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Coho salmon Oncorhynchus kisutch are semelparous spawners with highly synchronized oocyte growth. Plasma was collected from maturing female fish and plasma lipids were quantified by lipoprotein class. Stable 13C isotopes were employed to investigate the origins of plasma lipids. Plasma lipoproteins were partitioned into the very low density ( VLDL), low density ( LDL), high density ( HDL) and very high density ( VHDL) lipoprotein classes. Lipids from all lipoprotein classes increased between the lipid droplet and early vitellogenesis stages of oocyte growth and VHDL lipids continued to increase through mid vitellogenesis ( P < 0.05). During vitellogenesis, total plasma lipid concentrations were similar to the sum of lipoprotein lipids ( P = 0.51). Plasma vitellogenin determined by an ELISA method was very well correlated with VHDL lipid during vitellogenesis ( R 2 = 0.91, n = 15). In general, the δ13C values of plasma lipids reflected that of feed (exogenous) and muscle (endogenous) lipids when feed and muscle δ13C values were similar, and were intermediate when differences existed between the two lipid sources. With one exception, the δ13C values of lipids from all lipoprotein classes within a plasma sample were similar. Results indicate that during sexual maturity, lipids from all plasma lipoprotein classes originate from a common pool of exogenous and endogenous lipids. [ABSTRACT FROM AUTHOR]