318 Relative Value of Certain Larvivorous Fishes from the Punjab
ranged from 4 to 17 larvae per sq. ft. surface
area. The tank was, therefore, further stocked
with 50 Colisa lalia, 75 Ambassis baculis and 50
Barbus (Puntius) sophore. Study of gut contents
of two specimens of each species caught from the
tank daily, for three continuous days, showed
that the fish were devouring the larvae voraciously
and that the mosquito population had declined
(Table IV, a, b, c). On the third day it was
found to be two larvae per 4 sq. ft. surface area.
On the same day a fresh supply of 20 Colisa
lalia, 50 Ambassis baculis and 50 Barbus (Puntius)
sophore was introduced into the tank, and another
lot of 50 Ambassis baculis was added to it on the
7th day. On the 16th day, the tank was free
from mosquito larvae. The study of the gut
contents of these fish was, however, continued
till the middle of June. During this period it
was observed, that in Colisa the mosquito larvae
were present in 74 per cent of the fish examined,
in the case of Ambassis, 58.8 per cent had them,
while in Barbus (Puntius) sophore, 4.8 per cent
had remains of mosquito larvae.
The number of larvivorous fish put in the tank
to free it of the mosquito larvae, excluding those
taken out for study or picked out dead, was 300.
This number included 130 Barbus (Puntius)
sophore which are not active larvivores in the
field. It may, therefore, be recommended that
in order to achieve complete extermination of
mosquito larvae in the shortest possible time, a
population of one fish per 18 sq. ft. surface area
of the tank is needed, i.e., about 2500 fish per
acre are required to produce any tangible results.
TENACITY TO LIVE IN STAGNANT WATERS
It is an admitted fact that village ponds in
almost all cases receive some sewage. In order
to test the tenacity of the species to live in stagnant
waters, certain experiments were carried out in
the laboratory. It was observed that Colisa
lalia lives comfortably in stagnant waters. When
introduced in a bowl containing water from drain,
it remained alive till the duration of the experi-
ment, i.e. for one week, and did not show any
sign of distress. Under similar conditions
Ambassis baculis and Barbus (Puntius) sophore
died within five hours. The temperature of
water was 80ºF.
In another experiment, specimens of Colisa
lalia, Ambassis baculis and Barbus (Puntius)
sophore were kept in separate bowls, of 7 litre
capacity, containing ordinary tank water, water
from city sewage, and sewage water mixed with
tank water respectively. In tanks, which do not
receive any sewage and are occasionally repleni-
shed from canal, all these species live, flourish and
breed without restriction. In sewage water,
which has an offensive smell, and is dark in colour,
these fish die sooner or later, while in sewage and
tank water, mixed in equal proportion, Colisa
continues to live but the other two species live
only for a few hours. These results are tabulated
as under :—
Colour . . .
Odour . . .
Temperature . .
Dissolved oxygen .
Absorbed oxygen . .
Free carbon dioxide .
pH . . . .
Colisa lalia . .
Ambassis baculis . .
Barbus (Puntius) sophore
Tank water
Light yellow
No smell
84ºF.
2.66 c.c. p. 1.
0.11 c.c. p. 100 1.
2.275 c.c. p. 1.
7.8
Lives and breeds
Lives and breeds
Lives and breeds
Sewage water
Black
Offensive
80ºF.
0.243 c.c. p. 1.
0.45 c.c. p. 100 1.
23.763 c.c. p. 1
7.2
Lives for 33 to 48
hours
Lives for 5 to 13
minutes
Lives for 2 to 5 hours
Sewage water mixed
with tank water 1:1
proportion
Blackish
Offensive
81ºF.
1.61 c.c. p. 1.
0.34 c.c. p. 100 1.
10.997 c.c. p. 1.
7.4
Does not die
Lives for 7 to 10
hours
Lives for 6 to 11
hours
Remarks
All fish show signs
of distress in sewage
water.
Dissolved
oxygen, when fish
died, was 0.06 c.c.
p. 1.
Of all the three species, Colisa is the hardiest.
It does not confine itself to dissolved oxygen in
water for its respiration, as it possesses accessory
respiratory or labyrinthine organs above the
gills, which enable it to breathe atmospheric air
direct. It is on account of this air-breathing
capacity that it can live in foul water and can
easily be transported from place to place. Similar
observations have also been recorded by Prashad
and Hora [1936]. It may, however, be mentioned
that all the three species of larvivorous fish
mentioned above are hardy enough to live in such
stagnant waters as lie in the vicinity of towns
and villages.