It’s hard to forget the excruciating heat that blanketed the Pacific Northwest in late June 2021. Temperatures in Oregon, Washington and British Columbia soared to well above 100 degrees Fahrenheit, with Seattle of 108 degrees on June 28.

During the heat wave, also called a heat dome, scientists and community members alike noticed a disturbing on some beaches in Washington and British Columbia, both in the Salish Sea and along the outer coast. The observers quickly realized they were living through an unprecedented event and they organized to document the shellfish die-offs as they happened in real time.

Now, a team led by the 91Ě˝»¨ has compiled and analyzed hundreds of these field observations to produce the first comprehensive report of the impacts of the 2021 heat wave on shellfish. The researchers found that many shellfish were victims of a “perfect storm” of factors that contributed to widespread death: The lowest low tides of the year occurred during the year’s hottest days — and at the warmest times of day. The were published online June 20 in the journal Ecology.

“You really couldn’t have come up with a worse scenario for intertidal organisms,” said lead author , a research scientist at 91Ě˝»¨Friday Harbor Laboratories. “This analysis has given us a really good general picture of how shellfish were impacted by the heat wave, but we know this isn’t even the full story.”

The research team leveraged existing collaborations across tribes, state and federal agencies, academia and nonprofits. They devised a simple survey and five-point rating system (1 = much worse than normal to 5 = much better than normal) and asked participants to provide ratings based on their knowledge of a species in that location. In total, they gathered 203 observations from 108 unique locations, from central British Columbia down to Willapa Bay, Washington.

“The strength of this study and what it really highlights is the value of local knowledge and also the importance of understanding natural history,” said co-author , a 91Ě˝»¨associate teaching professor in environmental studies and aquatic and fishery sciences. “This is the first step and a snapshot, if you will, of what shellfish experienced on the beaches during the heat wave.”

The researchers found that each species’ ecology contributed to its general success or failure to survive the extreme heat. For example, some shellfish that naturally burrow deep beneath the surface, like butter clams, usually fared better than ones that typically ride out low tide just below the sand’s surface, such as cockles.

They also found that location mattered. Shellfish on the outer coast experienced low tide about four hours earlier than shellfish on inland beaches. For inland shellfish, low tide — or when the most shellfish were exposed — hit around solar noon, when the sun was directly overhead.

Additionally, air temperatures were much higher at inland sites compared to the outer coast, causing more stress on inland populations. For example, California mussels, found almost exclusively on the outer coast, mostly survived the heat while bay mussels, found in more inner coastal sites, were more likely to die from heat exposure. More water movement and wave action on the outer coast also likely helped lessen the impacts of the heat on shellfish along those beaches.

“The timing of low tide helps determine when and where organisms may be exposed to heat stress and can structure behavior and distribution. In this case, organisms at locations that are already exposed to air at the hottest time of day were very unlucky that temperatures soared so high,” said co-author Hilary Hayford, habitat research director at Puget Sound Restoration Fund.

Many shellfish don’t tend to move much on any given beach, so where they naturally live in the intertidal zone also contributed to their success or failure, the researchers found. For example, acorn barnacles that live higher on the shore generally were more impacted than clams and oysters that are lower on the beach and more likely to remain under water.

“Although this event had negative effects on marine life, there is hope that can be found in this work. Not all locations and species were affected equally, offering clues to pathways to resiliency in the future,” said co-author Annie Raymond, a shellfish biologist with Jamestown S’Klallam Tribe.

Perhaps most surprisingly, the researchers noticed interesting patterns in survival rates among shellfish on the same beach. In some locations, shellfish in the path of freshwater runoff on one section of beach survived, while others just a few miles away perished. If a tree hung over part of a beach and shaded the sand, those shellfish generally made it while others didn’t. Co-author , senior shellfish biologist with the Swinomish Indian Tribal Community, remembers seeing those patterns while walking the beaches of Skagit Bay and, in some locations, being surrounded by dead cockles in every direction.

“It was pretty unsettling, and I’ve never seen anything like it,” Barber said. She remembers exchanging emails with colleagues from around the region as they noticed similar mass die-offs on their local beaches, then realizing that they urgently needed to coordinate and document what was happening.

“This effort was a beautiful demonstration of how collaborators can come together with one common cause — which in our case was trying to understand what happened to these shellfish,” Barber said.

Because the heat wave occurred during the time frame when many shellfish are reproducing, the mass die-offs could impact those populations for at least several years, highlighting the need for long-term monitoring, the researchers said. And as climate change continues to produce more frequent extreme heat events, shellfish deaths like those of last summer may become more of a common reality.

“The Swinomish Indian Tribal Community is proud to be a leader in this important scientific research that assessed in real-time the devastating impacts to our shellfish resources from the unprecedented heat dome last summer. Shellfish are a priority first food that our tribal community relies on for spiritual and subsistence nourishment. Last summer’s extreme weather event reinforced to us that we must act faster to ensure climate resiliency for our community’s long-term health and well-being,” said Swinomish Tribal Chairman Steve Edwards.

“Once the effects of the heat wave started to become apparent, the collaboration that emerged was amazing as managers and scientists worked quickly to put together a rapid response to capture information,” said co-author Camille Speck, Puget Sound intertidal bivalve manager for Washington Department of Fish and Wildlife. “We still have so much to learn about the effects of the heat wave on Salish Sea marine ecosystems, and more work to do as managers to prepare for the next one and develop informed responses. These conversations are happening now, and it is our hope that we will be better prepared for whatever comes next.”

Other co-authors are Megan Dethier of the UW; Teri King of UW-based Washington Sea Grant; Christopher Harley of University of British Columbia; Blair Paul of Skokomish Indian Tribe; and Elizabeth Tobin of Jamestown S’Klallam Tribe. More than two dozen individuals contributed data to this project.

This analysis was funded by Washington Sea Grant with data contributions from tribes, state and federal agencies, academic institutions and nonprofits.

For more information, contact:

• Wendel Raymond, 91Ě˝»¨ and lead author: wraymond@uw.edu
• Sean McDonald, 91Ě˝»¨: psean@uw.edu
• Julie Barber, Swinomish Indian Tribal Community: jbarber@swinomish.nsn.us
• Camille Speck, WDFW (contact Ben Anderson, communications manager): Benjamin.Anderson@dfw.wa.gov
• Teri King, Washington Sea Grant: guatemal@uw.edu

Bulkheads and seawalls along the shores of Puget Sound help ease erosion and stabilize bluffs to protect waterfront properties.

But these walled structures also shrink beaches, reduce habitat for invertebrates and spawning fish and, indirectly, degrade conditions for iconic species like salmon and orcas. Many studies have shown this pattern at seawall sites around Puget Sound.

A new 91Ě˝»¨ shows that impacts at individual armored sites can scale up to have cumulative, large-scale effects on the characteristics of Salish Sea shorelines and the diversity of life they support. It is the first study to analyze sites broadly within Puget Sound and offers the most comprehensive look to date at the impacts of shoreline armoring on the Salish Sea ecosystem.

“Given the incredible variety and complexity of shorelines in the Salish Sea, to pick out patterns we had to go very broad and step back and squint, if you will,” said lead author , a research professor of biology at the UW’s .

“The huge challenge was searching for patterns in the noise. The shorelines are impacted by hundreds of different things and we were trying to see patterns driven by just one thing — armoring.”

The paper appeared online this month in the journal .

When researchers looked at sites from south, central and north Puget Sound, the data showed that armored beaches became slightly narrower and steeper over time, and larger pebbles replaced finer-grained sediment and sand. Additionally, in stretches of shoreline that were more heavily armored, even the unarmored areas showed similar impacts — less sand and more larger sediment on the beach.

“Changes to the shape and texture of a beach are subtle, slow to happen and can take decades,” Dethier said. “It took a big sample size and a range of beach types to see that geomorphic signal. To me, this is the big punch of the study.”

Of Puget Sound’s 2,500 miles of shoreline, more than one quarter are currently armored. Shorelines range from heavily armored, concrete-covered commercial ports to pristine, sandy beaches.

Erosion from bluffs and banks is a natural process and if left alone, most bluffs will erode and replenish the beaches with sand and gravel. Armoring stops or dramatically slows erosion, and gradual-sloped, wide, sandy beaches over time give way to pebbly, steeper shorelines that aren’t desirable to beach-spawning fishes — or humans.

“This new report by Megan and her team provides crucial information on shoreline armoring impacts that will be highly valuable in improving our management approaches to Puget Sound shorelines,” said Randy Carman, who works with the nearshore habitat program at the Washington Department of Fish and Wildlife.

“While we have often relied on conceptual models and studies conducted in other locales, we now have concise empirical data on armoring impacts from a large geography of Puget Sound.”

In addition to identifying cumulative effects, this paper also confirmed and observations that armoring impacts the ecology and structure of shoreline habitat in Puget Sound.

Specifically, armored beaches generally have fewer drift logs, algae, seagrass and other organic debris that naturally washes ashore than their unarmored counterparts. This vegetation provides a daily feast for crustaceans and insects, and indeed, fewer invertebrates were present at armored sites. Sandy beaches, which provide habitat for surf smelt and other forage fish to spawn, were replaced by coarser sediment in armored areas.

Ultimately, all of these changes in nearshore habitat probably alter the feeding and migration patterns of juvenile salmon in the Sound.

In designing the study, Dethier and her collaborators identified 65 pairs of sites around the Salish Sea. Each pair included one site with no shoreline armoring and another close by that had some degree of armoring such as bulkheads, seawalls or wood pilings. Additionally, each pair was within a distinct unit of shoreline, called a drift cell, and the percentage of armored shoreline in each drift cell varied.

They collected detailed data from each site, including the amount of natural debris (seagrass, algae); the number of logs deposited on shore; the presence of invertebrates such as insects and sand fleas; the size of beach sediment, ranging from sand to cobble; the amount of vegetation hanging over the shoreline; and the slope of each beach.

They found the effects of armoring were cumulative, because in shoreline drift cells that had a higher percentage of armoring, even unarmored sites showed impacts, including less sand and more larger sediment.

The data collection was exhaustive, Dethier said, and a single day in the field generated about three weeks of processing specimens in the labs — all before data analysis even began.

Because the effects of armoring are cumulative, it follows that reducing the number of bulkheads and seawalls throughout Puget Sound would improve the overall health of the ecosystem. Replacing concrete walls with softer, greener materials like logs, or replenishing armored beaches manually with truckloads of sand and gravel are options to lessen the impacts.

Even moving a seawall higher up on the shoreline would allow space for forage fish to spawn and natural tides to bring valuable nutrients to shore.

Many scientists agree the best option is to avoid putting in any new bulkheads and seawalls, which is what Hugh Shipman, a coastal geologist with the state’s , advocates in his work with landowners. Current laws for new armoring are set by the Department of Ecology under the , and local governments are in charge of regulating and approving projects, with additional permitting from Washington Department of Fish and Wildlife.

“We want to assure that more restrictive bulkhead policies are supported by the best science we can possibly have,” Shipman said. “This is the most significant paper we’ve seen that looks at the impacts on Puget Sound.”

Other co-authors are , formerly at Friday Harbor Laboratories and now at the University of Alaska, Fairbanks; , and Sarah Heerhartz of the UW’s School of Aquatic and Fishery Sciences; of the UW’s School of Oceanography; Aundrea McBride of the Skagit River System Cooperative; and Helen Berry of the Washington Department of Natural Resources.

This research was funded by Washington Sea Grant and the U.S. Environmental Protection Agency, through the Washington Department of Fish and Wildlife.

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For more information, contact Dethier at mdethier@uw.edu.