Surprise Finding Shows Coho Surviving Even in Temperatures Much Warmer Than Optimum

Coho salmon school in Little Sproul Creek.

Press release from the Eel River Recovery Project:

The Eel River Recovery Project (ERRP) has monitored water temperature throughout the Eel River watershed, checked flow conditions and also documented fish life since 2012.  The group’s temperature monitoring program is driven by volunteer interest and energy, and streams surveyed include the South Fork Eel River and selected tributaries in southern Humboldt and northern Mendocino County.  

In southern Humboldt, we have partnered with retired California Department of Fish and Game warden Larry Bruckenstein to monitor lower Sproul Creek where he and his wife Darcy reside, and the South Fork Eel downstream of Sproul Creek.  Bob and Barbara Froelich own land in Little Sproul Creek just upstream and are also ERRP monitoring partners.  In 2021, Sproul Creek lost surface flow in extensive reaches, and remained dry above the convergence with Little Sproul Creek as late as October 20, when water temperature gauges were retrieved.  

In 2022, it was a completely different story.  In early July when temperature gauges were placed, the flow of Sproul Creek was robust and there were numerous coho salmon juveniles feeding there.  No coho had been seen in lower Sproul Creek otherwise since 2012.  Little Sproul Creek is in late recovery from past logging and flood events and maintains cool water temperatures and flow throughout summer.  Coho had been documented there previously in several years, but not since 2018.  There were numerous coho juveniles in good condition present during an October 2 survey when gauges were being collected.  The most surprising find was that coho salmon were still present in main Sproul Creek at the mouth of Little Sproul, and further downstream at the Bruckenstein’s.  The maximum of lower Sproul Creek water temperature during summer was 73.9 F, which is near clinical lethal temperatures for coho and well above their recognized optimum of  62.2 F.  Natural selection is always at work, and survivors exhibited temperature tolerance that will likely be necessary for the species to adapt to climate change.  

Unfortunately, the fish community of lower Sproul Creek when probes were retrieved also included many small pikeminnow juveniles and California roach that outnumbered salmonids, representing another selective pressure.  Steelhead were present at all Sproul Creek locations during this year’s surveys, but few in number because record low rainfall from January to March prevented adult spawning migrations into tributaries.

Phil Petrovich and his pal Momo

Further up the South Fork watershed, ERRP has many monitoring partners and is sharing data with the newly formed Northern Mendocino Ecosystem Recovery Alliance.  Volunteer Phil Petrovich helped place and retrieve temperature probes in tributaries from Rattlesnake Creek to Red Mountain Creek, and main South Fork locations in this reach.  Cedar and Red Mountain creeks are monitored because they are cold-water refugia for salmon and steelhead juveniles.  Both have their headwaters in the critically important Red Mountain Wilderness.

Data show that South Fork water temperatures drop substantially below Cedar Creek, and its flow is nearly equal that of the river during critical drought conditions.  In addition to steelhead, Chinook salmon juveniles were seen at the mouth of Cedar Creek, when temperature gauges were placed in late May.

Where as upper Rattlesnake Creek at Area 101 dried up in 2021, flows were sufficient to maintain connection in 2022 and to provide cool water temperatures for rearing juvenile steelhead and trout.  Rainbow trout and steelhead are the same species, with some adopting a sea-run life history and other’s remaining as residents, migrating opportunistically up and down tributaries and into main river channels with fluxes in flow.  Where steelhead could not have taken advantage of this year’s April rains because they were not staged to spawn, conditions would have been optimal for resident trout.

Coho in Little Sproul Creek

Twin Rock Creek joins Rattlesnake Creek from the east, and it is like a hidden jewel.  The watershed view from Highway 101 is of steep chaparral terrain, and does not suggest a cold trout stream ecosystem.  But its headwaters receive high rainfall and most of its course follows a steep shady canyon that helps keep its waters cool.  Holly Ferreta participates in monitoring at her place on Twin Rock Creek, and her pool had at least four dozen steelhead and native trout juveniles, ranging in size from two to 12 inches long.  Given two years of drought preceding, it was reassuring to see that these fish had survived.  Twin Rock is an example of why the Eel River is so resilient.  While main branches experience ecological stress during seasonal droughts, hidden refugia allow cold-water fish to persist.  The continuing function of Twin Rock Creek, which maintained flow in 2021, is also in part owing to good stewardship of watershed residents.

See www.eelriverrecovery.org to view the temperature report, https://vimeo.com/763220280 to see a video of Sproul Creek and www.facebook.com/EelRiverRecovery/ to see photos.