By Mark Hume, with Photography by Nick Didlick
There are places and times in the Strait of Georgia, on Canada's West Coast, where the water shifts from tranquil to menacing within minutes. The sea boils, deep water upwells and then is subsumed, standing waves form, tidal rips curl around points and huge whirlpools swirl to life, standing driftlogs on end before slowly sucking them under.
At Yuculta Rapids, in Desolation Sound, the ground shakes when the tide turns; in Blind Passage, north of Campbell River, boats come to a standstill even as their diesel engines strain to advance. In Active Pass, between Galiano and Mayne Islands, the tide moves with such power that even huge British Columbia ferries slough in the current.
The churning of the waters in Georgia Strait, a sheltered body of water between Vancouver Island and mainland British Columbia which is famous for its salmon fishing, is the result of a complex symphony of immense natural powers. The sun, the moon, an intricate interplay of tidal currents and the prevailing winds combine to shift ocean waters through a web of channels, bays, points and islands spread over a strait 222 kilometers long and 20 to 40 kms wide.
The Strait used to provide the best salmon fishing in the world. But over the past few decades, catch rates have declined. Chinook and coho runs have declined to many of the inside rivers, both on the East Coast of Vancouver Island and the mainland side. There are a multitude of reasons, including logging damage, dam building, mine pollution and urban/industrial development. There has also been a decline in ocean survival, which some believe suggests the Strait has changed, and become less able to support the billions of young salmon that arrive in it each spring from spawning rivers.
And there is much speculation about the impact Atlantic salmon farms are having on wild stocks.
All that notwithstanding, the Strait of Georgia remains a remarkable area, with protected waters where you can find some incredible sport fishing for salmon - particularly in the late summer and fall when adult fish migrate through the area. You can fly cast along kelp beds for coho or chinook, troll bucktails around the points, or wade tidal beaches casting for salmon or sea run cutthroat.
Although it is often as calm as a lake, this inland sea is a dynamic area that is often in flux and any fisherman should study the tide charts before heading out. The tides move the bait fish around, trigger “the bite”, and signal the start of runs into spawning rivers. Beach anglers looking for cutthroat know to be on the water to catch the rising tide, which often comes with the surface swirls left by rising cutties.
Flood currents that are known for their “their strong jet-like flows and heavy tidal rips” enter the northern end through Johnstone Strait. In the south, a river of brackish water floods out on the surface through Juan de Fuca Strait, while a wedge of rich, saline water bulls its way in underneath, bringing with it a massive infusion of nutrients. This feeds the bait fish, which feed the salmon.
As the sea moves it grinds around the coast, froths down narrow tidal channels, charging the environment with energy and nutrients. When the wind opposes the direction of the tides, violent rips form, creating natural hazards that are the number one cause of small boat accidents in the Strait. If you get the tide moving one way, and the wind gusting against it, a calm point can suddenly turn into a dangerously choppy sea.
For more than 60 years scientists have studied the movement of Georgia Strait’s waters, using data gathered from the most remarkable sources. They have traced the paths of special “drift cards”, they’ve studied satellite imagery, tracked oil slicks from shipping accidents, and in one instance, used the final location of human and other wreckage from the 1875 sinking of the SS Pacific to backtrack the tidal flow.
They’ve used current shear probes to measure speed and deep water instruments to calculate pressure. Since 1934 lighthouse keepers have kept daily records showing sea surface temperatures and salinity.
Scientists have run computer models that are so sophisticated they can simulate what would happen if they shut off Juan de Fuca Strait or any other outlet to measure the interconnected effects. It was through just such a simulation that they learned the outflow of freshwater from the southern Strait is driving a furious back eddy on the continental shelf. The eddy creates a deep upwelling, sending waves of nutrient rich water pulsing north along the West Coast of Vancouver Island. That water, laden with phytoplankton, supports salmon fisheries, draws in migrating gray whales and sustains huge populations of birds nesting as far north as Triangle Island, off the northern tip of Vancouver Island.
In a scientific paper, Richard Thomson of the Institute of Ocean Sciences, drew much of the oceanographic knowledge together, protraying an intricate and complex waterway where the water of Georgia Strait is driven by four key forces: the tides, the winds, the open ocean and the freshwater that pours in from numerous rivers and streams.
The southern Strait of Georgia, he says, is dominated by the immense flow of the Fraser River, which dumps into the Pacific near Vancouver, and provides 80 per cent of the freswater found in the Basin.
The plume of the Fraser, silt gray, soil brown and tinged with the green of the sea, spreads out across the Strait like a huge alluvial fan. In the spring that fan shunts billions of salmon fry out into the Straight, where they distribute along the shorelines of the Gulf Islands.
Although the fresh and salt water mingle where they meet, much of the brackish water slips over the sea water beneath it “like a decoupled slab,” spreading over the southern Strait, two to 10 meters thick.
The general movement of flow is out through Juan de Fuca Strait, a crucial link to the open Pacific. As the brackish water moves out, sea water pours in underneath, replenishing the Strait’s polluted waters over a period of months. (More than 80 percent of freshwater leaves through Juan de Fuca Strait, with about 17 percent exiting through Johnstone Strait, to the North. The inflow of rich Pacific water through Juan de Fuca in the summer supplies roughly 40 times the nutrient loading that comes from the Fraser River during runoff - an estimated 1,500 tonnes of nutrients a day. )
The flow of the Fraser determines the rate of flushing in the Strait. In winter, with snow locking precipitation in the mountains, the river drops and it takes 100 to 200 days for the Basin to flush. In summer, spring run-off cuts the flush period to as little as 50 days.
The reason it takes the Strait so long to “flush”, despite the powerful tidal currents, is that winds, land forms and opposing tidal floods cause the water within to rotate slowly.
Water that floods in from the North, through Johnstone Strait, favors the western side of the Strait while water moving from the South, through Juan de Fuca, favors the eastern side, creating a counter-clockwise circulation.
Tidal changes are the events that most fishermen key their days to.
There are two ebb and two flood tides every 25 hours as the Pacific reacts to the pull of the moon.
In his paper, Mr. Thomson describes the tide not as a series of local events, but as an elongated wave - 22,000 kilometers long - that “leans” up against the West Coast of North America. Although the ocean appears to gently rise as the tide comes in, in fact a giant wave is traveling along the outer coast at 740 kms an hour, steadily lifting the water level.
It takes the tide about 20 minutes to pass along Vancouver Island’s entire outer coastline. The same “tidal” wave, however, takes five hours to travel through the constricting maze of channels, passages and narrows inside the sheltered waters of the Strait of Georgia.
“This leads to significant differences in tidal elevations through the narrow passages within the system and produces some of the strongest tidal streams in the world,” writes Mr. Thomson.
Violent tidal streams in Seymour, Yuculta, Surge, Hole-in-the-Wall, Okosillo and Arran rapids are created as the tidal wave coming in from the South, and the one moving down from the North, race towards a meeting in the narrow channels of Johnstone Strait. It is not coincidence that these high energy areas are known for their good fishing.
The movement of water in Georgia Strait is also affected by weather patterns far out in the North Pacific. During the winter a system known as the Aleutian Low prevails, creating mostly southeasterly winds, while in summer the North Pacific High pushes winds from the northwest and west.
Juan de Fuca Strait is a funnel for westerly winds that are created as the land heats during the day. The sea breeze typically starts around noon, peaking in the afternoon, when sail boarders can be seen jumping from wave crests near Tsawwasen.
Fishermen who spend time searching the waters of the Straight of Georgia would be wise to study the tides, not just in terms of when the high and low marks are - but also in relation to the seasons, the winds and the outflow rates of the inside rivers. These are elements the salmon respond to, like instruments to a conductor.
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