###### Abstract
India ranks as the third-largest fish producer globally and the second-largest in aquaculture, second only to China [14]. In India, over 4 million metric tons of this waste are generated annually and are often disposed of in the environment, including landfills and water bodies. The amount of waste produced during fish processing ranges from 20% to 60%, depending on the species [15; 16]. In the recent past, Indian fisheries have witnessed a paradigm shift from marine-dominated fisheries to inland fisheries, with the latter emerging as a significant contributor to fish production from 36% in the mid-1980s to 70% in the recent past. Within inland fisheries, a shift from capture to culture-based fisheries has paved the way for a sustained blue economy [14]. From 2014-2015 to 2018-2019, India's fisheries sector grew by 10.88% annually. Over the past five years, India's fish production has grown by 7.53% annually, reaching 137.58 lakh metric tons in 2018-2019 to 220 lakh metric tons by 2024-2025, with an annual growth of about 9% in fish production [17]. India has around 2.36 million hectares of tanks and ponds, where culture-based fishery is predominant and contributes to the maximum share in total fish production. The current production from tanks and ponds is 8.5 million MT. As a significant contributor towards production, the department has prioritized to expand the horizontal area under tanks and ponds to achieve a target production of 13.5 million MT [14]. One of the essential items in freshwater aquaculture is the water requirement, either in hatcheries or in grow-out systems. It is estimated that freshwater aquaculture is at present utilizing 9.5 billion m\({}^{3}\) of water under the existing 0.745 million ha pond area, which is expected to increase to 36.2 billion m\({}^{3}\) at the complete development of the remaining 1.755 million ha of existing water bodies. An additional creation of 25% of new ponds, i.e., 0.439 million ha, will be achieved, thus considering the water availability of 70-80 mhm. A share of 5% (3.6 mhm) must be available for fresh aquaculture [18]. In addition, the water requirements for fish processing operations generate potentially large quantities of waste and by-products from inedible fish parts and endoskeleton shell parts from crustacean peeling process, viz., particles of flesh, skin, scales, bones, visceral mass (viscera, air bladder, gonads, and other organs), head, fins, shells, or liquid stick water. Depending on the species processed, the solid wastes make up 30-40% of the total production [19]. At present, the management of fishery wastes is one of the main problems of the greatest concern and impact on the environment [20; 21]. In recirculating aquaculture systems, fish sludge and wastes pose a disposal challenge due to their high organic content and moisture, making incineration costly and landfill disposal unsustainable [22]. These effluents, typically high in nutrients, result in algal blooms, offensive odor, acutely lethal discharges, localized areas of anoxia, etc. [23]. The proper selection of disposal methods and waste processing for the reception of waste is of paramount importance. If large quantities of waste are discharged into a large receiving water body with inadequate mixing and poor water exchange, adverse environmental impact will be observed [24]. To promote sustainable and economical fish farming activities, utilizing the FW generated during farming is essential. Fish processing effluent treatment needs to be considered in order to obtain high-quality water requirements that allow its reuse or recycling for industrial processes [19]. The use of AD to treat fish waste aligns with efforts to promote circular economy practices, reducing environmental pollution while enhancing soil fertility using digestate. Due to its high biodegradability, FW is an ideal co-substrate for AD, providing a complementary nutrient profile that accelerates the breakdown of lignocellulosic materials and improves the overall biogas yield [25].