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Abstract
Due to the rarity of scientific information on Lates niloticus, Alestes dentex, Tilapia zillii, Oreochromis niloticus, and Sarotherodon galilaeus of Lake Nasser, the present work was suggested and aimed at:
i ) Estimation of the biological and population parameters, age composition, length distribution and age-length key of the fore mentioned species.
ii) Studying and modeling population dynamics, stock assessment and fisheries status of those species, hoping that this work would be beneficial for researchers and design makers.
The present results can be summarized as follows:
Length-weight relationship of Lates niloticus, Alestes dentex, Tilapia zillii, Oreochromis niloticus and Sarotherodon galilaeus was best described by the general power function equations (W in g and SL in mm):
Lates niloticus W = 6 x 10 -5 SL2.8246
Alestes dentex W = 5 x 10 -6 SL3.232
Tilapia zillii W = 8 x 10 -5 SL2.8958
O. niloticus 1984/1985 W = 6 x 10 -5 SL2.9414
O. niloticus 1994/1995 W = 1 x 10 -4 SL2.804
O. niloticus 1999/2000 W = 7 x 10 -5 SL2.9107
S. galilaeus 1984/1985 W = 2 x 10 -4 SL2.7534
S. galilaeus 1994/1995 W = 5 x 10 -4 SL2.5787
S. galilaeus 1999/2000 W = 2 x 10 -4 SL2.7716
Test of significance of the exponent (n) led to the conclusion that the growth in weight relative to fish size was negatively allometric (n statistically less than 3) for all the forementioned species except for Alestes dentex where the growth in weight relative to fish size was positively allometric. There was no special trend in the condition factor of species studied with size increase.
The relationship between SL and scale radius (S) for all species under study was found to be linear. Such a relationship was best described, according to the least square method, by the following equations:
Lates niloticus : SL = 59.364 + 55.056 S
Alestes dentex : SL = 75.404 + 44.034 S
Tilapia zillii : SL = 76.72 + 20.796 S
O. niloticus 1984/1985: SL = 14.902 + 39.55 S
O. niloticus 1994/1995: SL = 5.053 + 41.115 S
O. niloticus 1999/2000: SL = 13.169 + 37.068 S
S. galilaeus 1984/1985: SL = 64.089 + 26.026 S
S. galilaeus 1994/1995: SL = 50.345 + 29.915 S
S. galilaeus 1999/2000: SL = 51.585 + 26.724 S
Growth in length was studied for all species under study, annual fluctuations in growth rate were evident. The results show that in all age groups the mean back-calculated lengths did not exactly conform with the corresponding observed ones. Also, it is clear that the whole species under study attained their greatest length increment at age group I.
Growth in weight also was studied for all species under study and wide variations in increments in weight with age were revealed where Lates niloticus attained the greatest part of its weight during the 13th years of life, Alestes dentex attained the greatest part of its weight during the 3rd & 4th years of life, Tilapia zillii attained the greatest part of its weight during the 1ST years of life (32.03%). O. niloticus attained the greatest part of its weight during the 3rd year of life in 94/95 sample and 4th year of life for 84/85 and 99/00 ones. S. galilaeus attained the greatest part of its weight during the 1st and 2nd years of life in 84/85 sample, 3rd year of life for 94/95 and 4th year of life for 99/00.
The frequency and percentage frequency of fish specimens in each age group show that there was an overlapping between age groups and the majority of fishes was found to belong to age groups I and II.
The population parameters of all species under study of the present work were estimated. Such parameters exhibit the status of species fisheries and facilitate the intra and inter specific comparison through time and space. As regard Von Bertalanffy growth model, the curvature parameter, K, indicated that Tilapias and Alestes (k=0.120-0.325) approach their asymptotic length (L∞) faster than Lates niloticus (k=0.069). This is in a close agreement with the fact that the short-lived animals are characterized by a high K-value and the long-lived ones are characterized by a low K-value (Beverton and Holt, 1959).
The yield per recruit of Lates niloticus, Tilapia zillii, Alestes dentex, Oreochromis niloticus and Sarotherodon galilaeus were estimated by means of Gulland equation (1969). The results show that in L. niloticus, at the present level of fishing mortality (0.18), age at first capture (0.74 year) and natural mortality (0.17), the yield per recruit was estimated to be 1421.2 g. The increase of current Tc to be 2 and 4 year, led to MSY/R of 1776.2 and 2142.7 g at fishing mortalities of 0.12 and 0.2 respectively.
In Alestes dentex, at the present level of fishing mortality (0.1), age at first capture (1.35 year) and natural mortality (0.70), the yield per recruit was estimated to be 43.15 g. To evaluate the effect of changing the age at first capture “TC” on the yield per recruit of Alestes dentex, Tc reduced to be 1 year, this led to MSY/R of 173.99 at fishing mortality 3.6. By increasing Tc to be 2, the maximum yield per recruit obtained was corresponding to higher values of fishing mortality. The results show that in T. zillii at the present level of fishing mortality (0.58), age at first capture (1.47 year) and natural mortality (0.79) the yield per recruit was estimated to be 110.4 g. To evaluate the effect of changing the age at first capture “Tc” on the yield per recruit of T. zillii, Tc was increased to be Tc = 2 and decreased to be Tc=1, the maximum yield per recruit obtained was corresponding to higher values of fishing mortality, and it is preferable to decrease the age at first capture to 1 year because the values of yield per recruit were higher when Tc=1 than those obtained when Tc=1.47 and Tc=2 in the corresponding values of fishing mortalities
The results show that in Oreochromis niloticus 84/85, at the present level of fishing mortality (0.24), age at first capture (1.26 year) and natural mortality (0.67), the yield per recruit was estimated to be 311.3 g. Fig. 39 shows that the maximum sustainable yield per recruit (615 g) is corresponding to higher values of fishing mortality (5.0). The increase of current Tc to be two years, revealed that the maximum sustainable yield per recruit (594 g) is corresponding to higher values of fishing mortality (6.0). By decreasing Tc to be one year the maximum yield per recruit obtained was 599.5g and corresponding to fishing mortality of 2.7. This means that the increase of age at first capture is not associated with increase of MSY/R
In O. niloticus 94/95, at the present level of fishing mortality (0.7), age at first capture (1 year) and natural mortality (0.47), the yield per recruit was estimated to be 402.4 g. Fig. 40 shows that the maximum sustainable per recruit yield obtained was 404.1g at fishing mortality of 0.9.
This means that the present level of fishing mortality (0.7) is lower than that which gives the maximum yield per recruit by about 22.2% and by increasing of the level of fishing mortality from 0.7 to 0.9 (22.2%) the yield per recruit will be increased from 402.4g to 404.1g. it is clear that there is no big difference between the yield per recruit at the current fishing mortality (0.7) and that gives the maximum sustainable yield per recruit (0.9). The increase of current Tc to be 1.5 and 2 years, led to MSY/R of 434.98g and 455.95g at fishing mortalities equal to 1.3 and 2.4, respectively. This means that the increase of age at first capture is associated with increase of MSY/R
In O. niloticus 99/00, at the present level of fishing mortality (0.31), age at first capture (0.9 year) and natural mortality (0.42), the yield per recruit was estimated to be 350.5 g. Fig. 41 shows that the maximum sustainable yield per recruit (427.4g) can be obtained at fishing mortality of 1.3. The increase of current Tc to be 2 and 3 years, revealed that the maximum sustainable yield per recruit (MSY/R) is corresponding to higher values of fishing mortality in both cases. This means that the increase of age at first capture is associated with increase of MSY/R The results show that in Sarotherodon galilaeus 84/85, at the present level of fishing mortality (1.31), age at first capture (1.41 year) and natural mortality (0.40), the yield per recruit was estimated to be 456.2 g. Fig. 61 shows that the maximum sustainable yield per recruit (468.07g) can be obtained at fishing mortality (4.9). It is clear that there is no big difference between the yield per recruit at the current fishing mortality (1.31) and that gives the maximum sustainable yield per recruit (4.9). The increase of current Tc to be 2 year, revealed that the maximum sustainable yield per recruit (465.5 g) is corresponding to higher values of fishing mortality (6.0). This means that the increase of age at first capture is associated with increase of MSY/R. The decrease of current Tc to 1 year, led to MSY/R of 461.7 g at a value of fishing mortality equal to 2.4.
In S. galilaeus 94/95, at the present level of fishing mortality (3.78), age at first capture (1.01 year) and natural mortality (0.62), the yield per recruit was estimated to be 317.4 g. Fig. 68 shows that the maximum sustainable yield per recruit (MSY/R) obtained is 421.03 g at fishing mortality equal to 0.8 corresponding to higher values of fishing mortality. The increase of current Tc to be 1.5 and 2 years, led to MSY/R of 484.1 and 530.68 g at fishing mortalities equal to 1.2 and 2.3 respectively. This means that the increase of age at first capture is associated with increase of MSY/R.
In S. galilaeus 99/00, at the present level of fishing mortality (0.47), age at first capture (0.8 year) and natural mortality (0.36), the yield per recruit was estimated to be 323.3 g. Fig. 70 shows that the maximum sustainable yield per recruit (MSY/R) is 360.9 g at fishing mortality of 2.3. The increase of current Tc to be 1.5 and 2 years, revealed that the maximum sustainable yield per recruit (MSY/R) is corresponding to higher values of fishing mortality(6.0) in both cases.
The yield Isopleths exhibited that the maximum sustainable yield per recruit of Lates niloticus, Alestes dentex, T. zillii, O. niloticus 84/85, 94/95, 99/2000 and S. galilaeus 84/85, 94/95, 99/2000 lies between 0.15-0.55, 0.20-0.60, 0.15-0.55, 0.20-0.60, 0.20-0.55, 0.15-0.55, 0.15-0.50, 0.20-0.60 and 0.15-0.50 of Lc/L∞ espectively.
Length-based Thompson and Bell analysis of Lates niloticus, Alestes dentex, Oreochromis niloticus and Sarotherodon galilaeus are shown in the related Tables and Figures. These Tables and Figures show MSY, MSE, their corresponding biomass, the biomass relative to MSY and X-factor for all species studied. Using the current recruitment derived by VPA MSY of Lates niloticus, Alestes dentex, Oreochromis niloticus 84/85, 94/95, 99/00, Sarotherodon galilaeus 84/85, 94/95, 99/00, were 2217.21, 4384.63, 59945.14, 17204.09, 21589.32, 16069.36, 15870.98 and 24209.71 ton at a multiplication factor (XFactor) of 0.46, 8.54, 11.65, 1.61, 4.49,2.11, 0.69 and 5.20 respectively. This means that the current fishing pattern should be highly increased to 0.46, 8.54, 11.65, 1.61, 4.49, 2.11, 0.69 and 5.20 respectively to obtain MSY. On the other hand, to reach MSE, the multiplication factor should be 0.41, 4.11, 4.64, 1.09, 2.69, 2.11, 0.54 and 1.86 for Lates niloticus, Alestes dentex, Oreochromis niloticus 84/85, 94/95, 99/00, Sarotherodon galilaeus 84/85, 94/95 and 99/00, respectively. The results of the present study were compared and discussed with other authors in different regions of the world. This study also answer on some of lake research requirements mentioned in the workshop held in Aswan by ICLARM in cooperation with the Fishery Management Center of the High Dam Lake Development Authority in June, 1998.