Salmon eggs

Over 100 years ago, in 1910, workers began construction on the first of two hydroelectric dams that would eventually be built on the Elwha River, on Washington State’s Olympic Peninsula. Before the dams were built (the lowest just five miles from the river’s outlet on the Strait of Juan de Fuca), the Elwha was home to robust populations of several species of Pacific salmon. After the dams were built, most of the river was cut off from the ocean – salmon could no longer migrate back to freshwater to spawn, reproduce, and nourish the streams where they were born.

Last August, three years after the largest dam-removal project ever conducted in the U.S. began, the last section of the uppermost dam was demolished, and just weeks later, salmon were back in the upper Elwha.

Scientists anticipate that salmon will continue to follow their migrations upstream and recolonize the upper Elwha River; as they do so, some will encounter a mysterious population of fish living in Lake Sutherland, a small lake connected to the Elwha River by a creek that comes in above the location where the lower dam used to be.

Those mysterious fish are Oncorhynchus nerka, also known as sockeye salmon or kokanee. Sockeye salmon and kokanee are distinct populations of the same species that tend to either migrate to the ocean and return to freshwater streams to spawn, a trait scientists call ‘anadromy’ (sockeye salmon), or spend their entire lives in freshwater (kokanee). The population of Oncorhynchus nerka in Lake Sutherland was landlocked by the Elwha Dam for a hundred years, but researchers were not sure whether their ancestors were sockeye salmon trapped above the dam when it was built, or kokanee that might never have migrated to the ocean and back at all.

Now, a team of scientists believes they have the answer, and they came to their conclusion based on a humble clue – the size of the eggs the Lake Sutherland Oncorhynchus nerka produce.

As the researchers recently reported in the journal Ecological Research, they compared the Lake Sutherland fish eggs to eggs produced by several populations of sockeye salmon and kokanee from Alaska, Washington, British Columbia, and New Zealand. Kokanee eggs tend to be smaller than those of sockeye salmon (just as kokanee adults tend to be smaller than sockeye salmon adults). The Lake Sutherland fish themselves were typically about a foot long, the same size as the adults of the kokanee populations and half as big as the sockeye salmon adults.

Their eggs, however, were much larger than the kokanee eggs – and well within the range of the sockeye salmon eggs. The growth of the adult fish in Lake Sutherland appears to have been limited by their inability to access the ocean, and, based on the size of their eggs, it’s likely that the Oncorhynchus nerka in Lake Sutherland are descendants of sockeye salmon.

The scientists note that this has “immediate relevance to the restoration” of salmon in the Elwha River, because “it would mean that other traits linked to anadromy might also remain in the population, facilitating the resumption of anadromy in this population.”

Now that the dams on the Elwha River have come down, several species of salmon will once again be able to migrate upstream to spawn – and some Oncorhynchus nerka may finally be able to make it downstream to the ocean, before returning to freshwater to start the cycle anew.

The Elwha Dam in October 2011, about a month after the removal project began. 

(Image by Sam Beebe via Flickr/Creative Commons license)


Sometime in the next five or six weeks, the ice on the Tanana River in Nenana, Alaska, 55 miles southwest of Fairbanks, will break up. Every year since 1917, local residents have hosted a contest, called the Nenana Ice Classic: anyone who purchases a ticket can guess the exact date and time the ice will go out, and the closest guess wins the pot – in recent years, so many people have entered that multiple people have picked the right minute, and they’ve had to divide the prize money.

Last year’s winners split $363,627.

With that kind of cash on the line, it’s no surprise that the organizers of the Nenana Ice Classic have kept meticulous records. We know when, exactly, the ice on the Tanana broke up outside Nenana each year for almost the last hundred years – that’s the kind of archive that ecologists dream about, because it’s a long enough record to allow us to see changes over time.


The ice on the Tanana River in Nenana, Alaska, breaks up between mid-April and mid-May each spring. Since 1917, when the Nenana Ice Classic began, the average trend has been toward earlier ice-out dates. 

Sources: Data from the National Snow and Ice Data Center and the Nenana Ice Classic

(Figure by Emily Benson)

Environmental cues that organisms use to time their migrations or developmental milestones are changing as the world’s climate changes: plants are blooming earlier than ever before, frozen rivers are thawing sooner and sooner, and in some places, salmon are returning to freshwater to spawn weeks earlier or later than they have in the past.

That can be a problem for organisms that rely on the salmon, and their eggs, for food – if those animals don’t know when the salmon will be arriving, they might miss their chance to chow down. Enough mismatches in timing, and some species might face a serious threat to their survival.

A group of scientists working in a coastal Alaskan stream recently investigated the migration timing of Dolly Varden, a type of fish that often lives in the same streams as salmon, and which sometimes depends on salmon for food. As the authors write, “[w]here salmon remain at historical levels of abundance, Dolly Varden can acquire the majority of their annual energy intake by gorging on salmon eggs.”

The researchers recently reported their results in the journal Freshwater Biology. They compared the timing of Dolly Varden migrations to salmon migrations over ten years, and they also analyzed environmental conditions, like water temperature and precipitation, to see if Dolly Varden were responding to environmental cues (in which case they might be at risk of missing the salmon migration), or to the movement of salmon themselves.

They found that Dolly Varden seem to synchronize the timing of their migration with that of salmon. Dolly Varden migrations “appear to be cued directly by salmon migration rather than environmental conditions,” suggesting that Dolly Varden are less vulnerable to a timing mismatch than they might be otherwise.

Still, not all animals will be as lucky as Dolly Varden. The authors point out that Dolly Varden can likely see or smell salmon as they return to freshwater to spawn, alerting them to their presence; other migrating animals can’t be assured that the resources they depend on will await them at the end of their journey, and must rely on environmental cues as a proxy. Those organisms are the ones most vulnerable to a timing mismatch, and the ones most likely to suffer negative consequences as environmental indicators – like the date the ice goes out on the Tanana River – continue to shift in time.

Dolly Varden appear to base their migration timing on when salmon are migrating rather than on environmental cues. 

(Image by cinaflox via Flickr)

Salmon steaks

Picture a salmon steak, maybe marinated in olive oil and spices, grilled just long enough to crisp the edges, served with roasted asparagus and wild rice – that’s a dinner I would be happy to serve (and eat).

Salmon have long been a key source of protein for the people living in the Pacific Northwest and Alaska, in addition to being culturally and economically significant. Yearly runs of salmon returning from the ocean to reproduce in the freshwater rivers and streams where they were born represent a pulse of nutritiously rich food that supports river ecosystems as well as people – adult salmon die after they spawn, releasing nutrients incorporated from the ocean into the freshwater systems where the fish originated.

In rivers, the first recipients of this marine bounty are the macroinvertebrates that colonize and consume salmon after they’ve spawned (and which will eventually serve as food for the juvenile fish that hatch from the salmon eggs). As reported recently in the journal Aquatic Sciences, a team of scientists from the University of Notre Dame investigated how the precise location of salmon carcasses influences the macroinvertebrates that devour them.

The researchers, working in streams on Prince of Wales Island, Alaska, set out salmon steaks in four different areas of each stream – buried in the streambed, resting on top of it, lying out of the water on a gravel bar, and placed above the water among the trees next to it. They found that different types of insects were present in the four habitats – as they expected, terrestrial locations (the gravel bar and the riverbank) featured macroinvertebrates that specialize in consuming carrion, while underwater locations were dominated by generalist consumers, aquatic insects that were likely already living in those locations and opportunistically consumed the salmon steaks.

Despite high rates of decomposition, the scientists also found that, by the end of their experiment (two weeks in some locations, four weeks in others), the remaining salmon was still high in nutrients. They interpret this to mean “that salmon carcasses may provide a resource supporting a succession of consumers over an extended period of time.” In other words, the pulse of energy that a salmon run brings to a river may be less of a spike and more of a slow release, sustaining different types of organisms as it plays out.

Salmon are more than a source of fillets and steaks for human plates – they also nourish the river ecosystems where they’re born by returning there to spawn at the end of their lives. 

Salmon are born in freshwater streams, grow large in the ocean, then return to their natal streams to spawn and expire. 

(Image by Todd Gordon Brown via Wikimedia Commons)