Indian Journal of Geo Marine Sciences Vol. 48 (12), December 2019, pp. 1870-1880

Life History and Population Dynamics of Tenualosa ilisha of Sundarban Estuary in Bay of Bengal, India for Sustainable Fishery Management

Sachinandan Dutta1*, Kunal Chakraborty2, and Sugata Hazra1

1School of Oceanographic Studies, Jadavpur University, Kolkata, West Bengal, India. 2Indian National Centre for Ocean Information Services, Hyderabad, Andhra Pradesh, India.

*[E-mail: duttasachi@gmail.com]

Received 25 April 2018; revised 24 July 2018

The population structure of Hilsa Shad (Tenualosa ilisha), a choicest table fish, in the estuaries of India, Bangladesh and Myanmar of Bay of Bengal has been studied by different methods and each provided complementary data on population structure. Considering the present scenario of climate change, increasing pollutant load in Indo–Bangladesh estuarine region and its effect on reproduction and maturation of anadromous Hilsa Shad, a thorough and detailed understanding of the life history of Hilsa is a pre requisite criterion. Results of such studies would be important for sustainable management of this highly economic biological resource. The present paper deals with the aspects of life history and population dynamics of Hilsa Shad in Sundarban estuaries. The data collection was done during the period of June 2011 to March 2012 at Frasergunje Fishing Harbour and offshore of northern Bay of Bengal. The length and weight of total 617 Hilsa fish were measured under this study. Monthly variations of length and weight, length frequency distribution, monthly variation of the allometry coefficient, movement pattern, and catch per unit effort were estimated. The exploitation rate of Hilsa species was found to be 0.78 and the maximum sustainable yield was 11700.18 tonnes whereas the annual catch was 18126.00 tonnes. Highest weight of adult Hilsa was recorded during the monsoon i.e. the months of June, July and August. The result of relative yield per recruitment indicated that the mortality due to current fishing period and pressure were high. Widespread fishing of juvenile and growing Hilsa (< 500 g) declined the Hilsa population considerably losing the economic advantage. The present paper dealt with life history parameters like, growth, exploitation, mortality, stock assessment for sustainable management of Hilsa population. In our study we have observed that Hilsa fish, especially Jatka (<500 gms and <230 mm) is over harvest, from West Bengal coastal areas.

[Keywords: Bay of Bengal; Exploitation rate; Hilsa; Life history; Sustainable management]

Introduction

Five species of tropical shads (Tenualosa ilisha, T. toil, T. macrura, T. reevesii, T. thibaudeaui from the Clupeidae family) live in the estuaries and coastal waters of tropical Asia1. Among the above-mentioned five species, Hilsa Shad (Tenualosa ilisha) is the most economically important species and its habitat is marine water, freshwater and brackish water. The nature of Hilsa species is pelagic-neritic and anadromous. Hilsa spends most of its life in the inshore areas of the sea and undertakes extensive migrations ascending the estuaries and rivers for the purpose of breeding2. In India, Hilsa migrate upto Allahabad before Farakka barrage contraction and reduction in Hilsa catch recorded at upstream of Farakka barrage3,4. The distribution of the species is very high in GBM (Ganges-Brahmaputra-Meghna) basin of northern Bay of Bengal, it is also found in Mayanmar, Vietnam, West coast of India, Pakistan,

Kuwait and Iran5. Large fish schools of Hilsa Shad have been reported throughout its geographical range, by fishermen at various times of the year in coastal waters, but, the origin and movement of these fish have never been investigated6. The growth, mortality estimates derivations have suggested that Tenualosa ilisha has a high resilience to exploitation7. This fish is generally categorized based on its distribution, abundance, habitat, life history characteristics and the amount of harvest.

In order to study the population response or, in general, the life history pattern of an economically important species, it is necessary to consider both the human impact and the environmental forcing8. Life history theory offers a promising framework for building bridges between demography and fundamental aspects of species biology9.

The life history of Tenualosa macrura has been studied for the purpose of its management1,10. Growth

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and life history studies were conducted on Hilsa in Hooghly Estuarine System11, in the waters off the coast of Bangladesh12,13in India14and in Sindh, Pakistan15.

In this paper, we have studied the length weight relationship along with the month wise length weight variation, length frequency distribution, weight frequency distribution, allometry coefficient. Moreover, Catch per Unit Effort (CPUE) of Hilsa and migration pattern along with catch structure in Bay of Bengal, West Bengal was examined. The basic objective of this paper was to study the life history pattern of Hilsa species to utilise it in sustainable usage and conservation. In this light, the management of Hilsa species was taken into consideration. The managerial recommendations were provided with the help of the obtained results for the most uncertain climatic zone of northern Bay of Bengal (Sundarban Estuary). Mandal et al.16 nicely described the fish diversity of Sundarban estuary in their review work, but the present study is conducted in marine part of Sundarban estuary.

Materials and Methods The present study was conducted during June, 2011

to March, 2012 (Fishing ban is observed during 15th April to 31st May on the east coast of India) at Frasergunje Fishing Harbour and northern Bay of Bengal (NBOB) of West Bengal (WB) (Fig. 1). The length and weight of 617 Hilsa fish were measured under this study. In order to study the migration pattern of Hilsa, we have used the fish catch and effort data obtained from fishermen in NBOB, WB.

Length-weight data of T. ilisha were measured by metric scale and a digital weighing machine of Wensar (TTB-3), (providing a maximum measuring capacity of 3 kilograms and an accuracy of 0.1 gram). The harvest data was collected from the gillnetters fishermen of northern Bay of Bengal. We measured total length of each fish. Total length means the measurement of the fish from the lower jaw to the tip of the tail by laying it normally i.e., without any stretching17. The on-board sea surface temperature (SST) was measured in C unit. The salinity was

Fig. 1 — Map of the northern part of the Bay of Bengal with bathymetry and Frasergunje Fishing Harbour.

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measuredin ppt (parts per thousand) unit. The SST and salinity were measured by the fishermen regularly with the help of a hand held digital thermometer (H-9269) ranged between C°50 and C200 and Erma Hand Refractometer ranged between 0-100 ppt. The month wise catch and effort data were obtained from the fishermen of Frasergunje Fishing Harbour. CPUE was calculated in kg/hour unit.

All computations were carried out using Microsoft Office 2010 and MINITAB 13.31 software package. The Pearson co-relation was used for all statistical calculations.

The frequency distribution of length and weight was calculated. We have used the following non-linear form of the length-weight relationship:

baL=W ,

where, W is the weight of the fish, L is the total length of the fish, b is the exponent describing the rate of change of weight with respect to length and a denotes the weight at unit length. The ‘ a ’ and ‘b ’ were calculated from the baL=W equation. Fulton’s condition factor was calculated using the following formula:

35 /10 LW=C ,

where, W = the weight of the fish in grams; L = the standard length of the fish in millimeters.

Ford18 – Walford19 equation was used to estimate the von Beralanffy growth parameters; asymptotic length (L) and VBGF growth constant (K).

The equation is:

tKK

t LeeL=L )1(1

where, Lt and Lt+1are the total length at age t and t+1 respectively. By plotting Ltagainst Lt+1, the resulting slope )Keb and the intercept )1( KeLa

Total mortality )(Z could be estimated using parameters of the von Bertalanffy Growth Function (VBGF) as follows:

'LL

LLK=Z

,

where, K is the VBGF growth constant, L is the

asymptotic length, L is the mean length of the fish population and 'L represents the mean length at entry into the fishery.

The natural mortality (M) of Hilsa Shad was calculated using Pauly’s M-empirical equation20 as follows:

Log10M=.0066 – 0.279Log10L8 + 0.6543Log10K + 0.4634Log10T, SD (logM)=0.245 where, the mean habitat temperature (T) is considered as 27.8 °C.

The fishing mortality rate (F) was calculated as

MZ=F and the rate of exploitation (E)was calculated using the quotient between fishing and total mortality: ZF=E /

21. The length of Hilsa depending on age was

estimated from the von Bertalanffy growth equation:

)e(L=LtK(t

t

)01

, where,

tL =Length at age t, L = Asymptotic length,

K= Growth coefficient, 0t = Age at length zero and

0t was calculated from the following expression:

K)Log(L=)tLog( 1.0380.2750.3920

.

Again, the weight of Hilsa depending on age was estimated from the von Bertalanffy growth equation:

3)01 )e(W=W

tK(t(

t

,

where, tW =Weight at age t , = Asymptotic weight,

K = Growth coefficient and 0t = Age at weight zero.

The age at maturity (mT ) was estimated according

to Richter and Efanor22.

)+)(M(=T m

0.160.72

11.52

The lifespan (Ls) of Hilsa was calculated from

0

3t+

K=Ls

.

The recruitment pattern was figured out by the backward projection of the length-frequency data, using the von Bertalanffy growth equation and the estimated growth parameters (

L and K )23. The total annual stock size, average standing stock

size and MSY of T. ilisha was estimated. In this

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regard, the initial exploitation rate (U) was estimated using the following equation24,25:

)eZ(F=U Z1/ .

In order to estimate the annual catch (Y) of Hilsa, the landing data was collected from the Department of Fisheries, Govt. of WB. The total annual stock and standing stock were calculated usingU , F andY . Finally, the approximate MSY was calculated using the following relationship, proposed by Gulland26,

tt BZ=MSY 0.5 ,

where,

tZ is the instantaneous total mortality in the

year t and tB is the standing stock size in the year t.

The relative yield-per-recruit and biomass-per-recruit were estimated based on the Beverton and Holt27 model. Relative yield-per-recruit R)(Y ' / was computed from:

,)3m+1(/U)2m+1(/3U+m)+1(/3U1

EU=R/Y32

K/M'

where, )L(L=U c /1 and

cL is the mean length of

fish at first capture. In standard case cL is taken as

50L ,

Z)(K=K)(ME)(=m ///1

and ZF=E / .

Relative biomass-per-recruit R)(B ' / was estimated from the following equation:

FR)(Y=RB '' /// .

The Hilsa movement pattern was traced out by

utilising the fisherman catch data as well as by using Geographic Information System (GIS) and Remote Sensing in this study. An attempt was made to map the position of the major stock of Hilsa. The geographical positions of high (>150 kg per haul) Hilsa catch zone was incorporated in a geo-referenced image of BOB in order to create an output map for clear understanding of the Hilsa migration pattern. Moreover, during the study period, we recognized the movement of Hilsa stock in the marine and lower estuarine part of Hugli-Matla estuary of Sundarban Biosphere Reserve. Results Length and weight frequency distribution

The minimum and the maximum length of Hilsa of our sample were 208 mm and 504 mm respectively. For presenting a length frequency distribution the seven length classes of 50 mm width was considered with uniform step length 50. The class 258–308 mm comprises the majority of specimen throughout the year (Fig. 2).

The weight frequency distribution of T. ilisha was divided into fifteen classes with minimum weight 94.10 g and maximum weight 1382.20 g with uniform

Fig. 2 — Monthly length frequency distributions of Hilsa between June-2011 and March-2012 with an interval of 50 mm and minimum length class 208 mm and maximum length class 508 mm.

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class width 100 g. The weight class 195 – 295 g comprises the majority of Hilsa fish throughout the year and consequently, it was obvious that maximum Hilsa fish belong to this class (Table 1). The weight decreased during post monsoon season. Length weight relationship

The length-weight relationship of T. ilisha was statistically significant (p<0.0001) and estimated as W = 0.00002×L2.86, R2=0.92 (Fig. 3).

The month wise variation of length-weight relationship was clearly depicted in Figure 4.

It was to be noted that the length of Hilsa did not vary significantly throughout the year (Fig. 5). The average length was measured .. 44.72301.18 DS± mm. It was clearly observed that the weight of adult Hilsa

was highest during the monsoon i.e. the months of June, July and August. The weight of Hilsa was approximately 450 g during monsoon whereas during the rest of the year the weight observed was 300 g. Allometry coefficient

The allometric coefficient of Hilsa was increased during monsoon season (June-August, Fig. 4). At the end of the monsoon, it started decreasing, and subsequently, during the post monsoon season, it remained almost uniform. It was also observed that allometric coefficient again starts increasing during the pre-monsoon season. Condition factor

The condition factor varied between 0.9 and 1.2. During June to October, the condition factor of Hilsa was increased from 1.0 to 1.2, which clearly indicates that the fish were in good condition indicating a favorable and healthy habitat for living. However, it decreased during December to March. The condition factor of Hilsa increased with decreasing salinity and increasing temperature (Fig. 6). The habitat mean temperature and salinity were ..0.14727.765 ES± and ..0.36321.938 ES± , respectively. Catch per Unit Effort

The values of CPUE were varying significantly during monsoon, pre-monsoon and post monsoon season. During the monsoon period, CPUE increased steadily. It started from June and continued till October, but in the post-monsoon season CPUE decreased drastically (Fig. 7). This implies that the

Fig. 3 — Total length weight relationship of Tenualosailishain 2011-12.

Table 1 —Weight frequency distribution of Tenualosailishain June, 2011 to March, 12 of West Bengal coast, weight is divided into 15 class of 100 g interval with lower limit of weight class of 95 g and higher limit of weight class of 1395 g

Gram June July August September October November December January February March 95 g 1 195 g 3 3 13 21 9 8 20 6 3 13 295 g 3 13 2 23 38 26 59 22 44 40 395 g 2 29 4 15 5 3 26 17 17 15 495 g 1 15 3 8 5 2 7 2 1 5 595 g 3 5 6 5 2 2 695 g 1 6 4 2 1 1 795 g 2 4 1 2 3 895 g 2 2 3 1 1 995 g 1 2

1095 g 2 1 1195 g 1295 g 1395 g 1

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abundance of the Hilsa Shad was low in the fishing zone during November to March, possibly due to an offshore migration of the fish population i.e. into areas outside of the scope of fishing zone. The mean CPUE was .E.S0.354±3.455 kg/hour, which was

excessively low, which indicates a low mean concentration in the fishing area. The Hilsa CPUE was positively correlated with SST

0.0270.6922 =p,=R and negatively correlated with salinity 0.0090.7722 =p,=R .

Fig. 4 — Monthly length-weight relationships of Tenualosa ilishain 2011-12.

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Mortality and exploitation

The total mortality (Z) of the Hilsa population was 3.15, using von Bertalanffy Growth Function (VBGF), in West Bengal coast. The natural mortality (M) is 0.71, (95% CI: 0.24-2.1) obtained from Pauly’s M-empirical equation, where the mean habitat temperature was considered as 27.8 °C. The fishing mortality (F) was thus 2.44, with a 95% CI of 2.9-1.05, when assuming a correct Z- value. Using the expected values, fishing mortality appears to be substantially higher than the natural mortality, which indicates that the population was highly overexploited with an exploitation rate (E) of Hilsa was 0.78. However, the estimate of natural mortality includes a large uncertainty and so does the exploitation rate (95% CI: 0.3-0.9). Length and weight at age

We had estimated the length and weight of Hilsa depending on its age at WB coast (Table 2). The mean length of a one-year-old Hilsa was 300 mm, and that of a 1.5 year old was 379 mm (Fig. 8). It was also observed that, at the age of five years, Hilsa could

Fig. 5 — Monthly variation of mean length and weight of Hilsawith error bar from Frasergunje fishing harbour of West Bengalcoast from June, 2011 to March, 2012.

Fig. 6 — Monthly variation of the condition factor with SSTand salinity.

Fig. 7—Monthly CPUE of Hilsa in the gill net fishing boat of Frasergunje Fishing Harbour, in 2011-12, compared to SSTand Salinity.

Table 2—The length and weight at different age of Hilsa.

Age in year Weight (g) Length (mm) 0 0 0

0.5 61.51 181.17 1 281.55 300.81

1.5 566.73 379.81 2 833.80 431.98

2.5 1049.61 466.43 3 1210.78 489.18

3.5 1325.78 504.20 4 1405.58 514.12

4.5 1459.99 520.67 5 1496.68 524.99

Fig. 8—Length and weight at age of Hilsa from West Bengal coast.

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grow up to 524 mm in West Bengal coast. However, the mature Hilsa was available at the age of 1.26 year with a length up to 345.96 mm. The one-year-old Hilsa weighs 281 g and by the age of 2.5 years, it grows up to 1 kg (Fig. 8). It was observed that we could get a five-year-old Hilsa weighing up to 1496.68 gm in WB coast. Recruitment pattern

The recruitment pattern of Hilsa was only one pulse and it was during the monsoon season from June to August (Fig. 9). The maximum percentage of Hilsa recruitment was 17.72 and was found in the month of July. Catch structure

Hilsa is an anadromous fish, which means that it migrates from ocean to fresh water river during monsoon season. If we look at the seasonal catch of Hilsa, it was clear that during monsoon season the amount of catch increases (Fig. 10). In order to estimate the seasonal fluctuation of Hilsa catch we

collected market sale data of Diamond Harbour fish market, West Bengal. Relative yield and relative biomass per recruit

The relative yield and relative biomass per recruit were plotted with respect to exploitation rate (E) (Fig. 11). It was obvious that the relative yield increased with increasing rate of exploitation. The result of relative yield per recruitment indicated that the current fishing mortality was high and most of the juvenile Hilsa were caught by the fishermen, which decline the fish population and also losing the economic advantage. Estimation of MSY

The estimation of total annual stock, standing stock and maximum sustainable yield (MSY) was calculated using annual data of catch collected from Department of Fisheries, Government of West Bengal. The total annual catch of Hilsa from West Bengal was 18126 tonnes during 2011 fishing year. Based on the estimated F- and Z- values the annual stock and standing stock were expected to be in the order of 24000 tonnes and 7400 tonnes respectively. Finally, the MSY(11700tonnes) is in accordance with the Gull and sequation however it is also in accordance with the conservative suggestion of Beddington and Cook. These data include high uncertainties, mainly due to the uncertain M- estimate, which requires confirmation. Movement pattern

The pattern of Hilsa movement was studied with the help of fishermen catch data. Most of the Hilsa catch was concentrated in NBOB and near the

Fig. 9—Recruitment pattern of Hilsa Shad from West Bengalcoast with one pick during June to August, 2011.

Fig. 10 — Hilsa sold from Diamond Harbour Fish marketon 2011-12.

Fig. 11 — Beverton and Holts Relative Yield per recruit and Biomass per recruit of Hilsa from West Bengal during 2011 to 2012.

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Sundarban estuary. We collected Hilsa fish catch data from the ten (10) gill net fishing trawlers of Fresergunje Fishing Harbour. We had plotted the fishing catch data of Hilsa from June to October, 2011 in order to identify the areas of Hilsa fishing in BOB (Fig. 12). From the fishermen catch data; we had mapped the monsoon Hilsa fishing zone. During the months of June and July, Hilsa was observed to migrate towards maximum freshwater input pathway driven by the monsoons i.e., near to the coastline. At the end of the monsoon, i.e. October onwards, the fish population was found to be scattered in more offshore in the deep water, which marks the beginning of the retreating migration towards open sea. Discussion

The present paper deals with Hilsa life history parameters like, growth, exploitation, mortality, and stock for its management. The growth allometry of Hilsa is negative as the value of ‘b’ is 2.86. Roomian

and Jamili28 pointed out 2.97=b of Hilsa growth in Iran, whether in Bangladesh water the growth allometry was positive (b=3.38)13. Considering the monthly variation in the condition factor, it was indicated that during the monsoon months, Hilsa was found to be well fed, whereas during pre and post monsoon periods, Hilsa was slim and slender in form.

If the value of E is more than 0.50, we can conclude that the fish is overharvested from a particular area during some specific period29. In our study, we had found that the exploitation rate (E) of Hilsa was 0.78. Though this value was still highly uncertain and required confirmation, the studied population was potentially largely overexploited. Hashemi et al.7 estimated the rate of exploitation (E) of Hilsa as 0.72 in Coastal Waters of Iran and Nurul Amin et al.30 found the exploitation (E) of Hilsa as 0.66 in Bangladesh water, indicating over exploitation of these population. Dutta et al.14 projected a threat of overexploitation of Hilsa resources of northern Bay of Bengal in near future.

Fig. 12 — High catch zone of Hilsa between June and October 2011 to understand their movement pattern.

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The effort is increasing day by day and the catch getting decline. As a result, the mean The Catch per Unit Effort (CPUE) of Hilsa was very low31. The estimated mean CPUE was 45.7 kg/boat/day during the peak period (September-October).CPUE hypothesis was influenced by diverse factors other than abundance like environmental conditions, fishing methods, fishing equipment, fisher behavior, management and economic factors32. In the present study, the low CPUE was influenced by fishing methods, as bottom trawling in shallow water was increased enormously by management practices, that result the destroy of Hilsa habitat in shallow water.

Rahman and Cows33 observed two pulses of the recruitment pattern of Hilsa in Bangladesh water, one major pulse during March-May and one minor pulse during November and January. Nurul Amin et al.34 reported more or less continuous recruitment with one major peak from June to September. The results obtained by us indicate only one pulse of Hilsa in WB during June to August.

The MSY estimated for the Hilsa population in this study was as uncertain as the estimated exploitation rate; however, it may show the order of magnitude the annual catch should be reduced for a sustainable fishery. The results obtained by us indicate that unless proper stock assessment and improved fishery management techniques were employed, the livelihoods of about 380 thousand Hilsa fishermen would come under threat. Management

The successful management of a system is often identified with its successful interaction with biological, economic, social, and political objectives35. The Hilsa fishery is the most significant fishery from the socio-cultural and economic point of view in West Bengal. Therefore, a proper management leading to a sustainable utilization of this vulnerable biological resource is extremely essential. The management options of the Hilsa fishery are as follows: I The mean length and weight of the caught hilsa

population is very low, which is due to use of small mesh sized net. Therefore, banning catch of the small fishes called “Jatka” (<500 gms and <230 mm.) and enforcement of fishing regulation restricting mesh size (>90mm) of fishing net and type of fishing gear in shallow depth (<50m) should be considered.

II The annual stock is being declined as we harvest more than the MSY. Proper stock assessment with

notification for total allowable annual catch is an essential task. Prohibition of catch beyond a permissible limit should be formulated to enhance the overall annual stock.

III Declaring ‘Sanctuary’ for Hilsa with proper identification of spawning and breeding grounds as the Sundarban estuary, work as a nursing ground of the fish. Hilsa sanctuary should be declared considering the areas of high importance taking breeding and spawning as a main factor.

IV The mean CPUE is very low, so we should confine the fishing effort.

V The Hilsa population is overharvested during this study period, so intensive fishing should be penalised.

On such suggestions, recently in 2013 the Government of West Bengal has enforced new fishing regulations36 exclusively for Hilsa. Treating Hilsa as a flagship species, the entire estuarine–marine ecosystem of northern Bay of Bengal can be managed sustainably in a long run. Acknowledgement

The authors are also greatly indebted to the Indian National Center for Ocean Information Services (INCOIS) for funding the Potential Fishing Zone validation project, which led to systematic data collection. The second author is also grateful to the Director, INCOIS for his encouragement and unconditional help. We also extend our gratitude to the field staffs and marine fishermen of West Bengal for their help during data collection under adverse condition. References 1 Blaber, S. J. M., Brewer D. T., Milton D. A., Merta G.S.,

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