World Journal of Fish and Marine Sciences 4 (1): 54-59, 2012ISSN 2078-4589© IDOSI Publications, 2012DOI: 10.5829/idosi.wjfms.2012.04.01.6196

Corresponding Author: Sachinandan Dutta, School of Oceanographic Studies, Jadavpur University, Kolkata 700 032, India.Tel: +91-9143684917, Fax: +91-33-21426242.

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Population Structure, Mortality Rate and Exploitation Rate of Hilsa Shad (Tenualosa ilisha) in West Bengal Coast of Northern Bay of Bengal, India

Sachinandan Dutta, Sourav Maity, Abhra Chanda and Sugata Hazra

School of Oceanographic Studies, Jadavpur University, Kolkata 700 032, India

Abstract: The population dynamics of Hilsa was investigated by using FiSAT II software of FAO on lengthbased data, was collected from the three main fish landing centers of West Bengal between June 2010 andMarch 2011. The asymptotic length (L8) was 477.75 mm and the von Bertalanffy Growth Function (VBGF)growth constant (K) was 1.900 yearG . The total mortality (Z) was 1.98 yearG and the fishing mortality (F) was1 1

0.73 yearG . The natural mortality (M) of Hilsa Shad was 1.25 yearG by using Pauly’s M empirical equation,1 1

where the mean habitat temperature was 27.9 °C. FiSAT shows the observed extreme length was 455.00 mmwhere predicted maximum length was 453.58 mm and the growth performance index (2') was 3.637. The averagesize group of Hilsa population was observed to be comparatively smaller than that found in Bangladesh water.The exploitation rate (E) was found to be 0.37 (< 0.50). However, the apparent under exploitation of Hilsapopulation, (though E =0.555) may switch to overexploitation in future, if sustainable fishing is notmax

implemented.

Key words: Population Dynamics % Length Based Data % Mortality % FiSAT II % Overexploitation

INTRODUCTION population dynamics of Hilsa from Bangladesh by

Estimation of growth and mortality of exploitable Nurul Amin [18], Ahmed et al. [19], Amin et al. [13]species is very important since stock assessment and and Rahman and Cowx [20]. Similar studies had beenmanagement rely on these population parameters [1]. done by Hashemi et al. [21] and Roomian and Jamili [22]Hisha Shad (Tenualosa ilisha) is one of the most common in Iran. The population dynamics of Hilsa has beenand important anadromaous fish of West Bengal. It is also comparatively less studied in this part of the country.a very important commercial fish and one of the most In India only few authors researched the populationpopular fish in Bengali cuisine. Several scientists have dynamic of Hilsa viz. Reuben et al. [23]. In the presentworked on different biological aspects of Hilsa Shad viz. study, asymptotic length(L8), growth coefficient(K),Pillay and Rosa [2], Hora [3], Pillay [4, 5], Jones [6, 7], total mortality(Z), natural mortality(M), fishingMathur [8], Bhanot [9], De and Datta [10]. Tenualosa mortality(F), exploitation rate (E), relative yield perilisha inhabits coastal shelf, estuaries and freshwater recruit(Y/R), relative biomass per recruit(B/R), maximumrivers in Indonesia, Sumatra, Myanmar, Bangladesh, India, length, growth performance indexes (2'), requirementPakistan, Kuwait, Iraq and Iran. Out of these nations, pattern, probability of capture, virtual population analysismaximum Hilsa fishes have been harvested from (VPA) were evaluated by using FiSAT II softwareBangladesh. Extensive Hilsa fishing is conducted from package to understand the status of Hilsa population inBangladesh and comprises 75% of the total world catch of Northern Bay of Bengal of West Bengal coast. New FAO-this species [11] and represents about 25% (2,50,000 t) of ICLARM software for (mainly length-based) fish stockthe total fish production in Bangladesh [12]. In the recent assessment, the FAO-ICLARM Stock Assessment Toolspast, the fishery in inland waters of Bangladesh had been or “FiSAT” package, integrating the ICLARM’s Completedeclining, but the total catch has remained stable (about ELEFAN, FAO’s LFSA and various other routines,2,00,000 t annually) due to an increase in catch from the released in mid-1992 [24] have been used during themarine sector [13]. Several studies had been done on present study.

Rahaman et al. [14], Nurul Amin et al. [15-17], Halder and

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MATERIALS AND METHODS The natural mortality of Hilsa Shad was calculated by

The total length of T. ilisha were measured weekly habitat temperature was 27.9 °C.between June 2010 and March 2011 by using a metricscale from the major marine fish landing centers of West Log M = 0.0066-0.279 Log L8 + 0.6543 Log K +Bengal viz. Kakdwip, Namkhana and Frasergunje 0.4634 Log T. (Figure 1) for the population dynamics study. The fisheswere collected by gill net from the Northern Bay of The fishing mortality rate (F) was then calculated byBengal. The fishes were randomly selected for its the difference between (Z) and (M) from the equation;measurement. A total of 464 T. ilisha were measured for F = Z-M and the rate of exploitation E was calculated bythis present study. The minimum and maximum length was the quotient between fishing and total mortality: E = F/Z155 mm and 412 mm respectively. [26]. The growth performance index was calculated by

Then the data was put month wise into the FiSAT II using the equation of 2’ = log K + 2 log L8 [27]. In thein an interval of 50 mm. The analysis was done in FiSAT present study we also estimated the probability of captureII (FAO-ICLARM Stock Assessment Tools) software from gill net selection criteria. One year recruitment(Version 1.2.2). The von Bertalanffy Growth Function and pattern was also figured out in Hilsa population and thelength frequency were plotted based on ELEFAN I which length-structure virtual population analysis was done byis used to understand the seasonal oscillation along with using M, K, L8, Ft parameters as an input in FiSAT-II. the estimation of the L8, K and R . The predicted maximumn

length from extreme values was computed. RESULTS AND DISCUSSION

Fig. 1: Map of Northern Bay of Bengal with three fish al. [21] estimated L8=43.32 cm and K= 0.78 yearG andlanding centers. Roomiani and Jamili [22] calculated L8= 42.74 cm and

using Pauly’s M empirical equation [25], where the mean

10 10 10

10

Different types of population parameters of Hilsapopulation were calculated by using the FiSAT II.

Growth Parameters: The L8 of 477.75 mm and K of 1.9yearG from K scan (Figure 3) routine were observed by1

using direct fit of length frequency data in ELEFAN I andthe response surface (R ) was found to be 0.674 for then

von Bertalanffy growth curve (Figure 2). The growthperformance index (2') was observed to be 3.637. Rahamanand Cows [20] estimated 2' was 3.47 for marine populationof Hilsa and Roomiani and Jamili [22] calculated 2' as 3.14in Hilsa. The L8 and K values of Hilsa were different fromother authors. Nurul Amin et al. [16] recorded L8 60.00 cmand K=0.82 yearG in Bangladesh water while Hashemi et1

1

Fig. 2: von Bertalanffy growth curve for Tenualosa ilisha.

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Fig. 3: K-scan routine for determining the best growth ilisha in West Bengal Coast.curvature which giving the best value ofasymptotic length with growth performance The fishing mortality was observed to be less than theindices in Tenualosa ilisha. natural mortality that indicates the Hilsa has not been

Fig. 4: The cumulative probability of the extreme length. population of Bengal coast was 0.37 and the E value

K= 0.77 yearG in coastal water of Iran. In the Ganga- than 0.50 then it could be concluded that the fish is1

Brahmaputra-Meghna (GBM) basin the Hilsa population overharvested from the particular area during this periodwas found comparatively at much higher density in [28]. The exploitation rate of Hilsa were 0.66 [14], 0.67 inBangladesh water because of more freshwater input and male, 0.68 in female, 0.61 in adult and 0.61 in juvenile [18]it was also established [16] that Hilsa population was in Bangladesh water, whereas 0.72 [21] and 1.8 [22] inoverexploited from Bangladesh water. Iranian water. So it is clear that the Hilsa was overfished

Maximum Length Estimation: The observed extreme overfishing stage (E =0.555) in West Bengal, Indialength (Figure 4) in Hilsa population was 455 mm and the during the study period. predicted extreme length was 453.58 mm and the range at95% confidence interval was 434.43 to 472.73. Recruitment Pattern: The recruitment pattern (Figure 6)

Mortality Estimation: The total mortality (Z) was 1.98 breeding season of Hilsa as it indicates the strongyearG from length-converted catch curves (Figure 5), the seasonal recruitment. The prolonged recruitment was1

natural mortality (M) was 1.25 yearG and the fishing noticed in June to October with a peak in September,1

mortality (F) was 0.73 yearG in case of Tenualosa ilisha. which marks the highest amplitude of Hilsa in counts.1

Fig. 5: Length-converted catch curve for Tenualosa

over harvested in West Bengal coast of Bay of Bengal butNurul Amin et al. [16] estimated M less than F (M=1.28and F=2.49) in Bangladesh water of Bay of Bengalindicating that the Hilsa was over harvested fromBangladesh water. Reuben et al. [23] estimated theexploitation and mortality rate of Hilsa along the northeastcoast of India. The natural mortality was 0.678 and thetotal mortality was 1.71. According to them theexploitation rate of Hilsa was 0.5883. The increase in thenatural mortality than the fishing mortality may be due toincrease in predation rate.

Exploitation Rate: The exploitation rate (E) in Hilsamax

recorded as 0.555. If the value of E would have been more

in Bangladesh and Iran but Hilsa population is yet to richmax

of Hilsa population was significantly high in the primary

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Fig. 6: Recruitment pattern of Tenualosa ilisha in Fig. 8: The virtual population analysis in Tenulosa ilishaNorthern Bay of Bengal. in West Bengal coast.

Fig. 7: Logistic selection curve for probability of capture, curves for Tenualosa ilisha from West Bengalshowing 25%, 50% and 75% selection length of Coast using the selection ogive option (E =Tenualosa ilisha in Northern Bay of Bengal. 0.507, E = 0.241, E = 0.555).

Probability of Capture: Probability of capture was 305 and 405 mm and the size class which faced maximumestimated from the type of net (gill net) used for fishing. fishing mortality (F = 1.47) was 355 mm. From VPA it wasFrom this finding it is estimated that the maximum indicated that the survival rate was highest in 155 mm sizeprobability of capture class size lies between 305 mm and group Hilsa, so the juvenile capture in Bengal coast has405 mm (Figure 7). The selection length of 25 % or L was been less during this study period which indicates that25

237.06 mm, 50 % or L was 265.55 mm and the 75 % or L the Hilsa population is not under immediate threat due to50 75

was 294.03 mm. From the probability of capture analysis overfishing.using selection curve the estimated optimum length ofHilsa (TL, mm) at first capture (Lc = L ) were 322 and 335 Relative Yield per Recruit and Biomass per Recruit: The50

mm, TL, respectively for river and marine [20]. relative yield-per-recruit (Y/R) and biomass-per-recruit

Virtual Population Analysis: Virtual population analyses The L /L8 value is 0.351 and M/K value are 0.789 for(VPA) are very useful tool which allow the reconstruction Hilsa (Figure 9). The Y/R was 0.065 and the B/R was 0.314.of the population from total catch data by age or size. In The selection ogive routine of FiSAT-II was used in thethis present study we estimated VPA from length based analysis of relative yield per recruit and biomass perdata (Figure 8). From this analysis we observe that the recruit analysis (Figure 10) and produced values ofmaximum numbers of Hilsa Shad were caught between E = 0.507, E = 0.241, E = 0.555.

Fig. 9: Relative yield-per-recruit and biomass-per-recruit

10

50 max

(B/R) were determined as a function of L /L8 and M/K.50

50

10 50 max

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Fig. 10: Isopleths, showing optimum fishing activity bothin terms of fishing effort and size of first capture(depicted with a star in the central curve) ofTenualosa ilisha.

CONCLUSION

According to Froese [29] overfishing can beprevented by following certain rules. All of the mature fishpopulation should be caught, those who have attainedoptimum length but the mega-spawners should not betargeted at all. In this present study, it is observed thatthe percentage of mature and optimum length size fisheswere caught with maximum number and the percentage ofmega-spawner catch were found to be minimum. Due tothis reason overfishing of Hilsa population could not beestablished in West Bengal coast during this period, butthe E value of 0.555 denotes symptoms ofmax

overexploitation in near future. If any population isaffected by the ‘recruitment overfishing’, the populationmight be seriously hampered to attain the sustainableyield in the long run [30], so the fishing regulation mustbe strong for Hilsa to make this species sustain for a longtime in future. On the other hand, the average Hilsa sizefound here was quite smaller than Bangladesh, whichindicates that it could be a different subspecies or race ofHilsa. Further long term monitoring is required to establishthe Hilsa population dynamics, as well as phylogeneticstudy should be done in the West Bengal coast.

ACKNOWLEDGEMENTS

The authors are very grateful to the Indian NationalCenter for Ocean Information Services of ‘Ministry ofEarth Science’, Government of India for funding thePotential Fishing Zone Validation Project which enabled

the authors to conduct the study. We also thank the EcoSystem for Life, a joint India-Bangladesh initiative byIUCN which facilitated some field studies during 2011. Weare also thankful to the all field staffs and fishermen forhelping us during collection and measurement of thespecimens on board and in harbours.

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