Fish-processing waste is a heterogeneous and underutilized biomass containing lipids, proteins, minerals, and biodegradable organic matter that can serve as feedstocks for the production of fuels, bioproducts, and recovered nutrients. However, conventional valorization approaches often focus on individual products, resulting in incomplete resource utilization and continued generation of residual waste. This review critically examines the potential of an integrated fish-waste biorefinery based on sequential fractionation and conversion of different waste components. Particular emphasis is placed on lipid recovery and biodiesel production, followed by utilization of the defatted biomass for anaerobic digestion and biogas or biomethane generation, with protein recovery and digestate-based nutrient recovery considered as complementary pathways. The review evaluates the biochemical basis, process parameters, pretreatment, and co-digestion strategies influencing anaerobic digestion, together with the effects of feedstock composition, free fatty acids, ammonia, long-chain fatty acids, and salinity on process performance. The environmental and techno-economic implications of integrated processing are also critically discussed, including energy consumption, greenhouse-gas emissions, logistics, product quality, process complexity, and the need for life-cycle and economic assessment. Particular attention is given to the trade-off between recovering energy-rich lipids for biodiesel and retaining them for methane production, highlighting the importance of evaluating total energy and resource recovery rather than individual product yields. The review identifies standardized feedstock characterization, low-energy extraction, reusable catalysts, inhibition control, optimized co-digestion, and pilot-scale validation as key research priorities. Overall, sequential valorization offers a promising pathway for transforming fish-processing residues from a disposal problem into a multi-product biorefinery, although its environmental and economic superiority must be demonstrated through integrated mass, energy, techno-economic, and life-cycle assessments.
