The how and why of salmon migration


This is the season we celebrate salmon returning to their natal streams and rivers right here in Seattle, but how do salmon find their way home? Before we tackle that, though, a larger question: why do they do it?

The ultimate purpose for salmon to return to their home streams and rivers is to reproduce and ensure the survival of their offspring. Simple enough. But why is returning to the natal site part of the process? Consider the alternative: swimming upstream to just any old river could have some pitfalls. A random river might not have suitable sites for spawning, but a salmon’s birthplace is already a proven success for spawning. It may not have mates of the same species. Or conditions might not favor that type of salmon. For all these reasons, we can see why salmon navigate their way home.

In recent years, studies have shown that in the open ocean environment, salmon use the magnetic field of the Earth to guide their migration. This helps them move from the coastal areas near their spawning grounds to rich feeding areas, and then back again toward the end of their lives. For example, most of the salmon returning to Seattle-area rivers right now are coming from feeding grounds in Alaska, but some may be traveling from as far as Japan.

Salmon use both the intensity and the inclination of Earth’s magnetic field to orient themselves. Unlike their navigation by sense of smell (discussed below), this ability appears to be genetically inherited by a salmon, not learned along its migration.

salmon at the Seattle Aquarium

Young salmon learn the smell of their home stream, possibly even memorizing it at various points along the way, as they migrate toward the ocean. As adults returning to freshwater, when they encounter that familiar smell, it stimulates them to swim upstream. So there may be some “testing of the waters” as salmon migrate home. If they swim up the wrong river, that memorized scent of their birth stream will fade, decreasing their drive to swim upstream. Then they may travel downstream for a bit, until they encounter that home stream smell again. The more they sense the smell of their birthplace, the more they swim upstream. It’s a bit like playing that child’s game of “hot and cold.”

There are still many unknowns in the famous story of the salmon swimming upstream. Evidence exists that salmon from different reaches of the same river will tend to migrate to the same stretch where they originated. But do they return to the very same nest site where they were hatched? How close do they get? At some point, that urge to return home will be up against other factors: selecting a nest site, selecting a mate, using remaining energy stores.

Interested in learning more about salmon migration and salmon in general? Join us for the Cedar River Salmon Journey to talk to trained naturalists while watching salmon spawn! Click here for remaining dates, times, locations and directions.

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A historic moment for climate change

one world ocean

Last week, in a historic move, the Paris Agreement on climate change was formally ratified by the European Union, ensuring the Agreement had enough support for entry into force in early November. The ratification is essential for galvanizing the climate action that will help restore the health of Earth’s one ocean.

First things first: what exactly is the Paris Agreement?

The Paris Agreement is the world’s first comprehensive climate agreement, and represents the first time that all countries have agreed they will act together to limit the rise in global temperatures. Countries signing the Agreement agree to hold the increase in global average temperature to well below 2°C and to pursue efforts to limit the temperature increase to 1.5°C. For the U.S., this means cutting emissions by up to 28 percent by 2025 as compared to 2005 levels.

Wait, wasn’t the Paris Agreement agreed upon in December?

At least 55 countries representing 55 percent of global emissions were needed to formally sign on in order for the agreement to be binding. As of October 13, 78 nations had officially ratified the agreement.

The work is just beginning.

Participating countries must begin to determine how to set these ambitious climate change strategies into motion. For the U.S., this means developing new and innovative low-carbon solutions and deploying new energy technologies. As U.S. Secretary of Energy Ernest Moniz stated, “With each country doing their part to meet the Paris Agreement, we can leverage science and technology to combat climate change, improve quality of life around the world, and create unprecedented opportunities through a low-carbon economy.”

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A closer look at the wonder of salmon migration

sockeye salmon

Many people know that the broad term “salmon” encompasses several different species. Seven of those are found here in the Pacific Northwest: chinook, coho, chum, pink, sockeye, steelhead and cutthroat. And, within those seven species in Washington state are a whopping 486 distinct populations—each one a scientifically designated, biologically distinct group of individuals (e.g., Lower Columbia River spring chinook; Skagit River coho) adapted to specific streams, estuaries and other conditions.

When people join us for the Cedar River Salmon Journey each October to see salmon spawn, they’re witnessing the journey of a specific group of salmon, through specific conditions that only the Cedar River provides. Because of varying conditions from river to river, each salmon stock has slightly different timing for their reproduction: when they are signaled out in the ocean to return, when they start to move upriver, and when their eggs hatch.

What dictates this timing? What clocks do salmon set their watches by? Thomas P. Quinn explains in his book, The Behavior and Ecology of Pacific Salmon & Trout, “Unlike many freshwater fishes, whose migration and reproduction are strongly influenced by environmental conditions such as temperature, flow or rainfall, in salmon these events are more strongly controlled by genetic factors, as adaptation to the long-term average conditions that prevail in the waters affecting each population. From the ocean, the salmon cannot assess conditions in the river, so selection favors salmon migrating at the right time.” This partly explains why the salmon in the Seattle Aquarium’s exhibits start to mature, even without the environmental cues of the ocean or a river to guide them.

This also explains why it’s so difficult to transplant a population of salmon from one place to another. Their genes determine reproductive timing and, over centuries, those genetics have been honed to a particular stream or river.

One exception to this is the population of Cedar River sockeye. Sockeye are the most numerous salmon to spawn in Cedar River, and this sockeye population was transplanted from Baker Lake stock in the 1930s, after the construction of the Lake Washington ship canal and Ballard Locks. That engineering project lowered the level of Lake Washington by over eight feet and dried up the Cedar River’s natural course of flow to Puget Sound—through the Black River and into the Duwamish. Sockeye salmon were a perfect choice to plant in the Cedar River, which now flowed into Lake Washington, as they are the salmon species whose young rear in a lake before heading out to saltwater.

Cedar river salmon journey

Interested in learning more about salmon migration and salmon in general? Join us for the Cedar River Salmon Journey to talk to trained naturalists while watching salmon spawn! Remaining dates include October 8, 9, 15, 16, 22 and 23 from 11am to 4pm at four locations along the Cedar River in the Renton and Maple Valley areas. Click the link for details!

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How low can you go? Puget Sound edition!

How low can you go

“How deep can you dive?” It’s a question frequently asked by Aquarium visitors attending one of our daily diver shows. And it prompted us to look into depths overall—specifically, what you might see at various depths in Puget Sound.

First, some perspective. The Space Needle, that towering icon of the Seattle skyline, is 650 feet tall. Puget Sound, at its lowest point just north of Seattle, is almost a full third deeper: 930 feet. And there’s plenty going on between the surface and the low point. For instance…

0­–300 feet deep:

Sunlight limits bull kelp growth to depths of about 80 feet. Scuba divers are typically certified at depths down to 130 feet, but this can be increased with specialized equipment and gas mixtures. The shallow-dwelling cabezon (a cool-looking fish that lays poisonous eggs!) has been observed down to 250 feet. Sea otters can descend to depths of up to 300 feet. Marine mammals can induce bradycardia, slowing their heart rates from 55–120 beats per minute to 4–15 beats per minute to conserve oxygen while diving. In this state, their bodies shunt blood toward the brain and heart, preserving these essential functions during a deep dive. Marine mammals also collapse their lungs and store oxygen in their muscle tissue when diving, to prevent pressure-related injuries and help them sink rapidly (alcids do this too—see the next section for more details!).

300–600 feet deep:

At this range, we might find some of the same animals seen on the beach at low tide, such as the leather star. Down to 300 feet, this star can find its prey, such as anemones and sea pens. The average depth of Puget Sound, by the way, is 450 feet. Amazingly, the common murre has been documented at a depth of 492 feet below the surface. Alcids, or diving birds, have a variety of adaptations that allow them to dive deeply, including: special muscles for flattening their feathers and pushing out trapped air to reduce buoyancy; a clear nictitating membrane, or “inner eyelid” to enhance their vision underwater; and the ability to slow their heart rates during dives, reducing their need for oxygen.

600–900 feet deep:

Below 600 feet is considered “the twilight zone” because little light penetrates at this depth. Photosynthesis is not possible. You might be surprised to learn that surface-dwelling salmon can be seen at depths of up to 820 feet. They typically spend time near the surface to find their food, but may need to dive that deep to avoid becoming food themselves.

900–1,200 feet deep:

The deepest part of Puget Sound is off Point Jefferson, five miles northwest of Seattle, where the sea floor drops to 930 feet. Rockfish, on average, are found at depths of around 900 feet—but they’ve been documented well past 3,500 feet and are adapted for extreme high-pressure, low oxygen environments. Orcas average 1,000 feet.

Interested in learning more? Visit the Seattle Aquarium and don’t miss our daily diver shows!

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A fond farewell to sea otter pup Rialto


September 19 was a bittersweet day for us at the Seattle Aquarium, as we said goodbye to Rialto and wished him a lifetime of health and happiness in his new home at the Vancouver Aquarium.

The Aquarium has a long history with sea otters—that’s part of the reason we were selected for Rialto’s care and rehabilitation after he was orphaned. He’s the third young sea otter that we’ve raised: Lootas came to us after being orphaned at approximately six weeks of age in 1997; Calypso was about eight weeks old when she was brought to the Aquarium in 2003; and, of course, Rialto joined us in early August.

“Rialto was much more challenging than the others because he arrived sick and emaciated and required the involvement of more veterinary professionals,” says Aquarium Curator of Conservation Research Dr. Shawn Larson. “He had a 20 percent chance of survival during his first couple of weeks here, and now he’s got a clean bill of health.”

Even healthy, Rialto requires 24-hour care—that’s the case for any young sea otter pup. “They have a basic routine that gets repeated over and over,” Shawn says. “Sleep, wake, bathroom, feed, play, groom, sleep again. The only time his caregivers could step away even for a moment was when Rialto was sleeping, usually about an hour at a time, and his naptimes were used for making more formula and catching up his laundry.” (Rialto’s formula, the consistency of heavy whipped cream or yogurt, was composed of a blended surf clams, water and puppy replacement formula—yum! Towels were used to groom him and line his “waterbed,” a cushion filled with cool water.)

Rialto is currently the only Washington-born sea otter under human care—and as such, he’s serving as an ambassador for Washington sea otters in the wild. Our state’s sea otter population was hunted to extinction by 1910. In 1969–1970, Alaskan sea otters were introduced to the Washington coast. It’s thought that as few as 10 survivors from those translocations founded the current Washington sea otter population, which now numbers over 1,700 animals. Such translocations account for over 35 percent of the sea otters alive today: approximately 30,000 animals in Washington, British Columbia and southeast Alaska. Translocation is the most successful management tool used to conserve sea otter populations.

Rialto underwent quite a transformation during his weeks at the Aquarium, going from tiny and desperately ill to growing and thriving. By the time he left us, he had all of his baby teeth and was voraciously eating 25–28 percent of his body weight each day via formula and solid foods like clams, squid and shrimp. He loved playing with toys (favorite: ice toys with delicious squid heads in them!) and frequently slept with an ice pillow—mimicking what he might have experienced in the wild while nestled against the salt water-coated fur of his mother.


Rialto will still need 24-hour care in his new home at the Vancouver Aquarium, and is slated to begin integrating with the other sea otters there when he’s about four months old. Getting him to the point where he ready to travel required the time and expertise of a dedicated and passionate team. To close this post, we’d like to share highlights shared by some of those involved in Rialto’s care:

“I really enjoyed observing Rialto’s curiosity for the world grow. When he first arrived, he really seemed to only see what was right in front of him. By the end of his stay, he was very curious of everything and wanted to explore the world.”

—Amy Green




“Rialto will be greatly missed but we’re thrilled for the Vancouver Aquarium to receive such an amazing animal to educate and excite their visitors about sea otters and other marine life. We can’t wait to visit him!”

—Julie Carpenter




“Taking care of Rialto has been one of the highlights of my 30 years here at the Aquarium. My favorite moments are giving him his bottle of formula on my lap. I will miss him so much!”

—Carol Jackson




“There is nothing more rewarding than giving a helpless sick animal another chance at life to become a beautiful healthy individual again. It didn’t matter what Rialto was doing—sleeping, drinking from his baby bottle, learning to groom himself or swimming and playing in the water—he always made me smile. Rialto was a true sweetheart!”

— Caroline Hempstead



“My favorite part about working with Rialto was watching him instinctively learn how to use his body to swim like a sea otter. It’s amazing that we were able to rehab Rialto to give him a second chance as well as educate future generations about Washington sea otters.”

—Lindy McMorran



You can keep up with Rialto’s continuing recovery by watching Vancouver Aquarium’s baby otter cam.

Additional information:
Farewell, Rialto the sea otter, “You’re a Canadian Now”—The Seattle Times

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