Wednesday, July 29, 2009

Resident Orcas Live in a Dynamic Environment

Bubbles (Photo by Erin Heydenreich)
Do orcas use bubbles similarly to other dolphins? We really don't know for sure - nor can we say with certainty if they use their sonar ability to detect currents and water masses. However, we do have some idea of the nature of the marine environment where our local resident orcas spend much of their time, and we can speculate on the rest as we continue to explore the intelligence of these animals. Future posts will delve more deeply into the marine environment, but for now we are staying focused on the qualities that characterize intelligence in mammals.

Resident orcas spent a lot of their time in Haro Strait which is the body of water that runs along the west side of San Juan Island, generally from the Strait of Juan de Fuca and up to Boundary Pass by the Gulf Islands. Underneath is a deep trough, which bumps up against San Juan Island and rises steeply along the area where the whale watch park is located (about where the orca icon is placed). On either end of the strait are shallow areas, called "sills".

Haro Strait Bathymetry
Divins, D.L., and D. Metzger, NGDC Coastal Relief Model,

Salmon returning from the ocean on their way to spawn up in the Fraser River (near Vancouver B.C.) come through the Strait of Juan de Fuca, then most of them head up Haro Strait. Orcas are present frequently during the time the salmon are running, and often appear to hunt them right along the edge of the island, over the sills or any geographic feature that tends to force the fish into an area where they are easier to catch - and fortunately brings the whales close to shore where we can see them.

When watching the whales you might notice that the surface of the water shows some subtle variations in texture - but those surface changes can belie the intensity of what may be going on beneath. Masses of water collide in this region, the tides push through, underwater waves stream, and curtains of bubbles get dragged down below at impressive speeds.

Haro Strait off Whale Watch Park (Photo by Elliot Whiting)

These graphs illustrate the dynamic nature of Haro Strait, and show a mass of bubbles moving at a clip of 50 centimeters per second downward and extending to a depth of almost 100 meters. That is a little over a foot and a half per second, to a depth of just under 300 feet. This type of event is transitory, but occurs frequently. It is certainly possible that these bubbles and vertical currents could be detected by the orcas - the entrained bubbles were measured using equipment that works similarly to dolphin sonar.

Entrainment of bubbles.
(Graph prepared by the Ocean Dynamics Laboratory at the University of British Columbia (except for the added clip art) To be accurate the whale icons should be about half the size shown, but they are too hard to see when made to scale).

The middle graph shows the temperature difference of the water masses, and the blocks of salmon illustrate that as the fish move from salt to fresh water on their journey home, they move around the water column to track the river source (the river water is warmer and lighter than regional ocean temperatures - we will cover river plumes and salmon in later posts).

The bottom graph shows the speed of the currents as they moved vertically up and down the water column, and the whale icons show the direction of movement.

If you have spent much time whale watching, you will be familiar with the whales' ability to pull a vanishing act at times...they just seem to submerge and disappear, or surface a mile a way. Maybe the orcas are hitching rides on deep waves or currents.

It is certainly possible...

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Candace Calloway Whiting