There's a lot of action out in the Pacific Ocean this week. Millions of salmon heading back towards the coastal waters of British Columbia, back from an annual journey thousands of kilometers out into the north and western Pacific.
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Since last spring, salmon have been hitching a ride on the currents known as the North Pacific or Alaskan Gyre. Imagine a huge swirl of water running counter-clockwise up from British Columbia, mirroring the Alaskan shoreline, then moving down around Eastern Russia to Japan and returning eastward back to British Columbia. It is actually far more complex than a big, swirling donut of water—there are a multitude of smaller currents acting within it.
Evolution has led them to spawn in the relative stability of a fresh water environment (cool, clean and well oxygenated freshwater is perfect for incubating their eggs), followed by a move to the ocean to take advantage of the abundant food available in the marine environment.
Up the rivers connected to the North Pacific, alevin (young salmon) are hatching from eggs deposited last fall. This stage of the salmon's development sees the tiny fish still attached to yolk sacs, their only source of nutrition. That means these young salmon are surviving on stored energy from the Pacific ocean, collected in the mother's eggs while foraging in the open ocean. Still just a few centimeters long, the salmon alevin remain in the gravel bed for protection. Soon they will emerge from the streambed to venture out for food, with many becoming prey themselves.
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