Coho salmon were discovered in a body of water in California for the first time following revitalization efforts for the endangered fish species known as Mill Creek.

A century-old dam was taken down from Mill Creek, a tributary that flows through Santa Cruz Mountains in Central California, just over a year ago.

Scientists now claimed that the creek is already displaying signs of revitalization, including an unexpected find.

Last month, aquatic ecologists with the Sempervirens Fund, one of several conservation organizations that share ownership of the 8,532-acre San Vicente Redwoods forest, discovered 15 coho salmon fry and 12 juvenile steelhead trout swimming in the creek.

Both species are federally endangered, with the latter species discovered there for the first time.

The abandoned dam had previously been impeding the migratory fish's ability to swim upstream. Additionally, it contained the granite sediment that served as salmon spawning grounds.

Unexpected Find

Ian Rowbotham, the senior land stewardship manager from Sempervirens Fund, said that the very first fish they discovered in Mill Creek that day was a coho, which they had not anticipated seeing there.

However, when it removed the dam in October of last year, the Sempervirens Fund considered the creek as promising ideal spawning habitat for coho salmon.

The length of time it took the salmon to arrive there surprised them.

Matthew Shaffer, a spokesperson for the Sempervirens Fund, said that the team has no way of knowing the amount of cobble that went downstream or whether it was caused by the presence of salmon in the creek and their accidental ascent upstream.

However, it is rather amazing that they discovered the fish species within a year after a year of unprecedented flooding and the work they had done over the previous ten years.

According to the Sempervirens Fund, the steelhead trout's presence was also unusual as it was the first time in more than a century that the endangered species had traveled upstream from the dam site.

Ecologists Mike Podlech and Jim Robins, along with the fund's field operations manager, Melisa Cambron Perez, and biologist Sean Cochran from the California Department of Fish and Wildlife, confirmed sightings of both species.

The scientists were able to collect fin clip specimens from the salmon under Cochran's guidance for DNA testing, which can shed light on the fish's origins and reveal vital details about their migratory patterns.

Although the scientists said more proof is required, it is a likely sign that the dam removal is resulting in healthier conditions and better habitat for the fish to thrive.

Podlech said it is reassuring to know that the removal of the dam has made it possible to access additional habitat in case the species eventually extends its range upstream in Mill Creek.

The Serious Decline of Coho Salmon and Heavy Downpours

The coho salmon entered the list of endangered species in 2005.

According to the California Department of Fish and Wildlife, the species has been in "serious decline" since the middle of the 20th century as a result of threats like habitat loss, overfishing, interactions with hatchery fish, and climatic factors like a lack of precipitation.

Overexploitation happens when fish are depleted beyond the capacity of the species to repopulate.

Due to the drought, sand bars have recently prevented coho salmon from reproducing in creeks between the Monterey and Golden Gate bays, where they usually spawn.

According to the Marin County-based Salmon Protection and Watershed Network, the species is "on the verge of extinction."

But in January, coho salmon made a historic comeback in the Bay Area, spawning for the first time in 18 years in the waters of Montezuma Creek, located in Forest Knolls, a small tributary of the San Geronimo Valley.

Eric Ettlinger, an ecologist from the Marin Municipal Water District, stated in June that the coho salmon population in Lagunitas Creek may have been boosted as a result of significant rainfall amounts in late 2021.

Fish surveyors found 330 coho egg nests, the second-highest number ever found in that period, during one of the biggest salmon runs the agency had seen in the previous ten years.

The fish were able to lay their eggs at California's Mill Creek thanks to the same run of intense rainstorms that started in October.

The storms contributed to broadening the creek bed and releasing sediment downstream just two weeks after the dam was taken down.

Read also: New Research Discovers Historical Data That Saved Pacific Salmon From Predators 

Further Efforts

The Sempervirens Fund's scientists are hoping that ongoing actions, like getting rid of invasive plant species, such as Clematis vitalba and putting in more "large woody debris" (branches and trees that fall into the creek and lessen water flow), will help to enhance and foster a habitat in which the fish can eat, spawn, hide from predators, and find refuge.

Shaffer said that in addition to being critically endangered, coho salmon are also losing their ability to find healthy habitat due to climate change.

They are looking for locations where the fish species can spawn and also ultimately survive as a species because those conditions are constantly changing more quickly than ever.

It has been determined that this watershed is crucial. They think it may be a stronghold for the species because the body of water is at the southern end of their range, which increases the likelihood that they will visit year after year.

Shaffer said that the redwood forest canopy, which maintains cool conditions so that other plant and animal species can be preserved as well, is also being monitored and protected by the Sempervirens Fund.

He said that given the difficulties brought on by climate change, they believe their team can truly tip the scales in situations like this so that the habitat is flourishing, which is much tougher today than it was five years ago.

He continued that it is a race against time to reach these kinds of milestones, SFGate reported.

Related article: 'Extremely Exciting': Endangered Salmon Returns to California for the First Time Since 2004