Energy and Resource Conversion of Fish Waste: Current Status and Future Prospects

Abstract

The global fish industry is an essential contributor to food security and the economy; however, it produces considerable quantities of fish waste. Depending on species and processing methods, 20–80% of fish biomass is discarded as waste, amounting to almost 64 million tons annually. Conventional disposal methods such as landfilling, incineration, and wastewater discharge cause environmental pollution and greenhouse gas emissions. Therefore, for sustainable development, it is important to valorize fish waste into renewable energy and value-added products. Hence, this review aims to encourage the technical development of fish waste into bioenergy and bioresources by thoroughly studying key technologies for converting it into biogas, biodiesel, fertilizer, animal feed, and biochar. Although several studies have explored these technologies, most remain disjointed and lack a systematic assessment. Therefore, this work attempted to critically evaluate the technological principles, advantages, limitations, and optimization strategies of current valorization approaches. Furthermore, several recommendations have been proposed, including conducting pilot-scale trials, producing higher value-added products within a biorefinery system. Overall, this review provides a holistic outlook on the energy and resource conversion of fish waste, supporting sustainable waste management.

1. Introduction

The global seafood industry constitutes a vital sector for food security, employment, and economic development, with worldwide production exceeding 180 million tonnes annually [1]. Fish processing generates substantial quantities of by-products, including viscera, heads, bones, scales, skin, fins, and processing wastewater; the fraction generated depends strongly on species, product type, and processing intensity [2]. These streams contain recoverable proteins, lipids, minerals, and organic matter, but inadequate management can contribute to odour, high oxygen demand, nutrient discharge, greenhouse-gas emissions, and sanitary risks [3].

In recent years, the concept of a circular bioeconomy has gained significant traction as a framework for sustainable waste management. Under this paradigm, waste is viewed not as a disposal problem but as a renewable resource capable of generating energy, materials, and chemicals. Fish waste is particularly amenable to biorefinery approaches because of its diverse and nutrient-dense composition. The high protein content makes it suitable for feed and fertilizer production, the lipid fraction is a promising feedstock for biodiesel, the mineral-rich bones and scales can be converted to biochar and phosphorus fertilizers, and the organic matter is highly biodegradable for biogas production [4].

Numerous technological pathways have been investigated for fish waste valorization. Anaerobic digestion (AD) represents a mature biological process for converting organic matter into methane-rich biogas, which can be used for heat and power generation or upgraded to biomethane for injection into natural gas grids [5]. Composting and hydrolysis processes transform fish waste into nutrient-dense organic fertilizers suitable for sustainable agriculture [6]. Thermal processes such as pyrolysis can produce biochar from fish bones and scales, creating functional materials with exceptional adsorption and catalytic properties [7]. The oil fraction of fish waste, rich in omega-3 fatty acids, can be transesterified into biodiesel, a renewable alternative to petroleum diesel [8]. Additionally, the excellent amino acid profile of fish proteins supports their use in animal nutrition through fish meal, fish silage, and protein hydrolysates [9]. Beyond these bulk products, fish waste is increasingly recognized as a source of high-value biomolecules including collagen, gelatin, chitin, and bioactive peptides with applications in food, cosmetics, and pharmaceuticals [10].

Despite extensive research on individual pathways, a comprehensive assessment is still needed to identify where each route is technically, environmentally, and economically most appropriate. This review therefore synthesizes process performance and limitations while explicitly considering feedstock heterogeneity, resource inputs, product value, scale-up barriers, and opportunities for integrated biorefineries. The literature was updated through September 2026, incorporating recent studies on fish waste biorefineries [11], integrated oil extraction–anaerobic digestion–nutrient recovery [12], biodiesel and biopolymer production [13], fish waste-adapted bioaugmentation [14], hydrothermal carbonization [15], systematic evidence on anaerobic digestion [16], and the latest assessment of fish waste biodiesel challenges, energy demand, LCA, and techno-economics [17]. The overarching goal is to support transition from linear disposal toward context-specific cascading recovery that maximizes resource value while avoiding environmental burden shifting.

Fish Waste Heterogeneity as a Determinant of Process Selection

Fish waste should not be treated as a uniform feedstock. Its moisture, protein, lipid, ash, salt, and mineral contents vary with fish species, age, season, capture or aquaculture conditions, processing method, storage time, and the specific fraction collected [2,3]. Viscera and trimming residues are generally richer in readily degradable protein and lipids, whereas bones and scales contain more ash, calcium phosphate, and collagen; processing wastewater and sludge are substantially more dilute and may contain salt or cleaning chemicals [3]. This variability directly affects route performance. High protein and low C/N ratios increase ammonia-inhibition risk during anaerobic digestion [16], while lipid-rich fractions can improve methane or biodiesel potential but also increase long-chain-fatty-acid inhibition and oxidation sensitivity. Bone-rich fractions are better suited to phosphorus recovery, biochar, or mineral products, whereas fresh, hygienically controlled protein fractions may retain greater value as feed or protein hydrolysates. Seasonal changes in supply and composition also influence storage requirements, reactor loading, product consistency, and plant capacity utilization. Consequently, representative compositional characterization and feedstock segregation should precede route selection, and reported performance should be interpreted in relation to the actual waste fraction rather than generalized to “fish waste” as a whole.

2. Fish Waste to Biogas

2.1. Fundamentals and Four-Stage Metabolic Pathway

Anaerobic digestion (AD) is the most technologically mature route for converting fish waste into renewable biogas. As depicted in Figure 1, the process proceeds through four microbially mediated stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. In hydrolysis, extracellular enzymes cleave proteins, lipids, and carbohydrates into soluble monomers (amino acids, long-chain fatty acids, sugars) [18]. During acidogenesis, fermentative bacteria convert these monomers into volatile fatty acids (VFAs), alcohols, CO₂, H₂, and NH₃ [19]. Acetogenesis then oxidizes higher VFAs into acetate, CO₂, and H₂ under syntrophic association with hydrogenotrophic methanogens. Finally, methanogenesis produces CH₄ via acetoclastic and hydrogenotrophic pathways, yielding biogas typically containing 50–70% CH₄ and 30–50% CO₂ [5]. The biomethane potential of solid fish waste is considerable, reaching 300–500 mL/g VS under mesophilic conditions (35–38 °C, pH 6.8–7.5) [20]. However, the high biodegradability of fish waste is a double-edged sword: its low C/N ratio (3–7) and rapid protein hydrolysis predispose the system to ammonia accumulation and process instability [19].

2.2. Key Inhibitory Challenges: Ammonia, Lipids, and Process Instability

The stable operation of fish waste AD is governed by a constellation of interacting inhibitory factors. Ammonia toxicity is the principal constraint. The rapid deamination of fish proteins releases NH₄⁺, which at elevated pH and temperature shifts to free NH₃—a membrane-permeant toxin that disrupts the proton motive force of methanogenic archaea [21]. Mesophilic digestion is generally preferred, as thermophilic operation (50–55 °C), while kinetically faster, increases the NH₃ fraction and amplifies inhibition [19]. Lipid accumulation poses a secondary challenge: fish viscera and heads release long-chain fatty acids (LCFAs) that adsorb onto biomass, cause flotation, and inhibit methanogenesis [22]. Furthermore, high salinity and residual H₂S from processing residues can exert additional stress. The interplay between rapid acidification (VFA accumulation), low buffering capacity, and ammonia toxicity often leads to reactor failure in mono-digestion systems [23].

2.3. Mitigation Strategies for Anaerobic Digestion Inhibition

To mitigate the inhibition challenges in fish waste anaerobic digestion (AD), two complementary strategies—co-digestion and pretreatment/additive enhancement—have been widely investigated. Co-digestion with carbon-rich substrates, including lignocellulosic biomass, agricultural residues, or manure, represents the most effective and widely adopted approach, as it adjusts the C/N ratio toward the optimal range (20–30), dilutes ammonia and long-chain fatty acids (LCFAs), and improves buffering capacity and microbial diversity [23]. For instance, co-digestion of fish waste with water hyacinth using cow dung as inoculum significantly enhanced biogas production, with the FW/WH ratio serving as a critical performance determinant [24], while blending with waste activated sludge [25], whey and manure [26], or bagasse [27] has demonstrated synergistic methane yields exceeding the weighted averages of individual substrates. Similarly, the anaerobic co-digestion of strawberry and fish waste, optimized via response surface methodology, confirmed that proper mixing maximizes biogas yield while keeping volatile fatty acid (VFA) accumulation in check [28]. Agricultural residues such as wheat straw and corn stover are particularly effective in this regard, as they provide structural support for microbial biofilms and reduce LCFA inhibition through adsorption [23]. Beyond co-digestion, pretreatment and additive strategies can further accelerate hydrolysis and improve process stability. Thermal pretreatment (120–170 °C) disrupts protein tertiary structures and cell membranes, enhancing organic matter bioavailability, though excessive heating risks generating Maillard-derived recalcitrant compounds [29]. More recently, biochar-mediated enhancement has emerged as a particularly promising strategy; the addition of bamboo-derived hydrochar to fish waste AD significantly improved biogas yield by promoting direct interspecies electron transfer, reducing VFA accumulation, and stabilizing the syntrophic microbiome [30], while the co-digestion of fish processing waste with hydrothermal carbonization liquid fraction further demonstrated that biochar acts as a conductive conduit for syntrophic electron exchange [31]. Additionally, mineral additives such as zeolite and magnetite have been shown to adsorb ammonium ions and shift the microbial community toward more ammonia-tolerant consortia [32]. Nevertheless, co-digestion is not automatically beneficial: carbon-rich co-substrates can dilute biodegradable fish organics, introduce slowly hydrolysable lignocellulose, and increase transport or preprocessing requirements [16,23]. Pretreatments and conductive additives also add energy, material, and recovery costs and should therefore be evaluated using net methane gain, process stability, and whole-system energy balance rather than methane yield alone.

2.4. Microbial Dynamics, Reactor Scale-Up, and Economic Outlook

The performance of fish waste AD is profoundly influenced by inoculum selection. Adapted seeds (e.g., from piggery or high-protein waste digesters) consistently outperform non-adapted municipal sludge, achieving higher methane yields and faster acclimation [33]. Gradual stepwise feeding during start-up allows microbial communities to adapt to elevated ammonia, preventing process shock [34]. At the reactor scale, continuous and semi-continuous systems are essential for industrial deployment. Early work on the continuous co-digestion of cattle slurry and fish offal established long-term feasibility but underscored the need for careful organic loading rate control [35]. Innovative designs such as self-agitated anaerobic baffled reactors enhance phase separation and mixing, mitigating the challenges of high-strength fish waste streams [21]. Two-stage configurations that physically separate acidogenesis from methanogenesis are particularly attractive, allowing each stage to operate at its optimal pH and HRT. Recent work further shows that fish waste-adapted bioaugmentation can improve mono-digestion stability and methane yield, but performance depends on the feed-to-microbes ratio and augmentation dosage, adding an inoculum-management requirement for scale-up [14].

From an economic perspective, fish waste biogas offers a dual benefit: renewable energy generation and waste stabilization. The conversion of fish waste into biomethane for artisanal fishing communities provides a decentralized, circular energy solution [36]. Economic feasibility analyses indicate that integrated biorefineries producing biogas alongside fertilizer and feed co-products are substantially more viable than single-product pathways when co-product markets and integration benefits justify the additional costs [37]. The digestate from fish waste AD is a nutrient-rich organic fertilizer that can close agricultural nutrient loops. Pre-ensiling of fish waste prior to AD serves the dual purpose of stabilizing highly putrescible material and enhancing subsequent methane yield [38]. Future research should prioritize the development of robust ammonia-tolerant methanogenic consortia, the integration of AD with nutrient recovery technologies (e.g., struvite precipitation), and life cycle assessments that quantify the net environmental benefits of coupling fish waste biogas with aquaculture and agriculture. A 2025 integrated tuna-waste biorefinery combined oil extraction, anaerobic co-digestion, and struvite recovery, illustrating that methane production can be coupled with high nutrient recovery [12]; however, solvent selection, residual solvent effects on digestion, chemical demand for precipitation, and market value of multiple co-products determine whether such integration is environmentally and economically favorable.

3. Fish Waste to Fertilizer

3.1. Nutrient Composition and Agronomic Value of Fish Waste

Fish waste is not merely a disposal burden, it is a concentrated nutrient package that can effectively close the loop between seafood processing and agricultural production. The raw material of fish waste is characterized by high organic nitrogen (proteins, amino acids), substantial phosphorus locked in bones and scales as hydroxyapatite [Ca₅(PO₄)₃OH], and appreciable potassium, calcium, magnesium, and trace micronutrients [3]. The protein-rich soft tissues (viscera, muscle residues) provide rapidly mineralizable organic nitrogen, while the skeletal fraction represents one of the most concentrated organic sources of phosphorus available to agriculture. Unlike synthetic N-P-K fertilizers, which can acidify soils, deplete organic matter, and leach into waterways, fish-based amendments release nutrients in a slow, biologically controlled manner that tracks plant demand more closely, thereby improving nutrient use efficiency and reducing off-site losses [6]. The nutrient recovery and recycling potential from fishery waste has been comprehensively reviewed, with evidence that these organic inputs can simultaneously substitute for chemical fertilizers while boosting soil carbon stocks and microbial diversity. Representative production methods and reported agronomic outcomes are summarized in

Table 1. Fish waste-derived fertilizer products, production methods, and agronomic performance.

Product	Production Method	Key Nutrients	Target Application	Reported Agronomic Effect	Reference
Fish waste compost	Thermophilic aerobic composting (20–60 d, 50–65 °C) with straw/seaweed	N (2.5–4%), P (0.5–1.5%), humic acids	Soil amendment, organic potato, lettuce, horticulture	Improved tuber yield, root elongation, soil microbial biomass	[31,32]
Seaweed-fish co-compost	Co-composting with seaweed (K, Mg, polysaccharides)	N-P-K plus micronutrients, soil structure improvers	Horticulture, organic agriculture	Enhanced water retention, synergistic yield increase	[33]
Liquid fish fertilizer	Microbial fermentation/hydrolysis (5 L-pilot scale)	Amino acids, peptides, soluble minerals	Foliar spray, fertigation, hydroponics	Tomato: increased plant height, leaf area, fruit yield; improved soil microbial activity	[34]
Traditional fermented fish waste (Gunapaselam)	Indigenous fermentation	Organic N, P, microbial metabolites	Vegetable crops (eggplant, tomato)	Enhanced vegetative growth, flowering, comparable to chemical fertilizer	[35]
Fish bone-derived P fertilizer	Calcination/low-temp pyrolysis of clean bones	P (5–12% as hydroxyapatite), Ca	P-deficient soils, slow-release P	Agronomic effectiveness comparable to triple superphosphate	[36]
Fish bone biochar	Pyrolysis (400–700 °C, N2 atmosphere)	P (slow-release), Ca, porous carbon matrix	Acidic soils, soil amendment, carbon sequestration	Resists P fixation, improves water retention, enhances microbial habitat	[37,38]

.

3.2. Composting and Co-Composting: The Dominant Pathway

Among the conversion routes shown in Figure 2, composting remains the most accessible and widely practiced method for stabilizing fish waste into a land-applicable product [39]. The process relies on thermophilic aerobic decomposition: under proper aeration and moisture management, temperatures reach 50–65 °C over 20–60 days, which is sufficient to inactivate pathogens and weed seeds while jump-starting the humification process. However, fish waste alone is notoriously difficult to compost. Its moisture content hovers around 70–90%, and its C/N ratio of 3–7 is far below the 25–30 ideal for rapid composting. Without amendment, the material compacts, turns anaerobic, and emits offensive odors and ammonia [40]. These operating conditions should be treated as indicative rather than sufficient proof of sanitization: effective pathogen reduction requires adequate temperature–time exposure throughout the pile, while inadequate aeration or moisture control can increase odour, methane, nitrous oxide, and ammonia losses, and nitrogen volatilization also reduces fertilizer value [6,41].

The solution is co-composting with carbon-rich bulking agents. Agricultural residues such as straw, sawdust, and corn stover are commonly used, but seaweed deserves special mention. Co-composting with seaweed is a particularly elegant approach: the seaweed absorbs excess moisture, provides complementary potassium and magnesium, contributes polysaccharides that improve soil structure, and adds a balanced carbon source that brings the C/N ratio into a manageable range. The evaluation of compost from seaweed and fish waste demonstrated that the final product possessed excellent physical properties, including enhanced water retention and the presence of humic substances that stimulate root development [42]. Agronomic trials on organic potato crops showed that seaweed-fish compost significantly increased tuber yield and soil microbial biomass compared to unamended controls, confirming that marine-derived nutrients work synergistically when returned to soil [43]. Beyond potatoes, humic acids extracted from fish-based composts have been shown to enhance lettuce growth by chelating micronutrients and stimulating root plasma membrane H⁺-ATPase activity, providing a mechanistic basis for the observed growth promotion [44]. The composting of fish offal and biosolids in northwestern Patagonia offers a real-world example of how decentralized, low-technology composting can serve remote fishing communities—turning a disposal problem into a local soil amendment with minimal capital investment [45]. However, the required bulking-agent ratio is site- and feedstock-specific. Seaweed can introduce salts and high electrical conductivity, while excessive lignocellulosic bulking material can slow stabilization. Moisture, C/N ratio, aeration, salinity, maturity indices, and crop response should therefore be optimized jointly rather than maximizing fish waste inclusion [43,46].

3.3. Liquid Fish Fertilizers: Rapid Nutrient Delivery

For applications requiring faster nutrient availability, solid composting is not always the best fit. Liquid fish fertilizers—produced through hydrolysis and microbial fermentation—fill this niche by delivering amino acids, short-chain peptides, and soluble minerals directly to plant leaves and roots. This bypasses the weeks-to-months mineralization lag typical of solid organics, making liquid fish products suitable for foliar feeding, drip irrigation, and even hydroponic systems. The main constraints are batch-to-batch nutrient variability, odour and microbial stability during storage, possible biogenic amine formation, salinity, and the need for hygienic control. Fermentation/hydrolysis conditions, filtration, storage stability, and nutrient analysis should therefore be standardized before fertigation or foliar use, especially when products are applied to food crops [6,42].

The scaled-up bioconversion of fish waste to liquid fertilizer using a 5 L ribbon-type reactor demonstrated that controlled fermentation can achieve nearly complete liquefaction of fish biomass within a few days, with the final product stable enough for storage and distribution [47]. The microorganisms driving this process need not be exotic; researchers have identified gut-associated bacteria from earthworm viscera—including Bacillus and Lactobacillus species—that accelerate protein hydrolysis and suppress spoilage pathogens, effectively improving both product quality and shelf life [48]. Field-level evidence supports the efficacy of these liquids. The bioconversion of marine trash fish to organic liquid fertilizer and its application on tomato showed significant improvements in plant height, leaf area, and fruit yield, with performance comparable to or exceeding that of commercial chemical fertilizers [49]. A direct comparison of chemical fertilizer, fish offal fertilizer, and manure on tomato and onion further demonstrated that fish-derived organic fertilizer achieved comparable yields while significantly improving soil microbial activity and organic carbon content—an advantage that chemical fertilizers simply cannot replicate [50]. The traditional fermented fish waste preparation known as Gunapaselam has also been documented to enhance eggplant vegetative growth and flowering, suggesting that indigenous fermentation knowledge can be validated and scaled into modern biofertilizer production [51].

3.4. Fish Bone Phosphorus as Fertilizer: Closing the P Cycle

Perhaps the most strategically important component of fish waste fertilizer is the phosphorus concentrated in bones and scales. With global phosphate rock reserves depleting and geopolitical supply risks mounting, recovering phosphorus from organic waste streams is no longer just an environmental priority—it is a food security imperative. Fish bones contain phosphorus as hydroxyapatite, a calcium phosphate mineral that can be converted into a slow-release fertilizer through calcination or low-temperature pyrolysis. This approach has been explored as a low-tech, circular-economy solution for regions with limited access to commercial fertilizers [52].

The agronomic performance of fish-derived phosphorus is well documented. The effect of dewatered fish sludge and manure solids as phosphorus fertilizer demonstrated that fish-derived P can match triple superphosphate in use efficiency on certain soils, confirming that the organic matrix does not necessarily impede plant uptake [53]. This finding underscores the broader potential of fish waste to help close global phosphorus cycles and reduce dependence on mined phosphate [3]. Nevertheless, phosphorus availability depends on particle size, thermal history, hydroxyapatite crystallinity, and soil pH. Excessive calcination or high-temperature pyrolysis can lower P solubility, whereas insufficient treatment may leave unstable organic matter. Plant-available P and crop response therefore provide a more meaningful basis for application than total P concentration alone [48,53].

When fish bones are pyrolyzed rather than merely calcined, the result is a biochar with a unique dual identity: a porous carbon matrix and an embedded calcium phosphate skeleton. This bone biochar resists phosphorus fixation in acidic soils—a major limitation of conventional water-soluble P fertilizers—and simultaneously improves water retention and provides habitat for beneficial soil microorganisms [41]. It is a soil amendment and a fertilizer in one material, an example of the multifunctional products that integrated fish waste biorefineries can deliver.

4. Fish Waste to Biochar

4.1. Production and Unique Physicochemical Characteristics

Biochar derived from fish bones and scales is produced through pyrolysis [54] or carbonization at 400–800 °C under an inert atmosphere (N₂ or limited O₂), following washing, drying, and size reduction in the raw feedstock (Figure 3). The resulting biochar exhibits a distinctive mineralogy compared to lignocellulosic-derived biochars: the natural hydroxyapatite content of fish bones and scales is partially retained during pyrolysis, creating a porous, calcium- and phosphorus-rich carbon matrix with abundant surface functional groups [7,55].

Table 2. Summary of biochar production from fish waste components and their key properties.

Feedstock	Pyrolysis Temp. (°C)	Yield (%)	Surface Area (m2/g)	Pore Volume (cm3/g)	Major Minerals	Key Application	Reference
Fish bone	400–600	35–50	50–200	0.05–0.20	Ca5(PO4)3OH, CaCO3	Phosphorus fertilizer, soil amendment	[46]
Fish scale	300–700	30–55	100–800	0.10–0.50	Hydroxyapatite, collagen-derived C	Heavy metal adsorption, catalysis	[6]
Fish waste (mixed)	350–550	25–45	20–150	0.03–0.15	Ca, P, Mg, K	General adsorbent, soil amendment	[40]
Tilapia bone	500–700	40–60	80–250	0.08–0.25	Ca-P complexes	Tetracycline adsorption	[42]

summarizes the production conditions and key properties of biochar from various fish waste components. Higher pyrolysis temperatures generally increase surface area and carbon content but reduce oxygen-containing functional groups; for fish bone biochar, the degree of hydroxyapatite crystallization and porosity is strongly temperature-dependent [46]. A major process limitation is that conventional pyrolysis requires prior drying of high-moisture fish residues and can therefore impose a substantial energy penalty; mineral-rich feedstocks also yield ash-rich chars with lower carbon yield than lignocellulosic biomass. Recent hydrothermal carbonization (HTC) of fish waste offers an alternative for wet feedstocks because it avoids complete pre-drying, but heat recovery, pressurized equipment, process-water treatment, and the fate of dissolved organics and nutrients must be included in system assessment [15].

4.2. Soil Amendment and Phosphorus Delivery

The primary agronomic application of fish bone biochar lies in its dual function as a soil amendment and a slow-release phosphorus fertilizer. When incorporated into soil, the porous carbon matrix improves water retention, aeration, and cation exchange capacity, while the embedded calcium phosphate phases release P in a controlled manner that resists fixation in acidic soils [41]. The thermal conversion of fish bones into fertilizers and biostimulants has been demonstrated as a low-tech, circular-economy approach suitable for regions with limited access to synthetic fertilizers [52]. Changes in heavy metal bioavailability from soil amended with biochars further indicate that fish bone biochar can simultaneously stabilize soil contaminants and provide nutrients, making it a multifunctional soil amendment [56]. Application rates must nevertheless account for ash content, alkalinity, salinity, nutrient-release kinetics, and soil type. Long-term field evidence remains limited, so carbon persistence, nutrient availability, and contaminant mobility should be assessed together rather than inferred from short-term pot experiments.

4.3. Environmental Remediation: Heavy Metal and Organic Pollutant Removal

Fish waste biochar exhibits exceptional performance in environmental remediation due to its high surface area, phosphate-rich surface chemistry, and abundant functional groups. For heavy metal removal, fishbone and fishbone-derived biochar have demonstrated high adsorption capacities for cadmium, lead, and other toxic metals, with phosphate groups forming stable surface complexes with metal cations [57]. Fish scale biochar has been successfully applied for fluoride removal from groundwater through ion exchange and surface complexation with calcium species [58], and for uranium extraction via temperature-tuned porous structures [59,60]. For organic pollutants, tetracycline adsorption by tilapia fish bone-based biochar has been optimized with mass transfer modeling [61], while fish-scale biochar functionalized with H₃PO₄ showed ultra-efficient adsorption of diclofenac sodium through synergistic surface complexation and pore-filling mechanisms [62]. Additionally, fish scale-based biochar with defined pore sizes has achieved highly efficient adsorption of ciprofloxacin from aqueous solution [63]. Most adsorption capacities, however, are measured in controlled synthetic solutions. Competing ions, natural organic matter, pH, and complex wastewater matrices can reduce performance, while regeneration, desorption, spent-sorbent management, and possible phosphate or metal release affect practical sustainability [57,61].

4.4. Catalytic Applications in Advanced Oxidation Processes

Beyond passive adsorption, fish waste biochar is increasingly recognized as a functional catalyst support and catalyst for advanced oxidation processes (AOPs). The natural N and P content of fish scales enables self-doped, heteroatom-rich carbon frameworks that activate peroxymonosulfate (PMS) and peroxydisulfate to generate reactive sulfate radicals (•SO₄⁻) and hydroxyl radicals (•OH) for degrading recalcitrant organic pollutants [64,65]. The transformation of fishbone biowaste into biochar as an efficient persulfate catalyst has been demonstrated for the degradation of organic pollutants, with the phosphate-rich surface facilitating radical generation [64]. Furthermore, functionalized Fe-N co-doped fishbone biochar has been applied for the remediation of phthalate-contaminated marine sediment, highlighting the potential for in situ environmental remediation [66]. Biphasic apatite-carbon materials derived from pyrolysed fish bones have also shown effective adsorption of persistent pollutants and heavy metals, combining mineral precipitation and carbonaceous adsorption mechanisms [67]. Catalytic applications further require attention to oxidant consumption, active-site deactivation, catalyst recovery, leaching, reusability, and transformation-product toxicity. These factors may dominate operating cost and environmental impact in real water treatment even when high pollutant-removal percentages are obtained in batch tests.

5. Fish Waste to Biodiesel

5.1. Feedstock Characteristics and Oil Extraction

Fish waste contains 5–30% oil depending on species, rendering it a promising non-edible feedstock for biodiesel production that avoids food-fuel competition [68]. The crude fish waste oil is characterized by high free fatty acid (FFA) content (>2–3 wt%), substantial phospholipids, and a fatty acid profile dominated by long-chain polyunsaturated fatty acids (PUFAs) such as EPA and DHA, which confer excellent lubricity and cetane number but compromise oxidative stability [69,70]. Oil extraction is typically achieved via pressing, thermal rendering, solvent extraction, or supercritical CO₂; the latter offers a solvent-free alternative that preserves oil quality while eliminating downstream purification steps [71]. Marine oils have been physicochemically characterized as potential biodiesel feedstocks, with waste fish oil from processing plants representing a low-cost, geographically concentrated resource in coastal regions [4,72,73].

5.2. Transesterification Routes: Three Process Pathways

As illustrated in Figure 4, three dominant process routes have been developed to convert fish waste oil into fatty acid methyl esters (FAME), each addressing the high-FFA challenge differently.

Table 3. Comparison of biodiesel production from fish waste oils.

Feedstock Source	Oil Content (%)	Catalyst	Methanol/Oil Ratio	Temp. (°C)	Yield (%)	Reference
Rohu processing waste	12–18	H2SO4/NaOH	9:01	60	92.5	[64]
Fishmeal plant waste	8–15	KOH	6:01	65	89.3	[65]
Mixed non-edible + fish oil	10–20	KOH	12:01	55	95.2	[64]
Fish waste (transesterification)	15–25	Clay (heterogeneous)	15:01	350	85.6	[7]
Fish waste oil	18–22	Nano-magnetic	9:01	60	94.1	[66]
Fish waste oil (supercritical)	20–30	None	40:01:00	280	91.8	[67]

compares representative process conditions and yields.

(1)

Acid–base two-step process is the most established route for high-FFA fish waste oil. Acid esterification with H₂SO₄ reduces FFA to <1 wt% (50–65 °C, 1–2 h), followed by alkaline transesterification with NaOH/KOH to produce high-purity FAME and glycerol [74]. While technically mature and scalable, this approach generates acidic wastewater and requires neutralization steps.

(2)

Heterogeneous catalysis eliminates the need for liquid catalysts and simplifies separation. A particularly innovative strategy is the use of calcined fish scales (CaO) as a low-cost, waste-derived solid catalyst. Fish scales are rich in calcium phosphate; calcination at 700–900 °C converts them into active CaO, which catalyzes one-step transesterification at 60–65 °C with easy recovery and reusability [8]. This approach exemplifies true circularity: the waste material itself becomes the catalyst for fuel production. Nano-magnetic catalysts (e.g., CaO/Fe₃O₄) have also been developed, enabling magnetic separation and high yields (~94%) at moderate temperatures [75].

(3)

Supercritical methanol (SCM) process operates at 240–350 °C and 15–30 MPa without any catalyst, achieving rapid conversion (10–30 min) regardless of FFA or water content [76]. This route eliminates catalyst costs and separation steps but demands higher energy input and pressurized equipment. The pyrogenic transformation of oil-bearing biomass into biodiesel without lipid extraction represents a related thermal approach that could be adapted for fish waste [77].

5.3. Fuel Properties and Emission Performance

The lipid content of fish waste varies substantially with species and tissue fraction, rendering it a promising non-edible feedstock for biodiesel production that avoids food-fuel competition [68]. The crude fish waste oil is characterized by high free fatty acid (FFA) content (>2–3 wt%), substantial phospholipids, and a fatty acid profile dominated by long-chain polyunsaturated fatty acids (PUFAs) such as EPA and DHA, which can improve lubricity but may compromise oxidative stability [69,70]. Oil extraction is typically achieved via pressing, thermal rendering, solvent extraction, or supercritical CO₂; the latter avoids conventional organic extraction solvents but requires pressurized equipment and subsequent separation and purification [71]. Marine oils have been physicochemically characterized as potential biodiesel feedstocks, with waste fish oil from processing plants representing a low-cost, geographically concentrated resource in coastal regions [4]. This wide range itself is a major design constraint: species, season, tissue fraction, storage, and rendering history alter oil yield, free fatty acid content, moisture, and fatty-acid composition. Low-oil wastes can become uneconomic once extraction, drying, solvent recovery, and refining are included, whereas high-FFA oils require pretreatment before alkaline transesterification [13,17].

6. Fish Waste as Feed

6.1. Nutritional Potential and Value of Fish Waste as Feedstock

Fish waste is an exceptionally valuable feedstock for animal nutrition, unlike plant proteins, fish-derived proteins possess high digestibility (>90% in many species) and favorable essential amino acid ratios that closely match the requirements of fish, poultry, and swine [78]. The lipid fraction is particularly notable for its high concentration of long-chain omega-3 polyunsaturated fatty acids (EPA and DHA), which confer immunomodulatory, anti-inflammatory, and growth-promoting properties in farmed animals [79]. The bioactive compounds from marine processing byproducts—including peptides, enzymes, and minerals—have been reviewed as functional ingredients that can enhance feed conversion efficiency and disease resistance [78]. However, the rapid autolysis and spoilage of fresh fish waste necessitate immediate preservation through acidification, fermentation, or drying to prevent protein degradation and biogenic amine accumulation (particularly histamine) [80]. The suitability of a particular waste stream for feed nevertheless depends on freshness, traceability, salt and ash contents, lipid oxidation, contaminants, pathogen control, and species-specific nutritional requirements. Composition and digestibility vary with fraction and processing, so replacement levels established for one fish waste product or animal species should not be generalized [9].

6.2. Fish Silage: Preservation, Processing, and Nutritional Quality

Fish silage is the most widely adopted and cost-effective method for preserving fish waste as a liquid or semi-solid feed ingredient. The process involves acidifying the waste with mineral acids (formic, sulfuric, or propionic acid) or fermenting with lactic acid bacteria (LAB) in the presence of a carbohydrate source (molasses, fruit pomace) to achieve a pH of 3.5–4.5, which inhibits spoilage bacteria while allowing endogenous proteases to hydrolyze proteins into peptides and free amino acids [81]. The production and utilization of fish silage have been extensively reviewed by FAO, establishing it as a technically mature strategy for small-scale and industrial operations alike [82]. Operational challenges include rapid initial spoilage, the need for prompt pH control, corrosion and safe acid handling, carbohydrate requirements for biological silage, biogenic amine control, and the high mass and transport cost of a wet product. Drying improves storage and handling but can add substantial energy demand and may alter nutrient quality [80,83].

The use of fish waste to silage preparation and its application in animal nutrition has been comprehensively evaluated, demonstrating that properly preserved fish silage maintains nutritional quality for 6–12 months without refrigeration [84]. The role of selected lactic acid bacteria on organic acid accumulation during wet and spray-dried fish-based silages revealed that Lactobacillus and Pediococcus strains can dominate fermentation, producing sufficient lactic acid to suppress pathogens while preserving protein integrity [85]. The chemical and nutritional qualities of dried fermented fish silages have been established for tilapia feeds, with protein digestibility comparable to or exceeding that of conventional fish meal [86]. The biotransformation of fish waste into a stable feed ingredient through controlled acidification has shown that pH reduction arrests microbial spoilage while enhancing protein solubility [80]. Silage production from fish waste in cannery factories using mineral acid, organic acid, and biological methods demonstrated that all three approaches yield stable products, though biological methods are preferred for organic certification [87]. Representative fish waste-derived feed products and their nutritional applications are summarized in

Table 4. Fish waste-derived feed products and their nutritional applications.

Product	Production Method	Key Nutrients	Target Species	Inclusion Level/Effect	Reference
Acid-preserved fish silage	Mineral/organic acid (pH 3.5–4.5)	Protein (15–20%), lipids	Tilapia, catfish	Up to 50% fishmeal replacement without growth loss	[84]
LAB-fermented fish silage	Lactic acid bacteria + carbohydrate	Protein, organic acids, peptides	Shrimp, broilers	Enhanced growth, gut health, feed conversion	[85]
Fish protein hydrolysate	Enzymatic/acid hydrolysis	Peptides, free amino acids	Sea bass, barramundi	Improved immunity, non-specific disease resistance	[86]
Fish silage oil	Oil extraction from acid silage	Omega-3 (EPA/DHA)	Tilapia, abalone	Alternative to fish oil; enhances immunity	[76]
Co-dried fish silage	Mixing with absorbent + drying	Protein, minerals	General aquaculture	Extended shelf life, reduced moisture, easy handling	[87]

.

6.3. Fish Protein Hydrolysate and Functional Feed Applications

Beyond simple preservation, enzymatic or acid hydrolysis of fish waste can produce fish protein hydrolysate (FPH)—a highly digestible, bioactive ingredient containing small peptides and free amino acids that are readily absorbed across the intestinal epithelium. The effects of different levels of fish protein hydrolysate in the diet on the non-specific immunity of Japanese sea bass demonstrated that FPH supplementation enhanced lysozyme activity and disease resistance, likely due to the presence of immunomodulatory peptides [88]. The growth, biochemical response, and liver health of juvenile barramundi fed fermented and non-fermented tuna hydrolysate confirmed that fermented hydrolysate could effectively replace fishmeal while improving gut histology and antioxidant status [89]. The evaluation of co-fermented apple-pomace, molasses, and formic acid-generated sardine-based fish silages as fishmeal substitutes for juvenile European sea bass revealed that co-fermentation with fruit byproducts improved palatability and nutritional balance [90]. Fish viscera silage has also been characterized for tambaqui juveniles, showing high nutrient digestibility and energy availability, confirming that viscera—a major waste fraction—can be efficiently converted into a high-quality feed [91]. Furthermore, fish waste-derived feeds have been successfully applied across a broad taxonomic range. In aquaculture, rainbow trout silage has been evaluated as an immune stimulant and feed ingredient for Mozambique tilapia, with positive effects on growth performance and immune parameters [92]. Fish silage oil from rainbow trout processing waste has been used as an alternative to conventional fish oil in formulated diets, providing essential fatty acids and reducing feed costs. Rainbow trout silage oil has also been shown to enhance immunity in South African abalone (Haliotis midae), demonstrating cross-species applicability [79]. In poultry, the evaluation of increasing concentrations of fish waste silage in diets for broiler chickens showed that inclusion up to 10–15% improved growth performance and intestinal morphology [93]. The preparation of silage from Spanish mackerel and its evaluation in broiler diets further confirmed acceptability and nutritional adequacy [94]. In livestock, the chemical composition and standardized ileal digestibility of protein and amino acids from fish silage in pigs have been determined, confirming excellent protein quality for monogastric animals [95]. The processing and evaluation of nutritive value of fish silage for feeding Omani sheep demonstrated that properly preserved fish silage is safe and nutritious for ruminants [96]. The effect of dietary supplementation of fermented fish silage on serum biochemical parameters of broiler Japanese quails showed positive health biomarker profiles [97]. In crustacean aquaculture, the dietary replacement of fishmeal by fish silage in Pacific white shrimp (Litopenaeus vannamei) indicated that moderate replacement levels (up to 25–30%) could be achieved without compromising growth, with mTOR signaling pathways implicated in the growth response [98]. The microbial fermentation of abalone waste has been shown to improve digestibility, gut health, and immunity in freshwater crayfish (Cherax cainii), highlighting the potential of processing waste from one aquatic species to feed another [99]. Commercial use also requires control of enzyme cost, degree of hydrolysis, bitterness, peptide-size distribution, microbial stability, and drying or concentration energy. The optimal inclusion rate remains diet- and species-specific, and nutritional benefits must be balanced against amino-acid profile, lipid oxidation, mineral load, and regulatory limits.

In conclusion, fish waste represents an outstanding, underutilized protein resource for animal feed. Through acid preservation, lactic fermentation, enzymatic hydrolysis, or co-drying, highly perishable fish waste can be stabilized into nutritionally equivalent or superior alternatives to conventional fish meal and fish oil. Future research should focus on optimizing inclusion rates for different species, developing low-energy drying protocols, and establishing quality control standards for histamine and biogenic amines to ensure regulatory compliance and market acceptance.

7. Fish Waste to Other High-Value Products

7.1. Collagen, Gelatin and Bioactive Peptides

Fish skin and scales represent one of the most valuable and underutilized sources of collagen and gelatin in the food, cosmetic, and pharmaceutical industries. Collagen is the most abundant structural protein in fish connective tissue, and its extraction from processing waste—particularly salmon, cod, and tilapia skins—has gained substantial commercial interest. Unlike mammalian-derived collagen, marine fish collagen is free from religious constraints (kosher/halal limitations), exhibits lower immunogenicity, and possesses a unique amino acid composition rich in glycine, proline, and hydroxyproline [10]. The cosmetic potential of marine fish skin collagen has been explored for skincare, wound healing, and tissue engineering applications, with marine collagen showing superior moisture retention and cell compatibility compared to terrestrial sources [100]. However, collagen yield, molecular integrity, gel strength, thermal stability, color, and odor depend on species, body fraction, pretreatment, and extraction conditions. Marine collagen often has lower thermal stability than mammalian collagen, and acid/alkali extraction, purification, desalting, and drying generate water and chemical demands that must be considered at scale.

Gelatin and hydrolysates derived from farmed salmon skin by-products have been characterized, revealing excellent functional properties including gel strength, emulsification, and foaming capacity [101]. The protective effect of gelatin and gelatin hydrolysate from salmon skin on UV irradiation-induced photoaging of mice skin demonstrated that marine collagen peptides can inhibit matrix metalloproteinase activity and promote dermal collagen synthesis, suggesting applications in anti-aging cosmeceuticals [102]. Beyond structural applications, Atlantic salmon co-product-derived protein hydrolysates have been identified as a source of antidiabetic peptides, with in vitro and in vivo studies showing dipeptidyl peptidase-IV (DPP-IV) inhibitory activity comparable to commercial antidiabetic drugs [103]. The improvement of glycemic control in streptozotocin-induced diabetic rats by Atlantic salmon skin gelatin hydrolysate confirmed these bioactive properties, positioning fish waste as a feedstock for functional food and nutraceutical development [104]. Atlantic salmon waste has been further identified as a unique source of biofunctional protein hydrolysates, with emerging production technologies and promising applications in food and pharmaceuticals [105]. The valorization of aquaculture by-products of salmonids to produce enzymatic hydrolysates has demonstrated that process optimization and chemical characterization can yield peptides with confirmed antioxidant, antihypertensive, and antimicrobial activities [106].

7.2. Chitin, Enzymes and Marine Bioactive Compounds

Fish scales and crustacean processing shells (shrimp, crab) are rich in chitin, a natural polysaccharide that can be deacetylated to chitosan—an invaluable biopolymer with extensive applications in water treatment, food preservation, agriculture, and biomedicine. While chitin extraction from fish scales is less established than from crustacean shells, the natural calcification of fish scales (calcium phosphate-chitin composite) offers unique advantages for bone tissue engineering scaffolds and controlled-release drug delivery systems [78]. The bioactive compounds from marine processing byproducts, including chitin derivatives, peptides, and pigments, have been comprehensively reviewed as valuable resources for the functional food and pharmaceutical industries.

Fish waste is also a source of industrial enzymes. Fish scales have been identified as a potential substrate for the production of alkaline protease and amino acid-rich aqua hydrolysate by Bacillus altitudinis, demonstrating that the waste itself can support microbial enzyme production for downstream bioprocessing [107]. The preparation, properties and preservation of lactic acid fermented shrimp heads further established that crustacean waste streams can be simultaneously preserved and valorized for chitin and protein extraction [108]. These enzymes can be used in detergent formulation, leather processing, and food protein hydrolysis, creating an internal demand within the biorefinery framework. For enzyme production, variable substrate composition, sterilization, fermentation control, recovery, and purification can outweigh the low cost of the waste substrate. The value of isolated enzymes should therefore be compared with less processing-intensive uses of the same protein fraction.

7.3. Advanced Materials and Non-Traditional Applications

Beyond biological and chemical products, fish waste has found innovative applications in construction and advanced materials science. Recycling fish scale powder in improving the performance of asphalt has been explored as a sustainable utilization strategy, with scales acting as a natural polymer modifier that improves the rheological properties, rutting resistance, and thermal stability of asphalt mixtures [109]. The organic-inorganic composite structure of fish scales (collagen-calcium phosphate) provides crack-bridging and self-healing capabilities that are superior to conventional mineral fillers.

A sustainable approach to scalable production of a graphene-based flame retardant using waste fish deoxyribonucleic acid (DNA) has been reported, exploiting the high phosphorus and nitrogen content of fish DNA to create advanced carbon materials [110]. Fish DNA is rich in phosphate backbone groups, which are effective flame-retardant elements, making it an excellent precursor for synthesizing nitrogen-phosphorus co-doped graphene. This represents a cutting-edge example of molecular-level valorization, where the genetic material of fish waste is transformed into high-performance functional materials.

The lipid fraction of fish waste also contains valuable phospholipids and sterols. A review on phospholipids and their main applications in drug delivery systems highlighted the biomedical potential of marine phospholipids, which form stable liposomes and enhance bioavailability of hydrophobic drugs [111]. The role of sterols in membranes, established in foundational lipid biochemistry, underpins the biological value of fish-derived sterols as functional food ingredients and pharmaceutical precursors [112].

Table 5. High-value products from fish waste and their applications.

Product Category	Source (Fish Waste Fraction)	Production Method	Key Properties	Target Applications	Reference
Collagen/Gelatin	Skin, scales, bones	Acid/alkaline extraction, enzymatic hydrolysis	Low immunogenicity, high biocompatibility	Cosmetics, wound healing, food packaging	[99]
Bioactive peptides	Skin, muscle residues	Enzymatic hydrolysis, fermentation	DPP-IV inhibition, antioxidant, antimicrobial	Functional foods, nutraceuticals, pharmaceuticals	[103]
Chitin/Chitosan	Scales, crustacean shells	Deproteinization, demineralization, deacetylation	Biocompatibility, antimicrobial, film-forming	Water treatment, biomedical scaffolds, food packaging	[75]
Industrial enzymes	Viscera, scales	Microbial fermentation	Protease, lipase, chitinase activity	Detergent, leather, food processing	[105]
Asphalt modifier	Scales	Milling, powdering	Organic-inorganic composite, thermal stability	Road construction, pavement engineering	[107]
Flame retardant	DNA from waste tissue	Pyrolysis, carbonization	N-P co-doped carbon, intumescent char	Polymer additives, construction materials	[108]
Phospholipids/Sterols	Oil fraction	Solvent extraction, supercritical CO2	Liposome formation, membrane stability	Drug delivery, functional foods	[109]

summarizes the principal high-value products derived from fish waste, their production methods, and target applications.

8. Cross-Pathway Comparison, Route Selection, and Sustainability Trade-Offs

8.1. Feedstock-Driven Route Selection and Critical Comparison

No single valorization pathway is universally preferable. Route selection should balance the biochemical composition and hygienic quality of the waste against product value, capital intensity, energy and chemical demand, scale, local infrastructure, and market access. Clean and fresh protein-rich fractions can justify feed or high-value protein recovery because these retain more embedded value than conversion to energy. Lipid-rich streams favor oil extraction and biodiesel when oil content is sufficiently high and oxidation is controlled. Dilute, mixed, or lower-value putrescible organics are better suited to anaerobic digestion, particularly where heat/power or biomethane can be used locally. Bone- and scale-rich fractions favor phosphorus recovery, biochar, collagen/mineral materials, or hydrothermal processing. Composting is attractive where land application is available and low capital expenditure is important, whereas pyrolysis/HTC produces more transportable carbon/mineral products but requires thermal equipment.

8.2. Environmental and Economic Considerations: From Technical Yield to LCA and TEA

Life-cycle assessment (LCA) is therefore needed to quantify burden shifting across waste collection, transport, preprocessing, conversion, energy use, chemicals, emissions, and product substitution. Results are highly sensitive to the functional unit, system boundary, allocation of avoided waste treatment, and credits for displaced energy, feed, fertilizer, or materials. Techno-economic analysis (TEA) should complement LCA by reporting capital expenditure, operating expenditure, feedstock logistics, energy and chemical prices, product revenues, scale, capacity factor, and sensitivity to seasonal supply and co-product prices. Existing economic analyses show that integrated biorefineries can outperform single-product systems, but profitability is strongly dependent on local product prices and policy assumptions [37]. Recent 2025–2026 reviews likewise identify harmonized mass/energy balances, continuous-scale data, TEA, and LCA as priorities before fish waste valorization can be described as sustainable at industrial scale [16,17].

A technically high conversion yield does not necessarily correspond to the lowest environmental burden or the strongest business case. Anaerobic digestion generally has modest external energy demand but may require mixing, heating, pretreatment, gas upgrading, digestate management, and co-substrate transport [16]. Composting has low capital intensity but can lose reactive nitrogen and emit NH₃, CH₄, and N₂O when aeration and moisture are poorly controlled [12]. Pyrolysis requires drying and process heat, whereas HTC avoids extensive pre-drying but uses pressurized hot water and generates an aqueous stream requiring treatment or recovery. Biodiesel adds oil extraction, alcohol, catalyst, washing/separation, and solvent recovery, while supercritical routes trade catalyst use for substantially higher temperature and pressure [69]. Feed and high-value recovery can preserve greater material value, but refrigeration, acidification, enzymatic processing, purification, and especially drying may dominate energy and operating costs [14].

9. Challenges and Future Prospects

Despite the extensive research on fish waste valorization, several challenges hinder industrial-scale implementation. For biogas production, ammonia inhibition remains the primary technical barrier, requiring advanced reactor designs, bioaugmentation with tolerant microbial consortia, or the integration of biochar for enhanced process stability. For biodiesel production, the high unsaturation and FFA content of fish waste oil necessitate complex pretreatment or specialized catalysts, increasing production costs. For feed applications, the risk of biogenic amine accumulation (particularly histamine) and the seasonal variability of fish waste supply pose challenges for consistent product quality.

Future research should prioritize: (1) Process integration: Developing multi-product biorefineries that synergistically combine biogas, biodiesel, feed, fertilizer and biochar production to maximize economic returns and minimize waste. (2) Advanced pretreatment: Investigating microwave, ultrasonic, and enzyme-assisted pretreatments to improve the efficiency of lipid extraction, protein hydrolysis, and anaerobic digestion. (3) Circular economy metrics: Conducting comprehensive life cycle assessments and techno-economic analyses to identify the most sustainable and profitable valorization routes for different fish waste streams. (4) Policy and regulation: Establishing quality standards for fish waste-derived products, particularly feed and fertilizer, to facilitate market acceptance and international trade. (5) High-value product extraction: Advancing green chemistry approaches, including the use of deep eutectic solvents and supercritical fluids, for extracting collagen, bioactive peptides, and other nutraceuticals with minimal environmental impact.

The sustainable integrated in situ transesterification of microalgae for biodiesel production and associated co-products has been reviewed, providing a model for how fish waste biorefineries could be designed to maximize resource recovery [111]. The effect of moisture on in situ transesterification of microalgae for biodiesel production highlights the importance of process parameter optimization [112,113]. Lipid recovery from wet oleaginous microbial biomass for biofuel production has been critically reviewed, with insights applicable to fish waste oil extraction [114,115].

10. Conclusions

Fish waste represents a significant and underutilized resource that can be converted into a diverse array of energy carriers and value-added products through biological, thermal, and chemical processes. This review has systematically examined the current status and future prospects of fish waste valorization into biogas, fertilizer, biochar, biodiesel, animal feed, and high-value products including collagen, gelatin, and bioactive peptides. Anaerobic digestion of fish waste offers a mature pathway for biogas production, but its efficiency is constrained by ammonia inhibition, which can be mitigated through co-digestion with carbon-rich substrates and the addition of conductive biochar. Fish waste composting and hydrolysis produce nutrient-dense fertilizers that can support sustainable agriculture while closing nutrient loops. Biochar derived from fish bones and scales exhibits exceptional adsorption and catalytic properties due to its unique phosphate-rich mineral composition, making it valuable for environmental remediation. Biodiesel production from fish waste oil is technically feasible, but economic viability depends on catalyst development and process integration. Fish silage and protein hydrolysates remain important feed ingredients, though quality control and preservation technologies require further optimization. The extraction of collagen, gelatin, and bioactive peptides represents the highest-value valorization pathway, but scalability and cost remain significant barriers.

The future of fish waste management lies in integrated biorefineries that combine multiple conversion pathways to maximize resource recovery and minimize environmental impact. By transitioning from linear waste disposal to circular bioeconomy models, the seafood industry can simultaneously reduce pollution, generate renewable energy, and create valuable products. Achieving this vision will require continued interdisciplinary research, technological innovation, and supportive policy frameworks that incentivize sustainable waste management practices. The comprehensive valorization of fish waste is not merely an environmental necessity but an economic opportunity that can enhance the resilience and sustainability of the global seafood industry.