Abstract: Monthly data of length composition for Tenualosa ilisha landed between April 2006 to March 2007 in north of Persian Gulf were used to estimate the growth, mortality and exploitation parameters of the stock. Maximum total length and weight were 43 cm and 949 g, respectively. Nonlinear least square fitting provided a complete set of von Bertalanffy growth estimate: L8 = 42.74 cm total length, K= 0.77 and t0 = -0.21 years-1. The estimated value of total mortality based on length converted catch curve using these growth parameters is Z= 2.55 years-1. Natural mortality based on growth parameters and mean environmental temperature (T = 23 C) is 0.75 years-1. Fishing mortality computed for T. ilisha was 1.8 years-1. The exploitation rate (E) was 1.8 years-1. The higher value of E indicates over fishing during in period. Population dynamics and stock assessment of Tenualosa ilisha in Iran waters was studied using the length-frequency based analysis of FiSAT software to evaluate the growth parameters, mortality rates, exploitation rate and Maximum Sustainable Yield (MSY). The study showed that T. ilisha is in over fishing pressure.
INTRODUCTION
Among fishes of the family Clupeidae (which includes the shads), about 29 species are diadromous, most of them anadromous, though one is amphidromous and two are caradromous. The hilsa shad, Tenualosa ilisha, belonging to the sub-family Alosinae of the family Clupeidae (Mazumder and Alam, 2009).
Hilsa shad (Tenualosa ilisha), an important diadromous fish species of South and South-East Asia, is habitant in Persian Gulf too. Hilsa shad supports a commercial fishery and in the early 1970s composed more than 95% of the total commercial catch (Coad et al., 2003).
Hilsa shad is widely distributed in almost the major rivers, estuaries and marine waters of Iran. It is capable of withstanding a wide range of salinities and migrating long distances upstream (up to 1287 km). The majority of the population feed and grow mainly in the sea before migrating to fresh water for spawning. Juveniles develop and grow in fresh water, but soon migrate to the ocean, where they spend most of their lives (Haroon,1998). The reduction of the depth and discharge of rivers due to construction of dams and barrages has affected the spawning, feeding and migration of this fish (Mizanur, 1997).
In Persian Gulf this fish migrate in Arvand and Bahmanshir rivers for spawning. The average annual landing of this species in 2006 was 4989.83 t and constituted about 15% of Khuzestan's total commercial fish landing. It supports a large fishery that is very important to Bangladesh and comprises 75% of the total world catch of this species and represents about 25% (250,000 t) of the total fish production in Bangladesh (Salini et al., 2004).
The delicious taste and contribution of this fish has lead to its position as one of the most economically important fish in this country (Mazumder and Alam, 2009).
Different aspects of biological studies on Hilsa have been done by different authors in world and Iran (Pillay, 1957; Pillay and Rosa, 1963; Islam and Talbot, 1968; Marammazi, 1994; Marammzi et al., 1998; Rahman and Cowx, 2006) but few study has been done on the stock assessment of this species in Iran but in Bangladesh many works has been done (Amin et al., 2005, 2008; Mazid et al., 2007; Haldar and Amin, 2005).
Length-based methods have lately found widespread use for the estimation of growth in fish populations, especially in tropical areas, because of the various disadvantages presented ageing techniques such as otolith or scale examinations or tagging experiments. As for other tropical fish, ageing of hilsa is problematic, because of the absence of annual rings on scales (Ahmed et al., 2008).
Growth studies are an essential instrument in the management of fisheries resources because these studies contribute to estimates of production, stock size, recruitment and mortality of fish populations (Issac, 1990). The fundamental purpose of fisheries stock assessment is to provide a background for the elaboration of management plans envisioning optimum long-term exploitation of natural aquatic resources (Hilborn and Walters, 1992). FAO-ICLARM Stock Assessment Tools (FiSAT) has been most frequently used for estimating population parameters of fin-fish and shell-fish (Al-Barwani et al., 2007).
The present study was undertaken to estimate key parameters of stock assessment and population dynamics of T. ilisha.
MATERIALS AND METHODS
Totaly, 10071 fish were collected monthly from April 2006 to March 2007 from two commercial landing centers of Khuzestan Province, Choebdeh in Abadan and Sajafi in Hendijan (Fig. 1). These samples of T. ilisha were caught by gillnet. Total Length (TL) was taken to the nearest mm for all fish and Total Weight (TW) of individual fish to the nearest 1 g was measured wherever possible.
The relationship between length (TL) and weight (TW) was estimated using linear regression analysis. To convert the exponential curve (W = aLb) to linear, both variables were transformed using ln x. The line of best fit for the linear relationship was described by Pauly (1983): ln TW = lna + blnTL.
The length frequency data were pooled into groups with 2 cm length intervals.
Growth was investigated by fitting the Von Bertalanffy growth function to length frequency data. The von Bertalanffy growth equation is defined as follows (Sparre and Venema, 1998):
where, Lt is length at time t, L8 the asymptotic length, K the growth coefficient and t0 is the hypothetical time at which length is equal to zero. FiSAT (FAO-ICLARM Stock Assessment Tools) programme (Gayanilo et al., 2003) provided estimated L8 and K. Asymptotic length (L8) was estimated using the methods of Powell-Wetherall (Pauly, 1983).
| Fig. 1: | Location of landing centers where T.ilisha were sampled |
L – L' = a + bL', L∞ = - a/b
and Z/K= - (1+b)/b |
where, L is defined as the mean length, computed from L' upward, in a given length-frequency sample, while L' is the limit of the first length class used in computing a value of L.
t0 was estimated by employing the equation of Pauly (1980):
Log (-t0) = - 0.3922 - 0.2752 Log L∞
- 1.038 Log K |
And the growth performance index (φ') was calculated using the following formula (Pauly and Munro, 1984):
φ' = Log10 K + 2 Log10
L∞ |
Total mortality rate (Z) estimated by using length converted catch curve method, which has been incorporated in to the FiSAT programme (Gayanilo et al., 2003). Natural mortality (M) calculated using the equation of Pauly (1980) which incorporates water temperature and the VBGF growth parameters L8 and K. The annual mean water temperature for the study was 23°C.
Log M = -0.006 - 0.279log L∞ + 0.6543logk
+ 0.4634 log |
The Clupeidae have values of M generally differing from those obtained through the proposed equations. It is tempting to attribute this schooling behavior that is so strongly developed in this group (Nikolsky, 1969). Correction factors 0.6 are given for factor M. The instantaneous fishing mortality (F) was taken as the difference between total and natural mortality: F=Z-M.
The exploitation ratio (E) is equal to the fraction of fishing mortality. Maximum Constant Yield (MCY) is defined as the maximum constant catch that is estimated to be sustainable.
| Table 1: | Guide to the relationship between natural mortality, M and natural variability factor, c |
The MCY represents the average catch that can be taken from a stock taking into account the natural variability inherent in the particular stock. MCY is calculated by the equation, MCY = c Yav where c is the natural variability factor related to natural mortality (Table 1) and Yav is the average catch across the appropriate time series. Resource status was evaluated by comparing of the fishing mortality rate with target (Fopt) and limit (Flimit) Biological Reference Points (BRP) which were defined as: Fopt = 0.5M and Flimit = 2/3M (Patterson, 1992).
The total annual stock size, average standing stock size and also MSY of T. ilisha were estimated. For this purpose, at first exploitation rate (U) was estimated using the equation given by Beverton and Holt (1957) and Ricker (1975) as U= F/Z (1-e-z). To estimate the annual catch (Y), the landing data of Hilsa shad were collected from the Fisheries Resources of Iran, then by using the values of U, F and Y the total annual stock (Y/U) and average standing stock or Bt (Y/F) were determined.
The approximate MSY was then calculated using the relationship proposed by Gulland (1977).
MSY= Zt x 0.5 x Bt |
where, Zt is the total mortality in the year t and Bt the standing stock size in the same year.
RESULTS
The linear regression analysis of the length-weight data allowed the estimation of the constants, a and b of the length-weight relationship represented by the equation:
W = 0.012 L2.968 |
with a regression coefficient R2 = 0.916 (Fig. 2)
Growth parameters of von Bertalanffy growth formula for T. ilisha were estimated as L8 = 42.74 cm and K= 0.77 year-1. Figure 3 shows the method of powell-wetherall plot for T. ilisha in Iran. Calculated growth performance index (φ') was found to be 3.14.
The yearly growth curve of this species using the von Bertalanffy growth parameters was calculated (Fig. 4).
The value of t0 was taken as -0.21 and φ' was estimated from the parameters as 3.14.
The annual instantaneous rate of total mortality (Z) derived from length-frequency catch curve was 2.55 year-1 (Fig. 5), the annual instantaneous rate of natural mortality (M) derived from the Pauly (1983) equation was estimated as 0.75.
| Fig. 2: | The length-weight relationship curve for T. ilisha |
| Fig. 3: | Estimation of L8 using of Powll-Wetherall plot for T. ilisha |
| Fig. 4: | Growth curve of T. ilisha |
Annual instantaneous rate of fishing mortality was 1.8 years-1 and the exploitation rate (E) was 0.7. The annual instantaneous rate of fishing mortality (F =1.8 year-1) was considerably greater than the target (Fopt = 0.37) and limit (Flimit = 0.5) biological reference points, suggesting that the stock was heavily overexploited.
The estimate of MCY was calculated here using the more reliable time series of commercial catch data from 1994 -2006. The resulted in an estimate of MCY for T. ilisha fishery of 2248 t.
M = 0.75, c = 0.6 (Table 1), Yav
(1994-2006) = 3747 t |
| Fig. 5: | FISAT graphic output of the catch curve analysis |
MCY= 3747x0.6 = 2248 t |
The estimate of MSY was calculated using the relationship proposed by Gulland (1977). The resulted in an estimate of MSY for T. ilisha 2653 t.
DISCUSSION
It was assumed for the analysis that sampling was randomly done despite the fact that the migration of pelagic fish stocks might have affected the representative of the samples and that bias could have been due to the introduction by the schooling behaviour of migratory species.
The length-weight relationship observed during the present study is given below:
W= 0.012 L2.968 |
The b parameters values in the weight-length model, W = aLb are close to 3 for the T. ilisha indicating isometric growth (King, 1995) (Table 2).
The reasons for the variation of b in the different regions are said to be due to seasonal fluctuations in environmental parameters, physiological conditions of the fish at the time of collection, sex, gonadal development and nutritive conditions in the environmental of fish (Biswas, 1993). The values of L8 and K were calculated as 42.74 cm and 0.77 year-1. Present results also appeared to be of the right order in comparison with a range of published estimate of von Bertalanffy growth equation derived from length frequency (Table 3). For T. ilisha, differences in growth rates between regions indicated stock separations (Quddus et al., 1984) which has, in some cases, supported a genetic difference (Mizanur, 1997). In general, the correlated parametrics values adjust themselves to provide a similar growth pattern represented by φ' (Sparre and Venema, 1998). The index of phi prime is suitable for comparing and computing the overall growth performance of different species of fish stocks (Amin et al., 2009).
Notably, the values estimated for T. ilisha were comparable to those for other stocks of T. ilisha in different regions, suggesting a similar growth pattern across different population (Table 3).
The data set for estimating Z by the length converted catch curve method should satisfy the primary assumption that the stock was is in equilibrium (Al-Hosni and Siddeek, 1999). In a declining stock, such as that of T. ilisha, this assumpting may have been violated because of a declining trend in recruitment tends to under estimate Z by roughly the some percentage of decline.
| Table 2: | Length-weight relationship of T. ilisha in different area of world |
| M: Male and F: Female | |
| Table 3: | Summery of the growth parameters estimates of T. ilisha in different area |
| M : Male and F : Female | |
| Table 4: | Summery of the mortality parameters estimates of T. ilisha in differente area |
| M : Male and F : Female | |
Thus, the true values for F and E should have been higher than what we mentioned above. Reliable estimate of M can only be obtained for an unexploited stock (Al-Hosni and Siddeek, 1999).
The total mortality of T. ilisha estimated in this study differs from results of others (Table 4). This is maybe a result of different sampling regions and different rates of exploitation of T. ilisha. Also, as Sparre and Venema (1998) point out, if a length- based method is applied to a migratory stock, without taking the effects of migration into account, the results can be highly biased.
M values correlate strongly with growth parameters. In tropical fishes tend to have, for any asymptotic size, higher values of M than other fishes. This is similar for K, tropical fishes tend to have, for any value of K, higher values of M than temperature fishes (Pauly, 1998).
The fishing mortality rate of 1.8 year-1 was substantially greater than both the target (Fopt =0.37 year-1) and limit (Flimit = 0.5) biological reference points. These results are important to fisheries management authorities as they suggest that the resource is heavily overexploited and in addition to a revision of mesh size regulations, a substantial reduction in fishing effort would also be required if management objectives are to be achieved. Patterson (1992) observed that the fishing rate satisfying optimal E level of 0.5 tended to reduce pelagic fish stock abundance and hence,the former author suggested that E should be maintained at 0.4 for optimal exploitation of those stocks. According, the T. ilisha fish stock appears to have been overexploited (E = 0.7).
The values of MSY and MCY calculated as 2653 and 2248 t, were greater than commercial catch. The results indicating that the stock of T. ilisha is heavily exploited. It is necessary to immediately impose fishing regulation on the stock and this can be done by gradually increasing the mesh size of the gears or by restricting fishing for certain seasons or declaring fish sanctuaries in certain areas, especially in spawning areas. Although high fishing intensity and/or pollution could have caused stock depletion, the biological and ecological causes of this disappearance remain unknown (Zhang et al., 2009).