S. Nazemroaya - Department of Fisheries, College of Natural Resources and Marine Science, Tarbiat Modares University, Noor, Islamic Republic of Iran.
M. A. Sahari - Department of Food Technology, Collage of Agriculture, Tarbiat Modares University, P. O. Box: ۱۴۱۱۵- ۳۳۶, Tehran, Islamic Republic of Iran.
M. Rezaei - Department of Fisheries, College of Natural Resources and Marine Science, Tarbiat Modares University, Noor, Islamic Republic of Iran.

Changes in the fatty acid composition and biochemical indices of mackerel (which has a substantial lipid content) and shark (which has negligible lipid content) fillets stored at - ۱۸°C for up to six months were measured. Lipid content was measured (۶.۳۵% and ۱.۳۸%) in mackerel and shark, respectively; however it decreased during frozen storage in both fish species. In analysis of fatty acids the amount of PUFA, especially -۳ ones, was more predominant in mackerel than shark, nevertheless, fatty acid composition has changed in both species during frozen storage. The amount of saturated fatty acids in contrast with unsaturated fatty acids increased due to oxidation of PUFA. The decrease in PUFA compounds (۴۰.۱% and ۲۳.۹۴%) was as follows: -۳ (۴۸% and ۴۲.۸۳%), -۳/ -۶ ratio (۴۱.۳۶% and ۵۰%), PUFA/SFA ratio (۵۶% and ۴۲.۲۳%) and EPA+DHA/C۱۶ ratio (۵۵.۵۵% and ۴۶.۶۶%) in mackerel and shark, respectively. For both species, tiobarbituric acid (TBA), peroxide (PV), free fatty acids (FFA) and total volatile base nitrogen (TVB-N) values were significantly (P< ۰.۰۵) increased with storage time. The results showed that, among these indices, changes in the PV and TBA in mackerel were significantly (P< ۰.۰۵) larger than in shark; but changes of FFA and TVB-N in shark were significantly (P< ۰.۰۵) higher than in mackerel. It means that oxidative and hydrolytic deterioration are promoter factors of biochemical changes in mackerel and shark, respectively.

Frozen storage, Biochemical changes, Carcharhinus dussumieri, Fish fillet, Scomberomorus commersoni