Age, growth and harvestable size of Labeo rohita (Ham.) from Lake Jaisamand. (Dist. Udaipur) were determined with the use of key scales. Six annual rings.
Indian J. Fish., 45(2) : 169-175, Apr.-Jun., 1998
Age, growth and harvestable size of Labeo rohita (Ham.) from the lake Jaisamand, Rajasthan, India SHALENDRA SINGH, L. L. SHARMA AND V. P. SAINI Department Rajasthan
of Limnology Agricultural
University New Campus,
and
Fisheries,
University, Udaipur — 313 001, India
ABSTRACT Age, growth and harvestable size of Labeo rohita (Ham.) from Lake Jaisamand (Dist. Udaipur) were determined with the use of key scales. Six annual rings or annuli were used to estimate selected growth parameters i.e. annual length increase (h), annual increase in weight (w), index of species average size (Qh), index of population weight growth intensity (QCW), specific growth rate (CI), specific rate of weight increase (Cw), growth constant (Clt) and growth characterisitc (Cth). These growth parameters indicated higher values of QCW (52.60 g) and Qh (10.18 cm). The W was found to increase upto fifth year of life, while Cth increased only upto third year and declined thereafter. Similarly, the Clt and average Clt observed during the initial phase of life i.e. upto second year were comparatively more than the last phase i.e. fifth to seventh year. On the basis of growth parameters the harvestable size of 46 cm has been calculated for Labeo rohita of Lake Jaisamand. Introduction _ , .. . i . T Scale studies aimed a t age and growth determinations are not very common in India. Menon (1955) worked on the age and growth of Labeo fimbriatus from Mettur reservoir in southern India by examining scales. Johal and Tandon (1985, 1987a), studied the scales in La6eo ro/iito. Natarajan and Jhingran (1963), Kamal (1969), H a n u m a n t h a Rao (1974) and Pathani (1981) restricted their studies mainly for back calculation of lengths. In the present study an attempt has been made to investigate the key-scale of
Labeo rohita for finding information regarding size, scale rings and related d i m e n s i o n s f o r c a l c u l a t i n g i e n g t h a t the t i m e of a n n u l u g
formation
Material and methods The key scales were obtained from selected samples of 35 fishes of size range 31.20 to 78.10 cm collected from commercial landings of Lake Jaisamand (Rajasthan) during 1994-'95. The laboratory study on scale was conducted using CZ VEB DOKUMATOR. The primary, secondary and tertiary radii of scale were studied. The methods sug-
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Shalendra Singh et al.
gested by LeCren (1951) were followed for the estimation of correlation between different parameters offish scale. For the back calculation, the following formula was used. Sn L -a = — (L - a) S
wherein : Ln - 1 = total length of fish at ultimate and penultimate ages; Wn, Wn1 = weight at ultimate and penultimate ages; j = juvenile; a = adult; h = absolute increase in length and t2 and t( = time intervals between ultimate and penultimate age classes.
where, Ln = length offish in cm when annulus 'n' was formed, L = length of fish in cm at the time of capture; Sn = radius (cm) of annulus; S = total scale radius (cm) and a = correction factor (this mode is calculated by plotting the graph between total fish length and scale radius).
In the present study the structural details of typical cycloid type of scales were evident. From these results an attempt was made to establish relationship between the scale height and total length of the fish and also with radius of the scale (Table 1).
The following growth parameters were calculated (Johal and Tandon, 1985). Growth characteristics (Cth) = Log L -Log L - 1 , T s n s n x L -1 0.4343
Growth constant (Clt) = Log Ln - Log Ln - 1 0.4343
t2 + t t 2
X
Specific rate of linear growth (CI) = L
n ~ L~ " L -1
1
x 100
n
Specific rate of growth increase (Cw) = W - W -1 n
n
inn
y
100
W - 1 n
Index of population weight growth C = 1 intensity (QCW) = ^ + & Index of species average size (Qh) = h - 1 X nj + a nj + a
Results and discussion
The fish weight indicated a high V value of 0.979 with the height of scale. In other cases too the 'r' value was always above 0.90 except for the fish length vs scale radius, where this was 0.889. It is evident from Table 1 that the value of 'n' (exponent) varied between 0.3493 and 1.1369 for different parameters. The highest value was, however, seen for fish weight vs height of scale. Further, the studies of growth rings or annuli in the scales of Labeo rohita showed a maximum of six rings. It is worth mentioning here that Johal and Tandon (1987 b) observed spawning marks on the scale valid for age deterTABLE 1. Regression for selected morphometric parameters with scale readings in Labeo rohita of Jaisamand Lake 1. Fish weight in g (W) vs scale height in cm (SH) Log W = -0.89748 + 0.3723 log SH. r = 0.979 2. Fish weight in g (W) vs scale radius in cm (SR) Log W = -0.8962 + 0.3493 log SR. r = 0.937 3. Fish length in cm (L) vs scale height in cm (SH) Log L = -1.5775 + 1.1369 log SH. r = 0.903 4. Fish length in cm (L) vs scale radius in cm (SR) Log L = -1.5117 + 1.0502 log SR. r = 0.889 5. Scale height in cm vs scale radius in cm (SR) Log SH = -0.0449 + 0.8924 log SR. r = 0.9506
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Age, growth and harvestable size of Labeo rohita
mination. According to these authors, factors like spawning stress, high temperature and scarcity of food are responsible for the formation of annuli.
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Bhatnagar (1979) found that the scales of Labeo fimbriatus showed clear rings which were annular in nature. These rings are reported to be formed due to starvation. Similar studies have been made by Jhingran (1959, 1969) and Rao and Rao (1972).
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On the basis of scale studies and back calculations for lengths (Table 2), the back calculated length (L) in cm, annual length increment (h) in cm, annual increase in weight (w) in g, index of species average size (Qh), index of population weight growth intensity (QCW), specific rate of linear growth (CI), specific rate of weight increase (Cw), growth constant (Clt) and growth characteristic (Cth) were calculated for Labeo rohita and are depicted in Table 3. From these calculations it is evident that with the increase in age there is decrease in the specific rate of linear growth, specific rate of weight increase and annual length increment. On the contrary, the annual increase in weight (w) has shown an increasing trend upto 5th year of life. In the 6th year, it declined slightly and increased again in the 7th year. Table 3 also indicates the average annual length increment of 10.18 cm and weight increment (Qw) of 52.60 g. It is interesting to note that Johal and Tandon (1985) while studying growth parameters of Labeo rohita from three water bodies of northern India observed Qh to vary between 10.67 and 11.19 cm. From this point of view, the average length increment (Qh) calculated in the present study is fairly comparable despite being slightly lower than the
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