Population dynamics of the juvenile hilsa shad (Tenualosa ilisha) in the nursery ground of the Meghna River have been studied on the basis of the length cohort analysis of 8023 specimens. The growth parameters viz; asymptotic length (Lα), curvature character (K) and initial time (t0) were found to be 30.69 cm, 1.2 yrˉ¹ and 0.45 yrˉ¹ respectively. Curvature parameter indicates that jatka is a fast growth performer. The natural, fishing and total mortality were found to be 1.37 yrˉ¹, 1.41 yrˉ¹ and 2.78 yrˉ¹ respectively. Survival rate (S) was found to be 6.2%. A small difference was found between the age at first capture (Tc) and the recruitment age (Tr). Stocks of jatka seem to be overexploited and need to be conserved

Haldar, G.C.

Mazid, M.A.

Miah, M.S.

Rahman, M.A.

Aquatic Commons

Bangladesh]. Fish. Res., 4(1), 2000: 75-81 
Some aspects of population dynamics of juvenile hilsa shad 
(Tenualosa ilisha Ham.) from the Meghna river, Bangladesh 
M.S. Miah, M.A. Rahman*, G.C. Haldar and M.A. Mazid 
Bangladesh Fisheries Research Institute, Riverine station, Chandpur-3602,Bangladesh 
*Co1Tesponding author 
Abstract 
Population dynamics of the juvenile hils a shad (Tenualosa ilisha) in the nursery ground of 
the Meghna river have been studied on the basis of the length cohort analysis of 8023 
specimens. The growth parameters viz; asymptotic length (L= ), curvature character (K) 
and initial time(t0 ) were found to be 30.69 em, 1.2 yr·1 and 0.45 yr·1 respectively. Curvature 
parameter indicate that jatka is a fast growth performer. The natural, fishing and total 
mortality were found to be 1.37 yr·1, 1.41 yr·1 and 2. 78 yr·1 respectively. Survival rate (S) was 
found to be 6.2%. A small difference was found between the;age at first capture (T J and the 
recruitment age (Tr). Stocks of jatka seems to be overexploited and need to be conserved. 
Key words: Juvenile hilsa, Population dynamics, Meghna river 
Introduction 
J atka is the juvenile stage of the hilsa shad. It is the most exploited fishery in 
Bangladesh water of rivers, estuaries and the coast. As an open water fishery, its 
exploitation is a crucial phase for sustainable yield.· Recruitment is a continuous 
biologically renewable process in the open water system. If by any means, the recruitment 
process impaired the adult population will decline in the long run. Biomass is regenerated 
both from recruitment and tissue growth. Nonetheless, it is necessary to have established 
clear and strong relationship between the adult stock and-the recruiting young to obtain a 
sustainable yield from any fishery (Cushing 1968). Therefore, jatka fishery has a significant 
effect on hilsa production year after year. The growth parameters differ from species to 
species and also vary from stock to stock within the same species and age (Pauly 1980a). 
The aim of this investigation is to provide practical guidelines to understand the impacts 
of juvenile hilsa population as a capture fishery. 
Materials and methods 
Length-frequency data collection 
Length-frequency data of 8023 individuals of jatka were collected from the nursery 
ground of the Meghna river in and around Chandpur during December 1993 through 
M.S. Miah et al. 
April1996. Length-frequency data were collected both by experimental fishing as well as 
from commercial catches. In each month adequate number of length-frequency data were 
randomly collected. Total length (em) was measured from the tip of the snout to the 
posterior most margin of the caudal fin. Experimental fishing was done by a beach seine 
net (150m x 18m, 0.75cm mesh) manually hauled by 12 people. Commercial catches were 
done by a large seine net (400m x 30m, 0.75cm mesh) employing about 60 fishermen for 
hauling. 
Data analysis 
Length-frequency data were analyzed by using the Microstat and Excel-statistica 
computer software packages program. Growth parameters of jatka fishery viz; asymptotic 
length (L.,J, curvature parameter (K) and initial time (t0 ) when jatka begins to grow just 
after hatching, were estimated on the basis of the Von-Bertalanffy growth analysis by using 
the following growth equation model; 
L( t) = L= [1-e·k(t-to)J ...................... , ...................... (i) 
A linear regression analysis was done b~tween the two variables X and Y using the formula; 
Y =a+ bX; ......................................................... (ii) 
Considering L1L/L1t as X and mean length [L'(t)] as Y. Asymptotic length of jatka was 
calculated by the formula; 
L"" =-a/b ............................................................ (iii) 
Mean length [L' (t)] was converted by the formula of -ln[1-L' (t)/ L=] and was denoted 
as Y1 variable. In this case the independent variable (X1) was denoted as assumed cohort 
age (t). By the regression analysis between X1 and Y1 variables, Curvature parameters (K) 
was calculated as K=slope, b1 and the initial age as t0 =-a1/b1• After determining the values 
ofL"", K and t0 length data converted into age data by using the Inverse Von-Bertalanffy 
growth equation (1938); 
t(L)= t0-1/k*In [1-L' (t)/ Loc] ........................... (iv) 
The growth pattern of jatka was determined by plotting length against age. 
Total mortality 
Total mortality was calculat~d by the "linearized length- converted catch curve" 
method. The linearized length converted catch curve formula was; 
76 
Population dynamics ofT. ilisha 
Where t= 1/K*In[(L"'-L1)/(L"'-L2)], X = t[(L1 + L2)/2) = t0-1/K*In[1-(L1 + L2)/L"'] and 
Y = ln C(L1,L2))/ t(LDL2), C(LDL2) is the number of fish caught, L1 and L2 were the lower 
and upper limit of each length class, 'a' was the intercept and the slope b ( -Z) was the total 
mortality. 
Natural and fzshing mortality 
Natural mortality (M) for jatka fishery was calculated by Pauly's empirical formula 
(Pauly 1980 b) through various combinations of L, K and average annual 
temperature(T0 c). Here annual temperature was considered only for the period of 
November to April when }atka is available in the riverine nursery ground. Natural 
mortality was calculated by the following formula; 
M= 0.8*exp[-0.0152-0.279*In L"' +0.6543*In K + 0.463*In T] ...... (vi) 
Fishing mortality rate (F) was derived by subtracting the natural mortality from the total 
mortality, since Z=M + F, where Z, M and F were the instantaneous rate of total, natural 
and fishing mortality. 
Exploitation rate 
The rate of exploitation of jatka in the Meghna river was calculated by the formula; 
Exploitation rate, E=F/Z ..................................................... (vii) 
Results 
Length-frequencies of 8023 specimens of jatka from the riverine nursery grounds of 
the Meghna river were analyzed and shown in Fig.1. 
U) 
c:: 
0 
'g 
~ 
"' 
"' .!) 0 
0 
0 
·~ 
VI 
"' 'tl 
c: 
OJ 
::J 
0" 
~ 
~ 
c: 
"' -' 
2200 
1800 
1400 
1000 
600 
,. 2,-1-
200 \-0+ ~\·5"+ l 
-200 
0 4 8 12 16 20 24 
Length (em) 
Fig. 1. Length cohort analysis of jatka population for riverine 
nursery ground of the Meghna river. 
28 
Length-frequency analysis revealed that the length cohorts of jatka population were 
<2, <9, <15, <19 and <22 em respectively (Fig.l). Cohort assumed age (t), cohort mean 
77 
M.S. Miah et al. 
length [*.1L/.1 t)], growth rate *(.1L/.1t) and individual mean length [L(t)] were shown in 
Table 1. Through a regression analysis between Y (.1L/.1t) and X (L(t)), the intercept a 
and slope b were found to be 36.22 and -1.18 respectively. According to the Von-
Bertalanffy growth equation asymptotic length (Loc) of jatka was 30.69 em, cohort mean 
length [L(t)] converted into -ln [1-L(t)/ Loc] denoted as Y1 and assumed cohort age (t) 
denoted as X1 were put into a simple regression. The values of curvature parameter K and 
initial time t0 were found as 1.2 yr-1 and 0.45 yr (Table 2). 
Table 1. Growth rate of Tenualosa ilisha as a function of cohort age 
Aget .1t Cohort .1L(t) Growth rate L(t+M)+ L(t)/2= L'(t) 
years mean length L(t+.1t)-L(t)/t= .1L/.1t 
em 
o.5+ 0.25 <2 7.0 28 5.5 
0.75+ 0.25 <9 6.0 24 12 
I.o+ 0.25 <15 4.0 16 17 
1.25+ 0.25 <19 3.0 12 20.5 
1.5+ <22 
N=4, x= 13.75 a (intercept)= 36.22, b= (slope)= -1.18 
Table 2. Calculation of Curvature parameter (K) and Initial time (t0), (using Loc = 30.69 em) 
Cohort age t (year) Cohort mean length -In[l-L(t)/ Loc] 
(X)X1 L(t) em (Y)Y1 
o.s+ 2 0.06 
0.75+ 9 0.35 
1.0+ IS 0.67 
1.25+ IS 0.97 
1.5+ 22 1.26 
a1 (intercept)= -0.54, b1= (slope)= 1.2yr1, (Curvature parameters) and initial 
time t0= -a/b1= 0.54/1.2= 0.45yr. 
The values of curvature parameters are indicating that jatka is a fast growth performer. 
The growth rate ofjatka was calculated by using Inverse Von-Bertalanffy growth equation. 
The length of jatka was converted into age (t). Growth is the change of absolute increase in 
length and weight with respect of age. The instantaneous rate of increase in length with 
respect of age ilL(t) has been estimated by the Von-Bertalanffy growth equation. The 
growth curve of jatka was shown in Fig.2. 
The growth pattern indicating that the instantaneous rate of growth in the younger 
phase of life was found much higher. Jatka attain up to 28 em within 2 years. But 7-15 em 
attain within one year. It was also found that maximum peak catch attain with 8-12 em size 
groups (Fig. 1). 
78 
Population dynamics ofT. ilisha 
24 
E' 
-':'.. 20 
Ill 
~ 
.!!l 16 
'0 
"& 12 
c: 
.3 
8 
0.5 1.5 2.5 3 3.5 
Age (t) in years ~ 
Fig. 2. Growth curve of jatka from the riverine nursery ground. 
Total mortality rate (Z) 
The instantaneous rate of total mortality was estimated by means of the length 
converted catch curve method. The total mortality rate (Z) was found to be 2.78 yr-1, 
which indicating a high value. The length converted age based catch curve was shown in 
Fig.3. 
51. 
N" 
...1 
::::; 
0 
.s 
I 
11.5 r---------------------, 
10.5 
9.5 
8.5 
7.5 
6.5 
5.5 
--- ---:.:-. ... -~·- --
' ' I 
I 
I 
I 
I 
I 
I 
I 
I 
I 
-------- ----·~-- ---
"' I I 
" I I I 
I I 
4.5 '------~---'--1 -----'----~----' 
-0.2 0 0.2 0.4 0.6 0.8 1.2 
Age at Length group t(L 1 +L2)/2 
Fig. 3. Length Converted Catch Curve used for estimation of total mortality of jatka fishery. 
The descending part of the catch curve was not calculated due to too close to L"' and 
hence it was avoided. The high Z value for jatka fishery was not a good sign to obtain a 
sustainable yield. From the Z value it was calculated that the survival rate of jatka fishery 
in the riverine nursery ground was S = e-z x 100 = e-2·78 x 100 = 6.2%, which was very low, 
which indicating that the hilsa population catch will be seriously hampared in the long 
run. 
Natural and fishing mortality 
The natural mortality was calculated by the Pauly's empirical formula (1980b) and it 
was shown in Table 3. Where L"' = 30.69 em and K= 1.2 yr-1 were used in the calculation. 
Mean natural mortality M was found to be 1.37 yr-1 was found. The instantaneous rate 
of fishing mortality as derived from the values ofZ and M, was obtained F=Z-M=l.41yr-1• 
79 
M.S. Miah et al. 
Table 3. Natural mortality calculated by Pauly's empirical formula (1980b) for 
various ambient water temperature (month-wise, during the jatka season) 
30.69 
K 
1.2 
M= 
Nov. 
23 
1.45 
Ambient water temperature (T0C) 
Dec. Jan. Feb. Mar. 
19 17 20 22 
1.33 1.28 1.39 1.36 
Mean M = 1.37yr1, M= O.B*exp [-0.0152-0.279*/n L +0.6543 InK +0.463/n'f'JC] 
Exploitation ratio 
Apr. 
24 
1.48 
Exploitation ratio, E = 0.51 has been estimated on the basis of the relationship 
E=F/Z, which tends to overexploitation for the jatka fishing. Tends of overexploitation is 
not ~ good sign for any open water fishery. 
Discussion 
Length-frequency analysis of jatka population reveals that major share of jatka fishery 
in the riverine nursery ground were caught between the sizes of 8-12 em in November to 
April and the peak were 9-10 em size group. In the riverine nursery ground, the abundance 
of jatka was found from November to April and peak in March in each year (Rahman et. al. 
1995) which supports the present findings. 
The first capture age (TJ was calculated as Tc =0.5 yr (6 months). and the recruitment 
size was 8-9 em, so the recruitment age was Tr=0.70 yr (8.4 months). It was seen that the 
differences between Tc and Tr was very small (2.4 months). The results for Tr and Tc 
showed that Tc age was smaller than Tr age, which was not true for the proper 
management. Therefore, to obtain a sustainable hilsa population, Tr age must be less than 
Tc age. Beverton and Holt (1957) reported that in the open water system the fish 
population are affected by the natural mortality during the ages between Tr and Tc. But in 
case of jatka fishery in the Meghna river Tr to Tc period suffers from both by natural (M) 
and fishing mortality (F). So, it can be said that due to fishing pressure at the recruitment 
phase, the total mortality might be high for jatka population. 
A stock is supposed to be optimum when Eopr=0.51 but when E value is more than 0.5, 
the stock of a fish population is overfished (Gulland 1965). So, it appears that the stock of 
jatka fishery tends to cross the overfished level in the riverine nursery ground of the 
Meghna river. 
Csirke and Sharp (1984) opined that if any population affected by high mortality in 
their recruitment phase the population might be seriously hampered in the long run. So, as 
the fishing pressure on the jatka fishery. was on the way to cross the optimum fishing 
condition, it is urgently needed to control fishing at their recruitment phase. 
80 
Population dynamics ofT. ilisha 
References 
Bertalanffy, L. Von., 1938. A quantitative theory of organic growth (Inquiries on growth laws. II). 
Hum. Biol., 10(2): 181-213. 
Beverton, R.J.H and S.J. Holt, 1957. On the dynamics of exploited fish populations. Fish. Invest. 
Minist. Aqric.Fish. G.B. (2 Sea Fish.), 19: 533pp. 
Csirke, J. and G.D. Sharp (eds), 1984. Reports of the expert consultation to examine change in 
abundance and species composition of neritic fish resources, San Jose, Costa Rica, 18-29 April 
1983. FAO Fish.Rep., 291(1): 102 pp. 
Cushing, D.H, 1968. Fisheries biology: a study in population dynamics. Madison, University of 
Wisconsin Press. 200pp. 
Gulland, J.A., 1965. Estimation of mortality rates. Annex to Arctic fisheries working group report 
ICES C.M. Doc. 3 (mimeo). 
Pauly, D., 1980a. A selection of simple methods for the assessment of tropical fish stocks. FAO 
Fish.Circ., 729:54p. Issued also in French. Superseded by FAO Fish. Tech. Pap.(234). 
Pauly, D., 1980b. On the interrelationships between natural mortality, growth parameters and mean 
environmental temperature in 17 5 fish stocks.]. Cons. ClEM, 39(2): 175-92. 
Pauly, D., 1983. Some simple methods for the assessment of tropical fish stocks. FAO Fish.Tech.Pap, 
234: 520pp. 
Rahman, M.A., M.S. Miah, M.J.Rahman and G.C. Haldar, 1995. Jatka (Juvenile Tenualosa ilisha) 
fishery in Bangladesh. Indian Journal of Fisheries, 42(3): 13-16. 
(Manuscript received 5 October 1997) 
81 


Some aspects of population dynamics of juvenile hilsa shad (Tenualosa ilisha Ham.) from the Meghna river, Bangladesh

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Some aspects of population dynamics of juvenile hilsa shad (Tenualosa ilisha Ham.) from the Meghna river, Bangladesh

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