Page 4 - HydrographyReadMe

Environmental Limits

The amount of dissolved oxygen in the water is a particularly important parameter for fish.
Although a hypoxic limit of about 2 ppm of O2 (a concentration of about 63 μM) indicates
conditions that can lead to death (and this is rarely found in the Strait of Georgia), the growth
of actively swimming fish can be adversely affected at concentrations much higher than this
hypoxic limit. In fact, approved BC Water Quality Guidelines suggest that an instantaneous
threshold of about 5 ppm is required to prevent harm to all life stages of aquatic life, and a
30-day mean of no less than 8 ppm (around 250 μM) is recommended. In our plots, we show
a limit of 6 ppm as conservative estimate of the instantaneous threshold (solid white line), and
the 8 ppm limit as an estimate of this longer-term mean limit (dash-dotted white
line). Staying in waters above these limits often means fish are restricted to quite shallow
waters, especially in the late summer.

However, warm temperatures can also affect fish, and the optimal range for salmon and
other fish at different life stages are generally under about 17°C (see also here). This limit (in
the upper few meters of the Strait's waters in summertime) is also indicated with a solid white
line in temperature plots.

Although the Strait is large and deep, you may be surprised at how small a region is ideal for
fish, especially later in the summer.