The hydrobiological parameters have a major role in inland fish production, and they are the key factors for developing management strategies. The ecology and biota in reservoirs are mainly dependent on the habitat characteristics of the resources. The physicochemical characteristics of the water and soil determine the productivity of a reservoir (Montanhini Neto et al. 2017). The analysis of water quality parameters revealed that the reservoirs are conducive for fisheries enhancement. The variation of temperature in the present study follows the seasonal pattern of typical Indian agro-climatic conditions with the highest in summer or the pre-monsoon period. Several authors also observed the same pattern of temperature variations in different reservoirs of India (Narayana et al. 2008; Garg et al. 2009; Verma et al. 2011; Prabhahar et al. 2012). The higher value of nutrients in the pre-monsoon seasons during the present study might be due to evaporation which increases the nutrient content of the water. In accordance with the present findings, many authors also observed the same pattern of seasonal variations of nutrients in reservoirs and lakes (Gorham 1961; Rajashekhar et al. 2007; Vetriselvi et al. 2011; Pawar and Shembekar 2012). BOD which is an important parameter for the assessment of the ecological health of aquatic ecosystem was higher in premonsoon and monsoon periods which is directly related to the availability of water in the reservoir and the accumulation of organic load through the runoff from the catchment during the rainy season. The microbial load is also an important parameter for assessing the water quality parameters. The microbial load was higher in the summer which is due to the enhancement of the growth of the microbes at the higher temperature during pre-monsoon period.
Stock enhancement programs help to augment the existing fish stock and yield from reservoirs (Cowx 1994; De Silva and Funge-Smith 2005; Sarkar et al. 2020). Fisheries enhancement and conservation measures were strictly observed in the reservoir through the introduction of Reservoir Fishery Policy-2003 during the year 2004-05 which increased fish production. Moreover, during the year 2009-10 Reservoir Development programme implemented by National Fisheries Development Board (NFDB), India through FFDAs in three districts of Odisha (Sambalpur, Bargarh & Jharsuguda) included Hirakud reservoir. During the year 2010-11 & 11–12, advanced fingerlings were stocked by FFDA, Sambalpur, Bargarh and Jharsuguda under the programme to compensate the loss of juveniles due to indiscriminate fishing. The stocking continued during the year 2012-13 to 2015-16. In the present study a considerable higher fish production during the year 2010-11 to 2015-16, showed the systematic implementation of the fingerling stocking programme by the National Fisheries Development Board, India. The high fish yield may be attributed to high natural productivity (Sugnnan 1995; Das et al. 2008; Sarkar et al. 2018). The enhancement strategy in Indian reservoirs largely depends on indigenous carp species, viz. G. catla, L. rohita, C. mrigala, L. calbasu, L. fimbriatus, and C. cirrhosus besides, the minor carps such as L. bata, Systomus sarana, etc. and catfish species (De Silva and Funge-Smith 2005). In accordance with the present finding, the positive impact of stocking in the different categories of reservoirs is reported in India (Sugunan and Katiha 2004; Sarkar et al. 2020; Lianthuamluaia et al. 2021) and in other parts of the world (Phan and De Silva 2000; Nguyen et al. 2001, 2005; Wijenayake et al. 2005; Amarasinghe and Nguyen 2010; Pushpalatha and Chandrasoma 2010; Pushpalatha et al. 2017). According to Sarkar and Mishal (Sarkar and Mishal 2017), the potential fish yield for a large reservoir is100kg/ha/yr. The earlier report in Aliyar reservoir stated that the stocking programme impacted the fish catch with an increased yield from 26.7 to 136 kg/ha/yr (Selvaraj et al. 1990). Through sustainable stocking and fishing management programmes in Suvarnavathy reservoir of Karnataka, the fish yield increased from 116 to 197 kg/ha/yr (Rao et al. 2013). Similar results are also reported in Uttar Pradesh (150 and 140 kg/ha/yr in Gulariya and Bachhra reservoirs), and Rajasthan (94 kg/ha/yr in Budha Beratha reservoir) where the fish yield increased as a result of stocking (Sugunan 2011). If the recruitment of new stock and conservation measures are taken up simultaneously and continued for some more years there will be a spectacular increase in fish production of the reservoir. The increasing trend of fish production in the reservoir is well supported by the ARIMA model which also predicted the upward trend in fish production.
In India, staggered information on fish yield potential and habitat data of large reservoirs are available. Consequently, development of an integrated baseline database on ecology, morpho-edaphic factors, climatic parameters, harvest composition, biodiversity and yield potential in GIS platform is essential for calculating the optimum stocking rates and sustainable production from large reservoirs. The present analysis and observation indicate the prospect of enhancing fish yield and production from large reservoirs through stocking enhancement. Indian reservoirs are suitable for fisheries enhancement but they still remain understocked and unscientifically managed to give ample scope for fisheries development by bridging the gap between fish production potential and yield. There are several issues in implementing successful enhancement programmes in large reservoirs. The varying water level is also a challenge in estimating the effective water area for successful stocking program (Sarkar et al. 2018). The use of GIS and remote sensing will help to resolve this issue and thus assist in decision making. Moreover, the use of mobile based apps for fish catch estimation holds great potential in central Indian reservoirs (Karthikeyan et al. 2020).
The stocking was found to be ineffective in some of the large reservoirs. The role of auto stocking in fish yield and identification of breeding grounds are important researchable issues in large reservoirs. The food web based modeling (Behera et al. 2020) can be attempted for optimising the fish yield. The application of Lorenzen model (Lorenzen 2000) also holds great potential in large Indian reservoirs. Fishing is also difficult in many large reservoirs; socustomization of fishing gear and crafts are also essential for efficient exploitation of stocked fish (Sugunan 2011).
There are several challenges in the ecological assessment of large reservoirs. The water level influences productivity and the productivity is in turn, influenced greatly by external random factors. The lack of comprehensive historical data makes it difficult to estimate the temporal changes in ecology (Sarkar et al. 2018). Lack of scientific knowledge base on auto recruitment, migratory patterns and site fidelity of fishes, and complex food web dynamics in large reservoirs is still a challenge (Lianthuamluaia et al. 2019; Sandhya et al. 2019).
Fish play an important component in maintaining the ecological health of the aquatic ecosystem. Conservation of fish diversity needs to be taken into consideration while managing reservoirs or lakes. The present observations of the fish diversity indicated that the number of species has declined. Job et al. (Job et al. 1955) recorded 86 fish species and Singh (Singh 2014)) recorded 56 species from Hirakud region, but during the present study, only 40 species were recorded. Habitat alteration is one of the most important factors for the loss of fish diversity (Shrestha 1990; Dehadrai et al. 1994; Sehgal 1994). Many fishes need a particular environment and ecological conditions for breeding, migration and larval development to complete their life cycle. Habitat alteration can greatly hamper the biological process of many fishes to increase their populations. Water flow and water availability are among another important factors influencing fish breeding and growth. Reservoirs are not constructed for fish production, the water use in the form of electric generations, irrigation and flood control play a very important role in the water availability of reservoirs. The fluctuations of water availability in reservoirs due to different water use are among the major causes of reduction in fish species richness. Pollution or deterioration of water qualities can be important factors causing loss of fish diversity (Dehadrai et al. 1994). Due to the increase in human populations and uncontrolled anthropogenic activities our environment including the aquatic environment is deteriorating which has drastically impacted biodiversity including fish. Illegal fishing and overexploitation are also among the main causes for the loss of fish diversity in inland waters (Dehadrai and Ponniah 1997). Climate change impacted fish diversity directly or indirectly. Irregular rainfall patterns greatly impacted water availability and water flow which can greatly influence the breeding biology of fish. An increase in temperature also impact water availability through evaporation which can influence the fish biology of many fishes.
The present study indicated that most of the water quality parameters were in the favorable range for fish and associated flora and fauna. However, the species richness of fish has gone down due to several reasons.The fingerling stocking in medium and large reservoirs was rendered ineffective in India (Sreenivasan 1984). But the findings of the present study reveals that large reservoirs are also suitable for fisheries enhancement.The impact of stocking depends on the habitat quality, management practices, quality of seed, seed stocking ration, species performance and carrying capacity of a water body. The National Fisheries Development Board, India has suggested stocking of an average 1000 fingerlings/ha in Indian reservoirs, however, for sustainable production the management practices depending on the ecology and climatic condition of the water resources are required. The location-specific management practices are essential to achieve the production goals. Based on the present study, observations and discussion with the stakeholders, the following recommendations are suggested:
Fishing ban period /closed season should be practiced in the reservoirs to avoid the catch of brooders. The priority fish habitat suitable for breeding grounds should be identified and protected. Advanced fingerling stocking (~ 120 mm in size) is advocated, which may ensure better survival and yield. The carrying capacity should be estimated to determine the optimum stocking and production. Nursery and rearing facility in suitable shoreline of reservoir along with seed raising in enclosures are recommended. The application of ecological modeling for devising stocking strategy, species selection and the combination would help the managers to utilize the fish production potential. The dynamics of physical, chemical and biological attributes of reservoirs are still not well documented which may be done at the periodical interval to understand the changing pattern in the context of environmental challenges. The management of the reservoir should be done in a participatory mode so that the local fishers should be involved in implementing the regulatory measures for fisheries enhancement and development. A sound policy framework on reservoir fisheries for management, governance and institutional arrangement would be important in accomplishing the potential of multi-use reservoir.