Chinook salmon are a threatened species of anadromous fish, migrating thousands of miles to breed. In the face of climate change, a minority group of juvenile salmon have adopted a novel life history trait that, so far, has been the life
support of the entire Californian population. Now, the innovative young salmon face a new threat of habitat loss, as waters warm and impassable dams block their access to cooler waters.
Chinook salmon, or king salmon, are the largest salmon species found in any ocean. Consequently, they make an appealing haul, not just for fishermen, but for bears, orcas, birds of prey, seals and other predators. This makes the salmon a
keystone species, one which plays a critical role in the ecology of an area. Chinook salmon are only found in the Pacific Ocean, so while you will struggle to find one in a British fishmonger, they are frequent in shops and restaurants all
over the Americas.
While they spend the majority of their life in marine waters, Chinook salmon are born in freshwater and return to spawn (breed) at the end of their lives. This migration makes the salmon an anadromous (upward-running) species—one which
migrates upriver, from saltwater to freshwater.
Crudely speaking, Chinook salmon can be categorised into two ecotypes (or subspecies)— spring-run and fall (or autumn)-run. Fascinating research at the University of California (Santa Cruz) has shown that these two ecotypes are not separate
ecotypes at all, but part of one larger, diverse population. The Chinook’s migratory timing is determined by just one short piece of DNA, depending on which version a fish inherits from its parents, not dissimilar to us having blue or brown
Spring-run salmon leave the sea earlier in the year than their fall-run counterparts. Such an array of migratory timing gives the Chinook salmon an advantage, by spreading the risks associated with these migrations over time and space.
Biologists have coined this ‘the portfolio effect’, whereby a species with a broad portfolio of traits is more likely to survive in a fluctuating environment. The salmon population is not putting all their eggs in one basket—nor one river.
In the California Central Valley, spring-run salmon are the dominant Chinooks, though their numbers are far from that of a thriving population. These Californian spring-run fish are up against overfishing, habitat loss and, in recent years,
impassable dams. As a result, the Central Valley spring-run salmon are listed as 'threatened' according to the Endangered Species Act (ESA).
The California Central Valley has a thriving agricultural section, growing over half of US produce. Stretching for 450 miles north to south, it dominates central California. Unsurprisingly, a vast amount of irrigation infrastructure is in
place to help crops grow. Thirsty crops, such as rice and almonds, rely on fields and orchards being flooded. In California’s warming climate, droughts are more frequent, and more and more water is being diverted for irrigation.
Dams and water diversions provide less water, not just for the salmon, but for a huge range of species. Worse still, shallower streams are more susceptible to extreme heating. Water temperature is known to affect salmonid (trout and salmon)
growth, survival and physiology. In a warmer environment, high-elevation streams offer salmon cool-water respite from adverse temperatures, called thermal refugia.
With a warming climate, many species have been recorded to shift their phenology, referring to a change in the timing of life cycle events at the population level. This could mean a whole range of things, such as daffodils flowering earlier
in the year, or juvenile salmon migrating later to avoid the summer heat. A shift in phenology simply means the timing of the trait has changed.
Take the time of year the young salmon start their migration. Research over a ten-year period by Dr Cordoleani and her team has revealed that, as juveniles, our spring-run salmon in the Central Valley exhibit three distinct migratory
phenotypes—three observable traits when it comes to their sea-ward migration (‘phenotype’ is the biologist’s word for observable characteristics).
Dr Cordoleani found that any given juvenile was either an ‘early’, ‘intermediate’ or ‘late’ migrant, broadening the species portfolio of juvenile migration times. Early migrants are those who leave the natal stream, that is, the stream
where the fish hatches, within one month of hatching. Intermediate migrants emigrate from the natal stream after two or four months, and late migrants, fascinatingly, stay in the natal stream for the whole summer, leaving five or seven
months after they hatch.
This obviously makes the late migrants larger and older than their brothers, sisters and cousins, who left months before them, at the start of emigration. You would expect their larger size to give the late migrants a head start in the game
of life, as a large size reduces intraspecific (Chinook versus Chinook) competition.
And you would be right, late migration was indeed the most successful option. Dr Cordoleani found that 60% of returning adults were late migrants, averaged over the ten-year period. This is remarkable, considering that the minority of
juveniles were late migrants, representing only 10% of young salmon.
The migratory journeys of the salmon were revealed by measuring the strontium in the fishs’ otoliths, the closest equivalent body part a fish has to ears. Strontium is a metallic element and different rivers in the Central Valley have
distinct strontium profiles, which reflect in the deposits in a fish’s ‘ear’.
This means that the strontium deposits in a fish’s ‘ear’ act as a geographic tag, telling Dr Cordoleani where a fish has been. The otoliths are also a helpful way to measure the size of a fish, as there is a pretty clear correlation between
fish size and otolith size. Hence, we know that late migrants were larger than the earlier migrants.
But is their success just due to their size? Well, perhaps in part, but the late migrants emigrate at a cooler time of year which, in a warming climate, is the real head start in life. The years 2012, 2014 and 2018 brought intense droughts
and ocean heat waves to the Pacific Coast near the Central Valley. In these years, late migration was ‘functionally the only strategy to survive to adulthood’, states Dr Cordoleani (2021). In 2012, all the returning salmon were late
It seems that in hotter years, ever more frequent as the climate warms, late Chinook migrants act as the life support for the spring-run Chinook population—bolstering the species from collapse, despite being the minority group initially.
This surely highlights the importance of supporting both common and rare phenotypes within a species to broaden their portfolio for future resilience.
But here is the catch—as stream temperatures in the Central Valley warm, our late migrants will be vulnerable to habitat loss. Remember, late migrants make the most successful adults, so losing them means losing many healthy adult fish
later down the line.
While late migrants have adapted to avoid the migratory corridor in the heat of summer, this trick relies on rearing grounds and natal streams being cool in the summer months. By 2040, stream temperatures in the California Central Valley
are predicted to increase by 0.6°C, and by a whole degree by 2080.
With no intervention, late migrants will be left with less than half of the suitable habitat accessible during Dr Cordoleani’s study period. Man-made dams, such as the Nimbus Dam on the American River (near Sacramento), act as impassable
barriers to high-elevation pools and streams, which are deep enough to stay at a suitable temperature.
The remaining areas downstream of dams are often shallower than they should be, as more water is diverted for irrigation. These shallow channels heat up far quicker than the deeper streams our salmon are used to. It is currently unclear
whether the salmon can adapt their thermal tolerances fast enough to keep pace with the predicted warming, potentially rendering the salmon’s current thermal refugia as inhabitable.
The loss of high-elevation thermal refugia is especially harmful to the spring-run Chinook salmon population, as it is likely to increase mortality rates in late-migrant juveniles who, as we have seen, are the life support of the Chinook
population in the changing climate.
But think of the river as half full, not half empty. Providing access above dams in a number of rivers, including the American and the Sacramento rivers, would triple the area of suitable rearing habitats for the late migrants.
Ebersole (2020) summarised managing thermal refugia very well in his paper, saying they are ‘making the most of a bad situation’. One such strategy is being used in Oregon in order to protect the endangered Steelhead trout.
In 2009, thousands of juvenile trout died following reduced flow in the Fifteenmile Creek. In response, the Fifteenmile Action to Stabilise Temperature (FAST) programme was launched, aiming to balance the needs of the fish and the farmers.
The state uses predictive models of rainfall and streamflow to forecast stream characteristics. When the water temperature is predicted to exceed the threshold for trout survival, irrigators are automatically called. Irrigators reduce their
withdrawal and are reimbursed for doing so.
The FAST programme has been a success, with no documented fish deaths in the Fifteenmile Creek during Oregon’s 2015 drought, despite nearby areas seeing fish fatalities on-mass following two droughts over a 16-day period.
A similar programme could be launched in the Central Valley. Of course, better than this would be to remove dams altogether, but this would be met with outrage from locals and governments; as mankind loses control of the water flow,
limiting financial gain in many industries.
Ebersole also reports that re-establishing beaver populations is being used across the USA to restore aquatic habitats. In the 19th century, the North American beaver was almost eradicated from Western states through habitat
extermination and hunting for their pelts and fur. The reintroduction of beavers to California Central Valley could bring deeper pools in areas downstream from the man-made dams, giving the salmon access to thermal refugia.
There is still a long way to go until the Central Valley spring-run Chinook salmon are safe. However, knowing that the rare late migrant juveniles carry the most importance to the population’s success in drought-conditions is a step in the
right direction for successful conservation efforts.
The success of the late migrants underscores that conservation work cannot just focus on helping the most abundant salmon types, for they, like us, are part of a dynamic population which interacts with the physical environment in a wealth
Cordoleani, Florence (2021) 'Threatened salmon rely on a rare life history strategy in a warming landscape, Nature Climate Change,' Volume 11, pages 982–988.
Ebersole, Joseph (2020) 'Managing climate refugia for freshwater fishes under an expanding human footprint,' Frontiers in Ecology and the Environment, Volume 18, pages 271-280.
Isaak D. (2017) 'The NorWeST Summer Stream Temperature Model and Scenarios for the Western U.S.: A Crowd-Sourced Database and New Geospatial Tools Foster a User Community and Predict Broad Climate Warming of Rivers and Streams.' Water
Resources Research, Volume 53, pages 9181–9205.