Northwest Fisheries Science Center

Hypoxia

Hypoxia (dissolved oxygen concentrations < 1.4 ml/L) is common in bottom waters across the continental shelf off Oregon and Washington during the summer months (Figure HYP-01). The presence of hypoxic waters can be lethal to benthic invertebrates and may displace demersal fish species (Grantham et al. 2004).

Plot showing relation between the Coastal Upwelling Index and coho salmon survival Figure HYP-01.  Oxygen concentration in bottom waters at a baseline station NH 05. Hypoxia is defined as waters with oxygen concentrations <1.4 ml/L, and is observed only during the coastal upwelling season, especially during Jun-Sep.

Along the Newport Hydrographic (NH) Line, hypoxic waters tend to occupy the lower 10 - 30 m of the water column (Figure HYP-01). Spatially, hypoxic bottom waters can cover the entire width of the shelf (Figure HYP-02), but is less common in shallower areas (< 30 m depth) where wind and wave action helps to aerate the water column.

Juvenile salmon tend to reside in the upper layer of the water column and are likely not directly influenced by hypoxia.

Dissolved oxygen maps.  Figure HYP-02. Dissolved oxygen maps. Distribution of the minimum dissolved oxygen values (ml/L) during the June BPA surveys from 2007 - present. A level of 1.4 ml/L dissolved oxygen is used to identify hypoxic waters (outlined with bold contour line).

The greatest extent of hypoxic bottom waters typically occurs in August and September. Based on regional surveys conducted since 2006, hypoxic bottom waters can cover up to 62% of the shelf north of the Newport Hydrographic line (Figures HYP-02 and HYP-03).

Dissolved oxygen maps.  Figure HYP-03. The area of the shelf containing hypoxic bottom waters during the May, June, August and September surveys. The shelf area surveyed is 15,750 km2. The shelf area and region of hypoxia were calculated from maps similar to those shown in Figure HYP-02.