Little fish, big splash: Coho salmon’s Mill Creek debut stuns conservationists

Mill Creek dam flows free one year after dam removal.
Mill Creek dam flows free one year after dam removal.
(Via Ian Bornarth)

Coho salmon have been documented for the first time in Mill Creek, to the surprise of ecologists and conservationists alike. Their appearance in the San Vicente Redwoods stream comes just one year after the removal of the Mill Creek dam.

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Beneath the cool water of Mill Creek in the San Vicente Redwoods, silvery coho salmon have been spotted for the first time ever. Their appearance comes as a welcome surprise to conservations and ecologists, who returned to the creek in September to survey the site following the removal of the Mill Creek dam a year earlier.

For over a century, the Mill Creek dam had segmented its namesake stream. Conservationists hoped removing the dam would allow steelhead trout to migrate upstream, reclaiming decades of lost habitat. Removing the dam would also free trapped sediment, which conservationists hoped could create spawning grounds for endangered coho salmon.

Scientists identified 15 juvenile coho salmon at the base of Mill Creek, near its junction with San Vicente Creek, as well as 12 juvenile steelhead trout, including some upstream of the dam’s removal. As exciting as it is to see the coho so soon after the dam’s removal, aquatic ecologist Mike Podlech says, it is not yet clear whether the appearance of the fish is related.

A GIF of coho salmon spawn in Mill Creek in the Santa Cruz Mountains
(Via Melisa Cambron Perez / Sempervirens Fund)

Regardless, Podlech adds, restoration of this habitat is critical. Turn-of-the-century redwood logging, water diversion, development, mining and damming all resulted in extensive habitat damage to streams in the San Vicente Redwoods. Today, coho are on the verge of becoming extinct; streams are at best sparsely populated, if not devoid of their presence entirely.

Monterey Bay encompasses the southernmost tip of the coho salmon’s range. Streams here are canopied by redwoods, whose fallen limbs create necessary shelter for coho. Gravel basins provide ideal nest habitat: Female cohos expertly move pebbles using rapid tail-fin movements to create round depressions — known as “redds” — that are perfect for salmon eggs.

Coho salmon are anadromous fish: Born in freshwater streams, they migrate down and out to the ocean, then return home to spawn. Coho stay with their young until they die, and the cycle begins anew.

Coho salmon have been a “vital part of the ecosystem for millions of years,” said Ian Rowbotham, senior land stewardship manager at the Sempervirens Fund, which spearheaded the effort to remove the dam. The cyclical nature of their lives produces “a cycle of bringing nutrients from the ocean back to inland sites where some of those nutrients just aren’t readily available. And so they form kind of a building block of that ecosystem.”

Bringing down the Mill Creek dam

The disruptive nature of the Mill Creek dam paired with the potential enticement of fish to return made its removal an obvious conservation project. The necessity of this project was further underscored by the fact that the dam, 12 feet high and 25 feet wide, was “a century-old mistake,” Rowbotham said. “The information we have was that the dam wasn’t put far enough up in the watershed for gravity flow for where it needed to go. So it was basically not useful right from the beginning.”

Mill Creek dam.
Mill Creek dam has stood for over a century preventing steelhead trout from migrating upstream, and sediment from flowing downstream.
(Via Ian Bornarth)

After nearly a decade of preparation by the Sempervirens Fund, in collaboration with three other groups that manage the San Vicente Redwoods — the Peninsula Open Space Trust, Save the Redwoods League and the Land Trust of Santa Cruz County, as well as the Resource Conservation District of Santa Cruz County and a partnering tribal land trust, the Amah Mutsun Tribal Band — the dam fell in a matter of minutes.

Oct. 4, 2021, the day of dam removal, happened to be Melisa Cambron Perez’s first day on the job as field operations manager. “It was a little bit chaotic,” she recalled. Two imposing excavators made light work of knocking down the wall of the dam and redistributing granite boulders and soil along the original contours of the stream. Then Mill Creek began flowing freely for the first time in over 100 years.

Perez described being onsite as breathtaking: “Everything was green. This was an area that had a very low-severity burn [from the CZU fires]. So a lot of the understory was coming back and the redwoods looked amazing.”

Shortly after the removal of the dam, the area was soaked by historic two-week-long October storms. The timing was ideal, Rowbothan noted: The rainstorms held off until after the dam removal, and “we saw a ton of sediment start to move downstream, right off the bat.”

‘Dude, get out of the way!’

Rowbotham returned at the end of the winter season, in March, with a team of engineers, ecologists and biologists to check on the progress of the former Mill Creek dam site.

As they were assessing the area where Mill Creek meets San Vicente Creek, one field ecologist was greeted by a pushy trout. “A steelhead literally starts trying to swim up the creek,” Rowbotham recalled, “right next to his foot, to the point where we’re like, ‘Dude, get out of the way!’”

A juvenile coho salmon
A juvenile coho salmon. Juvenile coho have characteristic “parr” marks, bluish-gray vertical strips along their sides.
(Via Ian Rowbotham / Sempervirens Fund)

For Rowbotham, who had “never seen particularly large fish at all in that segment [of Mill Creek] before,” it was “a very special moment where it feels like you’re starting to see change in a really positive way.”

The positive news didn’t end there. In September, Rowbotham and Perez joined Podlech, ecologist Jim Robins and California Department of Fish and Wildlife biologist Sean Cochran to see how the dam’s removal was affecting fish.

Shocking all present, the very first fish found was a juvenile coho salmon. “It was just amazing,” Perez said. “There [was] such excitement over seeing such small fish.” Added Rowbotham, “we had anticipated that, with removing the dam, access upstream probably [would be] more so for steelhead … so finding coho on Mill Creek was a huge surprise for sure.”

From left, Sean Cochran, Jim Robins, Melisa Cambron Perez and Mike Podlech at Mill Creek the day of the coho discovery.
(Ian Rowbotham / Sempervirens Fund)

One thing remains murky: whether the dam’s removal is directly responsible for the presence of coho in Mill Creek. As Podlech explains, the area in which coho were documented is very near the San Vicente stream, a known habitat for coho. The team doesn’t yet know whether the juvenile coho found in Mill Creek were the product of coho spawning in Mill Creek or whether the eggs were swept from the adjacent San Vicente Creek into Mill Creek.

Regardless, removal of the dam is crucial to the well-being of the stream and its fishy inhabitants. “Should coho start moving upstream,” Podlech said, “it’s very reassuring to know that the dam is gone so the dam is not going to get in the way in the future.”

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