Managing Tilapia in Large Reservoirs – A Case Study From Jaisamand Lake, Udaipur (India)
N.C. Ujjania M.P.S. Kohli* and L.L. Sharma**
Krishi Vigyan Kendra, Kota
*Central Institute of Fisheries Education, Mumbai
**Department of Aquaculture, College of Fisheries
MPUAT, Udaipur, Rajasthan
Abstract:
The variations of Tilapia population in Jaisamand Lake, Rajasthan have been analyzed in the present study. Samples of Tilapia between size range 17-43.5 cm. Length and 80-1620 g, weight were collected and various biological parameters like length-weight relationship, age and growth, gastrosomatic index, and gonadosomatic index were studied. The length-weight relationship revealed a strong correlation (r = 0.986). The back calculation from the scale studies established the presence of 0 to +3 year class fishes in the catch. The gastrosomatic index ranged between 0.909 – 2.619 and the gonadosomatic index was found to be 0.304 for male and 0.568 for female. The declining trends of carps and cat fish population in the annual landings due to the stiff competition by Tilapia during last three years were also analysed. The present study calls for regular monitoring of fish population data in order to ensure protection of commercially important fishes as well as to keep the Tilapia population under suppressed condition using appropriate biomanipulation technique.
Introduction:
The Jaisamand Lake situated 56 km South East of Udaipur in Rajasthan state has been reported to have high fish production comparable to the most productive standing water bodies (Durve, 1976). The age old existence of the reservoir and specific morphometric features (DL > 3) provide opportunities for the high biological production in this water body which has been supporting fisheries of valuable carps and cat fishes. In 1990 the occurrence of exotic fish Tilapia (Oreochromis mossambicus P.) was noticed in fish catch (Anon, 1995) probably, on account of accidental entry with seed of Indian Major Carp. The reported occurrence of this fish has been cause of concern for the fishery biologists (Jain and Gupta, 1994; Singh, 1994; Anon, 1995). The fish production data for the past years (fig.1)suggest that there has been alarming increase in the total number of Tilapia landings with a simultaneous impact on the total production of the valuable Indian Major Carps and cat fishes as evident from the data (Durga and Srivastava 2008).
It has been reported from other parts of country that Tilapia is overpopulating and dominating the fishery of reservoirs and lentic water bodies in particular. The prolific breeding with strong parental care and omnivorous feeding habits of Tilapia are the two main reasons behind its gradual dominance and spatial establishment. In this context, it needs a concerted effort on the part of fishery managers to assess the biological impact and simultaneous shadowing of other fish population in natural or manmade ecosystems.
In the present paper Tilapia problem of Jaisamand Lake has been analyzed with particular reference to the results of certain remedial measures adopted for control of Tilapia.
Material and Methods:
For the biological and catch analysis the fish samples were collected from ‘Namla’ landing center of the lake during the fishing year 1996-97. Subsequent observations on fish catch have been made recently during October, 2007 on fish landing center. Data on fish stocking was calculated from the records of state fisheries department.
Length-Weight relationship:
For length-weight relationship 550 samples were collected. Total length, Standard length and weight of fish were measured following the method of Le Cren (1951) at the landing center for calculating length-weight relationship.
Age and Growth:
To determine age and growth a separate study was conducted on fish scales. For this purpose 5 scales from each specimen of different size groups were collected. The length of the fish at the time of formation of annuli could be estimated through Bagenal and Tesch (1978).
Gastrosomatic Index:
The Gut contents were analysed quantitatively and qualitatively. The food items were expressed in terms of percentage number on “eye estimation” (Pillay, 1952). The value of Gastrosomatic Index was calculated using the formulae given by Desai (1970)
Gonadosomatic Index:
For the study of gonadosomatic index, weight of gonads and weight of body (both male and female) were observed. Gonadosomatic index (GSI) was estimated with the help of the formula given by (Valadykov, 1956; Hopkins, 1979).
Results and Discussion:
As shown in Table 1 and Fig. 2 there was strong relationship between length and weight, the highest ‘r’ value (0.986) was found between total length and weight of fish of length group I while lowest value of ‘r’ (0.771) was found between standard length and weight of fish of length group II. Similarly, the value of growth exponent ‘n’ was appreciably high (3.414) for the total length and weight of fish of length group I and low (2.374) for the standard length and weight of fish of length group II. As regards, the constant term ‘a’ of the ‘Y’ equation (Table 1) the highest value (-0.592) was observed between standard length and weight of fish belonging to length group II while the lowest value (-2.306) was seen between total length and weight of fish of length group II (De and Datta 1990); Basu et al 1996 have reported the length-weight relationship of many fish species. In Table 1, as evident, that the value of tabulated ‘t’ was lesser than the calculated ‘t’. Pratap and Makamba (1987) found that there was correlation between length and weight of O. urolepsis C. wherein the value of ‘n’ was 3.03 and value of ‘r’ was 0.960. Considering this the value of ‘n’ (exponent of regression) as well as value of ‘r’ (coefficient of correlation) obtained for Tilapia of lake Jaisamand appears to be fairly satisfactory.
In Tilapia the cycloid type of scale were characterized by distinct marking which represents the growth periods. Sehgal et al (1979), De and Datta (1990) and Chacko and Krishnamoorthy (1954) reported the growth parameters in certain fresh water fishes. Johal and Tandon (1985 and 1987) and Singh (1994) used the scale for the study of growth parameters in many fish species. It may be seen that most of the specimens belong to the age group or year class +2 followed by year class +1 and +3 (fig 2). The results further indicate that average length of Tilapia at time of capture was 28.2, 35.0 and 43.0 in year class +1, +2 and +3 respectively. The back calculated length and weight of fish are also mentioned in the Table 1, which are 26.4, 32.3 and 38.8 cm and 330, 625 and 1057 g for year class +1, +2 and +3 respectively.
Gut content of Tilapia sampled in the present study showed that it consumes a variety of food items ranging from macrophytes and algae to plankton and detritus. The quantity of items consumed by the fish varied in different samples thus indicating lack of a fixed menu. In the stomach contents filamentous algae varied from 5-10%, cellular algae 10-20% to 60-70% and zooplankton varied between 10-15% to 30-40% zooplanktonic food was composed of cladocera, rotifers and copepods. Besides this, food contained plant tissue, detritus and insects that varied from 10-20% to 80-90% in the diet of Tilapia of this Lake. In this regard the gastrosomatic index of tilapia was found to vary between 0.909 - 2.619.
The gonadosomatic index of male and female fish observed 0.304 and 0.568 respectively which could be considered fairly well.
After the reported occurrence of Tilapia in Jaisamand Lake, further studies have shown that during 1997 Tilapia attained sizable population in this lake amounting up to 97% in the total fish landings (Ujjania, 1997). Such high population of Tilapia obviously affected other fishes due to overlapping niches. Moreover, due to intra-species competition size reduction in Tilapia was also noticed. Considering such grave situation and for protecting the interest of Tribal fishermen a technical committee was formed by the TRADCF Ltd. Udaipur. Subsequently, the committee made following recommendations:
-Selective fishing of Tilapia with the use of appropriate gears.
-A temporary ban on catching, catfishes in Jaisamand Lake. Thus, committee recommended stopping use of long lines.
-It was also suggested to stock advance fingerlings of Indian major carps.
On the advice of technical committee RTADCF obtained permission to operate zero mesh net for removing juveniles of tilapia. Thus, a sizable stock of under sized tilapia (and of course others also) were removed from Jaisamand lake. These low priced small fishes were sold as dried fish. Simultaneously advance fingerlings of Indian major carps were also stocked in a sizable number in this lake from 1990 onwards.
In the monsoon of 2006 excessive rains and subsequent overflow of small perennial waters in the catchment caused movement of advance size yearlings into the lake Jaisamand. This stock has effectively suppressed the tilapia proliferation in this lake which is interesting.
It is further observed that due to occurrence of Tilapia in sizeable numbers in Jaisamand density of aquatic weed also shown noticeable reduction. Thus, at present aquatic macrophytes are observed in major quantity.
For continuous suppression of Tilapia population in Jaisamand following measures are suggested:
- Identification of unoccupied ecological niche for optimization of fish production.
- Calculation of maximum sustainable yield as well as setting up fish stocking ratio for formulating management policy.
- Maintaining adequate pressure on Tilapia with the use of biomanipulating techniques.
- Ue of selective fishing gears for harvesting optimum number of Tilapia from Jaisamand Lake.
- To work out feasibility for achieving sex reversal in wild waters as that of Jaisamand to control further expansion of Tilapia.
Acknowledgement:
The authors express their gratitude to the Manager, RTADCF, Udaipur and Deputy Director (Fisheries), GOR, Udaipur for providing the necessary help for the preparing the manuscript.
References:
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Table: 1. Regression equations for length on weight of Tilapia from Lake Jaisamand
S. N. / Length groups / Equation / ‘r’A / Total length v/s weight
1. / 15 – 30 cm / Log W = -2.306 + 3.414 Log TL / 0.986**
2. / 30 – 35 cm / Log W = - 1.655 + 2.953 Log TL / 0.892**
3. / 35 – 45 cm / Log W = - 1.752 + 3.010 Log TL / 0.864**
B / Standard length v/s weight
1. / 15 – 30 cm / Log W = -1.671 + 3.157 Log SL / 0.982**
2. / 30 – 35 cm / Log W = - 0.592 + 2.374 Log SL / 0.771**
3. / 35 – 45 cm / Log W = - 1.043 + 2.699 Log SL / 0.807**
**Significant at 1% level
Table: 2. Total fish production of Jaisamand Lake and percentage contribution of Tilapia
S. No. / Year / Total catch(t) / Tilapia catch
(t) / (%)
1985-1986 / 210.8
1986-1987 / 141.0
1987-1988 / 234.0
1988-1989 / 185.3
1989-1990 / 260.7
1990-1991 / 287.1
1991-1992 / 329.1 / 0.1
1992-1993 / 361.1 / 0.1
1993-1994 / 355.3 / 21.0 / 6.0
1994-1995 / 225.0 / 58.0 / 25.7
1995-1996 / 263.6 / 91.2 / 35.0
1996-1997 / 105.7 / 55.0 / 52.0
1997-1998 / 159.7 / 99.0 / 62.0
1998-1999 / 139.3 / 100.3 / 72.0
1999-2000 / 327.3 / 284.8 / 87.0
2000-2001 / 352.4 / 341.8 / 97.0
2001-2002 / 350.9 / 343.9 / 98.0
2002-2003 / 207.7 / 196.1 / 94.4
2003-2004 / 212.4 / 191.2 / 90.0
2004-2005 / 29.8 / 20.9 / 70.0
2005-2006 / 252.1 / 54.2 / 21.5
2006-2007 / 307.6 / 57.8 / 18.8
(Number of fishing days are 200-250/year, *Tilapia first time seen in small quantity)
Figure 1. Total fish production of Jaisamand Lake and contribution of Tilapia
Figure 2. Occurrence of various size groups of Tilapia in Jaisamand Lake
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