"They are very resilient and opportunistic. That's why we have hope. We just have to give them half a chance," she said.
The salmon live much of their lives in the ocean, but they swim far upstream to spawn. In the process, they're a key part of the food chain, including for bears and whales, and they are important to indigenous groups and fisheries along the U.S. West Coast.
Human infrastructure, including dams and water diversions, were already affecting their streams, reducing the flow and reducing access to the coldest habitats that can serve as a hiding place for salmon during heat waves or drought. Global warming is now intensifying those impacts.
The salmon populations that have persisted in Western rivers since the dam-building era have adapted to some of that warming, and their sensitivity to climate factors has been incorporated in conservation plans, Crozier said.
But beyond 2 degrees Celsius of warming (3.6 degrees Fahrenheit) compared to the pre-industrial era, all bets are off, she said, because then the chances increase for significant changes in the ocean that could lead to a catastrophic failure of salmon populations.
Timing Is Everything
Some salmon migrations coincide with high spring runoff from melting mountain snows, while juvenile salmon return to the sea in sync with seasonal plankton blooms off the coasts.
Global warming is already disrupting those cycles for some salmon populations, including sockeye that swim 900 miles to spawn in streams high in the mountains of Idaho.
To spawn successfully, they need exactly the right combination of stream flows and temperatures at exactly the right time of year. But warmer temperatures are rapidly changing the timing of snowmelt and runoff in Western mountains, making it harder for the fish.
Snake River sockeye are listed as endangered. In some streams, only a small number reach their spawning grounds. Their overall numbers grew slightly between 2008 and 2014 (the most recent numbers available), thanks mainly to conservation measures, including the introduction of hatchery-raised fish to bolster the population after Snake River sockeye nearly disappeared from the region.
Sometimes they need human help to reach their goal, so they're transported past dams in an "assisted migration," which might become an important (and expensive) strategy to adapt to global warming for other species, as well.
Chinook salmon in California's Central Valley face even more daunting challenges, and some of those populations might be the first to blink out, said University of California, Santa Cruz, researcher Mark Carr, who studies salmon in their coastal habitat.
"These runs down in central California may simply not persist in the face of a changing climate and water conflicts. Can't say they are lost causes, but they are the most likely candidates," said Carr, a co-author on the new study.
"It's frustrating," he said. "I work on many species other than salmon, so it's pretty overwhelming to try to identify how to mitigate or adapt to the growing impacts to so many species simultaneously. It's even more frustrating to know that some policies, particularly the current administration's, are fully counterproductive to the work."
The Central Valley Chinook also have to compete with humans for water, and they are already losing that contest. The greatest salmon declines are where the greatest conflicts over water occur, including the demand for agricultural water in the Central Valley.
"California has a long history of destroying the freshwater ecosystems required to maintain strong salmon runs," Carr said. "If we want salmon around in the future, we need to start working to ensure we have healthy freshwater ecosystems that will better tolerate the changing environmental conditions."
Climate Change Threatens Salmon
The new study covered 33 salmon populations along the U.S. Pacific Coast, from the Mexican border to the Canadian border, assessing how local environmental conditions will change and whether salmon populations will be able to adapt to the changing climate.
The research spells out several ways that global warming endangers the fish. Among them:
- Young salmon die when the water warms above a certain threshold, and droughts can leave salmon stranded or exposed to predators by low water levels.
- Flooding can also flush eggs and young fish from their nests, so the scientists included projections of how global warming will affect extreme atmospheric river rain storms in California as one of the ways to measure the growing threat.
- Warmer stream temperatures have also increased outbreaks of fish disease that can affect salmon, including pathogenic parasites. In May, a toxic algae bloom along the coast of Norway killed 8 million farmed salmon at an estimated cost of about $82 million. In Alaska's Yukon River, a parasite linked with global warming has taken a big toll on the salmon fishery. And in recent weeks, local indigenous observers in Alaska have posted numerous reports of dead salmon in rivers in the western part of the state, as water temperatures reached record highs during Alaska's record-setting heat wave.
- Salmon are also sensitive to changes in ocean currents that carry nutrients, as well as sea level rise, which affects the physical connection between ocean and stream ecosystems, like coastal wetlands in California. Some salmon populations living near the edge of the range of suitable conditions will start to cluster in rivers near the coast, unable to reach their historic spawning grounds unless "access to higher-elevation habitats is restored and habitat quality in rearing areas and migration corridors is improved," the scientists wrote.
Crozier said scientists worldwide have been documenting "almost synchronous declines in salmon populations. Time after time, we see the same patterns of long-term decline." For example, global research shows that climate change is expected to reduce reproductive success and jeopardize salmon migration.
Most types of fish are affected by global warming. Research last year showed many species important to U.S. coastal communities will move hundreds of miles northward during the next few decades.
Some salmon will also move northward seeking cooler waters, and that is bad news for West Coast tribes whose place-based fishing rights are linked to pre-colonization fishing grounds. Native American communities can't just relocate to another area to catch their allocation if the fish move away, said Tom Moore, an oceanographer with the Northwest Indian Fisheries Commission.
Maintaining any salmon populations will require significant restoration efforts to make sure they have large areas of connected habitat, the researchers wrote.
Other conservation strategies include releasing hatchery-spawned salmon, boosting streamflows at the right time with water releases from reservoirs, and even assisted migration, in which some fish are trapped, transported over dams and then released in rivers above the dams.
Carr said the study will help conservation efforts because it shows exactly when and how salmon vulnerability is highest at different stages, with the freshwater environment the key.
To sustain themselves, adults need to be able to successfully migrate to spawning habitat, and survival of the eggs and larvae require cool water temperatures, appropriate gravel structure, sufficient water flow and oxygen while eggs are in the sediments. And finally, there has to be enough water flow to allow them to migrate back to the sea.