The Salish Sea

Definition and Geography

The Salish Sea is defined as the interconnected body of water comprising Puget Sound, the Strait of Juan de Fuca, the Strait of Georgia, and the waterways surrounding the San Juan Islands. It was jointly named in 2009 by the U.S. Board on Geographic Names and the Geographical Names Board of Canada, honoring the Salish peoples who have lived along these shores for millennia.

The name does not replace existing names — Puget Sound is still Puget Sound. "Salish Sea" describes the whole system as a single ecological and hydrological unit, which is how the water actually behaves.

Physical Dimensions

Tidal Exchange with the Pacific

The entire Salish Sea breathes through a single opening: the Strait of Juan de Fuca. Twice daily, billions of gallons of Pacific water enter and leave through this channel, driven by mixed semidiurnal tides. The narrow passes between the San Juan Islands create some of the strongest tidal currents in the world — Deception Pass runs up to 8 knots.

Population Status

The Southern Resident Killer Whales are a genetically distinct population of approximately 73 individuals, listed as endangered under both the U.S. Endangered Species Act (2005) and Canada's Species at Risk Act. They are divided into three family groups:

Diet: Salmon Specialists

Unlike transient (Bigg's) orcas that hunt marine mammals, Southern Residents are salmon specialists. Chinook salmon make up approximately 80% of their diet during summer months. This extreme dietary specialization is the root of their vulnerability — as Chinook populations decline, the orcas decline with them.

The orcas use echolocation to hunt individual salmon, often at depth. Vessel noise directly competes with their sonar, reducing foraging efficiency by an estimated 25% in heavy traffic.

Threats

• Prey depletion: Chinook salmon runs across the Pacific Northwest are at historic lows. No prey recovery = no orca recovery.
• Vessel noise and disturbance: Commercial shipping, whale-watching boats, and recreational vessels create acoustic interference with foraging echolocation.
• Contaminants: PCBs, PBDEs, and other persistent organic pollutants bioaccumulate in blubber. When orcas metabolize fat during food-scarce periods, toxins re-enter their bloodstream.
• Declining Chinook runs: Habitat loss, dams, and warming ocean conditions continue to reduce the salmon stocks these orcas depend on.

Five Species of Pacific Salmon in the Salish Sea

The Snohomish River System

The Snohomish River watershed is one of the most productive salmon systems in Puget Sound. Its two major tributaries — the Skykomish and the Snoqualmie — drain the western Cascades, collecting snowmelt and rain that sustains salmon from egg to ocean and back.

The river we track via USGS gauge 12150800 (Snohomish River near Monroe) carries all five salmon species from the mountains to the Sound. Flow conditions directly affect migration success:

Hatcheries and Restoration

Two major hatchery systems supplement wild salmon runs in the Snohomish watershed:

• Tulalip Tribes hatcheries: Co-manage salmon recovery under treaty rights; operate hatcheries on the Skykomish River system. Tribal harvest management is governed by the Boldt Decision (1974).
• WDFW (Washington Department of Fish and Wildlife): Operates state hatcheries focusing on Chinook and Coho supplementation, working to maintain genetic diversity while boosting run sizes.

The tension between hatchery production and wild stock recovery is one of the central debates in Pacific salmon management. Hatchery fish can compete with and dilute the genetic fitness of wild populations, but without supplementation, some runs might disappear entirely.

Residents and Visitors of Puget Sound

Harbor Seals: The Locals

Harbor seals (Phoca vitulina) are the most common marine mammal in the Salish Sea, with an estimated population of 14,000+ in Washington waters. They haul out on Jetty Island — the sandbar directly across from Mukilteo's waterfront — to rest, thermoregulate, and nurse pups.

Pupping season runs from June through September. During this time, apparently "abandoned" pups on shore are almost always waiting for their mothers to return from foraging. The standard guidance: observe from 100+ feet, never touch.

The Return of Humpbacks

Humpback whales were hunted to near-extinction in the North Pacific. Since the moratorium on commercial whaling (1966 for humpbacks), populations have recovered dramatically. Their increasing presence in the Salish Sea — rare before 2010, now seen regularly — is one of the clearest marine conservation success stories in the region.

However, their return creates new challenges: humpbacks feeding in the same waters as heavy commercial shipping face entanglement and strike risks. The recovery of one species reveals the complexity of managing an ecosystem, not just individual populations.

Mixed Semidiurnal Tides

The Salish Sea experiences mixed semidiurnal tides — two high tides and two low tides each day, but with unequal heights. This is exactly what our NOAA station 9447659 measures at Mukilteo. The "mixed" part comes from the declination of the moon: when the moon is north or south of the equator, the two daily highs (and two lows) differ in height.

Typical tidal range at Mukilteo: 2.5–4.0 meters (8–13 feet). Extreme tides during king tides can exceed 4.5 meters.

Intertidal Zones

The intertidal zone — the strip of shore exposed and submerged by tides — is organized into bands defined by how much air exposure each level tolerates:

Eelgrass Meadows: The Underwater Nurseries

Eelgrass (Zostera marina) is not a seaweed — it is a true flowering plant that grows underwater, forming dense meadows in shallow, protected waters. These meadows are critical nursery habitat for juvenile salmon, herring, and hundreds of invertebrate species.

Eelgrass provides structure (hiding places from predators), food (epiphytic algae on the blades), and oxygen (photosynthesis). It also stabilizes sediment, reducing erosion. The Snohomish River estuary supports some of the most extensive eelgrass beds in Puget Sound.

Sea Star Wasting Disease

Between 2013 and 2016, a devastating epidemic called Sea Star Wasting Syndrome (SSWS) swept the Pacific coast from Alaska to Baja California. Affected sea stars developed lesions, lost limbs, and disintegrated within days. The ochre sea star (Pisaster ochraceus) — a keystone predator that controls mussel populations — was especially hard hit.

Puget Sound populations were devastated. The cause was identified as a densovirus (Sea Star-associated Densovirus, SSaDV), potentially triggered or amplified by warming water temperatures. Recovery has been slow and uneven. The loss of sea stars allows mussels to dominate, squeezing out other intertidal species — a textbook demonstration of keystone predator effects.

The State of Puget Sound

The Puget Sound Partnership, established in 2007, is the state-led effort to recover the health of Puget Sound by 2050. The partnership coordinates across hundreds of agencies, tribes, and organizations to track and address the ecological indicators that define Sound health: orca population, salmon returns, water quality, habitat extent, and human well-being.

The uncomfortable reality: despite decades of effort and billions in investment, most indicators are flat or declining. The Sound is not getting healthier at the pace needed.

Pollution Sources

6PPD-quinone: The Tire Chemical Killing Coho

In 2020, researchers at the University of Washington identified 6PPD-quinone — a chemical produced when the common tire preservative 6PPD reacts with ozone — as the cause of decades of unexplained Coho salmon die-offs in urban streams.

The discovery is remarkable for several reasons:

• 6PPD has been in tires since the 1960s. Nobody thought to look at tire wear particles as a specific toxicant.
• 6PPD-quinone is lethal to Coho salmon at concentrations as low as 0.8 micrograms per liter — levels easily reached in stormwater.
• Chinook salmon appear more resistant, but sublethal effects are still being studied.
• The chemical forms on road surfaces and washes into waterways with every rainstorm.

Shellfish Growing Areas

Water quality in the Salish Sea directly determines whether shellfish can be safely harvested. The Washington State Department of Health classifies shellfish growing areas based on fecal coliform bacteria levels — the standard indicator of human health risk from contaminated water.

When rain events overwhelm stormwater systems or trigger combined sewer overflows, shellfish beds are closed. These closures are the most immediate, visible connection between land use, weather, and marine ecosystem health. Every impervious surface in the watershed — every road, roof, and parking lot — contributes to the runoff that closes the beds.

Every data stream we collect at the Mukilteo Observatory connects to the Salish Sea ecosystem:

A microclimate observatory is not just a weather station. It is a lens into the health of the ecosystem where atmosphere, freshwater, and ocean meet. Understanding the Salish Sea gives every data point we collect its ecological context.