Stable oxygen and carbon isotope ratios (δ 1 8 O and δ 1 3 C) from archived otoliths of Pacific halibut, Hippoglossus stenolepis, were measured to examine the most recent regime shifts in British Columbia and the Gulf of Alaska. δ 1 8 O values of these otoliths ranged from -1.5 to +2.8%% VPDB, and were consistent with the life stages and migration behavior of halibut. δ 1 3 C varied from -3.3 to +0.9%% VPDB, but did not show a transition from the juvenile to the adult stage as does δ 1 8 O. Evaluation of δ 1 8 O and δ 1 3 C values of mature halibut (ages 8-12) indicated that the 1977 regime shift might have a warming impact on the northeast Pacific fish stocks. In contrast, a possible regime shift around 1990 with a bottom seawater temperature decrease of about 2 o C might have occurred in both the areas. The connection between stable isotope variations in otoliths and the climate regime shifts is thus potentially useful in studying the population dynamics of Pacific halibut, and decadal scale (e.g., the last 20-30 years) ocean environmental changes.