(Watanabe et al., 1980; Kitajima et al., 1980a,b; Oka et al., 1980, 1982; Watanabe et al., 1982; Fukusho et al., 1984, 1985). Subsequently, EPA and/or DHA have been shown to be essential for various marine fish larvae, such as ayu (Kanazawa et al., 1981), red sea bream (Izquierdo et al., 1989; Watanabe et al., 1989b), striped jack (Watanabe et al., 1989c), and gilthead sea bream (Koven et al., 1989), as well as for the larvae of one freshwater fish, striped bass (Webster and Lovell, 1990).

The essential fatty acids function as components of phospholipids in all biomembranes and as precursors for eicosanoids that fulfill a variety of metabolic functions. Biomembranes must be in a fluid state to function properly at various temperatures. Membrane fluidity depends on the proper balance of saturated and unsaturated fatty acids as components of membrane phospholipids (Bell et al., 1986). The role of dietary n-3 PUFA during homeoviscous regulation, whereby fish alter their biomembrane phospholipid composition in response to changes in environmental temperature, has been demonstrated by Hazel (1984). During acclimation to cold-water temperatures the total amount of phospholipid in the fish biomembrane does not change, however changes occur in the relative proportion of individual phospholipids, in the fatty acid composition of the phospholipids, and in the distribution of fatty acids within the phospholipid molecules. For example, in rainbow trout transferred from 20° to 5°C, the proportion of phosphatidylethanolamine increases with a corresponding decrease in phosphatidylcholine in both liver and gills (Hazel, 1979, 1985). Similar changes have been observed during cold adaptation in membranes of goldfish intestine (Miller et al., 1976) and carp muscle (Wodtke, 1981). Conversely, during adaptation to higher temperatures, the proportion of phosphatidylethanolamine decreases in trout gill membranes and phosphatidylcholine increases (Hazel and Carpenter, 1985). Thus changes in the ratio of the two major membrane phospholipids, phosphatidylcholine and phosphatidylethanolamine, can be used in fish as an index of proper adaptation to changes in environmental temperatures.

During cold adaptation, the relative amount of n-3 PUFA increases in trout liver membrane phospholipids, whereas the amount of saturated fatty acids decreases and the amount