population in the former section was
estimated to be between 350 and 400 animals. Even if the populations inother sections are included, the total population seems to be only 450 to 600
animals. The calculated mortality rate, reproductive rate and environmental
factors suggest that the population is still decreasing. For the survival of thispopulation, the stop of the killing of this dolphin and the control of water athigher level in winter are suggested.

INTRODUCTION
The dolphin of the genus Platanista is found in the lndus River and in the GangesBrahmaputra Rivers. There is found a difference of the shape of nasal crest between the dolphins in the two separate rivers (Pilleri and Gihr, 1971). Though
Kasuya (1972) reported the difference of the proportion of tail length, as he did
not measure the lndus dolphin but referred the data prepared by Pilleri (1970)
his conclusion is still doubtful.
It seems to be generally accepted that these two separate populations, started
from a single origin, have been isolated long enough to attain some morphologicaldifferenciation. However, there are two contrary opinions on their classification.
One is to consider them distinct species (Pilleri and Gihr, 1971), and the other is toclassify them into two subspecies of a single species (Kasuya, 1972). At the present
status of knowledge on these animals, we consider it reasonable to take the latteridea until more distinct morphological or ecological difference is found between thetwo, or until it is suggested that they do not freely interbreed even when the geographical barrier is not present.
The lndus dolphin was known to have been distributed in the wider range ofthe tributaries (Anderson, 1878). However, since the start of the construction of
permanent barrages for irrigation, the dolphin population have been split up into
separate smaller populations (Pilleri, 1970). And in some sections of the river,the population has been decreased or extinct (Pilleri, 1972; Roberts, 1972; Taberet al, 1967). The alarming situation of this dolphin was already noticed by Pilleri(1972) based on the observation made in 1969 and 1972 at the lower part of theSci. Rep. Whales Res. Inst.,

No. 27, 1975, 81-94.

82

KASUYA AND NISHIWAKI

section blocked by Sukkur and Guddu Barrages. The total population of thisdolphin has been considered not exceeding 700-1,000 (Anon., 1973).
The present study was conducted in December 1974 to know the present rangeof distribution of the dolphin and to estimate the population.
RANGE OF DISTRIBUTION
The presence of the dolphin was checked at various bridges or barrages. However,as the dolphin is usually scarce near the barrage even if it is situated in the section
where the presence of many dolphins is confirmed by other method (e.g. Sukkur or
Kashmore Barrage), it was tried to collect more informations from the fishermenor people living by the river. Furthermore, if possible the length of about 1 to
3 km along the stream was observed at each spot.
Section 1, up of Jinnah Barrage
The Jinnah Barrage is situated near Kalabagh Town (71°34'N, 35°59'E).
This section was observed on Dec. 15. The wide stream was present only within
about 5 km above the barrage, and above the range the river enters into a gorge.
Though it was reported by Anderson (1878) that the dolphin was present in the water
above Kalabagh, the dolphin was not observed by us. The people in the town
denied the presence of the dolphin. The dolphin seems to be absent in this partof the river in recent years.
Section 2, between Chasma and Jinnah Barrages
This part was not observed by us. But considering the small distance between
two barrages and the scarcity of water below Jinnah Barrage, the dolphin will not
be distributed in this section.
Section 3, between Taunsa and Chasma Barrages
This section was observed at Dera Ismail Khan and Taunsa Barrage on Dec.
14.
The water of the Ind us by Dera Ismail Khan was about 500 m in width andseemed to be fairly deep suggesting the possibility of the presence of the dolphin.
However we could not find it. A man living on the river bed told that he does not
see the dolphin in winter season but he had once seen an animal in summer season.
At the north side of Taunsa Barrage the water was abaut 1,200 m wide. But
there was found no dolphin in the area of about 2 km along the river.
From the above informations we consider that the dolphin might be absent inthis section or if present the number must be almost negligible.
Section 4, from Guddu Barrage to Taunsa Barrage (R. Indus) and to Panjnad Barrage (R.
Panjnad)This section was studied at 5 spots on 7 occasions as mentioned below.
Though no animal was observed at the down stream of Taunsa Barrage (Dec.
Sci. Rep. Whales Res. Inst.,
No. 27, 1975.

83

POPULATION OF INDUS DOLPHIN

14) and Panjnad Barrage (Dec. 13 and 17), it does not necessarily indicate theabsence of the dolphin in this section because the water was scarce below the barrages and the dolphin will not migrate there.

32°

+
II)

"

"O

.£

TAB "'-DGK•

+

+

0

50

100

150 200km+
Fig. 1. Map of the Indus. Black bar indicates the barrage, numeral the numberof the section in text, and the arrow the check point of Platanista. BEG, Begari.
CHA, Chachran. CHB, Chasma Barrage. DGK, Dera Ghazi Khan. DIK,
Dera Ismail Khan. EMB, Emerson Barrage. GUB, Guddu Barrage. HYD,
Hyderabad. ISB, Islam Barrage. JHM, Jhang Maghiana. JIB, Jinnah
Barrage. KAR, Karachi. KHU, Khushab. KHW, Khanki Weir. KOB,
Kotri Barrage. MIT, Mithankot. MUL, Multan. MUZ, Muzaffargarh.
PAB, Panjnad Barrage. RAB, Rasul Barrage. ROH, Rohri. SEH, Sehwan.
SHA, Shahpur. SUB, Sukkur Barrage. TAB, Taunsa Barrage.

Sci. Rep. Whales Res. Inst.,
No. 27, 1975.

84

KASUYA AND NISHIWAKI

By Dera Ghazi Khan, the observation was made on Dec. 14 at the boat bridgeconnecting Dera Ghazi Khan and Muzaffargarh. The water there was only about
100 m wide and probably less than few meters in depth. No dolphin was foundin the ranges of about 1 km at the both sides of the bridge.
Other observations were made along the stream of the length of about 3 km
at the boat bridge between Mithankot and Chachran on Dec. 13 and 17. On thisspot the water was more plenty than that at Dera Ghazi Khan situating about 150
km north. The width was about 500 m and the current was fast (2 to 3 km/hr ).
At the first visit one small dolphin (about 120 cm in body length) was observed
swimming leisurely at about 100 m north of the bridge, and another slightly larger
animal was seen at about 500 m south of the bridge. In case of the second visit,
two small individuals were sighted near the latter spot.
At the water just above the Guddu Barrage, there was found no dolphin(Dec. 12).
From the above informations it is safe to conclude that some individuals of
the Indus dolphin are distributed in this section of the Ind us River. The range ofthe distribution in winter season will be from Guddu Barrage to the confluence of
the Indus and the Panjnad. The length of this part is about 100 km. As this partof the section receives the water from the two rivers, the water level will not be too
low to support Platanista in winter season.

Section 5, between Sukkur and Guddu Barrages
The presence of the dolphin in this section was confirmed from boats, and theestimation of the population is made in the next section of this report.
Section 6, between Kotri and Sukkur Barrages
The river of this section is devided into two by Ghulam Mohammed Barragesituated about 12 km north of Kotri Barrage. During our trip we only confirmedthat the water is very scarce at just below Sukkur Barrage and at Sehwan Town,
which is situated slightly south of the midpoint between Sukkur and Kotri Barrages.
This indicates the probable absence of the dolphin in the upper part of the section.
However, as reported by Pilleri (1972), Mr. Roberts sighted one animal near thenorth of Ghulam Mohammed Barrage and suggested that the population of thedolphin in the section will be quite small. He also states that Platanista is not expected down stream of Kotri Barrage. These informations are considered to becorrect.
Section 7, from Emerson Barrage to Rasul Barrage (R. Jhelum) and to Khanki Weir (R.
Chenab)The section was studied on Dec. 16 at the bridge on theJhelum River connecting Khushab and Shahpur. This bridge is situated nearly at the midpoint between
the two barrages. At this point the stream was only from 100 to 150 m in widthand shallow. No dolphin was observed.
The other observation was made by the bridge crossing the Chenab River at
Sci. Rep. Whales Res. Inst.,
No. 27, 1975.

POPULATION OF INDUS DOLPHIN

85

the north of Jhang Maghiana This bridge is situated about 32 km upper streamof Emerson Barrage. In spite of the close distance from the lower barrage, thewater was only bout 70 m wide and was very shallow.
It is reasonable to expect no dolphin in this section, as already suggested by
Taber et al (1967) through the observation of the Ravi and Chenab Rivers.

Section 8, between Panjnad and Emerson Barrages
The Ravi River, a tributary of the Chenab River, was almost dry in this season(Dec. 16) leaving small ponds on the bed.
At the bridge connecting Multan and Muzaffargarh the stream was about
150 m in width and shallow, and no dolphin was observed on two occasions (Dec.
13 and 16). At Panjnad Barrage an information was collected on Dec. 17 froma fishermen telling that the dolphin is absent in this section.
No dolphin seems to be distributed in this section.
POPULATION IN THE SECTION BETWEEN SUKKUR
AND GUDDU BARRAGESThe field observation of the Indus River of this section was conducted in the period
from Dec. 7 to Dec. 12, using one (from Guddu Barrage to Begari) or two (fromBegari to Rohri) local boats. The size of the boats was about 2.8 m wide and6.5 m long. The boats cruised only in daytime with a stop for lunch, and usually
2 or 3 biologists watched the front direction of the boat while it was slowly rowed
down the stream.
On Dec. 7 1974 our boats left the camp on the east bank of the Indus situating
about 1 km south of Begari and arrived at Begari after 10 minutes. Then we tooka jeep at Begari for Guddu Barrage. At Guddu Barrage a boat was arranged andwe left that place at 1526 of the same day. After spending 2 nights on the boat, wearrived at the camp near Begari at 1900 of Dec. 9. On Dec. 9, as the observation
was stopped at sunset (1735), we could not observe the river of about 3 km. On
the next day (Dec. 10), we changed the boat and left the camp for Rohri, and arrived there at 0920 of Dec. 12 after spending 2 nights on the boats.
Though it was dry season, the width of the water was more than 200 m (estimated by eye) at any part of the section. However the width was small enoughand the weather was calm to keep all the width in the range of observation, except
when the water was split into two streams by sand bank or by island. On such cases
the observation was always made on the larger stream. The speeds of the current
was 2. 7 km/hr (Dec. 8), 1.5 km/hr (Dec. 9), or 5.0 km/hr (Dec. 11) at the shore on
the faster side of the stream.
The stream probably corresponds to the arm of the Indus running the eastside of Tappu Island reported by Pilleri (1970, 1972) was narrow and formed asort of canal, but was distinguished by fairly wide dry river bed. When Pilleri
visited Tappu Island in 1969 the east arm was much larger than the west, but it
reduced the size by 1972 to be as wide as the west arm (Pilleri, 1972). Possibly theSci. Rep. Whales Res. Inst.,
No. 27, 1975.

86

KASUYA AND NISHIWAKI

east arm attained the present condition by continuing the reduction of the size
after Pilleri visited there in 1972.
When the dolphin was found, the number of surfacings was counted for eachindividuals untill the animal passes the level of the boat. The distinction of theindividual was made considering the position and interval of surfacings, the size ofthe animal, and sometimes the swimming direction. Though most of the animals...
2:··--------------10 Dec.·- --- - - -- - - - - - - ··:

0

...

• I

__ _..12Dec .

....
.... --"'\.i ..
..:. •"'~ ~!

Miyassaghot

~

:.~

'.

\

~

'

11 Dec.-----GUB

..

.:i
',

Ferry(Kire)-·-~.._-.a..i&.-~Baiya

2.7km/hr...
8 Dec.

(

Keti Shah Bela)
Forest9 Dec.

060

120min-...... ',' ~.
• 0;~

BEG 7 Dec.

Fig. 2. Schematic figure of the lndus between Guddu Barrage and Begari (bottom)
and between Begari and Rohri (top). Direction of the stream and cruising hours areindicated instead of direction and goegraphical distance. Circle at the left sideof the river indicate the dolphin observed in cruising hours, and that at the rightthe animals observed while anchored. Numbers in parenthesis indicate theroughly estimated number of dolphins and number of surfacings in high density
area. BEG, Entrance of Begari Canal. GUB, Guddu Barrage. ROH, Rohri.

Sci. Rep. Whales Res. Inst.,
No. 27, 1975.

87

POPULATION OF INDUS DOLPHIN

seemed to be solitally, the area with high density was usually restricted to some deep
places, and the dolphin was scarce in the shallow and rapid stream. In some cases
many dolphins were found in a small area (e.g. 100 to 500 m in diameter), and thedistinction of the individuals was difficult. Even in these cases it was not sure if
the animals were forming a school or only assembling randomly at one spot. These
cases are shown with parenthesis in Table 1. In this table the lines are arranged in
the order of from the upper stream to the lower stream.

Interval of respiration
One juvenile dolphin (about 120 cm in body length), which was released after

5
50900, 6 Dec.

10
5
L..

0630, 6Dec.

.01930, 5Dec.

QJ

E 5
::J

z

1730, 5Dec.

10
510~'--~~~~;:;Jrn~~fil-_i_-1..._,_-L_l~53-0~.-5~D-ec~.~
5om·~~~~mi::.;~~L...'--'-----'--'--'-_L__L-l._J

0

10

20

30

40

50

60

70

80

Interval in seconds
Fig. 3.

Frequency of respiration interval of a juvenile P. g. indi.

the observation, was driven into a small inlet. And the interval of surfacing for
respiration was measured for 5 to 20 minutes at various hours of a day in order toget some clue on distinguishing the individual dolphins in the river. This resultis shown in Figs. 3 and 4.
The respiration interval showed wide variation from 4" to 81". Judging
from the inconspicuous peaks of the frequency, there seems to be three modes of
respiration interval. The one is at 20" or less, the second at about 40", and the thirdat 80". The frequency of the shorter interval had decreased in the second day.
This may be related to the decrease of excitement caused by the driving.
The observations of freely swimming animals give slightly different feature.
The mean interval of 9 respirations of a median size animal was 64". The 4 intervals of another median size animal ranged from 10" to 80" with a mean of 54". On
the other hand the mean interval of 12 respirations of one small animal, which was
swimming slowly at one spot, was 87". These informations suggest that the ratio
Sci. Rep. Whales Res. Inst.,
No. 27, 1975.

88

KASUYA AND NISHIWAKIof the longer interval may increase in the freely swimming animals, but that there
is no significant difference of the mean length of the interval between small andlarger animals. These features are also confirmed by the data reported by Pilleri(1970), in which the highest frequency was at 60", the next peak at 10", and theaverage was 42".
The respiration in night time was checked by the respiration sound. Thisshows that the respiration in the night is strongly biased to the shorter side.

Estimation of the population
The numbers of dolphins and of surfacings observed in the Indus between
1130
11 I I
I I I
11 I I I I 11
11 111II
0900
111 I I I I I 111 I 111 I I I 11 I 111
11 11
I I
0630
11 11 I I 1111 I 11 11 11 1111 11 11 I I I I I I
11
1930
111111111111 11111111 11 I
1730
II II 1111111 1111111 111111 Ill! I 1111 1111111
1530
11 I 11 1111 111 I I I II I 1111 11 I 111 Ill 11 111 I 11

0

100

200

300

400

500

600

700

800

II

11

900 1000 1100

Seconds
Fig. 4.

Respiration pattern of P. g. indi based on same data in Fig. 3.

Guddu Barrage and Rohri are shown in Table 1. The rough estimation of numberof dolphins we sighted is made, as shown bellow, from the total number of surfacingsand number of surfacings with the distinction of each individuals.
( 420+ 117) !~~ =256
However this does not give the total number of dolphins in the observed water,
because the overlooked animals are neglected. Accordingly the following method
was used in this study.
Excluding the count of the dolphins made at the places where the dolphinswere too concentrated to get reliable distinction of each individual, if the numberof dolphins are plotted against the number of surfacings of each individual, there
is found a tendency decreasing toward the left (Fig. 5). The frequency of animals
sighted twice is higher than that expected from the linear decrease. This may be
because, if the first surfacing is sighted, a larger attention is apt to be paid for thesecond surfacing. If the least square is used for the points between 1 to 5 surfacings,
the relationship y= -15.90x+82.l, where y is the number of dolphins and x is
Sci. Rep. Whales Res. Inst.,
No. 27, 1975.

POPULATION OF INDUS DOLPHIN

89

the number of surfacings, is obtained. However the frequency of the animals
with 6 or 7 surfacings does not come on the extended straight line. Possibly thesepoints will include the cases where two or more animals were erroneously considered
as a single animal. If each case with 6 (7 cases) or 7 (3 cases) surfacings is considered to be represented by 2 animals with 5 or less surfacings, the real number ofanimals discussed here should be 192, and above formula should be parallelly slided
up for (7+3) x2+5=4.0 and can be shown as follows.

80

(/)

''

c

''

E

'

_9-60

0
'O

0

'

,.

''

'

40

''

L.

<!>

•

.Q

E
20
:::J

''

'

''

'

•'''

z

00

2

3

4

'

•

•

5

6

7

Number of surfacings
Fig. 5. Frequency of the number of observed surfacings of each individual of
Platanista in the section between Guddu and Sukkur Barrages. Solid line indicates
y=-15.90x+82.l, and dotted line y=-15.90x+86.1.
TABLE 1. NUMBER OF DOLPHINS AND OF SURFACINGS OBSERVED
BETWEEN GUDDU BARRAGE AND ROHRI IN DEC. 1974
Date
7
7
8
9
9
7
10
10
10
10
11
11
12
12

Hours ofobservation
2°01 1
8130"
8°241
201
9°041
101
151
4°24'
19'
2°341
131
9°00 1

While at stopsurfacings

dolphins

20

10

61

28

124
(29)
(40)
66
(48)
29

53
(10)
(13)
24
(15)
9

90

48

30
420
(117)

12
182
(38)

Sci. Rep. Whales Res. Inst.,

surfacings

dolphins

8

51

1°421

Total

No. 27, 1975.

While cruising

6

3

3

2

90

KASUYA AND NISHIWAKI

y=-15.90x+86.l .................................... (1)The estimation of the number of overlooked dolphins is obtained from formula (1)as the value of y=86.l corresponding to x=o. The real number of dolphinsrepresented by one surfacing is obtained as follows.
182+7+3+86.l
420

0.661 ................................. (2)From this figure and total number of surfacings counted, the total number of dolphins should have been present in the observed area is obtained as follows.
( 420+ 117) x 0.661 =355.0 ............................... (3)This is the minimum estimate of dolphins in the Indus River between Guddu Barrageand Rohri. In order to estimate the total number of dolphins in the section, thenumber of dolphins in the unobserved area must be added.
There are three occasions where the smaller branch of the stream was not observed in spite of the possible presence of the dolphin. The length of time of thosecases were 20' (Dec. 8), 19' (Dec. 9), and 8' (Dec. 9). And the number ofspoutings
counted in the corresponding main streams were 0, 16, and 0 respectively. Assuming that the same number of dolphins are distributed both in the smaller branch
and larger main stream, the following number is calculated as the number of dolphinsmight have been present in the smaller stream.
(O+ 16+0) x 0.661=10.6 ................................. ( 4)However this can be a overestimate, because it is more reasonable to expect thesmaller number of dolphins in the smaller branch.
While the cruise of about 1 hour at the upper part of Begari, the observation
was not made because of the night time. The number of dolphins in this area canbe estimated from the number of surfacings counted in each one hour period atboth sides of this area. In the upper area there were observed 47 surfacings in one
hour, and in the lower area 64 surfacings. The mean is 50.9 surfacings. This shallrepresent
50.9x0.661=33.6 .................................... (5)However this estimation is also considered to be higher, because the water of theunobserved area was so shallow to have difficulty in cruising at night and the waterat both sides of this area was deeper.
Another estimation must be made for the area between Rohri and Sukkur
Barrage (about 2 km), where we could not cruise. As the number of surfacings
counted during the cruise of the same distance just above Rohri was 14, the numberof the dolphins in the areas between Rohri and Sukkur Barrage is obtained as follows.
14x0.661=9.3 ....................................... (6)Sci. Rep. Whales Res. Inst.,
No. 27, 1975.

POPULATION OF INDUS DOLPHIN

91

By totalling the figures (3), (4), (5), and (6), 409 is obtained as the numberof dolphins between Guddu and Sukkur Barrages. As this figure is a maximum
estimation, more reasonable estimation can be between this figure and that of (3),or about 380 animals.ii

Size composition
Table 2 shows the number of dolphins and of the surfacings classified by thesize of the dolphin in the section between Guddu and Sukkur Barrages. The
juvenile animals smaller than about 140 cm in body length are classified into
" Small ", the large individuals presumably exceeding 180 cm and having theelongated rostrum were classified into "Large'', and the remaining dolphins into
" Intermediate ". Though the frequency of the three sizes are calculated by two
methods, one by the number of observed surfacings and the other by the number ofidentified individuals, there is observed no significant difference between the ratios
obtained by two methods. And the approximate ratio of small, intermediate, andlarge animals is 70 %, 20 %, and 10 % respectively. According to Kasuya (1972)
TABLE 2. RELATIVE ABUNDANCE OF THE THREE SIZES OF PLATANISTA
IN THE SECTION BETWEEN GUDDU AND SUKKUR BARRAGES
No. of surfacings

Small
Intermediate
Large
Total

No. of dolphins

no.

%

no.

%

299
89
40
428

69.9
20.8
9.3
100.0

97
32
17
146

66.4
21.9
11. 7
100.0

the female Platanista in the Brahmaputra River attains the sexual maturity at thelength between 170 and 200 cm and grows about 50 cm larger than the male, andthe rostrum is especially long in mature females. So it is considered that the individuals classified into large animal in Table 2 represent the mature females, andthe intermediate premature animals of both sexes and mature males. This meansthat the number of mature females is only 10 %of the total population. Even under
the unreasonable assumption that all the intermediate animals are sexually mature,
the percentage of the mature femals could be 15 % of the total population (an evensex ratio is assumed). In other words, the number of mature females is considered
to be surely less than 30 % and probably about 20 % of all the females in the section
between Guddu and Sukkur Barrages.
DISCUSSION

It is indicated that the dolphin is distributed also in the lower part of the section at
the north of Guddu Barrage. Though its length is almost same with that of thel) The fitness of Poisson distribution to the points in Fig. 5 is worse.
28 instead of 86 and the population between 269 and 309.
Sci. Rep. Whales Res. Inst.,
No. 27, 1975.

It gives overlooked animal

92

KASUY A AND NISHIWAKI

entire section between Guddu and Sukkur Barrages, the water in the former area
is more scarce than in the latter. So the total number of the dolphin in that part
must be smaller than that in the section between Guddu and Sukkur Barrages.
Possibly it will not exceed the half of the population in the latter section or about
200 animals. If the density of 2 animals in 3 km is applied for the area, the population can be about 100 animals. As the number of dolphins in the section between Kotri and Sukkur Barrages and that between Taunsa and Chasma Barrages
is almost negligible, the total number of Platanista in the lndus River is estimated tobe between 450 and 600. This is the lowest estimation has ever been made on thispopulation.
The ratio of sexually mature females to the total females was obtained in theformer section. The corresponding figures of delphinids are 69.1 % (calculated
from the data in Sergeant, 1962) in Globicephala melaena, 68.3 % (Kasuya et al, 1974)
in Stenella attenuata, and 58.5 % (Kasuya, 1972) in S. coeruleoalba. However these may
sometimes have a bias derived from the segregation of the animals by different growthstage (Kasuya et al, 1974). The real maturity ratio of female S. attenuata calculated
from the life table in Kasuya et al (1974, Table 24) is 59.0 %· These values are extremely high compared with that of Platanista in the Indus. A possible explanationof this phenomenon is the high mortality rate. If the mean age at sexual maturity is
known, the mortality is roughly estimated. Kasuya (1972) suggested on Platanista
in the Brahmaputra River that the age at the attainment of sexusal maturity infamales might be 10 years of slightly less as in the case of other toothed whales. If
9 years is applied for the age of the female Platanista in the lndus River at the attainment of sexual maturity, there can be the following equations.
H"e-µx dx
He-µx dx
M=I-e-µNo. of mature females
No. of immature females

Where µ is the annual mortality coefficient, M the annual mortality rate, x the ageof the animal in years. As the number of mature females is 20 %of the total females,
the annual mortality rate of 0.164 is obtained from the above equations. Even for
the assumption of improbably high ratio of mature females 30 %, the annual
mortality rate is 0.125 Though the mortality can change by the age of the animal,
the above calculation will give some idea on the mean mortality rate of Platanista inthe past. There is another problem in this estimation. The mean mortality thus
calculated is largely influenced by the change of the annual recruit, and in the strict
sence this value can indicate the real mean mortality only when the population isstable. In a decreasing population the mortality will be underestimated, and in a
increasing population overestimated. However, as the environment of the dolphin
has been changing worse as mensioned below, it is unreasonable to suspect theincreasing population. Accordingly the mortality estimated here seems to becorrect or an underestimation.
The next problem in analysing the population of the dolphin is the recruitment rate of Platanista. Kasuya (1972) showed that the parturition and mating
Sci. Rep. Whales Res. Inst.,
No. 27, 1975.

POPULATION OF INDUS DOLPHIN

93

seasons of Platanista in the Brahmaputra River are in winter season and that thecalves born in the early winter are weaned by the beginning of the next dry (winter)season. If this reproductive cycle is correct for Platanista in the Indus River, themean calving interval will be at least 2 years. Then the gross annual reproductive
rate is calculated as follows, assuming the even sex ratio and the ratio of mature
females 20 % obtained above.

Even in case of the unreasonably high ratio of mature female~ 30 %, the gross annual
reproductive rate is only 0.075.
The net reproductive rate is obtained as the difference of gross reproductive
rate and the mortality rate. It is 0.05-0.164=-0.114 and surely less than 0.0750.125= -0.050. Though this calculation is rough, it will be correct to conclude
that the population of Platanista in the Indus River has been decreasing at the rateof about 10% per annum. As the environment does not seem changing better, thedecrease of the population will be continuing.
One possible reason of the high mortality of this dolphin population can be
the unknown amount of the catch by local fishermen. Another and more importantfactor seems to be the change of the environment caused by the development of theirrigation system as indicated by various authors (Anon., 1973; Pilleri, 1972;
Roberts, 1972; Taber et al, 1967). The cause of the disappearance of the dolphin
in most of the part of the Indus seems to be related directly to the construction of
the barrages for irrigation system. The dolphin population, before the construction of the barrages, will have expanded the range of distribution in summer season
and retreated to main stream in winter season as in the case of Platanista in theGanges-Brahmaputra Rivers (Kasuya and Haque, 1972). However the construction of barrages blocked the seasonal movement and devided the population into
smaller units. Then, as most of the water is used for irrigation in each section, only
the subpopulations in few sections where the water level is not too low in winterseason have survived at the lower population level. It is told that the water decreases even at Begari so low in February not to allow the use of the local fishingboat. Begari situates at the middle of the section between Guddu and Sukkur
Barrages where most of the Platanista population is left. This condition will be badfor the survival of the dolphin. Furthermore, if the utilization of water for irrigation increases in future, the survival of this dolphin population will be impossible.
As the conclusion following two acts are suggested for the survival of Platanista
gangetica indi (Blyth, 1859). One is to stop the poaching of this dolphin. For thispurpose the educational propaganda will be needed to deny the erroneous conception on the medical efficiency of the dolphin oil. The other is to keep the water ofsome section above the certain level even in the winter season. This seems to bepossible only through the accomplishment of the economical utilization of the water
resources.
Sci. Rep. Whales Res. Inst.,
No. 27, 1975.

94

KASUYA AND NISHIWAKIACKNOWLEDGMENTS
Greatest thanks are due to Mr. T. A. Khan and Dr. R. L. Brownell Jr., who gave
us valuable suggestions in planning our research trip to Pakistan and kindly checked
the draft. Dr. F. Yamasaki, Dr. T. Kamiya, and Dr. A. Takemura are also acknowledged for their cooperations offered for our study.
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ANONYMOUS, 1973. Indus dolphin or susu, Platanista indi (Blyth, 1859). in Red Data Book vol. 1 (Mammalia):
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BLYTH, E., 1859. On the great rorqual of the Indian Ocean with notice of other cetals, and of the sirenia
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KASUYA, T., 1972. Growth and reproduction of Stenella caeruleoalba based on the age determination bymeans of dentinal growth layers. Sci. Rep. Whales Res. Inst., 24: 57-79.
KASUYA, T., 1972. Some informations on the growth of the Ganges dolphin with a comment on the Indus
dolphin. Ibid., 87-108.
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KASUYA, T., N. MIYAZAKI, and W. H. DAWBIN, 1974. Growth and reproduction of Stenella attenuata in the
Pacific coast of Japan. Ibid., 26: 157-226.
PILLER!, G., 1970. Observations on the behaviour of Platanista gangetica in the Indus and Brahmaputra
Rivers. Investigations on Cetacea, ed. G. Pilleri, 2; 27-69, Berne.
PILLER!, G., 1972. Field observations carried out on the Indus Dolphin Platanista indi in the winter of 1972.
Ibid., 4: 23-29, Berne.
PILLER!, G., and M. GIHR, 1971. Differences observed in the skull of Platanistagangetica (Roxburgh, 1801)
and indi (Blyth, 1859). Ibid., 3(1): 13-21, Berne.
ROBERTS, T.J., 1972. A brief examination of ecological changes in the province of Sind and their consequences on the wildlife resources of the region. Pakistan J. Forestry, 22: 89-96.
SERGEANT, D. E., 1962. The biology of the pilot or pothead whale Globicephala melaena (Traill) in Newfoundland waters. Bull. Fish. Res. Board Canada, 132: 1-84.
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Sci. Rep. Whales Res. Inst.,
No. 27, 1975.