Chinook, coho, chum, sockeye, pink, and steelhead— these Pacific salmon have always been a defining element of the Northwest region. Ecologically, their distribution in the upper left corner of the lower 48 is a vital source of nutrients for marine and terrestrial predators alike. Seals, whales, otters, bears, and wolves all feast on these beings as they make their way upstream to spawn as adults, at times transporting their carcasses onto land and therefore lending marine-derived nutrients to the land surrounding their natal streams, fertilizing the forest. And of course, the salmon remains that decay in-stream feed the base of the food chain by nourishing aquatic macroinvertebrates.
Indigenous cultures in the Northwest have appreciated the unique place of salmon in this environment since time immemorial. For many, the fish played a major role in determining seasonal subsistence patterns, and influenced their worldviews. Newer Euro-American arrivals to the area didn’t at first place much value on the abundant salmon, using carcasses to fertilize their farm fields rather than more directly making use of them. However, as smoking, and especially canning technologies emerged, commercial interest in the species took off and a booming industry began.
More recently, in 2007, commercial salmon fisheries contributed more than $2 billion to the economies of Russia, Japan, Canada, and the United States, directly employing 35,000 people. Meanwhile, recreational fisheries have also come to benefit the economy, especially in terms of local scales. When anglers travel to fish the many relatively pristine bodies of water in the Northwest, they bring money into rural communities that offer lodging, food, and gear. In addition, fishing license fees in many states contribute to habitat restoration goals to improve the sport into the future.
Unfortunately, climate-change related pressures on salmon habitats are mounting. Because of their unique migratory life cycle spent in both fresh and salt waters, Pacific salmon will feel the hurt of these changes at every stage of their life. However, not all is lost: increasing habitat restoration efforts may serve as a buffering force against these stressors.
Climate Change Impacts to Freshwaters of the Northwest, the Pacific Ocean, and Human Communities
Varying precipitation regimes and increasing air temperatures associated with climate change are expected to intensify the Northwest’s periods of summer drought, increase exceptionally heavy downpours, and reduce snowpack.
Drought is often defined by extremes in low streamflow, and the Northwest is not a stranger to these episodes. Under a high emission pathway, summer runoff is projected to decline 5.3% between 2011 and 2050 as compared to what was seen between in the timeframe of 1966 to 2005.
On the other side of the spectrum, extreme precipitation events are projected to increase by both frequency and intensity. These events generally occur between October and March, and are often caused by atmospheric rivers, defined as narrow, elongated swaths of warm, moist air originating in the tropics that carry large amounts of water vapor to mid-latitudes. The effect of atmospheric rivers on our terrestrial rivers often results in floods that pose a risk to human development. The number of atmospheric river days declared “most extreme” during winter is expected to quadruple by the end of this century under a high emissions pathway.
The combination of intensifying droughts and extreme precipitation events will significantly change the timing of water supply and demand to human communities with summer agricultural growing seasons. Many western communities rely on snowmelt in the spring to serve their water needs over the dry summers, but as winters thaw earlier on and less snow falls overall, this reliance will need to be rethought.
Meanwhile, climate change impacts to the oceans are manifesting in warming seawater surface temperature, changes in ocean circulation, acidifying conditions, and sea level rise.
In 2014, a large, persistent area of water more than 3.6 degrees Fahrenheit above the normal range was detected in the Northeast Pacific Ocean, and soon was dubbed “the Blob.” The Blob extended towards the West Coast shore in 2015, and continued towards the Gulf of Alaska in 2016. These abnormally warm waters caused a large bloom of toxic algae that impacted the commercial fishery on Dungeness crab—Oregon’s state crab. At the same time, warmer water fish species were seen on the Oregon Coast in unusually large quantities.
One ocean circulation pattern off the West Coast of North America all the way from Baja California to Vancouver Island is often credited for the nutrient-rich conditions that allow for productive fisheries: upwelling. Upwelling occurs during the spring and summer when winds blow strongly from the north, pushing surface waters offshore, and causing deep waters to replace them. These deep waters are colder and more nutrient-rich. It is projected that upwelling events may become weaker with climate change, but with lots of variability.
Carbon dioxide emitted into the atmosphere is absorbed in the ocean. This causes a chemical shift in which the amount of carbonate ions decreases, hydrogen ions increases, pH decreases, and acidity increases. This impairs the ability for calcifying organisms—such as corals and shellfish—to build vital body structures. Declines in these populations have food-web wide effects, impacting the fisheries of multiple species that humans rely on.
Sea-level rise is occurring due to the thermal expansion of water molecules, glacier and ice sheet mass loss, and other changes in land water storage. In response, the frequency, depth, and extent of tidal flooding is increasing, impacting human developments on coastlines. In addition, saltwater is intruding coastal groundwater sources that supply drinking and irrigation water. Further, higher storm surges are occurring, also in response to the increasing frequency and intensity of atmospheric rivers, with an often more dramatic impact to human developments than flooding associated with the tides.
In addition to the already described direct impacts humans will face due to climate change, many livelihoods within the Northwest are at risk. Given that this region’s economy is still considered dependent on natural resources, people here can be described as frontline communities, or those that experience the first and often worst effects of climate change. In Oregon in 2013, commercial seafood and manufacturing accounted for $614 million in sales, supporting thousands of jobs. In 2015, a study of Columbia River Basin tribes, including Oregon’s Confederated Tribes of Warm Springs and the Confederated Tribes of the Umatilla Indian Reservation, found that primary concerns in regards to climate change impacts included the quality and quantity of fresh water, snowpack, conditions for salmon spawning, and fishing rights. In a more positive perspective, because of the vested interest of the many involved in natural resource industries and native communities, the region has a foundation for building community resilience to climate change impacts.
Climate Change Impacts at Every Stage of the Salmon Life Cycle
Adult female salmon lay their eggs in nests they build—called redds—in the gravel streambeds. Redds create prime temperature and oxygen conditions for fertilized eggs to develop by allowing for optimum flow-through of water. After the eggs hatch, they retain their yolk sac in what is called the alevin stage of their life, and use its’ remaining nutrients until their mouth develops and they can eat small aquatic macroinvertebrates on their own. At this point, they are called parr, due to oval marks on their sides that help them camouflage in the gravel.
Different species of Pacific salmon will reside in freshwater from anywhere between a week and a couple years. During this time, they are vulnerable to the hydrologic regime changes expected to come with climate change.
Extreme precipitation events increase the amount and speed of the water in the stream quickly. This makes it hard for juveniles to navigate to calmer sections where they won’t get washed downstream. These strong waters can also scour out redds, removing the eggs and alevin from their vital habitat. In addition, more sediment is brought into streams during these events, and eventually is deposited in the gravel, which can in turn smother redds.
During periods of drought and extremely low streamflow, juveniles residing in freshwater are at a greater risk of catching diseases in warm, shallow water. If the water temperatures get above 68 degrees Fahrenheit, it is lethal. Shallow water also poses a greater risk of predation, as it is easier for terrestrial animals to see into the water and find the young, defenseless fish.
If the parr survive all these challenges, they move onto their next life stage in the mixed salt and freshwater environments of estuaries. Here they undergo a metabolically expensive process called smoltification, in which their physiology changes so that they can survive in saltwater. At the same time, they change their diets from aquatic to estuarine and marine macroinvertebrates in this more nutrient rich environment.
Changes in hydrologic regimes due to climate change could desynchronize the migration timing for salmon from freshwater to the ocean. Extreme precipitation events could push parr into estuaries before they are developmentally ready for this change in environment. In addition, the timing of nutrient availability and arrival of smolts into the ocean may be less than ideal, causing slowed growth.
As sea levels rise and inundate wetland areas surrounding estuaries, there may be fewer habitats for smolts to utilize to hide from predators.
Increased water temperatures in estuaries pose another threat for smolts, increasing their thermal stress and susceptibility to disease and predation. In Yaquina Bay in Newport, a 5.4 degree Fahrenheit increase in air temperature would lead to a 1.3-2.9 degree Fahrenheit warming of estuarine waters. The upper portions of the bay would experience up to 40 more days of not meeting the water temperature criteria set by the state for the protection of rearing and migrating salmon.
Warming waters in the ocean present a challenge to adult Pacific salmon, which spend 1-5 years there depending on the species. Warmer seawater surface temperatures—as realized during the Blob—cause thermal stress, and increased susceptibility to disease and predation. Because of this, the range of Pacific salmon in the ocean is projected to shift polewards.
Changes in upwelling patterns and acidifying conditions will change the timing and availability of food for salmon. One common salmon prey are pteropods, or sea snails. These are organisms that build shells, and are especially vulnerable to the decreased availability of carbonate ions in the water column. It is expected that large dissolution events of pteropods may triple by the mid-century.
When they are reproductively mature, Pacific salmon return to their natal streams to spawn. When they enter freshwater, they cease eating and focus all their time and energy on moving upstream. Most species undergo this migration in the fall, such as Chinook, coho, sockeye, chum, and pink salmon. Some steelhead return to spawn in the winter, and others return in the summer but hold in freshwater until the early fall to spawn. Similarly, spring Chinook return to freshwater in the spring but wait to spawn until the early fall.
Reduced summer flows of rivers could delay the upstream migration of adult salmon, in effect shortening the growing period of their future offspring. If they do not delay their migration, adults that enter shallow, warmer waters, may be more likely to die before spawning due to an increased susceptibility to diseases and higher metabolic costs. Spring Chinook and summer steelhead are especially vulnerable to these conditions.
Habitat Restoration as a Mitigation Strategy
While the impacts projected from climate change spell out a rather dire situation for the Northwest’s most beloved fish, habitat restoration is one way to protect Pacific salmon into the future. In the last decade, billions of dollars have been spent in the US on this noble endeavor on both coarse and fine scales, following the federal listing of many species of salmon as endangered.
Coarse-scale restoration activities involve watershed-level changes, such as reforestation and the reduction of impervious surface cover. These actions can decrease the load of fine sediments that enter streams—projected to increase with the extreme precipitation events associated with climate change—lessening the smothering of redds.
Fine-scale restoration actions occur within a stream’s water body, including the reconnection of historic side channels, the removal of dikes and culverts that block fish passage, and the restoration of natural bank conditions through planting of vegetation. Removing obstacles that prohibit streams from flowing across a greater proportion of their historical floodplains can improve conditions during times of both extreme low and peak flows. Adding vegetation to stream banks can reduce water temperatures—especially on narrow streams—by providing shade.
Many case studies have been done in watersheds across the Northwest to model the effects of climate change on streams and salmonids at the same time as restoration activities.
If full riparian restoration were to occur in Southern Oregon’s Rogue River Basin, peak summer stream temperatures could actually decline by a little less than one degree Celsius according to one model, even with the warming air temperatures projected with climate change. Similarly, Northern California’s Salmon River could see a quarter of a degree Celsius decline in peak summer temperatures with restoration and climate change co-occurring. Eastern Oregon’s John Day River could see a 3.8 degree Celsius reduction in peak summer water temperatures with restoration taking place co-incident with climate change. In Idaho’s North Fork of the Salmon River, riparian restoration could reduce predicted stream warming by half.
In the Snohomish River Basin of Northwest Washington, one model predicted that the Chinook salmon population would decline by 40% by 2050 given climate change impacts if no restoration were to occur. But with full restoration of the watershed, that decline would be limited to just 5%. Further, modeling work in the Grande Ronde River Basin of Northeastern Oregon showed that there could actually be an increase in the abundance of the juvenile stages of Chinook salmon by 20% in a major tributary, Catherine Creek, and by 63% in the Upper Grande Ronde River.
In general, restoration activities tend to be concentrated in the lower reaches of watersheds, where more habitat degradation historically occurred. However, climate change impacts are expected to be greatest at the higher elevations of basins, where relatively pristine habitat conditions currently exist. This means that there is a possibility that the impacts of climate change and restoration together could shift salmon distributions to lower elevations locally. While this would certainly have its own ecological impacts to the watersheds of the Northwest, stream restoration could prove an effective tool in finding the resilience necessary for the region’s flagship fish and its’ surrounding culture to survive into the next chapter of the Anthropocene.
By Ari Blatt