Photo:  Pacific Underwater: Salmon don't grow on trees, but trees grow on salmon

Credit: Jitze's Photostream via Flickr

By Panos Grames, Communications Specialist

The month of October sees the boundaries of the Pacific Ocean push far beyond its shores. Every coastal British Columbian recognizes the seasonal dump of water from the Pacific, lifted through transpiration, which cascades onto the coastal rainforest. But, there's another part of the ocean currently surging hundreds of kilometers inland: Pacific salmon.

October's Pacific rains create prime spawning conditions for wild Pacific salmon. It's also a banner month for the towering trees of British Columbia's coastal temperate rainforests. However, you might be surprised that it's the salmon, not the rain, that make it such a great month for the trees.

This story of salmon, bears and trees illustrates the interconnected web of life, and has aptly spawned the name "The Pacific salmon forest".

Right now, millions of chum, pink, chinook, and coho salmon are leaving the Pacific Ocean to swim up rivers both all along the British Columbia coast. Waiting alongside these rivers are thousands of bears poised to fatten up for their winter hibernation.

Each bear carries hundreds of kilos of fish out of the streams and into the forest to consume. With the abundance of fish, the bears can afford to selectively gorge on their favourite parts of the fish, the eggs (or roe), brain, skin and back muscles, before they move on to the next fish. Sometimes they eat as little as five per cent of the fish, leaving the rest to decompose on the forest floor. That's where the relationship between the bears and the fish connects to the trees.

Tree growth in coastal rainforests is limited by the availability of the element nitrogen. Gardeners can tell you that fish fertilizer is a potent and effective way to increase the health and size of your garden plants, and the same goes for these trees. The fish left behind by the bears are packed with nitrogen, which fertilizes the trees and helps them grow to their impressive size. It sounds simple, yet the science behind it is almost as fascinating as the process itself.

Several years ago, scientists, led by Dr. Tom Reimchen of the University of Victoria, tracked uptake of nitrogen from salmon in coastal forests. They did this by following the nitrogen isotope 15, which is found almost exclusively in marine environments. It turns out that the great trees of coastal temperate rainforests—such as Douglas fir, Sitka spruce, western hemlock and western red cedar—owe a good deal of their girth to the tsunami of nutrients carried from the ocean by salmon, then into the forest by the bears. In rivers with robust salmon populations, Reimchen's team discovered peak years where individual Sitka spruce derived 80 per cent of their nitrogen from the isotope found in ocean-going salmon.

Just like there's more to the birds and the bees than what they do with flowers, there's much more to this story, too. Millions of insects also eat the leftover salmon, bolstering invertebrate populations, which in turn feed birds and other small forest creatures. Dr. Reimchen's team found insects with more than half their nitrogen coming from an ocean source, and a greater abundance and variety of both insects and plants near salmon bearing streams.
Gulls, wolves, eagles, osprey, crows, pine martin and dozens of other species also take advantage of the rich bounty of salmon. Not a shred of nitrogen goes unused by nature; even the bears' urine casts another shower of nitrogen into the forest ecosystem.

The rivers themselves distribute the salmon further. Remnants of salmon float downstream, where they are eaten by aquatic insects and smaller fish, finally drifting to the river's mouth. In the estuary, crabs and other marine scavengers devour the final remnants. Just as the Pacific Ocean doesn't end at its shores, the salmon forest doesn't end at the edge of the wood. You can find it in the birds flying by your window, in the two-by-fours that hold up your home, and in the breath you just took.

Like the coastal rainforest, salmon weaves just as completely through the fabric of First Nations culture, coastal communities and the whole of Canadian life. Clearly, it's perfect timing for today's release of the Cohen Commission report on salmon, and The David Suzuki Foundation hopes the Government of Canada dedicates proper resources to implement recommendations that truly serve to rebuild and maintain our wild pacific salmon runs.

If you live on the West Coast of British Columbia, make sure you get out in the coming weeks to witness the magic of the Pacific salmon forest (but be bear aware!). You can ask locals in the know, or check this sheet from the Department of Fisheries and Oceans to find salmon runs in your area. Enjoy.

October 29, 2012

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