THE DEPTH DISTRIBUTION OF THE CAT-FISHES,
TACHYSURUS THALASSINUS (RUPP.) AND T. TENUISPINIS (DAY),
IN THE NORTH-WESTERN BAY OF BENGAL.
K. V, SEKHARAN*
Central Marine Fisheries Research Institute; Sub-station, Waltair,
Visakhapatnam-3
,
ABSTRACT
During 1964-67, a study was made of the depth distribution of- the catfishes, Tachysurus thalassinus (RuPP-) and T. tenuispinis (Day) in the grounds
up to 100 m deep in the north-western Bay of Bengal, based on the catches of
exploratory trawlers. On an annual average, both species have peak abundance
in two depth-ranges, the first being 30-39 or 40-49 m and the second, 60-69 or
70-79 m. Both species have high abundance in shallow grounds < 50 m deep)
in April-June and October-December and in deeper grounds in; July-September.
In September-December the sizes available, of both species, increased with increase
in depth. T. thalassinus of small size was recorded only from the 17»40', 19°10'
and 19°40' zones. No inter-zonal difference was found in the size of the other speeies.
INTRODUCTION
In an account of exploratory trawling from 1961 to 1965 on the continental shelf along the north-western Bay of Bengal, Sekharan et al. (1968)
gave estimates of the relative abundance of cat-fishes, as a group, in the grounds
up to 100 m deep. Work on the biology of the cat-fishes of this region was
started in 1964. It was seen that the cat-fishes represented in the traWl catches
consisted mainly of two species, Tachysurus thalassinus (Rlipp.) and T. tenuispinis (Day). The seasonal abundance, and possible annual catch, in commercial trawling, of each species has been dealt with by the author (Sekharan,
1968). The present account deals with the catch per hour and the lengthgroups of each species according to depth in different zones in the area, based
on the observation of exploratory trawling from 1964 to 1967.
MATERIAL AND METHODS.
The present account is based on observations on the catches of the
Government of India trawlers, m. t- Ashok, m. v. Champa and m. v. Sea Horse,
stationed at Visakhapatnam and the log reports of the skippers. The log
reports gave the data for cat-fishes as a group. The species composition was
*
Present Address: Central Marine Fisheries Research Institute, Cochin-I8.
194
K. V. SEKHARAN
estimated by shipboard observations whenever possible. When this was not
possible, the following procedure was adopted for estimating it. (i) For daily
voyages the catch heaps on the deck could normally be split into hauls (there
were usually only 2-3 hauls on each day), (ii) In respect of long voyages, the
cat-fishes in the fish-hold could be divided into 4-5 layers, the topmost layer
assigned to the last few hauls, the bottom one to the first few hauls and the
others to the intermediate hauls containing cat-fishes, as recorded in the log
sheets. The disadvantage here is of course that the same composition would
be assigned to more than one haul. However, the species composition so arrived at compared favourably with that observed in the same depth-ranges and
areas during shipboard work.
The mid-value of the depth-range of a haul was regarded as the depth
of the haul. The depth-wise data were summed up for 10 m intervals. The
length measurements were grouped into 2 cm size-classes. Ashok used a trawl
net with larger mesh than the other two vessels; hence the data of Ashok on
the one hand and Champa and Sea Horse on the other are treated separately.
The zones are the same as those mentioned by Sekharan et al. (1968).
ANNUAL CATCH PER HOUR IN DIFFERENT DEPTH-RANGES
The annual depth-wise catch per hour in the intensively fished zones is
given in Fig. 1.
a) T- thdassinus. The c.p.h. in all zones and for all vessels has
maxuna in two depth-intervals, the first usually in the 30-39 or 40-49 m and
the second in 60-69 of 70-79 m.
b) T. tenuispinis. The depths of maximum c.p.h. are roughly the
same as in the case of the other species; only in certain zones and in certain
years the depth of the maximum c.p.h. of the two species differed by about
10 m, which can be expected even on sampling considerations.
SEASONAL CHANGES IN THE DEPTH-WISE C.P.H.
Only the zones fished for 6 months or more each in a year have been
taken into account here. The 17°40' - 19''40' zones fall into this category. The
monthly depth-wise c.p.h. are represented in Figs. 2-5. The depths of the maximum
cp.h. of various months are joined by lines. Following Sekharan et al. (1968)
the grounds up to 50 m deep are referred to as shallow and those more than
50 m in depth as deep. The figures show that the shallow and deep regions
have alternating periods of abundance of cat-fishes.
a)
T. thalassinus.
i) m.t. Ashok: Generally April-June and October-November are the
periods when c.p.h. is higher in shallow regions than in the deeper grounds
195
DEPTH DISTOIBXJTION OF CAT-FISHES
18M0'
17* 40'
FI6 1A
19* <0'
19*10'
18*<0'
1964
O100
O
*0
100
A S H 0 K
kkdlUl
NF
Mif^i,^^^
Ixli
•Jfi[|[|[lr-,NF
J
Jk
m^M-
J I
1965
160
40
-Nf.—
160
100
100
40
- l l f i - n n —NF,-.-
40
CC
D 40
O
u
Jfl[l,T.!;'^-r
65
S5
^7*iCf
hi 20
a.
J
Ji Id
40
•NF-
NF
NF
65
65
95
35
85
NF
Nf
40
65
95
35
IT
65
65
95
1965.20
1966 20
25
55
17*40'
85
FIG1C
18*10'
1964
20
-
.n-NF
- 20
HORSE,,
• 20
. mm.rU,
19-67
J ^ — - NF - 7 -
55
NF
LJJ.
1966
C HA M P A
20' .
lis
40
NF
18*10'
18*10'
19.64
1-
-NF
n-NT-n-NF-
196'
1965
nJLiJt tm rti ^L
-
•
J^LdfclMnJL^w rill I
if 20
.
1966
35
F I 6 1B
i.rfi rfi r* ni.HI J
20
X
-NF -
Nfr
NF
,35
35
^
J
l-NF.
25
II 85„ J
55
T
H
20 1
1965s £ A
— Mr —
25
A
55
N
85
G
25
E
55
S
(M)
FIG. 1. Annual catch per hour (kg) of T. thalassinus (white bare) and T. tenuispinis
(black bars) in different depth-ranges of various zones. (NF = No Fishing)
and July-September and November-December, the period when the reverse trend
is seen. The table below sums up the pattern in diflferent zones.
Months when c.p.h. is higher
in deeper grounds than in shallow
Zone.
in shallow grounds than in
grounds.
deeper grounds.
Mar. (some years),
17°40' March - June, October
July-September, Nov.-Feb.
18° 10'
March - June, Oct., Nov,
Mar. (some years), Aug., Sept.
18°40' April, May, September
Feb., Mar., June., July, Oct
Feb.,
March, July, Oct.
19" IC April - June, Sept., Nov., Dec.
September.
19''4(y
March.
85
K; V.'SEKHARAN
196
I
I I
I I I
I I
l7'4Cf
25
I
I I
I
(1967)
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18 10' (1967*
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A
a
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18-23
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A 30-35
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a i r 4 a (1966)
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25
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o
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S
FIG. 2. Catch per hour (kg) of T. ihalassinus recorded by m.t. Asliok from different
depth-ranges. (The depths of maximum c.p.h. in various months are joined by
lines).
The difference between zones may, to some extent, be due to sampling
error. However in 1964 and 1965 when all zones were fished, the offshore
shift of the best c.p.h. in March-April was found to be more marked in the
northern zones, 18°40' and 19° 10', than in the southern zones. On the other
hand the shift in July-September of the best c.p.h. to deeper grounds is well
expressed in all zonesii) m.v. Champa and m.v. Sea Horse. The pattern is roughly the sarne
as that shown by the data of Ashok.
'
197
DEPTH DISTRIBUTION .OF.rCAT-FISHES
I I'l
25
r 111
f"i »
I I ' l |i '|i«'i"i r"i"' 'f r i . ' i " i ' I ' T " i ' .1 r
' 'lUI'
I
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I
I
°
1967
I
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I
19*40'
55 •0 0 O
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85
I 4 I I > hi t I > I li
1966
3
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25
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(1964) .
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9
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(1965)
•
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1965
1965 o
o o
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(1964)
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J A S O N D J F M A M J J A S O N D
M
0
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t _ ' .
•
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FIG. 3. Catch per hour (kg) of T. tenuispinis recorded by m. t. Ashok from different
depth-ranges (details as in Fig. 2).
b)
•
•
!
•
J r M A M J J A S 0 NTT
T. tenuispinis.
i) m.t. Ashok. April-June and October-December are the periods
when c.p.h. is higher in the shallow region than in the deeper region and JulySeptember, the period when it is higher in the deeper grounds. The following
table gives the general pattern of shift in maximum c.p.h. in different grounds.
Period in which c.p.h. is higher
Zone.
in shallow grounds than in
in deeper grounds than in shallow
deeper grounds.
grounds.
17°*0' March - June, Oct.
July - Sept., Nov. - Jan.
18°10' March, May, Oct., Nov.
March (Some years), Aug., Sept. I
18°40'
April, May, Sept.
Feb., March, June, July, Oct.
19*10' Sept;, Nov., t)ec.
March - Jiily^ Oct: "
'.'•
I9''40'
Oct. -Dec:
i
September; - - ;
198
K. V. SEKHARAN
> O O O A O
• O ) A P A
"^35
UJ
1965
o o
-»15-
I'cA* a a oJ
•
BO 0 O
17* AO'
<
GCd5
15
1964
X 35h4 o
o o
Ixl
•
7595
_J
A
M
0
N
T
H
S
• •
o
'
lAo'
FMAMJJ
O M 0
NT
*
1
AS6ND
H S
FIG. 4 A . Catch per hour of T. thalassinus recorded from different depth-ranges by m.v.
Champa (details are in Fig. 5A).
FIG. 4 B . Catch per hour (kg) of T. thalassinus recorded from different depth-ranges by
m.v. Sea Horse (details as in Fig. 5A).
It may be noted that the shift of the best c-p.h. to deeper grounds in
July-September is seen in all zones.
ii) m.v. Champa and m.v. Sea Horse. The shifts in c.p.h. have the
same general pattern as those mentioned in regard to Ashok.
199
DEPTH DISTRIBUTION OF CAT-FISHES
I fi-i
0 6
25
t967
i 55
85I
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1966
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17*40 (1964).
VSt Oo o o a
ooA
•o'
• • ) A/o\a o
6 6 o 0 V ^ " • •/a\o A5510 69 A
A o
O Ji
Ji o • - • . « A >3 »
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85
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A
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s
B
FIG. 5A. Catch per hour (kg) of T. tenuispinis recorded from different depth-ranges by
m.v. Champa.
FIG. 5B. Catch per hour (kg) of T. tenuispinis recorded from different depth-ranges by
m.v. Sea Horse.
LENGTH-RANGES RECORDED IN DIFFERENT DEPTHS
The size-ranges recorded from different depth-intervals are shown in
Table 1. The data of Champa and Sea Horse showed that in September-December, with increase in depth there is perhaps an increase in the size of the two
species. Apart from this, there was no indication of change of size of the
two species in relation to depth.
1. Length-ranges (cm) of Tachysunis thalassinus (T. thai.) and T. tenuispinis (T. ten.)
recorded from different depth intervals during 1964-67.
TABLE
.!
Depth - range (m) •
•-
a) m.t. Ashok.
17''40'
18''10'
T.thal.
T.ten;
T.thal.
T.ten.
20-29,
>' 16-47
no data
' 20-43
30-39
14-47
14-39
40-49
8-47
50-59
18-40 /
T.thal.
T.ten
19-'10'
T.thal.
T.ten.
19''40'
T.thal.
T.ten
tj
no data
no data
no data
no data
no data
14-39
14-31
16-47
14-43
22-43
-do-
-do
-do-
18-47
26-35
fS,
.-!;
14-43
16-47
14-43
16-47
20-45
14-47
14-41
16-49
16-39
?5
16-47
16-43
1&-47
14-43
16-47
14-43
14-47
14-43
16-39
14-39
i
60-69
14-43
14-43
no data
no data
18-39
14-39
16-47
14-43
16-39
no data
•z
70-79
18-41
14-43
-do-
-do-
18-43
18-31
16-47
no data
no data
-do-
80-89
18-41
14-43
-do-
-do-
no data
no data
20-47
-do-
-do-
-do-
90-99
18-41
no data
-do-
-do-
-do-
-do-
no data
-do-
-do-
-do-
100 m & above. —
-do-
-do-
-do-
-do-
-do-
-do-
-do-
-do-
-do-
201
DEPTH DISTRIBUTION. OF .CAT-FISHES
TABLE
J. {Continued.)
b)
Depth-range
m.v. Champa
17«40'
T.thal.
T.ten.
10-19'
C)
18°10'
-'
T.thal.
Tten.
—
m.v. Sea Horse
17040/
T.thal.
T.ten
no data
20-29
14-43
12-41
16-25
no data
16-37
12-39
30-39
10-47
14-47
10-39
12-41
12-43
12-43
40-49
8-43
12-47
12-37
12-47
14-43
12-45
50-59
10-45
14-55
12-35
14-39
14-47
18-43
60-69
16-41
12-43
no data
14-45
no data
no data
70-79
no data
no data
-do-
24-39
-do-
-do-
J80-89
22-31
-do-
-do-
Ho data
-do-
16-29
90-99
no data
-do-
-do-
-do-
-do-
no data
But inter-zonal differences are found in the size-range of T.thalassinks.
The small-sized fish were landed by Ashok only from the southern zone 17°40'
and the two northern zones 19°10' and 19°40'. In all zones, May-July and
November-February were the main periods of occurrence of young fish. In
regard to T. tenuispinis no marked inter-zonal difference in. size-range was
found; in all zones, young fish were landed mainly in May-July and OctoberDecember. The smallest of the size-groups of this species represented in the
catches of Champa and Sea Hqrse \yas not recorded from the landings of Ashok,
which used a net with larger mesh size, as already mentioned^
DISCUSSION
In the account of exploratory trawling from 1961 to 1965 in northwest Bay of Bengal, Sekharan et al. (1968) showed that cat-fishes as a group
had greater abundance in the grounds more than 50 m deep than in shallow
grounds in March-April and July-September and that the shift of the concentration of these fishes to deeper grounds is more marked and regular in JulySeptember than in March-April. The present data for 19i64-67 show the same
phenomenon, when individual species are taken into account. The shift is
perhaps related to the upwelling reported to take place in the area (Prasad,
1952; La Fond, 1954).
•
The present study also showed that the size groups caught from various
jdepths are practically the same, except during the last 3-4 months of the year.
202
K. V. SEKHARAN
when the sizes in deeper grounds tended to be larger than those in the shallow
grounds. The inter-zonal differences in the size of T. thalassinus are striking.
Fish belonging to lower limit of the size range recorded during the study were
found only in the 17°40', 19''10' and 19"40' zones. These zones also gave
higher annual catch per hour of this species than the other zones (Sekharan,
1968). Thus, from the point of view of the size-range available and the annual
c.p.h. two groups of T, thalassinus may be made out: one around 17''40' and
the other around 19°40 '.
The annual c.p.h. values show that the two species attain high level of
relative abundance in practically the same depth-ranges. This may be expected
on a consideration of their diet, the food spectrum of the two species being
different (Mojumder, personal communication). However the monthly c.p.h.
shows that the two species may attain dominance in two different depths during
a particular period. The indication given by annual c.p.h. can therefore be
regarded only as the average condition.
ACKNOWLEDGEMENTS
The co-operation and help received from the skippers and crew of the
Government of India trawlers, m.t. Ashok, m.v, champa and m.v. Sea Horse
are gratefully acknowledged. The author also thanks Dr. S. V. Bapat and
Dr. R. N. Bhattacharya for the facilities extended for this work.
REFERENCES
"LA. FOND, E . C. 1954. On upwelling and sinking of! the east coast of India. Andhra
Univ. Mem. in Oceanogr., !:78-85.
PRASAD, R. R. 1952. Preliminary observations of the surface temperature gradients and
light penetration in the upper 20O feet of water in the Bay of Bengal. PTOC. Indian
Acad. Sci., 36A:61-69.
SEKHARAN, K. V. 1968. Cat-fish resources of the coasts of Andhra Pradesh, Orissa and
West Bengal. In Proceedings of the symposium on the living resources of the
seas around India. Spec. Publ, C.M.F.R. Institute, Coc/ii/i:517-536 (Issued in 1973)
SEKHARAN, K. V., M. S. MUIHU, K. VENKATASUBBA RAO, V. RAMAMOHANA RAO, P. MOJUM-
DER & S. REUBEN. 1968. Exploratory trawling on the continental' shelf along the
north-western Bay of Bengal. In Proceedings of the symposium on the living
resources of the seas around India. Spec. Publ, C.M.F.R. Institute, Cochin:2S0-3i7
(Issued in 1973).