Salmon spend most of their lives in salt water. From the moment they move downstream to the estuary and Puget Sound as juveniles, they are exposed to the marine environment. Many salmon rear in estuaries to grow quickly as they feed on the abundant food webs in these productive ecosystems. Once salmon leave the river system they move through the marine environment, often near the shore. Beaches and nearshore habitats provide refuge from predators and a dynamic productive environment with abundant food to continue growth. Some salmon may remain residents within the Puget Sound and Salish Sea, but most will continue westward to the Pacific Ocean.
Once salmon enter the Pacific Ocean, they can distribute widely across an enormous range, depending on age and species. The Pacific Ocean is a vast and dynamic environment that both is affected by global climate change, and affects regional climate in the Pacific Northwest. This ecosystem supports Chinook to grow to massive sizes, sometimes historically over 100 pounds – very large fish for their age. These size fish were common in historic fish runs but are exceedingly rare today. Their prolific growth in just a few years is fueled by the productive ocean ecosystem and allows salmon, particularly Chinook salmon to carry many thousands of eggs and produce many offspring when they spawn. Chinook today are smaller, due to selective fishing that targets large fish, and more recently potentially due to shifts in saltwater and ocean conditions do not allow them to grow as large.
Figure 18. Most of the Energy of the biosphere is absorbed by the atmosphere. This graph shows the relative heat anomalies (from historic average) in the ocean and land + atmosphere. Increased heat in the ocean affects weather and climate, as well as sea level rise (via thermal expansion), and ocean ecosystems (phytoplankton, zooplankton, and food chains including salmon). Murphy et al. 2009
The ocean and marine ecosystem play an important role in climate change. The ocean is affected by a changing atmosphere, but it also affects climate patterns at global scales, from carbon storage, to sea level rise, to ocean currents, pressure systems, and the jet stream. It is difficult to predict exactly how the ocean ecosystem will change but we can predict how likely changes will affect salmon.
Sea levels around the Puget sound have risen by about 8 inches since 1900 (NRC, 2012; figure 19). As the climate changes sea level rise is expected to accelerate. Predicting future sea level rise is a complicated, probabilistic science. Due to uncertainty in the rate that the Greenland and Antarctic ice will melt, predictions of sea level rise are highly variable.
Figure 19. Historic (1900-2008) monthly sea level rise for Seattle, Wa. We have observed roughly 8 inches (20 cm) of se level rise since 1900. The “relative sea level” is relative to the average sea level (zero) of the period in question.
We discuss Sea level rise projections in terms of the probability of exceeding a certain elevation. The most recent projections for Washington State suggest, at the current rate of greenhouse gas emissions (high), there is a 50% chance of exceeding 2ft of sea level rise by 2100 (Miller et al. 2018). There is an 83% chance of exceeding 1.4 ft and a 10% chance of exceeding 3.1ft (Table 1). Recent research has led to an increase in high-end projections, and it is possible future research will do the same (Kopp et al. 2014).
Table 2. Absolute sea level rise projections, in feet, for Washington State. Projections are expressed in terms of the “probability of exceedance” for three different time periods (2050, 2100, 2150) and two different greenhouse gas Scenarios (RCP 4.5 [low], and RCP 8.5 [high]). Projected changes are assessed relative to contemporary sea level, defined as the average sea level over the 19-year period from 1991-2009. Projections for 2050 and 2100 under the high greenhouse gas scenario (RCP 8.5 – highlighted below) are also shown in Figure 20 below (Miller et al. 2018).
The higher the elevation in question, the lower the likelihood. For instance, there is a 1% probability of exceeding 4.8 ft by 2100. These are estimates for absolute sea level rise (Figure 20). This is the height of the ocean relative to a fixed reference point. However, the earth’s crust is not a fixed reference point. Complex geologic processes cause the land to rise and fall over time relative to the ocean. Even within Washington State these processes result in significant variability in sea level rise between different locations. In most of Puget Sound, the land is subsiding, which increases sea level rise. On much of the peninsula, the land is uplifting, decreasing the effect of sea level rise. For the Snohomish River delta, elevations with equivalent probabilities are about 0.2ft higher than the absolute sea level. Here, near the Tulalip Reservation, there is a 50% chance of exceeding 2.2ft by 2100 (Miller et al. 2018, Figure 21).
Figure 21. Vertical land movement best estimate rates (left) expressed in feet per century and their uncertainties (1 standard deviation, right) as estimated for Washington’s coastline (Miller et al. 2018).
Sea level rise has major implications for the human population, which has built cities along the shores of the Puget Sound. Effects on salmon are less direct, but they are significant and many are associated with human responses to sea level rise. Like flooding, the usual human responses to sea level rise are bad for salmon. The typical response is to armor beaches and waterfronts with sea walls to prevent erosion and direct wave energy away from the shore and the toes of bluffs. These approaches squeeze nearshore habitat for salmon and other prey species they depend on the beach between rising seas and armored shorelines (Figure 22).
Directing energy back toward the water with sea walls decreases bluff erosion and starves beaches of the sediments needed to maintain healthy beach substrates. Armored beaches also create a high-energy environment that can erode the beach face harm juvenile fish and diminish a vital refuge juvenile fish depend on during their growth and migration to deeper marine waters.
Beaches are also important ecosystems that support the marine food web. Small forage fish that young salmon feed on also require the proper substrates and beach conditions to reproduce. One aspect of salmon recovery involves the nourishment of beaches and the removal of armoring to naturalize beach habitats so they can support juvenile salmon and the forage fish they depend on later in life. Sea level rise will increase the need to restore beach processes. This will be an important conflict in the coming century if we are to recover salmon and the ecosystems of the Salish Sea in general.
Similar to beaches, as the sea level rises, if estuaries do not have room to move to higher elevations in river systems, they will be squeezed between rising seas and the levees that protect land upstream. Estuary habitats are vital for salmon survival and estuarine habitat is already 70-90% smaller than it was historically (Figure 23). Restoring estuaries, removing levees, and expanding estuarine habitat in the higher estuaries will be important to accommodate sea level rise and adapt to climate change. Furthermore, many of these lower estuarine habitats were converted to farmland. As the sea level rises, and groundwater in low elevation farmlands in historic estuaries rise with it, farmland will become far less productive, and should be targeted for restoration.
Figure 23. Aerial photograph of the Skagit River delta, in the Puget Sound area of Washington, illustrating changes between 1850 (left) and 2010 (right). In 1850 the delta included extensive wetlands providing important habitat for salmon spawning (orange). By 2010 most of the delta had been “reclaimed” for development by a system of dikes and levees (red), greatly reducing the habitat available to salmon. Left: courtesy of Brian Collins, University of Washington. Right: Eric Grossman, USGS. (https://walrus.wr.usgs.gov/climate-change/lowNRG.html)
Sea level rise primarily affects salmon by threatening to reduce habitat in estuaries, and adjacent to beaches and nearshore habitats. Because the Puget Sound is extensively developed by humans, especially along the shoreline, as sea levels rise, the already limited existing habitats in these areas will be squeezed between rising seas and human desires to maintain property. In an undeveloped landscape, the habitats would shift upslope with sea level rise. Strategies usually involve getting people off land and upslope away from the lands that will be affected by sea level rise to make space for habitats to shift
There are a few large scale climate cycles that naturally influence ocean conditions in the North Pacific, including the Pacific Decadal Oscillation (PDO), the North Pacific Gyre Oscillation (NPGO), and the El Nino Southern Oscillation (ENSO). These interacting and overlapping oscillations in water temperature and pressure systems influence ocean currents, air and water temperatures, plankton assemblages and distributions, upwelling patterns, and marine survival rates of salmon. PDO, referred to as having warm and cold years, is correlated with salmon survival (Mantua et al. 1997; Figure 24).
Figure 24. Pacific Decadal Oscillation (PDO), 1925-present. Blue bars represent cold years, red bars represent warm years.
Understanding ocean conditions is exceedingly complicated due to the vast numbers of interacting climatic and ecological cycles, and our lack of data describing such a vast ecosystem. We know that ocean conditions have profound impacts on marine survival, or the number of fish that survive the ocean to return to rivers. In some years, marine survival is much better than others, and salmon runs are better. We are not very good at prediction when marine survival is poor, but we are getting better, and are using more conservative estimates to ensure populations are not overharvested . Generally, as the atmosphere warms, the ocean will warm with it. A warmer north Pacific, like what was observed in 2015 during “The Blob,” will result in a significant increase in marine mortality. Along with other factors associated with warm water conditions, such as upwelling and plankton communities, these changes will challenge the survival of salmon populations (Figure 25).
Figure 25. Illustration of how basin-scale and local-scale physical forces influence the northern California Current and resultant food web structure. PDO = Pacific Decadal Oscillation. NPGO = North Pacific Gyre Oscillation. ENSO = El Nino-Southern Oscillation (Peterson et al., 2015)
It is clear that PDO cycles affect salmon survival. However, the impacts of climate change on the natural variations that determine ocean conditions are not known, and the effect of ocean conditions on salmon is not well understood. While there are informative correlations between survival and ecosystem indicators, changes in any indicator can confound relationships between others. The ways in which salmon are impacted will depend on their life stage while in the ocean ecosystem, how long they spend in the ocean, and other ocean variables like plankton communities. Further study is important to understand how climate change will affect salmon, and is likely already doing so. There are few direct actions we can take to influence ocean conditions, but continuing to better measure and understand what is happening in the ocean and how that affects salmon survival is extremely important. Our effects on the atmosphere are likely to have profound impacts on the ocean food web, so anticipating those changes and responding to them now instead of once they are observed is important. As with all threats from climate change, the most effective solution is to stop it from happening.
Gases in the atmosphere exchange between the air and the water in the ocean. They are at an equilibrium state where gas enters water and off gases to the atmosphere. However, as the atmospheric concentration of carbon dioxide (CO2), increases more carbon dioxide absorbs into the ocean. Due to this process, the CO2 concentration in ocean and marine waters projected to increase 150-200% by 2100 based on current CO2 emission scenarios (TNC, 2016).
Figure 26. Conceptual diagram comparing the state of carbonate ions in the ocean under the lower-acid conditions of the late 1800s and the higher acid conditions expected in 2100 (https://www.britannica.com/story/ocean-acidification-how-carbon-dioxide-is-hurting-the-seas)
As the ocean acidifies, marine species that form calcium-based shells (like shellfish) are directly impacted because more acidic conditions inhibit the formation of shell and actually can dissolve it. These species are not just clams and oysters, which are vital to the marine food web, but also species in the water column, such as plankton and juvenile crab, which are important food sources for juvenile salmon, as well as forage fish adults depend on. As the ocean acidifies and populations of these important elements of the ocean food web are challenged, it will change food availability for salmon during their smolt and ocean life cycle phases. For instance, acidification will reduce the availability of crab larvae, an important food source for juvenile salmonids and the forage fish adult salmonids depend on. The role affected species play in supporting Puget Sound salmon, and the entire food web in general raises significant concerns about how acidification could affect the entire Puget Sound and ocean food web as the climate changes (Ecology, 2012).
Image: This video shows the difference in swimming behavior and shell dissolution between a pteropod in seawater with low surface CO2 conditions and that of a pteropod exposed to elevated CO2 conditions (https://www.pmel.noaa.gov/co2/story/Ocean+Acidification).
Ocean acidification will result in major shifts in marine and ocean food webs. The exact nature of these changes in such a complex system is not known, and effects on salmon are difficult to quantify, but major perturbations in food webs invariably challenge populations of predators at the top of the food chain. Because ocean acidification is a passive effect of increased carbon dioxide in the atmosphere, the only effective way to stop it is to decrease atmospheric carbon dioxide concentrations by lowering emissions and increasing carbon uptake into biotic carbon sinks.
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