Results of mesh selectivity experiments on B. tor are presented in this paper. Selectivity curve on the basis of maximum girth of fish in relation to perimeter of mesh was worked out. The optimum girth/mesh perimeter ratio was found to be 1.31. A linear regression of G+0.445L=12.8 was fitted for conversion of length to girth

George, N.A.

Kesavan Nair, A.K.

Khan, A.A.

Mathai, T.J.

Aquatic Commons

e Optimum 
of Ba.rbus 
h size 
(Ha mi 
e Capture 
n) 
A. A. KHAN, N. A. GEORGE,* T. JOSEPH MATHAI** AND 
A. K. KESA VAN NAIR* 
Bur/a Research Centre of Central Institute of Fisheries Technology, Burla - 768 017 
Results of mesh selectivity experiments on B. tor are presented in this paper. 
Selectivity curve on the basis of maximum girth of fish in relation to perimeter of mesh 
was worked out. The optimum girth/mesh perimeter ratio was found to be 1.31. A 
linear regression of G + 0.445L = 12.8 was fitted for conversion of length to girth. 
Mesh selectivity curve is important from 
management point of view of fi:.heries and 
also to obtain accurate information concer-
ning fish population from catch data, to 
protect the immature fishes and to improve 
the exploitation of exploitable stock. Mesh 
selectivity of giU nets have been discussed 
by Olsen (1959), Regier & Robson (1966), 
McCombie & Fry (1960), Ishida (1962), 
Kitahara (1971) and Hamley & Regier (1973). 
These workers have made use of either the 
length frequency distribution of fishes to 
fit the selectivity curve or the maximum girth 
of fish in relation to perimeter of mesh. An 
attempt is made here to suggest the optimum 
mesh sizes. 
Materials at.d Method§ 
The fisrung experiments were carried out 
in Gobindsagar reservoir from October, 1969 
to November, 1970 and a total of 561 valid 
observations were made. Mesh sizes experi-
mented at a depth range 7 m to 40 m in the 
fishing experiments are 40 mm, 47.5 mm, 
50 mm, 52.5 mm and 55, mm and 60 mm bar. 
The design details and specification of the 
gear are as given by Khan et al. (1975). 
A surf boat OAL 18' was used for condu-
cting the fishing experiments. Two hundred 
and sixtyfive B. tor were caught among the 
other fishes, predominantly L. diplostoma, L. 
* Central Institute of Fisheries Technology, 
Cochin-682 029. 
'"':' Research Centre of Central Institute of Fisheries 
Technology, Panaji - 403 001, Goa. 
bata and Mystus. seenghala. Details of data 
on length, gill girth, maximum girth and 
weight of each :fish caught by the different 
mesh sizes were recorded. 
Results ar.d Discussion 
The catches in numbers for the total num-
ber of nets operated for all mesh sizes are 
presented in Table l. Only net . of mesh 
sizes 50 mm bar were taken to estimate the 
optimum girth/ mesh perimeter ratio, as 
sufficient number representing all size groups 
were available only with this net. Linear 
relationships of 'girth on length' and 'length 
on girth' were worked out by the method of 
least squares. 
Mccombie & Berst (1969) have chosen 
girth in preference to length, to investigate 
the relation between selectivity and the fit 
of the fish to the mesh. According to them 
a ratio based on the relation between fish 
girth and mesh perimeter would seem t<? be 
the most direct measure of fit. McComb1e & 
Fry (1960) plotted the frequency of capture 
aoainst the ratio of maximum girth of fish 
t; mesh perimeter. McCombie & Berst 
(1969) fitted the gill net.selectivity ~urves,.on 
the basis of maximum girth of fish m relat10n 
to perimeter of mesh, by the method of 
moments. Following these authors maxi-
mum girth/mesh perimeter ratios were worked 
out for each individual fish for the mesh 
size 50 mm. The frequency distribution of 
these are shown in Table 2. The mean worked 
out to 1.31 and the standard deviation to 
0.147. The skewness as measured by the 
FISHERY TECHNOLOGY 
OPTIMUM MESH SIZE FOR THE CAPTURE OF BARRUS TOR 93 
Table 1. No. of Barbus tor caught in different 
size groups 
Length 40 47.5 50 52.5 55 60 
group mm mm mm mm mm mm 
330 1 
350 1 
370 1 
390 4 2 
410 21 4 
430 8 2 4 2 
450 6 6 3 1 
470 6 5 5 3 1 
490 1 3 8 14 
510 3 2 11 4 
530 2 1 25 6 
550 1 16 3 
570 2 22 4 2 
590 12 l 
610 6 1 
630 4 3 4 
650 3 2 1 
670 4 1 
690 
710 1 1 2 
730 1 1 
750 1 
Total 55 27 125 37 12 9 
Tab1e 2. Frequency distribution of maximum 
girth/mesh perimeter ratios 
Maximum girth/ No. of Theoretical 
mesh perimeter fishes frequency 
1.0 5 4.8 
1.1 7 12.4 
1.2 30 25.4 
1.3 39 33.4 
1.4 23 27.8 
1.5 13 14.9 
1.6 5 5.1 
1.7 3 1.2 
third moment worked out to 0.00098. The 
theoretical frequencies obtained on fitting 
the normal frequency function (Hoel, 1957), 
- J.. ( x - 1 . 31) 
2 0.147 
)2 
f (x) = e 
0.147 ..; 2 1f 
Vol. 26, 1989 
where 'x' is the girth/perimeter ratio and are 
given in column (3) of Table 2. The good-
ness of fit was tested x 2 • The frequencies 
in the first and last classes were pooled with 
the adjacent classes as the expected frequency 
in these classes were less than 5 (Table 2). 
The x2 now worked out to 5.01 which is not 
significant for 3 degrees of freedom. Thus 
the normal approximation was found satis-
factory. The fitted normal curve corres-
ponding to the percentage frequencies along 
with the observed percent frequencies are 
shown in Fig. l. The model girth-perimeter 
>-
" c 
!!5 
u 
30 0 
0 
~ 20 
lL 0 
~ 
10 
0 
o..__-'-_ __._ _ _,_ _ ___,__ _ __. __ _ 
0·8 1·0 12 1-4 1·6 1·8 
Girth/perimeter 
Fig. 1. Gillnet selectivity curve on the basis of 
maximum girth of fish i.n relation to peri-
meter of mesh. 
ratio is found to be 1.31. Using this ratio, it 
is possible to determine mesh sizes to exploit 
any desired size of fish. If a specified length 
is perferred, the length-girth relationship 
can be used to fix the mesh size. This rela-
tionship for B. tor was found to be 
L = 1.85 G + 67.6 
where L is the estimated mean length for a 
girth measurement, G. Standard error of the 
regression coefficient was 0.052 (2.8 % of the 
regression coefficient) with 95 % confidence 
interval for the regression coefficient ranging 
from 1.75 to 1.95. Hamley & Regier (1973) 
used a linear relationship G = al + b to 
estimate girths (G) from length (L) by regres-
sion of girth samples of Walleye. They 
obtained a relationship of the form, 
G = 0.56 L - 1.7 
From a regression of maximum girth on 
total length, this relationship for B. tor was 
obtained as 
G = 0.445 L + 12.8 (Fig. 2) 
The regression coefficient was found to be 
highly significant (r = 0.908 being highly 
94 A. A. KHAN, N. A. GEORGE, T. J. JOSEPH MATHAI, AND A. K. KESA VAN NAIR 
significant for 265 degress of freedom). The 
95 % confidence interval for the regression 
coefficient is 0.420 to 0.470. The standard 
error of the regression coefficient was only 
2.8 % of the same. Since the normal fit 
was found adequate in the present case, the 
optimum girth to perimeter ratio can be 
taken to be 1.31. This optimum ratio can 
be used to fix the mesh size to catch a fish 
of a specified girth. If only length measure-
ments are available the regression equation 
0~-1~00~~20=0---'--=3=00~~4~0=0-'--=so~o~6~0~0~7o~o~s~oo"-'­
Fish length (mm) 
Fig. 2. Regression between maximum girth and 
length 
G = 0.445 L + 12.8 can be applied to get 
the corresponding girth measurement and 
the optimum mesh size worked out. Using 
the regression relationship L = l .85 G + 
67.6 and the optimum maximum girth to 
mesh perimeter ratio of 1.31, the optimum 
maximum girths and lengths of fish caught 
by nets of the mesh size considered is presen-
ted in Table 3. 
Table 3. Optimum maximum girths and 
lengths of Barbus tor caught by 
d(fferent nzesh sizes 
Mesh size 
40 
47.5 
50 
52.5 
55 
60 
Optimum 
maximum 
girth 
mm 
209.6 
248.9 
262.0 
275.l 
288.l 
314.4 
Estimated 
optimum 
length 
mm 
455.4 
528.l 
552.3 
576.5 
600.8 
649.2 
No fish was caught for girth/perimeter 
ratio greater than 1.7. The optimum ratio 
of 1.31 compares well with the girth/peri-
meter ratio of yellow perch of Senth Bay, 
estimated by McCombie & Berst (1969). 
In conclusion, selectivity curve on the 
basis of maximum girth of fish in relation to 
perimeter of mesh for the species B. tor was 
worked out. The optimum girth/mesh peri-
meter ratio was found to be 1.31. For con-
version of length to girth, a linear regression 
of G = 0.445 L + 12.8 was found useful for 
B. tor. The optimum maximum girths and 
the corresponding lengths (total) of B. tor 
caught by the experimental mesh sizes have 
been presented on this basis. 
The authors are thankful to Shri M.Rajendranathan 
Nair, Director, Central Institute of Fisheries Tech-
nology, Cochin for permission to publish the paper. 
References 
Hamley, J.M. & Reigier, H.A. (1973) J. 
Fish. Res. Bd Canada 30, 817 
Hoel, P.G. (1957) Introduction to Jvlathe-
matical Statistics,Asia Publishing House, 
Bombay 
Ishida, T. (1962) Bull. Hokkaido Reg. 
Fish. Res. Lab. 25, 20 
Kitahara, T. (1971) Bull. Jap. Soc. Sci. 
Fish. 37, 4 
Khan, A.A., George, N.A. & Pandey, O.P. 
(1975) Fish. Technol. 12, 64 
McCombie, A.M. & Berst, A.H. (1969) 
J. Fish. Res. Bd Can. 26, 2681 
McCombie, A.M. & Fry, F.E.J. (1960) 
Trans. Am. Fish. Soc. 90, 337 
Olsen, S. (1959) J. Fish. Res. Bd Can. 16, 339 
Regier, H.A. & Robson, D.S. (1966) J. 
Fish. Res. Bd. Can. 23, 423 
FISHERY TECHNOLOGY 


On the optimum mesh size for the capture of Barbus tor (Hamilton)

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On the optimum mesh size for the capture of Barbus tor (Hamilton)

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