Ethyl acetate extracts of seaweeds were chromatographically fractionated to yield 14-methyl pentyl tetrahydro-8-hydroxy-10 methylnaphthalene–8-carboxylate (1) and tetrahydro-4-isopropyl-9-(9, 14-dimethyldec-9-enyl)-pyran-1-one (2) from Sargassum ilicifolium, whereas Padina gymnospora afforded dihydro-2-(10-(hydroxymethyl)-7,15-dimethyl-9-oxoundec-11-enyl)-2-methyl-2H-pyran-1(4H)-one (3) and 1-(decahydro-1-hydroxy-7-methyl-8-vinylnaphthalen-2-yl)ethanone (4) as major constituents. Compound 1 displayed significantly higher antioxidant activity (IC50 < 1 mg/mL, p< 0.05) comparable to other analogues (IC50 > 1 mg/mL). The C20-22 polyunsaturated fatty acid (C20-22PUFA) concentrate (CFA) prepared from the dogfish liver oil was added with the studied compounds and physiochemical properties and fatty acid composition during an accelerated storage were evaluated. No significant reduction in C20-22 PUFAs (∼ 6%) in the CFA treated with 1 as compared to that with the control (∼ 35%) was recorded. A greater induction time was observed for the CFA supplemented with 1 (6.8 h) than other compounds (≤ 6 h) and control (∼ 1.6 h), maintaining the oxidation indices of the CFA within desirable limits.