Fatty Acids in Waste Tissues: The Nutraceutical Value of Gonads and Livers from the Moroccan Hypophthalmichthys molitrix and Cyprinus carpio Fishes

**Background:** Fish consumption provides essential fatty acids that reduce cardiovascular disease risk. However, increasing fish consumption generates substantial waste, and recycling fish by-products aligns with circular economy principles. Carp species (Cyprinidae) account for ~45% of global aquaculture production. This study evaluated the fatty acid profiles of liver and ovary tissues (typically discarded) from Moroccan Hypophthalmichthys molitrix (silver carp) and Cyprinus carpio (common carp) at mature and immature stages, comparing them with edible fillet tissues to assess nutraceutical potential.

**Methods:** Fish were collected from Deroua Fisheries Station (Beni Mellal-Khenifra, Morocco) in April 2019. Mature silver carp were selected from ≥46 cm length; mature common carp from ≥54 cm length. Gonadosomatic index (GSI) confirmed maturity: common carp mature GSI 18.34 ± 1.12, immature GSI 7.08 ± 1.94; silver carp mature GSI 0.67 ± 0.26, immature GSI 0.08 ± 0.03. Lipids were extracted using the Bligh and Dyer method. Fatty acid methyl esters (FAMEs) were analyzed by GC-MS on a 5% diphenylpolysiloxane column. Quantification used methyl tricosanoate (C23:0) as internal standard. Sixteen fatty acids were measured (LOQ 0.3 mg/100 g tissue). Fat quality indices (h/H, AI, TI) were calculated. Statistical analysis used Student's t-test (p < 0.05 significant).

**Key Results:** Total lipid content increased from immature to mature stages in all tissues. Mature common carp gonads contained 2.56 ± 0.48 g lipid/100 g; liver 1.27 ± 0.22 g/100 g; immature values were 0.88 ± 0.68 and 0.83 ± 0.08 g/100 g, respectively. SFAs were highly abundant in all tissues (range 30–54%), with preferential accumulation in liver (~44% mature and immature common carp; 38% mature silver carp; 54% immature silver carp). Mature common carp gonads contained 207.63 ± 16.61 mg SFA/100 g (36% of total FA); silver carp gonads 170.30 ± 8.52 mg/100 g (31%). MUFAs were the most abundant fatty acids (range 33–58% of total FA). Mature silver carp fillet contained the highest MUFA (119.43 ± 3.67 mg/100 g, 57%, p = 0.0006). PUFAs preferentially accumulated in gonads: 180.36 ± 26.92 mg/100 g in mature common carp (23–31% of total FA) and 130.93 ± 12.98 mg/100 g in mature silver carp. ω-3 FAs were significantly abundant in mature common carp gonads (119.84 ± 23.40 mg/100 g, p = 0.006); ω-6 FAs were significantly abundant in mature silver carp gonads (117.48 ± 12.31 mg/100 g, p = 0.04). Major PUFAs identified: DHA (4.13–14.37%), EPA (3.3–6.25%), arachidonic acid (1.11–13.11%), linoleic acid (4.40–20.11%). PUFA/SFA ratio ranged from 0.40 to 1.06 in fillets. AI ranged from 0.23 to 0.53 (all below recommended 1.0); TI ranged from 0.33 to 1.55 (below 1.0 for all tissues except liver). Mature common carp fillet showed significantly low AI (p = 0.01) and TI (p = 0.03). h/H ratio ranged from 0.74 to 3.61, above 1 for most gonad and fillet tissues.

**Clinical Implications:** The fatty acid profiles of Moroccan common and silver carp tissues, particularly the PUFA content in mature fillets and gonads, support cardiovascular health benefits. AI and TI values below 1.0 indicate good antiatherogenic and antithrombogenic properties. The high h/H ratio (>2 in gonads) suggests favorable hypocholesterolemic effects. Importantly, non-edible tissues (gonads and livers) contain substantial amounts of bioactive fatty acids (EPA, DHA, AA, LA) comparable to or exceeding fillet levels. This supports repurposing fish processing by-products for nutraceutical production, functional foods, or aquaculture feed, reducing waste while providing high-value lipid extracts. The differential ω-3/ω-6 accumulation between species (common carp gonads richer in ω-3; silver carp gonads richer in ω-6) enables targeted oil production for specific nutritional applications.