Despite the overwhelming importance of reservoirs in the inland fisheries of India, a reliable estimate of the area under this resource is still elusive, causing serious constraints to the R & D activities. The available estimates made by various agencies are conflicting and wide off the mark. The National Commission on Agriculture (NCA) has estimated the total area under reservoirs at 3 million ha during the mid-sixties and projected its growth to 6 million ha by 2000 AD (Anon., 1976). Bhukaswan (1980) put the figure at 2 million ha. Srivastava et al. (1985) compiled a list of 975 large and medium reservoirs in the country with an estimated area of 1.7 million ha. One of its major shortcomings is the exclusion of small reservoirs, especially those in Tamil Nadu, Karnataka and Maharashtra.
Enumeration of the medium and large reservoirs is relatively easy, as they are less in number and the details are readily available with the irrigation, power and public works authorities. However, compilation of data on small reservoirs is a tedious task as they are ubiquitous and too numberous to count. The problem is further confounded by ambiguities in the nomenclature adapted by some of the States. The word tank is often loosely defined and used in common parlance to describe some of the small irrigation reservoirs. Thus, a large number of small manmade lakes are designated as tanks, thereby precluding them from the estimates of reservoirs. There is no uniform definition for a tank. In the eastern States of Orissa and West Bengal, pond and tank are interchangable expressions, while in Andhra Pradesh, Karnataka and Tamil Nadu, tanks refer to a section of irrigation reservoirs, including small and medium sized water bodies. In fact, some of the tanks in Tamil Nadu and Karnataka are much bigger than Aliyar and Tirumoorthy reservoirs.
David et al. (1974) defined the peninsular tanks as water bodies created by dams built of rubble, earth, stone and masonry work across seasonal streams, as against reservoirs, formed by dams built with precise engineering skill across perennial or long seasonal rivers or streams, using concrete masonry or stone, for power supply, large-scale irrigation or flood control purposes, which is obviously tedious and inadequate. Irrespective of the purpose for which the lake is created and the level of engineering skill involved in dam construction, both the categories fall under the broad purview of reservoirs, i.e., man-made lakes created by artificial impoundment of surface flow. From limnological and fisheries points of view, the distinction between small reservoirs and tanks seems to be irrelevant. Moreover, numerous small reservoirs fitting exactly into the description of the south Indian tanks are already enlisted as reservoirs in the rest of the country. Therefore, the large tanks have been treated at par with reservoirs for the purpose of this study.
In Andhra Pradesh, the tanks and small reservoirs are segregated either arbitrarily or based on yardsticks that have no limnological relevance. For instance, all the small reservoirs in the State, created before independence and those without a masonry structure and spillway shutters are called tanks. Tanks in Andhra Pradesh are classified as perennial and long seasonal. Of the 4 604 perennial tanks, 1 804 in Srikakulam, East Godavari and Krishna districts, having average size less than 10 ha, are not considered as reservoirs in this study. The remaining 2 800 tanks covering a total area of 177 749 ha have been reckoned as reservoirs.
In Tamil Nadu, the tanks are classified as short seasonal and long seasonal. The latter, also known as major irrigation tanks, have an average size of 34 ha and retain water for 9 to 12 months a year. Major irrigation tanks of Chengalpattu MGR and Salem districts are larger with average area 222 and 156 ha respectively. A total of 8 837 major irrigation tanks of Tamil Nadu with water surface area of 300 278 ha have been included under small reservoirs. Similarly, 4 605 perennial large water bodies in Karnataka, listed as major irrigation tanks are brought under the ambit of reservoirs.
Fish Seed Committee of the Government of India (1966) termed all water bodies of more than 200 ha in area as reservoirs. David et al. (1974) while classifying the water bodies of Karnataka State, considered impoundments above 500 ha as reservoirs and named the smaller ones as irrigation tanks. In the former USSR, reservoirs up to 10 000 ha area are assigned the status of small reservoirs, whereas in USA they may range from 0.1 ha to several ha (Bennet, 1970). In China, where reservoirs are classified on the basis of storage capacity (Lu, 1986), those holding more than 100 million m3 of water are classified as large reservoirs, 10 to 100 million m3 as medium and 0.1 to 10 million m3 as small reservoirs. Assuming an average depth of 10 m, small reservoirs of China are in the size range 10 to 1 000 ha.
Reservoirs are classified generally as small (<1 000 ha), medium (1 000 to 5 000 ha) and large (> 5 000 ha), especially in the records of the Government of India (Sarma, 1990, Srivastava et al., 1985), which has been followed in this study. All man-made impoundments created by obstructing the surface flow, by erecting a dam of any description, on a river, stream or any water course, have been reckoned as reservoirs. However, water bodies less than 10 ha in area, being too small to be considered as lakes, are excluded.
After removing the anomalies in nomenclature, especially with regard to the small reservoirs, by bringing the large (above 10 ha) irrigation tanks under the fold of reservoirs, India has 19 134 small reservoirs with a total water surface area of 1 485 557 ha (Table 1.1). Similarly, 180 medium and 56 large reservoirs of the country have an area of 527 541 and 1 140 268 ha respectively. Thus, the country has 19 370 reservoirs covering 3 153 366 ha (Table 1.2).
The State of Tamil Nadu accounts for maximum number (8 895) and area (315 941 ha) of small reservoirs, followed by Karnataka (4 651 units and 228 657 ha) and Andhra Pradesh (2 898 units and 201 927 ha). Medium reservoirs constitute less than 1% of the total number of units and 17% of the total area. Madhya Pradesh with 169 502 ha tops the table in respect of medium reservoirs. Andhra Pradesh, Rajasthan, and Gujarat have more medium reservoirs than Madhya Pradesh, though the water area in these States is much less. Karnataka has a preponderance in number (12) of large reservoirs. Nevetheless, the 7 reservoirs in Andhra Pradesh in this category are much larger and have a water area of 190 151 ha (Table 1.2).
The predominance of reservoirs in the peninsular States, viz., Tamil Nadu, Karnataka, Andhra Pradesh, Kerala, Orissa and Maharashtra is elucidated by Figs 1.1 to 1.4. These six States account for more than 56% of the total reservoir area in the country. Of the 19 134 small reservoirs, 17 989 (94%) are located there, contributing 63% of the total water area. Similarly, 34% of the medium reservoirs is distributed in these States.
Figure 1.1 Distribution of small reservoirs in India
Figure 1.2 Distribution of medium reservoirs in India
Figure 1.3 Large reservoir in India
Figure 1.4 Distribution of reservoirs (all categories) in India
|States||Small reservoirs||Irrigation tanks||Total|
|Tamil Nadu||58||15 663||8 837||300 278||8 895||315 941|
|Karnataka||46||15 253||4 605||213 404||4 651||228 657|
|Andhra Pradesh||98||24 178||2 800||177 749||2 898||201 927|
|Gujarat||115||40 099||561||44 025||676||84 124|
|Uttar Pradesh||40||20 845||-||197 806||40||218 651|
|Madhya Pradesh||6||172 575||-||-||6||172 575|
|Orissa||1 433||66 047||-||-||1 433||66 047|
|Kerala||21||7 975||-||-||21||7 975|
|Rajasthan||389||54 231||-||-||389||54 231|
|North East||4||1 639||-||600||4||2 239|
|Total||2 331||551 695||16 803||933 862||19 134||1 485 557|
|Tamil Nadu||8895*||315 941*||9||19 577||2||23 222||8906||358 740|
|Karnataka||4 651*||228 657*||16||29 078||12||179 556||4 679||437 291|
|Madhya Pradesh||6||172 575||21||169 502||5||118 307||32||460 384|
|Andhra Pradesh||2 898*||201 927*||32||66 429||7||190 151||2 937||458 507|
|Maharashtra||-||119 515||-||39 181||-||115 054||-||273 750|
|Gujarat||676*||84 124*||28||57 748||7||144 358||711||286 230|
|Bihar||112||12 461||5||12 523||8||71 711||125||96 695|
|Orissa||1 433||66 047||6||12 748||3||119 403||1 442||198 198|
|Kerala||21||7 975||8||15 500||1||6 160||30||29 635|
|Uttar Pradesh||40**||218 651||22||44 993||4||71 196||66||334 840|
|Rajasthan||389||54 231||30||49 827||4||49 386||423||153 444|
|Himachal Pradesh||1||200||-||-||2||41 364||3||41 564|
|Northeast||4**||2 239||2||5 835||-||-||6||8 074|
|West Bengal||4||732||1||4 600||1||10 400||6||15 732|
|Total||19 134||1 485 557||180||527 541||56||1 140 268||19 370||3 153 366|
* Including large irrigation tanks
** Not exhaustive
The production propensity of a reservoir is determined by a set of key environmental parameters, especially the water and soil quality which, in turn, are functions of the geo-climatic conditions under which it exists. Thus, the geography, climate, topography and a number of physiographic parameters play a vital role in bestowing the reservoirs their intrinsic productive potential. India, being a country of continental proportions, its reservoirs are spread over various types of terrains, and soil types exposed to diverse climatic conditions and they receive drainage from a variety of catchment areas.
The land area of India covers 3 287 728 km2, half of which lying above the Tropic of Cancer and the rest in the tropics. The southern limit is as close to equator as 8° 4' N. The climate varies from the warm tropical in the south to the temperate in the north. The landscapes include some great mountains, extensive alluvial plains, riverine wetlands, plateau lands, deserts, coastal plains and deltas. The main soil types are alluvial, deep and medium black, red and yellow, laterite, saline and desert, and forest and hill (Fig. 1.5). Almost all conceivable forms of vegetation, including tropical evergreen, littoral and swamp, tropical moist deciduous, tropical thorn, montane sub-tropical, Himalayan and alpine are present in various parts of the country.
The major physiographic divisions of the country are the Himalayas, the Indo-Gangetic plains, the Vindhyas, the Satpuras, the Western Ghats, the Eastern Ghats, coastal plains, the deltas and the riverine wetlands (Fig. 1.6). The alignments of hills and their elevation have profound influence on the prevailing winds and thereby the distribution of rainfall in the country. India receives, on an average, 105 cm of rainfall every year, which is one of the highest in the world for a country of comparable size. Total amount of rainfall received annually is estimated at 400 million hectare meters (mhm), out of which 230 mhm goes back to the atmosphere as evapotranspiration, leaving 170 mhm to impregnate the rivers through surface flow (110 mhm) and regeneration (60 mhm). The temporal and spatial distribution of rainfall exhibits wide variations within the country.
More than one million km2 of the country's geographical area receives inadequate rainfall. This includes the deserts, the semi-arid regions of north India and the rain shadows of the Western Ghats (Fig. 1.7). Large rivers like the Godavari, the Krishna, the Pennar and the Cauvery pass through extensive tracts of low rainfall areas and hence carry much less water than the rivers passing through high rainfall areas like the northeast and the west coast. In the northeast, the Khasi and Jaintia hills receive a bountiful 1 000 cm of rainfall annually and the Brahmaputra valley gets precipitation to the tune of 200 cm. Rainfall up to 1 142 cm recorded in Cherrapunji and Mawsyngram in the region is one of the highest in the world. In the west coast of India, heavy rainfall occurs along the slopes of the Western Ghats, where during the southwest monsoon, rainfall of very high order is recorded on the windward side which rapidly decreases towards the leeward side. The Indo-Gangetic plains and the Himalayas also receive rainfall above the national average.
Seasonal distribution of rainfall in India is worth noticing. The Western Ghats, Assam, parts of sub-Himalayan West Bengal and some higher elevations of Himalayas up to Punjab have more than 100 rainy days a year, while in extreme west Rajasthan the number of rainy days are less than 10 (Rao, 1979). The south-west monsoon season extending from June to September is the principal rainy season in the country as a whole when 75% of the annual rainfall is received. In more than one third of the country, 90% of the rainfall and thereby the surface flow is limited to a very brief period of 2 to 3 months. This extreme seasonality in rainfall distribution makes the irrigation reservoirs a sine qua non for agriculture in India, especially in the rainshadows of the peninsular India. People inhabiting this area learned to store water by erecting barricades across minor stream and rivulets from time immemorial. In recent years, with the advent of modern hydraulic structures, larger and more complex dams came into being. The steep gradient and heavy discharge of water in the mountain slopes of Western Ghats, the northeast and the Himalayas offer ideal opportunities for hydro-electric power generation. A large number of such projects have come up in these regions in recent years. Thus, the reservoirs have become a common feature in the Indian landscape, dotting all river basins, minor drainages and seasonal streams.
Figure 1.5 Soil msp of India
Figure 1.6 Phusiographic features of India
Figure 1.7 Rainfall map of India
Reservoir is a man-made ecosystem without a parallel in nature. Though essentially a combination of fluviatile and lacustrine systems, a close examination of the biotope reveals that it has certain characteristic features of its own. The riverine and lacustrine characters coexist in reservoirs, depending on the temporal and spatial variations of certain habitat variables. For example, the lotic sector of the reservoir sustains a fluviatile biocoenos, whereas the lentic zone and the bays harbour lentic communities. During the months of heavy inflow and outflow, the whole reservoir mimics a lotic environment whereas in summer, when the inflow into and outflow from the reservoir dwindle, a more or less lentic condition prevails in most parts of it. Another unique feature of reservoirs that makes them distinctly different from their natural counterparts is the water renewal pattern marked by swift changes in levels, inflow and outflow.
In India, most of the precipitation takes place during the monsoon months which contribute substantially to the surface flow. During this period, due to a heavy inflow of water into the system, all outlets of the dam are usually opened, resulting in total flushing. This process dislodges a considerable part of the standing crop of biotic communities at the lower trophic level and disturbs the natural primary community succession. The sudden level fluctuations also affect the benthos by exposing or submerging the substrata. Factors determining the water and soil quality in reservoirs are different from those of natural lakes. In the latter, the basin soil plays a predominant role in determining the chemical water quality through soil water interphase. In the reservoirs, on the other hand, the nutrient input from the allochthonous source often determines the water quality, nutrient regime and the basic production potential. This is because of the fact that the catchment of parent rivers is very often situated far away from the reservoir, under totally different geoclimatic conditions. Deep drawdown, wind-mediated turbulence, locking up of nutrients in the deep basins, etc., are but some of the factors that impart the uniqueness to the reservoir ecosystem. Besides, the varying purpose and design of the dams make the reservoirs different in their hydrographic and morphoedaphic characteristics, with implications on the production potential. Some salient features of the reservoir ecosystem are depicted in Table 1.3.
The ecology of reservoirs is radically different from that of the parent rivers. Dams alter river hydrology both up- and downstream of the river. The obstruction of river flow and the consequent inundation trigger off sudden transformation of lotic environment into a lentic one. The riverine community is subjected to changes akin to the secondary community succession. A number of organisms perish, some migrate to more hospitable environs, and the more hardy ones adapt themselves to the changed habitat. There is usually an initial spurt of plankton and benthic communities due to the increased availability of nutrients released from the decay of submerged vegetation. This trophic burst is also on account of the saproxenic lacustrine species filling the vacant niches created by the disappearance of saprophobic riverine taxa. As the effects of trophic burst wean away, the reservoir passes into a phase of trophic depression and the final fertility is regained after a few years. Habitat variables responsible for a reservoir's productivity can be summed up into climatic, morphometric, and hydro-edaphic factors.
|Positive/augmentative factors||Major effects unknown||Negative/reductive factors|
|High shoreline development (coves,bays,bays etc.)||Sedimentation of inorganic material||Low transparency in floods due to inorganic turbidity.|
|Low mean depth (less than 18 m)||High rate of evaporation||High mean depth|
|Existence and extent of marginal vegetation||Contributions of autochthonous nutrients||Erosion in the reservoir water shed area|
|Optimum nutrient levels||High surface temperature during summers (in northem India)||Reduction of quantity of water flowing into reservoir|
|Nutrient enrichment during floods||Low water temperature during winter (In northern India)||Large water level fluctuations creating large aridal (barren littoral)|
|Moderate to long growing season||Aquatic community interrelationship||Low level of dissolved oxygen in parts of hypolimnion|
|High frequency of phytoplankton blooms||Pollution in the reservoir water- shed|
|Moderately developed macrophyte community||Phytoplankton biomass mainly blue greens|
|Periphyton abundant||Relative low fish species diversity indicating low stability and a potentially low resilience to stresses|
|Well established plankton and benthos||Unbalanced fish populations favouring predatory and trash species|
|Tree and bush cleared||Low abundance and diversity of terrestrial vegetation hence early successional stage|
|Conditions permitting passage of migratory fish||Relatively low environmental heterogeneity|
|Introduction of fishes adapted to lentic conditions||Low diversity of plankton and benthos|
|Employment of modern fishing gear and optimization of fishing effort||Low diversity of aquatic macro- phytes|
|Enforcement of fishery regulations||Exposure of fish nests during drawdowns|
(After Jhingran, 1988)
The Indian reservoirs are exposed to a wide range of climates from the temperate Himalayas in the north to the extreme tropical in the southern peninsula. From Gobindsagar in Himachal Pradesh to Chittar in Tamil Nadu, they spread over the southern slopes of Himalayas, the Indo-Gangetic plain, the Vindhyas, the Satpuras, the Western and Eastern Ghats and the Deccan plateau.
Apart from being the main factor influencing the prevailing climate of the region, the latitudinal location is important in determining the quantum of solar radiation available at the water surface for primary productivity. Natarajan and Pathak (1983) estimated the amount of solar radiation available at four reservoirs within 31° 25' N and 11° 28' N and the rate at which the solar energy was converted into chemical energy. Incident solar energy available at the surface varied from 213 x 104 cal m-2 yr-1 in Bhavanisagar (10° 28' N) to 172 × 104 cal m-2y-1 in Gobindsagar (31° 25' N). Jhingran (1990) observed that 0.2 to 0.68% of the incident solar energy was fixed as chemical energy by the primary producers in five reservoirs, viz., Gobindsagar (Himachal Pradesh), Ramgarh (Rajasthan). Rihand (Uttar Pradesh) and Bhavanisagar (Tamil Nadu). It is often the qualitative and quantitative abundance of the producer communities that determines the photosynthetic efficiency rather than the actual amount of solar energy available. For instance, Nagarjunasagar despite receiving solar energy at the rate of 205 x 104 cal m-2 yr-1, fixes chemical energy to the extent of 0.29%, whereas in Gobindsagar 0.68% of the 172 × 104 cal m-2 yr-1 is being fixed by the producers (Table 1.4).
|Reservoir||Area (ha) at FRL||Latitude||Available light energy (cal m-2 yr1)||Available chemical energy (cal m-2yr1)|
|Bhavanisagar||7 285||11° 25'||213 × 104||8 781 (0.41%)|
|Nagarjunasagar||2 8474||16° 4'||205 × 104||5 959 (0.29 %)|
|Rihand||4 6538||24°||188 × 104||3 970 (0.20 %)|
|Ramgarh||1 265||27° 12'||183 × 104||8 236 (0.49 %)|
|Gobindsagar||16 867||31° 25'||172 × 104||11 696 (0.68 %)|
(After Jhingran, 1990)
Prevailing climatic factors including air temperature, wind velocity, rainfall, etc. play an important role in the biological productivity of a water body. The wide seasonal variations in air temperature is the predisposing factor in the thermal features of the north Indian and peninsular reservoirs. In contrast to the reservoirs of the north, their southern counterparts are characterised by the narrow range of fluctuations in water and air temperature during different seasons, a phenomenon which prevents the formation of thermal stratification. Normally, the thermal gradient occurs when high air temperature during summer warms up the upper layer. In peninsular India, there is no winter worth its name and the air temperature remains comparatively high during the whole year. During summer, when surface water gets heated up, the prevailing high temperature at the bottom does not offer any scope for thermal resistance by the warm upper layers. Thermal stratification is limnologically important because in thermally stratified lakes, the water above and below thermocline does not mix up and thereby rich nutrients at the bottom layer get locked up. A warm bottom layer also facilitates rapid decomposition of organic matter, thereby accelerating the process of nutrient release.
The deep basin of Nagarjunasagar, despite 40% of its capacity being dead storage, does not favour the formation of thermocline. Apart from the high water temperature throughout the year, the continous drawdown from deeper layers and the wind and wave mediated turbulence facilitate mixing up of water column. This is true to most of the reservoirs in the States of Andhra Pradesh, Karnataka, Kerala, Tamil Nadu, Orissa and Maharashtra. However, seven reservoirs in the upper peninsula comprising south Bihar, Gujarat and Madhya Pradesh undergo transient phases of thermal stratification during the summer stagnation, depending upon the other parameters such as the depth of the basin, water abstraction pattern and the wind. Konar reservoir situated beyond the Tropic of Cancer has distinct epi-and hypolimmon during the summer months. Similarly, a well-defined thermocline is reported from Gobindsagar. Apart from the solar warming of the top layer, which remains as a separate thermal regime, the inflowing Beas water that joins the reservoir at the lotic sector does not get mixed up, retains its cool character and remains as a separate layer at the bottom.
The amount of rainfall determines the rate of inflow into the reservoir, and hence plays a crucial role in bringing in the water replenishment and nutrient enrichment. More often, rainfall in the catchment of the river situated hundreds of km away from the reservoir affects the inflow rate. Another important climatic factor with implications on thermal and chemical regimes of the reservoir is the wind. It helps distribution of heat and equalisation of temperature in the water column. Wind velocity is very high in monsoon and premonsoon months in most reservoirs in India (Natarajan, 1979). Wind-induced turbulence is important in churning of the reservoirs and thereby facilitating the availability of nutrients at the trophogenic zone.
Reservoir morphometry is a function of the height of the dam and the topography of the impounded areas. Apart from the nature of the basin and the characteristics of the terrain, it is the design of the dam and the water use pattern that decide the influence of morphometric and hydrographic features on the aquatic productivity. Most of the hydel reservoirs on the mountain slopes of Western Ghats, Himalayas and the other highlands are deep, with steeper basin walls than the irrigation impoundments.
One of the most important morphometric considerations is the mean depth, that is believed to determine the productivity of reservoirs (Hayes, 1957, Rawson, 1952). This is based on the well known dictum that the shallower lakes have greater part of their water in the euphotic zone, facilitating greater mixing and circulation of heat and nutrients and hence higher productivity. A large portion of water in deep lakes serves as a nutrient sink at the bottom, where organic matter accumulates and thus the nutrients become unavailable at the photosynthetic zone. Mean depth calculated from the capacity and area varies from 5.2 in Panchet to 58 m in Gobindsagar among the large reservoirs. Medium reservoirs in the country have mean depth ranging from 2.3 m (Poondi) to 24.0 m (Bhatghar). Hope Lake in Tamil Nadu has an exceptionally deep basin of 37.7 m, while the small reservoirs in India have a mean depth range of 2.1 m (Vidur) to 14.57 (Badua). Mean depth in case of hydel reservoirs of the mountain slopes is invariably high, compared to the irrigation reservoirs of the plains and plateaus. The two largest impoundments in the country Viz., Hirakud and the Gandhisagar have very low mean depths of 11.3 which is one tenth of Gandhisagar in area has a mean depth of 32 m.
The mean depth, however, does not show any direct correlation with productivity, either at primary or fish level. Vidur, despite being one of the shallowest (mean depth 2.1 m) of all reservoirs in the country, does not support rich plankton community. Likewise, Kulgarhi and Govindgarh reservoirs in Madhya Pradesh exhibit propensities towards oligotrophy in spite of their shallowness. On the other hand, Gobindsagar, the deepest reservoir, has the highest productivity among large reservoirs. Medium deep reservoirs like Amaravathy (13.7 m), Aliyar (16.8 m), and Tirumoorthy (11 m) develop regular blooms of plankton.
Shoreline and volume development indices
A highly crinkled shoreline, as indicated by the high values of shoreline development index, is believed to be indicative of productive nature of the water body. An irregular shoreline encompasses more littoral formations and areas of land and water interface. High shoreline indices of Hirakud (13.5), Gobindsagar (12.26), Tilaiya (9.12), Konar (8.78), Nagarjunasagar (7.89) and Rihand (7.04) are accompanied by a moderate to rich plankton community.
Ratio between the maximum depth and mean depth, often described as volume development index denotes the depth of basin in relation to the nature of basin wall. An index value less than 1 suggests basin wall convex towards water. No perceptible relation exists between this parameter and the productivity of Indian reservoirs.
Hydrographic changes have a direct bearing on productivity, as sudden changes in water level, inflow and outflow directly affect the biotic communities. It has been observed that plankton, benthos, and periphyton pulses coincide with the months of least level fluctuations and all these communities are at their ebb during the months of maximum level fluctuations and water discharge. Percentage of shallow areas (littoral formation) which varies at different levels, depending on the contour, is also an indicator of productive nature of the lakes.
Storage and release of water from dams are dictated by the requirements of irrigation, power generation and other primary purposes of the dam, rather than any considerations related to fisheries. The spillway discharge, apart from dislodging the standing crop of plankton, removes the oxygenated clear water at the top layer, leaving the oxygen-deficient, turbid bottom water. Similarly, the deep drawdown removes the decomposing material including nutrients.
The oligotrophic tendencies shown by some of the reservoirs are mainly due to the poor nutrient status and other chemical deficiencies. In most of the cases, poor water quality is a direct reflection of the catchment soil. All reservoirs in Kerala portray a low status in terms of specific conductivity (<50 μmhos) and total alkalinity (<50 mg 1-1) with the attendant low primary productivity and plankton. The rivers of Kerala drain the hills of Western Ghats with lateritic and humus soils deficient in N, P and Ca. The eastern slopes of these hills drain the rivers feeding Hope Lake, Manimuthar, Pechiparal and Peruchani, all deficient in ions. Sathanur, Krishnagiri and Vidur reservoirs receiving drainage passing predominantly through cultivated area have higher levels of alkalinity and hardness. Similarly, in Madhya Pradesh the water is soft to medium soft with less mineral salts, due to geo-chemical reasons. Even small lakes with shallow bottoms, more often than not, do not show signs of productivity due to the poor chemical make up of the catchment. Soils in Madhya Pradesh are normally deep black, medium black, shallow black and mixed red and skeletal, low in nitrogen and phosphorus.
Catchment of Ravishankarsagar comprises rocky, denuded forests and upstream rivers are intercepted by impoundments, which further deprive the water of the suspended matter. Allochthonous enrichment with minerals and nutrients of the reservoir is very low, resulting in low standing crop of plankton. Limestone and other calcareous rocks underlying the water course in the Deccan plateau are responsible for the predominantly hard water character of many of the reservoirs on the Krishna and Cauvery in Andhra Pradesh and Tamil Nadu. The acidic nature of the water of the north eastern reservoirs Kyrdemkulai, Nongmahir and Barapani is attributable to the acidic soil of the reservoir bed and in the catchment.
Most of the Indian reservoirs are characterised by low levels of phosphate and nitrate. Phosphate very seldom exceeds 0.1 mg 1-1 in reservoirs free from pollution. However, the reservoirs of Rajasthan have particularly high levels of phosphate, ranging from traces to 0.929 mg 1-1. They receive phosphate from the rain washings derived from soils types like brown hills, grey brown hills, red and yellow and desert soils. In the highly eutrophic reservoir of Mansarovar in Madhya Pradesh, phosphate levels of 4 to 13 mg 1-1 were recorded.
Nitrate nitrogen in water in Indian reservoirs is mostly in traces and seldom exceeds 0.5 mg 1-1. Lack of nutrients in water, especially the nitrate nitrogen and phosphate, does not seem to be indicative of low productivity. In many cases, despite their virtual absence, the production processes are not hampered. In Amaravathy, Bhavanisagar, Gandhisagar, Ravishankarsagar and many other reservoirs, moderate to very high primary productivity is reported, although the phosphate in water is either absent or present in a very low concentration. In the tropical reservoirs, phosphate level in water has limited scope as an indicator of productive traits. This phenomenon is attributed to rapid turnover of nutrients (Ehrligh, 1960; Abbot, 1967) and their quick recycling, as seen from the high metabolic rates. Hayes and Phillips (1958) showed that 95% of the phosphorus could be taken up by the phytoplankton within 20 minutes, while some algae could convert inorganic phosphate into organic state in less than one minute.
Unlike the nutrients like phosphate and nitrate, the measure of total dissolved solids in the form of total alkalinity and the specific conductivity reflects the production propensities of reservoir satisfactorily, with the exception of Amaravathy which, despite very low levels of specific conductivity (38 to 63 μmhos), total alkalinity (7 to 84 mg 1-1) and total hardness (18 to 50 mg 1-1), supports a very rich plankton community and a good stock of fish.
The ranges of notable abiotic factors indicating their productivity status are given in Table 1.5. A close examination of physico-chemical data pertaining to more than 100 reservoirs in the country leads to the conclusion that none of the morphometric, edaphic, and water quality parameters can be used as a dependable yardstick to predict the organic productivity to any degree of accuracy, production propensities of each reservoir being determined by a variety of factors. The vertical gradient of some of the chemical parameters, especially the dissolved oxygen, however, conveys the status with a higher level of accuracy.
|Alkalinity (mg 1-1)||40–240||<40.0||40–90||>90.0|
|Nitrates (mg 1-1)||Tr.-0.93||Negligible||Upto 0.2||0.2–0.5|
|Phosphates (mg 1-1)||Tr.-0.36||Negligible||Upto 0.1||0.1–0.2|
|Specific conductivity (μmhos)||76–474||Upto 200||>200|
|(with minimal stratification : i.e.,>5°C)|
|Available P (mg/100 g)||0.47–6.2||<3.0||3.0–6.0||>6.0|
|Available N (mg/100 g)||13.0–65.0||<25.0||25–60||>60.0|
|Organic carbon (%)||0.6–3.2||<0.5||0.5–1.5||1.5–2.5|
(After Jhingran, 1990)
The bacterial decomposition of organic matter at the bottom is well reflected by the high rate of oxygen consumption. A corresponding increase in oxygen at the trophogenic upper zone gives clues of the high rate of photosynthesis. Almost all productive reservoirs in the country, irrespective of their geographic location, have a klinograde oxygen curve.
In most of the cases, the oxycline is accompanied by a vertical stratification of other chemical parameters such as pH, carbon dioxide, total alkalinity and specific conductivity. The tropholytic zone has a steady supply of free carbon dioxide, which reacts with carbonate to produce bicarbonates. This results in an increase of bicarbonates towards the bottom. Similarly, due to the increase in the hydrogen ions, the pH drops rapidly. Thus, the increase in total alkalinity, specific conductivity and CO2 and the decrease in pH values towards the bottom layers act as useful indicators of productivity.
Rate of primary productivity in reservoirs is very high due to the warm tropical conditions available in most parts of the country . Many workers consider 1% of the total carbon produced at the phytoplankton phase as the potential fish production from a water body, although almost all reservoirs produce much less fish than their potential.
The highly seasonal rainfall and heavy discharge of water during the monsoons result in high flushing rate in most of the reservoirs which does not favour colonisation by macrophytic communities. Similarly, inadequate availability of suitable substrata retards the growth of periphyton. Plankton, by virtue of drifting habit and short turnover period, constitutes the major link in the trophic structure and events in the reservoir ecosystem. A rich plankton community with well-marked seral succession is the hallmark of Indian reservoirs.
Blue-green algae from the mainstay of plankton community in vast majority of the man-made lakes studied. The overwhelming presence of Microcystis aeruginosa in Indian reservoirs is remarkable. The productive water of Gangetic plains, Deccan plateau, south Tamil Nadu and Orissa invariably have good standing crop of Microcystis. A common feature of all these reservoirs is the bright sunshine, isothermal water column, klinograde oxygen curve and an extensive catchment area, draining a calcium rich, forested or cultivated land. The species is almost ominipresent in the southern peninsula, except in the reservoirs of Karnataka and Kerala, which tend to be oligotrophic and have poor plankton count with desmids and other green algae as the main constituents. Reservoirs of Rajasthan receiving scanty rainfall and poor flushing rate favour macrophytes and despite being productive do not harbour blooms of Microcystis. The oligotrophic lakes of the northeast have a desmid-dominated plankton community (Fig. 1.8).
Altitude plays a decisive role in the distribution of Microcystis. Gobindsagar, the highly productive high altitude temperate reservoir, supports a rich community of Ceratium sp. instead of Microcystic. Similarly, the tropical Markonahalli situated at an altitude of 731 m above MSL, has Ceratium sp. as the major constituent of plankton.
Most of the reservoirs have three plankton pulses conciding with the post-monsoon (September to November), winter (December to February) and summer (March to May) seasons. The monsoon (June-August) flushing disturbs and often dislodges the standing crop of plankton. However, no sooner the destabilising effects wean away (as the dam outlets are closed), the allochthonous nutrient input favours an accelerated plankton growth. As the post-monsoon merges into winter, the turbulence decreases and water becomes clean, the plankton community progresses through a series of seral successions to culminate in a peak. The summer maxima coincide with the drastic drawdown, bringing the deep, nutrient-rich areas into the fold of tropholytic zone. The temperature, bright sunlight and rapid tropholytic activities also accelerate the multiplication of plankton during summer. In some cases, only two pulses(i.e., the post-monsoon and summer) are seen. However, the shallow, nutrient-rich reservoirs in the southern tip of the peninsula, by virtue of the fast turnover of nutrients and availability of sunshine and warmth, sustain a permanent bloom of plankton.
Figure 1.8. Dominant phytoplankton in reservoirs (State-wise)
The ubiquitous blooms of Microcystis in reservoirs in peninsular India are an example of a lacustrine biocoenose giving way to fluviatile ones in an impoundment. Studies have indicated that Chlorophyceae and Bacillariophyceae constituted the main components of riverine plankton. On reservoir formation and the consequent transformation of lotic environment into the lentic system, saprophobes disappear from the scene giving room for the rapid multiplication of saproxenes. Microcystis, finding a favourable note with the new environment, bursts into blooms, outnumbering all other forms into in significance. In many reservoirs, orientation of lacustrine and fluviatile plankton can be clearly discerned from the composition of plankton in lotic, lentic and the cove sectors. The fluviatile lotic sector, although recording a lower plankton density, often shows better diversity and evenness indices, compared to the lentic and bay sectors, the still waters of which are characterised by higher concentration of dominance (C) and low evenness(J') (Sugunan, 1991).
Aquatic macrophytes do not figure prominently in the community structure and trophic events of the reservoirs in India, barring some exceptional circumstances, such as low water renewal, ageing of reservoir and pollution stress. In most of the reservoirs they are totally absent or their population is too insignificant to be taken into account. Mostly, they are restricted to isolated patches of Vallisneria, Hydrilla and mats of Spirogyra, found in the protected bays and coves. Yerrakalava in Andhra Pradesh, Ramgarh in Rajasthan and Sayajisarover in Gujarat are the examples of macrophytic growth due to low flushing rate. Small irrigation reservoirs in Uttar Pradesh. viz., Bachhra and Baghla are also known for the luxuriant growth of macrophytes.
Hussainsagar in Andhra Pradesh and Mansarover in Madhya Pradesh harbour thick vegetation which thrives due to the hyper-eutrophication. These reservoirs are well-advanced on their way to change to swamps. Age of the reservoir seems to have an influence on the macrophyte community. Vanivilas Sagar, and Markonahalli reservoir in Karnataka formed in 1901 and 1939 have a luxuriant growth of macrophytes. Similar age-related macrophyte growth can be observed in Yerrakalava, Ramgarh, Jaisamand, Hussainsagar and Fatehsagar reservoirs. Reservoirs of Rajasthan exhibit a seasonal rhythm in aquatic weeds, their population peaking in summer and declining during the monsoon season.
In the irrigation tanks of Karnataka, the qualitative and quantitative distribution of macrovegetation depends, to a large extent, on the physiographic divisions and the soil types. Thick vegetation comprising the littoral, submerged and emergent types is the characteristic feature of the tanks of the costal plains and the Malnad region. In the transitional zone between plateau and hills, marked by the presence of laterite and red soil, the tanks are fertile and plankton-rich. Weeds are sparse and whenever present, do not choke the water. In the tanks in the black soil zones, the weeds are mostly submerged and emergent types such as Hydrilla, Chara Nitella etc. In most of the tanks in the red soil area, which are seasonal, the vegetation is limited to littoral areas.
Macrophytes offer substrata for an array of insects, molluscs and other invertebrate fauna, and thereby contribute to the species diversity of a water body. Nevertheless, the presence of weeds is considered to be undesirable from fisheries point of view. They accumulate large quantities of inorganic nutrients early in the season, depriving the phytoplankton of their share of nutrients. The floating vegetation utilizes the incident solar radiation for its photosynthesis and makes it unavailable to the phytoplankton communities. Submerged weeds provide shelter for minnows and weed fishes which compete with major carps for food. Excessive growth of macrophytes cause high rate of decomposition of dead plants at the bottom, creating anaerobic conditions. Problems are further confounded, if the water surface is matted by the floating vegetation which prevents light penertration. Instances of fish mortality in summer under such circumstances are reported from Hussainsagar and the reservoirs of Rajasthan. A major deleterious effect of weeds is its physical obstruction they cause to a variety of fishing gear.
The submerged plants commonly encountered in Mansarovar are Ceratophyllum demersum, Potamogeton crispus, P. perfoliatus, P. pectinatus, P. natans, P. nodosus, Najas graminaea, N. minor, Hydrilla verticillata, Vallisneria spiralis, Chara intermedia, Nitella gracilus. Floating weeds comprising Eichhornia crassipes, Lemna sp.. and Spirodela and the emergent Ipomoea aquatica, I. reptans, Polygonum glabrum, Typha angustata, and T. elephantia are reported. Hussainsagar is choked mainly with the water hyacinth, while the reservoirs of Rajasthan harbour a rich flora, dominated mainly by Hydrilla, Ceratophyllum, Potamogeton, Najas, Azolla and Ipomoea. The marginal areas of Yerrakalava reservoir in Andhra Pradesh have Hydrilla and Chara, while the deeper zones harbour Vallisneria and Spirogyra
Benthic invertebrates fauna show an erratic distribution in Indian reservoirs. The main factors that retard this community are the predominantly rocky bottom, frequent water level fluctuation and the rapid deposition of silt and other suspended particles. In spite of this, a number of reservoirs harbour rich communities of benthic invertebrates. The sequence of dominance of benthic communities closely follows the soil fertility pattern, the pre-impoundment debris often providing suitable habitats. The benthic community succession especially that of chironomids is sometimes used to characterize habitat changes. High shoreline development, variable slopes and vegetation act as favourable factors for the development of a rich assemblage of benthic organisms.
Small, shallow reservoirs of the Gangetic basin, such as Bachhra and Baghla are particularly rich in benthic fauna mainly due to the favourable substratum, rich in organic matter and the absence of swift changes in water level. In Bachhra reservoir, the standing crop of benthos registered a steady growth from 490 to 1 894 individuals m-2 during the last 10 years. Baghla, another small reservoir in Uttar Pradesh, has a population of benthic invertebrates represented by Chironomus, annelids and molluscs. The deep Rihand reservoir in the Ganga basin has a poor benthic community.
Reservoirs of Karnataka, such as Tungabhadra, Markonahalli, Hemavathy, Vanivilas Sagar and Krishnarajasagar have impressive populations of benthic organisms. So are the reservoirs of Himachal Pradesh and Rajasthan. Trends in Tamil Nadu and Madhya Pradesh are erratic. The local conditions, rather than a general geo-climatic features of the area, determine the density of benthic populations in their reservoirs.
Chironomid larvae, being a saprophobic, quickly fill the niches vacated by the saproxenes during the tranformation of habitats. They form the most important constituent of benthos, reported from all soil types and geographic locations and depths. Gastropods and annelids from the next important groups. Viviparus bengelensis enjoys country-wide distribution.
Among the biotic communities of the reservoir ecosystem, periphyton is the least reported upon. It constitutes an important component of food for the browsing fishes which contribute substantially to the total fish biomass of the tropical reservoirs. Apart from the limited littoral region in reservoirs, it is the frequent level fluctuations that prevent the growth of periphyton on natural substrata. Significantly, rich periphyton, whenever reported, coincides with relatively stable reservoir levels. There are reports of rich periphyton deposits on anchored boats, rafts, etc. that move down along with the receding water level. The fixed substrata either get totally exposed when water level decreases or they are submerged too deep for the communities to survive when level goes up. Propensities for rich settling rates of periphyton have been established through experiments with the artificial substrata, such as glass slides (David et al., 1975; Jha, 1979; Sugunan and Pathak, 1986).
Despite the cataclysmic faunistic changes associated with the impoundment, Indian reservoirs preserve a rich variety of fish species. The ichthyofauna of a reservoir basically represents the faunal diversity of the parent river system. On the basis of studies conducted so far, large reservoirs, on an average, harbour 60 species of fishes, of which at least 40 contribute to the commercial fisheries. The fastgrowing Indo-Gangetic carps, popularly known as Indian major carps, occupy a prominent place among the commercially important fishes. More recently, number of exotic species have contributed substantially to commercial fisheries. Broad categorisation of the species is as follows:
The Indian major carps: Labeo rohita, L. calbasu, L. fimbriatus, Cirrhinus mrigala, Catla catla,
The mahseers : Tor tor, T. putitora, T. khudree, Acrossocheilus hexagonolepis
The minor carps including snow trout and peninsular carps: Cirrhinus cirrhosa, C. reba, Labeo kontius, L. bata, Puntius sarana, P. dubius, P. carnaticus, P. kolus, P. dobsoni, P. chagunio, Schizothorax plagiostomus, Thynnichthyes sandkhol, Osteobrama vigorsii,
Large catfishes: Aorichthys aor, A. seenghala, Wallago attu, Pangasius pangasius, Silonia silondia, S. childrenii,
Featherbacks : Notopterus notopterus, N. chitala,
Airbreathing catfishes: Heteropneustes fossilis, Clartas batrachus,
Murrels : Channa marulius, C. striatus, C. punctatus, C. gachua,
Weed fishes : Ambassis nama, Esomus danrica, Aspidoparia morar, Amblypharyngodon mala, Puntius sophore, P. ticto, Oxygaster bacaUa, Laubuca laubuca, Barilius barila, B. bola, Osteobrama cotio, Gadusia chapra.
Exotic fishes : Oreochromis mossambicus, Hypophthalmichthys molitrix, Cyprinus carpio specularis, C. carpio commun, Gambusia affinis, Ctenopharyngodon idella.
Most of the catfishes, featherbacks, air breathing fishes,murrels and the weed fishes enjoy a country-wide distribution, while that of the major carps, minor carps and mahseers (Tor putitora, T. tor, Acrossocheilus hexagonolepis) varies according to river basins. The Indian major carps, catla (C. catla), rohu (L. rohita) and mrigal (C. mrigala) constitute the important native ichthyofauna of the rivers of the Gangetic system. These rivers also harbour Labeo bata, P. sarana, P. chagunio, and C. reba, Tor putitora, Labeo dero and the snow trouts (Schizothorax spp.) form the dominant riverine fish fauna of Indus system. Mahseers, especially the chocolate mahseer. Acrosscheilus hexagonolepis are also found in the streams associated with all the major river systems of the country. Indigenous fishes of the peninsular rivers include Cirrhinus cirrhosa, C. reba, Labeo kontius, L.fimbriatus, P. dubius, P. sarana. P. carnaticus, P. kolus, P. dobsoni, T. tor, T. sandkhol and 0. vigorsii.
Fish faunistic diversity of a reservoir at a given time is the result of the impact of a series of man-made and natural changes on the native fauna of the parent river. Riverine fish fauna is subjected to a series of habitat changes such as water current, turbidity levels, fishing pressure, loss of breeding grounds and the changes in fish food organisms due to lake formation. The original fauna changes and hardy fish species take advantage of the vacant niches. In many reservoirs, transplantion of fishes from other basins and introduction of exotic species have led to further radical changes in the species set up.
The three Indian major carps have been stocked extensively in reservoirs all over the country for many decades and, in many instances, they have established themselves in reservoirs far away from their original habitat. Sathanur reservoir in Tamil Nadu has a naturalised population of catla that contribute 80 to 90% of the total catch. This Indo-Gangetic carp has eclipsed all indigenous fish fauna including Labeo fimbrtatus, which dominated the scene by contributing 36% of the catch during the mid 1960s. Similarly, introductions of the silver carp in Gobindsagar, common carp in Krishnarajasagar and tilapia (Oreochromis mossambtcus) in Amaravathy are examples of man induced changes in fish communities.
Formation of reservoirs have affected especially the following Indigenous fish stocks:
The mahseers, snow trouts and Labeo dero and L dyocheilus of the Himalayan streams.
The anadromous hilsa, the catadromous eels,and freshwater prawns of all major river systems.
P. sarana, T. tor,Tor mahanadtcus, T. mosal, L.fimbrtatus, L calbasu,and Rhinomugil corsula of the Mahanadi river,
P. dobsoni, P. dubius, P. carnaticus, C. drrhosa and Labeo kontius of the Cauvery basin.
P. kolus, P. dubius, P. sarana, P. porcellus, L.fimbrtatus, L. calbasu, L. pangusia and Tor kudree of the Krishna river system, and
The mahseers, eels and Osteobrama belangiri of the northeast (Fig. 1.9).
The pristine streams of the river Sutlej harboured at least 51 species of fishes including the (exotic) trout, Salmo trutafario, the snow trouts, Schizothorax spp. and several species of hillstream fishes (Anon.. 1989b). Most of them were unique due to the sub-temperate climate and the zoogeographic affiliations to the Himalayan region. The upper reaches of the Sutlej and its tributaries were particularly rich in T. putltora, L. dero, L dyocheilus and Schizothorax spp. A decline in the number of species and their populations has been reported on account of changed ecological conditions especially the silt deposition at the bottom and the stratification of water body. Apart from minnows, many native species, such as, Schizothorax plagiostomus, T. putitora, L. dero, L. dyocheilus are on the decline. Proliferation of the exotic carps, H. molitrix and C. carpio in recent year has further to the local species.
Figure 1.9. Indigenous fish species affected by reservoir formation in India
Before the creation of Hirakud reservoir, the parent river Mahanadi had a rich fish fauna of 103 species, comprising both plain and sub-montane forms with sizeable representation of carps and catfishes. The common species of the river were P. sarana, T. tor, T. mahanadicus T. mosal, L. fimbriatus and the Indo-Gangetic major carps. The endangered T. mosal and T. mahanadicus were protected in the temple tanks that were submerged during reservoir formation. Presently, the number of species has declined to 40, of which many may disappear. The worst affected are Tor mosal, Rhinomugil corsula and the freshwater prawn Macrobrachium malcolmsonii.
Fishes affected in the two reservoirs in the Krishna river system viz., Tungabhadra and Nagarjunasagar are P. sarana, and Labeo spp. Soon after the impoundment, the Tungabhadra reservoir harboured a good population of indigenous P. kolus that contributed up to a third of the total fish landings. P. dubius, P. sarana, P. porcellus, P. potail and Labeo pangusia were also present in large numbers. Most of these native species have either disappeared or declined drastically due to the absence of fluviatile environment and the changed trophic structure. The vacant niche has been filled by minnow-predator combination, due to the management lapse of not inducting the fast-growing species into the system.
Nagarjunasagar reservoir on the mainstream Krishna harboured rich populations of Labeo fimbriatus, Labeo calbasu and T. khudree in the earlier years of impoundment. On account of recurring breeding failure and habitat loss, these species have declined over the years, giving way to minnows which have shared the common niche with them. In the absence of any commercial fish to utilize the rich planktonic resources of the reservoir, they are mainly channelled through the detritus-molluscs chain to favour Pangasius pangasius and through grazing-predator chain to help the Silonia childreni populations. Thus, finally, these two catfishes have established a firm hold on the fish fauna.
Cauvery river system is the original abode of a number of fish species including P. dubius, P. carnaticus, C. cirrosa, C. reba, L. kontius, L. fimbriatus, Tor putitora and Acrossocheilus hexagonolepis which have been affected in various degrees by the impoundments, chiefly, Krishnarajasagar, Mettur, Bhavanisagar and Amaravathy reservoirs. During the last 25 years, the indigenous economic carps in Krishnarajasagar viz., Labeo spp., P. dubius and P. carnaticus have suffered setback due to the changed ecological conditions, especially the components of fish food biotic communities. These species, contributing more than 60% of the total catch during the 1950s, have given way to the transplanted exotic common carp, C. carpio, which found a favourable environment in the reservoir vis-a-vis feeding and breeding. At present, most of the energy transfer is channelled through the detritus/benthos chain, giving considerable edge to the common carp which is a prolific breeder and competitor to Cirrhinus spp. for food.
One of the earliest casualties of hydraulic structures is the Indian shad Tenualosa ilisha (the hilsa) which was affected as early as mid 19th century when the Upper and Lower Anicuts were constructed on Cauvery. These barrages had severly restricted the migration of hilsa by obstructing their pathways and construction of Mettur dam (Stanley reservoir) in 1935 completely stopped the hilsa run in Cauvery. Several fishes were affected by the Mettur dam. Puntius spp. which used to form 28% of the landings in 1943–44 faded out in the mid 1970s. Although the indigenous Cirrhinus cirrhosa took some initial advantage, it also could not survive. Low water levels during July for three consecutive years have most probably caused its decline. Similarly, Labeo kontius which was next only to C. cirrhosa in Cauvery also disappeared from the reservoir. The Gangetic carps transplanted into the reservoir also did not find roots there. C. mrigala was stocked in 1950–51 and appeared in catch during 1957–58, contributing up to 13.9% in 1966–67, only to fade into insignificance later. The some fate met Labeo rohita. Recruitment failure, water level changes, and predator pressure are the main reasons for the failure of Indian major carps in Stanley reservoir. In 1993, the total catch of 115 t in the reservoir comprised L. rohita (19%), Wallago attu (15%), other catfishes (14%), Puntius spp (14%) and C. catla (10%).
Bhavanisagar is the only reservoir in the Cauvery basin, where the indigenous species like Puntius spp., Tor putitora, T. tor, A. hexagonolepis, P. dubius, P. carnaticus, L. kontius and C. cirrhosa still hold together well. Their survival is mainly due to the uninterrupted breeding activities at Moolathurai and Nellithurai, especially when water is released from the upstream Pilloor reservoir. P. dubius ascends the river Moyar during the northeast monsoon and lays eggs in batches of 1 000 to 2 000 on the gravel beds. Similar breeding success has been confirmed in case of Cirrhinus reba, Labeo fimbriatus, Labeo calbasu, L. kontius and Puntius carnaticus.
Amaravathy and Sathanur with their prime fishes of tilapia and catla respectively are the examples of introduced fishes finding a favourable environment and propagating themselves into a dominant position
Positive impact of reservoirs on fish fauna
Many species of fish not only manage to adapt to the reservoir ecosystem but also find it congenial and flourish there, which is the main reason for the rise in the biomass of reservoir in the early stage of impoundment. However, most of the fishes that manage to multiply in the reservoir system are not very high in priority from the commercial and ecological point of view. Stocks of the small clupeid, Salmostoma phulo phulo and o. vigorsii, which support a flourishing dry fish trade in Nagarjunasagar and Tungabhadra reservoirs, multiply in a much higher scale than they do in the riverine ecosystem. The catfish, P. pangasius which was believed to be a catadromous migrant, has not only adapted itself to become a resident population in Nagarjunasagar, it has also become a very important component of the population. Ramakrishniah (1994) described many instances where reservoirs acted as sanctuaries by citing examples of Barilius bola in Tilaiya (Damodar), Mystus krishnensis, Osteobrama vigorsii, and Pseudeutropius taackree in Nagarjunasagar (Krishna), T. sandkhol in Nizamsagar (Godavari), Tor khudree and T. mussullah in Shivajisagar (Krishna), A. seenghala and T. putitora in Pong (Beas) and Vallabhsagar (Tapti).
A realistic evaluation of fish production from reservoirs in India is elusive. Compared to the impressive volume of data generated by the individual research workers and various institutions on limno-chemical variables and biotic communities, the estimates on fish catch remain grossly inadequate. Reliable fish catch statistics and yield estimates remain as the weakest link in the database on reservoirs. More discomfitting is the fact that the production figures available on most of the reservoirs are inaccurate and unreliable. This lacuna is imputable to a large number of factors, chiefly:
the multiplicity of agencies owning the fishing rights that pose difficulties in some States to gather data.
highly scattered and unorganised market channels, mostly under the clutches of illegal money lenders.
Ineffective cooperative set up,
diverse licensing/royalty/crop sharing systems practised by different State Governments, some of which include a free for all system, providing little scope for recording catch statistics, and
inadequate and poorly trained manpower at the disposal of State Governments/Cooperatives to collect catch data, follow statistically sound sampling procedures, unable to cover the whole reservoir.
The All India Coordinated Project on Reservoir Fisheries took the lead in evolving a collection methodology on the basis of stratified random sampling during 1971 to 1985. However, many State Governments were not able to follow the procedure and to continue with recording of catch data after the project wound up in 1985. Nevertheless, some States like Tamil Nadu and Madhya Pradesh, despite the enormity of the resource size, have a streamlined machinery to record catch statistics. Similarly, Himachal Pradesh has good documentation on catch. On the contrary, Karnataka, has very little information on the catch structure of its reservoirs. Figures of Andhra Pradesh seem to be off the mark, due to the free fishing system and inadequate methods of data collection. For instance, system followed in Nagarjunasagar virtually allows anybody to catch fish in the reservoir and freely sell it anywhere. Considering the size of the lake and the large number of remote landing centres, an effective monitoring of catch is almost impossible. This is equally true with regard to many reservoirs of Kerala, Maharashtra, Orissa, Karnataka and Uttar Pradesh.
Based mainly on the data obtained from various State Governments, the fish production particulars from 422 reservoir have been presented in Table 1.6. Fish yield figures of small reservoirs of Andhra Pradesh, as given by the State Fisheries Department are very impressive (188 kg ha-1), followed by those of Kerala, Madhya Pradesh, Tamil Nadu and Rajasthan in the range of 46.43 to 53.5 kg ha -1. Medium reservoirs of Rajasthan, on an average, produce fish at the rate of 24.47 kg ha-1, while Tamil Nadu, Maharashtra, Madhya Pradesh and Orissa record about half this yield. The two large reservoirs in Himachal Pradesh produce 35.55 kg ha-1 which is remarkably high, compared to other States.
The estimated fish yield of 291 small, 110 medium and 21 large reservoirs of the country are 49.90, 12.30 and 11.43 kg ha-1 respectively. Based on the catch of 422 reservoirs belonging to 10 states during 1992–93, the national fish production rate of Indian reservoirs is estimated as 20.13 kg ha-1. Applying this national average yield rates into the 1 485 557 ha of small, 527 541 ha of medium and 1 140 268 ha of large reservoirs in the country, their current production rate can be estimated as 74 129, 6 488 and 13 033 trespectively. A modest increase in yield rate up to 100, 75 and 50 kg ha-1 in respect of small, medium and large reservoirs, would ensure production of 148 556, 39 565 and 57 013 t. This would increase the production by 2.5 times i.e., from the present 93 650 t to 245 134 t (Table 1.7).
The present low level of fish production in Indian reservoirs can be attributed to inadequate management inasmuch as many of them have high propensities of production from a limno-chemical point of view. In many of the reservoirs, the high rate of the primary and secondary productivity is not channelled to fish production. Insufficient understanding of the reservoir ecosystem often comes in the way for adopting effective management measures.
|State||Small reservoirs||Medium reservoirs||Large reservoirs||Pooled|
|Number||Production (t)||Yield (kg ha-1)||Number||Production (t)||Yield (kg ha-1)||Number||Production (t)||Yield (kg ha-1)||Number||Production (t)||Yield (kg ha-1)|
|Category||Yield (kg ha-1)||Area (ha)||Present Production||Potential Production|
|Small||49.90||1 485 557||74 129||148 556|
|Medium||12.30||527 541||6 488||39 565|
|Large||11.43||1 140 268||13 033||57 013|
|Total||3 153 366||93 650||245 134|
Since fish production from reservoirs is essentially extractive in nature, the essence of management strategy lies in exploitation of natural stocks. Nevertheless, the ecosystem management provides different degrees of freedom for stock maniputation, depending on the size and class of the water body (Table 1.8). One of the possible criteria that can be used to differentiate between capture and culture fisheries is the extent of human intervention in the ecosystem management. While aquaculture systems provide maximum avenues for the man to monitor and change the habitat variables and the biotic communities at will, this freedom attenuates as we proceed from aquaculture to the culture-based and capture fisheries. In a large water body, managed on capture fishery norms, there is little room for altering the habitat variables and the scope for effecting change in biotic communities is limited to stocking and ranching, which have uncertain chances of success.
Relative contribution of culture and capture norms in management vary, depending on the category of the reservoir. Medium and large reservoirs are predominantly capture fisheries systems and the management norms are based on the principle of stock manipulation, adjustment in fishing effort, observance of conservation measures and gear selectivity. Selective stocking is resorted to for correcting imbalances in species spectrum and to fill the vacant ecological niches. The small reservoirs, on the other hand, are generally managed as culture-based capture fisheries, akin to extensive aquaculture, where the main accent is on stocking, fattening and harvesting. An imaginative stocking and harvesting schedule and right species mix hold the key for effective management of small reservoirs.
Inducting fast-growing extraneous species into the ecosystem to colonise the diverse niches is a necessary prerequisite of reservoir management. Since one of the primary aims of stocking is to ensure utilization of the enhanced food reserves, the ideal time to stock new species is the period of trophic burst. Any lapse in this important management measure causes the proliferation of trash fishes by taking advantage of the increased availability of fish food organisms, which in turn, may provide forage base for catfishes. Nagarjunasagar, Tungabhadra, Hirakud and a number of other large reservoirs in India are examples where the minnows, catfishes, murrels and other uneconomic fishes gained grounds in the early years, leading to establishment of long food chains. These reservoirs harbour good standing crops of plankton and benthos, which are not reflected in the fish output. Even intensive stocking at a later stage has failed to reverse the situation.
|Small reservoirs||Large reservoirs|
|Single-purpose reservoirs mostly for irrigation.||Multi-purpose reservoirs for flood-minor control, hydro-electric generation, large-scale irrigation, etc.|
|Dams neither elaborate nor very expensive. Built of earth, stone and masonry work on small seasonal streams.||Dams elaborate, built with precise engineering skill on perennial or long seasonal rivers. Built of cement, concrete or stone.|
|Shallow, biologically more productive per unit area. Aquatic plants common in perennial reservoirs but scanty in seasonal ones.||Deep, biologically less productive per unit area. Usually free of aquatic plants. Subjected to heavy drawdowns.|
|May dry up completely in summer. Notable changes in the water regime.||Do not dry up completely. Changes in water regime slow. Maintain a conservation-pool level (= dead storage).|
|Sheltered areas absent.||Sheltered areas by way of embayments, coves, etc. present.|
|Shoreline not very irregular. Littoral areas with a gentle slope.||Shoreline more irregular. Littoral areas mostly steep.|
|Oxygen mostly derived from photosynthesis in these shallow, non-stratified reservoirs, lacking significant wave action.||Although photosynthesis is a source of dissolved oxygen, the process is confined to a certain region delimited by vertical range of transmission of light (euphotic zone). Oxygen also derived from significant wave action.|
|Provided with concrete or stone spillway, the type and size of the structure depending on the size of the runoff.||Provided with more complex engineering devices.|
|Major carps breed in the reservoirs.||Breeding mostly observed in the headwaters or in other suitable areas of the reservoir.|
|Can be subjected to experimental manipulations for testing various ecosystem responses to environmental modifications.||Cannot be subjected to experimental manipulations.|
|Trophic depression phase can be avoided through chemical treatment and draining. Cycle of fish production can be repeated as often as the reservoir is drained.||Trophic depression phase sets in.|
|The annual flooding during rainy season may be compared to overflowing of floodplains. Inundation of dry land results in a release of nutrients into the reservoir when it fills up, resulting in high production of fish food through decomposition of organic matter, predominantly of plant origin, leading to higher fish growth and survival.||Loss of nutrients occurs as they are locked up in bottom sediments. Rapid sedimentation will reduce benthos production.|
|Through complete fishing or overfishing of seasonal reservoirs, no brood stock is left. Fish stock has to be rebuilt through through stocking. There is thus established a direct relationship between stocking rate and catch per unit of effort.||Prominent annual fluctuations in recruitment occur and balancing of stock number against natural mortality requires high density stocking of fingerlings. Their capture requires efficient capture methods.|
Since large and medium reservoirs are to be developed on the principles of capture fisheries, it is desirable to stock the species that may breed and ultimately get naturalised in the system through autostocking. This is imperative to meet the long-term objective of obtaining a sustained yield rate. Management involving persistent stocking in large water bodies not only pushes up the input cost, such systems also create many practical difficulties in raising the stocking material in adequate quantities. However, naturalisation of introduced species is quite often beset with many problems. The alien species should find the habitat conducive to its biological and physiological requirements. It should have an edge in the competition for food and finally, the environmental conditions should favour its requirements of spawning and larval development. Recruitment failure due to the erratic hydrographic conditions that break the breeding rhythm have been found to be the single major factor responsible for the failure of stocked fishes to hold out in reservoir.
In Konar, strong currents wash down the carp eggs into the deep zones of the reservoir leading to their destruction and resultant recruitment failure (Parameswaran et al., 1969). But more often, the fishes do not find suitable spawning grounds as in the case of many south Indian reservoirs. Since the breeding of major carps is governed to a great extent by the magnitude of monsoon floods, annual variations in this parameter affect their breeding and recruitment.
Jhingran (1988) has summarized the principles to be followed in the selection of species for stocking as:
The species should find the environment suitable for growth and reproduction.
It should be quick growing, ensuring high efficiency in food utilization.
A fishery comprising herbivores with a short food chain is preferable, as they have a better conversion of primary production to fish flesh.
The stocking density should be such that the food resources of the ecosystem are fully utilized and optimum population maintained, consistent with normal growth.
The size of the fingerlings to be stocked should be so chosen to get the desired results.
Seed should be readily available with minimal transportation cost.
Cost of stocking and managing the species must be such that the operation is economically viable.
One of the important considerations is to know the amount of food available in the new environment. This factor has a considerable bearing in determining stocking rates and hence production. Fish production from unit area is a product of individual growth rate and population density. From a study of growth rate of various species in a particular body of water, it is possible to assess their optimal stocking density. Efficient utilization of fish food communities increases the carrying capacity and therefore calls for a higher population density which is achieved by addition of species to the original fish populations. In addition, information on differences, if any, in the growth rates of the endemic and introduced species, and the time taken by the introduced species in attaining harvestable size would provide insight into the production dynamics of the system.
Both intra- and inter-specific competitions are to be considered in the stocking programme. Situations where two or more species use a similar resource, such as food or space, lead to overcrowding and poor growth rate. Under a higher than optimum stocking rate, though production may be high, the individual growth rate will be so small that to attain a marketable size a long growing period will be needed. On the other hand, if bigger fishes are needed, the rate of stocking should be lowered and a low production will have to be accepted. Similarly, when a marketable size is to be attained in a shorter period, stocking rate will have to be lowered to allow faster growth. Thus, a desired balance among stocking rate, population density and growth is to be maintained with enough flexibility so as to swing it to suit the changes in environmental factors. Such a plan must determine tentative stocking rates and population thinning, depending on the need(Jhingran, 1988).
The sizes of fingerlings that are stocked in Indian reservoirs come under the pre-recurit phase and so, up to the size of entering the exploited phase they are prone only to natural mortality. Therefore, a knowledge of the natural mortality rate is essential as due compensation can be provided for it while computing optimum stocking rates. Jhingran and Natarajan (1969), while evaluating the stocking rates of Damodar Valley Corporation (DVC) reservoirs, arbitrarily recommended a compensation factor for natural mortality @ 25% for reservoirs with no large predaceous fishes and 50% for those harbouring large predators. While estimating optimum stocking rates for such populations, about which no reliable estimates of natural mortality are available, it is felt that assumption of higher natural mortality rate would be desirable as a little overstocking would be less harmful than understocking. Fish Seed Committee of the Government of India (1966) recommended the stocking rate for reservoirs at about 500 fingerlings ha-1 of the size range 40 to 150 mm.
The policies hitherto adopted in Indian reservoirs mainly consisted of stocking fingerlings of a species or a combination of species without any definite density levels or ratios based on the biogenic capacity of the reservoir. Rate of stocking and the species-mix are often determined by their availability, as can be seen from the case studies in the chapters ahead.
Basic productivity of the reservoir is dependent on the amount of solar energy available and the efficiency of the system to transform it into chemical energy. Besides, the energy conversion efficiency at trophic levels of consumers differs considerably from one reservoir to another, depending on the qualitative and quantitative variations in the biotic communities. Any conversion rate above 1% can be considered as good. In an ideal situation, the commercial species share the ecological niches in such a way that trophic resources are utilised to the optimum. At the same time, the fishes should belong to short food chain in order to allow maximum efficiency in converting the primary food resources into harvestable materials. But in reservoirs, such conditions seldom prevail.
Indian reservoirs, by and large, have a wide ranging representation of biotic communities. Phytoplankton comprising Cyanophyceae, Chlorophyceae, Dinophyceae and Bacillariophyceae dominate over the zooplankton such as copepods, cladocerans, rotifers and protozoans. Benthos is represented by insect larvae and nymphs, oligochaetes, nematodes and molluscs. There is a rich growth of periphyton on the submerged objects. The large magnitude of water level fluctuations does not favour the establishment of aquatic macrophytic communities. Significantly, many of the above niches with the exception of insects, Cyanophyceae and molluscs are shared between Indo-Gangetic major carps and trash fishes, focussing the need for controlling carp minnows and weed fishes. The ecosystem-oriented management policy places due emphasis on trophic strata in terms of shared, unshared and vacant niches. Two main pathways through which primary energy finds its way to fish flesh are the grazing chain and the detritus chain. Contribution by both the pathways to the total availability of the energy needs to be assessed for determining the species combination, most suited to the ecosystem. A large number of Indian reservoirs exhibit the detritus chain of energy transfer.
Prior to the development of carp seed production technology in India, natural spawn collected from rivers were stocked in reservoirs. Thus, the seed of Puntius spp., Cirrhinus spp. and Labeo spp. collected from the Cauvery were extensively stocked in the reservoirs of Tamil Nadu along with euryhaline species such as Chanos chanos, Etroplus suratensis and Megalops cyprinoides. Labeo fimbriatus, Cirrhinus cirrhosa, tilapia and Etroplus suratensis were the common stocking material in Kerala during the early years. With the advent of induced breeding, most of the States in India were able to raise the carp seed in large numbers by the 1970s and this resulted in a shift in species-mix in favour of catla, rohu and mrigal. Today, reservoir fisheries in India largely centre on development of carp fisheries. Their unmistakable role has been demonstrasted in the Gangetic as well as peninsular reservoirs. Major carps, by virtue of their feeding habits and fast growth rate are indispensable in reservoir management. However, the Indian major carps are ill-suited to utilize phytoplankton, the most dominant fraction of plankton. The remarkable ability of silver carp in efficient conversion of phytoplankton into fish flesh has been demonstrated in Kulagarhi and Getalsud reservoirs, despite the persisting doubts about the digestibility of Microcystis. However, introduction of exotic fishes in open waters is still a subject of controversy due to its possible deleterious effects on indigenous populations.
Development of endemic candidates as stocking material has not made much headway in the country although some of them have a proven track record in ensuring an efficient energy transformation rate. P. pangasius, subsisting on a molluscan diet is a species to be considered in the detritus-based, mollusc-rich reservoirs of the country. Puntius pulchellus, the peninsular species is a well-known macrophyte feeder and Thynnichthys sandkhol consumes Microcystis, the common alga in Indian waters. Diversification of stocking material is essential for establishment of a multi-species fish stock that utilize all food niches of the ecosystem. In reservoirs, where annual drawdown is not pronounced and water level fluctuations are not steep, phytobenthos and macrovegetation develop in various degrees. The grass carp, Ctenopharyngodon idella can be considered for such water bodies. The common carp is being stocked in many reservoirs. This sluggish fish does not survive normally in the warm, deep-basin reservoirs of the south, especially when infested with predators. However, this prolific feeder could carve out a place for itself in the reservoirs of the northeast, in Gobindsagar and in some of the peninsular reservoir like Krishnarajasagar. The fish being a mud-stirrer, is considered to be unsuitable for already turbid waters.
Tilapia, due to its records of rapid proliferation and consequent stunted growth in pond ecosystem, does not find favour with many fishery managers, although in Amaravathy and Malampuzha it has performed well. Stocking of prawn, Macrobrachium malcolmsonii has been tried in Tungabhadra and Konar (Natarajan, 1979b), where they could not survive and contribute to the commercial fisheries. Ahmed (1993) emphasised the need for extensive stocking of this prawn, by quoting instances of its self-sustaining populations in many reservoirs. However, the prawn's ability to complete its full life cycle in the freshwater phase is yet to be proved. T. putitora, L. dero, and the exotic species such as mirror carp, silver carp, grass carp, Tinca tinca and Carassius carassius are advocated for the high altitude reservoirs (Natarajan, 1979b).
The most important objective of stocking, i.e., to augment the yield, can be achieved only if the stocked fishes survive, grow and get caught in the fishing gear. This is achieved, to a large extent, in small reservoirs where the management centres round the stocking and recapture system. However, in larger water bodies, the recapture is uncertain on account of many reasons as mentioned earlier.
Impact of stocking in medium and large reservoirs
Experience in a number of medium and large reservoirs prompts us to conclude that the stocking programme can be termed as successful, only when the stocked fishes breed in the reservoir and contribute towards autostocking. In many cases, despite persistent stocking, the transplanted species did not show up in the catch, thereby rendering the expenditure incurred in stocking as waste. Only in a few instances the resources mobilised for stocking operation were compensated by generation of income through recapture of the stocked fishes.
Sreenivasan (1984) reviewed the impact of stocking in 10 reservoirs of Tamil Nadu. The stocked catla built up a naturalised population in Mettur reservoir. Just 10 000 fingerlings were stocked during 1922 to 1935, which formed the nucleus of a self-propagating stock and dominated the catch during the 1960s. Catla fisheries, however, suffered periodic setback due to breeding failures. The current contribution is as low as 10%. Recapture of two other stocked fishes viz., L. rohita and L. calbasu is reported to be adequate (Sreenivasan, 1984). However, stocking of L. fimbriatus (over 2 million), common carp (1 million), L. kontius (0.4 million), P. carnaticus(0.4 million), C. reba (several hundred thousands) and P. dubius (several hundred thousands) is believed to be wasteful, since they were never recaptured in any appreciable quantity.
In Bhavanisagar, although the transplanted catla showed up in the catch, they could not make any impact on the catch structure. As opposed to this, L. calbasu, another transplant, found the environment congenial for breeding and established itself in the reservoir, supporting a lucrative fishery for the last few decades. More than 2 million common carp fingerlings were stocked in the reservoir, of which only a few hundreds were recaptured. Intensive stocking of L. fibriatus did not make any dent on the fisheries, despite the fact that the fish was native to the system.
In Sathanur, a breeding population of catla has been successfully established through stocking and the fish contributes more than 80% of the total catch. The species has also eclipsed the indigenous L. fimbriatus by reducing its contribution from 36% to 1%. It is pertinent to note that the stocking done over 12 years involving 2 million fingerlings of indigenous species such as C. cirrhosa, L. kontius, C. reba and L. fimbriatus could not make any impact on restoration of their fisheries, primarily due to the inability of the fishes to breed and propagate themselves.
In Krishnagiri, although the increase in percentage of the stocked major carps has resulted in some initial increment in their contribution in the catch, the recapture was not commensurate with the stocking rate. Stocking of Gangetic carps, common carp and L. fimbriatus done in Vaigai, so far, has been described as wasteful (Sreenivasan, 1984), where presently, catla contribute 20% of the catch.
In Malampuzha and Peechi, the two medium reservoirs in Kerala not register any substantial increase in yield rate despite sustained stocking with Gangetic major carps, mainly on account of their failure to breed. In Malampuzha, despite a sharp increase in stocking density from 52ha-1 in the 1970s, and a corresponding increase in the percentage of major carps in the stocked fishes, the yield rate remains at a low level of 5.0 kg ha-1. Although catla grows to an impressive size, the contribution of major carps never exceeded 20% of the total catch. Considering the low yield rate, the quantity of major carps harvested does not commensurate the stocking effort. Similarly, in Peechi, 90% of the fingerlings stocked belong to the Indian major carps, especially L. rohita. But they are not reflected adequately in the catch and the yield rate remains at a low level of 4.5 kg ha-1.
In Nagarjunasagar, Andhra Pradesh, regular annual stocking at the rate of 50 000 to 833 000 fingerlings comprising catla, rohu and mrigal during the 1970s had little impact on the catch structure, as none of the stocked fishes could breed and contribute to recruitment. Similarly, stocking as a management option has failed in Tungabhadra reservoir situated on the same basin and Krishnarajasagar in the Cauvery basin. In Tungabhadra, the stocking rate during 17 years ranged from 1 to 11 ha-1, while it varied from 0.15 to 67 ha-1 in Krishnarajasagar. In both the reservoirs, the selection of species has always been arbitrary, the stocking density was inadequate and the stocked fishes failed to breed in the reservoir.
Gandhisagar in Madhya Pradesh is an example of the stocked fishes contributing to fish catch in a sustained manner through breeding and recruitment. During the 1950s through 1970s, on account of steady stocking of catla (2 million), rohu (1.3 million), and mrigal (1.1 million), the fish yield rose to 20.33 kg ha-1 from the initial 0.51 kg ha-1. Among the stocked fishes, catla contribute 60 to 70%, while mrigal and rohu form only 1 to 20%. Mrigal, despite being a part of the indigenous ichthyofauna, is on the decline. Ravishankarsagar in the same State was stocked, on an average, with 1.77 million fingerlings every year and yet the yield rate could not be raised above 8.3 kg ha-1, primarily due to the non-establishment of the carps. The amount spent on stocking has been much higher than the cost of recaptured fishes. Rihand reservoir in U. P. could build up a breeding population out of the initial stocking. Although the yield rate of 0.58 kg ha-1 is not impressive, 73 to 99% of the catch comprises C. catla.
The success of stocked Indian major carps in Ukai reservoir in Gujarat can also be attributed to their breeding in the reservoir. Apart from augmenting the reservoir fisheries through recruitment, the young ones of Indian major carps are also reported to escape through the outlet of the dam and contribute to stocking of downstream impoundments. In all the DVC reservoirs, viz., Konar, Tilaiya, Maithon and Panchet, stocking did not have any impact (Sreenivasan, 1984). Some breeding activities in respect of catla and mrigal have been reported, though low survival rate of the spawn and frequent breeding failure due to erratic monsoon prevented proper recruitment of the planted species.
Indian experience of stocking medium and large reservoirs suggests that by and large, the stocking becomes effective only when the stocked fishes propagate themselves. Moreover, this breeding population can be built up only if the stocking is resorted to during the early phase of the reservoir formation.
Impact of stocking in small reservoirs
In sharp contrast to the large and medium reservoirs, stocking has been more effective in improving the yield from small reservoirs as success in the management of small reservoirs depends more on recapturing the stocked fish rather than on their building up a breeding population. The smaller water bodies have the advantage of easy stock monitoring and manipulation. Thus, the smaller the reservoirs, the better are the chances of success in the stock and recapture process. In fact, an imaginative stocking and harvesting schedule is the main theme of fisheries management in small, shallow reservoirs. The basic tenets of such a system involve:
Selection of the right species, depending on the fish food resources available in the system.
Determination of a stocking density on the basis of production potential, growth and mortality rates.
Proper stocking and harvesting schedule including staggerred stocking and harvesting, allowing maximum growout period, taking into account the critical water levels.
In case of small irrigation reservoirs with open sluices the season of overflow and the possibilities of water level falling too low or completely drying up, are also to be taken into consideration.
Aliyar is a standing testimony to the efficacy of the management based on staggered stocking. The salient features of the management options adopted in Aliyar are:
stocking is limited to Indian major carps (earlier, all indigenous, slow-growing carps were stocked)
Increasing the size at stocking to 100 mm and above.
reducing the stocking density to 235–300 ha-1 (earlier rates were erratic ranging between 500–2 500 ha-1).
staggering the stocking, and
regulating mesh size strictly and banning the catch of Indian major carps < 1 kg in size
A direct result of the above management practice was an increase in fish production from 1.67 kg ha-1 in 1964–65 to 194 kg ha-1 in 1985.
Effective recapture of the stocked fishes renders the stocking more remunerative in small reservoirs. Successful stocking has been reported from a number of small reservoirs in India. In Markonahalli, Karnataka, on account of stocking, the percentage of major carps has increased to 61% and the yield increased to 63 kg ha-1. Yields in Meenkera and Chulliar reservoirs in Kerala have increased from 9.96 to 107.7 kg ha-1 and 32.3 to 275.4 kg ha-1 respectively through sustained stocking. In Uttar Pradesh, Bachhra, Baghla, and Gulariya reservoirs registered steep increase in yield through improved management with the main accent on stocking. An important consideration in Gulariya has been to allow maximum growout period between the date of stocking and the final harvesting, i.e., before the levels go below the critical mark. The possible loss due to the low size at harvest was made good by the number. Bundh Beratha in Rajasthan, stocked with 100 000 fingerlings a year (164 ha-1) resulted in a fish yield of 94 kg ha-1, 80% of which constituting catla, rohu and mrigal (Table 1.9).
|Reservoir||State||Stocking rate (number ha-1)||Yield (kg ha-1)|
Instances where intensive stocking of Indian major carps became ineffective in small reservoirs are very rare. In Govindgarh, despite stocking of Indian major carps at the rate of 19 to 390 fingerlings ha-1, the yield remained at 15.92 kg ha-1, during the '60s and the '70s. Large-scale escapement of fishes through the open weir is believed to be the main reason for this low fish yield. An estimated 1.1 million fingerlings of catla, rohu and mrigal were stocked into Badua reservoir, Bihar during the period 1975 to 1979. However, the fish yield from the reservoir during the period remained within 4 to 7 kg ha-1. Other management measures taken in the reservoir are not known. Sreenivasan (1984) reported disappointing recapture of major carps after their heavy stocking in Manjalar reservoir (Tamil Nadu). Proliferation of the tilapia, O. mossambicus is the main factor that prevented the major carps from getting a foothold in the fishery, probably due to their competition for food.
Presence of predatory and weed fishes poses impediments in survival and growth of economic species in many Indian reservoirs. Keeping these unwanted population under check is a very difficult management problem, especially in large reservoirs. A small population of predators helps to crop the trash fishes which compete for food with the economic species. A small predator population of Gobindsagar which keeps the minnows under check is a good example. However, no scientifically sound methods are available to keep a limited population of predatory species. Repeated use of gill nets of appropriate mesh size, use of long lines, traps, etc. are suggested for control of the uneconomic and undesirable populations. Manipulation of reservoir level with a view to checking the breeding and destruction of the young ones of predators and the minnows has been tried in several countries. However, this is not practicable in many Indian reservoirs since water release pattern is dictated by priority sectors like irrigation and power generation. Poisoning of selected sheltered areas arms and coves as practised abroad has also limited use in India due to the multiple use of water and objections from the other water users. David and Rajagopal (1969) reported that non-selectivity of shore seines helped in reducing catfish population in Tungabhadra reservoir by 76 to 81%. Alivi, the giant shore seine of Tungabhadra also removes the trash fish in large numbers. Judicious use of this gear, with a condition that the juveniles of economic species are released back, can go a long way in containing the trash fish population.
Recent findings of Kartha and Rao (1990) with regard to the efficacy of trawling in checking predators and trash fishes are of interest. Bottom trawling in Gandhisagar was found to catch 64 to 91% of the unwanted fishes and this has been recommended as a method to crop the predators and carp minnows. Nevertheless, applicability of this method is limited to places where the bottom is free from obstructions. Natarajan (1979b) suggested biological control of trash fishes by stocking two euryhaline species viz., Megalops cyprinoides and Lates calcarifier. Since these predatory fishes do not breed in freshwater, they cannot go out of control.