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REPRODUCTIVE BIOLOGY OF INDIAN SHAD (TENUALOSA ILISHA) IN THE BHAGIRATHI-HOOGHLY STRETCH OF THE GANGA RIVER SYSTEM

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Hilsa (Tenualosa ilisha ) is the major fishery component in the Bhagirathi-Hooghly river system accounting for about 15-20% of total fish landings. It is an anadromous fish, which migrates into the Bhagirathi-Hooghly river system from the Bay of
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  42  J. Inland Fish. Soc. India , 44 (2): 42-48, 2012  REPRODUCTIVE BIOLOGY OF INDIAN SHAD ( TENUALOSA ILISHA ) IN THE BHAGIRATHI-HOOGHLY STRETCH OF THE GANGA RIVER SYSTEM Utpal Bhaumik, Sritama Bose, B.B.Satpathy, R.K.Manna, A.K.Sahoo and C.M.Roshith Central Inland Fisheries Research Institute, Barrackpore, Kolkata 700120. (Received : 18.05.2012; Accepted : 03.11.2012) Hilsa ( Tenualosa ilisha  ) is the major fishery component in the Bhagirathi-Hooghly river system accounting for about 15-20% of total fish landings. It is an anadromous fish, which migrates into the Bhagirathi-Hooghly river system from the Bay of Bengal for breeding and spending their earlier part of life. Aspects like gonado-somatic index, fecundity, maturity and breeding season were studied during 2011 and 2012. The gonado somatic relation for male could not be drown due to highly fluctuating values and for female it was y=8.837+0.004x. Fecundity ranged from 44,002 to 15,54,894 in the size range of 208 mm to 475 mm and 105 g to 1175.5 g. The minimum fecundity value recorded was lowest ever recorded for the species. The first maturity size for males tallied with the previous record but the same was comparatively lower for the females. Breeding season varied with year and indicated linkage with the monsoon and intensity of flood. Keywords: Tenualosa ilisha, Gonado somatic index, Fecundity, Maturity, Ganga. Introduction Hilsa, the Indian shad Tenualosa ilisha (Hamilton) belongs to the sub-family Alosinae of family Clupeidae. The fish is major component in the Bhagirathi-Hooghly river system accounting for about 15-20% of total fish landings. Hilsa, the prime anadromous species of the Bay of Bengal region migrates to fresh water river habitats in Bangladesh, Mayanmar, India for breeding and spent early part of its life. In India, Bhagirathi-Hooghly river system stands one of the important destinations for the species. Because of its behavioral, biological and commercial importance, the species had been a subject of extensive research since last centuries. Investigation of various aspects of biology, migration and fisheries had been carried out in different river system of the country by different workers time to time. It is evident from their findings that the species differed in various biological aspects from one system to the other, which might be due to eco-environment variabilities. Likewise breeding season, spawning peaks and periods, age at the first maturity, fecundity and somatic growth pattern had also been studied and reported to vary in the Hooghly estuary by different workers. Hydro-ecological conditions of the Hooghly estuary have undergone conspicuous changes due to Ganga water diversion and subsequent adaptation of management measures through the Farakka barrage since mid seventies. Time scale data available in this regards (Sinha et al., 1996) revealed remarkable changes in salinity distribution pattern of the estuary, and also though not pronounced, in other hydrological characteristics. Biologically the fresh water flora and fauna also gradually got dominance replacing many of the brackishwater and marine forms. Since, these hydro-biological alternation are  43 existing over prolonged period these have turned to be phenomenal for the estuary. The present attempt was to investigate the influence and impact of changed eco environmental conditions of reproductive biology of hilsa in Hooghly  –   Bhagirathi River. Materials and methods Entire stretch (523.59) of Hooghly-Bhagirathi river from sea face onwards upto the feeder canal lock gate at Farakka was studied at fifteen sampling station viz., Farakka, Lalbagh, Nabadwip, Kalna, Balagarh, Tribeni, Hooghly ghat, Nawabganj, Baranagar Godakhali, Raichawk, Diamond Harbour, Harwoodpoint, Nischintapur and Freserganj in decending order (Fig. 1). Hilsa specimens were procured from all these stations round the year depending on availability. For better coverage and obtaining every size group of the fish, samples from landing stations and from fishing nets were collected. Thus, adult hilsa from the gill nets and juveniles caught through the bag nets were equally represented in the samples. Fortnightly sampling with coverage of full moon and new moon phases were followed. Fig.1. Hooghly-Bhagirathi river system and sampling sites Fish samples were immediately preserved at the field itself in ice box (if those were adults and bigger size) or in 5% formaldehyde for comparatively smaller fishes. For first hand sorting, the mature males were segregated at the sites by testing presence of milt on application of slight pressure on abdomen. Females comparatively bigger in size, could be identified from the shape of the abdomen and vent condition. The remaining fishes were dissected, examined and segregated in maturing males and females and immature ones without sex differentiation were discarded. Fishes above 100 mm size were considered for studying reproductive biology. Month wise as well as location specific data on various aspects of reproductive biology were recorded during in period of two years 2010-11 and 2011-12. Results and Discussion Fecundity Fecundity is one of the important aspects of reproductive biology of fish species, which provides information on the number of eggs in the ovary prior to the next spawning season. Altogether 212 females’  samples collected from the entire stretch were examined for determining fecundity of the individual. The ovaries with mostly mature eggs were considered for fecundity determination. Weight of demoisturised ovary for individual fish was recorded. A known amount (10% of total ovary weight) of egg samples from upper, middle and lower portion of both the ovaries were collected in equal portion and mixed up thoroughly. Representative (in weight) sample of the total eggs was then taken out and counted with naked eye for numerical abundance. Finally the total no of ova was calculated with necessary multiplications for fecundity estimation of the fish. In the present study fecundity of the samples was worked out and the number of ova ranged between 44,002 and 15, 54, 894 for the fishes in the size range of 208 to 475 mm in length and 105 to 1175.5 gm in weight. The fecundity values were finally plotted against total length and total weight (fig.2 and fig.3). A straight-line relationship of  44 fecundity was evident with length and also weight of the fish. The regression equation calculated by adopting least square method between total length and fecundity was log F=2.262+2.260 log L and that for total weight and fecundity was log F=3.153+0.968 log W. Respective correlation coefficient (r) values for total weight and fecundity were 0.5486 (P<0.001) and 0.6403 (P<0.001). The minimum fecundity recorded for the species during present studies was much less compared to the finding of Pillay (1958) and De (1986). The observed lower fecundity was obviously in the smaller size fishes (Table: 1) and of early maturity stage. Table: 1. Fecundity of hilsa ( Tenualosa ilisha ) in Hooghly-Bhagirathi river (n = 212) Total length(mm) Weight(g) Gonad weight(g) Fecundity % of total population Range mean range Mean Range Mean range Mean 208-250 221.5 105-352 164.84 6.35-50 18.02 65394-1067220 213221 8.02% 251-300 275.06 173-332 249.28 2.78-56.39 24.7 44002-569539 334436 32.55% 301-350 327.33 250-605 395.78 3.19-92.03 41.74 50003-1066904 510361 16.98% 351-400 377.36 142-957 606.87 4-155.97 77.81 67996-10085922 1013520 28.77% 401-445 418.22 631-1175.5 845.06 7.14-183.62 96.11 72312-1554894 986856 12.74% 457-475 466 1086-1112 1099 136-149.27 142.64 1399984-1428663 1414324 0.94% Fig. 2. Fecundity-length relationship of  Hilsa  ( Tenualosa ilisha) Fig. 3. Fecundity-weight relationship of  Hilsa  ( Tenualosa ilisha) Fig. 4. Monthly variation of mean fecundity of  Hilsa  Fig. 5. Fecundity of  Hilsa  at different length group  45 Fecundity of hilsa has been observed to be varying with months. Higher fecundity has been recorded during peak spawning months i.e February and September ( Fig. 4 ). Further, the study reveals that fecundity increases with increase in length of the fishes (Fig.5). This is in conformity with study conducted by Pillay and Rao (1962) in the Godavari estuary Gonado-Somatic Index (GSI)  The ratio between gonad weight and body weight was studied for both the females and males. Samples collected round the year from all the selected points and were studied for understanding the maturity condition and breeding season of the fish. The gonad weight recorded for the fecundity studies were used for drawing the correlation of gonad weight with body weight for the females whereas for males the data were generated afresh. Male fishes in the size range of 151-200 mm with distinguishable testes, numbering 212 were brought under the study. The mean GSI for the size range was minimum of 0.48 (Table:2). The GSI gradually increased with the growth of male fishes and the value started peaking up beyond 300 mm of length. But the correlation between GSI and body weight and total length of male gonad could not be drawn due to highly fluctuating values. The GSI vales for the males showed variation with the months (Fig. 6). There was an indication of a peak during January to March. For rest of the year the GSI values had marginal differences with slightly higher range between August and October, which along with the winter peak tallied with the peak breeding season of the fish. Pillay (1958) also could not find clear pattern of fluctuations in the GSI values of males but recorder low values for the same (0.2-0.4%) in rainy season and fairly high up to 2.3% in winter month. The values recorded by the Pillay (1958), was comparatively less, due to selective fishing, as mentioned by him. During present investigation the samples of male fish could be obtained and studied for wide size range and almost round the year. Equal numbers of females (212) were studied for GSI estimation. The gonado-somatic relationship for the females ranged between 0.76 and 21.47. It is interesting to note that the females in Hooghly-Bhagirathi riverine stretch were found to carry oocytes all through the year, but with variation in GSI values (Fig. 7). From GSI values two distinct periods of breeding, one during August to October and the other from January to February could be ascertained. However, variation in GSI values for the hilsa females in Hooghly estuary is not uncommon: De (1980) reported the same to be higher during September to Merch i.e.,  continuity of breeding period from monsoon to end of winter. Such availability may be attributed to the compulsive physiological adjustment out of environmental alteration influences for the particular season or years of observation. The mean GSI values for different size samples above 201 mm did not differ much (Table:3). Abundance of female above 450 mm and 1000 gm size were very low (<1.0%), which might be due to intensive fishing in lower zone allowing very few to pass through in the upper estuarine-riverine stretch. While drawing correlation between GSI and length and weight of the females, the same were found to be positive (Fig. 8). The respective regression values were Y=5.605+0.162X and Y=8.837+0.0074X. However, the individual GSI values even in the same size range were quite fluctuating.  46 Table 2. GSI of Male Hilsa at different size range (n=212) Table 3. GSI of Female Hilsa at different size range (n=212) Fig. 6. Male Gonado-somatic Indices of the  Hilsa Fig. 7. Gonado-somatic Indices of the  Hilsa ( Tenualosa ilisha )  Maturity and breeding season Pillay (1958) reported that hilsa in the Hooghly-Bhagirathi river system spawn intermittently and the fish have two distinct breeding season but the same fish do not spawn twice during the the year. The occurrence of two spawning has also been reported from the Ganga river system by Nair (1958), Mathur (1964) and Bhaumik and Sharma (2011). While studying the reproductive biology of hilsa in Hooghly estuary De (1986), De and Saigal (1989) and De et al,  (1994) observed presence of only a single size group of ova in a mature ovary, which is distinctly separate from immature one. Pillay (1958) experienced the problem of inadequate fish samples from lower estuary or foreshore areas of adjoin Bay of Bengal in eighties for determining size at first maturity but it could be achieved under the present investigation. Year round gill net fishing of hilsa in the foreshore region facilitated sampling programme vis-à-vis  collection of samples. For ascertaining size at first maturity initially GSI values were utilized and thereafter, the autopsy was carried out for confirmation. Altogether 424 fish samples were examined during the investigation. First maturity of male hilsa was recorded in the size range of 151 to 200 mm and the percentage of mature males gradually increased following the advancement in size. About 80% of male population was observed to mature at the size between 250 and 499 mm. The findings on first or smallest size at maturity of the males tallied with Pillay (1958) and as regard the bigger size males, the observation also confirmed (Pillay, 1958) the majority in mature condition is above 230 to 250 mm size. For maturity study of female hilsa, diameters of random samples of 300 oocytes were measured by the compound microscope fitted with micrometer and the readings were converted for actual values by using the magnification factors. The ova diameter pattern was studied from all the sampling points and for every month depending on availability. Size frequency of the ova was the main
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