Visitors to North Head Lighthouse and the Lewis & Clark Visitors Center in Cape D State Park often notice the sweep of other-colored freshwater that wraps around Cape D and north along the beaches before spreading west and merging into the waters of the Pacific Ocean. They and boaters off the Columbia Entrance see 'rip lines' in the water, along which fishing boats drive, hoping to catch salmon. There may be several species of seabirds, sealions, seals, and whales all feeding along the rips.
This is the Columbia Plume, a massive flow of freshwater that slowly merges with saltwater off our coast. It brings nutrients, sediments, and yes, garbage and pollutants, to local ocean waters. The Columbia River is so large that the mixing zone, where fresh and salt waters merge, occurs mostly offshore, not in the river itself. Mixing takes place over many days and thousands of square miles.
Columbia River water retains a distinct chemistry, and has been tracked offshore in the open ocean south to California and north off British Columbia. The productivity of the Plume results from two large nutrient sources coming together in one place. Upwelled water from the deep ocean meets Columbia water, and the result is a very large increase in productivity.
We take it for granted, living here, and fishing for seafood.
Where there are nutrients in salt water, there are phytoplankton, and zooplankton, and the animals that feed on them, and those that feed on those animals, all the way to humans fishing for salmon. Northern Herring spawn in the plume. Their young feed on plankton so the Plume is an optimal place for them to spawn.
Many fish, including salmon, seals, porpoises, and pelagic birds feed on these species. When Northern Herring school to feed, predators follow. On some days, even from shore, you can see hundreds of thousands to millions of birds feeding from the skies, often more than twenty species, swirling in huge masses above the fish schools in one great mega-flock.
Beneath the surface, and so out of sight, are fish and marine mammals, also feeding on herring. Mega-flocks form up regularly when herring group into large schools. They are common only in a few areas along the Pacific Northwest coast, and the Columbia Plume is one of the main areas where mega-flocks occur. Cormorants, loons, grebes, fulmars, gulls, murres and alcids participate in mega-flocks.
Other bait fish can also trigger mega-flock formation. These include Northern Anchovy and Pacific Sardine.
Where there are salmon, seal and porpoise, top predators show up, including sharks and Killer Whales. Killer Whales are regular visitors during early to late spring in the Columbia Plume. Gray Whales migrate north during spring; Killer Whales target the young calves. While some sharks are seasonal, many are here year round, and they also cruise the Plume for prey.
I met Dr. Jen Zamon of the NOAA Hammond Research Laboratory in winter 2015 to learn more about the Columbia Plume and mega-flocks of pelagic birds. One of my questions was where the large herring schools/mega-flocks occurred most often. Based on her research and others, mega-flocks occur anywhere over the continental shelf from the south end of the Olympic Peninsula to south of Tillamook Head. More than half the time, they are within 15 miles of shore.
|Photo by Dr. Jen Zamon of a mega-flock of pelagic birds feeding on bait fish north of North Head off Seaview, Washington. This flock was composed of several million birds of 20 species.|
Occasionally they are very close in, and this is when we can view them from land. I saw a mega-flock for the first time in Spring 2014, from the South Jetty viewing platform––it had around 400,000 birds. Flocks of more than one million birds are common. Jen saw a mega-flock off Beard's Hollow, easily visible from land. They also form inside Willapa Bay and Grays Harbor when large schools of bait fish enter the estuaries.
We live in ecologically uncertain times, so every time a mega-flock forms, it's reassuring to know that something this spectacular and prolific is still able to happen. Like icebergs, we see just the above-water portion. In the water and out of sight are hundreds of thousands of other predators, feeding on herring and other bait fishes. And other predators, feeding on them.