Impact Of Chronic Hypoxia On Food Ingestion, Growth And Condition Of Atlantic Cod, Gadus Morhua

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The Gulf of St Lawrence is an enclosed sea located in Eastern Canada. It is characterised by the presence year-round of a layer of cold water (temperature < 0 °C and average salinity = 32.4) (Lauzier & Trites, 1958; Gilbert & Pettigrew, 1997). In winter, the water column is a two-layer system (Koutitonsky & Bugden, 1991). The permanent cold layer mixes with surface waters via storm events and density changes of surface waters caused by cold winter air temperatures. As a result, water is below 0°C from the surface down to circa 100 m. Starting in April, the water column becomes a three-layer system, with a cold intermediate layer (CIL), due to the increasing temperature and lowering salinity of surface waters (Koutitonsky & Bugden, 1991) (Gilbert & Pettigrew, 1997). In summer, the CIL is found from circa 50 to 100 m. Both surface waters and the CIL are rich in oxygen. Waters deeper than the CIL are warmer: temperature is circa 2°C below the CIL, around 125 m, and increases with depth up to about 250 m, where it typically reaches 4 to 6°C, and remains relatively stable at greater depths (Bugden, 1991). Deep waters do not mix with the CIL or surface waters. This prevents them from getting replenished in oxygen. Furthermore, deep waters flow upstream. A variable mix of waters from the Atlantic and the Labrador Current enters the Laurentian channel, with oxygen levels varying between 60 and 70% saturation (Bugden, 1991; Gilbert & Pettigrew, 1997). As these waters progress toward the head of the Laurentian channel, they get progressively more depleted in oxygen, and levels of 20–30% saturation are typical of the western end of the Gulf of St Lawrence and of the estuary (Figure 1).