1. Introduction
The growing demand for sustainable and health-promoting food sources has prompted a reevaluation of underutilized resources within the seafood industry. It is estimated that only 50 to 60% of the product of marine animal catch end up being used for direct human consumption, and therefore, the production and disposal of co-products raises problems related to industrial logistics, environmental impact and even human health [
1,
2,
3]. In an era where concerns about overfishing, waste generation and disposal, along with environmental sustainability, are paramount, the utilization of marine animal co-products emerges as a logical and appealing solution. The rational use of these co-products may present viable and relevant opportunities for both nutritional and environmental challenges, and the identification of additional valorization routes for these raw materials may also come with economic upside [
4]. In this review, we will explore the expanding field of marine animal co-products and their potential as sustainable sources of healthy and bioactive lipids, providing a holistic perspective linking health and environmental objectives. It aims to represent a preamble for researchers, policymakers, and industry stakeholders interested in exploring the potential of marine animal co-products as valuable healthy lipid sources with economic upside, while contributing to reduce and minimize ecological impacts that still persist in the seafood industry.
Marine animal co-products encompass a diverse range of materials generated during seafood processing, covering a variety of species, including fish, crustaceans, and mollusks. They include biological matrixes like fish/crustacean/mollusk heads, bones, skin, shells and viscera, at times discarded or still underutilized. This vast resource pool represents a potential reservoir of lipids, including essential fatty acids, omega-3 polyunsaturated fatty acids (PUFAs), and other bioactive lipids waiting to be explored. It is estimated that a third of total of the omega-3 PUFA eicosapentaenoic acid (EPA) + docosahexaenoic acid (DHA) originated from global capture and aquaculture operations may still go to waste [
5]. Therefore, considering the benefits generally described for marine animal fats, including counteracting manifestations of cardiovascular disease [
6,
7,
8,
9] and metabolic syndrome [
10,
11,
12,
13], as well as inflammation [
13,
14,
15,
16], while enhancing the cognitive function [
17,
18,
19], these co-products represent additional resources that may contribute to the mitigation of important widespread conditions promoted by modern lifestyle. Fat from some marine animal co-products (especially the ones from fish) are already explored for the production of fish oils, but these often represent low-value end products. Fish oils may be produced from fish co-products (especially viscera) through a number of different processes, including rendering, pressing, microwave-assisted extraction, supercritical fluid extraction, solvent extraction, autolysis, and enzymatic hydrolysis [
20]. The fish oil market represented a total of 1905.77 million dollars in 2019 and is continually growing (Coppola et al., 2021), with fish oil produced from fish co-products representing 26% of total production in 2016 [
21]. However, the exploration of lipid fractions for other purposes than oil production alone, including the retrieval of phospholipid-rich fractions, the prospection for bioactivities and the identification of bioactive lipids, may pave the way for the development of specialized products targeting specific applications and/or human conditions. This will promote the repurposing of these co-products for higher end applications, contributing to their valorization in additionally profitable markets. The diversity of marine co-products, as well as the multitude and origin of species and the processing methods involved, can result in variations in lipid composition. Therefore, a more thorough characterization of marine animal co-product lipids will enable researchers and industry stakeholders to better tailor their applications, whether for nutritional enrichment, functional food development, or pharmaceutical purposes.
Simultaneously, the sustainability aspect of marine animal co-product utilization cannot be understated. The seafood industry faces increasing challenges related to resource depletion and waste generation and disposal. Overfishing/overharvesting represent, obviously, the most overarching problems, with many species at risk of depletion due to unsustainable practices [
22,
23,
24,
25]. By repurposing marine animal co-products as valuable lipid sources, this approach contributes to waste reduction, contributes to marine ecosystem conservation, and supports a more responsible and holistic resource management. Furthermore, a strategic use of marine animal co-products could play a role in fostering the circular economy within the seafood industry, a vital step towards reducing the industrial ecological footprint.
4. The Value of Marine Animal Co-Product Lipids for Human Health
The lipid content of marine animal co-products is generally recognized as being nutraceutically-rich, including PUFAs, omega-3 fatty acids and lipid-soluble vitamins. A summary of the beneficial health effects ascribed to lipids present in marine animal co-products is presented in
Figure 4. The lipid content in marine animal co-products, although not predominant, is still very substantial, especially, as described before, in certain fish and shellfish species (can represent more than 25% of co-products of meagre (
A. regius) [
84], sea bass (
D. labrax) [
83,
85], chinook salmon (
O. tshawytscha) [
93], gilthead sea bream
(S. aurata) [
81,
84], sardinelle
(S. aurita) [
87], salmon (
S. salar) [
83] or bluefin thuna (
T. thynnus) [
86]), crustaceans (can amount to more than 10% DW in shrimp co-products of Argentine red shrimp (
P. muelleri) [
45] or whiteleg shrimp (
P. vannamei) [
122,
123], and more than 15% in some lobsters, namely Australian lobster (
P. cygnus) [
116] and Australian rock lobster (
J. edwardsii) [
117]) and mollusks (more than 10% in Patagonian squid (
D. gahi) [
58,
140], argentinean shortfin squid (
I. argentinus) [
141], giant squid
(D. gigas) [
109] or common octopus (
O. vulgaris) [
57] co-products). Obviously, the higher the lipid content, the more justifiable it is to explore and take advantage of these co-product matrixes as possible sources of healthy lipids. Interestingly, the lipid content in some of the co-products already profiled is comparable, or, at times, even higher than that reported to the edible portions of the same species, with this feature already having been documented for either fish [
148], crustaceans [
149,
150] or mollusks [
151]. A comprehensive study comprising fish, crustacean and mollusk species showed that this is consistently the case, with co-products being more lipid-rich than edible muscle portions in all cases [
152].
Lipids from marine animals are usually associated with a healthy content in PUFA, omega-3 fatty acids, namely EPA and DHA [
153], with DHA- and EPA-containing phospholipids being often referred to as “marine phospholipids” [
154,
155]. In fact, from a chemical standpoint, marine lipids are generally more varied in their fatty acid content, and present longer chain fatty acids (besides the aforementioned prevalence of PUFA, omega-3 and EPA and DHA in particular) with regard to terrestrial plants and animals [
156]. The fact that human metabolism of alpha-linolenic acid of plant-derived origins to synthetize EPA
de novo being negligible [
69], and that the metabolism of EPA to DHA being virtually non-existent [
157,
158,
159], implies that EPA and DHA must be acquired from diet, making marine sources of these fatty acids even more critical from a nutritional standpoint. Moreover, the current human consumption and plasma levels of EPA and DHA are generally considered deficient, and definitely lower than those considered to be ancestral values, [
160,
161]. An intake of up to 0.5 g of EPA + DHA per day is recommended by health agencies to foster the prevention of cardiovascular diseases and other metabolic disorders [
69,
162,
163,
164].
The benefits of PUFA consumption have always been highlighted, normally in opposition to the detrimental effects of excessive saturated fatty acid consumption (namely increased risk of cardiovascular disease and type-2 diabetes) [
165]. Lately, the benefits of PUFA have been increasingly focused on the content in omega-3 fatty acids, and in EPA and DHA in particular [
157,
166,
167,
168]. Omega-3 fatty acids have been elevated to essentially wide-specter nutraceuticals, with reported beneficial impacts in eye disease, bone health, fetal development, cardiovascular disease, diabetes, cancer treatment/prevention, cognitive function, neurodegenerative diseases and inflammation [
69,
169,
170,
171,
172,
173]. Even though the more immediate association between omega-3 fatty acids and health promoting benefits concerning cardiovascular disease, although this association has been a matter of some controversy as of late [
174,
175,
176]. Currently, there have been some steps towards the differentiation of the effects of EPA and DHA, with EPA being shown to present additional beneficial effects than EPA/DHA formulations, at least for some specific cases [
176,
177]. In fact, the pharmacologies of EPA and DHA are distinct, with divergent effects on membrane structure, on lipoprotein oxidation and on the production of downstream metabolites that modulate the resolution of inflammation [
177]. Marine animal co-products were reported to generally present substantial omega-3 fatty acid content, comparable to the edible parts of the animals, therefore making good alternative sources for these specific nutrients.
Among fish co-products, those from marbled rockcod (
Notothenia rossii, >30% omega-3 fatty acids [
100]), blue mackerel (
S. australasicus, >35% [
94]) and Atlantic bluefin tuna (
T. thynnus, 29.9% [
86]), the viscera from sardinelle (
S. aurita, 26.1% [
87]) and the co-products of Atlantic herring (
C. harengus, 26.5% [
99]) present especially good opportunities as omega-3 sources. In crustaceans, exoskeletons from southern king crab (
L. santolla, 40% [
45]), exoskeletons and heads of Argentine red shrimp (
P. muelleri, >40% [
45]), a processing co-product from the Northern shrimp (
P. borealis) containing 37.1% [
49] or the cephalothorax of the Norway lobster (
N. norvegicus, 27.6% omega-3 fatty acids) [
51], present the highest percentages of omega-3. Mollusk co-products may represent especially promising sources of omega-3 fatty acids, with Patagonian squid (
D. gahi) co-products containing up to 48.6% [
140] and the co-products of great scallops (
P. maximus) displaying a remarkable 40.7% of omega-3 fatty [
139]. Concerning EPA, a processing co-product from
P. borealis (21.1 % EPA) [
49] and the exoskeletons of southern king crab (
L. santolla, 20.5%) [
45] presented the highest percentual EPA contents in crustaceans, as did the co-products of wild scallops (
P. maximus, 20%[
139]), Patagonian squid (
D. gahi, 17.2% [
140,
143]) and pen shell scallop (
P. rugosa, 17% [
109]) in mollusks. As for DHA, the roe (and male gonads) of blue mackerel (
S. australasicus [
94]) shells and heads of Argentine red shrimp (
P. muelleri [
45]) and especially the co-products of mollusks especially Patagonian squid
(D. gahi [
143]) and common octopus (
O. vulgaris [
57]) can present contents higher than 20% of this particular omega-3 fatty acid. Another way to look at the benefits that lipids from marine animal co-products may entail, has to do with the low n-6/n-3 fatty acid ratios consistently reported. A low n-6/n-3 ratio has been ascribed to promote beneficial effects in inflammatory conditions, cancer and in cardiovascular and neurological disorders [
178,
179]. A ratio of 4–5 to 1 or lower is recommended, although it is reported to be normally much higher in western diets [
180,
181]. Therefore, with many co-products studied presenting extremely low n-6/n-3 ratios, they should present nutritional characteristics that are optimal to mitigate the pernicious effects of modern diets in western countries.
With the exception of a few specific cases mentioned before, like shrimp hepatopancreas (including
P. vannamei and
P. japonicus, [
48,
120,
126]), the cephalothorax of the Norway lobster (
N. norvegicus) [
51], or the livers of squids (
L. bleekeri,
B. magister,
D. gigas [
145,
146,
147]), phospholipids are generally the major lipid class in marine animal co-products. Like in the general case of phospholipids from marine sources, phospholipids from marine animal co-products are also enriched in PUFA, namely EPA and DHA [
155,
182], with these being mostly incorporated in the
sn-2 chain [
183]. Some reports state that omega-3 fatty acids bound to phospholipid are more efficiently absorbed and more efficiently delivered [
184,
185], outperforming triglycerides as omega-3 fatty acid carriers [
154,
186,
187,
188], which, in their turn, were suggested to be better carriers than omega-3 fatty acids in the ethyl ester form [
70]. This would make formulations rich in “marine phospholipids” more valuable than common fish oils, where omega-3 fatty acids are present mostly in the triglyceride form, and ethyl esters to a lesser extent [
70]. In agreement with this view, dietary approaches including omega-3 PUFA structured phospholipids did, in fact, reveal an increased efficiency in the improvement of human health parameters when compared to commercial fish oils [
154,
182,
189]. The fact that omega-3 containing phospholipids are normally ignored in the fish oil industry and frequently removed as an impurity during degumming processes [
190] should be a matter of reflection and a driver for the reevaluation of how these resources are being exploited and valued. These characteristics of marine phospholipids are thought to be instrumental to the effects of omega-3 containing phospholipids in common features of aging and chronic diseases, as is the case of inflammation phenotypes, oxidative stress, neurodegenerative disease and immune cell aging [
105,
185,
191,
192]. Moreover, phospholipids are pivotal for signal transduction in disease [
193], and those including EPA and/or DHA have been specifically proposed to feature several health-promoting effects, namely counteracting cardiovascular disease, improving the brain function and neurodegenerative conditions, presenting antitumor activity and regulating lipid and glucose metabolisms [
154,
182,
194,
195,
196,
197]. Phospholipids have uses in the food manufacturing industry, namely as emulsifiers, antioxidants and stabilizers [
155,
198].
Plasmalogens are a unique class of phospholipids, displaying a structure containing a fatty alcohol with a vinyl-ether bond at the
sn-1 position, while being enriched in polyunsaturated fatty acids at the
sn-2 position of the glycerol backbone [
199]. They are ubiquitous in animal membranes, both in invertebrates and vertebrates [
200]. In the few studies that characterized marine animal co-products lipids at a molecular level, and particularly in mollusks [
60,
139], plasmalogens were described to be present in considerable amounts, specifically phosphatidylcholine and phosphatidylethanolamine plasmalogens. Plasmalogens were first proposed to be endogenous antioxidants and to be involved in the membrane bilayer formation [
201]. Lately they have also been proposed to have a beneficial impact on atherosclerosis, on the prevention of inflammation (neuroinflammation in particular), on the improvement of cognitive function, and on the inhibition of neuronal cell death [
201,
202,
203]. In fact, oral ingestion or plasmalogen replacement therapy were both put forward as novel strategies to target neurodegenerative diseases (namely Alzheimer's disease) [
201] and chronic inflammatory disorders [
204]. Taking all this into account, marine animal co-products may indeed represent a convenient source of quality and healthy phospholipid and plasmalogen fractions that may find value in the food, supplement and pharmaceutical industries.
Sterol content in marine animal co-products may be interesting from a valorization standpoint. In shrimp (
P. borealis [
49]), lobster (
N. norvegicus [
51]) and squid (
D. gahi [
140]) co-products, sterols were reported to represent a significant part of total lipids. This is particularly interesting since, at least in mollusks, anti-inflammatory activities from extracted sterols have been previously documented [
205,
206].
The content in carotenoids, and astaxanthin in particular, in the lipid fractions of crustacean co-products has been extensively explored and studied [
136,
207]. Astaxanthin, the main carotenoid present in these co-products, has been ascribed an especially potent antioxidant activity [
208], but also other biological properties such as anti-inflammatory, antiproliferative and anticancer activities [
207,
209]. Moreover, it has been proposed to present benefits in cardiovascular disease and inflammation contexts, while improving both lipid and glucose metabolism [
207]. Therefore, this is another way to value lipid fractions of marine animal co-products, especially crustaceans.
Marine animals, especially oily fish, have been reported to contain significant levels of tocopherol compounds [
181,
210]. In the case of marine animal wastes, squid co-products have in fact been explored for their contents in liposoluble vitamin E with interesting results [
58,
140,
143]. Vitamin E is a chain-breaking antioxidant [
58] and can exclusively be obtained from diet [
211]. It has been linked to many beneficial effects regarding conditions in general where oxidation plays a role, including cancer, ageing, arthritis and cataracts [
212]. It was also shown to be effective in the prevention of chronic inflammation and in the inhibition of platelet aggregation [
213].
Finally, it is important to highlight that, more than just a potential to display beneficial effects based on their composition, some lipid fractions from marine animal co-products have in fact been ascribed interesting biological activities. Phospholipid extracts from shrimp heads, codfish roe, and squid gonads were tested for their antithrombotic, anti-stroke, anti-inflammatory, pro-angiogenic, and cardioprotective activities with promising results [
214]. Another study highlighted the anti-inflammatory activities of extracts from gloomy octopus (
Octopus tetricus) viscera, squid (
Sepioteuthis australis) heads, Australian sardine (
Sardinops sagax) viscera/heads, salmon (
S. salar) heads and school prawn (
Penaeus plebejus) viscera/heads and [
152]. Also, phospholipid extracts from the brain of skipjack tuna (
K. pelamis) were shown to display macrophage-activating activity by inducing pro-inflammatroy cytokines, therefore being suggested as possible boosters for human immunity [
215]. Lipid extracts from different organs (stomach, liver, brain, and skin) of marbled rockcod (
N. rossii) and mackerel icefish (
C. gunnari) were tested for their potential in skin protection with promising results [
100]. Also, a phospholipid-rich extract from salmon (
S. salar) heads was shown to elicit favorable effects in rat models of metabolic syndrome [
105]. Moreover, an acetone extract of Northern shrimp (
P. borealis) industry processing wastes was reported to display neuroprotective effects via antioxidant and anti-inflammatory effects and by increasing neurotrophins [
216]. A lipid extract from Pacific white shrimp (
P. vannamei) cephalothorax was shown to display antioxidant and anti-inflammatory activities, especially when encapsulated by spray-drying [
217], while an hexanic extract of was shown to present significant antibacterial activity [
218]. A lipid extract from
P. borealis processing co-product, obtained by Soxhlet extraction showed the potential to elicit anti-adipogenic effects [
49]. Phospholipids from yet another shrimp co-product (heads of
P. vannamei), showed angiogenic, antithrombotic, anti-arrhythmia and anti-inflammatory activities in zebrafish models [
124]. Regarding mollusks, lipid fractions of common octopus (
O. vulgaris) co-products (viscera, pocket of ink, eyes) obtained by traditional methods (Folch extraction), were shown to possess antiproliferative and apoptotic effects on human breast cancer cell lines [
219]. Finally, cupped oyster (
M. gigas) co-product extracts were shown to decrease lipid cholesterol and triglyceride content in rat livers and suggested as potential lipid-lowering functional foods or supplements [
220,
221]. A summary of the biological activities reported for lipid fractions from marine animal co-products is depicted in
Figure 4.
5. The Value of Marine Animal Co-Product Lipids for Industry
Given their enticing chemical qualities in terms of composition, marine co-product lipids may find their way to diverse applications in the industry. A first and more obvious application would be the incorporation of marine animal co-product lipids into food products, enhancing their nutritional profile while also creating innovative and health-focused products that cater to evolving consumer preferences and dietary trends. In “functional” or “designer” foods, conceived to enhance human health and wellbeing, marine co-product lipids offer the possibility of enriching diets in omega-3 fatty acids and other bioactive compounds, while also improving their sensory attributes. The more obvious commercially available products that are fortified in omega-3 fatty acids are dairy products, namely yoghurt, milk drinks, margarines, spreads and fresh and ultra-high temperature milk [
222,
223]. However, there are also less evident examples of omega-3 enriched foods, such as meat, eggs, baked goods, beverages and even infant formulas; these products are generally marketed for their cardiovascular upside and brain-boosting properties [
224,
225]. In the food industry, marine animal fats have also been explored as additives to improve the characteristics of salad dressings and mayonnaise [
226] and yoghurt [
227], namely their oxidative stability. A specific case of a concrete application of marine animal co-product lipid extracts in the food industry, is the suggested incorporation of shrimp cephalothorax lipid extracts in food products, including soups, sauces, and meat or fishery products, where it would function as a food coloring agent and a functional ingredient [
217,
228,
229]. These shrimp lipid extracts display in fact interesting anti-inflammatory and antioxidant activities, significant coloring capacity, and relative stability under thermal treatment and refrigerated storage, therefore presenting intrinsical valuable qualities for the food industry [
217].
Taking into account their qualities, especially their abundance in omega-3 fatty acids, marine animal co-products also appeal to companies working in the field of nutraceuticals, and even in the more recent field of functional lipids. Omega-3 supplements are widely available in a variety of different commercial formulations from different brands, mostly as (fish) oils or as capsules, thought to promote an increase shelf time absorption [
230,
231,
232]. The global lipid nutrition market, mostly based on the promise of omega-3 fatty acids as health promoters, is expected to reach 17 million dollars by 2031, growing by 7.6% annually over the 2021-2031 period, driven by increasing demand [
233]. Therefore, the market actually exists for additional healthy lipid sources.
Phospholipids are part of the EU list of authorized food additives (E322 lecithin), and are used as emulsifiers and antioxidants in foods [
234]. Phospholipids have in fact been ascribed antioxidant properties during food processing depending on the amine composition of their head group and on their fatty acid composition [
235,
236]. Phospholipid sourcing is, nonetheless, limited. Food lecithins (basically a complex mixture of phospholipids from natural sources) are produced typically from oil degumming pastes (soy, sunflower and, more recently, rapeseed) and egg yolk [
169]. The fatty acyl compositions of the side chains of the phospholipids present in lecithins are relatively simple, not very varied, and display a low degree of unsaturation [
237]. More recently krill oil has been explored as a marine animal source of omega-3 rich phospholipids [
184]. However, given the promising perspectives of their use and the pressure created by increased demand [
238], marine animal co-products, generally presenting phospholipids as the most abundant lipid class, may provide a viable, still largely unexplored source of these compounds.
Lipids from co-products of marine animals may also be appealing to the fields of pharmacology and drug development. As previously mentioned, phospholipid extracts from co-products derived from fish, crustaceans and mollusks have been shown to display interesting biological activities, and therefore suggested as possible targets for pharmacological and clinical studies and development, namely in the context of inflammatory and cardiovascular diseases [
214]. Moreover, phospholipids have also attracted interest as drug delivery systems on the basis of their excellent biocompatibility and amphiphilicity [
239,
240]. Liposomes, intravenous lipid emulsions, micelles, drug-phospholipids complexes and cochleates are all phospholipid-based delivery vectors [
240]. Liposomes in particular have been particularly used as delivery systems in food, cosmetic and pharmaceutical applications [
241]. Given their specific characteristics, namely their high contents in omega-3 fatty acids, it was suggested that marine phospholipid liposomes could promote an enhanced bioavailability and activity of encapsulated functional compounds [
154].
In the specific case of plasmalogens, they have also garnered interest from a pharmacological perspective, having been proposed to display attractive functionalities as healthcare materials. In fact, they have been proposed to present promising characteristics to serve as constituents of functional membranes of biosensors, light-activated liposomes or nanoparticles with endosomal escape capabilities [
203]. Moreover, marine organisms have also been proposed as alternative sources of complex lipids (as plasmalogens) in alternative to more common sources, such as bovine brain, which use is now unwarranted due to past outbreaks of bovine spongiform encephalopathy [
203].
Finally, lipids from marine animal co-products may also be appealing for the cosmetics and personal care industry. In fact, marine animal derived lipids were shown to display good conditioning, moisturizing and emollient abilities [
242,
243]. Astaxanthin in particular has also showed promise for cosmetics, mainly because of its remarkable antioxidant and radical scavenging abilities [
135]. These properties also justify the interest in exploring the use of astaxanthin in nutraceutical and healthcare applications [
244,
245]. Finally, tocopherols obtention from shrimp and squid co-products should also gather obvious interest from both pharmaceutical [
246] and cosmeceutical [
247] industries.
Aquaculture systems are absolutely reliant on the production of feeds from both marine and terrestrial origin, with feed production being pointed as the most significant source of environmental impact in fed aquaculture production [
248]. Other than their documented benefits when consumed by humans, DHA and EPA omega-3 PUFA are essential ingredients with high demand in aquaculture (namely of marine species), where they promote the growth and overall health of farmed animals [
249,
250]. This is another way justifying the increased demand for omega-3 PUFAs, putting pressure on the supply through conventional sources [
251,
252]. Therefore, new omega-3 sources are needed to alleviate the pressure of an increasing demand for low environmental impact feed assuring the quality of aquaculture animals. The repurposing of marine animal co-products aligns with eco-intensification paradigms and can promote a lower Fish In: Fish Out (FIFO) ratio [
253]. In fact, oils produced from marine animal co-products (e.g., tuna fish side streams) have been ascribed beneficial effects when used in formulations, preventing excessive fat deposition in farmed fish [
86]. Also, products based on the processing side streams of shrimp and crab are already being used in animal feed to balance nutrient profiles [
254]. Other lipophilic compounds present in the lipid fractions of marine animal (in this case crustaceans) co-products have also garnered particular specific interest in the aquaculture field, with astaxanthin being approved by US and EU authorities to be used as a colorant/dying agent in in animal feed and fish food, salmon in particular [
135,
255,
256].
6. Sustainability and Environmental Impact
Every year, the amount of discards from the world fisheries exceeds 20 million tons [
257], and therefore the disposal and recycling of such large amounts of biomass represents a challenging task. Measures like the recently enforced Landing Obligation of the European Common Fisheries Policy, show a trend for legislators to aim at mitigating environmental impact of fishing operations, but also imply more costs for the disposal of those additional specimens, that are now not permitted to be returned to sea [
258]. Therefore, repurposing marine animal food co-products should represent a justified sustainable and environmentally responsible practice, which aligns with the principles of a circular economy [
30] and with the United Nation’s Sustainable Development Goals [
259].
The identification and exploration of new healthy lipid sources is particularly important as global population keeps growing, and sustainable food supplies become more critical [
260]. A more rational use of marine animal co-products will reduce the waste generated by seafood processing by assuring that a larger portion of harvested marine animal biomass is utilized. This optimization of the use of marine animals from capture or farming can help to alleviate the pressure upon wild animal populations and contribute to the overall conservation of marine ecosystems [
30,
261]. Moreover, it can contribute to more eco-friendly seafood processing practices, allowing to also aim at broader environmental goals, such as reducing industry's carbon footprint, greenhouse gas emissions and alleviating the burden on landfill sites. In fact, in most instances, marine animal co-products are incinerated, composted, anaerobically digested, landfilled, returned to the sea or even simply abandoned [
21,
262], with consequent negative ecological and human health impacts. However, from both ecological and economic standpoints, the valorization and utilization of marine animal co-products for the collection of valuable compounds should always represent a preferential approach compared to all other options [
263]. Interestingly, downstream from the farmed animals themselves, effluents from the processing industry (fish canning in particular) were also explored as sources of omega-3 fatty acids [
264]. This is a new perspective onto the valorization of lipids from seafood processing that could also contribute to sustainability and reduce the environmental impact of such enterprises, increasing circularity and framing these economic activies under a bioeconomy paradigm.
The repurposing of marine animal co-products may also represent an extra revenue source for seafood processors and related industries, and an opportunity to foster innovation in the form of novel value-added products, and the diversification within the seafood industry with additional eco-friendly options. It also aligns with the broader goals of sustainable seafood certification programs and fisheries management initiatives [
265,
266]. Moreover, the exploration of these resources may represent a boost for local economies and lead to the creation of new jobs in the seafood industry. Ultimately, the incorporation of marine co-product lipids into the seafood value chain contributes to a more responsible and environmentally conscious, effective, and productive seafood industry.
Obviously, the extraction and valorization of lipids from marine animal co-product matrixes may not be appropriate or possible in all cases, namely for those matrices with poor lipid yields. However, for co-products with more promising features, lipid extraction should be explored in adequate and suitable frameworks. Lipid extraction could be included in a rational biorefinery pipeline approach, incorporating energy-efficient techniques, waste reduction, and recycling, making it attractive and economically sustained. There are already many particular instances of established high-value compounds being effectively harvested from marine animal co-products, such as collagen and gelatin, biopolymers, like chitin or chitosan, hydroxyapatite, carotenoids, pigments and protein, protein hydrolysates and bioactive peptides and minerals [
30,
31,
32,
33,
35,
42,
267,
268,
269]. Therefore, it may be feasible and even advisable to envision efficient biorefinery strategies to maximize resource utilization, ensuring that each type of co-product is utilized to its fullest potential and that all valuable compounds are accounted for and collected, while also minimizing costs and waste. There are several different approaches related to how lipid extraction could be incorporated in such procedures, and they would always depend on the specific biological matrix/co-product being targeted, on the presence of compounds of interest and on the specific needs of the market. In any case, lipid extraction could represent a key extraction step or a common step alongside other existing/well-established processes, such as collagen, chitin and chitosan and protein and protein-derivatives extraction depending on the source co-product and on the intended outputs of the biorefinery pipeline (
Figure 5). In the end of the process, recovered lipids could then be directed into various value chains, depending on their quality and composition, with the use of high-quality lipids being advocated for the production of functional foods, nutraceuticals, and dietary supplements, while lower-grade lipid extracts, or those co-products failing the requirements to maintain “food quality”, could be aimed for incorporation in feeds or even biofuel or biogas production [
83,
267,
270,
271]. Such pipeline approaches have already been proposed in the case of shrimp exoskeletons, although they did not encompassed lipid recovery [
272,
273] and, in the case of fish waste, with the production of fish oils as an integrated step [
274]. This inclusion in biorefinery platforms, aiming at extracting all possible compounds representing value, aligns very well with the “Zero Discards” mandate comprised in the Sustainable Developmental Goals of the United Nations, as well with EU policy agenda, which includes the Circular Economy Action Plan [
275], aiming to reduce raw materials and associated environmental pressures, the Bioeconomy Strategy [
276], targeting the exploitation of biomaterials in a sustainable manner, as well as the European Biorefinery outlook to 2030 [
277].
7. Challenges and Future Directions
Some of the challenges underlying a sustainable and profitable exploration of lipids derived from marine animal co-products have to do with the lack of available information. When writing the present review, other than estimates, it was challenging to find credible information on the true numbers of marine animal co-product production at global level, nor was it easy to find systematic studies calculating the carbon footprint and actual costs of disposal of such biomass by the industry [
278]. Both informations would likely reinforce the need to take further action to minimize waste that still persists associated with the use of marine animals and its co-products, as well as the potential economic upside that full use of these resources may represent, if rationally perceived and explored. Still in the topic of the lack of available information, and despite the many works available on the characterization of lipid fractions of marine animal co-products, especially in terms of their fatty acid profiles, there is still a lack of information on lipid composition at the molecular level. Few comprehensive studies have addressed the structural characterization of these biological matrices, with these being paramount to beter understand how important fatty acids of marine origin to which are ascribed important beneficial health effects (omega-3, EPA and DHA in particular) are distributed along lipid classes, since this has major implications in their absorption and general bioavailability. Moreover, there are only a few studies scanning the biological activities of lipid fractions from marine animal co-products, although the ones available have shown promising results. Identifying extracts displaying biological activity, and further investigating their composition and identifying the active lipids involved in the biological effects, would be another practical possibility to foster the valorization of these co-products. This would pave the way to possible high-end applications in the pharmacological and cosmeceutical industries. The main bottleneck is that, as of now, the mixing of co-products is still a common practice, as could be perceived in several characterization studies surveyed in this review; this practice hinders the discovery of specific lipid agents that are potentially nutritionally superior or display particular bioactivities, and consequently limits downstream applications.
There are also some technical problems related with the processing of marine animal co-products that may hinder the collection of lipids of interest. Marine animal co-products are generally highly perishable materials. These co-products are water and nutrient-rich, basically meeting the requirements for making good growth mediums for fungi and bacteria [
21,
279]. This means that, if not processed quickly, storage may result in the loss of raw material and nutritional value, and in the oxidation of compounds of interest (such as carotenoids and PUFA, highly prone to lipid peroxidation) [
48,
280,
281]. Taking into account the amounts of waste estimated to originate in the seafood industry, this could pose an obvious problem of scale that would have to be considered from operational and financial standpoints. The option to, in some cases, store these co-products at low temperature at integrated centralized facilities, could be advantageous, as small/artisanal fisheries, eventually unlike bigger operators, do not have the means to store and process these co-products. Centralized facilities may also be justifiable in the context of implementing biorefinery pipeline approaches.
One significant challenge for the widespread and profitable use of marine animal co-products lies in the technical aspects related to the lipid extraction itself. In fact, as of now, the standardization and optimization of green extraction procedures, guaranteeing quality and yield, are still a work in progress [
282,
283,
284]. Despite the availability of many different extraction techniques, selecting the most suitable method for a specific co-product remains a complex task, taking into account the heterogeneity and specificities that these co-products may present. Moreover, factors like cost-effectiveness, energy efficiency, and scalability must be carefully considered when appraising each possible approach. Solvent-based extraction is the most commonly used methodology for lipid extraction, especially procedures based on the Bligh and Dyer [
285] or the Folch [
286] methods. Although these are very efficient methods, both in terms of lipid yield and lipid classes covered, making them very convenient for characterization purposes, the use of noxious chemical solvents renders them unsuitable for the use in food/feed industries, or any other application related with direct human consumption, and thus there is a requirement for the use of food grade and non toxic solvents. Therefore, modern green lipid extraction techniques, compatible with human use, should be preferentially explored and applied in the processing of marine animal co-products with the objective of maximizing applicability. According to Directive 2009/32/EC of the European Parliament and of the Council of 23 April 2009 on the extraction solvents used in the production of foodstuffs and food ingredients, organic solvents currently allowed in the industry include propane, butane, ethyl acetate, ethanol, and acetone [
287]. Some ionic liquids, including deep eutectic solvents, are also considered green solvents [
288]. Therefore, all these could be potentially suitable alternatives to be included in lipid extraction strategies from marine animal co-products. Among some of the innovative green technical approaches that may be employed to obtain the lipid fractions of marine animal co-products, we may mention supercritical fluid-, enzyme-, microwave-, and ultrasound-assisted extraction techniques [
21,
289,
290,
291,
292]. In recent years, supercritical fluid extraction has gained traction as a leading option for lipid extraction from different biological matrixes including marine animal co-products, presenting a good efficacy in the recovery of omega-3-rich marine oils [
293,
294,
295,
296]. This technique presents operational conditions that are more favorable from an environmental and industrial processing viewpoint, such as sparing the need to use high temperature treatments that can lead to lipid oxidation/degradation, or the use of organic solvents [
230,
295]. However, supercritical extraction is more directed towards the isolation of neutral lipids, meaning that it preferentially targets the extraction of triglycerides; nevertheless, the use of modifiers, such as ethanol, may render this technique also applicable to the obtention of phospholipid rich extracts [
53,
139,
297]. However, in the end, the cost of adapting the application of these techniques to an industrial scale seems to represent the ultimate challenge for their use [
295]. As such, understanding the nuances of all these extraction techniques, enabling the use of sustainable and green solvents, optimizing and tailoring them to specific marine animal co-products is essential for devising integrated approaches optimizing yield, quality and profit. The establishment of a strong engagement between research and industry will be pivotal for the development and success of such strategies.
Another technical issue that cannot be overlookde has to do with interindividual variability in animals, which may be raised by factors such as age, sex, environmental conditions, season, nutrition and processing [
298] and ultimately may generate fluctuations in yield and composition of specific co-products. Moreover, another technical challenge may have to do with the potential presence of contaminants in marine animal co-products, namely those known to accumulate in marine animal biomass, such as metals (and metalloids), pesticides, polychlorinated biphenyls, dioxins and (micro)plastics [
117,
261,
299,
300,
301]. This may imply additional thorough quality control measures and investments in HACCP and decontamination procedures, ensuring the safety and purity of lipid extracts. Other technical issues are related to the stability of the biomolecules of interest along the extraction/biorefinery processes, which should be studied and assured. Other than that, it may also be needed to develop thorough studies regarding sensorial and bioavailability aspects, as well as possible interaction with other ingredients when incorporated in formulations.
Regulations regarding the disposal of marine animal co-products by fisheries or the seafood industry can be very uneven worldwide, and conflicting regulatory and economic drivers often create perverse incentives leading to practices that are not ecologically desirable [
302]. In the European Union the restrictions of the use of animal co-products were codified in 2001 by adopting Regulation (EC) No 999/2001 [
303]. Moreover, EU Regulation 1069/2009, as implemented by EU Regulation 142/2011 [
304], governs the collection, transportation, storage, handling, processing and use or disposal of all animal co-products, including fish material not destined for human consumption, finfish processing co-products and shellfish surpassing shelf life [
305]. This legislation includes safety regulations and sanitary practices that must be followed to ensure the suitability of co-products for various uses. The European Union is also actively enforcing policies to actively promote food and seafood circularity, namely within the scope of the Circular Economy Action Plan framed within the European Green Deal [
306]. However, available legislation must of course be dynamically adjusted in order to accommodate innovations regarding co-product processing and eventual novel products and processes that may arise from these applications. Other changes could contemplate introducing more accuracy and transparency in the reporting/communication of the amount of waste generated by the industry and associated costs of disposal, which would be very informative in the contexts of appraising environmental impacts and promoting a more efficient use of co-product as largely untapped resources. More than just govern the handling and disposal of marine animal co-products, regulatory considerations will also inevitably shape their utilization. Ensuring compliance with food safety and labeling regulations is critical for generating trust when these materials are eventually incorporated into functional foods and nutraceuticals. This is a task that requires a strong synergy and cooperation between research, industry and legislators. For this purpose, a reliable traceability system is paramount. Moreover, efficient marketing strategies and consumer acceptance may have to rely not only on nutritional attributes but also in the certification of ecologically-conscient, sustainable practices to engage stackeholders, particularly consumers.