European eels (Anguilla anguilla, L.) were fed on a commercial diet supplemented either with 15% by dry feed weight of menhaden oil (MO), an oil rich in highly unsaturated fatty acids of the n-3 series (n-3 HUFA), or with 15% by dry feed weight of coconut oil (CO), an oil composed primarily of saturated fatty acids (SFA). Following 90 days of feeding, the mean final masses of eels fed the two different oil supplements were similar, and higher than the mean final mass of a group fed the commercial diet alone. The diets created two distinct phenotypes of eels, distinguished by the fatty acid (FA) composition of their tissue lipids. Eels fed MO had significantly more total n-3 FA and n-3 HUFA in muscle and liver lipids than did eels fed CO, leading to higher n-3/n-6 and eicosapentaenoic acid/arachidonic acid ratios in the MO group. Measurements of O₂ uptake (M_O2) revealed that the MO group had a significantly lower routine metabolic rate (RMR) than the CO group. When exposed to progressive hypoxia, both groups regulated M_O2 at routine normoxic levels until critical water O₂ partial pressures that were statistically similar (9.62±1.08 kPa in MO versus 7.57±1.07 kPa in CO), beyond which they showed a reduction in M_O2 below RMR. The MO group exhibited a significantly lower M_O2 than the CO group throughout hypoxic exposure, but the percentage reductions in M_O2 below their relative RMR were equal in both groups. During recovery to normoxia, both groups exhibited an increase in M_O2 to rates significantly higher than their RMR. Throughout recovery, M_O2 was significantly lower in the MO group compared with the CO group, but the percentage increases in M_O2 relative to RMR were equal in both. During progressive hypoxia, neither group exhibited a marked ventilatory reflex response, both showed similar reductions in blood O₂ partial pressure and content, and similar increases in plasma lactate. The results indicate that, although the n-3 HUFA-enriched MO group had a significantly lower routine metabolic rate than the CO group, the difference in aerobic metabolism did not influence the European eel's homeostatic regulation of M_O2 in hypoxia.