Beam Reach

Salmon salinity stop?

Just happened upon this nice plot of the summertime salinity gradient in the Salish Sea.  I was surprised to see the average salinity gradient focused at the sill separating Haro Strait and the eastern Strait of Juan de Fuca.  While the variability due to tidal forcing must be dramatic, the steepest gradients are located at the southern end of Haro Strait, the vicinity of “Salmon Bank.”  I’ve come to believe that this same general area (centered around False Bay) is a focus of killer whale foraging, but no one (and no telemetry data for adult salmon) has clarified why the foraging (and presumably fish) might concentrate here.

Salinity gradient in the Salish Sea

This figure makes me wonder if (on average) this is where the Fraser Chinook are beginning their physiological adjustment to fresh water chemistry. A migrating salmon could pause at about this position in the estuary and access a wide range of salinities (changes of 2-6ppt) simply by moving between the surface and 100m at different stages of the tide.  Alternatively, they could sit still at one depth and have a smooth gradient of salinity wash past them with each tidal exchange.