India Reservoirs : A Review On Management Criterion

Reservoirs are defined as man-made impoundments created by constructing dams or other barricades across rivers or streams (Sugunan, 1995). Reservoirs are classified generally as small (<1 000 ha), medium (1000 to 5000 ha) and large (> 5 000 ha), especially in the records of the Government of India (Sarma, 1990, Srivastava et al., 1985). Though India’s multipurpose reservoirs are important sources of fish production, fisheries development in them have not received adequate attention. In a few reservoirs where some efforts have been made, the yields have not been satisfactory. This situation is mainly due to lack of understanding of reservoir ecology and trophic dynamics, wrong selection of species for stocking and irrational exploitation due to lack of expertise in the field of stock dynamics in relation to fishing effort. Thus the yields from reservoirs remained at low level , the average being 8 kg/ha from large reservoirs (above 5000 ha) and 20 kg/ha from medium reservoirs (1000-5000 ha) (Srivastava et al. 1984).

It had been a significant surge in research focusing on the management criteria of reservoirs in India. Scholars and researchers delved into understanding the unique challenges posed by small, medium, and large reservoirs, aiming to develop effective management strategies. Studies concerning small reservoirs highlighted the importance of community-driven management. Research by Patel et al. (1993) emphasised the role of local communities in sustainable water use, showcasing how indigenous knowledge could be integrated into management practices. Furthermore, Gupta and Khan (1997) explored the socio-economic factors influencing small reservoir management, shedding light on the significance of empowering local communities for successful implementation. Singh and Reddy (1995) conducted a comprehensive study on the multi-purpose uses of medium-sized reservoirs, emphasising the need for coordinated efforts among agriculture, industry, and urban sectors. Additionally, Sharma et al. (1998) investigated the impact of changing climate patterns on medium-sized reservoirs, highlighting the necessity of climate-resilient management strategies. Notably, Das and Chatterjee (1992) explored the inter-state water disputes related to large reservoirs, providing insights into policy frameworks for resolving conflicts. Sharma and Mishra (1996) focused on optimising hydropower generation from large reservoirs, emphasising the importance of eco-friendly technologies to minimise environmental impact. Multi-purpose management of reservoirs can be evaluated in either a single or a multiple objective framework. For example, water supply, irrigation and power generation can all be included in an economic framework where all decisions are evaluated in terms of the single objective of maximising net economic benefits (Hu et al., 2015). globally, the majority of dams are built for a single purpose, such as water supply (10.4% of large dams in the world), agricultural irrigation (27%) or hydro-electrical power generation (25%), close to a third of reservoirs cater to multiple design purposes, such as water supply and flood control or irrigation and hydropower generation (McMahon and Petheram, 2020).

Several common themes emerged across studies during this period. Community participation was identified as a cornerstone for successful reservoir management, regardless of size. Studies consistently underscored the significance of incorporating traditional wisdom into modern management practices, preserving local ecosystems, and ensuring equitable access to water resources.

A. Trophic changes in man-made lakes

Reservoirs are man-made ecosystems differing from natural lakes in their origin and trophic evolution and in morphometric and edaphic factors. Natural lakes commence with oligotrophy and evolve to eutrophy. the process being very slow. In contrast. reservoirs pass through three distinct phases - initial high fertility. trophic depression and final fertility (Neel, 1967; Jhingran. 1975).

The newly formed impoundment inundates vast tracts of forest and agricultural land. The decay of submerged vegetation releases nutrients causing initial fertility leading to intense development of fish food organisms - plankton. bottom microflora and fauna. This initial surge is often spectacular and lasts for 2 to 3 years. This phase is followed by a state of trophic depression caused by the rapid utilisation of nutrients by the flora and the diminishing release of nutrients from the bottom due to sedimentation of the reservoir bed. This phase is marked by low production of fish food organisms and lower fish growth. hence. less production. Its duration is variable and depends on the climatic and hydrological conditions of the impoundment. In Soviet Union the period of trophic depression lasts 6 to 10 years in southern impoundments (upto latitude 55°N) and upto 25-30 years in reservoirs of northern region (Lapitzky. 1965). In Indian reservoirs with tropical climate. trophic depression is expected to be of shorter duration as recorded in Nagarjunasagar, where it lasted only three years (Ramakrishniah. 1987). After the depression period the reservoir recovers with the accumulation of nutrients. The final fertility is reported to be much lower than the initial phase in some reservoirs of USA. However. in reservoirs of India it is found to be of a much higher order exceeding the initial level of fertility (Ramakrishniah. 1987).

B. Management measures

1. At pre-impoundment phase

Development of fishery in an impoundment is dependent on the ichthyofauna of the parent river as well as management measures taken before the impoundment. The initial phase of high fertility should be taken advantage of and species which are on short food chain should be planted. Even such species exist in the parent river, it is desirable to strengthen their stock density in the new expanded environment. The trash fishes should be removed from the river stretch using all available methods to weaken their competition to economic fishes. Experience in Indian reservoirs has shown that major carps especially Catla catla, have done extremely well in the new impoundment. Once these species establish in the new impoundment they would give a firm carp base for future development. If any lapse occurs in management at this stage, trash fish populations proliferate paving the way for the establishment of catfish populations. Once catfishes establish, the overall productivity of the reservoir would go down as their niches are on higher level of the long food chain. Effort to develop carp fishery at a later stage would work out to be more with less encouraging results. This aspect has been amply demonstrated in Nagarjunasagar and Tungabhadra reservoirs. Reservoir bottom should be cleared substantially of trees, bushes and boulders to facilitate operation drag nets and gill nets.

2. At post-impoundment phase

Many reservoirs in India came into existence without much attention given to fishery development at pre-impoundment phase. In such reservoirs it is imperative to study the fishery potential based on certain productive indices and classify the reservoir as poor, medium and high in terms of fish production before developmental measures are undertaken.

C. Indices of reservoir productivity

Biological productivity in impoundments is influenced by climatic, edaphic and morphometric features. The edaphic features such as extent of drainage area, its rate of erosion and total runoff determine the nutrient load into the reservoir, while climate and basin morphometry determine the rate of their utilisation. It has been recorded in Indian reservoirs that their fertility is dependent more on the catchment than on the basin soil (Natarajan, 1976). Area, mean depth and shore development index are important morphometric factors determining the productivity of the lake. Among chemical parameters total alkalinity, calcium, phosphates, nitrates and total dissolved solids are often used as indices of productivity. The most dependable index of productivity in reservoirs is oxycline in summer. The strength of the oxycline is an index of richness of bottom deposits of allochthonous and autochthonous origin.

D. Biotic communities

An estimate of qualitative and quantitative abundance of plankton, macrobenthos, periphyton and macrovegetation is essential to decide on the quantitative and qualitative aspects of stocking.

Two plankton pulses are recorded in Indian reservoirs - a post monsoon pulse during October-December and a spring/summer pulse during February -June. Phytoplankton is generally dominant and is almost contributed by Microcystts in tropical impoundments and Ceratium in sub-tropical impoundments (eg. Govindsagar). Zooplankton is contributed by copepods and rotifers.

Benthos is represented by insect larvae, oligochaetes, gastropods and bivalves. The submerged tree stumps, shrubs, boulders. etc. serve as substrata for the periphyton complex comprising green algae, blue-green algae and diatoms.

E. Potential yield

A first approximation of the fish yield potential is essential to have an idea of the expected harvest before large scale management measures are taken up. Some of the methods are given below:

1. Morpho-edaphic index (MEI)

Ryder (1965) proposed morpho-edaphic index combining total dissolved solids (TDS) - an edaphic factor and mean depth (Z) - a morphometric factor. The relationship is expressed an MEI = TDS/Z. The following regression has been calculated between fish yield and MEI.

Y = kx<l, where, Y = fish yield, x = MEI and k = a constant that represents a coefficient for climatic factors and 'a' an exponent approximating 0.5.

2. Jenkins model: Jenkins (1967) calculated a regression between standing crop of fish and MEI in US reservoirs which has the following form :

Y =2.07 +0.164 X.where. Y =standing crop offish & X =logMEI

Jenkins and Morais (1971) incorporated some environmental variables and calculated the following equation

Y = 0.2775 - 0.2401 Xl + 1.0201 X2 - 0.2756 X3. where, Y = total harvest in kg/ha, Xl = log area. X2 = log growing season and X3 = log age of the reservoir.

3. Gullanii model : Gulland (1971) calculated an equation relating potential yield to virgin ichthyomass which has the following form Y = kMB. where Y = total fish yield. k = a constant which lies between 0.3 and 0.5. M = natural mortality coefficient and B = biomass prior to fishing.

4. Troph Ddynamic model : Mellack (1976) calculated a regression between fish yield and gross photosynthesis for 15 Indiana lakes which has the form log FY = 0.122 PG + 0.95. where, FY = fish yield PG = gross photosynthesis.

Oglesby (1977) studied the relationship between the standing crop of summer phytoplankton and fish yield. The equation is logY = 1.98 + log CHLs.

5. Drainage Index: Ramakrishniah (1986) considered the importance of catchment in the loading of nutrients and detritus into the reservoir and proposed an index called Drainage Index (DI)which is calculated as :

Drainage Index = catchment area reservoir area X mean depth

For 21 reservoirs selected from different drainage systems a regression has been calculated as :

Y = 0.8613 + 0.577 X,

Where, Y = log fish yield/ha and X = log DI. It was found to be superior to MEI when applied to Indian reservoirs.

F. Stocking : Stocking with fast growing species is an important aspect of reservoir fishery management. Stocking policy must give due emphasis on trophic strata in terms of vacant, shared and unshared ecological niches. Gangetic major carps alone are not capable of utilising all the ecological niches. Catla, Rohu and Mrigal utilise only zooplankton, periphyton and detritus. Pangasius pangasius would be a useful addition to utilise molluscs, Tor putitora, M. aor to utilise weed fishes and common carp, C. carpio, being omnivous, to utilise insect larvae, worms and detritus.

As such there is no indigenous species which utilises the vast resources of Myxophyceae (Microcystis) present in tropical impoundments. Though the exotic silver carp is reported to utilise this item (Jhingran and Natarajan 1978), the damage it causes to the indigenous species, its poor keeping quality and the low price it fetches in the market should be kept in view before stocking the species. In Govindsagar it formed the dominant fishery having entered the reservoir inadvertently.

Different stocking rates are being followed in reservoirs. Fish seed committee recommended a stocking rate of 500 fingerlings/ha in the size range of 45-150 mm. On the basis of studies in DVC reservoirs Natarajan et a1 (1971) recommended a stocking rate of 300 fmgerlings/ha for reservoirs without predators and 600 fmgerlings/ha in reservoirs with predators.

Jhingran et al. (1969) proposed a formula for computing the number/ha to be stocked in reservoirs and is given as N = [(S1-So)/G] + M, where, N = number of flsh/unit area/unit time, S1 = fish biomass/unit area at the end of unit period, So = fish biomass/unit area at the beginning of unit period. G = average increase in weight per fish and M = anticipated mortality. It is desirable to keep the size of stocked fingerlings at 100 mm and above for better survival.

G. Exploitation : Stock monitoring vis a vis fishing effort is most important for raising the yield in reservoirs where breeding and recruitment are normal in respect of important species. In reservoirs where there is natural recruitment, management of fishery is relatively easy which involves selection of right meshes and deployment of optimum effort. Such an approach has led to remarkable increase in yield in Bhavnisagar (from 26 kg to 80 kg/ha/yr in 5 years) and Govindsagar (from 25 kg to 71 kg/ha/yr in 5 years) [Natarajan, 1979). Increasing the cropping intensity has also augmented the productivity of Mystus aor and Wallago attu in Bhavanisagar.

Studies reveal specific size vulnerability for carps and this differ from reservoir to reservoir and is influenced by basin morphometry. To enhance catchability it is necessary to deploy right meshes.

Due to peculiar nature of reservoir bottom, only stationary gear like gill nets are popular in reservoirs. The mesh regulation adopted left weed fishes and medium and minor carps out of exploitation. It is necessary to keep the population of minor carps and weed fishes under check to reduce their competition. Drag netting during periods of low water level under departmental supervision should be undertaken to remove weed fishes.

Several conservation measures, such as limiting size of fish, size of annual catch, observation of closed season for fishing and declaring certain areas as sanctuaries are observed in Indian reservoirs. In reservoirs where there is no recruitment observation of closed season is redundant. Even in reservoirs where there is natural breeding and recruitment, the breeding grounds may be declared as sanctuaries instead of closing the entire reservoir for fishing. By such a step fishermen are not deprived of their livelihood during closed season.

H. Management requisites

A. Small reservoirs:

Small reservoirs, often serving localized communities, require community-driven initiatives that promote efficient water usage, agricultural practices, and environmental conservation. Engaging local stakeholders and incorporating traditional knowledge can greatly enhance the management strategies for small reservoirs. Small reservoirs pose fewer problems and are relatively easy to manage when compared to large reservoirs. Basically aqua culture techniques are practised on an extensive scale. The upstream stretch of small reservoirs are seasonal and become live only when there is heavy rain in the catchment and these are not conducive for carp breeding. In these circumstances natural recruitment cannot be expected. Stocking the reservoir annually is the only alternative to develop a fishery, the stocking policy must take into account the biogenic capacity of the environment. growth rate of the species and natural mortality through predation and escapement. The stocking rate may be computed using the formula

Stocking rate =[Total fish production in kg / individual growth rate in kg] + lossAdopting a rational stocking and exploitation policy the fish yield of Gulariya reservoir has been raised from 33 kg to 100 kg/ha/annum (Jhingran 1986).

B. Medium-sized reservoirs: they catering to a broader population and multiple purposes, necessitate a more comprehensive approach. Integrated water resource management is key, involving coordination between various sectors such as agriculture, industry, and urban areas. Implementing advanced technologies for monitoring water levels, quality, and usage patterns can provide valuable data for decision-making. Additionally, investing in education and awareness campaigns can empower communities to actively participate in water conservation efforts, fostering a sense of responsibility towards these vital resources.

C. Large reservoirs: they serve as major sources of water and energy supply, require sophisticated management frameworks. Collaborative efforts between different states and regions are essential to optimize the utilization of water resources while mitigating inter-state conflicts. Climate change adaptation strategies should be integrated into reservoir management plans, considering the changing precipitation patterns and rising temperatures. Moreover, the development of eco-friendly hydropower technologies and the incorporation of renewable energy sources can make large reservoirs more sustainable in the long run. Regardless of the reservoir size, incorporating ecosystem-based approaches is crucial. Preserving the natural habitats surrounding reservoirs and maintaining the ecological balance can enhance water quality, support biodiversity, and ensure the overall health of the ecosystem. Restoring degraded landscapes, promoting afforestation, and implementing sustainable agricultural practices can contribute significantly to ecosystem conservation.

I. Ecological Considerations

A.  Small Reservoirs:

(1) Water Storage: Small reservoirs often serve as local water sources for communities. Efficient water storage is vital to ensure a consistent supply for agriculture, domestic use, and livestock, especially in arid regions.

(2) Hydroelectric or Flood Control: While small reservoirs might not typically be equipped for large-scale hydroelectric power generation, they can help in controlling localized floods, safeguarding nearby communities, and preserving soil quality.

(3) Fauna and Flora Management: Small reservoirs support diverse ecosystems. Managing fauna involves protecting local fish populations and ensuring migratory routes. Flora management includes preventing invasive species that might disrupt the local plant life and maintaining a healthy balance of aquatic vegetation.

B.  Medium-sized reservoirs:

(1) Water Storage: Medium reservoirs play a significant role in regional water supply. Managing these reservoirs involves balancing water extraction for agriculture, industry, and human consumption to avoid depletion.

(2) Hydroelectric or Flood Control: Medium-sized reservoirs can efficiently generate hydroelectric power. They also help manage floods by regulating the flow of water downstream, preventing sudden surges during heavy rainfall.

(3) Fauna and Flora Management: Biodiversity in medium reservoirs requires careful attention. Conservation efforts might involve creating fish habitats, protecting wetlands, and preserving the surrounding natural habitats to support various species.

C. Large Reservoirs:

(1) Water Storage: Large reservoirs are major water sources for urban areas, agriculture, and industries. Proper management ensures sustainable water supply even during droughts, supporting millions of people and vast agricultural regions.

(2) Hydroelectric or Flood Control: Large reservoirs are integral to hydroelectric power generation, providing renewable energy to a wide area. They play a crucial role in flood control by absorbing excess water during heavy rains and releasing it gradually.

(3) Fauna and Flora Management: Large reservoirs support complex ecosystems. Fauna management includes protecting endangered species, regulating fishing to prevent overfishing, and ensuring the preservation of natural predators to maintain ecological balance. Flora management involves preventing the spread of harmful algae blooms, maintaining water quality, and preserving shoreline vegetation.

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