October 11, 2013

The Return of the King (Salmon)

by Julia Rosen

At this very moment, beneath the swiftly-flowing surface of the mighty Columbia River, an extraordinary event is taking place: hundreds of thousands of salmon are fighting their way against the current, doggedly inching hundreds of miles upstream to the places of their birth. And this year, the Chinook (or King) Salmon are flocking to the Northwest waterway in droves. Biologists expect that by the time it’s all said and done, the run will likely quash the existing fall record of 600,000 fish when, sometime in the next few months, the millionth creature flops up the fish ladder at the Bonneville Dam, compelled by the biological imperative to reproduce.

Chinook salmon moving upstream. Credit: Dan Cook (USFWS)

Chinook salmon moving upstream. Credit: Dan Cook (USFWS)

Although salmon start and end their lives in the cool headwaters of freshwater streams like those that form the tributaries of the Columbia River, they pass the majority of their adult lives at sea. Most spend 2 to 5 years in the open ocean, venturing as far as the Gulf of Alaska and feasting on microscopic fare like copepods and ichthyoplankton (the scientific term for just-hatched juvenile fish). Finally, fattened for the journey, they return to their home rivers to spawn, and shorty afterward, to die.

The salmon’s epic final voyage contains all the elements of a classic adventure story: a noble mission, formidable challenges, dangerous enemies, unavoidable suffering, and ultimately, self-sacrifice. They must swim 1,000 miles upstream without eating a morsel, propel themselves against raging currents and up cascading waterfalls (sometimes into the hungry jaws of a bear), and successfully reproduce, all before the ticking timer of their inevitable death runs out.

Salmon leaping up Willamette Falls, a cascade on a tributary of the Columbia River. Source: NOAA Photo Library

Salmon leaping up Willamette Falls, a cascade on a tributary of the Columbia River. Source: NOAA Photo Library

But it is a journey they must undertake. Spawning is of course necessary to breed the next generation of salmon, but it is also vital to predators who rely on the glut of fish for a fall feast. The salmon run also acts as a major “nutrient pump” — rivers typically lose nutrients as they are swept downstream, but the salmon swim back full of nitrogen and phosphorus, which returns to inland wetlands, streams, and estuaries when they die and decompose.

In recent decades, however, new challenges have driven salmon runs to precipitously low numbers. Dams have blocked access to historical spawning grounds while fishing has reduced the population of healthy breeding adults. Other factors such as increasing water temperatures and changes in river sediment load further stress salmon populations. In 2002, the National Science and Technology Council Committee on Environment and Natural Resources estimated that access to 40% of salmon breeding grounds have been blocked and found that salmon reproduction fell below the replacement rate (the rate needed for parents to replace themselves with their offspring) as far back as 1984.

The life cycle of a Pacific Salmon takes it from rivers to the ocean and back again. At every steps, they face challenges of a changing world, shown in the shaded bubbles. Source: Washington State Recreation and Conservation Office

The life cycle of a Pacific Salmon takes it from rivers to the ocean and back again. At every steps, they face challenges of a changing world, shown in the shaded bubbles. Source: Washington State Recreation and Conservation Office

To address these problems, managers have mounted a massive hatchery campaign to supplement Pacific wild stocks with hatchery-raised animals. They have also pushed for placing limits on salmon catches to ensure that enough adults survive to spawn (all 5 species of Pacific salmon are listed as threatened under the Endangered Species Act). Last, they advocate for installing fish ladders at dams.

When the Bonneville Dam was completed in 1938, spanning the Columbia River 40 miles upstream from Portland, it included the first so-called “fishway.” Today, the Bonneville ladders provide safe passage for fish around the dam’s 77-foot-high walls, and attract hoards of tourists, and even an unwelcome crowd of hungry California sea lions. They also provide a means of quantifying the annual runs; fish counters watch the ladder pools during the day and examine videos of the evening traffic to tally the fish. This year, they have documented numbers that topped already high expectations.

Construction of the first fish ladder on the Washington side of the Bonneville Dam began in the 1930s. Source: Historic American Building Survey.

Construction of the first fish ladder on the Washington side of the Bonneville Dam began in the 1930s. Source: Historic American Building Survey.

The success of the Chinook salmon owes in part to a turn of good luck — ocean food sources have been plentiful in recent years and a weak El Niño kept waters favorably cool. However, the concerted effort of government agencies to bring salmon populations back from the brink deserves credit too. From fishermen with bulging nets to piscivores with sated bellies to onlookers at the Bonneville Dam, everyone in the Northwest is celebrating the return of the king.

LEARN MORE

Learn more about how animals like salmon develop behavioral and breeding patterns that promote survival in our module on Adaptation.

Check out the Bonneville Dam Fish Cameras to see if any fish are swimming up the ladders right now!

Watch this beautiful PBS documentary on the life cycle of wild Pacific salmon.

Julia Rosen

Written by

Julia Rosen is a freelance science writer and PhD student at Oregon State University. She received a Bachelor’s degree in Geological and Environmental Sciences from Stanford University before beginning her doctoral research on polar ice cores and climate change. In between, she did her “Master's” in backpacking around the world and skiing. Julia is a periodic contributor to Oregon State’s research magazine, Terra, and helps write blog content and develop learning modules for Visionlearning.