2. TAMIL NADU (Continued))

2.7 ALIYAR RESERVOIR (Fig. 2.6)

Aliyar dam was constructed in 1962 across the Aliyar river. Aliyar reservoir, situated between 10°15' and 10°30' N and 76°50' and 77° 10' E, covers 646.0 ha at the FRL of 320.04 m above MSL. Apart from a catchment area of 468.8 km², the lake receives water from Upper Aliyar reservoir through a hydro-electric power station at Navamali and the Parambikulam reservoir (Kerala) through a contour canal. The reservoir with its maximum depth of 36.5 m and mean depth of 18.2 m, exhibits annual water level fluctuations ranging from 13.15 m to 31.03 m at an average of 21 m per year (Table 2.13). Alyar reservoir was studied by the Central Inland Capture Fisheries Research Institute for eleven years from 1982 to 1992.

Figure 2.5. Sathanur reservoir, Tamil Nadu

Figure 2.6. Aliyar reservoir, Tamil Nadu

Compiled from Sreenivasan (1976) and Prabhavati & Sreenivasan (1979)

(CICFRI, Barrackpore)

Physico-chemical quality of water and soil

The basin soil is acidic (pH 6.42), comprising sand (24.1%), silt (29.0%) and clay (46.9%) with levels of available phosphate, available nitrogen and C/N ratio indicating medium level of productivity.

The catchment area of Aliyar mostly comprises the hill ranges of the Western Ghats with limited forest cover and negligible agriculture, resulting in low nutrient leaching into the reservoir. Water of Aliyar is warm (temperature at the surface within the range of 21.2 and 32.5 °C) and clear (transparency 108 – 182 cm) with a klinograde oxygen distribution and a discernible increase in the values of specific conductivity, bicarbonate, total alkalinity and free carbon dioxide from surface to bottom. The vertical stratification is more pronounced during the first and last quarters of the year due to lesser turbulence from wind and inflow. Higher values of specific conductivity, total alkalinity and dissolved organic matter are mostly noted in the lentic sector. Ammonia is present only at the bottom of the profundal region of the lentic sector. Carbonate alkalinity is absent during most of the times. The water is generally poor in nitrate (nil - 0.04 mg 1^-1), phospate, calcium and magnesium, but moderate to high values of silicates are recorded (Table 2.14).

While temperature, pH, transparency and dissolved oxygen levels depict a congenial environment for productivity, the alkaliphobic condition of the water and the low values of calcium and magnesium are retardants to productivity. The low to moderate levels of phosphate and nitrate in Tamil Nadu reservoirs probably indicate their constant utilisation. There is no sign of eutrophication in Aliyar reservoir. With the maturation of the reservoir, the dissolved oxygen concentration has been increasing. There is also a decrease in oxygen demand as the flooded vegetation is already decomposed (Sreenivasan, 1970 c). The alkalinity, specific conductivity and the pH do not show significant change in values over the years. Based on the water characteristics, the reservoir continues to be oligotrophic even after three decades of its formation.

(CICFRI, Barrackpore)

Plankton

The mean standing crop of plankton is estimated at 10.8 ml m^-3 within a range of 6.4 to 26.5 ml m^-3 (observation period 1983 to 1986). The characteristic feature of the reservoir is the Microcystis sp., blooms during low water levels. The blue-green constitute 84.32% of the total plankton of the reservoir (Fig. 2.7).

Figure 2.7. Percentage composition of plankton in Aliyar reservoir

The abundance of plankton in Aliyar reservoir is related to the water level; the low mean depth coinciding with a higher density of plankton, mainly contributed by Microcystis. The low mean depth of 2.6 m during March to June 1983 resulted in a Microcystis bloom of 95.95 ml m^-3. During 1984, when the mean depth remained high (6.1 to 14.9 m), there was no bloom. The blooms recorded in 1985 and 1986 also coincided with the lowest mean depth. Correspondingly, seasons having higher mean depth matched with low density of plankton population in the reservoir. The relatively high water temperature of the reservoir favours the dominance and abundance of the Microcystis. Since blooms can prove detrimental to the health and survival of the fish, the maintenance of minimum water level has to be reckoned as a management tool to ensure the health of the ecosystem and fish stock. For the common planktonic organisms, see Table 2.15.

Numerically, the plankton population is dominated throughout the year by Microcystis sp. The only other biota worth mentioning are Mougeotia, the filamentous alga, Cyclops (2.9%) and Keratella (2.5%). The diversity of species in the reservoir is poor with negligible sectoral variation. The year-round distinct dominance of Microcystis suppresses development of a species-rich plankton population in the reservoir.

Benthos

Bottom macrofauna : The average standing crop of bottom macrofauna in Aliyar, recorded for four years from the depths of 2 to 10 m, is estimated at 258 ind. m^-2 (3.18 g m^-2). Oligochaetes dominate both in terms of numbers (47.8%) and weight (86.8%). Shallows have a low density of macrobenthos and their concentration generally increases with depth. The maximum concentration of benthic organisms (2 331 organisms m^-2/48.45 gm^-2) was recorded at 30 m depth.

Primary productivity

During 1984 to 1986, the gross primary production rate in the reservoir ranged from 0.278 to 0.699 g C m^-2 day^-1 (net production: nil to 0.385 g C m^-2 day^-1). The average rate of energy transformation is estimated at 13 580 cal m^-2 day^-1. The incident solar energy of visible light at Aliyar reservoir works out at 2 150 000 cal m^-2 day^-1. Thus, 0.64% of available light energy is fixed by the producers in the reservoir. Considering that only about 40 to 62% of the energy fixed by the producers is stored by them and the rest is utilised for respiration and metabolic activities, the quantum of energy finally converted into fish flesh depends on the qualitative and quantitative abundance of biotic communities within the impoundment. The peak yield from the reservior @ 193.58 kg ha^-1 in 1989–90 reveals a conversion efficiency of 0.47% of the available primary energy in the reservoir. The rate of energy conversion at primary producer level as well as at the fish production level at Aliyar is considered higher than in any other Indian reservoir.

FISH FAUNA

The indigenous ichthyofauna of the reservoir consists of 40 species belonging to 13 families, besides the seven stocked species (Table 2.16).

No. 1 to 7 - Species stocked

Feeding habits of fishes

The stomach contents analysis of cirrhinus mrigala, Labeo rohita, L. fimbriatus, L. calbasu and Cyprinus carpio showed that they feed on bottom sediments (Table 2.17). C. catla is known to be zooplankton feeder. However, in Aliyar, the stomach contents of this species comprise blue green algae to the extent of 44 – 80%, followed by organic detritus (12.7%) and zooplankton (12 – 24%). The feeding orientation of fishes in Aliyar reservoir indicates the flexibility in food preference and adaptability of various fishes to the reservoir system.

Blue-green algae constitute almost 40% of the food of catla, mrigal, common carp, L. calbasu and mahseer (Tor). This is not surprising since 85 to 90% of the plankton population in the reservoir belongs to blue-green algae. The higher share of blue-green algae among the food components of these fishes may not mean a preference for blue-green algae, especially as the digestibility of the Microcystis found in the guts of the fishes is probably low. One could conclude that the fishes of Aliyar reservoir rely more on the detritus for their nourishment. Organic detritus at the bottom, constantly enriched by the plankton rain from the water column, is a rich food resource of the reservoir. Most of the plankton forms, especially Microcystis sp., recorded from the stomachs of detritivorous fishes are, in all probability, ingested from the bottom sediment phase.

A substantial proportion of the energy transformation from plankton phase to fish flesh seems to be channelled through the detritus chain. Thus, C. catla, among the plankton feeders and mrigal and common carp among the bottom feeders should form a substantial proportion of the regularly stocked fingerlings.

Maturation and fecundity of commercial carps

Commercially important fishes do not breed in the reservoir, therby necessitating continuous stocking. The investigations made by the central Inland Capture Fisheries Research Institute (CICFRI) reveal that the goands of the major carps reach only up to IV stage of maturity in the reservoir, after which they undergo resorption, leading to breeding failure, with the exception of a few specimens of C. mrigala, in the Vth stage of maturity. The failure of major carps to reach the ripe stage in the reservoir is further corroborated by the low gonado-somatic index (GSI) and fecundity. The maximum GSI observed is only 6.2 for catla, 9.8 for mrigal and 10.5 for rohu. Common carp, however, has a higher GSI (16.0). The relative fecundity (the number of ripe ova per kg body weight) is as low as 0.39 to 2.10 ×10⁵ for mrigal and 0.62–2.20 10⁵ for common carp.

Fisheries management

Current fish yield rate of Aliyar reservoir is one of the highest in the country. This reservior acted as testing ground for the field trials of the scientific management package developed by the CICFRI and the yield optimisation achieved thereof is a standing testimony to the validity of the package. The State Fisheries Department launched a stocking programme in the reservoir immediately after its formation in 1962 (Selvaraj and Murugesan, 1990). The stocking density was erratic in the earlier years with a high preponderance of medium and minor carps including Puntius carnaticus, p. dubius, Labeo Kontius, L. fimbriatus,L.calbasu, Cirrhinus reba, C. cirrhosa, Acrossocheilus hexagonolepis and Tor khudree malabaricus. Apparently, availability of fish seed in the vicinity and other logistics of seed supply governed the stocking density and the species-mix, rather than any ecological considerations. The size of fry stocked was usually so small that they fell an easy prey to predators.

The exploitation system followed during the early phase also left much to be desired. The fishing was done, both by the Department of Fisheries and private fishing units, on a royalty basis, without any restriction on mesh size. This resulted in indiscriminate exploitation of stocked fishes much below the desirable size. A cumulative effect of a number of irrational reservoir management practices resulted in very low yields, ranging from 2.67 to 54.7 kg ha ¹, at an average of 26.21 kg ha¹ yr¹ (Table 2.18). The reservoir was subjected to scientific investigations since 1983 and a number of measures were initiated to develop the fisheries. The results of scientific management started reflecting in the yield rate from 1985. The management options used (Table 2.19) for enhancing fish yield were:

1. Stocking was limited to Indian major carps

2. Size at stocking was increased to advanced fingerlings (>100mm)

3. Stocking density was reduced to 236–312 ha¹ (Av. 637 ha¹)

4. The stocking schedule was spread over different months of the year

5. A strict mesh regulation, banning nets < 50 mm in mesh bar (knot to knot),

6. Ban on catching Indian major carps less than 1 kg in size.

A direct outcome of the adoption of scientific management practices in Aliyar was a substantial hike in fish yield to the tune of 193.58 kg ha¹. The yield ranged during 1985–86 to 1989–90, from 77.75 to 193.58 kg ha¹, with major carps constituting 84.04 to 96.25% (Table 2.20).

*** Data not available (After Selvaraj and Murugesan, 1990)

(After Selvaraj and Murugesan, 1990)

(After Selvaraj and Murugesan, 1990; CICFRI Barrackpore)

2.8 STATUS OF RESERVOIR FISHERIES IN TAMIL NADU

Tamil Nadu is harnessing its water resources to the fullest extent by creating impoundments all over the State and this has resulted in the location of reservoirs in a variety of places ranging from the uplands of the west to the coastal deltas in the east. They are situated in diverse morpho-edaphic and geo-climatic environs, receiving drainage from varying types of catchment areas. All these factors add an element of diversity to the environmental conditions under which the reservoirs exist. Scientific data on various environmental variables and biotic communities are available on at least 21 reservoirs in the State. After a close scrutiny of 17 of them. Sreenivasan (1976) concluded that the production processes and the community succession in these reservoirs are so complex and interrelated that classifying them, based on the conventional methods of morphometric and edaphic production indices, was almost impossible. Most of the limnological information with predictive value has been found to be of limited application in these reservoirs.

Most of the man-made lakes in Tamil Nadu, except Pykara, Sandynulla and Mukerti, are situated in medium to low altitude, the elevation ranging from 38 to 483 m above MSL. Thus, almost all the reservoirs are exposed to the warm tropical climate with air temperature never dropping below 15°C. A high temperature regime throught the year, coupled with plentiful sunshine has compensated for much of the adverse attributes in the shallow reservoirs of Tamil Nadu, which explains the non-conformity with morphometric and chemical indicators of productivity. From the two mountain reservoirs viz ., Sandynulla and Pykara, situated at +2 120 and +2 036 m altitude respectively, the former is organically rich and highly productive, harbouring a permanent bloom of Microcystis, whereas the latter is truly oligotrophic, with low alkalinity, weak oxygen stratification and an aidic water with the presence of dissolved carbon dioxide even at the top layer (Sreenivasan, 1968). Manimuthar and Pechiparai, two small reservoirs in the plains remained oligotrophic mainly due to nutrient-starved catchments.

Shallow lakes are often considered to be more productive, as larger proportion of the water volume remains in the euphotic zone. This maxim does not hold good in respect of Tamil Nadu reservoirs. Neither mean depth nor the maximum depth has any correlation with productivity. Vidur reservoir, despite being the shallowest of all reservoirs (mean depth of 2.1 m), did not develop a rich plankton community, even after seven years of its impoundment. Deeper reservoirs like Amaravathy (13.7 m), Aliyar (16.8 m) and Tirumoorthy (11m), on the other hand, developed algal blooms as soon as the dams were sealed. Other morphometric indices like shoreline development and volume development also did not convey the status of productivity with any degree of accuracy. Sreenivasan (1976) examined the area, mean depth and volume, of a number of reservoirs in Tamil Nadu and found no correlation among the above variables and productivity.

Total dissolved solids, measured in terms of specific conductivity or the bicarbonate alkalinity, is an index of biogenic production potential. Eventhough this indirect measure of edaphic quality reflected the status, to an appreciable extent, there are many exceptions. For instance, Amaravathy and Tirumoorthy, despite their low values of total alkalinity, specific conductivity and hardness, have high organic productivity.

Thus, the productive nature of each reservoir is determined by different factors. For example, the low productivity of Manimuthar and Pechiparai is due to morphometric features (deep basin and low catchment area respectively), while the high rate of productivity in Sandynulla is due to the rapid eutrophication from urban wastes. However, a close examination of the reservoirs reveals certain definite indicators of productivity. The nutrients load of the inflowing waters being dependent on the nature of catchment, quality of the catchment area plays an important role in determining the nutrient status of a reservoir. The percentage of cultivated area is often taken as a criterion for assessing the catchment quality. The high bicarbonate alkalinity and hardness of Sathanur, Krishnagiri and Vidur reservoirs can be directly attributed to the geochemical status of its catchment (Sreenivasan, 1976). Similarly, the low values of alkalinity, conductivity and hardness of Hope lake, Manimuthar, Pechiparai and Perumchani are a reflection of the poor nutrient status of their catchment area.

Thermal regime and oxygen distribution, determining the vertical profile of chemical parameters give a more reliable measure of productivity status in the reservoirs under study. The chemical stratification is the measure of metabolic and organic processes and gives a more direct indication about the trophic status. Absence of proper thermocline is the characteristic feature of all reservoirs in the state. In many cases, the inflowing river water is much cooler than the standing water, as the former flows down form heavily wooded sheltered forests. This cooler incoming water, unlike in the temperate region, readily mixes with the lentic water. Other reasons for the ready mixing are the withdrawal of water from the bottom and the wind-induced turbulence.

Oxygen depletion at the bottom is a very reliable indicator of productivity in tropical waters, especially because it is unaccompanied by thermal stratification. The bacterial decomposition of organic matter at the bottom is hastened by high temperature. Similarly, the photosynthetic processes at the euphotic zone keep up a steady release of oxygen. All the productive reservoirs in the State have the klinograde oxygen distribution, while the oligotrophic Pykara, Uppar and Pambar reservoirs have the orthograde oxygen distribution. Where oxygen depleted water is discharged downstream, it gets reoxygenated within 100 meters downstream (Sreenivasan, 1970a) and its impact on riverine aquatic communities is negligible.

It is often believed that total alkalinity above 50 (Northcote and Larkin,1956) and 90 mg 1^-1 (Spencer, 1964) is indicative of high productivity. In Amaravathy, the inflowing water is poor in total alkalinity and hardness. However, the standing crop of plankton provides a continuous plankton rain to enrich the bottom, which in turn decomposes to release the nutrients and keep the carbon production going. Thus, it is the vertical gradient of these chemical parameters, rather than the ambient concentration in inflowing water, that indicates the status of productivity.

Changes in biotic communities

Construction of dams and the subsequent impoundment brings in drastic changes in the biotic communities at all levels. These changes at the micro-level have far reaching impact on the trophic structure and events of the ecosystem, affecting the survival of higher organisms, especially fishes. Sandynulla reservoir, despite its deep basin and high altitude, sustains a rich plankton community, represented mainly by Microcystis in blooms. Drastic reduction in diversity of planktonic organisms with a high index of concentration of dominance is characteristic of reservoirs which receive rich supply of nutrients. Most of the reservoirs in the state such as Stanley, Bhavanisagar, Amaravathy, Aliyar, Tirumoorthy, Vaigai, Vidur and Poondi have blooms of blue-greens round the year.

Apart from light penetration and nutrient loading, water renewal plays an important role in the abundance of planktonic organisms. Abraham (1980b) pointed out that plankton density in Bhavanisagar reservoir was rich during the months of low level fluctuations. In reservoirs with violent level fluctuations, the annual flood water sweeps away not only the nutrients but even physically dislodges all the passive communities like plankton from the system. It is after the closure of outlets that the plankton community starts developing again. The same is true with benthic and macrophytic communities.

Fish fauna

Fish fauna of the reservoirs in Tamil Nadu is comprised indigenous species, those transplanted from other river systems and the exotics. In most cases, the last two categories support commercial fisheries while the indigenous ones are on the decline. The gradual phasing out of indigenous fishes is a matter of concern. Reasons for their decline can be summarised as:

1. loss of habitat,

2. changes in fish food organisms

3. obstruction of migratory pathways,

4. loss of breeding grounds, and

5. lossing out in competition with the transplnted species.

The anadromous Indian shad (Tenualosa ilisha), the fishery of which has collapsed in the Cauvery above the Mettur Dam, is the most serious loss of indigenous fishes. The hilsa from Cauvery-Coleroon system used to ascend the River Cauvery for breeding. The three lower barrages viz., the Upper Anicut, the Grand Anicut and the Lower Anicut restricted hilsa runs even before the Mettur dam came up. After commissioning the dam, hilsa disappeared from the upstream stretches.

Another species that should attract the attention of conservationists is Puntius dubius, indigenous to Mettur, Bhavanisagar, Amaravathy and Krishnagiri reservoirs. In Amaravathy, it contributed 55% of the catch in the mid-960s and declined to 0.15% in the 1970s and thereafter disappeared completely. In Bhavanisagar, it still constitutes 5 to 10% of the total catch. Decline of P. dubius in the reservoirs is mainly due to recruitment failure. The genus Puntius had the maximum species diversity in the region which, in addition to 5 commercially important species, comprised a number of uneconomic fishes.

Mahseers, comprising Tor tor, T. putitora and Acrossocheilus hexagonolepis are also threatened. Cirrhinus cirrhosa, althought still caught in Bhavanisagar, Stanley and other reservoirs is on the decline. This fish faces severe competition from C. mrigala that has been introduced into the reservoirs. Tilapia Oreochromis mossambicus has been the most successful among the introduced fishes. At least 24 reservoirs in Tamil Nadu have tilapia either as the major component of fish catch or represent a dominant fishery as in Amaravathy, where tilapia outnumbers other fishes by 90 to 10. Catla is the major fishery only in five reservoirsviz., Sathanur, Ponnaniyar, Kannathudai, Manimukta and Gomukhi. The indigenous fishes still dominate in Stanley, Bhavanisagar(P. dubius), Manimuthar, Gadana, Chittar and Perunchani(Puntius spp.).

Fish Production

Sixty-nine reservoirs of Tamil Nadu produce 1 323 t of fish annually. Figures from the small reservoirs designated as irrigation tanks are not available. Small (<1 000 ha in size) reservoirs' share to this fish production is 760 t. Thus 57.4% of the production comes from small reservoirs which form just 26.8% of the total area. Production from unit area is also high in respect of the small reservoirs. The small reservoirs yield 48.5 kg ha^-1, calculated for FRL, 88.01 kg ha^-1,if calculated for mean area. Medium reservoirs have fish yield of 13.74 kg ha^-1 (FRL) and 42.20 kg ha^-1 (mean surface area). Large reservoirs have the lowest yields of 12.66 and 23.46 kg ha^-1, calculated at FRL and mean surface area respectively (Table 2.21). If all categories of reservoirs are pooled together, the Tamil Nadu reservoirs produce 22.6 kg ha^-1 (at FRL) and 48.04 kg ha^-1 (at mean surface are level).

Data pertain to 1993 Source: Govt. of Tamil Nadu

This is one of the highest yields in the country, but yields vary widely among the districts. Dindigul Anna district produces 156.81 kg ha^-1 from its 130 reservoirs. Thiruvannamalai Sambuvarayar, Kamarajar and Coimbatore districts have average yields rates above 50 kg ha^-1 (Table 2.22).

Fish yield potential

Fish yield potential, based on the primary production rates, has been estimated in respect of seven reservoirs (Sreenivasan, 1976). Tirumoorthy is the most efficient in converting solar energy (1.803%), followed by Amaravathy (1.442%) and Bhavanisagar (1.050%). The energy harvested as fish is estimated at 0.02931% to 0.2152% of the energy fixed by the primary producers. The best conversion of Sathanur is 0.2152% and considering the fact that up to 1% of the carbon can be harvested, there is sufficient room for incresing the fish yield. In Amaravathy, there is scope for a five-fold increase in the fish production. The mono-species fisheries of catla and tilapia in Sathanur and Amaravathy facilitate better conversion of primary production to fish flesh and consequently, these two species are better converters of energy (Table 2.23).

(After Screenivasan, 1976)

Increase in productivity

Nearly 60% of the reservoirs in the State of Tamil Nadu and all the tanks are less than 500 ha in size, making them ideal for extensive aquaculture. Management strategy in very small reservoirs and the irrigation tanks should centre around an imaginative stocking and harvesting schedule to allow the stocked fish maximum time to grow. In many cases, the reservoirs nearly or completely dry up during summer, eliminating chance for fish reproduction. This calls for put and take system of management, where the selected species of fishes are stocked and harvested, all within less than one year. Such systems can also be fertilized to enhance plankton production. The small reservoirs also offer opportunities to integrate aquaculture with animal husbandry and livestock farming. However, the possible conflicts with other water users may have to be reconciled.

Pollution

The reservoirs in Tamil Nadu do not face problems from pollution on a very large scale. In the absence of any organised sewage disposal system in small towns. the community wastes often find their way to the reservoir. From fisheries point of view, this organic loading within certain limits is acceptable. Nevertheless, the problem needs to be addressed from a public health and aesthetic point of view. Organic wastes pose serious eutrophication problems in Sandynulla, which receives city sewage from Ooty. Non-point sources, mainly agricultural wastes, are potentially harmful. Accumulation of pesticide residues in water or sediment phase, their bioaccumulation in organisms and biomagnification in different trophic levels have been not yet investigated in Tamil Nadu and need urgent attention.

Some such instances of pollution from industrial sources have been reported from many reservoirs with varying degrees of impact on the environment and biotic communities. Fish mortalities in Mettur dam area due to effluent discharge from the Mettur Chemical and Industrial Corporation were reported as early as 1949. The effluents contained high concentrations of dissolved and insoluble solids, chlorides and free chlorine and were highly alkaline. However, the effluents were dangerous only during low discharge from the dam when the offensive fluids remained entrapped in rock pools where they affected fish. Apart from this, a series of chemical plants at Mettur discharge their effluents into the Ellis surplus side, but none of them is known to affect the fish life downstream.

Waste discharge into the Bhavani river from the Seshasayee Pulp and Board Limited, Pallipalayam, manufacturing writing paper and boards using bamboos. wood and bagasse adopting the sulfate processes, has been studied and found to be innocuous by Sreenivasan (1979a and b). The South India Viscose, manufacturing viscose rayons and staple fibre, discharges 16 000 m³ of wastes into the river Bhavani at Sirumughal every year. The sulfite process of manufacturing is highly polluting. There have been reports of fish mortality in Bhavanisagar reservoir due to this effluent discharge. Low dissolved oxygen level and high carbon dioxide concentrations were recorded in affected areas. Combining effluents with the septic wastes from the residential colony and treatment of the combined wastes in stabilization ponds before discharging them into river have been suggested by Sreenivasan (1979b).

Exotic fishes

Among the fishes transplanted into reservoirs of the State, the tilapia, Oreochromis mossambicus has been the most successful. It has become so ubiquitious that no water body in Tamil Nadu is free from it. After the initial enthusiasm with which the people have accepted it, the fish has become very unpopular due to the stunted size. In ponds, myriad of tilapia flocking the entire water body is a very common sight. The small-sized tilapia does not fetch a remunerative price and only way to utilize them is conversion into fish meal. However, in the reservoirs, the problem of stunted growth has not yet arisen. In Mettur, where the fish forms the bulk of the catch, the individual size is more than 0.68 kg, which is readily accepted by the market. However, it is significant to note that the average size of tilapia decreased from 1.5 kg in 1968 to 0.68 kg over a period of 25 years. Excessive proliferation of tilapia in reservoirs could be kept under check by predators like Notopterus cinotala. Wallago attu, Channa spp. and eels, and frequent level fluctuation that exposes the breeding pits.

After heavy stocking of tilapia in Vaigai, Sathanur, Amaravathy, Krishnagiri and Manimuthar reservoirs in the early 1960s, fairly large sized fish were caught in gill nets. In Vaigai they formed 25–50% of the catch by weight soon after stocking. Presently O. mossambicusis the single largest component of the fishery in the reservoir. Opinion is divided on the utility of the fish for yield optimization in Indian waters.

Impact of stocking on the species spectrum and productivity

Catla catla, Cirrhinus mrigala, C. cirrhosa, Labeo rohita, L. calbasu, L. fimbriatus and Cyprinus carpio are the species being stocked in reservoirs. In Mettur, catla started appearing in the fisheries in 1943–44 and gained the third rank in ten years to form 16% of the catch. During the next decade, i.e., from mid 1950s to 1960s catla was the most dominant component of the catch. It had a fluctuating fortune, since then. Today, catla form nearly 10% of the catch in the reservoir. This is the fate of catla in most of the reservoirs, where it does not reproduce. The exception is Sathanur, where self-reproducing catla contributes up to 91% of the total catch, while the fish forms only 10–20% of the catch in many reservoirs with regular stocking. C. mrigala and L. rohita are the other two species which could not establish themselves in any appreciable extent. L.calbasu was successfully introduced in Stanley and Bhavanisagar.

In most of the reservoirs, stocking is practised on an ad-hoc basis. Introduction of new species should be done with due consideration of long-term and short-term objectives. In larger reservoirs, the first option should be to stock suitable fishes which have a reasonable chance to establish self reproducing stocks. The main criterion for selection should be to fill a vacant or underutilized niche. On the other hand, a small reservoir with no breeding facilities needs to be managed on a continuous stocking and harvesting basis. This is a culture-based fishery, where what is stocked is harvested. The stocking material is selected after an assessment of trophic structure i.e., fish feed availability. Such approach is adopted successfully in Aliyar.

The organically rich Aliyar reservoir has a permanent bloom of Microcystis which is not directly consumed by any species of fish, but it is added to the detritus. The detritivorous Cirrhinus mrigala (37%) and Cyprinus carpio (24%) together form 61% of the catch. followed by catla (21%) and Labeo rohita (16%). The Aliyar experience stressess the importance of utilising the detritus as a fish food resource, where direct plankton feeders are not available or cannot utilize blue-green algae. Job and Kannan (1980) corroborated this by studying the caloric value and chlorophyll content in detritus phase in Sathiar reservoir, where the introduced carps account for 98% of the total fish harvested from the lake. In both reservoirs, the fisheries is totally dependent on the stocked fish. The stocking and harvesting schedule is carefully planned and the fish is removed after attaining a certain size. Smaller fish caught accidentally are released back into the reservoir. This management approach has increased the yield in Aliyar from the initial 35 kg ha^-1 up to 192 kg ha^-1.

Craft and gear

The most common gear used by the fishermen in the reservoirs of Tamil Nadu is the gill net of entangling type. The Rangoon net is the most popular among them. which has been introduced into the state from Andhra Pradesh (Nayar, 1979). Another variety of gill net used extensively in reservoirs is udu vala, a narrow bottom set gill net, operated usually in the shallow areas of the reservoir. Gill nets of varying mesh sizes are used to catch different species of fish. The small–meshed nets are used for Oxygaster phulo, Osteobrama vigorsii and other small fish. Large–meshed nets are called catla nets in Tamil Nadu.

There is no uniform standard length or height for gill nets. For the sake of computing catch per unit of effort in Bhavanisagar, Ranganathan and Venkataswamy (1966) reckoned 20 pieces of Rangoon nets comprising three different mesh sizes and a coracle as a standard unit. Mesh sizes were 6.3 cm (50 m length), 5 cm (40 m length) and 10 cm (100 m length). The All India Coordinated Project on Reservoir Fisheries has considered 50 m hung length as a unit and the effort was identified in terms of cluster of 50 m units.

Apart from gill nets, simple long lines, pole and line and cast nets (both stringed and unstringed) are occasionally employed. Common coracle, a saucer shaped country craft made of split bamboo mat covered with buffalow hide is the most popular craft in reservoirs.

Post harvest arrangements

Most of the ice plants and cold storages in the State cater to the marine sector. Mettur, Bhavanisagar, Sathanur, Krishnagiri and Amaravathy reservoirs have ice plants and cold storages for storing fish before its despatch to the Howrah railway station in Calcutta. Various market channels are in vogue in respect of reservoir fishes. The agencies involved are primary cooperative societies, Tamil Nadu Fisheries Development Corporation (TNFDC), Department of Fisheries and the private traders. Networks involving one or more of the above agencies are also not uncommon. For instance, the TNFDC collects fish from Aliyar, Sathanur, Bhavanisagar and Tirumoorthy and sells it directly to the public or to merchants. There are also arrangements between the Department of Fisheries and TNFDC to dispose the catch in the event of glut. A substantial part of the catch from Sathanur, Mettur, Bhavanisagar and Aliyar is despatched to the Howrah market.

Cooperatives

Tamil Nadu has 598 primary fishermen's cooperatives with a total membership of 157 400, of which 250 are inland fishermen societies. These societies are not as efficient as their counterparts in agriculture, marketing, consumer and housing sectors, primarily because the fishermen cooperatives confine themselves mainly to the provision of cheap credit (long, medium and short-term) and working capital to fishermen (Parasuraman, 1979). They do not attract institutional finance due to legal restrictions. Fishermen cooperatives in the State are financed solely by the State Governmentin the form of share capital and loans in the long, medium and short-term categories. They normally do the following functions:

1. Distribution of essential commodities to their members with the working capital accommodations granted by State Government.

2. Taking small reservoirs on lease and let them on sub-lease to members on yearly basis.

3. Fish marketing, and

4. Distribution of yarn under subsidy schemes.

Figure 3.1. Distribution of reservoir in Kerala