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Prolonged swimming capacity (critical swimming speed, U crit , protocol) and metabolism were measured for 14 Atlantic cod Gadus morhua exposed to seven oxygen levels within the nonlethal range normally encountered in the Gulf of St Lawrence (35 to 100% saturation). Burstand-coast swimming was triggered earlier (at lower speeds) in hypoxia, and burst-and-coast movements were more frequent in hypoxia than in normoxia at low speeds. Furthermore, the metabolic scope beyond the metabolic rate at which Atlantic cod resorted to burst-and-coast movements decreased gradually as ambient oxygen concentration dropped. Overall, fewer burst-and-coast movements were observed in hypoxia while the distance swum in burst-andcoast mode remained c. 1% of the total distance swam in all tests. Oxygen availability had no effect on the rate of increase in metabolic rate with increasing velocity <50 cm s À1 , but limited swimming performances and metabolic rate at higher speeds. The prevailing low oxygen tensions on the bottom in the deep channels may impair the swimming capacity of Atlantic cod in the estuary and northern Gulf of St Lawrence. # 2007 Crown in Right of Canada

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